Planetary and Space Science - Accueil du LPGlpg-umr6112.fr/lpg/fichiers/tobie-g/PUBLICATION/Mitri15.pdf · lakes and seas located mostly in Titan ... Titan and determine how the alcanonological

The exploration of Titan with an orbiter and a lake probe

Giuseppe Mitri a,n, Athena Coustenis b, Gilbert Fanchini c, Alex G. Hayes d, Luciano Iess e,Krishan Khurana f, Jean-Pierre Lebreton g, Rosaly M. Lopes h, Ralph D. Lorenz i,Rachele Meriggiola e, Maria Luisa Moriconi j, Roberto Orosei k, Christophe Sotin h,Ellen Stofan l, Gabriel Tobie a,m, Tetsuya Tokano n, Federico Tosi o

a Université de Nantes, LPGNantes, UMR 6112, 2 rue de la Houssinière, F-44322 Nantes, Franceb Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), Observatoire de Paris, CNRS, UPMC University Paris 06,University Paris-Diderot, Meudon, Francec Smart Structures Solutions S.r.l., Rome, Italyd Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853, United Statese Dipartimento di Ingegneria Meccanica e Aerospaziale, Università La Sapienza, 00184 Rome, Italyf Institute of Geophysics and Planetary Physics, Department of Earth and Space Sciences, Los Angeles, CA, United Statesg LPC2E-CNRS & LESIA-Obs., Paris, Franceh Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United Statesi Johns Hopkins University, Applied Physics Laboratory, Laurel, MD, United Statesj Istituto di Scienze dell‘Atmosfera e del Clima (ISAC), Consiglio Nazionale delle Ricerche (CNR), Rome, Italyk Istituto di Radioastronomia (IRA), Istituto Nazionale di Astrofisica (INAF), Bologna, Italyl Proxemy Research, Rectortown, VA, United Statesm CNRS, LPGNantes, UMR 6112, 2 rue de la Houssinière, F-44322 Nantes, Francen Institut für Geophysik und Meteorologie, Universität zu Köln, Cologne, Germanyo Istituto di Astrofisica e Planetologia Spaziali (IAPS), Istituto Nazionale di Astrofisica (INAF), via del Fosso del Cavaliere 100, Rome, Italy

a r t i c l e i n f o

Article history:Received 17 January 2014Received in revised form18 July 2014Accepted 21 July 2014

Keyword:Titan

a b s t r a c t

Fundamental questions involving the origin, evolution, and history of both Titan and the broaderSaturnian system can be answered by exploring this satellite from an orbiter and also in situ. We presentthe science case for an exploration of Titan and one of its lakes from a dedicated orbiter and a lake probe.Observations from an orbit-platform can improve our understanding of Titan's geological processes,surface composition and atmospheric properties. Further, combined measurements of the gravity field,rotational dynamics and electromagnetic field can expand our understanding of the interior andevolution of Titan. An in situ exploration of Titan's lakes provides an unprecedented opportunity tounderstand the hydrocarbon cycle, investigate a natural laboratory for prebiotic chemistry andhabitability potential, and study meteorological and marine processes in an exotic environment. Webriefly discuss possible mission scenarios for a future exploration of Titan with an orbiter and alake probe.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Titan, Saturn's largest satellite, is a unique object in the SolarSystem, having a substantial nitrogen atmosphere, a surface with acomplex interplay of geological processes (Lopes et al., 2010) andan outer ice shell overlying a subsurface ocean (Iess et al.,2012; Mitri et al., 2014). The climate on this icy satellite boasts amulti-phase alcanological cycle where methane plays a role

similar to water on Earth. The Cassini–Huygens mission to theSaturn system, which is still ongoing after 10 years of operations,accomplished a first in-depth exploration of Titan (Lebreton et al.,2009; Coustenis et al., 2009a). The data revealed hydrocarbonlakes and seas located mostly in Titan's polar regions (Stofan et al.,2007; Hayes et al., 2008), which, combined with extensiveequatorial dune deposits (Lorenz et al., 2006), form a majorreservoir of organics in the Solar System. The presence of standingbodies of liquid, dissected fluvial channels (Lopes et al., 2010),tectonic features (Radebaugh et al., 2007; Mitri et al., 2010), vastdune fields (Lorenz et al., 2006) and putative cryovolcanic flows

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/pss

Planetary and Space Science

http://dx.doi.org/10.1016/j.pss.2014.07.0090032-0633/& 2014 Elsevier Ltd. All rights reserved.

n Corresponding author.E-mail address: [email protected] (G. Mitri).

Please cite this article as: Mitri, G., et al., The exploration of Titan with an orbiter and a lake probe. Planetary and Space Science (2014),http://dx.doi.org/10.1016/j.pss.2014.07.009i

Planetary and Space Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎

www.sciencedirect.com/science/journal/00320633

www.elsevier.com/locate/pss

http://dx.doi.org/10.1016/j.pss.2014.07.009



mailto:[email protected]





(Lopes et al., 2007; 2013) shows striking analogies with terrestrialgeological activity. Titan is the only other place in the SolarSystem, with the Earth, to have exposed lakes and seas, albeitwith different materials.

The initial discoveries of the Cassini–Huygens mission inspiredan extension of the mission in 2008, first as the Cassini EquinoxMission and then as the Cassini Solstice Mission to 2017, as well asmultiple proposals to return to the Saturnian system for furtherexploration of this enigmatic satellite. Two mission proposals,Titan and Enceladus Mission TandEM (Coustenis et al., 2009a)and Titan Explorer (Lorenz, 2009) for ESA and NASA respectively,merged to form the Titan Saturn System Mission TSSM (Cousteniset al., 2010; ESA, 2009) which sought further exploration of Titanwith both a dedicated orbiter and in situ probes (balloon and lakeprobe) and a large instrument suite. Ultimately ESA and NASAdecided to prioritize the Europa Jupiter System Mission (EJSM),which would later become ESA's first Large (L) L1 mission of itsCosmic Vision Program (2015–2025), the JUpiter ICy moonExplorer (JUICE) with a planned launch date of 2022 (Grassetet al., 2012). The demise of TSSM inspired mission proposalssmaller in scale and cost including the Titan Mare Explorer TiME(Stofan et al., 2010, 2013), the Titan Aerial Explorer TAE (Lunineet al., 2011) and the Journey to Enceladus and Titan JET (Sotinet al., 2011), using either a probe or an orbiter but not both, and asmall instrument suite. Building on this heritage, the explorationof Titan with an orbiter and a lake probe was presented as a WhitePaper in response to the 2013 European Space Agency (ESA)'s Callfor Science Themes for the Large-class (L) L2 and L3 missions(Mitri et al., 2013). This paper is based upon the fore-mentionedWhite Paper. The Online Supplementary material for this paperincludes the list of the authors and the supporters of the WhitePaper from the scientific community.

Future exploration of Titan with a dedicated orbiter and anin situ lake probe would address central themes regarding thenature and evolution of this icy satellite and its organic-richenvironment. While the term lake probe is used in this paperand in previous literature, it should be noted that the ideal targetof any future exploration of the standing liquid bodies on Titanwould mostly likely be one of the large seas found in the northpolar region rather than a lake. Bodies of standing liquid on Titanwith large dimensions are classified as seas (mare) rather thanlakes (lacus). Prior to an orbiter phase around Titan and an in situexploration of its lakes, mission scenarios could also involveinvestigations of the planet Saturn and its other satellites includ-ing the geologically active icy moon Enceladus. A companionWhite Paper proposed the exploration of Enceladus with flybysand Titan with an orbiter and a balloon probe (Tobie et al., 2014,in this issue). While the exploration of the Saturn system and itsmoons, Enceladus and Titan, was ultimately not selected for the L2and L3 missions, the scientific case for such exploration receivedstrong support from the scientific community (Mitri et al., 2013;Tobie et al., 2014, in this issue).

Section 2 summarizes the scientific objectives for Titan'sexploration and how future scientific investigations of Titanwith a dedicated orbiter and lake probe can address some of theseobjectives. In Section 3 we discuss how in situ explorationof Titan's lakes can provide an unprecedented opportunity to

understand the hydrocarbon cycle, investigate a natural laboratoryfor prebiotic chemistry and the limits of life, and study meteor-ological and marine processes in an exotic environment. In Section4 we present the scientific investigations, measurements andexample approaches. Finally, in Section 5 we review the missionconcept legacy and in Section 6 we present a mission scenario forthe future exploration of Titan with an orbiter and a lake probe.

2. Scientific objectives and investigations for a futureexploration of Titan

In spite of all its advances, the Cassini–Huygens mission, due toits limited instrumentation and timeline, among other things,cannot address some fundamental questions regarding Titan suchas (1) What are the complex organic molecules that form the dunematerial, and how have they been formed? (2) What is thecomposition of the lakes and seas? (3) What are the processesthat form and maintain the atmosphere? and (4) Is Titan endo-genically active? In fact, these central themes can be investigatedby a future mission using an orbiter and a lake probe.

To answer these questions among others, the scientific objec-tives for a future exploration of Titan include (Table 1)(i) Determine how Titan was formed and evolved, and its internalstructure (Goal 1); (ii) Determine the nature of the geologicalactivity on Titan (Goal 2); (iii) Characterize the atmosphere ofTitan and determine how the alcanonological cycle works (Goal 3);and (iv) Determine the marine processes and chemistry of Titan'slakes and seas (Goal 4).

2.1. Geophysics

The combination of measurements of the gravity field, rota-tional dynamics and electromagnetic field will contribute to theachievement of the scientific objective Goal 1 (Table 1). Static anddynamic gravity field measurements (Iess et al., 2010, 2012), andshape models (Zebker et al., 2009; Mitri et al., 2014) from theCassini–Huygens mission have greatly expanded our knowledge ofthe geophysical processes and structure of Titan's interior and as aconsequence its formation and evolution have also become betterunderstood. Data from the Cassini–Huygens mission suggest thateither Titan has no induced magnetic field or else it is not yetdetectable, while an intrinsic magnetic field is found to be absent(Wei et al., 2010). While there is still not yet a unique internalstructural model of Titan, constraints provided by gravity, topo-graphy and rotation data suggest that Titan's interior has somedegree of differentiation and possesses a water-ice outer shellseparated from its deep interior by a dense subsurface water ocean(Mitri et al., 2014). Thermal models suggest the presence of a high-pressure polymorph ice layer between the deep interior and thesubsurface ocean (Tobie et al., 2005; Mitri and Showman, 2008;Mitri et al., 2010).

Radio-tracking data registered during multi-flybys of Titan hasyielded gravity field measurements with a spherical harmonicexpansion to degree-3 (Iess et al., 2010, 2012). The quadrupole(J2 and C22) moments of the gravity field were used to constrainthe normalized axial moment of inertia which value was inferred

Table 1Scientific objectives for a future exploration of Titan.

Goal 1 Determine how Titan was formed and evolved, and its internal structureGoal 2 Determine the nature of the geological activity of TitanGoal 3 Characterize the atmosphere of Titan and determine how the alcanological cycle worksGoal 4 Determine the marine processes and chemistry of Titan's lakes and seas

G. Mitri et al. / Planetary and Space Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎2





using the Radau–Darwin approximation as MoIffi0:341470:0005(Iess et al., 2010). The high moment of inertia suggests that Titan'sdeep interior is formed of relatively low-density material(�2600 kg m�3) that is consistent with a deep interior differen-tiated and composed of silicate hydrates such as antigorite(Castillo-Rogez and Lunine, 2010; Fortes et al., 2007) or onlypartially differentiated and composed of a mixture of ice androcks (Iess et al., 2010). The state of the internal structure of Titanis still debated, and future investigations are therefore necessaryto determine its internal structure. The interior structure of Titancan be inferred from a combined measurement of the gravity field,rotational dynamics and electromagnetic field.

Future gravity measurements from a spacecraft (S/C) in a polarorbit will provide a significant improvement to the gravity fielddetermination with respect to Cassini's gravity measurements. Weconsider a nominal final orbit of the S/C around Titan as a 1500 kmaltitude circular near-polar orbit. Using such an orbit, the staticgravity field of Titan can be resolved to degree-10 and possiblyhigher orders. Also the gravity anomalies can be determined witha spatial resolution lower than 260 km allowing a better char-acterization of the density contrasts within the ice shell as well aswithin the deep interior.

Analysis based on Titan's shape suggests that Titan's outer iceshell is thicker at the equator and thinner at the poles (Nimmo andBills, 2010; Hemingway et al., 2013; Mitri et al., 2014). Estimatesfor the thickness of the ice shell vary from thin (50 km) to thick(200 km) (e.g., Tobie et al., 2005, 2006; Mitri et al., 2008), anddetermining which model is correct will require observationsbeyond the capability of Cassini mission. The thickness of Titan'sice shell has major implications for the thermal state of the crust.In fact, geological activities and processes on Titan could be relatedto the onset of thermal convection in the outer ice shell (Tobieet al., 2005, 2006; Mitri and Showman, 2008). Analysis of Titan'stopography suggests that the ice shell is currently in a conductivestate (Nimmo and Bills, 2010; Hemingway et al., 2013; Mitri et al.,2014); however, during Titan's evolution it is possible that the iceshell may have transitioned between a convective to a conductivestate (Mitri et al., 2008, 2010).

The radio science experiment during the Cassini missionallowed the measurement of the tidal Love number, k2. The tidalLove number k2 characterizes the tidal deformation of the interior.The relatively high measured tidal Love number k2 (0.58970.150)of Titan indicates that its outer ice shell is decoupled from thedeep interior by a global subsurface ocean (Iess et al., 2012). Thetidal Love number k2 (Iess et al., 2012) in conjunction withaltimetric measurements of the tidal deformation of the outerice shell (Love number h2 measurement) could be used todetermine the thickness of the ice shell (see similar experimentproposed for Europa by Wahr et al. (2006)) during a futuremission. Using a nominal polar circular orbit around Titan with a1500 km altitude, the measurement of the tidal Love number k2can be obtained with an expected uncertainty σk2 � 5� 10�4

improving the accuracy of the determination of k2 achieved duringthe Cassini mission by two orders of magnitude. In addition, radarsounder observations with a penetration depth up to �10 kmwitha vertical resolution of �30 m from an orbital platform coulddirectly determine the relict brittle–ductile transition of the iceshell revealing its thermal state.

Surface heights derived during the Cassini mission from theCassini RADAR altimeter (Elachi et al., 2004) and SAR-Topographytechnique (Stiles et al., 2009) are sparsely distributed and coveronly �1% of Titan's surface. Using the available altimetric data,Zebker et al. (2009) and Mitri et al. (2014) determined the shape ofTitan up to the seventh order of the spherical harmonic expansion.A quasi-global coverage of altimetric data with a �10 m verticalresolution for a future mission is necessary to refine the shape

model and gravity anomalies of Titan's interior and define thelevel of hydrostatic compensation of the shape and the outer iceshell. A refinement of the shape of Titan at �1 m verticalresolution will allow the characterization of the tidal deformationof the outer layer and the determination of the Love number h2.Radar altimetry is a suitable technique to determine the elevationof Titan's surface as demonstrated during the Cassini mission.Interferometric Synthetic Aperture Radar (InSAR) is also a possibletechnique that could be used to generate digital elevation models(DEMs) of the surface and to produce maps of surface deformationin combination with the imaging of the surface.

The measurement of the imaginary part of the tidal Lovenumber k2 using gravity field measurements from the radioscience experiment can be used to estimate the tidal quality factorQ ¼ k2

��

��=Imðk2Þ. The estimation of Q would be used to constrain

the present internal tidal dissipation of Titan providing informa-tion on the internal heat production as well as on the tidalevolution of Titan. The estimation of Q would also provideinformation on the viscosity of the ice shell and indirectly on thethermal structure as well as on the thermal state (conductive orconvective) of the ice shell.

The determination of rotational dynamics is crucial to under-stand the internal structure and geophysical processes on Titan.Evolution models of the orbital dynamics use the obliquity alongwith the eccentricity as a constraint. When the quadrupole gravityfield is known, the obliquity may also provide the moment ofinertia (Bills and Nimmo, 2008), a fundamental quantity toconstrain the internal structure of Titan. The spin rate and thephysical librations in longitude can provide essential informationon the degree of internal differentiation and the ice shell thick-ness. Currently the only information about the rotational dynamicsof Titan is derived from Cassini mission data (e.g., Stiles et al.,2008; Meriggiola et al., 2011) and astronomical observations(Seidelmann et al., 2006).

During Cassini observations the pole location was found to benot fully compatible with the occupancy of a Cassini state(Meriggiola et al., 2011). The estimated obliquity (0.31) is not inagreement with the predicted value (0.151) inferred from the polar,dimensionless moment of inertia (0.34) derived from gravity fieldmeasurements (Iess et al., 2010). The interpretation of the dis-agreement between the measured and predicted values is con-troversial and it seems to point to a differentiated internalstructure characterized by complex rotational dynamics. For thisreason the pole location in terms of obliquity and the occupancy ofa Cassini state should be investigated by a dedicated experiment.Physical librations in longitude are due to the eccentricity. Titan isin a 1:1 resonance and it is orbiting around Saturn at the same rateof its spin motion (Meriggiola et al., 2011). Since Titan's orbit iseccentric, the Saturn-satellite line and the long axis of the satelliteare not aligned and they differ by a libration angle. The librationamplitude is inversely proportional to the moment of inertia of thebody, and it is possible, if the quadrupole gravity field is known, toinfer the thickness (h) of the outer icy shell from the librationamplitude. The determination of the libration amplitude canprovide crucial information on the dimension as well as on theliquid/solid state of the shell and the deep interior. The rotationaldynamics of Titan can be inferred from tracking surface landmarksalong time using, for example, Synthetic Aperture Radar (SAR)images with a spatial resolution of �30 to 50 m, and/or fromgravity field measurements (see Cicalò and Milani (2012)).

Observations of Cassini's magnetic field on Titan show that itcurrently does not have an intrinsic field generated by a dynamo(upper limit is 2 nT surface field at the equator). Even though theCassini mission has not been successful thus far in inferring anelectromagnetic induction response from a subsurface salty oceanas measured during the Galileo mission for Europa, Ganymede and

G. Mitri et al. / Planetary and Space Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎ 3





Callisto, future, orbiting spacecraft can provide better informationon the interior (subsurface ocean) from this technique. Thediscovery of periodic changes in the quadrupole gravity field ofTitan at �4% of the static value indicates the presence of aninternal ocean (Iess et al., 2012). Further independent confirma-tion of Titan's subsurface ocean is necessary.

Detection of electromagnetic induction from the interior ofTitan induced by time-varying external magnetic fields couldprovide such a confirmation as has been successfully demon-strated in the discoveries of subsurface oceans of Europa, Callistoand Ganymede (Khurana et al., 1998; Zimmer et al., 2000; Kivelsonet al., 2002) and the magma ocean of Io (Khurana et al., 2011). Thebasic principle of electromagnetic induction relies on the fact thatsubsurface conductors generate eddy currents on their surfaces inresponse to time varying electric fields imposed by the changingprimary magnetic field. The secondary field generated by the eddycurrents is dipolar in nature if the conductor has a spherical shapeand can be easily characterized by a spacecraft bearing a magneto-meter. Modeling of the secondary field provides information onthe thickness and conductivity of the subsurface conductor. For theGalilean satellites, the time varying driving primary field isprovided by the rotating tilted magnetic field of Jupiter and itsmagnetosphere and has an amplitude ranging from 40 nT nearCallisto to 800 nT at Io. However, Saturn's internal magnetic field iscompletely axisymmetric and therefore does not impose a time-varying field on its satellites. However a cyclical magnetic field ofunknown origin has recently been detected; this field is supportedby current systems in Saturn's magnetosphere (Andrews et al.,2010). This magnetic field has an amplitude of 2–4 nT near Titanand has a period close to 10.8 h which is an ideal soundingfrequency. Assuming that the ice crust has a thickness of 100 kmor less, a highly conducting ocean would then create a secondaryinduced field with a magnitude of �0.6 to 1.3 nT just above Titan'satmosphere (assuming an R�3 falloff for the dipolar response)which would be detectable by a magnetometer. Another inducingfield with an amplitude exceeding 10 nT is imposed by theperiodic exits of Titan from Saturn's magnetosphere. The severalnT induction response to this signal would not be sinusoidal butwould be easily characterized by an orbiting spacecraft. Unfortu-nately, the plasma interaction of Titan with the magnetosphere ofSaturn creates magnetic perturbations with an amplitude of 10 nTor more (Wei et al., 2010), therefore, it has so far not been possibleto confirm the presence of an electromagnetic induction signalfrom Titan. This situation can be remedied if continuous observa-tions from an orbiter were available. The spatial forms andfrequency contents of magnetic fields generated by plasma inter-action and by electromagnetic induction are distinct and can beeasily separated by harmonic analysis if continuous observationsfrom an orbiter were available. Such an experiment will beperformed by the JUpiter ICy moons Explorer – JUICE spacecraftmission at Ganymede where researchers hope to disentangle theinducing field from its ocean with an amplitude of �60 nT fromthe much stronger permanent internal magnetic field with amagnitude of 1500 nT.

2.2. Surface processes and composition

A combination of measurements and observations from surfaceimaging, altimetric data, spectral data and radar sounding willenable the achievement of the scientific objective Goal 2 (Table 1).The Cassini mission has revealed for the first time the complexityof the surface processes and composition of Titan. However, it isstill not understood if Titan is endogenically active and if cryo-volcanic activity is present. Other unsolved questions includedetermining the role of the lakes, channel networks and dunefields in the alcanological cycle (e.g. Aharonson et al., 2012).

Sections 2.2.1 and 2.2.2 discuss how observations from theorbital-platform will improve our understanding of the geologicalprocesses and surface composition, respectively.

2.2.1. GeologyOur current knowledge of Titan's geology comes from Cassini

observations, which show a geologically complex surface strik-ingly similar to Earth's. Titan has been called the Earth of the outerSolar System. Titan has a dense atmosphere (1.5 bar) and thepresent-day surface–atmospheric interactions make eolian, fluvial,pluvial and lacustrine processes important on a scale previouslyseen only on Earth, despite vast differences in material propertiesand ambient conditions. These atmospheric and surface processesinteract to create similar geologic features to Earth's: lakes andseas filled with liquid, rivers caused by rainfall, and vast fields ofdunes caused by wind (e.g. Lorenz et al., 2006; Stofan et al., 2007;Hayes et al., 2008; Radebaugh et al., 2008; Barnes et al., 2008; Burret al., 2013). The scarcity of impact craters indicates that Titan'ssurface is relatively young, with estimates ranging from 200 Ma to1 Ga (Lorenz et al., 2007; Wood et al., 2010). Mountains indicatethat tectonic activity has taken place (Radebaugh et al., 2007; Mitriet al., 2010) although it may not necessarily be endogenic in nature(Moore and Pappalardo, 2011). Several putative cryovolcanicfeatures have been identified (e.g. Lopes et al., 2007, 2013;Soderblom et al., 2009) and the existence of some ongoingvolcanic activity has been proposed (Nelson et al., 2009a,2009b), though the cryovolcanism interpretation has been dis-puted (Moore and Pappalardo, 2011). The variety of geologicprocesses on Titan and their relationship to the methane cyclemake it particularly significant in Solar System studies.

The Cassini RADAR swaths have revealed a wide variety ofgeologic features, examples of which are shown in Fig. 1. However,interpretations of Titan's surface properties and geologic historyhave been hindered by observational challenges primarily relatedto its dense obfuscating atmosphere. Currently, the highest resolu-tion observations of Titan's surface come from SAR images with apixel-scale of �300 m. At this resolution, Cassini has imaged�43% of Titan's surface. Including lower resolution HiSAR data(�few km pixel scale), obtained at further distances from Titanthan standard SAR, Cassini's surface coverage increases to �60%.

A follow-up mission to Titan should image the surface at a pixelscale of 30 m or better with an almost complete surface coverageusing Synthetic Aperture Radar (SAR) technique. As has beenshown in both the terrestrial and Martian communities, resolutionincreases of an order of magnitude or better results in funda-mental advances in the interpretability of observed morphologies.Altimetric data with a vertical resolution of �10 m is necessary forthe geological interpretation of surface features. Both radar alti-metry and Interferometric Synthetic Aperture Radar (InSAR) aresuitable techniques to determine the elevation of Titan's surface.Radar sounder measurements with a penetration depth up to fewkm and a vertical resolution of 10–30 m are capable of character-izing the subsurface structures and determining their correlationwith surface features allowing the determination of the internalprocesses that form the surface geological features. In addition,radar sounder measurements will determine the thickness andstructure of surface organic-material deposits, which can be usedto ascertain the global volume of organic material on Titan.

The major geologic unit types and their possible genesis havebeen recently reviewed by Lopes et al. (2010). Below, we brieflydescribe each of the major geomorphic units that have beenidentified and, where appropriate, state how higher resolution datacould provide fundamental advancements in their interpretability.

Hummocky and mountainous terrain: This unit consists ofnumerous patches of radar-bright, textured terrains, some of






which have been interpreted to be of tectonic origin (Radebaughet al., 2007; Mitri et al., 2010). Among these features are longmountain chains, ridge-like features, elevated blocks that standgenerally isolated, and bright terrains of a hilly or hummockyappearance. This terrain is mostly found as isolated patches orlong mountain chains, small in areal extent. The exception isXanadu, a large area about 4500 km across. The hummocky andmountainous terrain unit, including Xanadu, is thought to be theoldest geologic unit exposed on Titan's surface (Lopes et al., 2010).Deciphering the origin of this feature class awaits high-resolutionimagery that can resolve the complex lineations and brightnessvariations that make up the hummocky appearance; altimetricdata can be used to determine the elevation of this terrain.

Impact craters: Few well-preserved impact craters have beenidentified on Titan. However, there are numerous structures on

Titan that may have been caused by impacts. Radar-bright com-plete or incomplete circles are seen all over the surface. Woodet al. (2010) identified 44 possible impact craters with a widevariety of morphologies and, since then, others have been recog-nized and mapped. Neish et al. (2013) found Titan's craters areshallower than those on Ganymede, probably due to eolianinfilling, with possible contributions from viscous relaxation andfluvial modification. Radar sounding can be used to determine thestructure of the craters. Detailed morpho-dynamic analysis ofTitan's craters, however, requires high resolution stereo modelsthat allow quantitative comparison with numerical simulations.

Dunes: Eolian transportation and deposition is a major andwidespread process on Titan. Features resembling linear dunes onEarth have been identified by SAR, covering regions hundreds,sometimes thousands, of kilometers in extent (Lorenz et al., 2006).

Fig. 1. Examples of geological features on Titan. Top row from left: Sinlap crater with its well-defined ejecta blanket. Middle: Crateriform structure (suspected impact crater)on Xanadu. Right: Ontarius Lacus near Titan's south pole. Middle row, left: Putative cryovolcanic flow Winia Fluctus (lobate, radar bright), note the dunes overlaying part offlow. The flow overlays radar-dark undifferentiated plains. Right: Dunes abutting against hummocky and mountainous terrain. Bottom left: mountains (radar bright) in theequatorial region. Right: Elivagar Flumina, interpreted as a large fluvial deposit, showing braided radar-bright channels. Deposits overlay radar-dark undifferentiated plains.






Dunes are dark to RADAR and Imaging Science Subsystem (ISS)and correlate well with dark surface units identified by the Visualand Infrared Mapping Spectrometer (VIMS). The dunes arethought to be made of organic particles (Soderblom et al., 2007),which may have originated as photochemical debris rained outfrom the atmosphere, deposited and perhaps later eroded byfluvial processes into sand-sized particles suitable for dune for-mation (Atreya et al., 2006; Soderblom et al., 2007). Titan's sanddunes represent a detailed geomorphic record of both current andpast wind regimes that can reveal global changes in climate andsurface evolution (e.g. Radebaugh et al., 2008). Studies of Titan'sdune morphologies, how the morphologies spatially vary andhow the dunes interact with local topography provide abasis for interpreting wind direction, sediment supply, and atmo-sphere conditions such as boundary layer depth. Thus dunesrepresent a morphologic unit where even slight increases in imageresolution can reveal patterns that have significant interpretiveimplications.

Channels: Many radar-bright and radar-dark channels areseen in SAR images, widely distributed in latitude and longitude.It is likely that many more channels exist on Titan, but are toosmall for the Cassini spacecraft SAR data to resolve. Channels areseen connecting some lakes at high latitudes and cutting thehummocky, mountainous terrain at lower latitudes, possiblyindicating orographic rainfall. Channels, lakes and dunes arethought to be the youngest geologic units on Titan (Lopes et al.,2010). The Labyrinthic terrains unit, identified by Malaska et al.(2010) is a probable end-member of the different types of channeland valley network morphologies identified on Titan. There arefew areas of this type of terrain exposed, mostly at high latitudes.Higher resolution images of Titan's channel networks can allowpattern analysis, such as branching angle distribution, that willaddress the strength of the bedrock and origin (e.g., overland flowvs. subsurface drainage) of the network (e.g., Burr et al., 2013).

Lakes: Hydrocarbon lakes are found preferentially in the polarregions at the current season on Titan (Stofan et al., 2007). WhileSection 3 discusses the benefits of in situ lake exploration, there isalso much to learn from studying the morphology and distributionof lacustrine features on Titan. Titan's lakes and seas are foundboth connected to and independent of regional and local drainagenetworks. They have shoreline transitions varying from sharp todiffuse and include a variety of plan-form shapes ranging fromnear-circular to highly-irregular. Exposed surface areas span fromthe limits of detection (�1 km2) to more than 105 km2 (Hayeset al., 2008). Along with the surrounding landscape topography,these lacustrine and features express a range of morphologiccharacteristics that constrain basin formation and evolution. Forinstance, the largest lakes, or seas, in the north polar region,consisting of Ligeia, Kraken and Punga Mare, have shorelines thatinclude shallow bays with well-developed drowned river valleys(Stofan et al., 2007). Such drowned valleys indicate that once well-drained upland landscapes have become swamped by rising fluidlevels where sedimentation is not keeping pace with a rising baselevel. These features also imply that liquid levels were previouslylower and stable long enough to allow the currently drownednetworks to be incised into the terrain. In contrast the largestsouthern lake, Ontario Lacus, expresses depositional morphologiesalong its western shoreline that include lobate structures inter-preted as abandoned deltas (Wall et al., 2010). Obtaining higherresolution images and topography data of Titan's lakes will allowquantitative morphology analysis that will lead to new hypothesesfor landscape formation and evolution.

Candidate cryovolcanic terrain: The Cassini's SAR swaths alsorevealed several lobate, flow-like features and constructs inter-preted as resulting from cryovolcanism (Lopes et al., 2007; Wallet al., 2009) and VIMS data also identified some features that were

thought to be cryovolcanic in origin (Sotin et al., 2005; Barneset al., 2006; Nelson et al., 2009a, 2009b). As more data wereacquired, particularly overlapping SAR coverage fromwhich stereocould be derived, several putative cryovolcanic units were re-assessed and thought to be of a different origin (Lopes et al., 2010)leading Moore and Pappalardo (2011) to argue that there may beno cryovolcanic units on Titan at all. However, radar stereogram-metry has revealed a tall mountain (Doom Mons) next to a deepelongated pit (depth 41 km), adjacent to several flow-like fea-tures. This is considered the strongest evidence for a cryovolcanicregion on Titan (Lopes et al., 2013). Higher resolution images andtopography are needed in order to further scrutinize potentialcryovolcanic features. Radar sounder measurements can distin-guish cryovolcanic features such as calderas from other featuresthat have similar morphology (e.g. impact craters) imaging thesubsurface structures.

Undifferentiated plains: There are vast expanses of plains,mostly at mid-latitudes, that appear relatively homogeneous andradar-dark, and are classified as plains because they are reallyextensive, relatively featureless, and generally appear to be of lowrelief. The units may be sedimentary in origin, resulting fromfluvial or lacustrine deposition, or they may be cryovolcanic, buttheir relatively featureless nature and gradational boundariesprevent interpretation using SAR data alone. Radar soundermeasurements will determine the layering of the undifferentiatedplains to determine if they are sedimentary or cryovolcanic inorigin.

Mottled plains: The mottled plains are irregularly shapedterrains having dominantly intermediate backscatter that appearto have relatively small topographic variations. Mottled terrainsmay be erosional and a mix of several other units, for example,some may be undifferentiated plains covered over in small areasby isolated dunes.

2.2.2. Surface compositionObservations of Titan's surface are severely hindered by the

presence of an optically thick, scattering and absorbing atmo-sphere, allowing direct observation of the surface within only afew spectral windows in the near-infrared. Based on the 1.29/1.08 μm, 2.03/1.27 μm and 1.59/1.27 μm band ratios measured byCassini's VIMS, three main spectral units can be distinguished onthe overall surface of Titan: whitish material mainly distributed inthe topographically high areas; bluish material adjacent to thebright-to-dark boundaries; and brownish material that correlateswith RADAR-dark dune fields (Barnes et al., 2007; Soderblom et al.,2007; Jaumann et al., 2008). Even though the spectral units aredistinct, their actual composition remains elusive. Bright materialsmay consist of precipitated aerosol dust composed of methane-derived organics (Soderblom et al., 2007) superimposed on water-ice bedrock. The bluish component might contain some water iceas its defining feature (Soderblom et al., 2007), even thoughorganics cannot be ruled out. Finally, the brownish unit revealslower relative water-ice content than Titan's average that iscompatible with an enhancement in complex organic materialand/or nitriles clumping together after raining onto the surface(Barnes et al., 2008).

A number of chemical species, including water ice, have beenproposed to exist on the surface of Titan including carbon dioxide,ammonia, ammonia hydrate, cyanoacetylene, benzene, hydrogencyanide, acetylene, acetonitrile, and liquid alkanes. But only a fewabsorptions have been unambiguously detected in Titan's methanewindows from remote sensing observations carried out duringCassini fly-bys. Evidence from VIMS infrared spectra suggests CO2

frost, which is probably condensed from the atmosphere (McCordet al., 2008). Liquid ethane has been identified in Ontario Lacus






(Brown et al., 2008) and, although the exact blend of hydrocarbonsin polar lakes and seas is unknown, liquid methane is almostcertainly present. Finally, the correlation between the 5-μm-brightmaterials and RADAR-empty lakes suggests the presence ofsedimentary or organic-rich evaporitic deposits in dry polarlakebeds (Barnes et al., 2011).

Spectral mapping of Titan's diverse surface at both high spatialand high spectral resolution could close a major gap in knowl-edge after the Cassini mission. Cassini's VIMS instrument isexpected to cover only a small percent of Titan's surface atkilometer-or-better resolution, even after the extended mission.Thus a prime element in the orbiter payload is a spectral mapper.In essence this instrument addresses two main goals: high-resolution near-IR imaging and spectroscopy of the surface. Otherspectral end-members will likely appear as a dedicated orbitermission improves the spatial resolution and coverage by orders ofmagnitude. An important augmentation beyond Cassini is thatthe spectral mapper should cover the wavelength range beyond5 μm (ideally up to 6 μm). This spectral region (in a methanewindow, where the atmosphere is quite transparent) is highlydiagnostic of several organic compounds expected on Titan'ssurface. Cassini VIMS data has already shown some tentativesignatures around 5 μm (possibly associated with aromatic and/or polycyclic compounds like phenanthrene, indole, etc.), how-ever robust identification is impossible due to inadequate spec-tral resolution. Cassini VIMS has shown that Titan's surface can beobserved and mapped at 5 μm (Sotin et al., 2012) despite the lowsignal-to-noise ratio of the VIMS instrument at this wavelength,the small amount of solar flux and the small surface albedo. Withthe current technologies, detectors in this wavelength domaincould provide high resolution mapping as well as diagnosticsignatures of several organic compounds likely to be present onTitan's surface either in the dune fields or on the shores of lakes(Stofan et al., 2007; Barnes et al., 2011). Finally, correlationsbetween high-resolution infrared hyperspectral images and sur-face images (e.g. SAR data) will enable the identification of terrainunits at various spatial scales, which are homogeneous both interms of surface composition (abundance of non-water-ice com-ponents compared to water ice) and surface properties (e.g.,roughness and porosity) (Tosi et al., 2010).

2.3. Atmosphere

An instrument suite, enabling both remote and in situ measure-ments and observations of Titan's atmosphere including, forexample, lidar and infrared spectrometers on an orbiter platformand mass spectrometers, physical property (temperature, pressure,humidity, wind speed) sensors and an imaging camera onboard alake-probe could be used to achieve scientific objective Goal 3(Table 1). Tobie et al. (2014, in this issue) discuss in detail thescientific case for the in situ examination of Titan's atmosphereusing a balloon probe.

Unlike the other moons in the Solar System, Titan has a densenitrogen-dominated (98.4%) atmosphere, presenting unique atmo-spheric dynamics, atmospheric chemistry and alcanonologicalcycle. Besides nitrogen, the atmosphere consists of 1.5% methane,as well as trace amounts of hydrocarbons such as ethane, acet-ylene, and diacetylene, benzene and nitriles, such as hydrogencyanide (HCN), cyanogen (C2N2) and cyanoacetylene (HC3N).Somewhat more complex molecules such as propane, butane,polyacetylenes, and complex polymeric materials might followfrom these simpler units based on experiments in the laboratory.In addition, several oxygen compounds, including water vapor, COand CO2 exist in the atmosphere, probably of exogenic origin. TheCassini–Huygens mission detected and measured many of theconstituents in Titan's atmosphere (Coustenis and Hirtzig, 2009).

The Cassini mission has revealed the presence of large organicmolecules with high molecular masses, above 100 amu, in thethermosphere and ionosphere. This is an indication of the photo-chemical production of Titan's aerosols starting in the upperatmosphere. While the identities of these molecules are stillunknown, their presence suggests a complex atmosphere thatcould hold the precursors for biological molecules such as thosefound on Earth.

The thermal structure of Titan's atmosphere is similar to theEarth's, with a troposphere, a stratosphere, a mesosphere and athermosphere; however these regions develop on a larger scaleheight due to Titan's lower gravity and lower temperatures.A detached layer of haze, the result of photochemical reactionsin the upper atmosphere, is visible at altitudes greater than�520 km (Porco et al., 2005). The Voyager spacecraft showedthe vertical structure of the haze to consist of a main layer, locatedat an altitude below 200 km, separated from a detached layer ofhaze at an altitude of �500 km by a gap of approximately 300 km;these two layers merge near the north pole (Porco et al., 2005).The extended haze layer has been found to have reduced itsaltitude from 500 km to �380 km between 2007 and 2010 (Westet al., 2011). Cassini's images also confirmed the existence of brightclouds in Titan's atmosphere, which were already inferred fromground-based observations (Brown et al., 2002). Titan's clouds areprobably composed of methane, ethane, or other simple organics.Their presence seems scattered and variable, interspersing theoverall haze.

A striking feature of Titan's atmospheric dynamics is thestrong stratospheric super-rotation in either hemisphere(Achterberg et al., 2011). The global profile of zonal windsbetween 100 and 500 km altitude can be reconstructed fromthe temperature data retrieved from Cassini Composite InfraredSpectrometer (CIRS), except very close to the equator (Achterberget al., 2011). On the other hand, the wind profile below 100 kmaltitude is known only at the entry site of the Huygens Probe.Huygens' Doppler Wind Experiment (Bird et al., 2005) found thatthe superrotation gradually weakens below 150 km altitude, butit also detected an unexpected layer of extremely weak windsnear 80 km altitude and a complex wind pattern near the surface.This demonstrates that in situ measurements can detect windpatterns especially in the troposphere that cannot be detectedfrom remote measurements in other parts of the atmosphere. Thewind speed and direction in selected areas of the troposphere canbe constrained by cloud tracking (Turtle et al., 2011a) but onlywhere clouds exist. The observed orientation and appearance ofdunes have been used to constrain the surface wind, but this ledto ambiguous interpretations on the prevailing surface winddirection (Lorenz and Radebaugh, 2009; Rubin and Hesp, 2009;Tokano, 2010). The wind direction near the surface and itspossible seasonal variation are crucial measurements for theangular momentum budget of the atmosphere as well aslength-of-day variation of the surface (Tokano and Neubauer,2005). It is unknown from atmospheric observations whetherthere is an Earth-like large angular momentum exchangebetween the atmosphere and surface, which is equilibrated onan annual average, or there is almost no angular momentumexchange. Geodetic observations should ideally be accompaniedwith wind observations to establish a link between possibleatmospheric forcing and response of the surface. Near-surfacewind measurements by a lake lander will be meaningful in thisregard since the wind at high latitudes is more indicative ofseasonal wind fluctuations than equatorial winds and there areno apparently active dunes at high latitudes that could be used toguess the wind direction.

Another major characteristic of Titan's meteorology is themethane weather, which most impressively manifests itself in






the visible clouds (Roe, 2012). During the southern summer mostconvective clouds were seen at high southern latitudes but themain cloud activity moved northward around the recent equinox(Schaller et al., 2009; Turtle et al., 2011a). More persistent cloudsat higher altitudes were seen near the north pole (Rodriguez et al.,2009). For many years, the equatorial region was devoid of visibleclouds and thus was expected to be dry. However, the verticalprofile of temperature and pressure (Fulchignoni et al., 2005) andmethane mole fraction (Niemann et al., 2005) measured byHuygens found that methane condensation and weak precipita-tion in the form of drizzles occur even near the equator where novisible cloud was present (Tokano et al., 2006). On the other hand,occurrence of convective clouds strongly depends on the globalcirculation pattern, static stability and methane humidity in thelower troposphere (Mitchell et al., 2011; Schneider et al., 2012).While the vertical temperature profile down to the surface can besounded by radio occultation (Schinder et al., 2012), the methanehumidity is difficult to measure remotely. In situ measurement ofthe near-surface methane humidity in the polar region wouldtherefore provide valuable insight into the mechanism behind theseasonality of Titan's methane weather. There is also evidence ofsubstantial methane rainfall on Titan, but the evidence is indirectin that it came from temporary surface darkening and temporalcorrelation with a cloud system (Turtle et al., 2011b). The pre-cipitation amount is unknown despite the huge role it plays forTitan's geomorphology. Only direct measurements of rainfall onthe surface would unambiguously show in which seasons it doesor does not rain in the polar region, how much it rains and thechemical composition of the rains.

The meridional circulation pattern in the troposphere pre-dicted by general circulation models (GCMs) differs from modelto model, particularly at high latitudes. To some extent the globalstatistics of clouds constrain the meridional circulation pattern(Rodriguez et al., 2009). However, clouds do not develop in theupwelling branch of the circulation if the near-surface air is toodry and not every condensation causes visible clouds. Therefore, amore certain quantity to verify the meridional circulation at highlatitudes is the surface pressure, which can be compared to thein situ surface pressure data at the Huygens landing site(Fulchignoni et al., 2005) after a sea-level correction. A surfacepressure higher and lower than the Huygens pressure wouldindicate downwelling and upwelling, respectively, in the seasonof observation.

Theoretical studies predict some significant changes when thenorth polar lake area approaches summer during the next fewyears. Seasonal change in the global circulation pattern may causesubstantial precipitation in the north polar region (Schneider et al.,2012), warming of the sea surface may cause a sea breeze/landbreeze depending on the sea composition (Tokano, 2009), fresh-ening of the near-surface wind may cause previously undetectedwaves on the seas (Hayes et al., 2013) and evaporation of a largeamount of methane from the seas might even cause tropicalcyclones (Tokano, 2013). These meteorological features causesubstantial surface pressure variations and thus can be detectedin situ by continuous surface pressure measurements and opticalmonitoring of the environment. The unknown exact chemicalcomposition in the sea, as well as in the air over sea, is importantfor many questions related to the polar region. Evaporation ofmethane from a sea itself is difficult to observe in situ unlessaccompanied with fog, but it can be quantified from the methanevapor pressure and methane partial pressure. The mole fractionsof other species in the sea and air over sea would indicate whetherthe lake and atmosphere are in short-term or long-term chemicalequilibrium, if any.

GCMs predict gravitational tides and various waves in thetroposphere, which affect the wind, pressure and temperature

near the surface (Tokano and Neubauer, 2002; Mitchell et al., 2011;Lebonnois et al., 2012). Some of the waves may become visible asclouds but most other waves may not. The most reliable data toverify these waves are in situ time series measurements of surfacewind and atmospheric pressure over several days. The variouswaves have different frequencies and amplitudes and may thus berecognized in the power spectra of the time series.

3. In situ exploration of a Titan sea

The Cassini spacecraft has unveiled a world that is both strangeand familiar to our own, with vast equatorial dune fields (e.g.,Lorenz et al., 2006), well-organized channel networks that drainfrom mountains into large basins (e.g., Burr et al., 2013) and,perhaps most astonishingly, lakes and seas (e.g., Stofan et al., 2007)filled with liquid hydrocarbons (Brown et al., 2008). Titan is theonly extraterrestrial body currently known to support standingbodies of liquid on its surface and is one of only three places in theSolar System, along with Earth and Mars, that we know to possesor have possessed an active hydrologic system (e.g., Atreya et al.,2006). Just as Earth's history is tied to its oceans, Titan's origin andevolution are chronicled within the nature and evolution of itslakes and seas and their interactions with the atmosphere andsurface. While Cassini has provided a wealth of informationregarding the distribution of liquid deposits on Titan (e.g., Hayeset al., 2008; Turtle et al., 2009; Aharonson et al., 2009), it hasprovided only the most basic information regarding their compo-sition (e.g., Brown et al., 2008) and role in Titan's volatile cycles(e.g., Lunine and Atreya, 2008). In order to address these funda-mental questions, which link the lakes to Titan's origin andevolution, we must visit them in situ. In situ exploration of Titan'slakes and seas presents an unprecedented opportunity to under-stand its hydrocarbon cycles through the abundance and varia-bility of the liquid's compositional components (methane, ethane,propane, etc.), glimpse into the history of the moon's evolution aschronicled by noble gas abundance (argon, krypton, xenon) andisotopic composition (e.g., 12C vs. 13C), investigate a naturallaboratory for prebiotic chemistry and the limits of life throughan examination of trace organics, and study meteorological andmarine processes in an exotic environment that is drasticallydistinct from terrestrial experience (scientific objectives 1, 2, 3,and 4, Table 1).

Titan's lakes are found mostly poleward of 551 latitude andencompass 1.2% of the surface that has been observed (�50%) byCassini's instruments (Hayes et al., 2008, 2011). Lacustrine featuresare predominantly found in the north, where 11% of the observedarea poleward of 551N (�60%) is covered by radar-dark lakes.Poleward of 551S, only 0.5% of the observed area (�60%) has beeninterpreted as potentially liquid-filled. This dichotomy has beenattributed to Saturn's current orbital configuration, in whichTitan's southern summer solstice occurs nearly coincident withperihelion and results in a 25% higher peak flux than what isencountered during northern summer (Aharonson et al., 2009).Over many seasonal cycles, this flux asymmetry can lead to nettransport of volatiles (methane/ethane) from the south to thenorth. As the orbital parameters shift, however, the net flux ofnorthward-bound volatiles is expect to slow and eventuallyreverse, predicting a liquid distribution opposite of today's in�35 kyr (i.e., more liquid in the south than the north). If thishypothesis is correct, the distribution of liquid deposits on Titan isexpected to move between the poles with a period of �50 kyr in aprocess analogous to Croll–Milankovich cycles on Earth. In situmeasurement and comparison between the relative abundance ofvolatiles that are mobile over these timescales (e.g., methane,ethane) vs. those that are involatile (e.g., propane, benzene), can be






used to test this hypothesis and understand volatile transport onthousand year timescales. Volatile transport over shorter time-scales (diurnal, tidal, and seasonal) can be investigated via in situmeasurements of the methane evaporation rate and associatedmeteorological conditions (e.g., wind speed, temperature, humid-ity). These measurements can be used to ground-truth methanetransport predictions from global climate models (e.g., Mitchell,2008; Tokano, 2009; Schneider et al., 2012).

In the north, 87% of the area of observed lacustrine deposits iscontained within the three largest seas, Kraken Mare (496,380 km2),Ligeia Mare (133,160 km2), and Punga Mare (49,400 km2) (Figs. 2 and3). Bodies of standing liquid on Titan with large dimensions areclassified as seas (mare) rather than lakes (lacus). The remainder ofnorth polar lakes approximately follow a lognormal distribution witha mean diameter of �10 km (Hayes et al., 2008). While Cassini has

provided a wide range of evidence that the largest of Titan's lakes areindeed liquid-filled basins (e.g., Brown et al., 2008), the returned datahave only been able to provide lower limits on their depth that arehighly dependent on the unknown dielectric properties of the boththe liquid and the lakebed (e.g., Paillou et al., 2008; Hayes et al., 2010,2011). In the absence of a substantial subsurface reservoir, thesemaria represent the dominant liquid hydrocarbon inventory on Titan.Prior to Cassini, a global ocean of liquid hydrocarbon was predictedto cover the surface and act as both a source of atmospheric methaneand sink for photolysis products such as ethane and higher orderhydrocarbon/nitriles (Lunine et al., 1983). The absence of this globalreservoir questions the long-term stability of Titan's atmosphere,which would be depleted in �107 years in the absence of a crustalsource or methane outgassing from the interior (e.g., Tobie et al.,2006). Determining the bathymetry of one of Titan's Mare, which canbe reliably accomplished in situ, is essential to understanding thetotal volume of liquid available for interaction with the atmosphere.Remote sensing radar techniques should also be used to determinethe lakes' bathymetry. The inventory of methane in Titan's maria,which requires knowledge of both depth and composition, willprovide a lower limit on the length of time that the lakes can sustainmethane in Titan's atmosphere (Mitri et al., 2007) and help toquantify the required rate of methane resupply from the interiorand/or crust. Similarly, the absolute abundance of methane photo-lysis products (e.g., ethane, propane) will determine a lower limit forthe length of time that methane has been abundant enough to drivephotolysis in the upper atmosphere and deposit its products onto thesurface and, ultimately, into the lakes. Furthermore, a comparisonamongst the relative abundance of photolysis products could revealfractionation processes such as crustal sequestration, as wouldhappen if one constituent (e.g., propane) were preferentially intro-duced into a repository such as crustal clathrate hydrate.

Similar to the Earth's oceans, Titan's seas record a history oftheir parent body's origin and evolution. Specifically, the noble gasand isotopic composition of the sea can provide informationregarding the origin of Titan's atmosphere, reveal the extent ofcommunication with the interior, potentially constrain the condi-tions in the Saturn system during formation, and refine estimates

Fig. 2. Distribution of lacustrine and fluvial morphologies in Titan's polar regions (see also Hayes et al. (2008) and Aharonson et al. (2012)). Blue polygons depict modernlakes and seas filled with liquid hydrocarbons. The red polygons show the locations of radar-bright basins interpreted as paleolakes. Cyan polygons depict the distribution ofalbedo-dark features that are above the radar noise floor and express lacustrine-like morphologies. These features are believed to represent anything from mud flats toshallow liquid deposits. Fluvial valleys are represented by blue lines. Dashed lines were drawn on ISS images while the remaining polygons were created using SyntheticAperture Radar (SAR) images generated from Cassini RADAR observations. SAR incidence angle and swath coverage are shown. (For interpretation of the references to colorin this figure legend, the reader is referred to the web version of this article.)

Fig. 3. Cassini SAR mosaic images of the north polar region showing Kraken, Ligeiaand Punga Maria. Black–yellow color map was applied to the single band data. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)






of the methane outgassing history. The Huygens probe detected asmall argon abundance in Titan's atmosphere, but was unable todetect the corresponding abundance of xenon and krypton(Niemann et al., 2005). The non-detection of xenon and kryptonsupports the idea that Titan's methane was generated by serpen-tinization of primordial carbon monoxide and carbon dioxidedelivered by volatile depleted planetesimals originating fromwithin Saturn's subnebula (e.g., Atreya et al., 2006). A competingmodel of methane origination, however, suggests that methane isprimordial and was sourced from planetesimals originating fromthe outer solar nebula (Mousis et al., 2009). This competing modelwas originally suggested as an attempt to explain the measuredratio of deuterium/hydrogen (D/H) (1:32� 10�4), which Mousiset al. (2009) suggest would require a D/H ratio in Titan's water icethat does match that of Enceladus. In order to support a primordialmethane source, xenon and krypton would both have to besequestered from the atmosphere. While xenon is soluble in liquidhydrocarbon (solubility of 10�3 at 95 K) and could potentially besequestered into liquid reservoirs, argon and krypton cannot.Therefore, the absence of measureable atmospheric kryptonrequires either sequestration into non-liquid surface deposits,such as clathrates (Mousis et al., 2011), or depletion in the noblegas concentration of the planetesimals that originally deliveredTitan's atmosphere (Owen and Niemann, 2009). The low, albeitmeasureable, argon-to-nitrogen ratio has been used to suggestthat Titan's atmospheric nitrogen was sourced from ammonia, asammonia is significantly less volatile than both molecular nitrogenand argon (Niemann et al., 2005), which have similar volatility.The presence of xenon in Titan's seas would indicate that theabsence of atmospheric krypton is a result of sequestration intocrustal sources (Mousis et al., 2011) as opposed to the result ofcarbon delivery via depleted planetesimals (Owen and Niemann,2009). In addition to noble gas concentration, isotopic ratios canalso be used to decipher the history of Titan's atmosphere. Forexample, the 13C/12C ratio of CH4 was used by Mandt et al. (2009)to conclude that methane last outgassed from the interior �107

years ago. However, this calculation assumes that the exposedmethane reservoir has an isotopic composition that is in equili-brium with the atmosphere. If the carbon isotope ratio of hydro-carbons in Titan's lakes/seas were found to be different than in theatmosphere, it would imply alteration of the isotopic compositionby evaporation and condensation and indicate a different time-scale for the history of methane outgassing.

Titan's lakes and seas collect organic material both directly,through atmospheric precipitation of photolysis products, andindirectly, through eolian/fluvial transport of surface materials(e.g., river systems flowing into the Mare). As a result, the lakesand seas represent the most complete record of Titan's organiccomplexity and present a natural laboratory for studying prebioticorganic chemistry. Titan's environment is similar to conditions onEarth four billion year ago and presents an opportunity to studyactive systems involving several key compounds of prebioticchemistry (Raulin, 2008). For instance, the complex aerosols pro-duced in Titan's atmosphere via photolysis, which are presumablytransported into the lake via pluvial, fluvial, and eolian processes,are similar to the initial molecules implicated in the origin of life onEarth. Other organics can act as tracers for specific environmentalconditions. For example, the presence of oxygen-bearing organicspecies would suggest that redox reactions have taken place andthat the organic deposits have, at some point, been in at leasttransient contact with liquid water. In general, simple abioticorganic systems exhibit monotonically decreasing abundance withincreasing chain complexity. Distinctive deviations from such adistribution can mark the signs of prebiotic chemistry and poten-tially represent the initial markers on a pathway toward self-replication and a minimally living system.

Finally, lacustrine settings on Titan represent exotic environ-ments that are being sculpted by many of the same meteorologicaland oceanographic processes that affect lake and ocean settings onEarth. In situ exploration of such an environment provides anopportunity to study these processes under vastly different envir-onmental conditions (scientific objectives 2 and 4, Table 1). Whilethe Huygens probe provided atmospheric data over equatoriallatitudes around winter solstice, models of Titan's climate predictthat polar weather could be quite different from the equatorialenvironment investigated by Huygens. In situ measurements ofsuch an environment would provide invaluable ground-truth forglobal climate models (GCM) of Titan's surface/atmosphere inter-actions. Ultimately, this could lead to a better understanding ofhow to accurately predict the evolution of our own climate systemand even how to model extreme climates on extrasolar planets, asTitan may represent a common climate system amongst solidbodies in Earth-like orbits around M-dwarf stars.

4. Scientific investigations, measurements and approaches

All of the previously discussed objectives of the proposedexploration of Titan and the broader Saturnian system (Table 1)can be achieved by complementary investigations of a dedicatedorbiter and a lake probe. The in situ exploration of one of Titan'sseas can contribute to the achievement of all of these scientificobjectives. Table 2 summarizes the scientific investigations, mea-surements and examples of approaches considered for an explora-tion of Titan from an orbiter and lake probe and ties them to thescientific objectives presented in Table 1.

5. Mission concept legacy

Since the Cassini spacecraft and Huygens probe arrived at Titanin 2004, there have been many proposed mission concepts using avariety of vehicles, including orbiters, landers, balloons and ofcourse, nautical probes, including floating, propulsion and sub-mersible types, to expand both orbital and in situ exploration ofTitan inspired by the initial discoveries of the Cassini Huygensmission. In spring of 2013, ESA put out a request for White Papersto the scientific community to propose Science Themes for itslarge (L)-scale L2 and L3 missions. The exploration of Titan with anorbiter and a lake probe was presented as a White Paper inresponse to this request, which is presented in an extended formin this paper (see Supplementary material). A companion WhitePaper was submitted, which presented the scientific case for theexploration of Titan with an orbiter and a balloon probe (Tobieet al., 2014, in this issue). The concept for a mission capable ofimplementing the science described in previous sections is in partbased on the Titan and Enceladus Mission TandEM (Cousteniset al., 2009a) and the Titan Explorer (Leary et al., 2007; Lockwoodet al., 2008) mission proposals for ESA and NASA respectively andwhich were merged into the joint space agency Titan SaturnSystem Mission Study TSSM (Coustenis et al., 2010; ESA, 2009)for consideration for a Large (L) class or Flagship type mission. TheTSSM mission proposal included an orbiter with two types ofprobe vehicles to enable in situ exploration of Titan: a balloon anda lake lander, with NASA responsible for the orbiter and ESAresponsible for the probe vehicles. The target of the proposed lakeprobe was the Kraken Mare. In addition, TSSM included a tour ofthe Saturnian system with flybys at Enceladus. The proposedmission would have had a launch date between 2023 and 2025,however ultimately the mission was not chosen for further studiesby the involved space agencies. This initiated several proposals forsmaller type missions in scale and cost for both lake, Titan Mare






Explorer (TiME) (Stofan et al., 2010, 2013) and balloon, Titan AerialExplorer (TAE) (Lunine et al., 2011) probes within NASA and ESArespectively. To reduce scale and cost, both proposals did notinclude an orbiter and both probes would use Direct-to-Earth(DTE) communication to relay data back to Earth. TiME was afloating lake probe that would drift across Ligeia Mare for 3–6months investigating lake composition and processes as well aslocal meteorology and atmosphere. TiME was a finalist for NASA'sDiscovery mission. TAE was a proposal in response to ESA's call fora medium (M) M3 class mission for its Cosmic Vision (2015–2025)program and included a pressurized balloon that would explore

the lower atmosphere of Titan's equatorial regions for 3–6 months.While both TiME and TAE discarded the orbiter, another NASADiscovery candidate, Journey to Enceladus and Titan (JET), jet-tisoned the probes and proposed examination of Enceladus' plumeand Titan's atmosphere and surface using only an orbiter with aninstrument suite consisting of two instruments and a radio scienceexperiment (Sotin et al., 2011). JPL prepared a study on Titan lakeprobe missions for the NASA Planetary Science Decadal Survey andinvestigated several potential mission architectures and nauticalprobe types for the exploration of Titan's lakes and seas includinga relay orbiter with a nautical probe with floating and submersible

Table 2Science Investigations, measurements and example of approaches.

Science investigations Measurements Example of approaches

Internal structure Determine the internal structure(Goal 1)

Gravity field S/C radio-tracking to measure static gravity fieldObliquity, libration, occupancy of a Cassini state Imaging (e.g. SAR) (S/C) with spatial resolution

�30 to 50 mS/C radio-tracking to measure static gravity field

Gravity anomalies in the ice shell anddeep interior (Goal 1)

High order gravity field components S/C radio-tracking to measure static gravity fieldTopography Altimetry/stereo imaging/interferometric topography

(S/C) with vertical resolution of �10 mPresent tidal quality factor Q (Goal 1) Time variation of the degree-2 gravity field S/C radio-tracking to measure time dependent

gravity fieldHydrostatic compensation (Goal 1) Gravity field S/C radio-tracking to measure static gravity field

Topography Altimetry/stereo imaging/interferometric topography(S/C) with vertical resolution �10 m

Subsurface ocean Extent of the subsurface ocean(Goal 1)

Time variation of the degree-2 gravity field (k2) S/C radio-tracking to measure static gravity fieldDetection of electromagnetic induction from asubsurface ocean

Magnetometer measurements (S/C)

Tidal deformation (h2) Altimetry and/or interferometric topography (S/C)with �1 m vertical resolution

Libration and obliquity Imaging (e.g. SAR) (S/C) with 30–50 m resolutionS/C radio-tracking to measure the static gravity field

Ice shell and geologyand composition

Thickness of the ice shell (Goal 1) Time variation of the degree-2 gravity field (k2) S/C radio-tracking to measure time dependentgravity field

Tidal deformation (h2) Altimetry and/or interferometric topography (S/C)with �1 m vertical resolution

Libration SAR imaging (S/C)S/C tracking to measure the static gravity field

Characterize the surface andsubsurface structures and thedetermine their correlation (Goals1 and 2)

Global mapping of the surface SAR imaging (S/C)Global topography Altimetry/stereo imaging/interferometric topography

(S/C)Profiles of subsurface structures and thickness of thesurface organic material layer

Radar sounding (S/C) with �10 km penetrationdepth and �30 m vertical resolution

Correlate the surface compositionand their distribution with surfacestructures (Goal 2)

Global mapping of the surface SAR imaging (S/C) with �30 to 50 m resolutionGlobal mapping of the surface composition Infrared spectral imaging (S/C) within the methane

band windows

Atmosphere Characterize the atmosphericcirculation and the composition ofthe atmosphere from orbit andduring probe descent (Goal 3)

Determine the dynamics and thermal state of theatmosphere including the clouds

Infrared spectra imaging of the atmosphere (S/C)Lidar (S/C)

Determine the composition Infrared spectra imaging of the atmosphere (S/C)Mass spectrometry with direct sampling from orbit(S/C) and during decent (L/P)Lidar (S/C)

Characterize the lake/atmosphereinteractions in situ (Goal 3)

Determine temperature, pressure, composition,evaporation rate and physical properties thatcharacterize lake and atmosphere interactions

Mass spectrometry (L/P)Physical properties package (L/P)

Lakes/seas Characterize lakes/seas and theircomposition/chemistry(astrobiological potential)(Goals 4, 1, 2, and 3)

Lake and sea composition, including low/high masshydrocarbons, noble gases, and carbon isotopes

Mass spectrometry/chemical analyzer (L/P)Atmosphere physical properties package(temperature sensor, barometer, anemometer)/meteorological package/thermo-gravimetry (L/P)

Exchange processes at the sea-air interface to helpconstrain the methane cycle

Atmosphere physical properties package(temperature sensor, barometer, anemometer)/meteorological package (L/P)

Presence and nature of waves and currents Physical properties package (L/P)Surface imaging (�250 μrad/pixel)

Properties of sea liquids including turbidity anddielectric constant

Lake physical properties package (turbidity anddielectric constant measurements) (L/P)

Sea depths to constrain basin shape and sea volume Sonar (L/P)Shoreline characteristics, including evidence for pastchanges in sea level

Surface Imaging (�250 μrad/pixel)Descent camera imaging/stereogrammetry (L/P)






components, a floating probe with DTE communication capabil-ities and submersible and floating probes both with relay com-munication capabilities (JPL, 2010).

Several other possible missions for in situ examination of Titanwere proposed in the literature including an airplane (AVIATR)probe that called for aerial reconnaissance of Titan's atmosphere(Barnes et al., 2010) and a propelled lake probe known as TitanLake In-situ Sampling Propelled Explorer TALISE (Urdampilletaet al., 2012).

6. Proposed mission concept

The proposed mission concept consists of two elements, thespacecraft (S/C), and a lake probe (lander). The S/C will carry ascientific payload consisting of remote sensing experiments and anEntry, Descent and Landing (EDL) module containing a lake landerequipped with an instrument suite capable of carrying out in situmeasurements of Titan's polar lakes. During the descent the probewill make in situ measurements of the atmosphere. Once asuccessful splashdown has been achieved, the lake lander will betake measurements, including the sampling of both the liquid ofthe sea and the low-atmosphere. A valuable augmentation may bethe capability to sample solid material on the shoreline or on theshallow sea-bed. The possible targets of the lake probe are LigeiaMare (781N, 2501W) and Kraken Mare (681N, 3101W). Fig. 4 showsthe design of the lake probe and the internal view with thelocation of the scientific instruments by the MasterSAT10 Team atUniversity La Sapienza (Italy).

The duration of the cruise from Earth to Saturn is estimated tobe on the order of 10 years or less, depending on the mass of thespacecraft and the gravitational pull options. Following orbitinsertion at Saturn, the S/C will conduct a tour of the satellitesystem, with the possibility of several flybys at Enceladus, whileflybys at Titan would be used both as a scientific opportunity andas a way to modify the trajectory in order to finally enter orbitaround Titan. A different mission scenario proposes that the orbitaround Titan could be achieved through an aero-capture maneu-ver, followed by other maneuvers to refine the spacecraft's final1500 km circular high-inclination orbit. Release and entry ofthe lake probe, and relay of its data to Earth will take place eitherduring (1) Titan flybys phase, or (2) Titan orbital phase. Thearchitecture of the mission will be defined by the scientists andthe agencies in dedicated studies. We considered as a possiblenominal final orbit of the S/C (orbiter) around Titan a 1500 kmaltitude circular near-polar orbit. A polar orbit optimizes the

gravity field measurements and maximizes the imaging coverageof the surface. A 1500 km altitude orbit has negligible drag effecton the S/C; however lower altitude orbits are also possibleincreasing the orbit maintenance. After orbit insertion aroundTitan, mapping of Titan with remote sensing experiments willbegin, lasting from several months to a few years. The lake probewill relay data to the orbiter, which will serve as the communica-tions link between the probe and Earth. Direct-to-Earth (DTE)communication from the lake lander (feasible with Earth viewfrom northern polar seas starting from 2036 to 2050 s and later)allows for augmented communication and tracking (Cousteniset al., 2009b). Scenarios for the lake probe are (1) the lake probewill have propulsion capabilities and mobility, (2) the lake probewill not have propulsion capabilities, rather it will float around thelake driven by winds or waves (although they are expected to berather small) and possible tides. Fig. 5 shows simulations of fivesplashdowns in Ligeia Mare and subsequent drift tracks usingwinds predicted by the GCM (Lorenz et al., 2012). The winddirection varies over the course of a Titan day, and so thetrajectories have loops and curves. For predicted winds in the

Fig. 4. Design of the lake probe and the internal view of the scientific instruments (Courtesy of the MasterSAT10 Team at University La Sapienza, Italy).

Fig. 5. Simulations of five splashdowns in Ligeia Mare and subsequent drift tracksusing winds predicted by the GCM (see Lorenz et al. (2012)). The wind directionvaries over the course of a Titan day, and so the trajectories have loops and curves.For predicted winds in the season shown (2023) the vehicle makes landfall on thewestern shore of Ligeia after several weeks, except in one instance where it reachesan island.






season shown (2023) the vehicle would make landfall on thewestern shore of Ligeia after several weeks, except in one instancewhere it reaches an island.

The power source both for the S/C and the lake probe is criticalfor this mission on Titan and the Saturn system. Preliminaryanalysis indicates that, to meet power requirements through solarpower, the orbiter would need large solar arrays (total surface130–200 m2). Given the opacity of Titan's atmosphere and the lowsolar elevation from the polar seas, the use of a solar poweredgenerator for the lake-probe is not feasible. The use of batteries forthe lake probe will only provide power to the lander on the orderof hours. The development of radioisotope space power systemswhich minimize or replace 238Pu fuel within the European SpaceAgency (ESA) funded programs includes the development of241Am fueled Radioisotope Thermoelectric Generator (RTG) (e.g.Ambrosi et al., 2012) that should be considered as a possiblepower source for a ESA-led Titan mission.

7. Conclusions

The exploration of Titan with a dedicated orbiter and in situlake probe will address fundamental questions involving theorigin, evolution, and history of both Titan and the broaderSaturnian system. Titan's ongoing organic chemistry has directapplicability to the early prebiotic chemistry on Earth, allowing forthe investigation of reactions and timescales inaccessible toterrestrial labs. Titan also represents a potentially common climatesystem amongst extrasolar planets and a detailed understandingof its nature and evolution will help to interpret data from thesedistant objects. Finally, it is important to emphasize the educa-tional potential of this mission concept. The novel idea of sendinga “boat” to an extraterrestrial sea, which is both innovative andawe-inspiring, can be used as an inspirational tool at all levels ofeducation and public outreach.

Acknowledgments

The authors thank the Reviewer Jani Radebaugh for her con-structive comments. Some of this work was carried out at the JetPropulsion Laboratory, California Institute of Technology, undercontract with NASA.

Appendix A. Supplementary material

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.pss.2014.07.009.

References

Achterberg, R.K., Gierasch, P.J., Conrath, B.J., Flasar, F.M., Nixon, C.A., 2011. Temporalvariations of Titan's middle-atmospheric temperatures from 2004 to 2009observed by Cassini/CIRS. Icarus 211, 686–698.

Aharonson, O., Hayes, A.G., Lunine, J.I., Lorenz, R.D., Allison, M.D., Elachi, C., 2009.An asymmetric distribution of lakes on Titan as a possible consequence oforbital forcing. Nat. Geosci. 2, 851–854.

Aharonson, O., Hayes, A.G., Lopes, R.M.C., Lucas, A., Hayne, P., Perron, T., Soderblom,L.A., 2012. Titan's Surface Geology. In: Mueller-Wodarg, I. (Ed.), Titan: Surface,Atmosphere, and Magnetosphere. Cambridge Planetary Science Series. Cam-bridge University Press, Cambridge (646 pp.).

Ambrosi, R.M., Williams, H.R., Samara-Ratna, P., Bannister, N.P., Vernon, D.,Crawford, T., Bicknell, C., Jorden, A., Slade, R., Deacon, T., Konig, J., Jaegle, M.,Stephenson, K., 2012. Development and testing of Americium-241 radioisotopethermoelectric generator: concept designs and breadboard system. Nucl.Emerg. Technol. Space 3042.

Andrews, D.J., Cowley, S.W.H., Dougherty, M.K., Provan, G., 2010. Magnetic fieldoscillations near the planetary period in Saturn's equatorial magnetosphere:variation of amplitude and phase with radial distance and local time.J. Geophys. Res. 115, A04212.

Atreya, S.K., Adams, E.Y., Niemann, H.B., Demick-Montelara, J.E., Owen, T.C.,Fulchignoni, M., Ferri, F., Wilson, E.H., 2006. Titan's methane cycle. Planet.Space Sci. 54, 1177–1187.

Barnes, J.W., Brown, R.H., Radebaugh, J., Buratti, B.J., Sotin, C., Le Mouelic, S.,Rodriguez, S., Turtle, E.P., Perry, J., Clark, R., Baines, K.H., Nicholson, P.D., 2006.Cassini observations of flow-like features in western Tui Regio, Titan. Geophys.Res. Lett. 33, L16204. http://dx.doi.org/10.1029/2006GL026843.

Barnes, J.W., Brown, R.H., Soderblom, L., Buratti, B.J., Sotin, C., Rodriguez, S.,Le Mouèlic, S., Baines, K.H., Clark, R., Nicholson, P., 2007. Global-scalesurface spectral variations on Titan seen from Cassini/VIMS. Icarus 186,242–258.

Barnes, J.W., Brown, R.H., Soderblom, L., Sotin, C., Le Mouèlic, S., Rodriguez, S.,Jaumann, R., Beyer, R.A., Buratti, B.J., Pitman, K., Baines, K.H., Clark, R.,Nicholson, P., 2008. Spectroscopy, morphometry, and photoclinometry ofTitan's dunefields from Cassini/VIMS. Icarus 195, 400–414.

Barnes, J.W., Bow, J., Schwartz, J., Brown, R.H., Soderblom, J.M., Hayes, A.G.,Vixie, G., Le Mouélic, S., Rodriguez, S., Sotin, C., Jaumann, R., Stephan, K.,Soderblom, L.A., Clark, R.N., Buratti, B.J., Baines, K.H., Nicholson, P.D., 2011.Organic sedimentary deposits in Titan's dry lakebeds: probable evaporite.Icarus 216, 136–140.

Barnes, J.W., McKay, C., Lemke, L., Beyer, R.A., Radebaugh, J., Atkinson, D., 2010.AVIATR: Aerial Vehicle for In-Situ and Airborne Titan Reconnaissance. In:Proceedings of the Lunar and Planetary Science Conference vol. 41, p. 2551.

Bird, M.K., Allison, M., Asmar, S.W., Atkinson, D.H., Avruch, I.M., Dutta-Roy, R.,Dzierma, Y., Edenhofer, P., Folkner, W.M., Gurvits, L.I., Johnston, D.V., Plette-meier, D., Pogrebenko, S.V., Preston, R.A., Tyler, G.L., 2005. The vertical profile ofwinds on Titan. Nature 438, 800–802.

Bills, B.G., Nimmo, F., 2008. Forced obliquity and moments of inertia of Titan. Icarus196, 293–296.

Brown, M.E., Bouchez, A.H., Griffith, C.A., 2002. Direct detection of variable tropo-spheric clouds near Titan's south pole. Nature 420, 795–797.

Brown, R.H., Soderblom, L.A., Soderblom, J.M., Clark, R.N., Jaumann, R., Barnes, J.W.,Sotin, C., Buratti, B., Baines, K.H., Nicholson, P.D., 2008. The identification ofliquid ethane in Titan's Ontario Lacus. Nature 454 (7204), 607–610.

Burr, D.M., Perron, J.T., Lamb, M.P., Irwin III, R.P., Collins, G.C., Howard, A.D.,Sklar, L.S., Moore, J.M., Adamkovics, M., Baker, V.R., Drummond, S.A.,Black, B.A., 2013. Fluvial features on Titan: insights from morphology andmodeling. Geol. Soc. Am. Bull. 125, 299–321.

Castillo-Rogez, J., Lunine, J.I., 2010. Evolution of Titan's rocky core constrained byCassini observations. Geophys. Res. Lett. 37, L20205.

Cicalò, S., Milani, A., 2012. Determination of the rotation of Mercury from satellitegravimetry. Mon. Not. R. Astron. Soc. 427 (1), 468–482.

Coustenis, A., Hirtzig, M., 2009. Cassini–Huygens results on Titan's surface. Res.Astron. Astrophys. 9, 249–268.

Coustenis, A., Atreya, S.K., Balint, T., Brown, R.H., Dougherty, M.K., Ferri, F.,Fulchignoni, M., Gautier, D., Gowen, R.A., Griffith, C.A., Gurvits, L.I.,Jaumann, R., Langevin, Y., Leese, M.R., Lunine, J.I., McKay, C.P., Moussas, X.,Mueller-Wodarg, I., Neubauer, F., Owen, T.C., Raulin, F., Sittler, E.C., Sohl, F.,Sotin, C., Tobie, G., Tokano, T., Turtle, E.P., Wahlund, J., Waite, J.H., Baines, K.H.,Blamont, J., Coates, A.J., Dandouras, I., Krimigis, T., Lellouch, E., Lorenz, R.D.,Morse, A., Porco, C.C., Hirtzig, M., Saur, J., Spilker, T., Zarnecki, J.C., Choi, E.,Achilleos, N., Amils, R., Annan, P., Atkinson, D.H., Benilan, Y., Bertucci, C., Be zard,B., Bjoraker, G.L., Blanc, M., Boireau, L., Bouman, J., Cabane, M., Capria, M.T.,Chassefiere, E., Coll, P., Combes, M., Cooper, J.F., Coradini, A., Crary, F., Cravens,T., Daglis, I.A., de Angelis, E., de Bergh, C., de Pater, I., Dunford, C., Durry, G.,Dutuit, O., Fairbrother, D., Flasar, F.M., Fortes, A.D., Frampton, R., Fujimoto, M.,Galand, M., Grasset, O., Grott, M., Haltigin, T., Herique, A., Hersant, F.,Hussmann, H., Ip, W., Johnson, R., Kallio, E., Kempf, S., Knapmeyer, M.,Kofman, W., Koop, R., Kostiuk, T., Krupp, N., Küppers, M., Lammer, H.,Lara, L.M., Lavvas, P., Le Moue lic, S., Lebonnois, S., Ledvina, S., Li, J., Livengood,T.A., Lopes, R.M., Lopez-Moreno, J.J., Luz, D., Mahaffy, P.R., Mall, U., Martinez-Frias, J., Marty, B., McCord, T., Menor Salvan, C., Milillo, A., Mitchell, D.G.,Modolo, R., Mousis, O., Nakamura, M., Neish, C.D., Nixon, C.A., Nna Mvondo, D.,Orton, G., Paetzold, M., Pitman, J., Pogrebenko, S., Pollard, W., Prieto-Ballesteros,O., Rannou, P., Reh, K., Richter, L., Robb, F.T., Rodrigo, R., Rodriguez, R.,Romani, P., Ruiz Bermejo, M., Sarris, E.T., Schenk, P., Schmitt, B., Schmitz, N.,Schulze-Makuch, D., Schwingenschuh, K., Selig, A., Sicardy, B., Soderblom, L.,Spilker, L.J., D. Stam, D., Steele, A., Stephan, K., Strobel, D.F., Szego, K., Szopa, C.,Thissen, R., Tomasko, MG., Toublanc, D., Vali, H., Vardavas, I., Vuitton, V.,West, R.A., Yelle, R., Young, E.F., 2009a. TandEM: Titan and Enceladus mission.Exp. Astron. 23, 893–946.

Coustenis, A., Lunine, J., Lebreton, J.-P., Matson, D., Erd, C.h., Reh, K., Beauchamp, P.,Lorenz, R., Waite, H., Sotin, Ch., Gurvits, L., Hirtizg, M., 2009b. Earth-basedsupport for the Titan–Saturn Mission. Earth Moon Planets 105, 135–142.

Coustenis, A., Lunine, J., Matson, D., Hansen, C., Reh, K., Beauchamp, P., Lebreton, J.-P.,Erd, C., 2010. The joint NASA–ESA Titan–Saturn System Mission (TSSM) study.In: Proceedings of the 40th Lunar and Planetary Science Conference, pp. 1060.

Elachi, C., Allison, M.D., Borgarelli, L., Encrenaz, P., Im, E., Janssen, M.A., Johnson, W.T.K., Kirk, R.L., Lorenz, R.D., Lunine, J.I., Muhleman, D.O., Ostro, S.J., Picardi, G.,Posa, F., Rapley, C.G., Roth, L.E., Seu, R., Soderblom, L.A., Vetrella, S., Wall, S.D.,Wood, C.A., Zebker, H.A., 2004. Radar: the Cassini Titan radar mapper. SpaceSci. Rev. 115 (1), 71–110.

ESA, 2009. TSSM in-situ elements. Assessment Study Report, ESA-SRE(2008)4,European Space Agency, 12 February 2009.

Fortes, A.D., Grindrod, P.M., Trickett, S.K., Vočadlo, L., 2007. Ammonium sulfate onTitan: possible origin and role in cryovolcanism. Icarus 188, 139–153.




http://refhub.elsevier.com/S0032-0633(14)00205-0/sbref1






















http://dx.doi.org/10.1029/2006GL026843

http://dx.doi.org/10.1029/2006GL026843

http://dx.doi.org/10.1029/2006GL026843














































































Fulchignoni, M., Ferri, F., Angrilli, F., Ball, A.J., Bar-Nun, A., Barucci, M.A., Bettanini, C.,Bianchini, G., Borucki, W., Colombatti, G., Coradini, M., Coustenis, A., Debei, S.,Falkner, P., Fanti, G., Flamini, E., Gaborit, V., Grard, R., Hamelin, M., Harri, A.M.,Hathi, B., Jernej, I., Leese, M.R., Lehto, A., Lion Stoppato, P.F., López-Moreno, J.J.,Mäkinen, T., McDonnell, J.A.M., McKay, C.P., Molina-Cuberos, G., Neubauer, F.M.,Pirronello, V., Rodrigo, R., Saggin, B., Schwingenschuh, K., Seiff, A., Simões, F.,Svedhem, H., Tokano, T., Towner, M.C., Trautner, R., Withers, P., Zarnecki, J.C.,2005. In situ measurements of the physical characteristics of Titan's environ-ment. Nature 438, 785–791.

Grasset, O., Dougherty, M.K., Coustenis, A., Bunce, E.J., Erd, C., Titov, D., Blanc, M.,Coates, A., Drossart, P., Fletcher, L.N., Hussmann, H., Jaumann, R., Krupp, N.,Lebreton, J.-P., Prieto-Ballesteros, O., Tortora, P., Tosi, F., Van Hoolst, T., 2012.JUpiter ICy moons Explorer (JUICE): an ESA mission to orbit Ganymede and tocharacterise the Jupiter system. Planet. Space Sci. 78, 1–21. http://dx.doi.org/10.1016/j.pss.2012.12.002.

Hayes, A., Aharonson, O., Callahan, P., Elachi, C., Gim, Y., Kirk, R., Lewis, K., Lopes, R.,Lorenz, R., Lunine, J., Mitchell, K., Mitri, G., Stofan, E., Wall, S., 2008. Hydro-carbon lakes on Titan: distribution and interaction with a porous regolith.Geopyhs. Res. Lett. 35, L09204.

Hayes, A.G., Wolf, A.S., Aharonson, O., Zebker, H., Lorenz, R., Kirk, R.L.,Paillou, P., Lunine, J., Wye, L., Callahan, P., Wall, S., Elachi, C., 2010. Bathymetryand absorptivity of Titan's Ontario Lacus. J. Geophys. Res.: Planets 115,9009.

Hayes, A.G., Aharonson, O., Lunine, J.I., Kirk, R.L., Zebker, H.A., Wye, L.C., Lorenz, R.D.,Turtle, E.P., Paillou, P., Mitri, G., Wall, S.D., Stofan, E.R., Mitchell, K.L., Elachi, C.,Cassini Radar Team, 2011. Transient surface liquid in Titan's polar regions fromCassini. Icarus 211, 655–671.

Hayes, A.G., Lorenz, R.D., Donelan, M.A., Manga, M., Lunine, J.I., Schneider, T., Lamb,M.P., Mitchell, J.M., Fischer, W.W., Graves, S.D., Tolman, H.L., Aharonson, O.,Encrenaz, P.J., Ventura, B., Casarano, D., Notarnicola, C., 2013. Wind drivencapillary-gravity waves on Titan's lakes: hard to detect or non-existent? Icarus225, 403–412.

Hemingway, D., Nimmo, F., Zebker, H., Iess., L., 2013. A rigid and weathered ice shellon Titan. Nature 500, 550–552.

Iess, L., Jacobson, R.A., Ducci, M., Stevenson, D.J., Lunine, J.I., Armstrong, J.W., Asmar,S.W., Racioppa, P., Rappaport, N.J., Tortora, P., 2012. The Tides of Titan. Science337, 457.

Iess, L., Rappaport, N.J., Jacobson, R.A., Racioppa, P., Stevenson, D.J., Tortora, P.,Armstrong, J.W., Asmar, S.W., 2010. Gravity field, shape, and moment of inertiaof Titan. Science 327, 1367.

Jaumann, R., Brown, R.H., Stephan, K., Barnes, J.W., Soderblom, L.A., Sotin, C.,Le Mouélic, S., Clark, R.N., Soderblom, J., Buratti, B.J., Wagner, R., McCord, T.B.,Rodriguez, S., Baines, K.H., Cruikshank, D.P., Nicholson, P.D., Griffith, C.A.,Langhans, M., Lorenz, R.D., 2008. Fluvial erosion and post-erosional processeson Titan. Icarus 197, 526–538.

JPL, 2010. Planetary Science Decadal Survey JPL Team X Titan Lake Probe Study,Final report, Jet Propulsion Laboratory, April.

Khurana, K.K., Kivelson, M.G., Stevenson, D.J., Schubert, G., Russell, C.T., Walker, R.J.,Polanskey, C., 1998. Induced magnetic fields as evidence for subsurface oceansin Europa and Callisto. Nature 395, 749.

Khurana, Krishan K., Jia, X., Kivelson, M.G., Nimmo, F., Schubert, G., Russell, C.T.,2011. Evidence of a global magma ocean in Io's interior. Science 332,1186–1189.

Kivelson, M.G., Khurana, K.K., Volwerk, M., 2002. The permanent and inductivemagnetic moments of Ganymede. Icarus 157 (2), 507.

Leary, J., Jones, C., Lorenz, R., Strain, R.D., Waite, J.H., 2007. Titan Explorer NASAFlagship Mission Study. JHU Applied Physics Laboratory, August.

Lebonnois, S., Burgalat, J., Rannou, P., Charnay, B., 2012. Titan global climate model:a new 3-dimensional version of the IPSL Titan GCM. Icarus 218, 707–722.

Lebreton, J.-P., Coustenis, A., Lunine, J., Raulin, F., Owen, T., Strobel, D., 2009. Resultsfrom the Huygens probe on Titan. Astron. Astrophys. Rev. 17, 149–179.

Lockwood, M.K., Leary, J.C., Lorenz, R., Waite, H., Reh, K., Prince, J., Powell, R., 2008.Titan explorer. In: Proceedings of the AIAA-2008-7071, AIAA/AAS Astrody-namics Specialist Conference. Honolulu, Hawaii, August.

Lopes, R.M.C., Mitchell, K.L., Stofan, E.R., Lunine, J.I., Lorenz, R., Paganelli, F.,Kirk, R.L., Wood, C.A., Wall, S.D., Robshaw, L.E., Fortes, A.D., Neish, C.D.,Radebaugh, J., Reffet, E., Ostro, S.J., Elachi, C., Allison, M.D., Anderson, Y.,Boehmer, R., Boubin, G., Callahan, P., Encrenaz, P., Flamini, E., Francescetti, G.,Gim, Y., Hamilton, G., Hensley, S., Janssen, M.A., Johnson, W.T.K., Kelleher, K.,Muhleman, D.O., Ori, G., Orosei, R., Picardi, G., Posa, F., Roth, L.E., Seu, R.,Shaffer, S., Soderblom, L.A., Stiles, B., Vetrella, S., West, R.D., Wye, L., Zebker, H.A.,2007. Cryovolcanic features on Titan's surface as revealed by the Cassini Titanradar mapper. Icarus 186, 395–412.

Lopes, R.M.C., Stofan, E.R., Peckyno, R., Radebaugh, J., Mitchell, K.L., Mitri, G.,Wood, C.A., Kirk, R.L., Wall, S.D., Lunine, J.I., Hayes, A., Lorenz, R., Farr, T.,Wye, L., Craig, J., Ollerenshaw, R.J., Janssen, M., Legall, A., Paganelli, F., West, R.,Stiles, B., Callahan, P., Anderson, Y., Valora, P., Soderblom, L., The Cassini RadarTeam, 2010. Distribution and interplay of geologic processes on Titan fromCassini RADAR data. Icarus 205, 540–588.

Lopes, R.M.C., Kirk, R.L., Mitchell, K.L., Legall, A., Barnes, J.W., Hayes, A., Kargel, J.,Wye, L., Radebaugh, J., Stofan, E.R., Janssen, M.A., Neish, C.D., Wall, S.D., Wood,C.A., Lunine, J.I., Malaska, M.J., 2013. Cryovolcanism on Titan: new results fromCassini RADAR and VIMS. J. Geophys. Res.: Planets 118, 1–20.

Lorenz, R.D., Wall, S., Radebaugh, J., Boubin, G., Reffet, E., Janssen, M., Stofan, E.,Lopes, R., Kirk, R., Elachi, C., Lunine, J., Mitchell, K., Paganelli, F., Soderblom, L.,Wood, C., Wye, L., Zebker, H., Anderson, Y., Ostro, S., Allison, M., Boehmer, R.,

Callahan, S., Encrenaz, P., Ori, G.G., Francescetti, G., Gim, Y., Hamilton, G.,Hensley, S., Johnson, W., Kelleher, K., Muhleman, D., Picardi, G., Posa, F., Roth, L.,Seu, R., Shaffer, S., Stiles, B., Vetrella, S., Flamini, E., West, R., 2006. The sand seasof Titan: Cassini RADAR observations of equatorial fields of longitudinal dunes.Science 312, 724–727.

Lorenz, R.D., Wood, C.A., Lunine, J.I., Wall, S.D., Lopes, R.M., Mitchell, K.L., Paganelli,F., Anderson, Y.Z., Wye, L., Tsai, C., Zebker, H., Stofan, E.R., 2007. Titan's youngsurface: initial impact crater survey by Cassini RADAR and model comparison.Geophys. Res. Lett. 34, L07204.

Lorenz, R.D., Radebaugh, J., 2009. Global pattern of Titan's dunes: Radar surveyfrom the Cassini prime mission. Geophys. Res. Lett. 36, L03202.

Lorenz, R.D., 2009. Titan mission studies – a historical review. J. Br. Interplanet. Soc.62, 162–174.

Lorenz, R.D., Newman, C.E., Tokano, T., Mitchell, J.L., Charnay, B., Lebonnois, S.,Achterberg, R.K., 2012. Formulation of a wind specification for Titan late polarsummer exploration. Planet. Space Sci. 70, 73–83.

Lunine, J.I., Stevenson, D.J., Yung, Y.L., 1983. Ethane ocean on Titan. Science 222,1229.

Lunine, J., Atreya, S., 2008. The methane cycle on Titan. Nat. Geosci. 1, 335.Lunine, J.I., Reh, K., Sotin, C., Couzin, P., Vargas, A., 2011. Titan aerial explorer: a

mission to circumnavigate Titan. In: Proceedings of the Lunar and PlanetaryScience Conference vol. 42, 1230.

Malaska, M., Radebaugh, J., Lorenz, R., Mitchell, K., Farr, T., Stofan, E., 2010.Identification of Karst-like terrain on Titan from valley analysis. In: Lunar andPlanetary Institute Science Conference Abstracts, vol. 41, pp. 1544.

Mandt, K.E., Waite, J.H., Lewis, W., Magee, B., Bell, J., Lunine, J., Mousis, O., Cordier, D.,2009. Isotopic evolution of the major constituents of Titan's atmosphere based onCassini data. Planet. Space Sci. 57, 1917–1930.

McCord, T.B., Hayne, P., Combe, J.-P., Hansen, G.B., Barnes, J.W., Rodriguez, S.,Le Mouélic, S., Baines, E.K.H., Buratti, B.J., Sotin, C., Nicholson, P., Jaumann, R.,Nelson, R., the Cassini VIMS Team, 2008. Titan's surface: search for spectraldiversity and composition using the Cassini VIMS investigation. Icarus 194,212–242.

Meriggiola, R., Iess, L., Stiles, B. W. 2011. The rotation of Titan by latest Cassini data.AGU Fall Meeting Abstracts.

Mitchell, J.L., 2008. The drying of Titan's dunes: Titan's methane hydrology and itsimpact on atmospheric circulation. J. Geophys. Res.: Planets 113, 8015.

Mitchell, J.L., Ádámkovics, M., Caballero, R., Turtle, E.P., 2011. Locally enhancedprecipitation organized by planetary-scale waves on Titan. Nat. Geosci. 4,589–592.

Mitri, G., Meriggiola, R., Hayes, A., Lefevre, A., Tobie, G., Genova, A., Lunine, J.I.,Zebker, H., 2014. Shape, topography, gravity anomalies and tidal deformation ofTitan. Icarus 236, 169–177. http://dx.doi.org/10.1016/j.icarus.2014.03.018.

Mitri, G., Bland, M.T., Showman, A.P., Radebaugh, J., Stiles, B., Lopes, R.M.C.,Lunine, J.I., Pappalardo, R.T., 2010. Mountains on Titan: modeling andobservations. J. Geophys. Res. 115, 10002.

Mitri, G., Showman, A.P., Lunine, J.I., Lopes, R.M.C., 2008. Resurfacing of Titan byammonia–water cryomagma. Icarus 196, 216–224.

Mitri, G., Showman, A.P., 2008. Thermal convection in ice-I shells of Titan andEnceladus. Icarus 193, 387–396.

Mitri, G., Showman, A.P., Lunine, J.I., Lorenz, R.D., 2007. Hydrocarbon lakes on Titan.Icarus 186, 385–394.

Mitri, G., Coustenis, A., Fanchini, G., Hayes, A.G., Khurana, K., Lebreton, J.-P., Lopes, R.M., Lorenz, R.D., Iess, L., Meriggiola, R., Moriconi, M. L., Orosei, R., Sotin, C.,Stofan, E., Tobie, G., Tokano, T., Tosi, F., 2013. The Exploration of Titan with anOrbiter and a Lake-Probe. White Paper written in response to the 2013European Space Agency (ESA)’s Call for Science Themes for the Large-class(L) L2 and L3 missions.

Moore, J.M., Pappalardo, R.T., 2011. Titan: an exogenic world? Icarus 212, 790–806.Mousis, O., Lunine, J.I., Pasek, M., Cordier, D., Hunter Waite, J., Mandt, K.E.,

Lewis, W.S., Nguyen, M.J., 2009. A primordial origin for the atmosphericmethane of Saturn's moon Titan. Icarus 204, 749–751.

Mousis, O., Lunine, J.I., Picaud, S., Cordier, D., Waite Jr., J.H., Mandt, K.E., 2011.Removal of Titan's atmospheric noble gases by their sequestration in surfaceclathrates. Astrophys. J. 740, L9.

Neish, C.D., Kirk, R.L., Lorenz, R.D., Bray, V.J., Schenk, P., Stiles, B.W., Turtle, E.,Mitchell, K., Hayes, A., Cassini Radar Team, 2013. Crater topography on Titan:implications for landscape evolution. Icarus 223, 82–90.

Nelson, R.M., Kamp, L.W., Matson, D.L., Irwin, P.G.J., Baines, K.H., Boryta, M.D.,Leader, F.E., Jaumann, R., Smythe, W.D., Sotin, C., Clark, R.N., Cruikshank, D.P.,Drossart, P., Pearl, J.C., Hapke, B.W., Lunine, J., Combes, M., Bellucci, G., Bibring,J.-P., Capaccioni, F., Cerroni, P., Coradini, A., Formisano, V., Filacchione, G.,Langevin, R.Y., McCord, T.B., Mennella, V., Nicholson, P.D., Sicardy, B., 2009a.Saturn's Titan: surface change, ammonia, and implications for atmospheric andtectonic activity. Icarus 199, 429–441.

Nelson, R.M., Kamp, L.W., Lopes, R.M.C., Matson, D.L., Kirk, R.L., Hapke, B.W.,Wall, S.D., Boryta, M.D., Leader, F.E., Smythe, W.D., Mitchell, K.L., Baines, K.H.,Jaumann, R., Sotin, C., Clark, R.N., Cruikshank, D.P., Drossart, P., Lunine, J.I.,Combes, M., Bellucci, G., Bibring, J.-P., Capaccioni, F., Cerroni, P., Coradini, A.,Formisano, V., Filacchione, G., Langevin, Y., McCord, T.B., Mennella, V., Nichol-son, P.D., Sicardy, B., Irwin, P.G.J., Pearl, J.C., 2009b. Photometric changes onSaturn's Moon Titan: evidence for cryovolcanism. Geophys. Res. Lett. 36,L04202.

Niemann, H.B., Atreya, S.K., Bauer, S.J., Carignan, G.R., Demick, J.E., Frost, R.L.,Gautier, D., Haberman, J.A., Harpold, D.N., Hunten, D.M., Israel, G., Lunine, J.I.,Kasprzak, W.T., Owen, T.C., Paulkovich, M., Raulin, F., Raaen, E., Way, S.H., 2005.



















































































































http://dx.doi.org/10.1016/j.icarus.2014.03.018











































The abundances of constituents of Titan's atmosphere from the GCMS instru-ment on the Huygens probe. Nature 438, 779–784.

Nimmo, F., Bills, B.G., 2010. Shell thickness variations and the long wavelengthtopography of Titan. Icarus 208, 896–904.

Owen, T., Niemann, H.B., 2009. The origin of Titan's atmosphere: some recentadvances. R. Soc. Lond. Philos. Trans. Ser. A 367, 607–615.

Paillou, P., Lunine, J., Ruffié, G., Encrenaz, P., Wall, S., Lorenz, R., Janssen, M., 2008.Microwave dielectric constant of Titan-relevant materials. Geophys. Res. Lett.35, 18202.

Porco, C.C., Baker, E., Barbara, J., Beurle, K., Brahic, A., Burns, J.A., Charnoz, S., Cooper, N.,Dawson, D.D., Del Genio, A.D., Denk, T., Dones, L., Dyudina, U., Evans, M.W.,Fussner, S., Giese, B., Grazier, K., Helfenstein, P., Ingersoll, A.P., Jacobson, R.A.,Johnson, T.V., McEwen, A., Murray, C.D., Neukum, G., Owen, W.M., Perry, J.,Roatsch, T., Spitale, J., Squyres, S., Thomas, P., Tiscareno, M., Turtle, E.P., Vasavada,A.R., Veverka, J., Wagner, R., West, R., 2005. Imaging of Titan from the Cassinispacecraft. Nature, 434; , pp. 159–168.

Radebaugh, J., Lorenz, R.D., Kirk, R.L., Lunine, J.I., Stofan, E.R., Lopes, R.M.C.,Wall, S.D., the Cassini Radar Team, 2007. Mountains on Titan observed byCassini RADAR. Icarus 192, 77–91.

Radebaugh, J., Lorenz, R.D., Lunine, J.I., Wall, S.D., Boubin, G., Reffet, E., Kirk, R.L.,Lopes, R.M., Stofan, E.R., Soderblom, L., Allison, M., Janssen, M., Paillou, P.,Callahan, P., Spencer, C., the Cassini Radar Team, 2008. Dunes on Titan observedby Cassini RADAR. Icarus 194, 690–703.

Raulin, F., 2008. Planetary science: organic lakes on Titan. Nature 454, 587–589.Rodriguez, S., Le Mouélic, S., Rannou, P., Tobie, G., Baines, K.H., Barnes, J.W., Griffith,

C.A., Hirtzig, M., Pitman, K.M., Sotin, C., Brown, R.H., Buratti, B.J., Clark, R.N.,Nicholson, P.D., 2009. Global circulation as the main source of cloud activity onTitan. Nature 459, 678–682.

Roe, H.G., 2012. Titan's methane weather. Annu. Rev. Earth Planet. Sci. 40, 355–382.Rubin, D.M., Hesp, P.A., 2009. Multiple origins of linear dunes on Earth and Titan.

Nat. Geosci. 2, 653–658.Schaller, E.L., Roe, H.G., Schneider, T., Brown, M.E., 2009. Storms in the tropics of

Titan. Nature 460, 873–875.Schinder, P.J., Flasar, F.M., Marouf, E.A., French, R.G., McGhee, C.A., Kliore, A.J.,

Rappaport, N.J., Barbinis, E., Fleischman, D., Anabtawi, A., 2012. The structure ofTitan's atmosphere from Cassini radio occultations: occultations from thePrime and Equinox missions. Icarus 221, 1020–1031.

Schneider, T., Graves, S.D.B., Schaller, E.L., Brown, M.E., 2012. Polar methaneaccumulation and rainstorms on Titan from simulations of the methane cycle.Nature 481, 58–61.

Seidelmann, P.K., Archinal, B.A., A’Hearn, M.F., Conrad, A., Consolmagno, G.J.,Hestroffer, D., Hilton, J.L., Krasinsky, G.A., Neumann, G., Oberst, J., Stooke, P.,Tedesco, E.F., Tholen, D.J., Thomas, P.C., Williams, I.P., 2006. Report of the IAU/IAGWorking Group on cartographic coordinates and rotational elements: 2006.Celest. Mech. Dyn. Astron. 98, 155–180.

Soderblom, L.A., Kirk, R.L., Lunine, J.I., Anderson, J.A., Baines, K.H., Barnes, J.W.,Barrett, J.M., Brown, R.H., Buratti, B.J., Clark, R.N., Cruikshank, D.P., Elachi, C.,Janssen, M.A., Jaumann, R., Karkoschka, E., Mouélic, S., Le Lopes, R.M.,Lorenz, R.D., McCord, T.B., Nicholson, P.D., Radebaugh, J., Rizk, B., Sotin, C.,Stofan, E.R., Sucharski, T.L., Tomasko, M.G., Wall, S.D., 2007. Correlationsbetween Cassini VIMS spectra and RADAR SAR images: implications for Titan'ssurface composition and the character of the Huygens probe landing site.Planet. Space Sci. 55, 2025–2036.

Soderblom, L.A., Brown, R.H., Soderblom, J.M., Barnes, J.W., Kirk, R.L., Sotin, C.,Jaumann, R., Mackinnon, D.J., Mackowski, D.W., Baines, K.H., Buratti, B.J., Clark,R.N., Nicholson, P.D., 2009. The geology of Hotei Regio, Titan: correlation ofCassini VIMS and RADAR. Icarus 204, 610–618.

Sotin, C., Jaumann, R., Buratti, B.J., Brown, R.H., Clark, R.N., Soderblom, L.A., Baines, K.H.,Bellucci, G., Bibring, J.-P., Capaccioni, F., Cerroni, P., Combes, M., Coradini, A.,Cruikshank, D.P., Drossart, P., Formisano, V., Langevin, Y., Matson, D.L., McCord, T.B.,Nelson, R.M., Nicholson, P.D., Sicardy, B., Le Mouelic, S., Rodriguez, S., Stephan, K.,Scholz, C.K., 2005. Release of volatiles from a possible cryovolcano from near-infrared imaging of Titan. Nature 435, 786–789.

Sotin, C., Lawrence, K.J., Reinhardt, B., Barnes, J.W., Brown, R.H., Hayes, A.G.,Le Mouélic, S., Rodriguez, S., Soderblom, J.M., Soderblom, L.A., Baines, K.H.,Buratti, B.J., Clark, R.N., Jaumann, R., Nicholson, P.D., Stephan, K., 2012.Observations of Titan's northern lakes at 5 μm: implications for the organiccycle and geology. Icarus 221, 768–786.

Sotin, C., Altwegg, K., Brown, R.H., Hand, K., Lunine, J.I., Soderblom, J., Spencer, J.,Tortora, P., JET, Team, 2011. JET: Journey to Enceladus and Titan. In: Proceedingsof the Lunar and Planetary Science Conference vol. 42, p. 1326.

Stiles, B.W., Kirk, R.L., Lorenz, R.D., Hensley, S., Lee, E., Ostro, S.J., Allison, M.D.,Callahan, P.S., Gim, Y., Iess, L., Perci del Marmo, P., Hamilton, G., Johnson, W.T.K.,West, R.D., Cassini RADAR Team, 2008. Determining Titan's spin state fromCassini RADAR images. Astron. J. 135, 1669–1680.

Stiles, Bryan W., Hensley, Scott, Gim, Yonggyu, Bates, David M., Kirk, Randolph L.,Hayes, Alex, Radebaugh, Jani, Lorenz, Ralph D., Mitchell, Karl L., Callahan, PhilipS., Zebker, Howard, Johnson, William T.K., Wall, Stephen D., Lunine, Jonathan I.,Wood, Charles A., Janssen, Michael, Pelletier, Frederic, West, Richard D.,Veeramacheneni, Chandini, The Cassini Radar Team, 2009. Determining Titansurface topography from Cassini SAR data. Icarus 202, 584–598.

Stofan, E.R., Elachi, C., Lunine, J.I., Lorenz, R.D., Stiles, B., Mitchell, K.L., Ostro, S.,Soderblom, L., Wood, C., Zebker, H., Wall, S., Janssen, M., Kirk, R., Lopes, R.,Paganelli, F., Radebaugh, J., Wye, L., Anderson, Y., Allison, M., Boehmer, R.,Callahan, P., Encrenaz, P., Flamini, E., Francescetti, G., Gim, Y., Hamilton, G.,Hensley, S., Johnson, W.T.K., Kelleher, K., Muhleman, D., Paillou, P., Picardi, G.,

Posa, R., Roth, L., Seu, R., Shaffer, S., Vetrella, S., West, R., 2007. Nature 445,61–64.

Stofan, E.R., Lorenz, R.D., Lunine, J.I., Aharonson, O., Bierhaus, E., Clark, B., Griffith, C.,Harri, A.M., Karkoschka, E., Kirk, R., Kantsiper, B., Mahaffy, P., Newman, C.,Ravine, M., Trainer, M., Waite, H., Zarnecki, J., 2010. Titan Mare Explorer (TiME):first in situ exploration of an extraterrestrial sea. LPI Contrib. 1538, 5270.

Stofan, E., Lorenz, R., Lunine, J., Bierhaus, E., Clark, B., Mahaffy, P., Ravine, M., 2013.TiME – The Titan Mare Explorer. In: Proceedings of the IEEE AerospaceConference. Big Sky, MT, March, paper #2434.

Tobie, G., Teanby, N.A., Coustenis, A., Jaumann, R., Raulin, F., Schmidt, J., Carrasco, N.,Coates, A.J., Cordieri, D., De Kok, R., Geppert, W.D., Lebreton, J.-P., Lefevre, A.,Livengood, T.A., Mandt, K.E., Mitri, G., Nimmo, F., Nixon, C.A., Norman, L.,Pappalardo, R.T., Postberg, F., Rodriguez, S., Schulze-Makuch, D., Soderblom, J.M., Solomonidou, A., Stephan, K., Stofan, E.R., Turtle, E.P., Wagner, R.J., West, R.A., Westlake, J.H., 2014. The science goals and mission concept for a futureexploration of Titan and Enceladus. Space Planet. Sci. (submitted forpublication).

Tobie, G., Lunine, J.I., Sotin, C., 2006. Episodic outgassing as the origin of atmo-spheric methane on Titan. Nature 440, 61–64.

Tobie, G., Grasset, O., Lunine, J.I., Mocquet, A., Sotin, C., 2005. Titan's internalstructure inferred from a coupled thermal-orbital model. Icarus 175, 496–502.

Tobie, G., Teanby, N., Coustenis, A., Jaumann, R., Raulin, F., Schmidt, J., Carrasco, N.,Coates, A.J., Cordier, D., De Kok, R., Geppert, W.D., Lebreton, J.-P., Lefevre, A.,Livengood, T.A., Mandt, K.E., Mitri, G., Nimmo, F., Nixon, C.A., Norman, L.,Pappalardo, R.T., Postberg, F., Rodriguez, S., Schulze-Makuch, D., Soderblom, J.M., Solomonidou, A., Stephan, K., Stopan, E.R., Turtle, E.P., Wagner, R.J., West, R.A., Westlake, J.H., 2014. The science goals and mission concept for a futureexploration of Titan and Enceladus. Planet. Space Sci., in this issue.

Tokano, T., 2009. Impact of seas/lakes on polar meteorology of Titan: simulation bya coupled GCM-sea model. Icarus 204, 619–636.

Tokano, T., 2010. Relevance of fast westerlies at equinox for the eastward elongationof Titan's dunes. Aeolian Res. 2, 113–127.

Tokano, T., 2013. Are tropical cyclones possible over Titan's polar seas? Icarus 223,766–774.

Tokano, T., Neubauer, F.M., 2002. Tidal wind on Titan caused by Saturn. Icarus 158,499–515.

Tokano, T., Neubauer, F.M., 2005. Wind-induced seasonal angular momentumexchange at Titan's surface and its influence on Titan's length-of-day. Geophys.Res. Lett. 32, L24203.

Tokano, T., McKay, C.P., Neubauer, F.M., Atreya, S.K., Ferri, F., Fulchignoni, M.,Niemann, H.B., 2006. Methane drizzle on Titan. Nature 442, 432–435.

Tosi, F., Orosei, R., Seu, R., Coradini, A., Lunine, J.I., Filacchione, G., Gavrishin, A.I.,Capaccioni, F., Cerroni, P., Adriani, A., Moriconi, M.L., Negrão, A., Flamini, E.,Brown, R.H., Wye, L.C., Janssen, M., West, R.D., Barnes, J.W., Wall, S.D.,Clark, R.N., Cruikshank, D.P., McCord, T.B., Nicholson, P.D., Soderblom, J.M.,Cassini VIMS and RADAR Teams, 2010. Correlations between VIMS and RADARdata over the surface of Titan: implications for Titan's surface properties. Icarus208, 366–384.

Turtle, E.P., Perry, J.E., McEwen, A.S., Del Genio, A.D., Barbara, J., West, R.A., Dawson,D.D., Porco, C.C., 2009. Cassini imaging of Titan's high-latitude lakes, clouds,and south-polar surface changes. Geophys. Res. Lett. 36, 2204.

Turtle, E.P., Del Genio, A.D., Barbara, J.M., Perry, J.E., Schaller, E.L., McEwen, A.S.,West, R.A., Ray, T.L., 2011a. Seasonal changes in Titan's meteorology. Geophys.Res. Lett. 38, L03203.

Turtle, E.P., Perry, J.E., Hayes, A.G., Lorenz, R.D., Barnes, J.W., McEwen, A.S.,West, R.A., Del Genio, A.D., Barbara, J.M., Lunine, J.I., Schaller, E.L., Ray, T.L.,Lopes, R.M.C., Stofan, E.R., 2011b. Rapid and extensive surface changes nearTitan's equator: evidence of April showers. Science 331, 1414–1417.

Urdampilleta, I., Prieto-Ballesteros, O., Rebolo, R., Sancho, J., 2012. TALISE: TitanLake In-situ Sampling Propelled Explorer. In: Proceedings of the EuropeanPlanetary Science Congress, p. 64.

Wall, S.D., Lopes, R.M., Stofan, E.R., Wood, C.A., Radebaugh, J.L., Hörst, S.M., Stiles, B.W., Nelson, R.M., Kamp, L.W., Janssen, M.A., Lorenz, R.D., Lunine, J.I., Farr, T.G.,Mitri, G., Paillou, P., Paganelli, F., Mitchell, K.L., 2009. Cassini RADAR images atHotei Arcus and Western Xanadu, Titan: evidence for recent cryovolcanicactivity. Geophys. Res. Lett. 36, L04203.

Wall, S., Hayes, A., Bristow, C., Lorenz, R., Stofan, E., Lunine, J., Le Gall, A., Janssen, M.,Lopes, R., Wye, L., Soderblom, L., Paillou, P., Aharonson, O., Zebker, H., Farr, T.,Mitri, G., Kirk, R., Mitchell, K., Notarnicola, C., Casarano, D., Ventura, B., 2010.Active shoreline of Ontario Lacus, Titan: a morphological study of the lake andits surroundings. Geophys. Res. Lett. 37, 5202.

Wahr, J.M., Zuber, M.T., Smith, D.E., Lunine, J.I., 2006. Tides on Europa and thethickness of Europa's icy shell. J. Geophys. Res. 111, E12005.

Wei, H.Y., Russell, C.T., Dougherty, M.K., Neubauer, F.M., Ma, Y.J., 2010. Upper limitson Titan's magnetic moment and implications for its interior. J. Geophys. Res.115, E10007.

West, R.A., Balloch, J., Dumont, P., Lavvas, P., Lorenz, R., Rannou, P., Ray, T.,Turtle, E.P., 2011. The evolution of Titan's detached haze layer near equinoxin 2009. Geophys. Res. Lett. 38, 6204.

Wood, C.A., Lorenz, R., Kirk, R., Lopes, R., Mitchell, K., Stofan, E., Cassini Radar Team,2010. Impact craters on Titan. Icarus, 334–344.

Zebker, H.A., Stiles, B., Hensley, S., Lorenz, R., Kirk, R.L., Lunine, J., 2009. Size andshape of Saturn's Moon Titan. Science 324, 921–923.

Zimmer, C., Khurana, K.K., Kivelson, M.G., 2000. Subsurface oceans on Europa andCallisto: constraints from Galileo magnetometer observations. Icarus 147, 329.










































































http://refhub.elsevier.com/S0032-0633(14)00205-0/other44457
































































































Documents

Planetary and Space Science - Accueil du LPGlpg-umr6112.fr/lpg/fichiers/tobie-g/PUBLICATION/Mitri15.pdf · lakes and seas located mostly in Titan ... Titan and determine how the alcanonological