12/19/2008

1

Mapping the Surface of TITANMapping the Surface of TITANChristopher Tsai

π . 2007

Presentation PreviewPresentation Preview

1. SAR Coverage of Titan

2. Looking at the Surface

3. A Titanic Catalogue of Features

4. Applications and Adjustments

Cassini-Huygens MissionCassini-Huygens Mission

• Titan, Saturn’s largest moon

• The Imaging Problem

• Wavelength λKu = 2.17 cm

• The Huygens Probe

The Cassini OrbiterThe Cassini Orbiter• Measurement Modes

– Altimeter– Radiometer– Scatterometer– Synthetic Aperture (SAR)

• Orbital Period– Initially Elliptical (120 days)– Finally Circular (7 days)

• Current Position

Cassini Scheduled Fly-Bys of TitanCassini Scheduled Fly-Bys of Titan

Orbit SAR Scatterometer DateTA October 26, 2004

T3 February 15, 2005

T4 March 31, 2005

T7 September 7, 2005

T8 October 27, 2005

T13 April 30, 2006

T16 July 21, 2006

T17 September 7, 2006

T18 September 23, 2006

T19 October 9, 2006

T21 December 12, 2006

T23 January 13, 2006

T25 February 22, 2007

T28 April 10, 2007

T29 April 26, 2007

T30 May 12, 2007

T36 October 2, 2007

T39 December 20, 2007

T41 February 22, 2008

T43 May 12, 2008

T44 May 28, 2008

Cassini SAR Coverage of TitanCassini SAR Coverage of Titan

[deg

rees

]

Cassini Synthetic Aperture Radar High-Resolution Coverage of Titan as of January 2007 90

60

30

0

TAT3T7T7T8T13T15T16T16T17T18T18T19estT19

T21A

T19e

T19 T19

TA

T18

T3

T17T16

T16

T20T23

West Longitude [degrees]

Lat

itud

e

360 300 240 180 120 60 0

-30

-60

-90

T19T20T21AT21BT23

T8 T13

T7

T7

T15T21B

T17

T23

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LOOKING AT TITAN’S SURFACELOOKING AT TITAN’S SURFACE

degr

ees]

90

60

30

0

West Longitude [degrees]

Latit

ude

[d

360 300 240 180 120 60 0

0

-30

-60

-90

A Closer Look at TitanA Closer Look at Titan• Swath Width of

120 – 450 km• No Pattern• Compress & Zoom• Features

FeaturesFeatures

Dunes

Craters

Circles

Lakes

Lines

Unknown

Surface TypesSurface Types

Xanadu – Titan’s Brightest Region

Mixed Surface on TA Edge of Xanadu

Shangri-La – One of Titan’s Darkest Regions

Creating a TITANic CatalogueCreating a TITANic Catalogue

Click around and obtain (x,y) coordinates of each vertex

Writing the CatalogueWriting the Catalogue

Lat

itu

de

[deg

rees

]

Dunes, Craters, and Lakes on the Surface of Titan, Detected with Cassini SAR

360 300 240 180 120 60 0

90

60

30

0

-30

-60

-90

LakesDunes Craters

Belet

SinlapMenrva

West Longitude [degrees]360 300 240 180 120 60 0

• Write (x,y) image coordinates• Extract latitude and longitude• Identify regions on map• Determine large regions,

eliminate small ones

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Dune Fields on TitanDune Fields on TitanLa

titud

e

Dunes on the Surface of Titan, Detected with Cassini SAR (February 2007)90

60

30

0

-30

West Longitude360 300 240 180 120 60 0

-60

-90

Applications of the Feature CatalogueApplications of the Feature Catalogue• Scatterometer Data

– Wide incidence angle coverage

• Overlapping data• Plotting backscatter

Latit

ude

Scatterometer Global Map

360 300 240 180 120 60 0

90

60

30

0

-30

-60

-90

Latit

ude

SAR-dunes (w) on scatterometry

360 300 240 180 120 60 0

90

60

30

0

-30

-60

-90

West Longitude

Latit

ude

Dunes (through T23) on ISS & SAR

360 300 240 180 120 60 0

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0

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-90

West Longitude [degrees]

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itu

de

[deg

rees

]

Dunes Located from SAR with Scatterometry Overlap

360 300 240 180 120 60 0

90

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Dunes Located from SAR Scatterometry-SAR Overlap

West LongitudeWest Longitude

0 10 20 30 40 50 60 70 800

0.5

1

1.5

2

2.5

3

3.5

4

Incidence Angle (deg)

Sigm

a0

A=0.24 n=0.48 D.Const=2.172 RMS slope=5.38 Albedo=0.357

Dune Surface fit by Hagfors Composite Model

West Longitude

Latit

ude

Dunes coincident with scatterometry

360 300 240 180 120 60 0

90

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30

0

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The Backscatter ModelThe Backscatter Model

( ) ( ) )()(1 _0_00 idifdifiqsdifi AA θσθσθσ ×+×−=

( )ii

iqs CC θθ

ρθσ 24_0 tanexp

cos)( −= ( ) i

ndifidif A θθσ cos_0 =

i

Incidence Angle of Radar

Normalized Radar Cross-Section, or Surface Reflectivity

Fresnel Power Reflection at Incidence

Parameter Inversely Related to Surface Roughness

Dielectric Constant, obtained by solving

RMS Surface Slope

Tilt Angle of Surface Facet

211

ερε

⎛ ⎞−= ⎜ ⎟⎜ ⎟+⎝ ⎠

1C

≈

iθ( )0 iσ θ

ρ( )2tan rmsC θ−=

ε( )tan rmsθ

rmsθ0 10 20 30 40 50 60 70 800

0.5

1

1.5

2

2.5

3

3.5

4

Incidence Angle (deg)

Sigm

a0

A=0.24 n=0.48 D.Const=2.172 RMS slope=5.38 Albedo=0.357

Dune Surface fit by Hagfors Composite Model

Possible Materials on TitanPossible Materials on Titan

Material εLiquid Hydrocarbons 1.6-1.9Solid Hydrocarbons 2.0-2.4CO2 Ice 2.2Snow 1.5-3.0

ε

Snow .5 3.0Water Ice 3.2Water-Ammonia Ice 4.5Terrestrial Rocks 3.0-9.0Meteoric Material 8.6Dielectric constants of possible surface materials on Titan.

(Thompson and Squyres, 1990; Lorenz et al., 2003)

Remember the Backscatter?Remember the Backscatter? Cataloguing Surface TypesCataloguing Surface Types• Categorize by reflectivity σ0

e [d

egre

es]

Regions with Average Reflectivity σ0 Higher Than 0.5 or Lower Than 0.1

90

60

30

0

Brightest Regions (σ0 > 0.5)

Darkest Regions (σ0 < 0.1)

Belet Shangri-LaQuivira

Menrva

Senkyo

Lakes

• Identify regions of similar σ0

• Plot backscatter

West Longitude [degrees]

Lat

itu

de

360 300 240 180 120 60 0

-30

-60

-90

XanaduQuivira

Tsegihi

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Backscatter by Surface ReflectivityBackscatter by Surface Reflectivity

• Advantages– Smaller regions– Fitting characterization– Refined backscatter map

• Drawbacks– Fewer data points– Vulnerable to randomness– Harder to fit the model– Standardization

The Standardization ProblemThe Standardization Problem

• Should be normalized…• Several pixels > 2 or 3

Backscatter σ0 > 0 ?

Brightness and Darkness• Scale inconsistency• Max and min ambiguityXanadu

File Types• Corrected vs. Uncorrected• Noise-Subtracted

Xanadu(T13)

σ0 ≈ 0.688

Xanadu(T3)

σ0 ≈ 1.102

The Next StepThe Next Step• Data – More passes will expand catalogue• Modeling – More points will improve fit• Inferences – Characterize dunes, lakes, etc.• Calibration – Standardize SAR data• Mapping – Divide surface into types

• Other forms of data– HiSAR– Altimetry Data

AcknowledgmentsAcknowledgments

FinFinFinFin