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INTRODUCTION TO GEOPHYSICS AND SPACE SCIENCE Günter Kargl Space Research Institute Austrian Academy of Sciences WS 2013

INTRODUCTION TO GEOPHYSICS AND SPACE SCIENCE Günter Kargl Space Research Institute Austrian Academy of Sciences WS 2013

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INTRODUCTION TO GEOPHYSICS AND SPACE SCIENCE

Günter Kargl

Space Research Institute

Austrian Academy of Sciences

WS 2013

AtmospheresAtmosphere: ἀτμός [atmos] "vapor" and σφαῖρα [sphaira] "sphere“

A gravitationally bound layer of gases around a solar system body.• Mechanical & chemical interaction with both the host body and the solar wind• May change over time or being lost due to erosion processes• Terrestrial Planets

• Venus, Earth, Mars• Gas Planets

• Jupiter, Saturn, Uranus, Neptune• Moons with atmospheres

• Titan, Triton, …• Special cases

• Mercury: Exosphere only• Pluto: Seasonal freezing of atmosphere• Comets: Thin gas cloud when close to sun

Video

Origin of atmospheres• Primordial atmospheres

• Reducing atmosphere accreted together with planet

• Early outgassing• Can be lost due to thermal

escape, heavy impacts, and solar wind stripping(T-Tauri phase of sun)

• Examples are gas planets and minor bodies (Titan, Triton, Pluto)

Secondary atmospheres• Outgassing, volcanism• Delivered by volatile rich

impactors (comets, asteroids)

• Compatible with actual isotope ratios

• Chemical alterations due to weathering processes (e.g. carbonate cycle with liquid water)

• On Earth accumulation of O2 due to biological processes

Composition• Earth: 1 bar, scale height ~7km

• 78.08% N2, 20.95% O2, 1.2% H2O, 0.93% Ar, 0.038% CO2 + trace gases

• Mars: ~0.6 mbar, scale height ~11km• 95.3% CO2, 2.7% N2, 1.6% Ar, 0.13% O2, 0.07% CO, 0.03% H2O,

0.013% NO

• Venus: 92 bar, scale height ~15.9 km• 96.5% CO2, 3.5% N2, 150ppm SO2, 70ppm Argon, 20ppm H2O

Including the carbon in carbonaterock Earth has almost the same totalamount of CO2 as Venus and Mars!

Venus atmosphere

Other Objects• Atmospheric composition

• MercuryNa, O, K, Ca, H, He, ?• Venus CO2, N2, SO2, H2SO4, CO, H2O, O, H2, H, D

• Earth N2, O2, H2O, Ar, CO2, Ne, He, CH4, K, N2O, H2, H, O, O3, Xe

• Mars CO2, N2, O2, CO, H2O, O, He, H2, H, D, O3

• Jupiter H2, He, H, CH4, NH3, CH3D, PH3, HD, H2O

• Saturn H2, He, CH4, NH3, CH3D, C2H2, C2H6

• Uranus H2, He, CH4, NH3, CH3D, C2H2,

• Neptune H2, He, CH4, NH3, CH3D, C2H2, C2H6, CO

• Pluto N2, CH4, ?

• Titan N2,CH4, HCN, organics

• Triton N2, CH4, ?

Barometric formula• Homosphere:

• All atmospheric constituents are mixed homogeneous due to local and large scale gas transport, convection and turbulences

• Maxwellian velocity distribution• Assuming perfect gas law

• Total Mass of atmosphere

• R0: planetary radius

• Hydrostatic equationdp = -gρdz• Perfect gas lawp = nkBTkB: Boltzman constantp: pressureρ: mass density ρ=nmn: number density

• Barometric formular:

• Atmospheric scale heightH = kBT/mg [km]

Atmospheric structure

• Structure defined by:• Temperature profile

• Absorption of radiation• Heat transport• Convection• Conduction

• Mixing state• Convection• Turbulences• Diffusion

• Ionisation state• Radiation

• Gravitational binding• Escape processes

Bauer & Lammer, Planetary Aeronomy,2004

Atmospheric structure picture

Troposphere• Troposphere

• Greek: τροπή = overturn• 80% of total atmospheric mass• Energy transfer with surface• Uniform mixing of the

components• 9 km (Poles) – 17 km (Equator)

height• linear decrease of the

temperature with height• Tropopause

• Constant (low) temperature• Prevents mixing with

Stratosphere

• Dry adiabatic laps rate

• γ : heat capacity ratio (1.4 for air)• R: universal gas constant• m: mass• g: gravity

• With water vapour the lapse rate is only -6.5 °C/km

Stratosphere• Stratosphere

• Increase in temperature due to absorption of UV by O3

• Inverse temperature gradient prevents convection

• Once e.g. CH4 or fluorinated hydrocarbons are there, they stay a long time (~50 – 100 yrs)

• Mixing mostly horizontally• Jet streams• Gravity waves

• Temperature

~200K < Tstr < 270 K

• Troposphere and stratosphere contain 99.9% of total atmospheric mass

• Stratopause• Upper limit where δT/δz < 0• Height ~ 50 km

Mesosphere• Mesosphere

• From Greek “middle”• Decreasing temperature due to

low radiative absorption but good emission (CO2)

• Height 80 – 90 km• Freezing of water produces high

cloud layers (Noctilucent clouds)• Still homogeneous mixing due to

turbulences• Strong zonal (East West) winds• Most meteorites desintegrate

above 80 km height

• Mesopause• Coldest part of the atmosphere

~173K• Close to “Homopause” or

“Turbopause” where the homogeneous mixing of the atmosphere due to turbulences ends

Thermosphere

IRxuvnnnv LQTKvpTvt

Tc

• Thermosphere• Greek θερμός = heat• Gas density ρ is low• Height from ~ 80 – 90 km up to

250 – 500 km depending on solar activity

• Temperature increase due to absorption of solar radiation• Max. temperatures up to

1500°C• Gas density so low that

thermodynamic temperature definition is no longer valid

• Atmosphere begins to separate constituents from homogeneous mixing

• Thermal balance in thermosphere

• vn: velocity of neutral atmosphere• p: pressure• Kn thermal conducivity• Qxuv: volume heat production

• LIR: Radiative loss

Temperature distribution

Exosphere

• Atmospheric molecules can escape from this region

• No longer homogeneous mixing

• Main constituents are Hydrogen, CO2 and atomic oxygen

• Isothermal region• Only lower boundary defined

as “Exobase” at 250 – 500 km• Where the mean free path of

a molecule is equal to the local scale height

• Highly variable due to solar activity

• Non-Maxwellian velocity distribution due to escape of high velocity particles

• All atmospheric parts below the exobase are summarized as the “Barosphere” i.e. where the barometric gas pressure law is valid

Atmospheric mixing• Transport effects

• Lower atmosphere• Homosphere =

homogeneous mixing of all constituents

• Convection• Gravity waves• Turbulences

• Upper atmosphere• Heterosphere• Principal process is

diffusion• Each constituent distributes

along its own scale height

• Minor constituents diffuse up or downwards depending on local sources or sinks

• Flux Fj:• qj and Lj are source and sink

processes respectively

• Dj: molecular diffusion coefficient

Atmospheric escapeMechanisms providing escape energy:• Thermal escape (Jeans escape) (e.g. Mars)

• Molecules in the exosphere can reach escape velocity• Depending on molecular mass i.e. hydrogen can escape more easily than

CO2 or N2

• Charge exchange H+* + H → H+ + H* + ΔE• Dissociative recombinationO2

+ + e* → N* + N* + ΔE

• Impact dissociation N2 + e* → N* + N* + ΔE

• Ion neutral reaction O+ + H2 → OH+ + H*+ ΔE

• Atmospheric sputtering H+sw + O → O* + H+

sw + ΔE

• Ion pick up O + hν → O+ + e• Ion Escape Ion escape via open magnetic field lines• Impact erosion Atmospheric loss due to impact of asteroid

etc.

Gas Planets: Jupiter

Ice Giant: Neptune

Icy Moons: Titan

• 98.4 % N2, 1.4 % CH4, ~0.1 H2

• Surface pressure 1.5 bar• Hydrocarbon can form in the

atmosphere an precipitate to the surface• Tholins• Methane rain

There is a possible cycle of precipitation and evaporation of methane comparable to the water cycle on earth