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

Atmospheres Atmosphere: ἀ τμός [atmos] "vapor" and σφα ῖ ρα [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 O 2 due to biological processes

Composition Earth: 1 bar, scale height ~7km 78.08% N 2, 20.95% O 2, 1.2% H 2 O, 0.93% Ar, 0.038% CO 2 + trace gases Mars: ~0.6 mbar, scale height ~11km 95.3% CO 2, 2.7% N 2, 1.6% Ar, 0.13% O 2, 0.07% CO, 0.03% H 2 O, 0.013% NO Venus: 92 bar, scale height ~15.9 km 96.5% CO 2, 3.5% N 2, 150ppm SO2, 70ppm Argon, 20ppm H 2 O Including the carbon in carbonate rock Earth has almost the same total amount of CO 2 as Venus and Mars! Venus atmosphere

Other Objects Atmospheric composition MercuryNa, O, K, Ca, H, He, ? VenusCO 2, N 2, SO 2, H 2 SO 4, CO, H 2 O, O, H 2, H, D EarthN 2, O 2, H 2 O, Ar, CO 2, Ne, He, CH 4, K, N 2 O, H 2, H, O, O 3, Xe MarsCO 2, N 2, O 2, CO, H 2 O, O, He, H 2, H, D, O 3 JupiterH 2, He, H, CH 4, NH 3, CH 3 D, PH 3, HD, H 2 O SaturnH 2, He, CH 4, NH 3, CH 3 D, C 2 H 2, C 2 H 6 UranusH 2, He, CH 4, NH 3, CH 3 D, C 2 H 2, NeptuneH 2, He, CH 4, NH 3, CH 3 D, C 2 H 2, C 2 H 6, CO PlutoN 2, CH 4, ? TitanN 2,CH 4, HCN, organics TritonN 2, CH 4, ?

Barometric formula

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 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

Stratosphere Increase in temperature due to absorption of UV by O 3 Inverse temperature gradient prevents convection Once e.g. CH 4 or fluorinated hydrocarbons are there, they stay a long time (~50 – 100 yrs) Mixing mostly horizontally Jet streams Gravity waves Temperature ~200K < T str < 270 K Troposphere and stratosphere contain 99.9% of total atmospheric mass Stratopause Upper limit where δT/δz < 0 Height ~ 50 km

Mesosphere From Greek “middle” Decreasing temperature due to low radiative absorption but good emission (CO 2 ) 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 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

Temperature distribution

Exosphere Atmospheric molecules can escape from this region No longer homogeneous mixing Main constituents are Hydrogen, CO 2 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

Distribution of minor constituents Density of a minor constituent in a N 2 atmosphere Idealised density distribution of a minor constituent in an isothermal atmosphere

Atmospheric escape Mechanisms 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 CO 2 or N 2 Charge exchange H +* + H → H + + H * + ΔE Dissociative recombinationO e * → N * + N * + ΔE Impact dissociationN 2 + e * → N * + N * + ΔE Ion neutral reactionO + + H 2 → OH + + H * + ΔE Atmospheric sputteringH + sw + O → O * + H + sw + ΔE Ion pick upO + hν → O + + e Ion EscapeIon escape via open magnetic field lines Impact erosionAtmospheric loss due to impact of asteroid etc.

Gas Planets: Jupiter

Ice Giant: Neptune

Icy Moons: Titan 98.4 % N2, 1.4 % CH 4, ~0.1 H 2 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