Introduction to Geophysics and Space Science

1. Introduction to Geophysics and Space Science Günter Kargl Space Research Institute Austrian Academy of Sciences WS 2013

2. Atmospheres Atmosphere: 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

3. 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)

4. 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

5. 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

6. Other Objects • Atmospheric composition • Mercury Na, O, K, Ca, H, He, ? • VenusCO2, N2, SO2, H2SO4, CO, H2O, O, H2, H, D • EarthN2, O2, H2O, Ar, CO2, Ne, He, CH4, K, N2O, H2, H, O, O3, Xe • MarsCO2, N2, O2, CO, H2O, O, He, H2, H, D, O3 • JupiterH2, He, H, CH4, NH3, CH3D, PH3, HD, H2O • SaturnH2, He, CH4, NH3, CH3D, C2H2, C2H6 • UranusH2, He, CH4, NH3, CH3D, C2H2, • NeptuneH2, He, CH4, NH3, CH3D, C2H2, C2H6, CO • Pluto N2, CH4, ? • Titan N2,CH4, HCN, organics • Triton N2, CH4, ?

7. 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 equation dp = -gρdz • Perfect gas law p = nkBT kB: Boltzman constant p: pressure ρ: massdensityρ=nm n: number density • Barometric formular: • Atmospheric scale height H = kBT/mg [km]

8. 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

9. Atmospheric structure picture

10. 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 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

11. 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

12. 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 desintegrateabove 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

13. Thermal balance in thermosphere vn: velocity of neutral atmosphere p: pressure Kn thermal conducivity Qxuv: volume heat production LIR: Radiativeloss Thermosphere • Thermosphere • Greek θερμός = heat • Gas density ρis 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

14. Temperature distribution

15. 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

16. 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: • qjand Lj are source and sink processes respectively • Dj: molecular diffusion coefficient

17. 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 CO2 or N2 • Charge exchange H+* + H → H+ + H* + ΔE • Dissociative recombination O2+ + 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.

18. Gas Planets: Jupiter

19. Ice Giant: Neptune

20. 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