introduction to geophysics and space science günter kargl space research institute austrian academy...
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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
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
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.