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