INTRODUCTION TO
GEOPHYSICS AND SPACE
SCIENCE
Günter Kargl
Space Research Institute
Austrian Academy of Sciences
WS 2015
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
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
Barometric formula
• Homosphere:
• Hydrostatic equation
• 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
dp = -gρdz
• Perfect gas law
p = nkBT
kB: Boltzman constant
p: pressure
ρ: mass density ρ=nm
n: number density
𝑚=
𝑀𝑎𝑡𝑚 =
𝑝
2
4𝜋𝑅
0
𝑔 𝑠
• R0: planetary radius
𝑛𝑗 𝑚𝑗 𝑛𝑗
• Barometric formular:
𝑚𝑔
𝑧
𝑝 = 𝑝0 𝑒𝑥𝑝 −
𝑧 = 𝑝0 𝑒𝑥𝑝 −
𝑘𝐵 𝑇
𝐻
• Atmospheric scale height
H = kBT/mg [km]
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 carbonate
rock Earth has almost the same total
amount of CO2 as Venus and Mars!
Venus atmosphere
Other Objects
• Atmospheric composition
• Mercury
• Venus
• Earth
• Mars
• Jupiter
• Saturn
• Uranus
• Neptune
• Pluto
• Titan
• Triton
Na, O, K, Ca, H, He, ?
CO2, N2, SO2, H2SO4, CO, H2O, O, H2, H, D
N2, O2, H2O, Ar, CO2, Ne, He, CH4, K, N2O, H2, H, O, O3, Xe
CO2, N2, O2, CO, H2O, O, He, H2, H, D, O3
H2, He, H, CH4, NH3, CH3D, PH3, HD, H2O
H2, He, CH4, NH3, CH3D, C2H2, C2H6
H2, He, CH4, NH3, CH3D, C2H2,
H2, He, CH4, NH3, CH3D, C2H2, C2H6, CO
N2, CH4, ?
N2,CH4, HCN, organics
N2, CH4, ?
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
𝑑𝑇
𝑚𝑔 𝛾 − 1
°𝐶
=−
= −9.8
𝑑𝑧
𝑅
𝛾
𝑘𝑚
• 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
• 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
• Thermal balance in thermosphere
𝐿𝐼𝑅 = 𝑛𝑥 𝑓 𝑇
• vn: velocity of neutral atmosphere
• p: pressure
• Kn thermal conducivity
• Qxuv: volume heat production
• Atmosphere begins to separate
constituents from homogeneous
mixing
• LIR: Radiative loss
 T

 vn  T   p  vn    K nT   Qxuv  LIR
 t

cv 
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:
𝑑𝐹𝑗
= 𝑞𝑗 𝑧 − 𝐿𝑗 𝑧
𝑑𝑧
• qj and Lj are source and sink
processes respectively
• 𝐹𝑗 = −𝑛𝑗 𝐷𝑗
1 𝑑𝑛𝑗
𝑛𝑗 𝑑𝑧
+
1
𝐻
+
• Dj: molecular diffusion
coefficient
𝐷𝑗 ∝ 𝑇1/2 𝑛−1
1+𝛼 𝑑𝑇
𝑇 𝑑𝑧
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
H+* + H → H+ + H* + ΔE
Dissociative recombination O2+ + e* → O* + O* + 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.
• Charge exchange
•
•
•
•
•
•
•
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