Planetary Atmospheres
Structure
Composition
Clouds
Meteorology
Photochemistry
Atmospheric Escape
EAS 4803/8803 - CP
17:1
Structure
Generalized Hydrostatic Equilibrium
z
P ( z) = P (0)e
" dr / H ( r)
#0
z
"( z) = " (0)e
# dr / H * ( r)
0
Io
$
Generalized Pressure Scale Height
!
H ( z)!=
kT ( z)
g p ( z) µa (Red
z) mSpot
amu
Generalized Density Scale Height
1 dT ( z) g p ( z) µa ( z) mamu
! *
=
+
H ( z) T ( z) dz
kT ( z)
1
EAS 4803/8803 - CP
17:2
Structure
Note: For an Isothermal Atmosphere
(or region of an atmosphere):
H ( z) = H * ( z )
Since
!
Remember that
!
dT ( z)
=0
dz
GM p
GM p
g p ( z) = 2 =
r
Rp + z
(
)
2
* So at small altitudes r ⇒ RP and gp(z) ≅ gp(Rp)
EAS 4803/8803 - CP
!
17:3
Structure
Most planets have near surface scale heights ranging
between ~10-25 km due to the similar ratios of
T/(gpµa)
Venus Earth
Tsurf (K) 737
Bond
Albedo
H (km)
Mars Jupiter Saturn Uranus Neptune
288
215
165*
135*
76*
72*
0.75
0.31
0.25
0.34
0.34
0.29
0.31
16
8.5
11
Credit NASA
24
47
25
23
* Temperature at 1 bar pressure
EAS 4803/8803 - CP
17:4
Structure
Of course, temperature actually does vary with height
If a packet of gas rises rapidly (adiabatic), then it will
expand and, as a result, cool
Work done in expanding = work done in cooling
mgm
C p dT
VdP =
dP
"
mgm is the mass of one mole, ρ is
the density of the gas
!
Cp is the specific heat capacity
of the gas at constant pressure
Combining these two equations with hydrostatic
equilibrium, we get the dry adiabatic lapse rate:
dT mgm g p g p
=
=
dz
Cp
cp
* On Earth, the lapse rate is about 10 K/km
EAS 4803/8803 - CP
17:5
Thermal Structure: Surface
What determines a planet’s surface temperature?
Incident
energy
Reflected
energy
Energy re-radiated
from warm surface
R
Sun
Absorbed energy
warms surface
Pin = (1 " Ab )#R
2
F•
r•2 AU
Pout = 4 "R 2#$T 4
Ab is Bond albedo, F is solar flux at Earth’s distance, r is distance of planet to
Sun, ε is emissivity, σ is Stefan’s constant (5.67x10-8 Wm-2K-4)
Balancing energy in and energy out yields:
!
EAS 4803/8803 - CP
!
1/ 4
% F (1 " A ) (
•
b
'
**
Teq = ' 2
& r• AU 4#$ )
17:6
Thermal Structure: Surface
•
•
•
•
Solar constant F =1300 Wm-2
Earth (Bond) albedo Ab=0.3, ε=0.9
Equilibrium temperature = 263 K
How reasonable is this value?
Body
Mercury
Venus Earth
Mars
Ab
0.12
0.75
0.3
0.25
Teq
446
238
263
216
Actual T
100-725
737
288
215
• How to explain the discrepancies?
• Has the Sun’s energy stayed constant with time?
EAS 4803/8803 - CP
17:7
Thermal Structure: Greenhouse Effect
• Atmosphere is more or less transparent to radiation
(photons) depending on wavelength – opacity
• Opacity is low at visible wavelengths, high at infra-red
wavelengths due to absorbers like water vapor, CO2
• Incoming light (visible) passes through atmosphere with
little absorption
• Outgoing light is infra-red (since the surface temperature is
lower) and is absorbed by atmosphere
• So atmosphere heats up
• Venus suffered from a runaway greenhouse effect –
surface temperature got so high that carbonates in the
crust dissociated to CO2 . . .
EAS 4803/8803 - CP
17:8
Thermal Structure: Albedo Effects
• Fraction of energy reflected (not absorbed) by
surface is given by the albedo A (0<A<1)
• Coal dust has a low albedo, ice a high one
• The albedo can have an important effect on
surface temperature
• E.g. ice caps grow, albedo increases, more heat is
reflected, surface temperature drops, ice caps
grow further . . . runaway effect!
• This mechanism is thought to have led to the
Proterozoic Snowball Earth
• How might clouds affect planetary albedo?
EAS 4803/8803 - CP
17:9
Atmospheric Thermal Structure
The atmospheric temperature profile is governed by
the efficiency of energy transport, which largely
depends on optical depth, τν . Remember that
heating by solar radiation is a ‘top-down’ process.
Optical depth (or transparency) is determined
physical and chemical processes in the
atmosphere and can change in time and in
altitude.
EAS 4803/8803 - CP
17:10
Atmospheric Thermal Structure
Other factors to consider:
Clouds can change the albedo, the optical depth,
and the local temperature (via
release/absorption of latent heat).
Surface variations/composition can effect albedo
and surface temperatures depend on the
thermal properties of materials and their
chemical interactions with the atmosphere
Geologic processes such as volcanism can greatly
impact the composition, as well as chemistry
and albedo (via dust grains and aerosals) of
the atmosphere.
EAS 4803/8803 - CP
17:11
Atmospheric Thermal Structure
Troposphere: Where
condensable gasses
form clouds. dT/dz < 0
Stratosphere:
dT/dz > 0
Mesosphere:
dT/dz < 0
Thermosphere:
dT/dz > 0
Exosphere: Roughly
Isothermal
EAS 4803/8803 - CP
17:12
Conduction
Atmospheric Thermal Structure
Radiation
Convection
EAS 4803/8803 - CP
17:13
Atmospheric Thermal Structure
EAS 4803/8803 - CP
Temperature
(schematic)
Altitude
Lower atmosphere
(opaque) is dominantly
heated from below and
will be conductive or
convective (adiabatic)
Upper atmosphere
intercepts solar radiation
and re-radiates it
There will be a temperature
minimum where radiative
cooling is most efficient
(the tropopause)
mesosphere
radiation
stratosphere
tropopause
clouds
troposphere
adiabat
Temperature
17:14
Altitude (km above 100 mbar height
Giant Planet Atmospheric Structure
Note position and order/composition of cloud decks
EAS 4803/8803 - CP
17:15
Atmospheric Thermal Structure
Radiation interactions are responsible for the structure we see:
• Troposphere
• absorbs IR photons from the surface
• temperature drops with altitude
• hot air rises and high gas density causes storms (convection)
• Stratosphere
• lies above the greenhouse gases (no IR absorption)
• absorbs heat via Solar UV photons which dissociate ozone (O3)
• UV penetrates only top layer; hotter air is above colder air
• no convection or weather; the atmosphere is stratified
• Thermosphere
• absorbs heat via Solar X-rays which ionizes all gases
• contains ionosphere, which reflects back human radio signals
• Exosphere
• hottest layer; gas extremely rarified; provides noticeable drag on
satellites
EAS 4803/8803 - CP
17:16
Terrestrial Planets Atmospheric Thermal
Structure
Mars, Venus, Earth all
• have warm tropospheres
(and greenhouse gases)
• have warm
thermospheres which
absorb Solar X rays
Only Earth has
• a warm stratosphere
• an UV-absorbing gas
(O3)
All three planets have
warmer surface temps
due to greenhouse
effect
EAS 4803/8803 - CP
17:17
Titan’s Atmospheric Thermal Profile
Balance between
greenhouse and anti-green
house effects:
Green house effects would
cause +21 K increase in
surface temperature over
Teq
Anti-green house from haze
layer absorption of sunlight
is responsible for -9 K
difference
So net ~12 K increase over
Teq
EAS 4803/8803 - CP
17:18