PY4022 Lecture 12-13: Planetary atmospheres oTopics to be covered: oAtmosphere composition....
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PY4022 Lecture 12-13: Planetary atmospheres Lecture 12-13: Planetary atmospheres o Topics to be covered: o Atmosphere composition. o Atmospheric pressure. o Atmospheric temperature. o Atmospheric retention.
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Transcript of PY4022 Lecture 12-13: Planetary atmospheres oTopics to be covered: oAtmosphere composition....
PY4022
Lecture 12-13: Planetary atmospheresLecture 12-13: Planetary atmospheres
o Topics to be covered:
o Atmosphere composition.
o Atmospheric pressure.
o Atmospheric temperature.
o Atmospheric retention.
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Primary atmospherePrimary atmosphere
o A planet’s primary atmosphere comes from nebular material in accretion disk.
o Mainly H, H2 and He.
o Trace elements also present in CO2, CH4, N2, H2O, NH3.
o If planet’s gravity isn’t strong enough or surface temperature is too large, these elements escape, leaving planet without an atmosphere.
o Solar wind can also drag material from the atmosphere.
o Relevant for planets without significant magnetospheres (e.g., Mars).
o For the terrestrial planets, most of the H escaped, leaving heavier gases such as argon, neon and ammonia concentrated near the surface.
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Secondary atmosphereSecondary atmosphere
o Rocks and planetesimals which combined to form each planet had trapped gasses.
o During formation, gases released from interior.o Differentiation caused them to rise to the outer
surface of the planet.o Released via volcanism.
o Comets/meteors containing water and gas collided with the planets (H2O, CH4, CO2).
o Volcanic gasses account for most of Earth's atmosphere. Primitive atmosphere contained H2, H2O, CO and H2S.
o Biological activity: photosynthesis converts CO2 to O2.
Mount Etna - March 2005 (credit Reuters/Irish Times)
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Atmospheric pressureAtmospheric pressure
o Assume hydrostatic equilibrium:
o As = P/RT and setting H = RT/ g =>
where P0 is pressure at surface and H is scale height.
o For Earth, H ~ 8 km.
o Scale height implies planets with low gravity or high temperature will have extended atmosphere.
o Can also write:
€
dP
dh= −ρg
€
P = P0 exp −1
Hdh
0
h
∫ ⎛
⎝ ⎜
⎞
⎠ ⎟
€
=0 exp −1
Hdh
0
h
∫ ⎛
⎝ ⎜
⎞
⎠ ⎟
International Civil Aviation Organisation (ICAO) Standard Atmosphere
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Atmospheric temperatureAtmospheric temperature
o Atmosphere not isothermal. Structured as function of height.
o Troposphere: Lowest region in atmosphere. On Earth, goes from ground to ~17 km. Weather and clouds form from trace elements of condensable gases. Temperature generally decreases with altitude.
o Stratosphere: T increases with altitude due to absorption of UV. Extends to ~50 km (on Earth). No clouds.
o Mesosphere: On Earth T quickly decrease with height
o Thermosphere: T increases with altitude due to strong UV flux. Includes the exosphere and part of the ionosphere. On Earth, T~1000K at 500 km.
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Atmospheric equilibrium temperatureAtmospheric equilibrium temperature
o At distance d from Sun, a planet of radius R receives: W
where Lsun is the solar luminosity in W.
o At 1 AU, Flux (F) = Lsun/ 4 d2 = 3.85 x 1026/ 4 (1.49 x 1011)2 = 1370 W/m2.
o But, a fraction (A) of power is reflected - A called planetary albedo (0 A 1).
o A = 1: Total reflection.
o A = 0: Total absorption.
o Fraction (1 - A) is absorbed by surface of planet.
o Rocks are poor reflectors and have low albedos,
ice is a moderate reflector.
€
Pabs =Lsun
4πd2× πR2
Planet A
Earth 0.37
Moon 0.12
Venus 0.65
Jupiter 0.52
Pluto 0.3
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Atmospheric equilibrium temperatureAtmospheric equilibrium temperature
o So, a planet of radius R and distance d from Sun will absorb:
where Fsun=1370/d2(AU). Assuming planet is blackbody will radiate energy back into space at a rate give by the Stefan-Boltzmann Law
W Eqn. 2
where is emissivity. Accounts for fact that planets not perfect blackbodies.
o In equilibrium, Eqn. 1 = 2. Rearranging gives,
o Temperature of planet is not related to how massive or its surface area.
€
Pabs = Fsun × πR2 × (1− A)
€
T =1370(1− A)
4εσ d2
⎛
⎝ ⎜
⎞
⎠ ⎟
1/ 4
W Eqn. 1
Pemitt = 4 R2 T4
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Atmospheric equilibrium temperatureAtmospheric equilibrium temperature
o Substituting for constants,
o E.g., for Earth, T = 248 K and for Moon, T = 269 K
o Observed temperatures are: Earth T = 288 K and Moon T = 252 K
o Earth is not a perfect blackbody:o Some solar heat is conducted into
surface rock and oceans - this is a form of ‘stored’ heat energy
o Earth has atmosphere which acts like thermal blanket, ‘trapping’ infrared radiation.
€
T = 278(1− A)
ε d(AU)2
⎛
⎝ ⎜
⎞
⎠ ⎟
1/ 4
Distance (AU)
0.1 1 10
Temperature (K)
10
100
1000
A = 0
Mercury
Venus
Moon
EarthMars
Jupiter
Saturn
UranusNeptune
Pluto
Tplanet = 278 { (1 - A) / ( )d au2 }1/4
slow rotation+
no atmosphere
( " ")runaway greenhouse effect
Perfect blackbody
= 0.9A
= , A albedo = = 1emissivity
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Greenhouse effectGreenhouse effect
o When sunlight reaches Earth, much passes to surface, because atmosphere is transparent to visible/very near-infrared.
o Ground absorbs V-NIR, and heats up.
o Then re-radiates energy. T ground lower than Sun’s surface, so radiation emitted at longer wavelengths (Wien’s Law) in the mid-IR (MIR).
o Atmosphere was transparent to V-NIR light, is opaque to the MIR. On Earth, H2O and CO2 absorb strongly in MIR.
o Energy trapped near surface. Eventually equilibrium is achieved, but at a higher T.
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Greenhouse effectGreenhouse effect
QuickTime™ and a decompressor
are needed to see this picture.
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Runaway Greenhouse effectRunaway Greenhouse effect
o Greenhouse effect is much more prominent on Venus.
o Venus has thick atmosphere of 96% CO2, 3.5% N2 and 0.5% other gases.
o Venus originally cooler and had greater abundance of water several billion years ago. Also, most of its carbon dioxide was locked up in the rocks.
o Because Venus was closer to Sun than Earth, water never liquified and remained in the atmosphere to start the greenhouse heating. As Venus heated up, CO2 in the rocks was “baked out”. Increase of atmospheric CO2 enhanced greenhouse heating and baked more carbon dioxide => runaway feedback loop.
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Atmospheric retentionAtmospheric retention
o Energy of a molecule in atmosphere can be written:
o A particle will escape from planet if has enough KE. Escape speed v = vesc, needed to escape from r = R is therefore:
o From kinetic theory, therefore,
o Lightest particles (H and He) have highest speeds and escape preferentially if T is large enough for particles to have vtherm > vesc.
€
E total = E k + E p =1/2mv 2 −GMm
r= 0
€
vesc =2GM
R
€
v therm =3kT
m€
1/2mv therm2 = 3/2kT
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Atmospheric retentionAtmospheric retention
o A planet will retain its atmosphere if
o The escape condition occurs when
o The region where this condition is met is called the exosphere.
o If surface temperature is large, planet will loose atmosphere. Also, small planets find it difficult to hold onto atmospheres.
o For a given planet or satellite of mass M and radius R the atmospheric retention condition is
Tatm < Tesc
€
v therm < vesc
€
3kT
m=
2GM
R
=> Tesc =2GMm
3kR
Random collisions
Ground
Atmosphere
ExosphereEscape
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Atmospheric retentionAtmospheric retention
o For a given molecule to be retained:
o Definition: m = mH
o where is molecular weight and mH is mass of H-atom (mH = 1.67 x 10-27 kg).o so, for hydrogen = 1, and for helium = 4o hence at a given temperature the He atoms will be moving slower than H atoms
o For Eartho Tatm = 288 K and vesc = 11.2 km s-1
o Hence, escape for all molecules with 4o So, don’t expect to find much H or He.
o For Jupitero Tatm = 134 K and vesc = 59.5 km s-1
o Hence, escape for all molecules with < 0.06o So, nothing escapes, since hydrogen with = 1 is the ‘lightest’ gas element. Observations
show that Jupiter is a H and He gas giant.
€
2GM
R>
3kT
m=> m >
3kTR
2GM
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Atmospheric retentionAtmospheric retention
Oxygen
Helium
Hydrogen
o As vtherm ~ m-1/2 and ~T1/2, light gases have higher speeds and hot gases have higher speeds.
o Gas giants are massive planets with high escape speeds and cold temperatures, so light gases such as H and He retained. Small rocky bodies are closer to the Sun, have higher temperatures and less mass, and so lack H and He - some have no atmosphere.
o Even if vtherm < vesc, some particles will escape due to the ‘high-speed’ tail of the Maxwellian distribution.
o For a planet to ‘hold’ an atmosphere over the age of the Solar System (~4.5 billion years), the escape condition is more like vesc > 10 vtherm
o The factor of 10 accounts for the high-velocity tail of the Maxwellian distribution of speeds.
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€
vesc =2GM
R
€
v therm =3kT
m
Retention of Atmospheric Gases
Temperature (K)
100 1000
Velocity (km/s)
0.1
1
10
100
PlanetsGalilean moonsTriton and TitanMinor Planets
NB: lines show ten timesmean molecular speeds
Jupiter
Saturn
Uranus
Neptune
Mercury
Moon
Venus
Earth
Mars
Pluto
Triton
Ceres
Titan
Vesta
Pallas
Hydrogen
Helium
H2O
N2
CO2
Xe
Atmospheric retentionAtmospheric retention
o Escape velocity:
o Thermal velocity:
o Consequences:
o Light elements escape more easily.o Hot planets “burn off” their atmosphere.o Small planets cannot hold onto atmosphere.
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Venus ExpressVenus Express
o What is the mechanism and driving force of the super-rotation of the atmosphere?
o What are the basic processes in the general circulation of the atmosphere?
o What is composition and chemistry of lower atmosphere and clouds?
o What is the past and present water balance in the atmosphere?
o What is the role of the radiative balance and greenhouse effect?
o Is there currently volcanic and/or tectonic activity on the planet
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
o Arrived at Venus in April 2006.
