Download Inner Planets` Atmospheres

The Inner Planets’
Atmospheres
• Atmosphere – gas in the form of individual atoms or
more typically, of molecules.
•Common molecules and their atomic wt:
•--carbon dioxide CO2 -> 12+2x16=44
•--Argon Ar (a noble gas) -> 40
•--nitrogen N2 -> 2x14=28
•--water H2O -> 16+2=18
•--methane CH4 -> 12+4x1=16
How are Density, Pressure, and
Temperature Related?
• Atmosphere’s are well-approximated by the
Perfect Gas Law
• Pressure (P) proportional to Temperature
and to Density
• P proportional to density x Temperature
How Does a Planet Retain an
Atmosphere?
• Surface gravity must be high enough and
• surface temperature must be low enough,
that the atmosphere molecules don’t leak
away during the 4.6 billion years since
formation.
Two Ways a Planet Loses
Atmosphere: First…Leakage!
Lighter molecules move faster, because on average Kinetic
Energy = Thermal Energy
• (½)m<v>2 = (3/2)kT
• For a given temperature, higher mass molecule means
lower velocity molecule, is what this equation is telling us
• So the lighter gasses leak away more quickly over time
• Molecules are continually bouncing off of each other and
changing their speed, but if the average speed is higher, a
few may be speedy enough to escape the planet’s gravity.
• So…. Slow leak! Like air from a bicycle tire
• Hydrogen and Helium = 97% of the mass of the solar
nebula, and these are the lightest and easiest molecules to
lose.
• Ergo, ALL the inner planets have THIN atmospheres
made of the rare HEAVY molecules
The Second way to Lose
Atmosphere…
• …maybe easier to understand - Impact Cratering!
Big comets and asteroids hitting the planet will
deposit a lot of kinetic energy which becomes
heat, blowing off a significant amount of
atmosphere all at once.
• This is a Big issue especially in dense areas (inner
solar system), and dense times (soon after
formation).
So Where did most of the
solar nebula material go?
• It’s hot close to the sun. So no ices. Only
the rocky material (~3% of the solar nebula)
could collect into self-gravitating objects.
Not hydrogen and helium since escape
velocities that are too low; these atoms are
blown away; calculations indicate this is
what halted planet formation,
• Atmosphere histories for each planet are
unique…as we’ll see
Early inner planet; a ball of lava
Mercury
• Smallest planet, only 3,000 mi across.
• 600F on daylight side, too hot to retain any
atmospheric molecules at all. Probably doesn’t
help that the sun is so close and solar storms can
rack the planet and help carry off any atmosphere
too.
• And
• Cratering shows it hasn’t had atmosphere for most
of solar system’s history
Mercury mariner
Mercury mud cracks
Venus
• Has thick CO2 atmosphere, 100 times
denser than earth’s. CO2 is the heaviest
common molecule.
• Greenhouse effect – CO2 transparent to
visible light coming down from the sun, but
opaque to infrared coming back off the
surface, hence heat comes in as light but
can’t easily escape. So…
• 900K on surface!!
• Let me draw you a picture…
Greenhouse effect
venusHST
Venus-surface2
Venus-surface4
Venera-right
Earth – Biggest Inner Planet, so
why so little atmosphere?
One Reason – Cataclysmic impact with
Mars-sized planet very early on. See here
• That’s one heck of an impact cratering
event! Primary atmosphere, if it had
one, was clearly gone at this point.
• But it’s not the whole story…
Earth’s Atmosphere; Initially
Rich in CO2, Methane, No Oxygen
What’s the OTHER reason so little atmosphere, and why is
CO2 such a tiny % (~0.3% today)?
** Life took CO2, pulled off the C and produced
O2, and organic and inorganic processes
produced CaCO3. Nice! This has been lowering
greenhouse gases at the same time the sun has
been increasing its luminosity – balance!
Aurora, iceland volcano
Mt. Aetna in italy
Ozone hole
moon
Mare humorum,
Clavius 160mi across
Mars – A Pure CO2
Atmosphere
• …But not much of it. Only 1% of Earth’s
atmospheric pressure. Why so little?
• Mars shows limestone rock, so some of the
CO2 got turned into rock in the ancient
oceans, we speculate
• Impact cratering – Mars is close to the
asteroid belt, and likely gets hit more than
the Earth. And, it’s already captured 2 of
‘em - can only do that if you lose some
orbital energy, like impacting.
marsHS
Mars globe, big craters
Olympic mons caldera
Mars valle marinaris
Mars continents
Mars solis plenum
Martian sand dunes
Mars gullies
Martian surface; pathfinder
Mars mud cracks
Martian rock; blueberries,
razorback
Mars BurnsCliffs
Mars frozen ice floes
Mars Rover “Curiosity” Finds
Clues…
• …As to how Mars lost so much atmosphere – it finds the
current atmosphere is much enriched in the heavy vs
lighter isotopes for Argon and Carbon, vs. the abundances
found in the older Martian rock found in Antarctica
• Lighter isotopes would be more easily lost to outer space
by thermal leakage, as at a given temperature, they move
faster.
• Thus, leakage to outer space over long periods of time
(vs. all at once, as in Impact Cratering) has played an
important role
• This supports indirectly the solar wind – weak
magnetic field theory for atmosphere lost, as this would
be a mechanism for enhanced loss to outer space
• See 2012 announcement here
How Does Mars’ Atmosphere
Change with Spin Axis Tilt?
• Mars spin axis tilt varies from near zero to well
over 45 degrees (!) because not stabilized by a
massive moon like we have (105-106 year cycles).
• When near zero, both poles are cold, resulting in a
Martian Ice Age, with ice extending over both
poles extensively.
• This pulls CO2 out of the atmosphere, resulting in
a thin atmosphere, colder, less greenhouse
warming, colder.
• Large tilt corresponds to thicker, warmer
atmosphere (study source: Laskar 2002)
• Today near 23 degrees, poles alternate getting icy
with the seasons leading to an intermediate
climate.
Quick Summary
• Atmospheres are retained by LOW temperature and HIGH gravity,
minimizing leakage into space
• Inner planets have thin atmospheres made of heavy molecules,
mainly CO2
• Except Earth, where life has taken CO2 out of the atmosphere (well,
except for homo sapiens) and turned it into limestone rock or buried
it into the mantle, leaving N2 as the dominant gas
• CO2 is a powerful greenhouse gas, absorbing the IR radiated by
planetary surfaces and inhibiting their ability to cool.
• Venus has had a runaway greenhouse effect. Will Earth follow?
Some day, but probably not for many millions of hundreds of
millions of years
Key Points: Chap 10 - Atmospheres of
Inner Planets
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Surface temperature and gravity determine how well you keep your atmosphere
Loss mechanisms: Leakage of lighter molecules, impact cratering, ablation by solar
wind
Atmospheric pressure increases with both temperature and with density
Understand the greenhouse effect!
Mercury, moon, too hot and low gravity to retain any atmosphere
CO2 dominates both Mars and Venus; heaviest common molecule
Earth atmospheric CO2 lost to diffusion into ocean, turned to CaCO3 by life. More on
Earth climate in next PowerPoint.
Earth’s atmosphere (troposphere, which is most of it) no thicker than a piece of paper on
a school room globe
Current global warming is being primarily caused by CO2 from human fossil fuel
burning leading to the greenhouse effect
Runaway greenhouse effect
Ice Ages caused by Milankovitch cycles in Earth orbit and axis tilt, related effect on
Mars
Stratosphere: requires heat source at upper levels (Earth: ozone absorbs solar UV)
Mars atmosphere has thinned progessively over 4.5B years due to no protection from
solar wind (weak mag field).
Mars and Venus both likely had oceans of water early in their history
Mars climate: denser warmer atmosphere when axis tilt is high, cold thin atmosphere
when tilt is small