1. 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

2. 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

3. 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.

4. 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

5. 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).

6. 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

7. Early inner planet; a ball of lava

8. 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

9. Mercury mariner

10. Mercury mud cracks

11. 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…

12. Greenhouse effect

13. venusHST

14. Venus-surface2

15. Venus-surface4

16. Venera-right

17. 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…

18. 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!

19. Aurora, iceland volcano

20. Mt. Aetna in italy

21. Ozone hole

22. moon

23. Mare humorum,

24. Clavius 160mi across

25. 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.

26. marsHS

27. Mars globe, big craters

28. Olympic mons caldera

29. Mars valle marinaris

30. Mars continents

31. Mars solis plenum

32. Martian sand dunes

33. Mars gullies

34. Martian surface; pathfinder

35. Mars mud cracks

36. Martian rock; blueberries, razorback

37. Mars BurnsCliffs

38. Mars frozen ice floes

39. 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

40. 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 ( 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.

41. 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

42. 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