Evolution of Climate - Venus and Mars

Venus

Venus has a thick CO₂ atmosphere, with little H₂O. Why?
Maybe Venus never had water? NO! Look at the D/H ratio -- it's 100x that of the Earth. Venus must have lost lots of water. How?

Runaway Greenhouse Effect:

The temperature and pressure of the atmosphere change with height. If water vapor (steam) exists, as it rises through the atmosphere it may encounter conditions which will cause it to condense back into liquid water and fall to the surface. The region where it condenses is referred to as the cold trap.

On Earth, the cold trap sits around 9-15 km above the Earth's surface.

On early Venus, the cold trap probably sat much higher, 100 km or further above the surface. Why?

This probably resulted in a runaway greenhouse in Venus' early history:

Water vapor could move into the upper atmosphere, where it could be dissociated into H and O, and the H lost.

Less water would be available to wash CO₂ out of the atmosphere, so the CO₂ concentrations would rise. This results in a stronger greenhouse effect, so the temperature rises.

Higher temperature means the cold trap moves higher, so water vapor is even more easily lost...

...so the cycle escalates.

So Venus lost its water, and its CO₂ content exists in its entirety in the dense atmosphere -- no carbonate rocks.
Venus was too close to the Sun to evolve like the Earth.

Mars

Mars has a thin CO₂ atmosphere, with little H₂O. Why?
But Mars clearly had water in some form early in its history. So let's do a thought experiment. How much of an atmosphere would it take to boost Mars' temperature up enough to support liquid water? It's a few bars of atmosphere (HW#6).

So let's go back to early Mars and assume it had a 5 bar CO₂ atmosphere, and liquid water. What happens?

Can recycling work? Without it, the CO₂ would be washed out within ~ 10 million years. But we see water flow marks in regions of low crater density -- what does that tell you about when water was present on Mars?

So recycling probably happened early on. But then what happened?

Atmospheric escape was significant (why?)

Recycling shut off (why?)

Atmospheric freeze-out (why?)

Question: if Earth had formed at 1.5 AU, would its atmosphere be more like Venus', Mars', or current Earth's?

Does this work?

This idea predicts that Venus should have all its CO₂ in the atmosphere, while the Earth and Mars have most of their CO₂ locked into carbonate rock. Do we see this?

	Venus	Earth	Mars
H₂O Atmosphere	60	3	0.02
H₂O Oceans/Polar Caps	0	250,000	5,000?
H₂O Rocks	160,000?	30,000	10,000?
H₂O total	160,000?	280,000	15,000?

CO₂ Atmosphere	100,000	0.4	50
CO₂ Polar Caps	0	0	10
CO₂ Rocks	0	100,000	>900??
CO₂ Total	100,000	100,000	>1000??

from Hartmann, Moons and Planets, 1999 Table 11-2
(units are 10^-9 kg/kg of planet mass)

So maybe it works. But the carbonate content of Mars is very small, and there may not have been sufficient outgassing to create a strong greenhouse.

Some argue that, based on this, theories of a warm, wet Mars are fading out, in favor of a Mars where most of the water is and was locked in a permafrost layer, and got released for a short period of time during volcanic or impact events.

At the same time, we now have evidence from the rovers that long-lived water did exist on Mars.

What's the answer? More work is needed...