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.

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

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...