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:
- 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?
So Venus lost its water, and its CO2 content exists in its entirety in the dense atmosphere -- no carbonate rocks.
- 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.
Venus was too close to the Sun to evolve like the Earth.
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?
- Atmospheric escape was significant (why?)
- Recycling shut off (why?)
- Atmospheric freeze-out (why?)
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?? |
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...