2012: Advent Atmosphere: Venus, Earth, Mars

by on December 3, 2012

After a few million years, when the solar system had cooled down a bit, it became cool enough that gases like water and carbon dioxide could be captured by the rocky inner planets instead of just flying back off into space. Again, if you just divided the inner solar system into a chunk for each planet you’d find approximately the same stuff in each area, so why do the planets look so different now?

The small differences in temperature at the different orbits – a few tens of degrees – were enough to set the planets up for completely different futures. What you get is a feedback loop, rather like the initial collection of the gas cloud into bigger and smaller planets.

If you start out too cold, then it’s too cold on the surface of the planet for ice to melt, and you don’t get a nice snug blanket of steam, and you stay cold and your water stays locked up in ice. That’s what things look like on Mars, where the pressure is 0.006 of Earth normal, and the temperature is actually three degrees below equilibrium*.

If you start out too hot, then the warming effect of an increasing atmosphere drives you past the point where liquid water can exist, and more water just means more steam in the atmosphere driving you even hotter. That’s Venus, where the temperature went up too fast for the pressure, and by the time there was a high enough pressure for water to start to condense out of the air and become liquid, it was too hot for that. Runaway greenhouse effect, with the result that Venus, with an atmospheric pressure of 0.93 of Earth normal, has a surface temperature of 735 Kelvin. Thermal equilibrium at Venus orbit is 328 Kelvin. The greenhouse effect is important.

Venus started out 50 degrees warmer than Earth, back when both of them were lumps of bare rock. When they started gathering their atmospheres, Venus was just slightly too hot for water to condense. Hot enough now to evaporate metal. No oceans. No possibility of our kind of life. And Mars? Mars has an equilibrium temperature fifty degrees lower than ours, the water it gained ended up stuck as ice and not contributing any greenhouse effect, and the whole planet is a frozen desert.

Somewhere in between those two problems lies Earth. If we didn’t have an atmosphere, then the heat that reaches us from the Sun would keep us at a nice balmly 255 Kelvin, or minus 18 degrees Celcius. When the atmosphere started to form and the pressure built up, it was in the right temperature bracket for water to condense out as liquid. After that, all the water you add just makes the oceans bigger and doesn’t increase the greenhouse effect. If our orbital distance from the Sun were much different either way, the Earth would not have ended up blue. (If it hadn’t ended up blue, we wouldn’t have evolved, and therefore wouldn’t be sitting here talking about how lucky we are that it did. Of course the Earth is remarkably well suited to our kind of life. That’s why we’re this kind of life.)

* Equilibrium being defined here as the temperature things should be based solely on how much energy they get from the Sun.

One Response to “Venus, Earth, Mars”

  • sienf says:

    Love the last two sentences! Very much “a puddle waking up one morning and thinking, “This is an interesting world I find myself in […]”” 🙂

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