"If you wanted to make Mars warm and wet again, you'd need carbon dioxide, but there isn't nearly enough if the polar caps are made of water," Ingersoll said. "Of course, terraforming Mars is wild stuff and is way in the future; but even then, there's the question of whether you'd have more than a tiny fraction of the carbon dioxide you'd need.
"This is because the total mass of dry ice is only a few percent of the atmosphere's mass and thus is a poor regulator of atmospheric pressure, since it gets 'used up' during warmer climates. For example, when Mars's spin axis is tipped closer to its orbit plane, which is analogous to a warm interglacial period on Earth, the dry ice evaporates entirely, but the atmospheric pressure remains almost unchanged."
A New Scientific Mystery
The findings present a new scientific mystery to those who thought they had a good idea of how the atmospheres of the inner planets compared to each other, according to Ingersoll. Planetary scientists have assumed that Earth, Venus, and Mars are similar in the total carbon dioxide content, with Earth having most of its carbon dioxide locked up in marine carbonates and Venus's carbon dioxide being in the atmosphere and causing the runaway greenhouse effect. By contrast, the eight-meter layer on the south polar ice cap on Mars means the planet has only a small fraction of the carbon dioxide found on Earth and Venus.
The new findings further pose the question of how Mars could have been warm and wet to begin with, according to Ingersoll. "Working backward, one would assume that there was once a sufficient amount of carbon dioxide in the atmosphere to trap enough solar energy to warm the planet, but there's simply not enough carbon dioxide for this to clearly have been the case."
There could be other explanations, Byrne said. "It could be that Mars was a cold, wet planet; or it could be that the subterranean plumbing would allow for liquid water to be sealed off underneath the surface."
In one such scenario, perhaps the water flowed underneath a layer of ice and formed the channels and other erosion features. Then, perhaps, the ice sublimated away, to be eventually redeposited at the poles.
At any rate, Ingersoll and Byrne said that finding the missing carbon dioxide, or accounting for its absence, was now a major goal of Mars research.
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