Mars is a dry, frigid, dusty, nearly airless place. A couple of billion years ago, though, it wasn’t much different from Earth. A string of space probes have proven that the Red Planet once sported rivers that carved huge canyons across the landscape — and even oceans, whose shorelines are still visible from orbit. Open water couldn’t have existed without a relatively thick atmosphere, and most of that Martian atmosphere leaked out into space long ago.
But some of that ancient atmosphere, it turns out, headed in the other direction. A series of measurements with ground-penetrating radar mounted on the Mars Reconnaissance Orbiter have revealed a massive deposit of frozen carbon dioxide — a Lake Superior’s worth of dry ice — buried under a layer of ordinary ice near the Martian south pole. “We knew there was some CO2 at the pole,” says Roger Phillips of the Southwest Research Institute in Boulder, Colo., lead author of a report on the discovery in the current issue of Science, “but there’s about 30 times more than we thought.”
It doesn’t stay buried, though. The reason, as the report explains, is that Mars wobbles slightly, so that every hundred thousand years or so, the poles tilt more toward the sun in summer, getting more direct sunlight than they ordinarily would. The extra heating turns the frozen CO2 back into a gas — enough to double the density of Mars’ sparse atmosphere. And then, as the planet straightens up, the CO2 freezes out again and the atmosphere thins.
On Earth, the same sort of wobbles help trigger the ends of ice ages, as warmer polar summers melt glaciers and liberate CO2 stored in the oceans. The CO2 acts as a greenhouse gas, warming the planet even more, which liberates even more CO2 in a feedback loop that brings the planet into a relatively balmy interglacial period.
