Buried under the south pole of Mars are the makings of one heck of a Halloween party.

Radar soundings from NASA's Mars Reconnaissance Orbiter (MRO) have identified huge dry ice deposits under the Red Planet's surface totaling roughly 10,000 cubic kilometers. That's enough to make a foggy cauldron at each household in the U.S. every Halloween for the next 20 million years or so, or to chill a few quadrillion coolers of frozen steaks during cross-country shipping.

More importantly to planetary scientists, that dry ice—solid carbon dioxide—is enough to change the climate of Mars. If all that CO2 were released as gas, as it may well have been in the past, it would nearly double the mass of the Martian atmosphere, increasing pressures across the planet and making Mars more hospitable to patches of liquid water.

Scientists had previously identified much smaller deposits of dry ice, totaling a few hundred cubic kilometers, near Mars's south pole. That CO2 is sequestered in shallow stores just a few meters thick. But a radar instrument on MRO was able to peer deeper, locating buried deposits hundreds of meters thick. The massive CO2 stores were reported in a study published online April 21 in Science.

Mars has a thin carbon dioxide–dominated atmosphere, but in the planet's geologic history that atmosphere is thought to have been much more robust. Some of Mars's ancient atmosphere has been lost to space, and some has been sequestered in carbonate minerals, but researchers postulated as early as 1966 that large amounts of atmospheric CO2 may also have gathered in solid form at the poles.

The planet appears to have a cyclical process of CO2 exchange between the polar caps and the atmosphere, driven by Mars's ever-changing axial tilt. Today, the Red Planet is tilted on its axis by about 25 degrees, roughly the same amount as Earth. But unlike our planet, Mars has no massive moon to stabilize it, so its axial tilt, or obliquity, varies greatly over long timescales, from about 10 degrees to more than 40 degrees.

When the obliquity is high, the poles absorb much more solar radiation in summer and the frozen caps of dry ice vaporize into the atmosphere. When the axial tilt is low and the poles go dark, the reverse happens. "The obliquity gets low enough that the whole atmosphere basically collapses," says lead study author Roger Phillips, a planetary scientist at the Southwest Research Institute in Boulder, Colo. "This stuff cycles between the atmosphere and the surface; we caught it kind of half and half."

The climate effects of releasing all that carbon dioxide into the atmosphere at periods of high obliquity, such as a time about 600,000 years ago when Mars's tilt and orbit conspired to drench the south pole in sun, are not yet clear. "We're still exploring that," Phillips says. "It's not the massive amount that people would like to see, that would lead to running water on the surface in rivers and creeks. It's not enough to send Mars into a tropical period." All the same, the global atmospheric pressure would increase by about 80 percent, which would expand the conditions under which water could exist as liquid without immediately vaporizing.

"If you double the amount of CO2 in the atmosphere, it's quite possible that you could have liquid water," says planetary scientist Philip James of the Space Science Institute in Boulder, who did not contribute to the new study. "People have suggested that this could happen, and now it looks like it could be possible. It's kind of neat."

James Head, a planetary scientist at Brown University, notes that the discovery shows that liquid water could have been more stable on Mars relatively recently in geologic time. "This finding alone could help to explain the gullies that we and others have mapped out from this period," he says.

Radar soundings are not the same as pulling up a drill sample for analysis, but the researchers say the signature of CO2 ice is convincing. "We went through a lot of work to prove" that the substance could not be porous water ice, which could mimic the radar signal, Phillips says, before concluding that it was indeed frozen CO2. "There's really nothing else that it could be," he says.