NASA's Mars Reconnaissance Orbiter has found a giant buried deposit of dry ice, which could be evidence that Mars once had a thicker atmosphere and was able to support more water on its surface.

The orbiter's ground-penetrating radar found the dry ice, which is frozen carbon dioxide, near the planet's south pole. The scientists think that when Mars' axial tilt increases, the carbon dioxide turns into a gas, thickening the atmosphere. The result would be more intense dust storms, but also a wider range of areas where liquid water could exist.

The newly deposit is about 3,000 cubic miles, or about the volume of Lake Superior, and holds enough carbon dioxide to nearly double the mass of the Martian atmosphere. Mars' atmosphere has a surface pressure less than 1 percent that at Earth's surface at the lowest altitudes. The air on Mars is 95 percent carbon dioxide, while that on Earth is less than 0.04 percent CO 2 .

We already knew there is a small perennial cap of carbon-dioxide ice on top of the water ice there, but this buried deposit has about 30 times more dry ice than previously estimated, said Roger Phillips of Southwest Research Institute in Boulder, Colo., in a statement. Phillips is deputy team leader for the Mars Reconnaissance Orbiter's Shallow Radar instrument and lead author of the study.

The scientists were able to tell that the subsurface deposit is CO 2 because the radar signature fit; different materials reflect radar waves differently. There are also visible pits that could be from the ice sublimating and leaving spaces that cause the surface to collapse - essentially sinkholes.

When you include this buried deposit, Martian carbon dioxide right now is roughly half frozen and half in the atmosphere, but at other times it can be nearly all frozen or nearly all in the atmosphere, Phillips said.

Mars currently has an axial tilt of about 25 degrees. (Earth is tilted at about 23.5 degrees). If in the past the planet was tilted more, sunlight would hit more of the poles and be more intense. That would boost the temperature and cause the frozen CO 2 to sublimate into gas.

A thicker atmosphere would be windier and lift more dust into the air. The increased pressure would allow liquid water to persist longer on the surface, in a wider range of areas, before freezing or boiling away. Another surprise is how fast it happens; modeling based on known variation in Mars' axial tilt seems to show that the mass of the planet's atmosphere can change in 100,000 years or less.

The increase in atmospheric density caused by the carbon-dioxide also would amplify some effects of the changes caused by the tilt. Researchers plugged the mass of the buried carbon-dioxide deposit into climate models for the period when Mars' tilt and orbit maximize the summer sunshine that hits the south pole. They found global, year-round average air pressure is approximately 75 percent greater than the current level.

A tilted Mars with a thicker carbon-dioxide atmosphere causes a greenhouse effect that tries to warm the Martian surface, while thicker and longer-lived polar ice caps try to cool it, said co-author Robert Haberle, a planetary scientist at NASA's Ames Research Center. Our simulations show the polar caps cool more than the greenhouse warms.

That is because Mars' atmosphere is too thin and dry to produce as strong a greenhouse effect as Earth's. On Earth there is a strong greenhouse effect, because of its thick, moist atmosphere. Mars' air is so thin and dry it doesn't hold nearly as much heat.