Mars today has a brutal environment — frigid, arid and, because of its very thin atmosphere, constantly bombarded by lethal radiation. But it was worse 600,000 years ago, according to new research that suggests the planet had a far dustier, stormier atmosphere.

“It was an unpleasant place to hang out,” said lead researcher Roger Phillips of the Southwest Research Institute in Boulder, Colo.

The evidence comes from the discovery of a huge underground reservoir of dry ice, or frozen carbon dioxide, at its south pole — much more than scientists realized. They suspect some of that store of carbon dioxide was once in Mars’ atmosphere, making it denser.

In the recent geologic past, when Mars’ axis tilted, sunlight reached the southern polar cap, melting some of the frozen carbon dioxide. This release would have made the atmosphere thicker and caused more dust to loft into the air, creating severe storms. Other times, carbon dioxide cycled back into the ground as part of a seasonal cycle.

The thicker atmosphere back then meant there were more regions on the planet where liquid water probably existed. Water is considered an essential ingredient for life.

The underground dry ice deposit, roughly the size of Lake Superior, was discovered using ground-piercing radar aboard NASA’s Mars Reconnaissance Orbiter, designed to probe below the crust. Researchers estimate it represents 30 times more carbon dioxide than previously believed.

“It really is a buried treasure,” said Jeffrey Plaut of NASA’s Jet Propulsion Laboratory, who was part of the discovery team reporting online last week in the journal Science. “We found something underground that no one else realized was there.”

Though the newfound store sounds like a lot, it’s only enough carbon dioxide to double the mass of the feeble Martian atmosphere if released — not enough to warm up the planet substantially or allow water to pool.

“The atmosphere would still be quite thin and would not have the density necessary to warm things up enough to have liquid water stable on the surface,” said Peter Thomas of Cornell University, who had no role in the mission.

The mystery of what happened to Mars’ atmosphere has long intrigued scientists. NASA plans to explore the upper atmosphere and study how gases are lost to space with a new spacecraft in 2013.