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LOS ANGELES — Think Mars today is a hostile place? It was worse 600,000 years ago, according to new research that suggests the planet had a dustier, stormier atmosphere. “It was an unpleasant place to hang out,” said lead researcher Roger Phillips of the Southwest Research Institute. He said Mars’ climate was probably a lot like the American Dust Bowl of the 1930s – but a lot worse. 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. There is an upside to that stormier climate: 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. Still, “it was not the balmy, tropical Mars” that existed even billions of years earlier, Phillips said. Mars today is frigid, arid and constantly bombarded by lethal radiation. Its atmosphere, made up mostly of carbon dioxide, is many times thinner than the top of Mount Everest on Earth. In fact, the Martian atmosphere is less than 1 percent of Earth’s. The red planet wasn’t always this unforgiving. A maze of gullies, canyons and river channels on the surface points to a warmer and wetter past very early in the planet’s history. The underground dry ice deposit, roughly the size of Lake Superior, was discovered using ground-piercing radar aboard the NASA Mars Reconnaissance Orbiter designed to probe below the crust. Researchers estimate it represents 30 times more carbon dioxide than previously believed. Its presence may help explain how most of the Martian atmosphere disappeared. “It really is a buried treasure,” said Jeffrey Plaut of the NASA Jet Propulsion Laboratory who was part of the discovery team reporting online Thursday 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.