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. 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 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.