Between 3.6 billion and 4 billion years ago, when Mars was warmer and wetter, water penetrated fractures in the subsurface rock. Saturated with carbon dioxide from the Martian atmosphere, carbonate minerals were deposited in the fractures. Living organisms also may have assisted in the formation of the carbonate, and were fossilized. Then, 16 million years ago, a huge comet or asteroid struck Mars, ejecting a piece of the rock. For millions of years, the rock floated through space, falling in Antarctica 13,000 years ago as a meteorite.