Scientists think Mars has a bad case of rust.
Here on Earth a gray-hued variety of iron oxide, a mineral called hematite, can precipitate in hot springs or in standing pools of water. Gray hematite is not the sort of rust you might expect to find on a desert-dry planet like Mars. But perhaps Mars wasn't always as dry as it is today. The discovery raised the tantalizing possibility that hot springs were once active on Mars. We think the deposit is fairly old.