On rusty-red Mars, a curious deposit of gray-colored hematite (a mineral cousin of common household rust) could hold the key to the mystery of elusive Martian water. The word “rust” conjures up images of things that are red –like Mars and old nails– but not all iron oxide is the same color. 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. There are many signs of ancient or hidden water on the Red Planet including flash-flood gullies, sedimentary layers … and hematite.