Geologists already know that water once existed on Mars.
On Monday, Pathfinder scientists presented evidence of massive floods 1 billion to 3 billion years ago. But the latest evidence hints that water may have existed there more than 4 billion years ago, the same era when life began on Earth. That's a type of volcanic rock named for Earth's Andes Mountains, where it is common. Such a rock could have been brought to the surface by volcanic activity or a meteorite impact. It is too early to tell for certain that Barnacle Bill is andesite.