The Opals that the Martian Sea Left in its Wake The question at this point isn't whether Mars was once covered in liquid water — it's just a question of how long that water was there before it evaporated. This image provides new evidence that water washed the Martian shores for a tremendously long time. Long enough to reshape the rocks in that crater and to leave behind whole beaches of pearly, hydrated silica commonly known as opal. The other interesting news is that this water may have evaporated fairly recently in geological time. According to Space:
These opaline silicates . . . formed where liquid water altered materials created by volcanic activity or meteorite impacts on the Martian surface. "This is an exciting discovery because it extends the time range for liquid water on Mars, and the places where it might have supported life," said CRISM principal investigator Scott Murchie of the Johns Hopkins Applied Physics Laboratory in Laurel, Md. "The identification of opaline silica tells us that water may have existed as recently as 2 billion years ago."
Assuming that water is a prerequisite for life, there might have been life forms on Mars as recently as a couple of billion years ago. That's long before homo sapiens evolved, of course, but it could mean that we might still find fossils from that life. Or even the descendents of it, living in urban hives beneath those opal beaches. New Signs that Ancient Mars Was Wet [via]