You probably know that Mars hasn't always been so barren, but have you wondered what it was like in its heyday? You almost wouldn't recognize it, according to an international team of scientists. They've used six years of atmospheric monitoring to determine that Mars had enough water to form a big but shallow (one mile deep) ocean that covered almost half of the northern hemisphere four billion years ago. The artist's rendition above gives you an idea of what it might have looked like -- the Red Planet would have been decidedly blue in places.
You sadly aren't likely to ever see Mars in that state again, since most of the water was lost to space. It's also possible that there was (and potentially, still is) more water locked underground. Even so, the findings could have a tremendous effect on the search for signs of life. The sheer abundance of water hints that Mars might have been habitable for longer than first thought, raising the chances that Curiosity and future rovers will find evidence of alien organisms.
[Image credit: ESO/M. Kornmesser]