NASA scientists are reporting definitive signs of liquid water on Mars' present-day surface.

"This, I think, gives a focus of where we should look more closely," said Alfred S. McEwen, professor of planetary geology at the University of Arizona and investigator of high-resolution images taken on NASA's Mars Reconnaissance Orbiter.

Mars is known to have had rivers, lakes, and perhaps an ocean a few billion years ago. Moisture on Mars' surface today, however, has been modest. Scientists have known for a long time that water still remains on Mars, but in frozen form. There have also been signs of liquid water, but nothing was proven until recently.