Gullies and narrow channels on the surface of Mars suggest the presence of flowing water, even if surface temperatures are normally too low for water to exist in liquid form. It’s possible that water bubbles up from underground where temperatures are higher.
Our ability to evacuate to Mars once we’re done wrecking the Earth depends on a lot, but the whole idea is a nonstarter if the fourth rock from the sun is dry. Ten years ago scientists discovered evidence of flowing water on Mars and we have reason to believe there’s plenty of the frozen variety, but we still haven’t caught Mars with its gullies wet.
Christian Science Monitor:
One of the main problems about proving whether or not water still flows on the surface of Mars “is that where we have seen gullies also tends to be where we don’t want to land spacecraft,” Meyer explained.
“The hope is that orbiter missions can catch something in the act involving liquid water near the surface today,” he said. “We do suspect there’s liquid water in the subsurface — it’s just depends on how deep you have to go.”