Critical to the question of life on Mars is the presence of liquid water.  Even more important than oxygen, water is critical to all life on Earth for a number of reasons.  If we can find liquid water on Mars, that greatly improves the odds of finding life.  Now we’ve known for a long time that Mars has got plenty of water frozen in the polar ice caps, and a barely detectable amount in its atmosphere, and we’ve seen features that may or may not have been caused by flowing water.

New observations returned from the Mars Reconnaissance Orbiter (MRO) suggest that the planet may be wet after all.  This series of images taken over the course of several years of a specific valley on Mars shows features that seem to flow downhill during the summer months, and vanish in winter.  The slope on which they’re seen is in the southern hemisphere and faces north, so that it gets plenty summer heat from the Sun.  It looks as if ice under the surface (something that’s been seen in meteor impacts, and in skidmarks from the Mars rovers) is melting in the Sun’s rays, and wetting the ground as it flows downhill.

Of course, merely looking that way doesn’t mean it IS that way.  To confirm we either need to collect samples, or get a much better look than the instruments on the MRO can provide.  So nothing is confirmed, but it’s certainly looking very positive!