Flowing Water on Mars?

A group of scientists using data from the Mars Reconnaissance Orbiter (MRO) has detected possible evidence of flowing water on the surface of Mars. A team led by Alfred McEwen has published this tantalizing result in the journal Science. They have found evidence of seasonal variations on steep slopes in the southern hemisphere of Mars that may be indicative of flowing water.

There is considerable evidence that Mars had a wet past, but most scientists would consider Mars a dry and (most likely) dead world today, at least on the surface. This discovery shows possible evidence for surface water today, and is we have seen on Earth, where there is water, there is life. Images from MRO show dark markings ranging in width from 0.5-5 meters (2-16 feet) on steep slopes that change seasonally. They call these markings recurring slope lineae (RSL). From late spring to early fall they observed the RSL appearing and growing in size; from fall through winter the RSL decrease in size and faded away. The theory is that as the surface heats up during the warmer seasons, water is more likely to be in liquid form on the surface. During the winter, the temperature is too cool for water to exist in liquid form, so it either freezes or sublimates (turns into gas).

The water flowing on Mars isn't exactly like a river or other bodies of moving water on the Earth. For one, this is not pure H2O - this water would be saturated with salts like calcium, magnesium, sodium or maybe iron sulfates.

It should be noted that there are still lots of unanswered questions about the RSL - why are these flows dark? Where is this water coming from? There are theories for the origins of the water, including absorption of water from the atmosphere by salts on the surface or subsurface water seeping out.

The existence of the these markings does not necessarily prove that there is flowing water on the surface of Mars. But the idea of water flowing on the surface of a planet other than Earth is intriguing, and could be astrobiologically significant.