The new discovery was made by comparing high-resolution photos taken by NASA's Mars Global Surveyor spacecraft, which fell silent in November 2006 after almost ten years of observing the planet from orbit. They show light-colored deposits in recent photos of two gullies that were not present in pictures taken in 1999. Michael Malin, the camera Principal Investigator, said in a NASA news conference that "The shapes of these deposits are what you would expect to see if the material were carried by flowing water." In a paper published in Science for November 8, 2006, Malin and his colleagues argue that the deposits appear to have been left by liquid water mixed with dirt or other material that flowed downslope for hundreds of meters before dispersing.
These light streaks could be actual deposits of ice still sublimating, or perhaps they are a salty deposit that remained after the water disappeared. Liquid water is not stable on the surface of Mars. If liquid is released from below the surface, it will simultaneously freeze and boil. However, liquid water escaping from the interior could have moved downslope for as much a half a kilometer before freezing solid.
Both of the observed flows emanate from pre-existing gullies on steep slopes. Thousands of similar gullies have been discovered, most of them apparently originating in sediment layers that may contain liquid water aquifers. There has been some controversy over the way the gullies form: from release of subsurface water or from melting of surface ice deposits. Recently the HiRISE camera on the new Mars Reconnaissance Orbiter has shown how small deposits of ice form in shaded gullies during the martian winter, but it is difficult to imagine these generating liquid water. If the interpretation of the fresh flows is correct, the explanation for gully formation in terms of escaping internal water is strengthened.
If there are currently aquifers with liquid water less than 100 m below the surface, this would have profound significance for the habitability of Mars. A deep subsurface biosphere exists on Earth, and recent investigations have found microbes populating ancient water more than 2 km below the Earth's surface. Now we have evidence of a possibly similar environment on Mars.
There are, of course, many questions about the detailed similarity between the subsurface environments on Earth and Mars. Oxygen gas in Earth's atmosphere and oceans, produced by photosynthesis, is utilized by almost all life today for its metabolism, directly or indirectly. Before there was free oxygen on Earth, however, early life forms must have managed without this gas. We don't know if any of these ancient life forms still exist. However, astrobiologists suggest that the life being discovered today in Earth's deep subsurface aquifers may also live on metabolic reactions that are independent of the oxygen in the atmosphere and oceans. If this turns out to be true, then the Earth's subsurface biosphere could be a useful analog for possible life forms on Mars.
In the past, scientists have thought that any martian biosphere that persists today is probably several kilometers below the surface, and hence inaccessible to our current space technology. The new results from Mars may overturn that pessimistic supposition. If there is water near the surface, and especially if that water is sometimes released on the surface, then a martian subsurface biosphere becomes much more accessible. Wouldn't it be exciting to visit one of these recent flows to search for evidence of frozen or freeze-dried microbes that were living just a few years ago!