The possible seasonal rills of running water on Mars……Credit: NASA, JPL
Earth has lots of liquid water, like oceans of it – though salty. Why would people be excited by briny water on Mars? However, for those intrepid, dreaming human beings who think of traveling to Mars and one day pitching camp there, the news from the NASA Mars Reconnaissance Orbiter (MRO) and reported in the journal Science [04 August 2011, paywalled, Seasonal flows on warm Martian slopes] is almost surprising: There may be, at times, running water on Mars.
Far and away most of the Martian planetary surface is too cold for water, liquid or frozen. With a mean temperature of -63 degrees Celsius, surface water or ice quickly sublimates in the cold dry atmosphere. It has long been thought, and then shown by satellite instruments, that water exists on Mars but only as subsurface ice. Now it looks like there may be exceptions.
Mars has seasons, much like Earth. It can get surprisingly warm at the height of summer in the middle latitudes, around 20 degrees Celsius (68F). It is there, in an area of the Southern hemisphere, where the MRO has taken a seasonal series of pictures of a slope with dark streaks, from .5 to 5 meters wide and hundreds of meters long, that appear with the Martian spring, grow bigger with the summer, and disappear with the coming of winter. There are several possible explanations (called hypotheses for scientists), for example the change could be caused by melting carbon dioxide; but the temperatures are too high to sustain CO2. At this point the one hypothesis that makes the most sense is liquid, salty water. Salty because the temperatures in the area during the ‘warm’ seasons are still too cold for pure water (it would freeze). However, the slopes where this occurs are struck directly by the summer sun and while the water may not be gushing, it is probably at or near the surface.
Probably. As with most science at this distance, the images and explanations are the best shot with the given evidence. That doesn’t mean it might not be something altogether different – a chemical reaction or the fluid motion of some other geologic material. In fact, there is already an interesting problem with the water hypothesis. The area of the flows was surveyed for the presence of water by the MRO’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) – none was found. This might mean the water is (mostly) subsurface. Then there is the dark color of the flows, which changes with the seasons. There are many possible explanations ranging from chemical reaction with water to the angle of light reflection. Such inconsistencies are the bread and butter of experimental science. The problem now goes to the laboratory where scientists will try to recreate the circumstances of the Martian flows. Someday, commitment to scientific budgets allowing, there may be direct sampling of the Martian soil and the mysteries will be resolved.
Meanwhile, there is provisional evidence of liquid water on Mars. This is not too surprising, really, given that by some estimates there are oceans of frozen water at or near the Martian polar caps. What may be causing the stir is that water in any form promises the possibility of life. This is especially true for liquid water, whether at the surface or deep underground in aquifers. The possibility of sustaining liquid water also increases the potential of Mars for human habitation – yes, the fabled Mars colonies.
Martian colonies are still very much science fiction. For now, science is doing its thing by following the water: