Mars is a cold dry planet, but its liquid water boils

Scientists have long puzzled over the source of dark streaks that show on the slopes of Mars. New research from France now suggests that those streaks may have been carved by boiling water.

Low atmospheric pressure is the key behind the boiling water. Mars pressure is about 1/100th of the atmospheric pressure on Earth. Under such low pressure water boils very easily, similar to boiling water at high elevations on Earth. For example, at the top of Mt. Everest, water boils at 161 degrees Fahrenheit rather than 212 degrees. According to study co-author Susan Conway, with the Martian atmosphere only 5 to 10 millibars, liquid water boils no matter what the temperature is.

The scientists did lab experiments in which they allowed a block of ice to melt and seep into the underlying sediment, then examined the patterns that emerged as the water ran downhill. When they tested how flowing water reacted on a slope made of loose, fine-grained sand that resembles Martian soil, they found that under low-pressure conditions, such as are found on Mars, the water boiled so vigorously that it tossed grains of sand into the air. The sand piled up until it collapsed and started avalanches. The lab’s test slope developed channels that looked very much like those seen on Mars.

The streaks, known as RSL, or recurring slope lineae, were previously thought to be caused by dust avalanches, or the venting of carbon dioxide gas. Now the researchers feel fairly certain that the source of the streaks is water. Conway says that the study results show that, under conditions such as found on Mars, a fairly small amount of water can move “a disproportionately large amount of sediment,” much more than is possible under conditions found on Earth.

Conway says that the discovery that less water is needed for creating such planetary features as RSL is both good news and bad news for the idea of life on Mars. While the results show that there is the possibility of liquid water on Mars, it also shows that there is likely very little and that it does not last long enough to be a good environment for microorganisms.

The findings from the research, which took place at the University of Nantes in France, and the National Center for Scientific Research, also in France, were published in the journal Nature Geoscience on May 2.