A new experiment suggests that the dark streaks on Mars' slopes may have been carved by boiling water, a finding that suggests the planet might still have liquid water. However, it also suggests that it might have less of it than previously believed.

Despite its current dry landscape, previous studies suggest that liquid water once ran on Mars' surface. The dark lines of the planet's slopes even suggest that water ran down them each spring. However, the current atmospheric pressure of the Red Planet is now too low for the existence of water, as it would simply boil it.

"A good example is that of Mount Everest - the atmospheric pressure at the top of Everest is 400 millibars, as opposed to around 1,000 millibars at sea level, and therefore water boils at 72 degrees Celsius [161 degrees Fahrenheit] rather than 100 degrees C [212 degrees F], meaning mountaineers cannot make a decent cup of tea," said Susan Conway, a planetary geomorphologist at the University of Nantes and co-author of the study. "On the Martian surface, the pressure is 5 to 10 millibars, meaning that liquid water boils no matter what the temperature is."

Although previous research suggested that the dark streaks on Mars' slopes were from dust avalanches or the venting of carbon dioxide gas, new experiments under simulated Martian conditions suggest that the unique features might stem from boiling water.

The team came to its conclusions by placing a block of ice on top of a slope composed of loose, fine-grained sand and watching the behavior of the resulting meltwater that made its way downhill.

"No one had performed such experiments before, so it was both exciting and intimidating to be able to break new ground," Conway said. 

"It takes a lot of testing to find a setup which adequately reproduces Martian conditions, yet can be observed and measured easily," she added. "For example, we had the problem where the ice block we used for our source of water kept falling down the slope as it melted, destroying the features created by the seeping water. So we had to design an ice-cube safety belt, which stopped it tumbling."

The results revealed that under Earth-like conditions, the flowing water had little effect on the slope, whereas under Mars-like conditions with lower air pressures, the water boiled and threw sand grains into the air, resulting in slopes with small channels similar to those observed on Mars. Furthermore, the amount of water needed to create these slopes was less than previously assumed.

"We already knew that water should boil under Martian conditions, but what we did not predict was that the boiling would be so intense where the water encountered the dry sediment," Conway said. "Our results show that a very small amount of water can actually move a disproportionately large amount of sediment, more than is possible under terrestrial conditions for the same amount of water."

The findings were published in the May 2 issue of Nature Geoscience.