NASA: streaks of salt on Mars mean flowing water, and raise new hopes of finding life

David Rothery, The Open University

Salty streaks have been discovered on Mars, which could be a sign that salt water seeps to the surface in the summers. Scientists have previously observed dark streaks (see image above) on the planet’s slopes which are thought to have resulted from seeps of water wetting surface dust. Evidence of salts left behind in these streaks as the water dried up are the best evidence for this yet. The discovery is important – not least as it raises the tantalising prospect of a viable habitat for microbial life on Mars.

I have lost track of how many times water has been “discovered” on Mars. In this case, the researchers have detected hydrated salts rather than salty water itself. But the results, published in Nature Geoscience, are an important step to finding actual, liquid water. So how close are we? Let’s take a look at what we know so far and where the new findings fit in.

Ice versus liquid water

Back in the 18th century, William Herschel suggested that Mars’s polar caps, which even a small telescope can detect, were made of ice or snow – but he had no proof. It wasn’t until the 1950s that data from telescopes fitted with spectrometers, which analyse reflected sunlight, was interpreted as showing frozen water (water-ice). However, the first spacecraft to Mars found this difficult to confirm, as water-ice is in most places usually covered by ice made up of carbon dioxide.

Part of Nirgal Vallis, a valley on Mars first seen on this image by Mariner 9 in 1972. This image is 120km from side to side. NASA

In the 1970s attention turned to the much juicier topic of liquid water on Mars, with the discovery by Mariner 9 of ancient river channels that must have been carved by flowing water. These channel systems were evidently very ancient (billions of years old), so although they showed an abundance of liquid water in the past they had no bearing on the occurrence of water at the present time.

‘Canals’ on Mars drawn by Percival Lowell in 1896. Percival Lowell/wikipedia