Hopes for liquid water on Mars are drying up, as researchers find fault with reports of damp sand

The expectation of scientists that Mars may still have liquid surface water appears to be evaporating.

Last autumn, Nasa scientists announced the strongest suggestion yet that the Red Planet may occasionally host patches of liquid water. This was indicated by the presence of “recurring slope lineae” (RSL) that appear seasonally, resembling streaks of damp sand, and contain perchlorate salts that usually get left behind when water evaporates.

The observations, made using Nasa’s Mars Reconnaissance Orbiter (MRO), didn’t offer direct evidence of water. But they confirmed scientists’ theories about how and where water might form on the planet’s surface, and offered hope that these wet patches could contain life - or at least show that something could have survived there once.

But a new study in the journal Geophysical Research Letters suggests that we shouldn’t pin our hopes of a Martian oasis on the RSLs just yet.

This year, planetary scientists Christopher Edwards and Sylvain Piqueux took a closer look at the feature using a thermal imaging instrument on board Mars Odyssey, another orbiter. They found no temperature differences between the dark RSL streaks and surrounding terrain - which suggests that the streaks aren’t really patches of wet sand streaming down a slope.

At best, they say, the RSLs could contain no more than 3 per cent liquid water - making them more like mildly damp, slightly salty dirt. And that’s an optimistic interpretation, Edwards said; it’s possible the RSLs contain no water at all.