Mysterious dark lines that appear on the slopes of Martian dunes and craters each warm season may be dry landslides instead of flowing water as previously thought.
Dark flows streak down hills and craters on Mars each summer, before fading away in the colder months. Initially, these lines – called recurring slope lineae – were thought to be evidence of liquid water flowing on the planet’s surface. But now, a team led by Colin Dundas of the US Geological Survey has found evidence for a simpler explanation: tumbling sand.
Dundas and his team looked for patterns in years of satellite images showing Mars’s recurring slope lineae extending and retreating. They didn’t use new data, but instead looked at existing data in a fresh way, says Anna Grau Galofre at the University of British Columbia in Vancouver.
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Analysing 151 lineae, they found that the flows behave more like dry sand than liquid water, and that the patterns in the lineae look similar to those made when dry sand tumbles down sand dunes on Earth, just on a larger scale.
“It’s actually really hard to explain how you would get liquid water,” says Galofre. Mars’s harsh surface conditions mean that while the planet has plenty of ice and permafrost, it is less hospitable to water.
Dry flows
It is also difficult to explain how Martian geology could return water to the top of hills, dunes and craters each season ready to form a new batch of lineae.
Having water trigger these flows also doesn’t fit other observations, Galofre says, because the striped patterns left behind don’t have the high salt concentrations we would expect from repeated deposition and evaporation.
Instead of trying to think up complex processes that might drive water to the top of slopes and wash away salt, Dundas and his team considered the strange nature of dry, granular flows that behave like fluids. They found that the lineae’s shapes and the distances they cover are consistent with sand flows observed on Earth.
Their theory also explains why lineae always occur at the top of slopes, particularly the top edge of dunes. The seasonal patterns could be tied to how spring and summer storms carry dust and sand.
But this may not be the last we hear about these lineae. “It’s possible that we find another observation somewhere else on Mars that puzzles us and we need to reconsider all the hypotheses and assumptions,” says Galofre. “That has actually happened on Mars a number of times.”
Nature
Read more: Boiling water on Mars could make the planet’s sand levitate
