Ice ice baby? Perhaps not
Martian images suggest another explanation for ridges on landslides.
Analysis of detailed three-dimensional images of an extensive landslide on Mars has led an international research team to question the theory that underlying layers of slippery ice are the only explanation for the long ridges found on landslides throughout the Solar System.
Writing in the journal Nature Communications, they suggest the unique structures on Martian landslides from mountains several kilometres high could have formed at speeds of up to 360 kilometres per hour due to underlying layers of unstable, fragmented rocks.
"Landslides on Earth, particularly those on top of glaciers, have been studied by scientists as a proxy for those on Mars because they show similarly shaped ridges and furrows, inferring that Martian landslides also depended on an icy substrate,” says first author Giulia Magnarini, from University College London (UCL), UK.
"However, we've shown that ice is not a prerequisite for such geological structures on Mars, which can form on rough, rocky surfaces.”
The team included researchers from UCL, the Natural History Museum in London, Ben Gurion University of Negev, Israel, and University of Wisconsin Madison (UWM) in the US.
Among them was a man who has actually been into space. Harrison Schmitt, now a professor at UWM, was one of the astronauts on Apollo 17 and completed geologic fieldwork while on the Moon in December 1972.
His latest work was closer to home, but significant in its own way. With colleagues he analysed NASA cross-sections of the Martian surface in the Coprates Chasma in the Valles Marineris to investigate the relationship between the height of the ridges and width of the furrows compared to the thickness of the landslide deposit.
The structures were found to display the same ratios as those commonly seen in fluid dynamics experiments using sand, they say, suggesting an unstable and dry rocky base layer is as feasible as an icy one in creating the vast formations.
Where landslide deposits are thickest, ridges form 60 metres high and furrows are up to 400 metres wide. The structures change as deposits thin out towards the edges of the landslide.
"The Martian landslide we studied covers an area larger than Greater London and the structures within it are huge,” says UCL’s Tom Mitchell.
“Earth might harbour comparable structures, but they are harder to see, and our landforms erode much faster than those on Mars due to rain.
"While we aren't ruling out the presence of ice, we know is that ice wasn't needed to form the long run-outs we analysed on Mars.
“The vibrations of rock particles initiate a convection process that caused upper denser and heavier layers of rock to fall and lighter rocks to rise, similar to what happens in your home where warmed less dense air rises above the radiator.
“This mechanism drove the flow of deposits up to 40 kilometres away from the mountain source and at phenomenally high speeds."