© NASA/JPL-Caltech/Univ.of Arizona
On Mars, there are seasonal landslides on steep slopes, especially in the equatorial regions and mid-latitudes. US researchers and the Viennese expert on planetary geology, Christian Köberl, propose a new hypothesis about the cause of these mass movements in the journal “Science Advances”. As observations in the dry valleys of the Antarctic and laboratory experiments show, salts in combination with water could cause the landslides.
For a long time, scientists have puzzled over the flow structures that regularly occur on some steep slopes on Mars in summer and are called “Recurring Slope Lineae” (RSL; roughly: “Recurring slope lines”). So far, two different educational processes have been suggested as an explanation for this – a “wet” and a “dry” one.
Dry material flows
Either meltwater from the permafrost could be responsible for this, “but many RSL occur in areas close to the equator where there is little or no permafrost,” said Köberl, Professor of Planetary Geology and Impact Research at the University of Vienna to the APA. In addition, landslides are only observed on slopes of more than 27 degrees, which would not be logical if the cause was water.
The other explanation is dry, granular material flows, similar to sand dunes – but this would again contradict the seasonal occurrence of the RSL. “The landslides observed cannot be fully explained with either of the two mechanisms,” says the expert.
Hybrid model as a solution
Janice Bishop from the Carl Sagan Center at the SETI Institute in California (USA) and Köberl and colleagues are now proposing a hybrid model that includes both wet and dry components of salty Martian soils. According to this, salts, especially sulphates and chlorides, would absorb existing water, expand and dissolve in the process and thus cause slipping.
“You don’t need too much water here, thin films of water under the surface are enough to make the grain boundaries slippery – a similar effect that you get when spreading salt in winter,” says Köberl. Due to the salts, this effect can also occur at temperatures of around minus 30 degrees Celsius, as is common in the Mars summer.
The geochemist and his US colleagues have observed similar processes in the Antarctic dry valleys, where they have been chemically investigating the composition of the sediments for years. These ice-free valleys are among the coldest and driest regions on earth, the surface of which is exposed to dry winds almost all year round. The conditions are very similar to those on Mars – “and it looks the same there,” said Köberl, remembering his stay in Antarctica.
Despite the drought, the permafrost soil in the McMurdo Dry Valleys in East Antarctica contains water ice, which in conjunction with high concentrations of sulfates and chlorides can form thin water films. The result is chemical weathering just below the surface and landslides. “We discovered correspondingly high salt concentrations in the Antarctic dry valleys,” said Köberl.
The researchers also based their hypothesis on laboratory experiments in which they showed that the salts formed thin, moving films of slushy water. These can expand and contract over long periods of time on Mars, weakening the fragile surfaces and ultimately causing landslides. Similar processes in which salts interact with gypsum and water in the subsurface and cause disturbances on the surface, including collapse phenomena and landslides, are also known from the Dead Sea in Israel and the Salar de Pajonales in the Atacama Desert in Chile.
For Köberl, this work is an important example that geological investigations on earth can be applied to other earth-like planets, and vice versa. “Geological processes take place in a similar way on the most varied of bodies in our solar system,” says the scientist.