In the big news of the day, a team led by SETI Institute scientist Janice Bishop has advanced a theory for why the recurring landslides on the surface of Mars happen. The new paper, published in Science Advances, is kind of a big deal among Mars scientists because this phenomenon, called Recurring Slope Lineae, has been a mystery for years.
Recurring Slope Lineae, or RSL, are seasonal landslides that appear to be some kind of flow feature, and previous theories thought they could be liquid debris flows or even dry landslides. But neither theory really explained how these RSLs work. Imaging data taken from the HiRise camera on the Mars Reconnaissance Orbiter has shown the expansion of RSLs over time, and studies have focused on Mars analog sites here on Earth. Space is hard, and we haven’t managed to send people to Mars, yet, obviously, to do the science in situ, so Earth is the next best thing.
The locations studied on Earth include dry valleys in Antarctica, the Dead Sea, and the Salar de Pajonales in Chile’s Atacama Desert. These locations have a similar combination of salts and water to what we have measured on Mars and make for good places to sample and observe the interactions between those components. What this research team found was that when salts interact with water underground, the surface above becomes unstable and prone to collapse.
In fact, in Chile, there are gypsum beds with huge voids underneath where the ground has been swept away.
Now let’s go into the lab. Per the press release: To test their theory, the team conducted lab experiments to observe what would occur if they froze and thawed Mars analog samples comprised of chlorine salts and sulfates at low temperatures such as would be found on Mars. The result was slushy ice formation near -50 °C, followed by gradual melting of the ice from -40 to -20°C.
This slush is thought to migrate underground on Mars, moving the water along between the various salts and sulfates. Bishop noted: I was thrilled to observe such rapid reactions of water with sulfate and chlorine salts in our lab experiments and the resulting collapse and upheave of Mars analog soil on a small scale, replicating geologic collapse and upheave features in karst systems, salt reservoirs, and edifice collapse on a large scale.
Here is the TL;DR version: We know there is water under the surface of Mars. We’ve talked about several recent discoveries locating subsurface reservoirs in the past few months even. And now it seems this subsurface water is responsible for mixing with salts in the soil and causing slope instability during warmer months. Bring in even that light Martian wind or dust storms, and you have enough force to cause the surface soils to slide.
Pretty neat stuff.
SETI Institute press release
“Martian subsurface cryosalt expansion and collapse as trigger for landslides,” J.L. Bishop et al., 2021 February 3, Science Advances