And then there is the mystery of Recurring Slope Lineae, known as RSL, that have intrigued scientists for years. These RSL are a form of landslide on Mars, but no one knows what causes them, said Janice Bishop, author of a new study on the phenomena.
“We see them from orbit by the dark streaks they produce on the ground and they tend to always occur on sun-facing slopes, which led geologists to think they were related to melting ice early on,” said Bishop, senior research scientist at the SETI Institute in California.
“The interesting thing is that they increase over months following dust storms and then fade away, and they appear to form repeatedly in the same regions. Also, a large number of these are forming in the equatorial part of Mars, where there is very little ice.”
Any ice in these regions would have to be in tiny frozen particles that exist between grains of soil below the surface.
These puzzling landslides have never been seen up close by a rover or lander, and until they can be investigated by a robotic explorer, scientists are using lab experiments and Martian analogs on Earth to try and understand them.
Some of the strange environments on Earth that are similar to that of Mars include the Atacama desert in Chile, parts of Antarctica and even the Dead Sea. These places show that surface collapse and landslides occur when salt interacts with sulfates or water underground.
“Antarctica and the Atacama are excellent analogs for Mars because they are ultra dry environments,” Bishop said. “Antarctica has the added benefit that it is super cold. Parts of Antarctica including Beacon Valley are actually on par with Mars for temperature and aridity.”
While water may have once been plentiful on the Martian surface billions of years ago, when the planet was warmer and still retained most of its atmosphere, the current surface of Mars is a freezing cold and barren landscape.
What lies beneath
However, Mars missions and imaging by orbiters have revealed that frozen salty water is below the surface — and that water could be driving activity that appears on the Martian surface, like the landslides.
Bishop and her colleagues collected samples from some similar environments on Earth, including Wright Valley in Antarctica, to test how salts and melting ice underground could cause chemical reactions that trigger these seasonal landslides on Mars. The researchers wanted to test if processes observed in places like Antarctica, where salty sediments can have an effect on surface soil, could be happening on Mars.
The scientists modeled the briny water beneath the surface of Mars in a lab by taking the collected soil samples and exposing them to water and chlorine salts and sulfates — all of which could exist beneath the Martian surface. This experiment resulted in the creation of thin, moving films of slushy water.
To model Martian temperatures where ice exists beneath the surface at the planet’s mid-latitudes, the researchers found slushy ice formed near negative 58 degrees Fahrenheit and a slow, gradual melting of the ice between negative 40 degrees Fahrenheit and negative 4 degrees Fahrenheit.
These temperatures may sound cold, but on Mars, they’re actually considered to be slightly warmer temperatures found near the equator — which could support briny water beneath the surface in the Martian spring and summer.
If this subsurface brine on Mars expands and contracts over time on Mars, it could weaken the surface and cause sinkholes, ground collapse and landslides.
Previously, scientists believed that flows of liquid debris or dry grainy material could be causing the landslides, but neither completely matched up with what scientists saw in the RSL.
However, if ice is melting just beneath the surface, that change would also alter the surface itself. The Martian surface is also at the mercy of wind and seasonal dust storms, which could also play a part in this phenomenon.
“During my fieldwork at Salar de Pajonales, a dry salt bed in Northern Chile, I have observed numerous examples of the action of salts on the local geology. It’s gratifying to find that it could play a role in shaping Mars as well,” said study coauthor Nancy Hinman, a professor of geosciences at the University of Montana, in a statement.
“If our hypothesis is correct, then RSL could be indicators for salts on Mars and for near-surface active chemistry,” Bishop said. “Most of us Mars scientists have considered modern Mars as a cold and dry and dormant place, shaped mostly by dust storms. This is certainly true of the surface, but our work shows that the subsurface could be much more chemically active than realized before.”
Bishop noted that this underground process that could result in landslides on the surface would be a slow and limited one.
While this brine would be too salty to support life, the experiments in the study support the idea that this subsurface liquid water can actually move around the salt and mineral grains. If that’s the case, water on Mars 4 billion years ago could have filtered down into the subsurface as a type of permafrost soil. This ice could have thawed and refrozen over time.
“It could be that more of this early water on Mars persisted longer than we realized below the surface,” Bishop said. “If true, this could indicate that the subsurface of Mars was habitable longer than the surface environment. It is difficult to estimate how long, but perhaps liquid water was present around soil grains below the surface until 3 or 2 billion years ago or even more recently.”
“Once we send rovers to Mars that can drill down into the surface, I think we will see signs of salt reactions below the surface — especially if we investigate some of the equatorial regions where RSL are occurring,” Bishop said.