Using Mars orbiter data, field observations and laboratory experiments, a team of researchers, including Peter Englert, professor in the University of Hawai‘i (UH) at Mānoa School of Ocean and Earth Science and Technology (SOEST), developed a new theory about what is causing landslides on the surface of Mars. Their research was published today in Science Advances.
Previous ideas suggested that liquid debris flows or dry granular flows caused this movement. However, neither model can completely account for the seasonal martian flow features known as Recurring Slope Lineae (RSL).
The team, led by Janice Bishop, SETI Institute Senior Research Scientist and member of the NASA Astrobiology Institute team, alternatively hypothesizes that small-scale ice melting underground causes changes that make the surface vulnerable to dust storms and wind. As a result, the features appear or expand on the surface of Mars. Further, the team believes that the thin layers of melting ice result from interactions between underground water ice, chlorine salts, and sulfates, which create an unstable, liquid-like flowing slush instigating sinkholes, ground collapse, surface flows, and upheave.
Mars analog field investigations on Earth, such as in the Dry Valleys of Antarctica, the Dead Sea in Israel, and Salar de Pajonales in the Atacama Desert, show that when salts including gypsum interact with water underground, it causes disruptions on the surface, including collapse and landslides.
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, gradual melting of the ice from -40 to -20 °C and thin layers of liquid-like water forming along grain surfaces.
Modeling the behavior of chlorine salts and sulfates, including gypsum, under low temperatures demonstrated how interrelated these salts are. It may be that this microscale liquid water migrates underground on Mars, transferring water molecules between the sulfates and chlorides, almost like passing a soccer ball down the field.
“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,” said Bishop.
This project arose out of work on sediments from the McMurdo Dry Valleys in Antarctica, one of Earth’s coldest and driest regions. As on Mars, the Dry Valleys’ surface is scoured by dry winds most of the year. However, subsurface permafrost contains water ice, and chemical alteration appears to be occurring below the surface.
Englert, who is based at the Hawai‘i Institute of Geophysics and Planetology in SOEST, managed efforts to analyze the chemistry of Antarctic sediments from several soil pits and cores, enhancing understanding of the salt enrichment in the near-surface layers.
“Because Dry Valley sediments are analogous to Mars sediments, our experiments can provide clues about the processes that may be occurring on Mars,” said Englert. “Elevated concentrations of chlorine salts and sulfates were found just below the surface in multiple locations we studied in Antarctica’s Wright Valley . The ubiquitous subsurface presence of these salts in Antarctica suggest their presence on Mars and their potential role in triggering landslide processes.”
Water ice has been detected below the surface on Mars within soil scooped up at the Phoenix landing site, as well as from orbit using radar measurements, and using neutron and gamma ray spectroscopy. More recently, HiRISE on the Mars orbiter has captured views of this near-surface ice at mid-latitudes.
In addition to helping explain Mars’ geological and chemical processes, the new theory also suggests that the martian environment continues to be dynamic — that the planet is still evolving and active — which has implications for both astrobiology and future human exploration of the Red Planet. The potential for thin films of water below the surface on Mars in salty permafrost regions opens new doors for exploring its chemistry.
“I am excited about the prospect of microscale liquid water on Mars in near-surface environments where ice and salts are mixed with the soil,” said Bishop. “This could revolutionize our perspective on active chemistry just below the surface on Mars today.”