Mars Has Bizarre Dunes Thanks to its Low Atmospheric Pressure and Strange Winds

In a recent study published in Nature Communications, an international team of researchers led by Stanford University used artificial intelligence (AI) to examine the formation of sand ripples and sand dunes of two distinct sizes on Mars. These formations might help scientists better understand Mars’ atmospheric history through examining the fossilized forms of these aeolian (windblown) structures using statistical analyses.

Windblown sand is common on both Earth and Mars, with the distinct difference being Mars has far less atmospheric pressure than Earth, on the order of 6.518 millibars (0.095 psi) compared to Earth’s 1013.5 millibars (14.7 psi), which is 0.6% of Earth’s atmospheric pressure. Two commonly observed formations of windblown sand are small crests known as “impact ripples” that result from sand grains impacting sand mounds, and the second form are much larger sand dunes that can span for several kilometers (miles).

The reason why Mars’ atmospheric history could be further examined from this study could be due to both an exact and consistent mathematical relationship between the lack of atmospheric pressure on Mars and the size of the windblown sand dunes and sand ripples on the Red Planet, which have been observed to occur at all sizes except for the smallest dimensions.

“This is particularly important because it is thought that Mars used to have a thicker atmosphere in the past, perhaps sustaining Earth-like surface conditions,” Dr. Mathieu Lapôtre, who is an assistant professor of geological sciences in the Stanford Doerr School of Sustainability and a co-author on the study, said in a statement. “However, it lost most of it, and we don’t really know when, how fast, and why.”

This study came about after scientists were puzzled over images from NASA’s Curiosity Mars rover in 2015 that observed similar windblown patterns on Mars’ surface. These include giant sand dunes along with smaller formations like the impact ripples seen on Earth but also formations about 10 times as big as these ripples, but smaller in size compared to sand dunes. Essentially, Curiosity observed a type of middle-sized sand formation never seen.  

NASA’s Curiosity Mars rover composite “selfie” of 53 combined images taken on January 19, 2016 from “Namib Dune”. (Credit: NASA/JPL-Caltech/Malin Space Science Systems)

One proposed hypothesis for these middle-sized sand formations could be from the ongoing growth of impact ripples due to the low Martian atmospheric pressure. Dr. Lapôtre and other scientists have previously suggested that these formations could result from what’s known as hydrodynamic (fluid motion) instability, which can be used for both liquid and air movements.

For the study, the researchers used AI and more than 130,000 high-resolution images of Mars to perform a quantitative analysis on one million barchan dunes, also known as crescentic dunes, on Mars to examine how their sizes and shapes vary across the Martian surface. Barchan dunes are common on both Earth and Mars and have been imaged extensively on the Red Planet by the HiRISE camera onboard NASA’s Mars Reconnaissance Orbiter.

A color-enhanced image of barchan dunes just west of Mawrth Vallis on Mars taken on December 30, 2013 by the HiRISE camera onboard the Mars Reconnaissance Orbiter. (Credit: NASA/JPL-Caltech/University of Arizona)

Their findings indicate that these middle-sized sand formations are not impact ripples, but instead are like miniature sand dunes whose growth ceases at a certain point due to the predicted change in the fluid-like airflow in the low atmospheric pressure close to the Martian surface.

“Impact ripples form on Mars exactly like they do on Earth, and have more or less the same size,” Dr. Lior Rubanenko, who is the lead author of the study while conducting the research as a postdoctoral scholar in geological sciences at Stanford, said in a statement. “This makes sense, since the mechanism that forms impact ripples has less to do with the properties of the atmosphere and more with the mechanics of sand transport.”

“Now that we know how the size of these ripples varies with atmospheric density and why, we can use the size of fossilized ripples in very old rocks to reconstruct the history of Mars’ atmosphere,” Dr. Lapôtre said.

As always, keep doing science & keep looking up!