Madrid. Between the mountainous dunes and small ripples on the desert surface of Mars lie intermediate-sized sand structures unlike anything on Earth.
Scientists at Stanford University used an artificial intelligence (AI) model to analyze a million Martian dunes and figure out how these sand ripples form on our planet on a scale (about a meter between crests) that previously seemed incompatible with the physics of how waves and dunes arise on Earth.
The results, published in Nature Communications, suggest that scholars can use fossilized versions of these structures to reconstruct the atmospheric history of Mars. This is because there is a precise and consistent mathematical relationship between atmospheric density and the size of wind-blown waves and dunes, except on the smallest scales.
“This is particularly important because it is believed that Mars used to have a thicker atmosphere in the past, perhaps maintaining surface conditions similar to those of Earth,” explained the study’s lead author, Mathieu Lapôtre, an assistant professor of geological sciences at the Stanford Doerr School of Sustainability. “However, he lost most of it, and we really don’t know when, how quickly, and why.”
On both Earth and Mars, windblown sand grains pile up in mounds of all different shapes and sizes, ranging from dunes that stretch for miles to small ridges barely high enough to hide a hermit crab.
On Earth, the crests of these smaller waves are typically only a few inches apart. They are common in deserts, on beaches, and on sandstones, preserved like the fingerprints of ancient winds. Scientists call them “shock waves” because they are the result of windblown grains splashing around mounds of sand like little torpedoes.
In 2015, the robot Curiosity, NASA, sent back images of similar patterns on the surface of Mars. In addition to the giant dunes, the photos showed smaller waves on two different scales: some were close to the size of shock waves familiar to similarly sized grains on Earth; others were about 10 times larger, but still smaller than dunes, which are formed more by airflow than sand impacts.
How these two distinct wavy scales came to coexist and coevolve on Mars has puzzled scientists ever since. According to one proposed explanation, the medium-sized structures are the result of continued growth from shock waves, made possible by very low air pressure on Mars. Contrary to the idea of a continuum, however, scientists had observed an unexplained absence of waves with crests 8 to 30 inches apart.
Lapôtre and other scientists have suggested that the shapes could be the result of a hydrodynamic instability already known to produce windblown dunes in deserts and similar undulating mounds in sandy riverbeds on Earth.
Researchers have also speculated that the size of the largest Martian waves and dunes, and those that form underwater on our planet, could be controlled by the same change or anomaly in air or water flow. This modification, which arises only after the mounds grow beyond a certain size, would be the result of the interaction between global atmospheric properties, density, for example, and local factors such as topography and wind shear speed. .
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