Carbon dioxide frost, not water, carves moving Martian gullies

A new study examining the surface of Mars suggests that movement of gullies on the surface of Mars is due to a pattern of freezing and thawing of carbon dioxide, as opposed to the actions of freezing and thawing water.

In an article published by The Geological Society of America, researchers from the University of Arizona and the Hopkins Applied Physics Laboratory detailed the findings from their observations of gullies and dunes on Mars.

Using images primarily captured by High-Resolution Imaging Science Experiment (HiRISE), and additional images from Context Camera (CTX) and Mars Orbital Camera (MOC), the researchers examined the changes in dunes and gullies on Mars over a time period of six Mars years.

Tracking the movements of these gullies, the group looked for indications based on patterns over time about what process might be responsible.

Their examination of the movement of 18 gullies in seven sand fields suggests that carbon dioxide is responsible based on the timing of frost patterns.

“Carbon dioxide frost annually cycles from the polar caps to the atmosphere and makes up the main component of the Martian atmosphere; we know this based on temperature and spectra measurements from both Earth-based and Mars-based observations,” Serina Diniega, lead author of this study, wrote in an email with The News-Letter.

Diniega had completed the work for this paper while a graduate student at the University of Arizona and has recently begun a post-doctorate fellowship at the NASA Jet Propulsion Laboratory.

On the surface of Mars, carbon dioxide is known to thaw and sublimate from the surface and polar caps directly into the atmosphere when temperatures are warm enough at certain times.

Subsequently, when temperatures decrease, some of the carbon dioxide in the atmosphere will begin to deposit and freeze back into ice layers on the polar ice caps and other surfaces.

According to Diniega, temperature measurements of the dunes collected by satellite suggest that the cycle of freezing and thawing of carbon dioxide is responsible for the movement of sand on the Mars surface.

“In winter, this frost will also accumulate on other surfaces (besides the polar caps). On these dunes, CRISM (a spectrometer on the Mars Reconnaissance Orbiter) measurements show that the surface temperatures (210K; water ice melts at 273K) and composition over these dunes during winter is consistent with carbon dioxide frost and estimates have been made about how much frost accumulates,” she wrote.

Diniega noted that water is very unlikely to account for the movement on the surface of Mars. “Although some water frost will also probably form and this water frost might melt later in the year (in spring), the very small amount of water available from the Martian atmosphere and the timing of gully activity (in winter) make it implausible that water frost or liquid is involved.”

In contrast to previous studies, Diniega notes that their work is the first to examine dune gullies specifically and to observe them within a constrained time frame.

“Although many gully studies have discussed mechanisms of gully formation and activity (which primarily focused on various water-driven mechanisms), this is the first study to evaluate possible mechanisms based on observations of the timing of activity,” she wrote.

Based on the results of their work, Diniega notes that their next step involves quantifying the changes on the surface caused by carbon dioxide.

“The main work that is needed now are quantitative models that can estimate how much carbon dioxide frost is needed to move material, and how the formation/accumulation or sublimation of frost leads to the large-scale movement of surface materials.”