The discovery of flowing water has highlighted how the Red Planet continues to change even to this day, but the latest observations show that water is not the only factor that erodes the surface of Mars.
Using NASA’s Mars Reconnaissance Orbiter (MRO), researchers from the Johns Hopkins University Applied Physics Laboratory (APL) have discovered that Martian gullies are likely not formed by flowing water, but by an erosion mechanism that depends on the thawing of water and carbon dioxide frost.
The gullies are cracks hundreds of meters long on the Martian surface that share an hourglass shape. They have an alcove at the top, a channel in the middle, and an apron of deposited material at the bottom. It is not surprising people thought they could be formed from water flowing.
The gullies definitely look different from the recurring slope lineae (RSL), seasonal darkening and fading that were proven to be caused by flowing brine last year. The RSL are short (several meters at the most) and they happen on the side of craters and canyons, without large debris movement.
This latest discovery about the gullies, published in Geophysical Research Letters, was possible due to two incredible instruments on board the MRO – the High Resolution Imaging Science Experiment (HiRISE) and the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM).
"The HiRISE team and others had shown there was seasonal activity in gullies – primarily in the southern hemisphere – over the past couple of years, and carbon dioxide frost is the main mechanism they suspected of causing it," said lead author Jorge Núñez of APL in a statement. "However, other researchers favored liquid water as the main mechanism."
The team used CRISM to detect the minerals present in the region, and these observations, combined with previously tested models, allowed scientists to have more confidence in the dry erosion mechanisms.
"On Earth and on Mars, we know that the presence of phyllosilicates – clays – or other hydrated minerals indicates formation in liquid water," Núñez added. "In our study, we found no evidence for clays or other hydrated minerals in most of the gullies we studied, and when we did see them, they were erosional debris from ancient rocks, exposed and transported downslope, rather than altered in more recent flowing water. These gullies are carving into the terrain and exposing clays that likely formed billions of years ago when liquid water was more stable on the Martian surface."
The Red Planet continues to surprise us with its “slowly but surely” changing surface. It might not have Io’s volcanos or Earth’s rivers, but Mars remains a fascinating and geologically active world.