In the 2000s, planetary researchers spotted ghostly, blue-white morning frost on the Martian surface in visible light images from NASA’s Mars Odyssey spacecraft. But using Mars Odyssey’s Thermal Emission Imaging System (THEMIS), the orbiter’s heat-sensitive camera, the frost appeared more widely, including in areas where none was visible. The scientists knew they were looking at frost that forms overnight and is made mostly of carbon dioxide. But why was this dry ice frost visible in some places and not others? Sorbonne Université’s Institut Pierre-Simon Laplace researcher Lucas Lange and his colleagues propose a surprising answer that may also explain how dust avalanches are triggered after sunrise on Mars.
These dark slope streaks resulted from dust avalanches in Acheron Fossae, Mars. The HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter captured them on December 3, 2006. Image credit: NASA / JPL-Caltech / University of Arizona.
“Odyssey’s morning orbit produces spectacular pictures,” said Dr. Sylvain Piqueux, a researcher at NASA’s Jet Propulsion Laboratory.
“We can see the long shadows of sunrise as they stretch across the surface.”
Because Mars has so little atmosphere, the Sun quickly warms frost that builds up overnight. Instead of melting, dry ice vaporizes into the atmosphere within minutes.
The study authors first noticed the cold-temperature signature of frost in many places where it couldn’t be seen on the Martian surface.
These temperatures were appearing just tens of microns underground — less than the width of a human hair below the surface.
“Our first thought was ice could be buried there. Dry ice is plentiful near Mars’ poles, but we were looking closer to the equator of the planet, where it’s generally too warm for dry ice frost to form,” Lange said.
In their paper, the researchers propose they were seeing ‘dirty frost’ — dry ice frost mixed with fine grains of dust that obscured it in visible light but not in infrared images.
The phenomenon led the them to suspect dirty frost might also explain some of the dark streaks that can stretch 1,000 m (3,300 feet) or more down Martian slopes.
They knew the streaks resulted from, essentially, dust avalanches that slowly reshape mountainsides across the planet.
They think these dust avalanches probably look something like a ground-hugging river of dust releasing a trail of fluffy material behind.
As the dust travels downhill over several hours, it exposes streaks of darker material underneath.
These dark streaks are not the same as a better-documented variety called recurring slope lineae, which recur in the same places, season after season, for weeks (instead of hours) at a time.
Once thought to result from briny water slowly seeping from mountainsides, recurring slope lineae are now generally believed to result from flows of dry sand or dust.
Mapping the slopes streaks for their recent study, the authors found they tend to appear in places with morning frost.
The team proposes the streaks resulted from the vaporizing frost creating just enough pressure to loosen the dust grains, causing an avalanche.
“Every time we send a mission to Mars, we discover exotic new processes,” said co-author Dr. Chris Edwards, a researcher at Northern Arizona University.
“We don’t have anything exactly like a slope streak on Earth. You have to think beyond your experiences on Earth to understand Mars.”
The team’s paper appears in the Journal of Geophysical Research: Planets.
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L. Lange et al. 2022. Gardening of the Martian Regolith by Diurnal CO2 Frost and the Formation of Slope Streaks. Journal of Geophysical Research: Planets 127 (4): e2021JE006988; doi: 10.1029/2021JE006988
