Solving the mystery of the disappearance of frost on Mars

(ORDO NEWS) — A new study using data from NASA’s Mars Odyssey orbiter may explain why Martian frost may be invisible to the naked eye and why dust avalanches appear on some slopes.

Last year, scientists were puzzled while studying images of the Martian surface taken at dawn by NASA’s Mars Odyssey orbiter.

When they looked at the surface with visible light – the kind that the human eye perceives – they saw a ghostly, bluish-white morning frost illuminated by the rising sun. But with the help of the orbiter’s heat-sensitive camera, the frost was visible more widely, including in places where it was not visible.

Scientists knew they were looking at frost, which forms overnight and is mostly carbon dioxide – essentially dry ice, which often appears on the Red Planet as frost rather than water ice. But why was this dry ice visible in some places and not in others?

In a paper published last month in the Journal of Geophysical Research: Planets, these scientists proposed a surprising answer that could also explain how dust avalanches that reshape a planet occur after sunrise.

From frost to steam

Launched in 2001, the Odyssey is NASA’s longest-running mission to Mars. On board is the Thermal Emission Imaging System (THEMIS), an infrared, or heat-sensitive camera that provides a one-of-a-kind view of the Martian surface. Odyssey’s current orbit allows a glimpse of the planet at 7 a.m. local Martian time.

Odyssey’s morning orbit makes for spectacular images,” said Sylvain Picot of NASA’s Jet Propulsion Laboratory in Southern California, who led the mission. “We can see the long shadows of the sunrise dragging across the surface.”

Because Mars has so little atmosphere (only 1% of Earth’s density), the sun quickly heats up the frost that forms overnight. Instead of melting, dry ice evaporates into the atmosphere within minutes.

Lucas Lange, a JPL intern working with Pico, first noticed the cold temperature signature of frost in many places where it was not visible on the surface. These temperatures appeared only tens of microns underground – less than the width of a human hair “under” the surface.

“Our first thought was that there might be ice buried there,” Lange said. “Dry ice is abundant near the poles of Mars, but we were looking closer to the planet’s equator, which is usually too warm for dry ice frost to form.”

In their article, the authors suggest that they observed “dirty hoarfrost” – dry icy hoarfrost mixed with fine grains of dust that darkened it in visible light but not in infrared.

Melting frost and avalanches

This phenomenon has led scientists to believe that dirty frost may also explain some of the dark bands that can stretch 1,000 meters or more down the Martian slopes. They knew that these streaks were essentially the result of dust avalanches that were slowly reshaping mountain slopes all over the planet.

Scientists believe that these dust avalanches are likely like a river of dust that ripples across the land, leaving behind a plume of fluffy material. As the dust travels downhill for several hours, streaks of darker material are exposed underneath.

These dark streaks are unlike the more documented variety called recurring slope lines, which repeat in the same places season after season, for weeks (not hours).

It was once thought to be the result of the slow seepage of salt water from the slopes of the mountains, but it is now generally accepted that the repeated lines of slopes are the result of flows of dry sand or dust.

Mapping slope bands for their recent study, the authors found that they tend to appear in areas where there is morning frost. The researchers speculate that the streaks result from evaporating frost building up enough pressure to loosen the dust grains, triggering an avalanche.

These hypotheses are further proof of how amazing the Red Planet can be.

“Every time we send a mission to Mars, we discover new exotic processes,” said Chris Edwards, co-author of the paper at Northern Arizona University in Flagstaff. “We don’t have anything like a slant on Earth. To understand Mars, you need to think more than on Earth.”

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