Martian Dry Ice Deposits Make Dust Flow Like Water

NASA’s eyes in the Martian sky see all kinds of amazing things, including subtle surface alterations linked to seasonal changes. Most recently, the Mars Reconnaissance Orbiter (MRO) captured more evidence of gullies on Mars. These cuts in the planet's surface are intriguing because gullies on Earth need water to form, but, on the red planet, gullies are caused by dry ice.

When the first evidence of gullies on Mars came in 2000, the discovery was met with great excitement. On Earth, a gully is formed by water running down a hillside, eroding soil as it trickles down. Seeing similar formations on Mars was thought to be strong evidence that liquid water existed on the red planet. It wasn’t out of the question; Mars does play host to water vapor and frozen water, even though we’ve yet to find liquid water on the planet.

“I thought the gullies on Mars indicated activity of liquid water," said Colin Dundas of the US Geological Survey's Astrogeology Science Center in Flagstaff, Arizona. "We were able to get many more observations, and as we started to see more activity and pin down the timing of gully formation and change, we saw that the activity occurs in winter."

Since the initial discovery of the gullies, Dundas and his collaborators have been using more sophisticated instruments to look at these surface formations, gaining a better understanding of what they are. A particularly useful instrument is the High Resolution Imaging Science Experiment camera on MRO. HiRISE combines high resolutions and signal-to-noise ratios with a large swath of viewing width to see surface features as small as three and a half feet. With this detailed camera, scientists can investigate deposits and landforms left in the wake of geologic and climatic processes, landforms like gullies.

Using HiRISE, scientists have examined gullies at 356 different locations on Mars since 2006. 38 of these sites revealed new channels and larger deposits at the end of existing gullies, signs that point to recent activity. By comparing time-stamped images, scientists have been able to match changes in gullies to seasonal carbon dioxide frost and temperatures—meaning the gullies on Mars are formed by frozen carbon dioxide and not liquid water.

Frozen carbon dioxide, which is more commonly known as dry ice, isn’t something we find naturally on Earth, but it’s common on Mars. And though it’s not a liquid, it can affect the Martian surface like a liquid. Dry ice sublimates; it moves directly from a solid to a gaseous state. As it sublimates, it can act as a lubricant for dry material, helping it flow along the surface. If that material is on a hill, it will flow down forming something that looks a lot like an Earth gully. It’s possible, too, that the seasonal buildup of dry ice frost could become heavy enough to start sliding down a slope.

In either case, the latest pictures from MRO add support the idea that the freshest gullies on Mars are due to naturally occurring dry ice cycles on the planet and not, as researchers previously believed, remnants of Mars’ wet past. "Much of the information we have about gully formation, and other active processes, comes from the longevity of MRO and other orbiters," said Serina Diniega of NASA's Jet Propulsion Laboratory. "This allows us to make repeated observations of sites to examine surface changes over time.”

So Mars might be cold and devoid of life (as far was we know, at least), but it's still far from a boring place, liquid water or not.