What’s carving active gullies on Mars? It’s not water

Scientists were shocked when they received the first images of Martian gullies. They were even more shocked as those gullies appeared to change over time. Their similarities to gullies seen on Earth were uncanny, but all of Earth’s gullies are formed by water runoff, and Mars is too cold and has too sparse an atmosphere to have liquid water on its surface. Whatever has been causing those changing gullies couldn’t have been water, so what was it? A new paper from Apolline Leclef of the Institut d’Astrophysique Spatiale at Université Paris-Saclay and her colleagues, available on the arXiv preprint server, shows how they are likely caused by CO₂ frost turning into a fluid.

To prove this idea, the authors turned to data from two of the longest-standing observers of Mars’ environment—Mars Express and the Mars Reconnaissance Orbiter (MRO). They looked at data collected by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on MRO and the OMEGA spectrometer on Mars Express, but focused on a very specific location. Known as Sisyphi Cavi, this system of deep polar pits is situated near the polar ice cap at Mars’ south pole. Crucially, despite being known to have active gullies itself, it is well outside the more temperate latitudes where gullies on Mars usually form.

Their first task was to completely disprove the theory that the gullies were driven by water. Water itself has a very specific spectral absorption band at 1.5 μm. That band was completely absent during the spring, when the CO₂ ice that blankets the planet’s surface during the winter vanishes. There were some detections of salts and clays, which typically require water to form, but only on the high plateaus and crater rims of the area, not anywhere near where the gullies are located. So, clearly, liquid water was not the cause.

Which turned the researchers’ attention to the next potential culprit—geysers. On Earth, we think of geysers as jets of hot water spewing up from inside the Earth’s surface. But on Mars, they have a different mechanism, known as the Kieffer geyser mechanism. In this mechanism, the wintry parts of Mars are blanketed in a sheet of transparent dry ice. During the spring months, increased sunlight starts to heat the dark soil at the bottom of those clear sheets of ice, sublimating some of the CO₂ underneath.

Eventually, after the pressure builds up enough, the ice ruptures and high-velocity jets of CO₂ gas erupt outward, bringing dark plumes of sand and soil with them. This results in the “dark spots” cameras on Mars Express and MRO have seen from orbit.

Geysers are certainly a plausible mechanism for creating gullies. However, according to the paper, the timing is off. The dark spots that are indicative of geyser action appear in late winter and peak around the spring equinox. However, gully activity doesn’t even begin until mid- to late spring, after the geysers have already become largely dormant. So it seems geysers aren’t the answer to where the gullies form either.

Since the two main theories don’t match the data, the authors point out a third one, which we’ll call the Air Hockey Effect. If you’ve ever played air hockey, you know a slab of solid material (like an ice sheet) can essentially float, and therefore move easily, when supported by a cushion of air from underneath. The authors think a sublimation-driven process on Mars, known as CO₂ frost fluidization, does the same thing on a larger scale to the CO₂ ice sheets on Mars, causing the gullies.

In this case, instead of geysering out through small cracks in the slab of ice, the gas itself acts as a lubricant, causing a stationary pile of dry ice and dirt to slide down a slope, becoming a fast-moving flow capable of carving alluvial-style aprons and deep trenches into the Martian landscape. Ultimately, the end result would look exactly like the gullies seen on the Red Planet today.

If true, this would prove that Earth-like features don’t always require Earth-like physics. The dynamic erosion posited at Sisyphi Cavi happens without a single drop of liquid water, and instead relies on the freeze-thaw cycle of the Martian atmosphere. While not great news for astrobiologists, planetary scientists and geologists can surely appreciate how the facts fit the evidence in this case, even if the headline isn’t as fantastic as “Liquid water found on Mars!”

Who’s behind this story?

Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

Full profile →

Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

Full profile →