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\u2019s gullies are formed by the water run-off, and Martian is too cold and has too sparse an atmosphere to have liquid water on its surface. Whatever has been causing those changing gullies couldn\u2019t have been water, so what was it? A new paper from Apolline Leclef of the Institut d\u2019Astrophysique Spatiale at Universit\u00e9 Paris-Saclay and her colleagues, available in pre-print on arXiv, shows how they are likely caused by CO2 frost turning into a fluid. <\/p>\n
To prove this idea, the authors turned to data from two of the longest-standing observers of Mars\u2019 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 near Mars\u2019 south pole. But crucially, despite being known to have active gullies itself, it is well outside of the more temperate latitudes where gullies on Mars usually form.<\/p>\n
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 um. That band was completely absent during the spring ice melt, where the CO2 ice that blankets the planet\u2019s surface during the winter vanishes. There were some detection of salts and clays, which typically require water to form, but only in the high plateaus and crater rims of the area, not anywhere near where the gullies are located. So pretty clear, liquid water was not the cause.<\/p>\n
Fraser talks about where liquid water could hide on Mars.<\/p>\n
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\u2019s surface. But on Mars, they have a different mechanism, known as the Kieffer geyser mechanism. As described above, the wintery parts of Mars are blanketed in a sheet of transparent dry ice. During the spring months, the increase in sunlight starts to heat up 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 geyser outwards, bringing dark plumes of sand and soil with them. This results in the \u201cdark spots\u201d cameras on Mars Express and MRO have seen from orbit.<\/p>\n
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 later winter and peak around the spring equinox. However, the gully activity doesn’t even begin until mid-to-late spring, after the geysers have already become largely dormant. So it seems geysers aren\u2019t the answer to where the gullies form either.<\/p>\n
Since the two main theories don\u2019t match the data, the authors point out a third one, which we\u2019ll call the Air Hockey Effect. If you\u2019ve 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, which is the cause of the gullies.<\/p>\n
Fraser talks about the lack of liquid water on Mars.<\/p>\n
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 carving deep trenches into the Martian landscape. Ultimately, the end result would look exactly like the gullies seen on the Red Planet today.<\/p>\n
If true, this would prove that Earth-like features don\u2019t 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\u2019t as fantastic as \u201cLiquid water found on Mars!\u201d<\/p>\n
Learn More:<\/p>\n
A. Leclef et al – Properties of Seasonal Ice at Sisyphi Cavi and Implications for Current Modification of Martian Gullies<\/a><\/p>\n UT – Melting Water in Mars’ Past Could Have Created Martian Gullies<\/a><\/p>\n