Science1 publisher2 min readPublished
Mars' polar gullies start moving after CO2 geysers pass their peak
Apolline Leclef's team read CRISM and OMEGA spectra at Sisyphi Cavi, a polar pit system with active gullies, and found no water band in the spring. They argue the gullies are cut by dry ice sliding on its own gas.
The Scientist · Science desk
What happened
- Apolline Leclef of the Institut d'Astrophysique Spatiale at Universite Paris-Saclay and colleagues argue in an arXiv preprint that Mars' active gullies are likely carved by CO2 frost turning into a fluid.
- The team combined data from the CRISM spectrometer on the Mars Reconnaissance Orbiter and the OMEGA spectrometer on Mars Express, and confined the analysis to one location.
- Water's absorption band at 1.5 micrometres was completely absent from the spring spectra, the season after the winter CO2 blanket has sublimated away.
- Salts and clays, minerals that typically need water to form, turned up only on the high plateaus and crater rims of the area, well away from the gullies themselves.
- Geyser dark spots appear in late winter and peak at the spring equinox, while gully activity does not start until mid to late spring, once the geysers are largely dormant.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- constraint The finding is anchored to a polar pit system picked because it sits outside the belt where most Martian gullies are, so extending the dry explanation to the mid-latitude gullies requires the same spectral test done there.
- decision For anyone allocating orbiter time or a landing site to the hunt for present-day brine, the hydrated-mineral plateaus and crater rims become the better bet and the active slopes the weaker one.
- capability A seasonal dry-ice fluidization process gives planetary geologists a way to account for slopes that visibly change from one year to the next on a planet that cannot keep liquid water at the surface.
Sisyphi Cavi is a system of deep pits near Mars' south polar ice cap, it has active gullies, and it lies well outside the more temperate latitudes where Martian gullies usually form [3]. A gully site where the climate case for meltwater is weakest is a good place to ask whether water leaves any trace at all.
Hydrated minerals did show up in the data. Salts and clays, which generally require water to form, were found on the high plateaus and crater rims, and not near the gullies [5].
Geysers were the harder alternative to remove, because Mars has a dry version of them. Under the Kieffer mechanism, a transparent slab of dry ice lets spring sunlight through to the dark soil beneath, carbon dioxide sublimates under the slab, and when pressure ruptures it the escaping gas throws sand and soil out as the dark spots seen from orbit [6]. Those spots appear in late winter and peak near the spring equinox [7]. The gullies move later [8].
That leaves the process the paper favours. Gas released beneath a stationary pile of dry ice and dirt acts as a lubricant, the pile slides downslope, and the flow becomes fast enough to cut alluvial-style aprons and deep trenches [9]. The phys.org account compares it to a puck on an air hockey table, riding on a cushion of gas [12].
One polar site, two spectrometers, and a water band checked in the spring, after the seasonal ice had gone [3][4]. A spring spectrum taken from orbit measures the surface; a thin transient brine at depth would not register in it. The paper is a preprint posted to arXiv and has not been through peer review [1].
Mars is too cold, and its atmosphere too thin, to hold liquid water on the surface [10]. What made the gullies persuasive was that they resembled Earth's runoff gullies, which are all cut by water, and that they changed over time [11]. If a dry process makes the same shapes, an image of a gully is no longer evidence of water, and a spectrum has to carry the claim. According to phys.org, the case at Sisyphi Cavi would show that "Earth-like features don't always require Earth-like physics" [14].
What to watch
- A peer-reviewed version of Leclef et al., and whether reviewers accept the seasonal timing argument as decisive against geysers.
- The same CRISM and OMEGA-style spectral check at mid-latitude gullies, during the weeks those slopes are active.
- Modelling that reproduces flow speed and apron geometry from CO2 fluidization on the slopes actually measured at Sisyphi Cavi.