Martian gullies were once hailed as evidence of liquid water on Mars. Now a new theory backed by experiment explains how they are formed by the flow of sand
In 1999, the Mars Global Surveyor spacecraft sent back some extraordinary images of the surface of the Red Planet. They showed gullies that had been carved into Martian hillsides in the mid-latitudes of the planet.
The same sort of gullies form on Earth and here their existence is the result of water erosion. Various planetary geologists immediately suggested that a similar process must be at work on Mars. Their thinking was that some fluid must be responsible.
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That’s an exciting idea but one that is fraught with problems. Recent evidence from Mars rovers suggest that liquid water once flowed on the surface of Mars but way back in the planet’s past. By contrast, the gullies spotted by the Global Surveyor are probably only a few million years old and over this timescale, the Martian atmosphere has been too cold and thin for liquid water.
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To counter this, the geologists say that some other heating mechanism could cause liquid water to flow from an underground aquifer. But the gullies look to be the result of repeated flows so if this explanation is correct, there has to be a mechanism that refills the aquifers. On Earth, that mechanism is rain. On Mars, such a mechanism is patently absent.
There is another possibility, however. Perhaps the gullies are caused by the flow of sand and dust. Similar gullies are known to occur on dunes on Earth but only when the angle of the hillside is above some critical threshold. The trouble with Martian gullies is that most of the hillsides are not steep enough for this process to occur.
Today, Yolanda Cedillo-Flores and Héctor Javier Durand-Manterola at the Universidad Nacional Autónoma de México suggest a solution to this conundrum. Their idea is that the gullies are formed when carbon dioxide in the ground sublimates, causing the sand to become fluidised.
This solves a number of problems. For a start, it allows the sand to flow on hillsides that are much less shallow than the critical threshold. It also explains how the gullies form by the action of repeated flows over long periods of time. Their idea is that carbon dioxide is deposited on the ground during cold periods and covered with windblown dust. The carbon dioxide then sublimates during warmer periods, causing the sand to flow downhill.
This theory also explains why the gullies form mainly at mid-latitudes and not at the equator or the poles. At the poles, it rarely gets warm enough to regularly heat sand-covered carbon dioxide and at the equator it is too warm for it to form. So the mid-latitudes are the best places for gullies to form.
Cedillo-Flores and Durand-Manterola attempt to seal the deal by recreate in their lab the conditions for martian gully formation by injecting air into a bed of sand. Sure enough, this process leads to the formation of martian-like gullies at angles well below the critical threshold (see picture above).
There are some important difference between the experimental gullies and the ones that form on Mars, such as their length. But the researchers say that this can be explained by the difference between the way air is injected into sand and the way carbon dioxide sublimates.
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Best of all, the new theory requires no special hypotheses about the Red Planet. “Our model does not require exotic conditions; only the ones found at present on Mars,” say Cedillo-Flores and Durand-Manterola.
That’s a convincing explanation for the formation of martian gullies. And it will be a substantial blow to those looking for evidence of liquid water on the surface of Mars (and the possibility of life it suggests).
Water may still be present on Mars but it is looking increasingly clear that there are better places in the Solar System for astrobiologists to focus their attention.
