TR: Martian Gullies Were Formed By Flow Of Sand

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TR: Martian Gullies Were Formed By Flow Of Sand

Post by bystander » Wed May 05, 2010 4:19 pm

Martian Gullies Were Formed By Flow Of Sand
Technology Review - 04 May 2010
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.

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.

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.
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Martian gullies: Produced by fluidization of dry material Are Martian gullies formed by water or not?
Bad Astronomy - 05 May 2010
The idea of liquid water on Mars is an enticing one. We know life on Earth needs liquid water, and if we find it on Mars… We know there’s plenty of frozen water on Mars; we see it there in abundance. But Mars is cold, and the air is thin, making liquid water on the surface difficult to achieve, let alone sustain.

But there’s been tantalizing evidence. Ever since Mars Global Surveyor got to the Red Planet in 1997, we’ve seen gullies sprinkled here and there. These gullies form on slopes near the tops of the hills, and are clearly the result of something moving downslope and digging furrows. But is that something water, or just sand and dust? The conclusions flip-flop back and forth; I’ve seen papers come out saying water-not-sand and others saying sand-not-water several times.

A new paper has just come out saying it’s sand and dust, and not water, that’s doing the trick. The authors did a clever experiment. They assumed that it was dry ice — frozen carbon dioxide — that was behind the gullies, and not water. As the dry ice turns into a gas in warmer weather, they supposed, it blows out of the ground and gets in between the sand particles, causing them to run downslope like a fluid. That’s a fair assumption, given how common dry ice is on Mars. They then set up a tub filled with Mars-like sand, piled it into a slope, and used an air pump to force air under the sloping pile. Sure enough, the sand flowed down, making gullies that seem very much like what’s seen on Mars!

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