Near-Polar Gullies

Near-Polar Channels on Mars
Water or CO₂?

A reduced-scale image of Malin and Edgett-style gullies caught in the act of flowing.
Extract from rectified version of MOC image M0906352
Courtesy NASA/JPL/Malin Space Science Systems

When Mike Malin and Ken Edgett first published their results that recent gullies had formed on Mars, I was as surprised and shocked as all other Mars scientists. Here was something totally unexpected - signs of recent fluid activity on Mars. By "Recent", we mean within the past 10,000 years or so (equivalent to the time on Earth since the passing of the last Ice Age), and by "fluid", Malin and Edgett clearly meant water (probably very salty water, or brine, which allowed it to flow at temperatures of -25 C or lower).

Their model was that subsurface water had seeped out at the edges of cliffs, and flowed downhill, carving the gullies. If true, it made Mars an immensely exciting planet, with active fluid systems at the present day and a very real possibility for microbial life to still be active at the present day, and recoverable at suitable surface sites.

Their results generated a flurry of activity in the scientific world and dozens of papers were published that attempted to show how the flows could be acheived, using subsurface brines with low melting points. Unfortunately, as with most work on water on Mars, these papers were rather optimistic and failed to take account of the locations and exact settings of the gullies:-

The gullies are situated mostly in the colder regions of the planet, and tend to be on cold poleward-facing slopes. If groundwater were responsible, you would expect the gullies in the warmer areas of Mars.

The gullies also show no link with volcanic features, while you would expect geothermal heat from volcanic terrains to play a role in melting ice in the subsurface.

And finally, the gullies begin at the tops of hills and sand dunes, rather than seeping out at the base like water should.

As a consequence of this, other models for the origin of the gullies were examined, some of which still rely on water, and some on CO2. What follows is a discussion of a key area on Mars where we can probably decide which fluid is responsible, because active flows have been photographed by the MOC camera.

This observation, unnoticed by Malin and Edgett, shows that in this area at least the flows are not merely "recent" but are Annual thaw features as the seasonal polecap retreats. Because we can measure the ground temperature at the time, and can see extensive deposits of CO2 snow and ice surrounding the flows, we know that they must be occurring at -125 C, far too cold for any aqueous (i.e. water-based) flow. As a consequence, thawing CO2 must be invoked as a gaseous lubricant for the flows.

Read on...

      Created: May 2002
      Last modified: May 2002
      Authorised by: Head, Earth Sciences

Maintained by: Nick Hoffman
Email: nhoffman@unimelb.edu.au