In this case the smoking gun may be flows, or steady trickles if preferred, of this gas, out of scarp edges, the result of annual warming and sublimation to some depth of the sub-surface dry ice. It would move along the topographic gradient set up below the surface, on the top of the remaining dry ice layer.

The sub-surface topographic gradients would be shaped by the ground closest to the scarp edge being warmed to a slightly greater depth than that further away, resulting in a sub-surface, truncated shallow basin � like very flat versions of the alcoves described by Malin and Edgett, but underground.

The dense gas would flow to the crater or canyon edge, then tip itself downslope, taking loose light loose material with it, and in time undermining the large boulders, to decametre size, that have been seen to have tumbled down some slopes.

Moreover, dune patterns converging on a breach at the top of one of the Gorgonum Chasm cliffs show that strong surface flows of air also tip themselves into the canyons, and probably contribute largely to erosion and transport below.

The flow may not all be one way either. All cliffs on this planet show air rotor effects, beloved of hang-glider pilots, where air comes up from below, giving a narrow lift band, just in front of and above the cliff edge.

On Mars there is a consistent line of dunes parallel to and just back from the top of many cliff edges, that looks like deposition of fine material in the obligatory down-draught of such rotors, just a short way back from the edge. Some of that dune would result from re-arranging the material on the top to fit this airflow, but some material would also have come up from below in major dust storms.

This picture would be one way to account for the prevalence of gullies facing predominantly away from the equator, and in high latitudes. Elsewhere, the gas would mostly dissipate too fast to flow katabatically.

I suspect, oddly, that we may see a high-temperature version of something similar to this gas-driven gully-formation on earth, on the ash-covered slopes of volcanoes, possibly such as Merapi in Java.

There, the flanks of the cone are scoured with deep, narrow channels radiating from the crater, which do not seem to have been formed by liquid water.

They may result from lava flows, but I suspect instead that a combination of sulphur and carbon dioxide in hot dense mode, together with entrained ash, flowing at the base of nuee ardentes, may be responsible. If so, dissipation upwards into the atmosphere may have been constrained by the blanketing ash.

As an aside, I keep wondering if given that the atmosphere if Mars is only 95 percent carbon dioxide, and that the balance, mostly nitrogen and argon, though remaining gaseous, may have large effects.

After all, water vapour is a small component of our atmosphere, but has proved to have quite some effect on landforms. As there is, however, only about a thousandth as much water on Mars, I think we should consider some experiments with simulated Martian atmospheres, temperatures and gravity, to check the behaviour of what is there now in large quantities, before concluding that we have found the culprit.

My conclusion is therefore that we probably have a very active sedimentary system on Mars, fully operational now. The engine driving it is airflow and the starter motor is gravity and temperature-controlled expansion and contraction, boosted occasionally by seismic impacts.

The entire set of features we see is then exactly what we would expect given a planet experiencing repeated fast winds and dust storms, and tectonic stress and the shocks from the occasional huge and more common small incoming missiles.

So, maybe there was almost no liquid water and hence no life on Planet Mars, ever.

Perhaps, given the ability of life to adapt to all sorts of environments with ferocious enthusiasm, we are better off without some tenacious Martian microbe hitching a ride home on a probe, and finding us and our planet a wonderful field for colonisation.

Given how troublesome life forms crossing from continent to continent have proved, maybe we really don't want Martian microbial weeds down here. An utterly lifeless planet might just be the best possible thing for us to start our own interplanetary colonisation career on.

Peter Ravenscroft is a geologist based in Queensland Australia. He holds a B.Sc, (geology, sub-majors chemistry and oceanography/marine geology,) from the University of Cape Town, South Africa. You may email him directly by nathydro@[email protected] - remove @nospam@ and replace with a single @.

The original version of this article is public domain and is available for reprinting via Peter. The HTML/edited version of this article as published on SpaceDaily is copyright SpaceDaily and cannot be reprinted without written authorization from SpaceDaily.

A DRY MARS? - Part 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9 - 10 - 11 - 12 - 13 - 14

References:
Fortes, A. D., June 1998. Wind, Water, Ice, and Gravity on Mars. Internet publication,

M. C. Malin, K. S. Edgett, M. H. Carr, G. E. Danielson, M. E. Davies, W. K. Hartmann, A. P. Ingersoll, P. B. James, H. Masursky, A. S. McEwen, L. A. Soderblom, P. Thomas, J. Veverka, M. A. Caplinger, M. A. Ravine, T. A. Soulanille, and J. L. Warren. NASA's Planetary Photojournal (http://photojournal.jpl.nasa.gov/), [MGS MOC Releases MOC2-180 to MOC2-183],[1 October 1999], [MGS MOC Release No. MOC2-241], [22 June 2000], [MGS MOC Release No. MOC2-236], [22 June 2000].

M. C. Malin, K. S. Edgett, Evidence of Recent Groundwater Seepage and Surface Runoff on Mars. Science, 20.6.2000, pp 2330-2335.

Malin, M.C., and Edgett, K. S., Evidence for Recent Liquid Water on Mars: Channels and Aprons in East Gorgonum Crater. MGS MOC Release No. MOC2-241, 22 June 2000.

Yen, Albert S., Murray, Bruce C. and Rossman, George R. Water Content of the Martian Soil. Laboratory Simulations of Reflectance Spectra. 1997.

MARSDAILY.COM
Exploring the Canyons and Cliffs of Mars
by Bruce Moomaw
Cameron Park - July 11, 2000 - In the first three installments of this series, I described the current theories now being offered to explain Surveyor's remarkable discovery of what looks very much like recent eruptions of liquid groundwater onto Mars' surface -- and in the coldest, most unlikely parts of the planet imaginable. There are at least five theories bouncing around at the moment (in two of which the stuff erupting isn't even water, but carbon dioxide).