Posted on Saturday 12 June 2004

Water exists in abundance on the planet Mars but all of it is locked up in the polar ice caps or is buried deep below the surface. In 2000, photographs coming back from the Mars Global Surveyor spacecraft, which has been orbiting the planet since 1997, caused a sensation because they provided the first evidence that water may also exist in a liquid form close to the Martian surface and might at times actually erupt from the ground and run in torrents down its mountain slopes. These Martian rivers would be short lived because water in its fluid state cannot exist for long on the Martian surface without either freezing in the incredible cold or rapidly boiling off into the thin atmosphere. However evidence of the possibility of liquid water excited many scientists because of what this could mean for the likelihood of life in or around these underground bodies of water. The idea of water bubbling to the surface, somewhat like geysers, has been described as the soda fountain view of Mars.

There are some problems with this theory, however. Chief amongst them is the fact that these ice thaws have not been observed in the equatorial regions but only towards the southern pole with latitudes of around thirty degrees of more. This is a region where the surface temperature rarely rises above -50 degrees centigrade (and is oftentimes much colder than even that) which raises the question of how this thawing could even occur. Volcanic activity as a heat source seems unlikely, it is thought to ceased at least a hundred million of years ago however there is still the possibility that remnant pockets of vulcanism do still exist on the planet.

An alternative suggestion is that these river-like erosion features are not caused by liquid flows at all but are instead caused by landslides of dry sand. This theory has been rejected in the past because, it is argued, grains of dust simply do not flow like a liquid. Troy Shinbrot of Rutgers University challenges this view and has been conducting experiments that demonstrate that if you take into account Mars' much weaker gravitational field and its much thinner atmosphere, things can behave very differently. In fact, in a low gravity environment sand does indeed flow like a liquid and can do so over long distances. In experiments using synthetic grains made from hollow spheres he claims been able to simulate most of the visible features of these Martian flows.

Over the past decade or more, contradictory evidence of Martian climate, indicating that surface temperatures seldom if ever approach the melting point of water at midlatitudes, and geomorphic features, consistent with liquid flows at these same latitudes, have proven difficult to reconcile. In this article, we demonstrate that several features of liquid-erosional flows can be produced by dry granular materials when individual particle settling is slower than characteristic debris flow speeds. Since the gravitational acceleration on Mars is about one-third that on Earth, and since particle settling speeds scale with gravity, we propose that some (although perhaps not all) Martian geomorphological features attributed to liquid flows may in fact be associated with dry granular flows in the presence of reduced gravity.

A number of recent studies have investigated evidence for liquid water on Mars. Much of the available evidence has concerned recent, or even contemporary, geomorphological features, such as eroded channels and gullies that are common signatures of terrestrial water flow. This evidence is difficult to reconcile with surface temperatures on Mars, which seldom exceed 50°C at latitudes and locations where these features are often found. This paradox persists despite recent Mars Rover data supporting the case for ancient surface water. Several authors have proposed possible resolutions to this contradiction, notably a recent analysis suggesting that some liquid-like flow features in Martian gullies may instead be associated with dry granular flow. In this article, we observe that gravity on Mars is 38% that on Earth, and behaviors of dry grains at reduced gravitational acceleration have never, to our knowledge, been catalogued. As we will show, reduced gravity has the effect of prolonging fluidization of particle flows by decreasing particle settling speeds as compared with debris flow speeds. We present data demonstrating that many features that have been attributed to liquid flow in Martian gullies can indeed be reproduced in terrestrial laboratory experiments designed to mimic reduced gravity...

Unfortunately the paper is password protected at the Proceedings of the National Academy of Sciences site but it is possible to see a text-only version of it via this Google cache.