Martian gullies: Produced by fluidization of dry material

The gullies on Mars were discovered in the year 1999.Since then several hypotheses have appeared trying to explain the presence of these gullies. The main hypotheses are the ones which suggest that some liquid, water or CO2, was responsible for modeling the gullies and ones that propose dry flows as the modeling agents. The aim of this work is to develop an alternative hypotetical mechanism of formation of Martian gullies. Our model proposes that the Martian gullies were formed as a result of a fluidization process of the material deposited on the slopes of the impact craters, plateaux and other geomorphologic structures. This fluidization is caused by the sublimation of carbon dioxide ice deposited in the form of snow, due to the daily and seasonal temperature changes. We also present the results of an experimental simulation. Structures similar to the Martian gullies were reproduced using the air injection mechanism, as a substitute to gaseous CO2, on a sandy slope. The Reynolds number for our experimental flow and for the flow in the Martian gullies was calculated and we found that they are of the same order, whilst the water flows have much higher Reynolds numbers. Taking into account the current environmental conditions for Mars, from our results it may be suggested that this mechanism is the possible modeling agent for the Martian gullies. Two of the most important characteristics of the proposed model are: a) It offers a simple explanation of how recurrent fluidization events occur in the same place, a necessary recurrence of the formation of a gully, offering a recharging mechanism for the system after every fluidization event, and b ) It considers the formation of gullies possible even in angles smaller than the angle of repose. These characteristics of the model solve these two problems present in current theories.

Very interesting paper. Thanks. Although, it's a bummer for Follow the Water enthustiasts. What I liked about the paper is that their experiment could be done by anyone at home. However, I'm still not persuaded that the proposed method of gully formation explains why gullies predominate in the 30-60 degree latitudinal ranges. It seems that conditions of rapidly sublimating CO₂ ice in sand and dust could also occur at far higher latitudes. The authors, to their credit, address this concern, arguing that the temperate latitudes have a greater diurnal and seasonal temperature range allowing for more rapid volitalization of CO₂ ice than the polar regions. Still, we know that CO₂ sublimation can be very rapid too at higher latitudes and evidenced by the rapid reptreat of the seasonal ice caps and the occurence of arianeiforms (spiders) in the southern polar regions. Maybe I'm comparing apples and oranges here, but I think the question needs further exploration.

Tom

This model suffers from the same problem as the dry gully hypothesis put forth by Shinbrot et al. (2004) (http://www.pnas.org/content/101/23/8542.abstract)—yes, you can get an alcove and an apron, but it's missing the key defining characteristic of gullies, which is the channel. Their experiments did not produce the sinuous, anastomosing channels often observed in martian crater wall gullies. They call some features in their experiments "channels," but terrestrial geologists studying landslides on sand dune faces wouldn't call those features channels. They're more like chutes. The gullies on Mars also aren't just simple landslides of loose sand/dust on slopes; in many places the channels cut into the underlying rock, which requires something able to erode such rock (i.e. liquid water).

Really good points. I noticed the lack of long, well defined channels too. The authors tried to address this by falling back on the model's inability to duplicate martian conditions exactly, particularly the continual releases of gas over time. It seems a weak argument to me.
Tom