24 December 2008

AGU Day 3: Fluvial Systems on Mars

Posted by Ryan Anderson

Well, I have survived holiday travel and am now home in Michigan with family. Before I head off to celebrate Christmas eve, here is a quick report from AGU. (I figure I need to get these blog entries posted before I completely forget what my notes mean!)

On Wednesday afternoon I spent my time in the Mars sessions on “Ground Truth for Orbital Data” and “Martian Fluvial Systems”. The ground truth session was not as exciting (to me) as the title sounded. It was primarily focused on how simultaneous observations of the atmosphere by landers and orbiters can really help to better our understanding of the atmosphere of Mars. Obviously, this is important science, but my notes are pretty light on it, so I will focuse on the second afternoon session about Martian Fluvial Systems.

Ginny Gulick gave an overview talk about Martian gullies, and showed one gully that she suspects may have water-ice in it. The best part of this presentation was the fantastic HiRISE images!


Nicolas Mangold presented his investigation of sinuous gullies and considered several possble formation mechanisms. The first possibility was “granular flows”, such as a sand or dust avalanche, but models and experiments showed that these never show the winding, snakelike shape of sinuous gullies. Next he considered debris flows, where water is carrying tons of rocks and mud with it, so that there is as much debris as tehre is water. These were much more consistent with the shape of the sinuous gullies on Mars. In particular, many of the gullies have raised edges called “levees” which are also formed by debris flows on earth. Many gullies on Mars do not have levees though, and Mangold suggested that these may be closer to mud flows.

Jay Dickson gave a presentation about how the gullies formed and suggested that they are more likely fed by the melting of ice and snow on the surface than by groundwater sources. Many gullies appear to start on the central peaks of craters and other places where it is pretty muc impossible to get any groundwater. Dickson suggested that the alcoves at the tops of the gullies act as cold traps, and as ice and snow accumulate ther and then melt, the gullies are formed. A similar process works in the dry valleys of Antarctica, as shown below.

A gully in the dry valleys of Antarctica.

Left: A gully on Mars; Right: A gully in the dry valleys of Antarctica.

Carl Allen gave an interesting talk about some interesting features in Vernal crater which he thinks might be spring deposits. He proposed the site at one of the earlier MSL landing site workshops, but the evidence for spring activity was too provisional and it did not make the cut. The image below shows the light-colored splotches that Allen thinks may be evidence of spring activity, and an aerial photo of the Dalhousie spring-mounds in Australia. Unfortunately the whole area is very dusty so it is impossible to tell the composition of the potential spring-mounds from orbit. I think the jury is still out on this discovery, but it is certainly interesting and worth looking for similar features elsewhere on Mars.


Another presentation that I found interesting was one on waterfall erosion by Mike Lamb. He presented an argument based on the simple physics of how flowing water causes blocks to topple, and claimed that channels on Mars that have always been thought to be due to groundwater sapping may actually be due to flow of water over the surface and erosion by waterfalls. His analogy was Box Canyon, Idaho, which appears to have been eroded by a waterfall.


Channels carved by waterfalls and those carved by groundwater both look the same: they are typically short and stubby with rounded ends. The problem with Lamb’s waterfall hypothesis for Mars is that, even assuming liquid water was stable on the surface, there is little to no evidence of flowing water leading up to the head of the stubby channels! I suspect that groundwater is the correct explanation, but it’s good to see people looking at alternatives.


That wraps up the highlights from Wednesday. By Thursday and Friday I was beginning to burn out, but I do still have some notes to write up into a post or two. Stay tuned!