http://www.psi.edu/pgwg/images/dec09image.html
On Earth, thaw slumps occur in permafrost areas like Alaska. This is an oblique aerial image of these features, taken from the site above:
The Natural Resources Canada has quite a nice (although I suspect somewhat old) cartoon to illustrate how these landslides work:
Essentially, the thaw of permafrost (ground that is usually frozen) allows the weak materials to fail and flow. This typically exposes a new face of frozen ground that, if the temperatures are high enough again, thaws and flows. Thus, over time, the back scarp of the landslide moves back into the hill - i.e. it retrogresses. This is what one looks like in a vertical view on Google Earth:
The features observed (by satellite obviously) on Mars have much the same morphology as retrogressive thaw slides on Earth:
The arrowed features are interpreted as pingos, which are only found in permafrost areas on Earth. Thus, the arcuate features upslope of the pingos are interpreted as the arcuate backscarp of the landslides. The existence of these features of course implies that at some point the ground thawed and there was liquid water present at some point in the past.
Dear Rose,
ReplyDeleteIn the aftermath of the earthquake the Chinese state mobilised an extraordinary team of geologists to map the landslides. These maps have then been used to help with the planning process during reconstruction. They have not got it uniformly right (it is fairly easy to cfome up with examples where buildings have been constructed in unsafe locations), but on the whole it has been a very impressive effort. Indeed, in all the earthquake affected areas in which I have worked, this is the one in which geologists have had the greatest input into the reconstruction process.
I think that it helps that the Chinese Premier, Wen Jibao, is a geologist and an engineer!