30 November 2009

Slumps caused by thawing ground on Mars and Earth

Posted by dr-dave

The Planetary Geomorphology Working Group of the International Association of Geomorphologists has a rather nice article online comparing the landforms caused by slumping during thawing of the ground with similar features that have been seen on Mars. The article is available here:

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.