Sisyphi Cavi

Sisyphi Cavi
Site of the Annual Flows

	71 ^oS 72 ^oS
5 ^oW 0 ^o 5 ^oE

A reduced-scale image of the Sysiphi Cavi area.
Click on image for larger version.
Image Courtesy PDS "Armchair Astronaut"

Sysiphi Cavi is a region of deep pits and depressions, carved into the Polar Layered Terrain (PLT) by unknown forces, although localised thawing of CO2 or H2O permafrost is one possibility. The walls of the pits are steep by Mars' standards (20 degrees or so), are up to 1 km in height, and are the site for ongoing erosion (often associated with deposits of dark dust at the foot of slope).

Many examples of the Malin and Edgett gullies are to be found here - the southernmost location noted to date. Malin and Edgett included an example from this area in their summary of gully morphologies and occurrences.

Unlike examples in more temperate zones, the gullies are not so restricted to poleward-facing slopes. Here, gullies occur frequently on east- and west-facing as well as on south-facing slopes. This alone suggests that mechanisms for gully formation are more active here then elsewhere on Mars.

My attention was drawn to this area for several reasons.

It lies within the seasonal polar cap of ~ 1m of CO2 snow and ice which thaws each spring
It has numerous gullies
It has been extensively imaged by Malin Space Science Systems, in their survey of gully occurences
It has some unusual gully forms that were first drawn to my attention by Greg Orme, an amateur observer of the planet.

In particular, I first saw MOC image M0906352, which excited me immediately. I could see here the signs of an intimate relationship between thick snow accumulation and the gullies. Of course, this by itself doesn't mean much. It could have been that the gullies were formed thousands of years ago in this area under different climatic conditions and were now passive collectors of winter snow in their sheltered alcoves. Regardless, I began a search of nearby images to examine this relationship.

There was a clear pattern to be seen in the data. Gullies were found in areas where snow cover persisted until late in the spring. On an individual slope, patches of gullies were directly related to patches of snow - but of course, this could work either way. Gullies could trap snow, or snow could somehow help cause the gullies. To distinguish this, one needs to look at a time sequence as the snow thaws.

Despite the extensive MOC image coverage of this area, and the relative closeness of ground tracks of the satellite in this near-polar area, repeat images of the same section of hillside are very rare. The satellite orbit is designed to step across the planet, not repeatedly pass the same track. Therefore one cannot build a time series of events in a single location, but instead a composite must be assembled from the many images of many different hillsides and gullies.

It was during the process of building this sequence that I made the most important observation that showed how everything worked in this area.

Read on...

      Created: May 2002
      Last modified: May 2002
      Authorised by: Head, Earth Sciences

Maintained by: Nick Hoffman
Email: nhoffman@unimelb.edu.au