Dao Vallis is a giant canyon carved into the side of the ancient volcano Hadriaca Patera. Imaging locations such as this can allow HiRISE to see the insides of Martian volcanoes.

In this case, the valley wall is covered with a mantling deposit that has been cut in many places by gullies. Only faint traces of rocky layers are visible. These might be extensively eroded ancient lava flows.

One striking aspect of this image is the dearth of impact craters, and the underlying surface is very ancient. However, the mantling deposit is young (or renews itself quickly).

Features in this (and other HiRISE images of similar mantling deposits) complicate one of the many models for how gullies form. Based on lower resolution images, it was suggested that the head (or amphitheater) of the gullies was filled with dirty snow when Mars' climate was different. As the base of this snow pack melted, small rivulets of water would erode the gullies. The smooth mantling deposits found in the gullies were speculated to be patches of this snow that has survived to the present day.

However, HiRISE sees large boulders in these deposits which would be difficult to have in a snow pack.



Written by: Laszlo P. Kestay  (13 June 2007)