Gullies in Terra Sirenum

This is a full-resolution subset of image TRA_000878_1410, covering an area
of approximately 220-by-500 m (730-by-1600 feet). It shows several gully
systems carved in the walls of an unnamed crater 10 km (6 miles) in diameter,
located in Terra Sirenum. Downhill is towards the bottom of the image, north
is up.

The gullies in this image are composed of multiple small tributaries, some as
narrow as 1 pixel (25 cm or 10 inches). Polygonal features similar to
polygons formed on terrestrial permafrosts (frozen soils) are ubiquitous on
the south-facing walls of this crater, both around the gullies and on some
gullies' walls. Whatever processes formed these gullies, they started long
enough ago so there has been time for polygonal features to develop along
their walls. Locally, some gullies' walls (west-facing walls in particular)
look puzzlingly dark in this black-and-white image. Nearby areas, imaged in
color by HiRISE, provided the evidence to solve this puzzle: the brighter
walls are covered in whitish-bluish frost and therefore look brighter in the
black-and-white image; frost-free materials (mostly on walls which, because
of their orientation, may get some direct sunlight) look darker. Some of the
gullies shown in this image, although well developed and locally deeply
incised, seem to fade away before reaching the crater's floor. 

Current leading hypotheses explaining the origin of gullies include erosion
from seepage or eruption of water from a subsurface aquifer, melting of
ground ice, or surface snow; and dry landslides. The nature of gullies will
be further investigated utilizing the high-resolution and color capabilities
of HiRISE.

(Note: Occasionally data is lost while being transmitted from Mars to Earth.
When this happens it shows up as a series of black pixels).