Gullies on Mars
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Martian gullies are small, incised networks of narrow channels and their associated downslope sediment deposits, found on the planet of Mars. They are named for their resemblance to terrestrial gullies. First discovered on images from Mars Global Surveyor, they occur on steep slopes, especially on the walls of craters. Usually, each gully has a dendritic alcove at its head, a fan-shaped apron at its base, and a single thread of incised channel linking the two, giving the whole gully an hourglass shape. They are estimated to be relatively young because they have few, if any craters. A subclass of gullies is also found cut into the faces of sand dunes, that are themselves considered to be quite young. Linear dune gullies are now considered recurrent seasonal features.
Gullies in the southern highlands of Mars, south of Argyre Planitia. 2014 image from the HiRISE camera on the Mars Reconnaissance Orbiter.
Group of gullies on north wall of crater that lies west of the crater Newton. The alcove and apron of one gully are labelled. These gullies are associated with moraine-like ridges at their downslope ends, which suggests they have formed at the site of a now-absent flowing ice. Note that they are cut into a mantle, which is much smoother than the rough-textured underlying material. Image taken by Mars Global Surveyor.
Gullies in crater in Phaethontis quadrangle, as seen by HiRISE under HiWish program
Gully on mound as seen by Mars Global Surveyor, under the MOC Public Targeting Program. Images of gullies on isolated peaks, like this one, are difficult to explain with the theory of water coming from aquifers because aquifers need large collecting areas.
Arcadia Planitia
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Arcadia Planitia is a smooth plain with fresh lava flows and Amazonian volcanic flows on Mars. It was named by Giovanni Schiaparelli in 1882 after the Arcadia region of ancient Greece. It dates from the Amazonian period's Arcadia formation's lava flows and small cinder cones. It includes a more recently developed large region of aeolian materials derived from periglacial processes.
Close view of expanded craters, as seen by HiRISE. After the impact, ice left the ground and widened the crater's diameter.
Wide view of group of gullies, as seen by HiRISE under HiWish program. Note that part of this image is enlarged in the following image.
Close-up of gullies, as seen by HiRISE under HiWish program. Arrow points to streamlined features. This enlarged view shows an excellent view of polygons which are common where ice-rich ground freezes and thaws.
Arcadia Planitia