Polygons, Crater Layers, and Defrosting Dunes
Polygons, Crater Layers, and Defrosting Dunes
PSP_008426_2595  Science Theme: Aeolian Processes
The north polar region is surrounded by a large sea (erg) of dark sand dunes that become covered by seasonal carbon dioxide frost. As the northern hemisphere begins to warm in the spring, the frozen carbon dioxide sublimates. The wind blows from an east-northeasterly direction and leaves dark streaks behind (exposed basalt) from the evaporating carbon dioxide. This image displays defrosting sand dunes in an unnamed crater.

The dune morphology in this image is complex. Because of the presence of the ice, it is difficult to determine all of the dune types. These jumbled dunes may result from erosion of the layers within the crater walls that act as a dune source.. However, two common types of dunes can be classified: the outer ring of the dune field is composed of chains of barchan dunes whereas the central area of the field contains transverse dunes.

Barchans are characterized by their crescent-shape with steep horns in the downwind direction. The transverse dunes have asymmetric, nearly parallel ridges and are oriented perpendicular to the wind direction.

Another feature of interest is the sublimating polygons that have very small ripples on top of them. Polygons are created from a freeze-thaw processes similar to features on Earth that undergo annual contraction of the permafrost regolith.

Written by: Circe Verba  (25 June 2008)
Acquisition date
14 May 2008

Local Mars time

Latitude (centered)

Longitude (East)

Spacecraft altitude
318.4 km (197.9 miles)

Original image scale range
32.1 cm/pixel (with 1 x 1 binning) so objects ~96 cm across are resolved

Map projected scale
25 cm/pixel

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Phase angle

Solar incidence angle
58°, with the Sun about 32° above the horizon

Solar longitude
71.5°, Northern Spring

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North azimuth:  103°
Sub-solar azimuth:  321.5°
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NASA’s Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, Calif., manages the Mars Reconnaissance Orbiter for NASA’s Science Mission Directorate, Washington. The HiRISE camera was built by Ball Aerospace and Technology Corporation and is operated by the University of Arizona.