Layers in Terby Crater
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Layers in Terby Crater

April 25, 2007

This Martian image shows a sequence of predominantly light-toned, layered, sedimentary rocks exposed by erosion on the floor of Terby Crater. Terby Crater is ~165 kilometers (~100 miles) in diameter. It's located on the northern rim of the Hellas impact basin in the southern hemisphere of Mars.

The layered sequence is ~2 kilometers (~1.2 miles) thick and consists of many repetitive, relatively horizontal beds. The beds appear to be laterally continuous, which means you can identify a given layer in many locations across the area.

Details in the layering seen in this HiRISE image reveal variations in the brightness of the layers and may indicate differing mineralogies. Based on the ease with which wind appears to erode these layers, they are believed to be composed mostly of fine-grained sediments. However, one or more of the beds is weathering to form meter(yard)-scale boulders that have accumulated downslope in fans of debris (see subimage, full resolution, approx. 200 m [218 yards] across; 880x1062, 3 MB). These larger boulders indicate the material in the layers may be stronger than just fine-grained sediments.

It's not clear how these layers formed, but it may have involved deposition by wind or volcanic activity. Another theory involves all or part of the Hellas basin being filled with ice-covered lakes at one time in the past. The layers we see may have formed as material that was suspended in the water dropped down to the bottom of the lake.

Observation Geometry

Image PSP_001596_1525 was taken by the High Resolution Imaging Science Experiment (HiRISE) camera onboard the Mars Reconnaissance Orbiter spacecraft on 28-Nov-2006. The complete image is centered at -27.3 degrees latitude, 74.3 degrees East longitude. The range to the target site was 256.4 km (160.3 miles). At this distance the image scale is 25.7 cm/pixel (with 1 x 1 binning) so objects ~77 cm across are resolved.

The image shown here has been map-projected to 25 cm/pixel and north is up. The image was taken at a local Mars time of 03:38 PM and the scene is illuminated from the west with a solar incidence angle of 68 degrees, thus the sun was about 22 degrees above the horizon. At a solar longitude of 142.3 degrees, the season on Mars is Northern Summer.

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