Craters in Fretted Terrain
4 October 2004
Robert P. Sharp, in a classic Journal of Geophysical Research paper, the fretted terrains of Mars are those in which, at about 200 to 400 meters per pixel scale, have "smooth, flat lowland areas separated from a cratered upland by abrupt escarpments" approximately 1 to 2 km (0.6 to 1.2 mi) high. As viewed from above, the fretted terrain troughs are nearly straight and carve-up old, heavily cratered terrain just north of Arabia Terra and part of northern Tempe Terra. The trough floors in the northern mid-latitude fretted terrain are heavily eroded. These floors were thought, on the basis of Viking orbiter images, to possibly have glaciers or some other form of flowing or creeping ground ice. However, Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) images have shown little evidence for flow on these pitted and lineated floors. Circular features that were probably once meteor impact craters have not been sheered or deformed, as they would have been if the material were flowing.
The MOC image shown here exhibits a few examples of the eroded forms of old craters on the floor of a fretted terrain valley. Crater 1 still retains the typical bowl shape of an impact crater, but its raised rims and ejecta blanket have been eroded away. Crater 2 is a shallow depression that might also represent the location of a meteor crater that has nearly eroded away. Feature 3 is a circular mesa; it is probably all that remains of a crater that was filled then eroded away, leaving behind a remnant of the material that filled the crater. Feature 4 is a small depression with a central mound--this, too, may have been an impact crater and the mound is a remnant of material that once filled the crater. In all, erosion appears to have been powerful enough to remove material that once existed above the present landscape, and altered the appearance of craters in this region.
The image is located near 40.2°N, 335.2°W. The 300 m scale bar is also about 985 ft across. Sunlight illuminates the scene from the lower left. The R. P. Sharp paper described here is, "Mars: Fretted and chaotic terrain," Journal of Geophysical Research,v. 78, n. 20, p. 4073-4083, 1973.