NASA Mars Picture of the Day: Craters in Fretted Terrain
Mars Global Surveyor Mars Orbiter Camera
MGS MOC Release No. MOC2-869, 4 October 2004
 NASA/JPL/Malin Space Science Systems |
As described back in 1973 by
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.
Malin Space Science Systems and the California Institute of Technology
built the MOC using spare hardware from the Mars Observer mission.
MSSS operates the camera from its facilities in San Diego, California.
The Jet Propulsion Laboratory’s Mars Surveyor Operations Project
operates the Mars Global Surveyor spacecraft with its industrial
partner, Lockheed Martin Astronautics, from facilities in Pasadena,
California and Denver, Colorado.
