Are these dunes? One of the most puzzling findings of the Mars Global Surveyor Mars Orbiter Camera investigation has been the discovery of many surfaces of sharp, parallel ridges and grooves that--at first glance--look like dunes, but upon closer inspection turn out to be something else. They aren't dunes because they occur too close together, their crests are too sharp, and their slopes are too symmetrical. In most places that they occur on Mars, they appear to be occurring within a specific layer of (usually) dark material. Exactly what processes make these ridges is a mystery, but it clearly involves some sort of erosion. Dark mesas in this picture of the floor of Melas Chasma in the Valles Marineris system are developing sharp, parallel troughs and pits that appear to eventually erode to become the fields of ridges seen throughout the rest of the image. Dark, ridged surfaces like this are common in the central floors of Valles Marineris and elsewhere in the equatorial regions of Mars, and present a type of surface that may need to be avoided by future Mars landers. This image, illuminated by sunlight from the left, covers an area 3 kilometers (1.9 miles) wide and 14.5 kilometers (9 miles) long. The scene is located near 8.8°S, 76.8°W and was acquired on March 22, 1999.