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Fretted terrain is a type of surface feature common to certain areas of Mars and was discovered in Mariner 9 images. It lies between two different types of terrain. The surface of Mars can be divided into two parts: low, young, uncratered plains that cover most of the northern hemisphere, and high-standing, old, heavily cratered areas that cover the southern and a small part of the northern hemisphere. Between these two zones is a region called the Martian dichotomy and parts of it contain fretted terrain. This terrain contains a complicated mix of cliffs, mesas, buttes, and straight-walled and sinuous canyons. It contains smooth, flat lowlands along with steep cliffs. The scarps or cliffs are usually 1 to 2 km high. Channels in the area have wide, flat floors and steep walls. Fretted terrain shows up in northern Arabia, between latitudes 30°N and 50°N and longitudes 270°W and 360°W, and in Aeolis Mensae, between 10 N and 10 S latitude and 240 W and 210 W longitude. Two good examples of fretted terrain are Deuteronilus Mensae and Protonilus Mensae.
Fretted terrain in Arabia Terra (Ismenius Lacus quadrangle), seems to transition from narrow straight valleys to isolated mesas. Most of the mesas are surrounded by forms that have been given a variety of names: circum-mesa aprons, debris aprons, rock glaciers, and lobate debris aprons. At first, they appeared to resemble rock glaciers on Earth. But scientists could not be sure. Even after the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) took a variety of pictures of fretted terrain, experts could not tell for sure if material was moving or flowing as it would in an ice-rich deposit (glacier). Eventually, proof of their true nature was discovered when radar studies with the Mars Reconnaissance Orbiter showed that they contained pure water ice covered with a thin layer of rocks that insulated the ice.
Besides rock-covered glaciers around mesas, the region had many steep-walled valleys with lineations—ridges and grooves—on their floors. The material comprising these valley floors is called lineated valley fill. In some of the best images taken by the Viking Orbiters, some of the valley fill appeared to resemble alpine glaciers on Earth. Given this similarity, some scientists assumed that the lineations on these valley floors might have formed by flow of ice in (and perhaps through) these canyons and valleys. Today it is generally agreed that glacial flow caused the lineations.
Fretted terrain in Aeolis Mensae is similar to that of Arabia Terra, but it lacks debris aprons and lineated valley fill. The Medusae Fossae Formation, a friable, layered material that is covered with yardangs surrounds parts of the fretted terrain in Aeolis Mensae.
The origin of fretted plateau material is not completely understood. It does seem to contain fine-grained material, and it has an almost total lack of boulders. This material contrasts with most of the Martian surface which is covered with the igneous rock basalt. Basalt breaks into boulders and eventually into sand. It is thought that when plateau material breaks down, the small-sized particles can be easily carried away by the wind. Erosion of plateau material seems to be much faster than other materials on Mars. Research presented in 2018 at a Lunar and Planetary Science Conference in Texas suggested that the erosion that formed fretted terrain was aided by water moving under the surface.