Fretted terrain

Fretted Terrain is a type of surface feature common to certain areas of Mars and discovered in Mariner 9 images. It lies between two different sufaces. 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 part of the northern hemisphere. Between these two zones is the fretted terrain, containing a complicated mix of cliffs, mesas, buttes, 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. ^[1] Fretted terrain is most common in northern Arabia, between latitudes 30°N and 50°N and longitudes 270°W and 360°W. ^[2] Parts of the fretted terrain are called Deuteronilus Mensae and Protonilus Mensae.

In fretted terrain the land seems to transition from narrow straight valleys to isolated measas. Most of the measa are surrounded by forms that have been called a variety of names: circum-mesa aprons, debris aprons, rock glaciers, and Lobate Debris Aprons. ^[3] 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). ^[4] Eventually, proof of their true nature was discovered by radar studies with the Mars Reconnaisance Orbiter showed that they contain 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.

Glaciers

Glaciers formed much of the observable surface in large area of Mars. Much of the area in high latitudes, especially the Ismenius Lacus quadrangle, is believed to still contain enormous amounts of water ice.^[5]^[6] It would be difficult to take a hike on the fretted terrain because the surface is folded, pitted, and often covered with linear striations. The striations show the direction of movement. Much of this rough texture is due to sublimation of buried ice. The ice goes directly into a gas (this process is called sublimation) and leaves behind an empty space. Overlying material then collapses into the void.^[7] Glaciers are not pure ice; they contain dirt and rocks.

Map of Ismenius Lacus quadrangle which is located just north of Arabia, a large bright area of Mars. It contains large amounts of ice in glaciers that surround hills.
Fretted terrain of Ismenius Lacus showing flat floored valleys and cliffs. Photo taken with Mars Orbiter Camera (MOC)on the Mars Global Surveyor.
Enlargement of the photo on the left showing cliff. Photo taken with high resolution camera of Mars Global Surveyor (MGS).
Valleys on the Ejecta Blanket from Cerulli Crater, as seen by HiRISE.
Clanis and Hypsas Valles, as seen by HiRISE. Ridges are probably due to glacial flow. So water ice is under a thin layer of rocks.
Coloe Fossae Dikes and/or Faults, as seen by HiRISE. Dikes and faults may have produced mineral deposits.
Tributary Glacier, as seen by HiRISE.
Coloe Fossae Lineated Valley Fill, as seen by HiRISE. Scale bar is 500 meters long.
Coloe Fossae Pits, as seen by HiRISE. Pits are believed to result from escaping water.