Elysium Quadrangle - Fossa/pit Craters

Fossa/pit Craters

The Elysium quadrangle is home to large troughs (long narrow depressions) called fossae in the geographical language used for Mars. Troughs are created when the crust is stretched until it breaks. The stretching can be due to the large weight of a nearby volcano. Fossae/pit craters are common near volcanoes in the Tharsis and Elysium system of volcanoes. A trough often has two breaks with a middle section moving down, leaving steep cliffs along the sides; such a trough is called a graben. Lake George, in northern New York State, is a lake that sits in a graben. Pits are produced when material collapses into the void that results from the stretching. Pit craters do not have rims or ejecta around them, like impact craters do. Studies have found that on Mars a fault may be as deep as 5 km, that is the break in the rock goes down to 5 km. Moreover, the crack or fault sometimes widens or dilates. This widening causes a void to form with a relatively high volume. When material slides into the void, a pit crater or a pit crater chain forms. On Mars, individual pit craters can join to form chains or even to form troughs that are sometimes scalloped. Other ideas have been suggested for the formation of fossae and pit craters. There is evidence that they are associated with dikes of magma. Magma might move along, under the surface, breaking the rock, and more importantly melting ice. The resulting action would cause a crack to form at the surface. Pit craters are not common on Earth. Sinkholes, where the ground falls into a hole (sometimes in the middle of a town) resemble pit craters on Mars. However, on the Earth these holes are caused by subsurface limestone being dissolved, thereby causing a void. The images below of Cerberus Fossae, Elysium Fossae and other troughs, as seen by HiRISE are examples of fossa.

Knowledge of the locations and formation mechanisms of pit craters and fossae is important for the future colonization of Mars because they may be reservoirs of water.

• Cerberus Fossae, as seen from THEMIS.

• Cerberus Fossae, as seen by HiRISE.

• Elysium Fossae, as seen by HiRISE. Scale bar is 500 meters long.

• Troughs to the East of Albor Tholus, as seen by HiRISE under the HiWish program.

• Portion of a trough (Fossae) in Elysium Planitia, as seen by HiRISE under the HiWish program. Blue indicates probably seasonal frost.