Medusae Fossae Formation
Medusae Fossae based on THEMIS day-time image
The Medusae Fossae Formation is a large geological unit of uncertain origin on the planet Mars. It is named for the Medusa of Greek mythology. "Fossae" is Latin for "trenches". Located roughly at , it straddles the highland - lowland boundary near the Tharsis and Elysium volcanic areas.
The Medusae Fossae Formation is a soft, easily eroded deposit that extends for nearly 1,000 km along the equator of Mars. Sometimes, the formation appears as a smooth and gently undulating surface, however in places it is wind-sculpted into ridges and grooves. Radar imaging has suggested that the region may contain either extremely porous rock (for example volcanic ash) or deep layers of glacier-like ice deposits amounting to about the same quantity as is stored in Mars' south polar cap. Using a global climate model, a group of researchers headed by Laura Kerber found that the Medusae Fossae Formation could have been formed from ash from the volcanoes Apollinaris Mons, Arsia Mons, and possibly Pavonis Mons. Further evidence for a fine-grained composition is the fact that the area gives almost no radar return. For this reason it has been called a "stealth" region. The formation is divided into three subunits (members) that are all considered to be of Amazonian age, the youngest era in martian geological history.
Layers in lower member of Medusae Fossae Formation, as seen by HiRISE. Location is Aeolis quadrangle.
The lower portion (member) of Medusae Fossae Formation contains many patterns and shapes that are thought to be the remains of streams. It is believed that streams formed valleys that were filled and became resistant to erosion by cementation of minerals or by the gathering of a coarse covering layer to form an inverted relief. These inverted stream beds are sometimes called sinuous ridges or raised curvilinear features. They have been divided into six classes: flat-crested, narrow-crested, round-crested, branching, non-branching, and multilevel. They may be a kilometer or so in length. Their height ranges from a meter to greater than 10 meters, while the width of the narrow ones is less than 10 meters.
Sinuous Ridges within a branching fan in lower member of Medusae Fossae Formation, as seen by HiRISE. Location is Aeolis quadrangle.
The surface of the formation has been eroded by the wind into a series of linear ridges called yardangs. These ridges generally point in direction of the prevailing winds that carved them, and demonstrate the erosive power of martian winds. The easily eroded nature of the Medusae Fossae Formation suggests that it is composed of weakly cemented particles, and was most likely formed by the deposition of wind-blown dust or volcanic ash. Layers are seen in parts of the formation. A resistant caprock on the top of yardangs has been observed in Viking, Mars Global Surveyor, and HiRISE photos. Images from spacecraft show that they have different degrees of hardness probably because of significant variations in the physical properties, composition, particle size, and/or cementation. Very few impact craters are visible throughout the area so the surface is relatively young.
Medusae Fossae Formation as seen with Mars Odyssey's THEMIS. Notice elongated formations called yardangs.
Yardangs in Medusae Fossae Formation with caprock labeled, as seen by HiRISE. Location is Aeolis quadrangle.
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