Medusae Fossae Formation

The Medusae Fossae Formation is a large geological formation of probable volcanic origin on the planet Mars. It is named for the Medusa of Greek mythology. "Fossae" is Latin for "trenches". The formation is a collection of soft, easily eroded deposits that extends discontinuously for more than 5,000 km along the equator of Mars. Its roughly-shaped regions extend from just south of Olympus Mons to Apollinaris Patera, with a smaller additional region closer to Gale Crater.

The Medussae Fossae Formation is part of an area called "stealth terrain" that produces little to no radar return, making it appear "stealthy" to radar signals. It is believed to be covered by a thick mantle of fine-grained, unconsolidated material, likely volcanic ash or dust.

The total area of the formation is equal to 20% the size of the continental United States. It is divided into three subunits (members) that are all considered to be of Amazonian age, the youngest era in martian geological history. The formation straddles the highland - lowland boundary near the Tharsis and Elysium volcanic areas, and extends across five quadrangles: Amazonis, Tharsis, Memnonia, Elysium, and Aeolis.

The origin of the formation is unknown, but many theories have been presented over the years. In 2020, a group of researchers headed by Peter Mouginis-Mark has hypothesized that the formation could have been formed from pumice rafts from the volcano Olympus Mons. In 2012, a group headed by Laura Kerber hypothesized that it could have been formed from ash from the volcanoes Apollinaris Mons, Arsia Mons, and possibly Pavonis Mons.

An analysis of data from the Mars Odyssey Neutron Spectrometer revealed that the western lobe of the Medusae Fossae Formation contains some water. This means that this formation contains bulk water ice. During periods of high obliquity (tilt) water ice was stable on the surface. By means of a re-analysis of data from Mars Express' MARSIS radar, Thomas Watters found evidence about the existence of large underground water deposits in Medusae Fossae up to 3.7 km thick and covered by hundreds of meters of dust.

Combining several gravity models of Mars with the MOLA topographic dataset allowed calculation of the density of the deposit; the value is 1.765 ± 0.105 g/cm³, similar to the density of terrestrial ignimbrites. This rules out significant amounts of ice in the bulk composition. In combination with the deposit's high content of sulfur and chlorine, it implies an explosive volcanic origin. The total volume of the deposit is 1.4 million km³; such a large deposit might have been emplaced in periodic eruptions over an interval of 500 million years.

In some places, the formation appears as a smooth and gently undulating surface, while in others 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. Further evidence for a fine-grained composition is the fact that the area gives almost no radar return.

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.