The Medusae Fossae Formation is a large geological formation of probable volcanic origin on the planet
Mars.[1] 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.[2]
The total area of the formation is equal to 20% the size of the continental United States.[3] It is divided into three subunits (members) that are all considered to be of
Amazonian age, the youngest era in martian geological history.[4]
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.
Origin and age
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.[5] 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.[6]
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.[7] 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.[8]
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.[9] 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 × 106 km3; such a large deposit might have been emplaced in periodic eruptions over an interval of 500 million years.[9]
Appearance and composition
In some places, the formation appears as a smooth and gently undulating surface, while in others it is wind-sculpted into ridges and grooves.[1] 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.[10][11] 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.[12]
Yardangs and dust
Comparisons of elemental composition suggest that the Medusae Fossae Formation has been a source of Mars' ubiquitous surface dust.[3] In July 2018, researchers reported that it may be the largest single source of
dust on the planet.[3]
The surface of the formation has been eroded by the wind into a series of linear ridges called
yardangs.[13] 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. Yardangs are parts of rock that have been sand blasted into long, skinny ridges by bouncing sand particles blowing in the wind.[14] Layers are seen in parts of the formation. A resistant caprock on the top of yardangs has been observed in Viking,[15]Mars Global Surveyor,[16] and
HiRISE photos.[17] 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.[18]
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^Kerber, Laura; Head, James W.; Madeleine, Jean-Baptiste; Forget, François; Wilson, Lionel (2012). "The dispersal of pyroclasts from ancient explosive volcanoes on Mars: Implications for the friable layered deposits". Icarus. 219 (1): 358–381.
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^Wilson, Jack T.; Eke, Vincent R.; Massey, Richard J.; Elphic, Richard C.; Feldman, William C.; Maurice, Sylvestre; Teodoro, Luís F.A. (2018). "Equatorial locations of water on Mars: Improved resolution maps based on Mars Odyssey Neutron Spectrometer data". Icarus. 299: 148–160.
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