The Medusae Fossae Formation is a large geological formation of probable volcanic origin on the planet

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...

. It is named for the

Medusa In Greek mythology, Medusa (; Ancient Greek: Μέδουσα "guardian, protectress"), also called Gorgo, was one of the three monstrous Gorgons, generally described as winged human females with living venomous snakes in place of hair. Those ...

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 The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can also ...

. Its roughly-shaped regions extend from just south of

Olympus Mons Olympus Mons (; Latin for Mount Olympus) is a large shield volcano on Mars. The volcano has a height of over 21.9 km (13.6 mi or 72,000 ft) as measured by the Mars Orbiter Laser Altimeter (MOLA). Olympus Mons is about two and a ha ...

Apollinaris Patera Apollinaris may refer to: Personal name * Apollinaris, a correspondent of Pliny the Younger (61–c. 112) * Apollinaris of Ravenna (flourished 1st or 2nd century), martyr and first bishop of Ravenna, the most prominent of several saints called ...

, with a smaller additional region closer to

Gale Crater Gale is a crater, and probable dry lake, at in the northwestern part of the Aeolis quadrangle on Mars. It is in diameter and estimated to be about 3.5–3.8 billion years old. The crater was named after Walter Frederick Gale, an amateur ast ...

. 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 Tharsis () is a vast volcanic plateau centered near the equator in the western hemisphere of Mars. The region is home to the largest volcanoes in the Solar System, including the three enormous shield volcanoes Arsia Mons, Pavonis Mons, and As ...

and

Elysium Elysium (, ), otherwise known as the Elysian Fields ( grc, Ἠλύσιον πεδίον, ''Ēlýsion pedíon'') or Elysian Plains, is a conception of the afterlife that developed over time and was maintained by some Greek religious and philos ...

volcanic areas, and extends across five quadrangles: Amazonis,

, Memnonia,

, and

Aeolis Aeolis (; grc, Αἰολίς, Aiolís), or Aeolia (; grc, Αἰολία, Aiolía, link=no), was an area that comprised the west and northwestern region of Asia Minor (modern-day Turkey), mostly along the coast, and also several offshore islan ...

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

. 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 Pavonis Mons (Latin for "peacock mountain") is a large shield volcano located in the Tharsis region of the planet Mars. It is the middle member of a chain of three volcanic mountains (collectively known as the Tharsis Montes) that straddle the ...

. 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 In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbi ...

(tilt) water ice was stable on the surface. 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

ignimbrite Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surro ...

s. 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.

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. 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. File:Medusae Fossae Remnant.jpg, Plateau and rootless cones (HiRISE) File:ESP 036510 1735layers.jpg, Mounds with layers, east of Gale Crater in

Aeolis quadrangle The Aeolis quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. The Aeolis quadrangle is also referred to as MC-23 (Mars Chart-23). The Aeolis quadrangle c ...

File:Wikiesp036510 1735layersmedussa.jpg, Layers and mounds, east of Gale Crater File:Wikiesp036510 1735moundsmedussa.jpg, Layers and a field of small mounds, east of Gale Crater File:Wikiesp036510 1735layersmound.jpg, Mound showing layers at the base, east of Gale Crater File:ESP 038302 1735medusaelayers.jpg, Surface features along a scarp in

Memnonia quadrangle The Memnonia quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. The Memnonia quadrangle is also referred to as MC-16 (Mars Chart-16). The quadrangle is ...

(HiRISE)

Inverted relief

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 Inverted relief, inverted topography, or topographic inversion refers to landscape features that have reversed their elevation relative to other features. It most often occurs when low areas of a landscape become filled with lava or sediment ...

. 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.

Yardangs and dust

Comparisons of elemental composition suggest that the Medusae Fossae Formation has been a source of Mars' ubiquitous surface dust. In July 2018, researchers reported that it may be the largest single source of

dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ...

on the planet. The surface of the formation has been eroded by the wind into a series of linear ridges called

yardang A yardang is a streamlined protuberance carved from bedrock or any consolidated or semiconsolidated material by the dual action of wind abrasion by dust and sand and deflation (the removal of loose material by wind turbulence.) Yardangs become e ...

s. These ridges generally point in direction of the

prevailing winds In meteorology, prevailing wind in a region of the Earth's surface is a surface wind that blows predominantly from a particular direction. The dominant winds are the trends in direction of wind with the highest speed over a particular point on ...

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 Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer ...

. Yardangs are parts of rock that have been sand blasted into long, skinny ridges by bouncing sand particles blowing in the wind. Layers are seen in parts of the formation. A resistant caprock on the top of yardangs has been observed in Viking,

Mars Global Surveyor ''Mars Global Surveyor'' (MGS) was an American robotic space probe developed by NASA's Jet Propulsion Laboratory and launched November 1996. MGS was a global mapping mission that examined the entire planet, from the ionosphere down through t ...

, and

HiRISE High Resolution Imaging Science Experiment is a camera on board the '' Mars Reconnaissance Orbiter'' which has been orbiting and studying Mars since 2006. The 65 kg (143 lb), US$40 million instrument was built under the direction ...

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 An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters ...

are visible throughout the area so the surface is relatively young. File:Medusae Fossae Formation.jpg, Yardangs in the Medusae Fossae (

THEMIS In Greek mythology and religion, Themis (; grc, Θέμις, Themis, justice, law, custom) is one of the twelve Titan children of Gaia and Uranus, and the second wife of Zeus. She is the goddess and personification of justice, divine order, fai ...

) File:Apollinarisandmedusae.JPG, Yardangs in

(

) File:WikiESP 039563 1730yardangs.jpg, Yardangs in

Arsinoes Chaos Arsinoes Chaos is a chaos terrain in the Margaritifer Sinus quadrangle on Mars. It is 200 km in diameter. Its location is 7.66 °S and 27.9 °W. Arsinoes Chaos was named after Arsinoe, a queen of ancient Egypt, daughter of Ptolemy and Berenice. ...

(HiRISE) File:WikiESP 039563 1730yardangsclose.jpg, Close-up, arrows point to transverse aeolian sand ridges (HiRISE) File:23664medussa.jpg, Yardangs in Amazonis (HiRISE) File:Yardangs in Medusae.jpg, Yardangs with caprock labeled, in

(HiRISE) File:WikiESP 035558 1830yardang.jpg, Yardangs near Gordii Dorsum, in the north of the formation (HiRISE) File:35558 1830yardangs.jpg, Yardangs near Gordii Dorsum (enlargement of the previous image) File:35558 1830yardangsclose.jpg, Yardangs near Gordii Dorsum (enlargement of the previous image) File:WikiESP 036429 1925yardangscrater.jpg, Yardangs near a crater in Amazonis, in the middle of the region File:Wikiesp036510 1735layersyardang.jpg, Yardangs showing layers, east of Gale Crater in

(HiRISE) File:46913 1800yardangs.jpg, Yardangs, in a crater in

Amazonis quadrangle The Amazonis quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. The Amazonis quadrangle is also referred to as MC-8 (Mars Chart-8). The quadrangle cov ...

(HiRISE)

External links

Graphical image of the geographical extent of the Medusae Fossae Formation
Nature Communications

References