{{Short description|Large geological unit of uncertain origin on Mars}} {{Infobox feature on celestial object |name = Medusa Fossae |image = Medusae Fossae based on day THEMIS.png |caption = Part of Medusae Fossae, from a [[Thermal Emission Imaging System|THEMIS]] day-time image |coordinates = {{coord|3.2|S|163.0|W|globe:mars_type:landmark|display=inline,title}} |length = 5,000 km }}

The '''Medusae Fossae Formation''' is a large geological formation of probable volcanic origin on the planet [[Mars]].<ref name=esa-mff>{{cite web|title=The Medusa Fossae formation on Mars|url=http://www.esa.int/Our_Activities/Space_Science/Mars_Express/The_Medusa_Fossae_formation_on_Mars/(print)|publisher=[[European Space Agency]]|date=29 March 2005}}</ref> 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&nbsp;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]].<ref>{{Cite journal |author=Lujendra Ojha |author2=Kevin Lewis |author3=Suniti Karunatillake |author4=Mariek Schmidt|date=July 20, 2018|title=Fig. 1, The Medusae Fossae Formation as the single largest source of dust on Mars|url=https://www.nature.com/articles/s41467-018-05291-5/figures/1|journal=Nature Communications|language=en|issn=2041-1723}}</ref>

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.<ref>Geologic context of the Mars radar “Stealth” region in southwestern Tharsis Kenneth S. Edgett, Bryan J. Butler, James R. Zimbelman, Victoria E. Hamilton. 1997 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. E9, PAGES 21,545-21,567. https://doi.org/10.1029/97JE01685</ref>

The total area of the formation is equal to 20% the size of the continental United States.<ref name="pmid30030425">{{cite journal |doi=10.1038/s41467-018-05291-5 |pmid=30030425 |pmc=6054634 |title=The Medusae Fossae Formation as the single largest source of dust on Mars |journal=Nature Communications |volume=9 |issue=1 |pages=2867 |year=2018 |last1=Ojha |first1=Lujendra |last2=Lewis |first2=Kevin |last3=Karunatillake |first3=Suniti |last4=Schmidt |first4=Mariek |bibcode=2018NatCo...9.2867O }}</ref> It is divided into three subunits (members) that are all considered to be of [[Amazonian (Mars)|Amazonian]] age, the youngest era in martian geological history.<ref>{{cite report |doi=10.3133/i1802B |citeseerx=10.1.1.457.7583 |title=Geologic map of the eastern equatorial region of Mars |first1=Ronald |last1=Greeley |first2=J.E. |last2=Guest |year=1987 |website=U.S. Geological Survey}}</ref> The formation straddles the [[martian dichotomy|highland - lowland boundary]] near the [[Tharsis]] and [[Elysium (volcanic province)|Elysium]] volcanic areas, and extends across five quadrangles: [[Amazonis quadrangle|Amazonis]], [[Tharsis quadrangle|Tharsis]], [[Memnonia quadrangle|Memnonia]], [[Elysium quadrangle|Elysium]], and [[Aeolis quadrangle|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]].<ref>{{Cite web|title=Scientists Float a New Theory on the Medusae Fossae Formation|url=https://eos.org/articles/scientists-float-a-new-theory-on-the-medusae-fossae-formation|access-date=2021-06-26|website=Eos|date=19 May 2020 |language=en-US}}</ref> 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]].<ref>{{cite journal |doi=10.1016/j.icarus.2012.03.016 |title=The dispersal of pyroclasts from ancient explosive volcanoes on Mars: Implications for the friable layered deposits |journal=Icarus |volume=219 |issue=1 |pages=358–381 |year=2012 |last1=Kerber |first1=Laura |last2=Head |first2=James W. |last3=Madeleine |first3=Jean-Baptiste |last4=Forget |first4=François |last5=Wilson |first5=Lionel |bibcode=2012Icar..219..358K }}</ref>

An analysis of data from the [[2001 Mars Odyssey|Mars Odyssey]] [[Neutron spectrometer|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.<ref>{{cite journal |doi=10.1016/j.icarus.2017.07.028 |title=Equatorial locations of water on Mars: Improved resolution maps based on Mars Odyssey Neutron Spectrometer data |journal=Icarus |volume=299 |pages=148–160 |year=2018 |last1=Wilson |first1=Jack T. |last2=Eke |first2=Vincent R. |last3=Massey |first3=Richard J. |last4=Elphic |first4=Richard C. |last5=Feldman |first5=William C. |last6=Maurice |first6=Sylvestre |last7=Teodoro |first7=Luís F.A. |bibcode=2018Icar..299..148W |arxiv=1708.00518 |s2cid=59520156 }}</ref> 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&nbsp;km thick and covered by hundreds of meters of dust.<ref>{{Cite web|url=https://www.esa.int/Science_Exploration/Space_Science/Buried_water_ice_at_Mars_s_equator|access-date=19 January 2024|website=[[European Space Agency]]|date=18 January 2024|title=Buried water ice at Mars's equator?}}</ref>

Combining several gravity models of Mars with the [[Mars Orbiter Laser Altimeter|MOLA]] topographic dataset allowed calculation of the density of the deposit; the value is 1.765 ± 0.105 g/cm<sup>3</sup>, similar to the density of terrestrial [[ignimbrite]]s.<ref name="Ojha2018a" /> 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 km<sup>3</sup>; such a large deposit might have been emplaced in periodic eruptions over an interval of 500 million years.<ref name="Ojha2018a">{{cite journal |doi=10.1029/2018JE005565 |title=The Density of the Medusae Fossae Formation: Implications for its Composition, Origin, and Importance in Martian History |journal=Journal of Geophysical Research: Planets |volume=123 |issue=6 |pages=1368–1379 |year=2018 |last1=Ojha |first1=Lujendra |last2=Lewis |first2=Kevin |bibcode=2018JGRE..123.1368O |doi-access=free }}</ref>

== 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.<ref name="esa-mff" /> 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.<ref>{{cite journal|last1=Watters|first1=T. R.|last2=Campbell|first2=B.|last3=Carter|first3=L.|author3-link=Lynn Carter|last4=Leuschen|first4=C. J.|last5=Plaut|first5=J. J.|last6=Picardi|first6=G.|last7=Orosei|first7=R.|last8=Safaeinili|first8=A.|last9=Clifford|first9=S. M.|last10=Farrell|first10=W. M.|last11=Ivanov|first11=A. B.|year=2007|title=Radar Sounding of the Medusae Fossae Formation Mars: Equatorial Ice or Dry, Low-Density Deposits?|journal=Science|volume=318|issue=5853|pages=1125–1128|bibcode=2007Sci...318.1125W|doi=10.1126/science.1148112|pmid=17975034 |last12=Phillips|first12=R. J.|last13=Stofan|first13=E. R.|s2cid=25050428 |doi-access=free}} *{{cite magazine |author=David Shiga |date=November 1, 2007 |title=Vast amount of water ice may lie on Martian equator |magazine=NewScientist |url=https://www.newscientist.com/article/dn12866-vast-amount-of-water-ice-may-lie-on-martian-equator/ |url-access=registration}}</ref><ref>{{cite journal|last1=Orosei|first1=R.|last2=Cantini|first2=F.|last3=Caprarelli|first3=G.|last4=Carter|first4=L. M.|author4-link=Lynn Carter|last5=Papiano|first5=I.|last6=Rossi|first6=A. P.|year=2016|title=Radar Sounding by MARSIS over Lucus Planum, Mars|journal=Lunar and Planetary Science Conference|issue=1903|pages=1869|bibcode=2016LPI....47.1869O}}</ref> Further evidence for a fine-grained composition is the fact that the area gives almost no radar return.

<gallery class="center" widths="380px" heights="360px"> 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]] 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]] (HiRISE) </gallery>

===Inverted relief=== [[File:Sinuous Ridges in Medusae Formation.jpg|Sinuous Ridges within a branching fan, in [[Aeolis quadrangle]] ([[HiRISE]])|thumb|right]]

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.<ref>{{cite journal |doi=10.1016/j.icarus.2009.04.003 |title=HiRISE images of yardangs and sinuous ridges in the lower member of the Medusae Fossae Formation, Mars |journal=Icarus |volume=205 |issue=1 |pages=198–210 |year=2010 |last1=Zimbelman |first1=James R. |last2=Griffin |first2=Lora J. |bibcode=2010Icar..205..198Z }}</ref>

===Yardangs and dust=== Comparisons of elemental composition suggest that the Medusae Fossae Formation has been a source of Mars' ubiquitous surface dust.<ref name="pmid30030425" /> In July 2018, researchers reported that it may be the largest single source of [[Martian soil#Atmospheric dust|dust]] on the planet.<ref name="pmid30030425" />

The surface of the formation has been eroded by the wind into a series of linear ridges called [[yardang]]s.<ref>{{cite book |doi=10.2110/pec.12.102.0169 |chapter=Duststones on Mars: Source, Transport, Deposition, and Erosion |title=Sedimentary Geology of Mars |pages=169–182 |year=2012 |last1=Bridges |first1=Nathan T. |last2=Muhs |first2=Daniel R. |isbn=978-1-56576-312-8 }}</ref> 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.<ref>{{cite web | url=http://www.uahirise.org/ESP_039563_1730 | title=HiRISE &#124; Yardangs in Arsinoes Chaos (ESP_039563_1730) }}</ref> Layers are seen in parts of the formation. A resistant caprock on the top of yardangs has been observed in Viking,<ref>{{cite journal |doi=10.1029/JB087iB02p01179 |title=Ignimbrites of Amazonis Planitia Region of Mars |journal=Journal of Geophysical Research: Solid Earth |volume=87 |pages=1179–1190 |year=1982 |last1=Scott |first1=David H. |last2=Tanaka |first2=Kenneth L. |bibcode=1982JGR....87.1179S }}</ref> [[Mars Global Surveyor]],<ref>{{cite journal |doi=10.1126/science.279.5357.1681 |title=Early Views of the Martian Surface from the Mars Orbiter Camera of Mars Global Surveyor |journal=Science |volume=279 |issue=5357 |pages=1681–1685 |year=1998 |last1=Malin |first1=M. C. |last2=Carr |first2=M. H. |last3=Danielson |first3=G. E. |last4=Davies |first4=M. E. |last5=Hartmann |first5=W. K. |last6=Ingersoll |first6=A. P. |last7=James |first7=P. B. |last8=Masursky |first8=H. |last9=McEwen |first9=A. S. |last10=Soderblom |first10=L. A. |last11=Thomas |first11=P. |last12=Veverka |first12=J. |last13=Caplinger |first13=M. A. |last14=Ravine |first14=M. A. |last15=Soulanille |first15=T. A. |last16=Warr En |first16=J. L. |bibcode=1998Sci...279.1681M |pmid=9497280 |doi-access= }}</ref> and [[HiRISE]] photos.<ref>{{cite journal |doi=10.1029/2008JE003076 |title=Origin of the Medusae Fossae Formation, Mars: Insights from a synoptic approach |journal=Journal of Geophysical Research |volume=113 |issue=E12 |pages=E12011 |year=2008 |last1=Mandt |first1=Kathleen E. |last2=De Silva |first2=Shanaka L. |last3=Zimbelman |first3=James R. |last4=Crown |first4=David A. |bibcode=2008JGRE..11312011M |doi-access=free }}</ref> 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 (geology)|cementation]]. Very few [[impact craters]] are visible throughout the area so the surface is relatively young.<ref>{{cite web | url=http://themis.asu.edu/zoom-20020416a | title=Medusae Fossae Formation &#124; Mars Odyssey Mission THEMIS }}</ref>

<gallery class="center" widths="380px" heights="360px"> File:Medusae Fossae Formation.jpg|Yardangs in the Medusae Fossae ([[THEMIS]]) File:Apollinarisandmedusae.JPG|Yardangs in [[Aeolis quadrangle|Aeolis]] ([[HiRISE]]) File:WikiESP 039563 1730yardangs.jpg|Yardangs in [[Arsinoes Chaos]] (HiRISE) File:WikiESP 039563 1730yardangsclose.jpg|Close-up, arrows point to [[transverse aeolian ridges|transverse aeolian sand ridges]] (HiRISE) File:23664medussa.jpg|Yardangs in [[Amazonis quadrangle|Amazonis]] (HiRISE) File:Yardangs in Medusae.jpg|Yardangs with caprock labeled, in [[Aeolis quadrangle|Aeolis]] (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:WikiESP 036429 1925yardangscrater.jpg|Yardangs near a crater in [[Amazonis quadrangle|Amazonis]], in the middle of the region File:Wikiesp036510 1735layersyardang.jpg|Yardangs showing layers, east of Gale Crater in [[Aeolis quadrangle|Aeolis]] (HiRISE) File:46913 1800yardangs.jpg|Yardangs, in a crater in [[Amazonis quadrangle]] (HiRISE) </gallery>

==Evidence of Water== Scientists are excited about a possible “oasis” of bulk ice in the equatorial region. Having a source of ice near the equator could make it easier for future human exploration. Landings near the equator need less fuel than at higher latitudes. We know that Mars has much frozen ground, but at some distance from the equator. Explosive volcanic eruptions can propel large pulses of water vapor from the volcano to higher levels of the atmosphere. These eruptions could deposit an ash-ice mixture, or a layer of ice covered in ash. Under certain conditions the ice may be preserved for long periods.<ref>Ayris, P. M. & Delmelle, P. The immediate environmental effects of tephra emission. Bull. Volcanol. 74, 1905–1936 (2012).</ref> Eruptions from Apollinaris Mons lead to dispersal ash and ice deposits forming around the Medussae Fossae Formation. Eruptions from Syrtis Major can also deposit material here. The water detected by orbiting instruments could be found in many different materials. Some are (1) adsorbed water onto regolith particles,<ref name="Feldman, W. C. 2004">Feldman, W. C. et al. Global distribution of near-surface hydrogen on Mars. J. Geophys. Res.: Planets 109, E09006 (2004)</ref><ref name="Malakhov, A. V. 2020">Malakhov, A. V. et al. Ice permafrost ‘“oases”’ close to Martian equator: planet neutron mapping based on data of FREND instrument onboard TGO orbiter of Russian-European ExoMars mission. Astron. Lett. 46, 407–421 (2020)</ref> (2) water incorporated into the mineral's crystal structure (i.e., hydrated minerals),<ref name="Malakhov, A. V. 2020"/> (3) OH and H<sub>2</sub>O located in the structure of salt hydrates,<ref>Basilevsky, A. T. et al. Search for traces of chemically bound water in the martian surface layer based on HEND measurements onboard the 2001 Mars Odyssey spacecraft. Sol. Syst. Res. 37, 387–396 (2003).</ref> (4) small amounts of water ice in the pores between regolith particles,<ref name="Malakhov, A. V. 2020"/> (5) hydrous alteration in an aqueous environment,<ref>Hood, D. R. et al. Contrasting regional soil alteration across the topographic dichotomy of Mars. Geophys. Res. Lett. 46, 13,668–13,677 (2019).</ref> (6) sulfate hydration in the shallow subsurface,<ref>Karunatillake, S. et al. Sulfates hydrating bulk soil in the Martian low and middle latitudes. Geophys. Res. Lett. 41, 7987–7996 (2014)</ref> (7) OH that is part of the structure of clays and trapped water between clay layers<ref name="Feldman, W. C. 2004"/> and/or (8) water interacting with cations located in the pores of zeolite mineral structure.<ref>Feldman, W. C. et al. Global distribution of near-surface hydrogen on Mars. J. Geophys. Res.: Planets 109, E09006 (2 004).</ref><ref>Hamid, S.S., Kerber, L. & Clarke, A.B. Precipitation induced by explosive volcanism on Mars and its implications for unexpected equatorial ice. Nat Commun 16, 8923 (2025). https://doi.org/10.1038/s41467-025-63518-8</ref>

==See also== * [[Aeolis quadrangle]] * [[Amazonis Planitia]] * [[Amazonis quadrangle]] * [[Geology of Mars]] * [[Groundwater on Mars]] * [[Impact crater]] * [[Yardangs on Mars]]

== External links == * [https://www.nature.com/articles/s41467-018-05291-5/figures/1 Graphical image of the geographical extent of the Medusae Fossae Formation], Nature Communications

== References ==

{{Reflist|30em}}

{{Geography of Mars}} {{Portal bar|Solar System}}

[[Category:Surface features of Mars]] [[Category:Amazonis quadrangle]] [[Category:Valleys and canyons on Mars]]