View source for Astronomy:Moons of Mars

{{short description|Natural satellites orbiting Mars}}
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| caption1 = Enhanced color image of [[Astronomy:Phobos (moon)|Phobos]] ([[Astronomy:Mars Reconnaissance Orbiter|MRO]], 23 March 2008)
| image2 = Deimos-MRO.jpg
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| caption2 = Enhanced color image of [[Astronomy:Deimos (moon)|Deimos]] ([[Astronomy:Mars Reconnaissance Orbiter|MRO]], 21 February 2009)
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The two '''moons of Mars''' are [[Astronomy:Phobos (moon)|Phobos]] and [[Astronomy:Deimos (moon)|Deimos]].<ref name="NYT-20200725">{{cite news |last=Andrews |first=Robin George |title=Why the 'Super Weird' Moons of Mars Fascinate Scientists - What's the big deal about little Phobos and tinier Deimos? |url=https://www.nytimes.com/2020/07/25/science/mars-moons-phobos-deimos.html |date=25 July 2020 |work={{wipe|The New York Times}} |access-date=25 July 2020 |archive-date=25 July 2020 |archive-url=https://web.archive.org/web/20200725094039/https://www.nytimes.com/2020/07/25/science/mars-moons-phobos-deimos.html |url-status=live }}</ref> They are irregular in shape.<ref>{{cite web |url=http://www.nasa.gov/exploration/whyweexplore/Why_We_27.html |title=NASA - Under the Moons of Mars |publisher=Nasa.gov |access-date=28 February 2013 |archive-date=29 March 2014 |archive-url=https://web.archive.org/web/20140329063957/http://www.nasa.gov/exploration/whyweexplore/Why_We_27.html |url-status=live }}</ref> Both were discovered by American astronomer Asaph Hall in August 1877<ref>{{cite book |chapter-url=http://www.uapress.arizona.edu/onlinebks/MARS/CHAP05.HTM |title=The Planet Mars: A History of Observation and Discovery |chapter=Chapter 5: 1877 |last = Sheehan |first = William |year=1996 |publisher=University of Arizona Press |bibcode=1996pmho.book.....S |access-date=28 February 2013 |archive-date=3 November 2017 |archive-url=https://web.archive.org/web/20171103164956/http://www.uapress.arizona.edu/onlinebks/MARS/CHAP05.HTM |url-status=dead }}</ref> and are named after the [[Religion:Greek mythology|Greek mythological]] twin characters [[Religion:Phobos (mythology)|Phobos]] (fear and panic) and [[Biography:Deimos (deity)|Deimos]] (terror and dread) who accompanied their father Ares into battle. Ares, the god of war, was known to the [[History:Ancient Rome|Romans]] as [[Religion:Mars (mythology)|Mars]].

Compared to the [[Astronomy:Earth|Earth]]'s [[Astronomy:Moon|Moon]], the moons Phobos and Deimos are small. Phobos has a diameter of 22.2&nbsp;km (13.8&nbsp;mi) and a mass of 1.08{{e|16}} kg, while Deimos measures 12.6&nbsp;km (7.8&nbsp;mi) across, with a mass of 2.0{{e|15}} kg. Phobos orbits closer to Mars, with a semi-major axis of {{convert|9377|km|mi|abbr=on}} and an orbital period of 7.66 hours; while Deimos orbits farther with a semi-major axis of {{convert|23460|km|mi|abbr=on}} and an orbital period of 30.35 hours.

== History ==

==={{anchor|Early speculation}}Early speculation===
[[File:PIA17352-MarsMoons-PhobosPassesDeimos-RealTime.gif|thumb|right|250px|''[[Engineering:Curiosity (rover)|Curiosity's]]'' view of the Martian moons: Phobos passing in front of Deimos – in ''real-time'' (video-gif, 1 August 2013)]]
Speculation about the existence of the moons of [[Astronomy:Mars|Mars]] had begun when the moons of [[Astronomy:Jupiter|Jupiter]] were discovered. When [[Biography:Galileo Galilei|Galileo Galilei]], as a hidden report about his having observed two bumps on the sides of [[Astronomy:Saturn|Saturn]] (later discovered to be its rings), used the anagram ''smaismrmilmepoetaleumibunenugttauiras'' for ''Altissimum planetam tergeminum observavi'' ("I have observed the most distant planet to have a triple form"), [[Biography:Johannes Kepler|Johannes Kepler]] had misinterpreted it to mean ''Salve umbistineum geminatum Martia proles'' (Hello, furious twins, sons of Mars).<ref>{{cite web|url=https://judgestarling.tumblr.com/post/62652246148/galileo-kepler-two-anagrams-two-wrong-solutions|title=Galileo, Kepler, & Two Anagrams: Two Wrong Solutions Turn Into Two Correct Solutions|website=Judge Starling|access-date=16 January 2023}}</ref>

{{anchor|Jonathan Swift}}
Perhaps inspired by Kepler (and quoting [[Kepler's laws of planetary motion|Kepler's third law of planetary motion]]), Jonathan Swift's satire ''Gulliver's Travels'' (1726) refers to two moons in Part 3, Chapter 3 (the "Voyage to [[Physics:Laputa|Laputa]]"), in which Laputa's astronomers are described as having discovered two satellites of Mars orbiting at distances of 3 and 5 Martian diameters with periods of 10 and 21.5 hours. Phobos and Deimos (both found in 1877, more than a century after Swift's novel) have actual orbital distances of 1.4 and 3.5 Martian diameters, and their respective orbital periods are 7.66 and 30.35 hours.<ref name=galileo>{{cite web|url=http://www.mathpages.com/home/kmath151/kmath151.htm|title=Galileo's Anagrams and the Moons of Mars|website=www.mathpages.com|access-date=29 October 2018|archive-date=12 June 2018|archive-url=https://web.archive.org/web/20180612142432/http://www.mathpages.com/home/kmath151/kmath151.htm|url-status=live}}</ref><ref name="ESA-Phobos"/> In the 20th century, V. G. Perminov, a spacecraft designer of early Soviet Mars and [[Astronomy:Venus|Venus]] spacecraft, speculated Swift found and deciphered records that Martians left on Earth.<ref>{{cite report |author=V. G. Perminov |date=1999 |title=The Difficult Road to Mars |url=https://archive.org/details/difficultroadtom00perm |publisher=NASA |docket=NP-1999-06-251-HQ |access-date=29 October 2018 |isbn=0-16-058859-6 |url-access=registration }}</ref> However, the view of most astronomers is that Swift was simply employing a common argument of the time, that as the inner planets Venus and Mercury had no satellites, Earth had one and Jupiter had four (known at the time), that Mars by analogy must have two. Furthermore, as they had not yet been discovered, it was reasoned that they must be small and close to Mars. This would lead Swift to make a roughly accurate estimate of their orbital distances and revolution periods. In addition, Swift could have been helped in his calculations by his friend, the mathematician John Arbuthnot.<ref>{{Cite journal |bibcode = 1925PA.....33..496L|title = The moons of Mars|last1 = Lamont|first1 = Roscoe|journal = Popular Astronomy|volume = 33|pages = 496|year = 1925}}</ref> 
 
[[Biography:Voltaire|Voltaire]]'s 1752 short story "Micromégas", about an alien visitor to Earth, also refers to two moons of Mars. Voltaire was presumably influenced by Swift.<ref>{{Cite book| edition = 2nd| publisher = University of Arizona Press| isbn = 9780816516414| last = Sheehan| first = William| title = The Planet Mars: A History of Observation and Discovery| location = Tucson| date = 1 September 1996| url-access = registration| url = https://archive.org/details/planetmarshistor00shee}}</ref><ref>Voltaire explained that since Mars is further from the Sun than Earth is, it could not make do with less than two moons. (Patrick Moore, 2000, ''The Wandering Astronomer'')</ref> In recognition of these 'predictions', two craters on Deimos are named Swift and Voltaire,<ref>{{gpn|5789|Swift}}</ref><ref>{{gpn|6431|Voltaire}}</ref> while on Phobos there is one named [[Earth:Planetary geology#Regio (astronomy)|regio]], ''Laputa Regio'', and one named planitia, ''Lagado Planitia'', both of which are named after places in ''Gulliver's Travels'' (the fictional [[Physics:Laputa|Laputa]], a flying island, and Lagado, imaginary capital of the fictional nation Balnibarbi).<ref>{{cite encyclopedia |entry-url=http://planetarynames.wr.usgs.gov/Page/PHOBOS/target |entry=Target: Phobos |encyclopedia=Gazetteer of Planetary Nomenclature |publisher=USGS Astrogeology Research Program}}</ref> Many of the [[Astronomy:Phobos (moon)#Craters on Phobos|craters on Phobos]] are also named after characters in ''Gulliver's Travels''.<ref name=PhobosCraters01a>{{cite encyclopedia |entry-url=https://planetarynames.wr.usgs.gov/SearchResults?Target=18_Phobos&Feature%20Type=9_Crater,%20craters |entry=Phobos Craters |encyclopedia=Gazetteer of Planetary Nomenclature |publisher=USGS Astrogeology Research Program |access-date=16 January 2023}}</ref>

==={{anchor|Discovery}}Discovery===
[[File:HallPortrait.jpg|left|thumb|upright|Asaph Hall III, discoverer of Phobos and Deimos]][[File:Usno-telescope-equalized-1.png|thumb|right|300px|The telescope used by Asaph Hall in the discovery of the Martian moons]]
Asaph Hall discovered Deimos on 12 August 1877 at about 07:48 [[Physics:Coordinated Universal Time|UTC]] and Phobos on 18 August 1877, at the US Naval Observatory (the Old Naval Observatory in Foggy Bottom) in [[Place:Washington, D.C.|Washington, D.C.]], at about 09:14 GMT (contemporary sources, using the pre-1925 [[Physics:Astronomical day|astronomical convention]] that began the day at noon,<ref name='Campbell "Beginning"'>{{cite journal|last=Campbell|first=W.W.|title=The Beginning of the Astronomical Day|volume=30|issue=178|pages=358|journal= Publications of the Astronomical Society of the Pacific|bibcode=1918PASP...30..358C|year=1918|doi=10.1086/122784|doi-access=free}}</ref> give the time of discovery as 11 August 14:40 and 17 August 16:06 Washington mean time respectively).<ref>{{Cite journal |bibcode=1877Obs.....1..181.|title=Notes|journal=The Observatory|volume=1|pages=181|year=1877}}</ref><ref>{{Cite journal|bibcode=1877AN.....91...11H|title=Observations of the Satellites of Mars|last1=Hall|first1=Asaph|journal=Astronomische Nachrichten|volume=91|issue=1|pages=11–14|year=1877|doi=10.1002/asna.18780910103|url=https://zenodo.org/record/1424707|access-date=1 July 2021|archive-date=1 October 2021|archive-url=https://web.archive.org/web/20211001044436/https://zenodo.org/record/1424707|url-status=live}}</ref><ref name="Morley1989">{{Cite journal |bibcode=1989A&AS...77..209M|title=A catalogue of ground-based astrometric observations of the Martian satellites, 1877-1982|last1=Morley|first1=T. A.|journal=Astronomy and Astrophysics Supplement Series|volume=77|issue=2|pages=209|year=1989}}</ref> At the time, he was deliberately searching for Martian moons. Hall had previously seen what appeared to be a Martian moon on 10 August, but due to bad weather, he could not definitively identify them until later.

Hall recorded his discovery of Phobos in his notebook as follows:<ref>{{Cite journal|title=Notes on some Points connected with the Progress of Astronomy during the Past Year|journal=Monthly Notices of the Royal Astronomical Society|volume=38|issue=4|page=206|date=8 February 1878|doi=10.1093/mnras/38.4.190}}</ref>
{{quote|
: "I repeated the examination in the early part of the night of 11th [August 1877], and again found nothing, but trying again some hours later I found a faint object on the following side and a little north of the planet. I had barely time to secure an observation of its position when fog from the River stopped the work. This was at half-past two o'clock on the night of the 11th. Cloudy weather intervened for several days.
:"On 15 August the weather looking more promising, I slept at the Observatory. The sky cleared off with a thunderstorm at 11 o'clock and the search was resumed. The atmosphere however was in a very bad condition and Mars was so blazing and unsteady that nothing could be seen of the object, which we now know was at that time so near the planet as to be invisible.
: "On 16 August the object was found again on the following side of the planet, and the observations of that night showed that it was moving with the planet and if a satellite, was near one of its elongations. Until this time I had said nothing to anyone at the Observatory of my search for a satellite of Mars, but on leaving the observatory after these observations of the 16th, at about three o'clock in the morning, I told my assistant, George Anderson, to whom I had shown the object, that I thought I had discovered a satellite of Mars. I told him also to keep quiet as I did not wish anything said until the matter was beyond doubt. He said nothing, but the thing was too good to keep and I let it out myself. On 17 August between one and two o'clock, while I was reducing my observations, Professor Newcomb came into my room to eat his lunch and I showed him my measures of the faint object near Mars which proved that it was moving with the planet.
:"On 17 August while waiting and watching for the outer moon, the inner one was discovered. The observations of the 17th and 18th put beyond doubt the character of these objects and the discovery was publicly announced by Admiral Rodgers."
|}}

The telescope used for the discovery was the 26-inch (66&nbsp;cm) refractor (telescope with a lens) then located at Foggy Bottom.<ref>{{cite web|url=http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/scopes/naval/index.php|title=Telescope: Naval Observatory 26-inch Refractor|website=amazing-space.stsci.edu|access-date=29 October 2018|archive-date=18 October 2013|archive-url=https://archive.today/2013.10.18-184452/http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/scopes/naval/index.php|url-status=live}}</ref> In 1893 the lens was remounted and put in a new dome, where it remains into the 21st century.<ref>{{cite web |title=The 26-inch "Great Equatorial" Refractor |url=https://www.usno.navy.mil/USNO/about-us/usnos-telescopes/the-26-inch-refractor |website=United States Naval Observatory |access-date=29 October 2018 |archive-date=29 October 2018 |archive-url=https://web.archive.org/web/20181029232436/https://www.usno.navy.mil/USNO/about-us/usnos-telescopes/the-26-inch-refractor |url-status=live }}</ref>

The names, originally spelled ''Phobus'' and ''Deimus'', respectively, were suggested by Henry Madan (1838&ndash;1901), Science Master of [[Organization:Eton College|Eton]], from Book XV of the ''[[Religion:Iliad|Iliad]]'', where Ares summons Fear and Fright.<ref>{{Cite journal|bibcode=1878AN.....92...47K|title=Entdeckung zweier Planeten|last1=Knorre|first1=V.|journal=Astronomische Nachrichten|volume=92|issue=3|pages=47–48|year=1878|doi=10.1002/asna.18780920305|url=https://zenodo.org/record/1424711|access-date=1 July 2021|archive-date=1 October 2021|archive-url=https://web.archive.org/web/20211001044437/https://zenodo.org/record/1424711|url-status=live}}</ref> The granddaughter of Henry Madan's brother Falconer Madan was [[Biography:Venetia Burney|Venetia Burney]], who first suggested the name of [[Astronomy:Pluto|Pluto]].

===Mars moon hoax===

In 1959, Walter Scott Houston perpetrated a celebrated April Fool's hoax in the April edition of the ''Great Plains Observer'', claiming that "Dr. Arthur Hayall of the University of the Sierras reports that the moons of Mars are actually artificial satellites". Both Dr. Hayall and the University of the Sierras were fictitious. The hoax gained worldwide attention when Houston's claim was repeated in earnest by a Soviet scientist, [[Biography:Iosif Shklovsky|Iosif Shklovsky]],<ref>'' Jefferson City Post-Tribune'' 4 May 1959</ref> who, based on a later-disproven density estimate, [[Astronomy:Phobos (moon)#Shklovsky's "Hollow Phobos" hypothesis|suggested Phobos was a hollow metal shell]].

===Recent surveys===

Searches have been conducted for additional satellites. In 2003, [[Biography:Scott S. Sheppard|Scott S. Sheppard]] and [[Biography:David C. Jewitt|David C. Jewitt]] surveyed nearly the entire [[Astronomy:Hill sphere|Hill sphere]] of Mars for irregular satellites. However scattered light from Mars prevented them from searching the inner few arcminutes where the satellites Phobos and Deimos reside. No new satellites were found to an apparent limiting red magnitude of 23.5, which corresponds to radii of about 0.09&nbsp;km using an albedo of 0.07.<ref>{{Cite journal| doi = 10.1086/424541| issn = 1538-3881| volume = 128| issue = 5| pages = 2542–2546| last1 = Sheppard| first1 = Scott S.| last2 = Jewitt| first2 = David| last3 = Kleyna| first3 = Jan| title = A Survey for Outer Satellites of Mars: Limits to Completeness| journal = The Astronomical Journal| date = November 2004| arxiv = astro-ph/0409522| bibcode = 2004AJ....128.2542S| s2cid = 45681283}}</ref>

==Characteristics==
<!---
[[File:Phobos & Deimos full thumb.png|thumb|left|Night sky of Mars showing Deimos (left) and Phobos (right) in front of [[Astronomy:Sagittarius (constellation)|Sagittarius]], as viewed by the Mars ''Spirit'' rover (26 August 2005).]]
--->
[[File:PIA17351-ApparentSizes-MarsDeimosPhobos-EarthMoon.jpg|thumb|right|250px|Apparent sizes of the moons of Mars, Deimos and Phobos, and the Moon as viewed from the ''surface'' of their respective planets (Mars' moons imaged by the ''Curiosity'' rover, 1 August 2013)]]
[[File:Moon_Phobos_Deimos.png|left|thumb|Size comparison between Phobos, Deimos and the Moon (right)]]
If viewed from Mars's surface near its equator, a full Phobos would look about one-third as big as a full moon on Earth. It has an [[Astronomy:Angular diameter|angular diameter]] of between 8' (rising) and 12' (overhead). Due to its close orbit, it would look smaller when the observer is further away from the Martian equator until it completely sinks below the horizon as the observer travels closer to the poles; thus Phobos is not visible from Mars's polar ice caps. Deimos would look more like a bright star or planet (only slightly bigger than how [[Astronomy:Venus|Venus]] looks from Earth) for an observer on Mars. It has an angular diameter of about 2'. The Sun's angular diameter as seen from Mars, by contrast, is about 21'. Thus there are no total [[Earth:Solar eclipse|solar eclipse]]s on Mars as the moons are far too small to completely cover the Sun. On the other hand, total [[Astronomy:Lunar eclipse|lunar eclipse]]s of Phobos happen almost every night.<ref>{{cite web|url=http://www.astrobio.net/news/index.php?name=News&file=article&sid=775|title=Astrobiology Exclusive News You Can't Afford to Miss - Astrobio.net|website=Astrobiology Magazine|access-date=29 October 2018|archive-date=20 February 2021|archive-url=https://web.archive.org/web/20210220082209/https://www.astrobio.net/news-briefs/?file=article&sid=775|url-status=usurped}}</ref>

The motions of Phobos and Deimos would appear very different from that of Earth's Moon. Speedy Phobos rises in the west, sets in the east, and rises again in just eleven hours, while Deimos, being only just outside [[Astronomy:Synchronous orbit|synchronous orbit]], rises as expected in the east but very slowly. Despite its 30-hour orbit, it takes 2.7 days to set in the west as it slowly falls behind the rotation of Mars.

Both moons are [[Astronomy:Tidal locking|tidally locked]], always presenting the same face towards Mars. Since Phobos orbits Mars faster than the planet itself rotates, tidal forces are slowly but steadily decreasing its orbital radius. At some point in the future, when it falls within the [[Physics:Roche limit|Roche limit]], Phobos will be broken up by these tidal forces and either crash into Mars or form a ring.<ref>{{Cite APOD |date=14 April 2008 |title=Phobos: Doomed Moon of Mars |quote=In 100 million years or so Phobos will likely be shattered by stress caused by the relentless tidal forces, the debris forming a decaying ring around Mars. |access-date=}}</ref><ref>{{Cite web|last=December 2017|first=Nola Taylor Redd 08|title=Phobos: Facts About the Doomed Martian Moon|url=https://www.space.com/20346-phobos-moon.html|access-date=13 June 2020|website=Space.com|date=8 December 2017|language=en|archive-date=19 March 2018|archive-url=https://web.archive.org/web/20180319103530/https://www.space.com/20346-phobos-moon.html|url-status=live}}</ref> Several strings of craters on the Martian surface, inclined further from the equator the older they are, suggest that there may have been other small moons that suffered the fate expected of Phobos, and that the Martian crust as a whole shifted between these events.<ref>{{cite web|url=http://www.nasa.gov/centers/goddard/news/topstory/2005/mgs_plates.html|title=NASA - New Map Provides More Evidence Mars Once Like Earth|website=www.nasa.gov|access-date=29 October 2018|archive-date=14 September 2012|archive-url=https://archive.today/2012.09.14-153601/http://www.nasa.gov/centers/goddard/news/topstory/2005/mgs_plates.html|url-status=live}}</ref> Deimos, on the other hand, is far enough away that its orbit is being slowly boosted instead,<ref>{{cite web|url=http://www.edb.utexas.edu/missiontomars/pdf/MarsSSBody.pdf|title=Mission To Mars - Project-based Learning|website=www.edb.utexas.edu|access-date=29 October 2018|archive-url=https://web.archive.org/web/20110617121245/http://www.edb.utexas.edu/missiontomars/pdf/MarsSSBody.pdf|archive-date=17 June 2011|url-status=dead}}</ref> akin to Earth's Moon.
{{Clear}}

===Orbital details===
{| class="wikitable" style="margin:1em auto;"
|- style="background:#efefef;"
! Label <br /> {{refn | group = note | Label refers to the [[Roman numerals|Roman numeral]] attributed to each moon in order of their discovery.<ref>{{Cite encyclopedia|entry=Planet and Satellite Names and Discoverers|encyclopedia=Gazetteer of Planetary Nomenclature|publisher=USGS Astrogeology Research Program|entry-url=https://planetarynames.wr.usgs.gov/Page/Planets|access-date=23 June 2022}}</ref>}}
! colspan=2|Name and pronunciation
! Image
! Diameter (km)
! Surface<br />area (km<sup>2</sup>)
! Mass (kg)
! Semi-major<br />axis (km)
! Orbital<br />period (h)
! Average moonrise<br />period (h, d)
! Eccentricity
! Inclination (°){{refn | group = note | Orbital inclinations are given to Mars's equator.}}
|-
| I
| [[Astronomy:Phobos (moon)|Phobos]]
| {{IPAc-en|ˈ|f|oʊ|b|ə|s}}<br />{{respell|FOH|bəs}}
| bgcolor=black| [[File:Phobos moon (large).jpg|50px|center]]
| {{convert|22.2|km|mi|abbr=on}}(27×21.6×18.8&nbsp;km)
|1,548&nbsp;km<sup>2</sup>
| 10.7{{e|15}}
| {{convert|9,377|km|mi|abbr=on}}
| 7.66
| 11.12 h (0.463 d)
| 0.0151
| 1.093
|-
| II
| [[Astronomy:Deimos (moon)|Deimos]]
| {{IPAc-en|ˈ|d|aɪ|m|ə|s}}<br />{{respell|DY|məss}}
| bgcolor=black| [[File:Deimos-viking1.jpg|50px|center]]
| {{convert|12.6|km|mi|abbr=on}}<br />(10×12×16&nbsp;km)
|483&nbsp;km<sup>2</sup>
| 1.5{{e|15}}
| {{convert|23,460|km|mi|abbr=on}}
| 30.31
| 131 h (5.44 d)
| 0.00033
| 0.93
|-
| colspan="12" |[[File:Mars Moons Orbit distance.jpeg|700px|center]]The relative sizes of and distance between Mars, Phobos, and Deimos, to scale
''(Load the image in full size to see both Moons of Mars.)''
|}

==Origin==
[[File:MoonsOfMarsImproved3.gif|thumb|left|200px|Animation illustrating the asteroid-belt origin for the moons]]
The origin of the Martian moons is still controversial.<ref name="burns">Burns, J. A. "Contradictory Clues as to the Origin of the Martian Moons," in ''Mars'', H. H. Kieffer ''et al.,'' eds., U. Arizona Press, Tucson, 1992</ref> Phobos and Deimos both have much in common with carbonaceous [[Astronomy:C-type asteroid|C-type asteroid]]s, with [[Physics:Electromagnetic spectrum|spectra]], [[Earth:Albedo|albedo]], and [[Physics:Density|density]] very similar to those of C- or D-type asteroids.<ref>{{Cite web|url=http://www.physorg.com/news115483748.html|title=New Views of Martian Moons|access-date=2 April 2011|archive-date=14 November 2011|archive-url=https://web.archive.org/web/20111114083834/http://www.physorg.com/news115483748.html|url-status=live}}</ref> Based on their similarity, one hypothesis is that both moons may be captured main-belt asteroids.<ref name="ESA-Phobos">{{Cite web|title=Close Inspection for Phobos|url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31031|quote=One idea is that Phobos and Deimos, Mars's other moon, are captured asteroids.|access-date=2 April 2011|archive-date=14 January 2012|archive-url=https://web.archive.org/web/20120114161949/http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31031|url-status=live}}</ref><ref name="landis">{{cite conference |last=Landis |first=G. A. |title=Origin of Martian Moons from Binary Asteroid Dissociation |conference=American Association for the Advancement of Science Annual Meeting |location=Boston, MA |date=2002 |arxiv=0903.3434 }}</ref> Both moons have very circular orbits which lie almost exactly in Mars's equatorial plane, and hence a capture origin requires a mechanism for [[Tidal circularization|circularizing]] the initially highly eccentric orbit, and adjusting its inclination into the equatorial plane, most probably by a combination of atmospheric drag and [[Physics:Tidal force|tidal force]]s,<ref name="cazenave">{{Cite journal|last=Cazenave |first=A. |author2=Dobrovolskis, A. |author3=Lago, B. |date=1980|title=Orbital history of the Martian satellites with inferences on their origin|journal=Icarus|volume=44|issue=3|pages=730–744|doi=10.1016/0019-1035(80)90140-2|bibcode = 1980Icar...44..730C }}</ref> although it is not clear that sufficient time is available for this to occur for Deimos.<ref name="burns"/> Capture also requires dissipation of energy. The current [[Astronomy:Atmosphere of Mars|atmosphere of Mars]] is too thin to capture a Phobos-sized object by atmospheric braking.<ref name="burns"/> Geoffrey Landis has pointed out that the capture could have occurred if the original body was a [[Astronomy:Binary asteroid|binary asteroid]] that separated under tidal forces.<ref name= "landis"/>

Phobos could be a second-generation Solar System object that [[Physics:Accretion (astrophysics)|coalesced]] in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as Mars.<ref name=ESA2010>{{Cite web|date=4 March 2010|title=Phobos Flyby Success|publisher=ESA|author=Martin Pätzold|author2=Olivier Witasse|name-list-style=amp|url=http://www.esa.int/esaSC/SEMIPX6K56G_index_0.html|access-date=4 March 2010|archive-date=7 March 2010|archive-url=https://web.archive.org/web/20100307015221/http://www.esa.int/esaSC/SEMIPX6K56G_index_0.html|url-status=live}}</ref>

Another hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large [[Astronomy:Planetesimal|planetesimal]].<ref name = "Craddock">Craddock, R. A.; (1994); ''The Origin of Phobos and Deimos'', Abstracts of the 25th Annual Lunar and Planetary Science Conference, held in Houston, TX, 14–18 March 1994, p. 293</ref> The high porosity of the interior of Phobos (based on the density of 1.88 g/cm<sup>3</sup>, voids are estimated to comprise 25 to 35 percent of Phobos' volume) is inconsistent with an asteroidal origin.<ref name = "Andert">{{Cite journal | last = Andert | first = T. P. | display-authors = 4 | author2 = Rosenblatt, P. | author3 = Pätzold, M. | author4 = Häusler, B. | author5 = Dehant, V. | author6 = Tyler, G. L. | author7 = Marty, J. C. | title = Precise mass determination and the nature of Phobos | journal = [[Earth:Geophysical Research Letters|Geophysical Research Letters]] | volume = 37 | issue = L09202 | publisher = [[Organization:American Geophysical Union|American Geophysical Union]] | date = 7 May 2010 | pages = n/a | url = http://www.agu.org/pubs/crossref/2010/2009GL041829.shtml | doi = 10.1029/2009GL041829 | access-date = 1 October 2010 | bibcode = 2010GeoRL..37.9202A | s2cid = 129002457 | archive-date = 26 June 2010 | archive-url = https://web.archive.org/web/20100626020003/http://www.agu.org/pubs/crossref/2010/2009GL041829.shtml | url-status = live | doi-access = free }}</ref> Observations of Phobos in the thermal infrared suggest a composition containing mainly [[Chemistry:Silicate minerals#Phyllosilicates|phyllosilicates]], which are well known from the surface of Mars. The spectra are distinct from those of all classes of [[Astronomy:Chondrite|chondrite]] meteorites, again pointing away from an asteroidal origin.<ref name = "Giuranna">{{Cite conference |first=M. |last=Giuranna |display-authors=4 |author2=Roush, T. L. |author3=Duxbury, T. |author4=Hogan, R. C. |author5=Geminale, A. |author6=Formisano |title=Compositional Interpretation of PFS/MEx and TES/MGS Thermal Infrared Spectra of Phobos |book-title=European Planetary Science Congress Abstracts, Vol. 5 |date=2010 |url=http://meetingorganizer.copernicus.org/EPSC2010/EPSC2010-211.pdf |access-date=1 October 2010 |conference= |archive-date=12 May 2011 |archive-url=https://web.archive.org/web/20110512174947/http://meetingorganizer.copernicus.org/EPSC2010/EPSC2010-211.pdf |url-status=live }}</ref> Both sets of findings support an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit,<ref name = "Blast">{{Cite web | title = Mars Moon Phobos Likely Forged by Catastrophic Blast | work = www.space.com | publisher = Space.com | date = 27 September 2010 | url = http://www.space.com/scienceastronomy/martian-moon-forged-by-catastrophic-blast-100927.html | access-date = 1 October 2010 | archive-date = 30 September 2010 | archive-url = https://web.archive.org/web/20100930013107/http://www.space.com/scienceastronomy/martian-moon-forged-by-catastrophic-blast-100927.html | url-status = live }}</ref> similar to the prevailing theory for the origin of Earth's moon.

The moons of Mars may have started with a huge collision with a protoplanet one third the mass of Mars that formed a ring around Mars. The inner part of the ring formed a large moon. Gravitational interactions between this moon and the outer ring formed Phobos and Deimos. Later, the large moon crashed into Mars, but the two small moons remained in orbit. This theory agrees with the fine-grained surface of the moons and their high porosity. The outer disk would create fine-grained material.<ref>{{Cite journal| doi=10.1038/ngeo2742| bibcode=2016NatGe...9..581R| title=Accretion of Phobos and Deimos in an extended debris disc stirred by transient moons| year=2016| last1=Rosenblatt| first1=Pascal| last2=Charnoz| first2=Sébastien| last3=Dunseath| first3=Kevin M.| last4=Terao-Dunseath| first4=Mariko| last5=Trinh| first5=Antony| last6=Hyodo| first6=Ryuki| last7=Genda| first7=Hidenori| last8=Toupin| first8=Stéven| journal=Nature Geoscience| volume=9| issue=8| pages=581–583| s2cid=133174714| url=https://hal.archives-ouvertes.fr/hal-01350105/file/Letter.pdf| access-date=29 February 2020| archive-date=9 March 2020| archive-url=https://web.archive.org/web/20200309100516/https://hal.archives-ouvertes.fr/hal-01350105/file/Letter.pdf| url-status=live}}</ref><ref>{{cite web |title=A giant impact: Solving the mystery of how Mars' moons formed |url=https://www.sciencedaily.com/releases/2016/07/160704144236.htm |website=ScienceDaily |access-date=1 August 2021 |date=4 July 2016 |archive-date=15 February 2021 |archive-url=https://web.archive.org/web/20210215005540/https://www.sciencedaily.com/releases/2016/07/160704144236.htm |url-status=live }}</ref> Simulations suggest the object colliding with Mars had to be within the size range of [[Astronomy:Ceres (dwarf planet)|Ceres]] and [[Astronomy:4 Vesta|Vesta]] because a larger impact would have created a more massive disc and moons that would have prevented the survival of tiny moons like Phobos and Deimos.<ref>{{Cite web |url=https://www.space.com/40344-crash-could-form-mars-moons.html |title=Giant Impact May Have Created Mars Moons {{!}} Space |website=Space.com |date=18 April 2018 |access-date=26 January 2020 |archive-date=26 January 2020 |archive-url=https://web.archive.org/web/20200126155716/https://www.space.com/40344-crash-could-form-mars-moons.html |url-status=live }}</ref>

Most recently, Amirhossein Bagheri and his colleagues from [[Organization:ETH Zurich|ETH Zurich]] and US Naval Observatory, proposed a new hypothesis on the origin of the moons. By analyzing the seismic and orbital data from [[Astronomy:InSight|Mars InSight Mission]] and other missions, they proposed that the moons are born from the disruption of a common parent body around 1 to 2.7 billion years ago. The common progenitor of Phobos and Deimos was most probably hit by another object and shattered to form Phobos and Deimos.<ref>{{Cite journal|last1=Bagheri|first1=Amirhossein|last2=Khan|first2=Amir|last3=Efroimsky|first3=Michael|last4=Kruglyakov|first4=Mikhail|last5=Giardini|first5=Domenico|date=22 February 2021|title=Dynamical evidence for Phobos and Deimos as remnants of a disrupted common progenitor|url=https://www.nature.com/articles/s41550-021-01306-2|journal=Nature Astronomy|volume=5|issue=6|language=en|pages=539–543|doi=10.1038/s41550-021-01306-2|bibcode=2021NatAs...5..539B|s2cid=233924981|issn=2397-3366|access-date=8 April 2021|archive-date=5 March 2021|archive-url=https://web.archive.org/web/20210305183459/https://www.nature.com/articles/s41550-021-01306-2|url-status=live}}</ref> But a recent paper suggests that it seems unlikely that Phobos and Deimos are split directly from a single ancestral moon.<ref>{{Cite journal |last1=Hyodo |first1=Ryuki |last2=Genda |first2=Hidenori |last3=Sekiguchi |first3=Ryosuke |last4=Madeira |first4=Gustavo |last5=Charnoz |first5=Sébastien |date=1 August 2022 |title=Challenges in Forming Phobos and Deimos Directly from a Splitting of an Ancestral Single Moon |journal=The Planetary Science Journal |language=en |volume=3 |issue=8 |pages=204 |doi=10.3847/psj/ac88d2 |arxiv=2208.04794 |bibcode=2022PSJ.....3..204H |s2cid=251442453 |issn=2632-3338 |doi-access=free }}</ref> They use N-body simulations to show that the single ancestral moon scenario should result in an impact between the two moons, leading to a debris ring in 10<sup>4</sup> years.
{{Clear}}

==Exploration==

=== Past attempts and proposals ===
While many Martian probes provided images and other data about Phobos and Deimos, only few were dedicated to these satellites and intended to perform a flyby or landing on the surface.

Two probes under the Soviet [[Astronomy:Phobos program|Phobos program]] were successfully launched in 1988, but neither conducted the intended jumping landings on Phobos and Deimos due to failures (although ''[[Astronomy:Phobos 2|Phobos 2]]'' successfully photographed Phobos). The post-Soviet Russian ''[[Astronomy:Fobos-Grunt|Fobos-Grunt]]'' probe was intended to be the first sample return mission from Phobos, but a rocket failure left it stranded in Earth orbit in 2011. Efforts to reactivate the craft were unsuccessful, and it fell back to Earth in an uncontrolled [[Astronomy:Atmospheric entry|re-entry]] on 15 January 2012, over the [[Earth:Pacific Ocean|Pacific Ocean]], {{wipe|Chile}}.<ref>{{cite news |url=https://www.bbc.co.uk/news/science-environment-16491457 |title=Russia's failed Phobos-Grunt space probe heads to Earth |website=BBC News |date=14 January 2012}}</ref><ref name="foxnews.com">{{cite news|url=http://www.foxnews.com/scitech/2012/01/15/russian-space-probe-to-crash-on-earth-within-hours/|publisher=Fox News Channel|title=Russian space probe crashes into Pacific Ocean|date=15 January 2012}}</ref><ref>{{cite news |url=http://www.nbcnews.com/id/46027901 |title=Russia asks if US radar ruined Phobos-Grunt space probe |website=NBC News |date=17 January 2012}}</ref>

In 1997 and 1998, the Aladdin mission was selected as a finalist in the NASA [[Astronomy:Discovery Program|Discovery Program]]. The plan was to visit both Phobos and Deimos, and launch projectiles at the satellites. The probe would collect the ejecta as it performed a slow flyby.{{cn|date=May 2022}} These samples would be returned to Earth for study three years later. Ultimately, NASA rejected this proposal in favor of ''[[Astronomy:MESSENGER|MESSENGER]]'', a probe to Mercury.

In 2007, the [[Organization:European Space Agency|European Space Agency]] and EADS Astrium proposed and developed a mission to Phobos in 2016 with a lander and sample return, but this mission was never flown. [[Organization:Canadian Space Agency|Canadian Space Agency]] has been considering the Phobos Reconnaissance and International Mars Exploration (PRIME) mission to Phobos with orbiter and lander since 2007. Since 2013 NASA developed the [[Astronomy:Phobos Surveyor|Phobos Surveyor]] mission concept with an orbiter and a small rover.<ref name=pandika-2012>{{cite news |url=http://news.stanford.edu/news/2012/december/rover-mars-phobos-122812.html | title=Stanford researchers develop acrobatic space rovers to explore moons and asteroids | work=Stanford Report | date=28 December 2012 | agency=Stanford News Service | access-date=3 January 2013 | author=Pandika, Melissa | location=Stanford, California}}</ref><ref name=tarantola-2013>{{cite news |url=https://gizmodo.com/5972507/nasas-developing-robotic-hedgehogs-to-explore-a-martian-moon | title=NASA's Developing Robotic "Hedgehogs" to Explore a Maritan Moon | work=Gizmodo | date=2 January 2013 | agency=Gizmodo | access-date=3 January 2013 | author=Tarantola, Andrew}}</ref> NASA's PADME mission was designed to conduct multiple flybys of the Martian moons, but was not chosen for development.<ref>{{cite news |url=https://www.nasa.gov/press-release/nasa-selects-two-missions-to-explore-the-early-solar-system/|title=NASA Selects Two Missions to Explore the Early Solar System|date=4 January 2017 |access-date=19 September 2017}}</ref> Also, NASA is assessing the [[Astronomy:OSIRIS-REx#OSIRIS-REx II|OSIRIS-REx II]], concept mission for a sample return from Phobos.<ref>{{Cite journal |bibcode = 2012LPICo1679.4017E|title = OSIRIS-REx II to Mars — Mars Sample Return from Phobos and Deimos|last1 = Elifritz|first1 = T. L.|journal = Concepts and Approaches for Mars Exploration|volume = 1679|pages = 4017|year = 2012}}</ref> Another sample return mission from Deimos, called ''Gulliver'', has been conceptualized.<ref>{{cite journal|url=https://ui.adsabs.harvard.edu/abs/2004cosp...35.3897B/abstract|title=The Gulliver sample return mission to Deimos|journal=35th Cospar Scientific Assembly|bibcode=2004cosp...35.3897B|access-date=3 September 2020|last1=Britt|first1=D. T.|last2=Robinson|first2=M.|author3=Gulliver Team|year=2004|volume=35|page=3897|archive-date=1 October 2021|archive-url=https://web.archive.org/web/20211001044436/https://ui.adsabs.harvard.edu/abs/2004cosp...35.3897B/abstract|url-status=live}}</ref>

=== Current proposals ===
[[Organization:JAXA|JAXA]] plans to launch [[Astronomy:Martian Moons eXploration (MMX)|Martian Moons eXploration (MMX)]] mission in 2024 to bring back the first samples from Phobos.<ref name='Homepage'>{{cite web |url=https://www.mmx.jaxa.jp/en/ |title=MMX - Martian Moons eXploration |website=MMX at JAXA}}</ref><ref>{{cite web|url=http://www.japantimes.co.jp/news/2015/06/10/national/science-health/jaxa-plans-probe-bring-back-samples-martian-moons/ |title=JAXA plans probe to bring back samples from moons of Mars|date=10 June 2015|newspaper=The Japan Times}}</ref> The spacecraft will enter orbit around Mars, then transfer to Phobos,<ref name='S Clark'>{{cite web |url=https://spaceflightnow.com/2017/11/20/nasa-confirms-contribution-to-japanese-led-mars-mission/ |title=NASA confirms contribution to Japanese-led Mars mission |first=Stephen |last=Clark |website=Spaceflight Now |date=20 November 2017}}</ref> and land once or twice and gather sand-like regolith particles using a simple [[Engineering:Pneumatics|pneumatic]] system.<ref name='CNES col'>{{cite web |url=https://mmx-news.isas.jaxa.jp/?p=432&lang=en |title=How to find the best samples on a moon: Building relationships and solving engineering challenges in France |website=JAXA News |date=4 December 2017}}</ref> The lander mission aims to retrieve a minimum {{cvt|10|g}} of samples.<ref name='sample mass'>{{cite web |url=https://mmx-news.isas.jaxa.jp/?p=331&lang=en |title=Gravity both too strong and too weak: landing on the Martian moons |website=JAXA News |date=31 August 2017}}</ref><ref name="fujimoto20170111">{{cite web |url=http://www.lpi.usra.edu/sbag/meetings/jan2017/presentations/Fujimoto.pdf|title=JAXA's exploration of the two moons of Mars, with sample return from Phobos|last1=Fujimoto|first1=Masaki|publisher=Lunar and Planetary Institute|date=11 January 2017|access-date=23 March 2017}}</ref> The spacecraft will then take off from Phobos and make several flybys of the smaller moon Deimos before sending the Return Module back to [[Astronomy:Earth|Earth]], arriving in July 2029.<ref name='S Clark'/><ref name='Homepage'/>

==Gallery==
<gallery class="center">
Image:Phobos.jpg|Phobos, with Stickney Crater on the right (2003).
Image:Phobosmgs.gif|Phobos (1998).<ref>{{cite web|url=http://www.jpl.nasa.gov/releases/98/mgsphobos.html|title=Martian moon Phobos hip-deep in powder|website=JPL|date=September 11, 1998|first=Diane|last=Ainsworth|access-date=29 October 2018|archive-date=12 December 2019|archive-url=https://web.archive.org/web/20191212095012/https://www.jpl.nasa.gov/releases/98/mgsphobos.html|url-status=live}}</ref>
Image:Phobos deimos diff.jpg|Comparison - [[Astronomy:Phobos (moon)|Phobos]] (top) and [[Astronomy:Deimos (moon)|Deimos]] (bottom) (2005).
</gallery>
[[File:PIA19396-PlanetMars-OrbitsOfMoonsAndOrbiters-20150504.jpg|thumb|center|600px|{{center|Orbits of moons and spacecraft orbiting Mars.<ref name="NASA-20150504-GW">{{cite web |last=Webster |first=Guy |title=Traffic Around Mars Gets Busy |url=http://www.jpl.nasa.gov/news/news.php?feature=4572 |date=4 May 2015 |work=[[Organization:NASA|NASA]] |access-date=5 May 2015 |archive-date=6 May 2015 |archive-url=https://web.archive.org/web/20150506163802/http://www.jpl.nasa.gov/news/news.php?feature=4572 |url-status=live }}</ref>}}]]

==See also==
* {{annotated link|Astronomy:Mars trojan}}
* [[Astronomy:Satellites of Mars|Satellites of Mars]]
* {{annotated link|Astronomy:Transit of Deimos from Mars}} 
* {{annotated link|Astronomy:Transit of Phobos from Mars}}

==Notes==
{{Reflist|group=note}}

==References==
{{Reflist|30em}}

==Further reading==
*[http://www.lpi.usra.edu/meetings/phobosdeimos2007/phobosdeimos2007.authorindex.shtml First International Conference On The Exploration Of Phobos And Deimos (2007)]
**[http://www.lpi.usra.edu/meetings/phobosdeimos2007/pdf/7044.pdf Pascal Lee – ''A Case for the Human Exploration of the Moons of Mars'' (2007)] (pdf)

==External links==
*[http://planetarynames.wr.usgs.gov/jsp/SystemSearch2.jsp?System=Mars Gazetteer of Planetary Nomenclature – Mars (USGS)]
*[https://thehappykoala.github.io/Harmony-of-the-Spheres/#/scenario/The%20Sun%20and%20the%20Martian%20System Interactive 3D visualisation of the orbits of Mars and its moons Phobos and Deimos]

{{Mars}}
{{Solar System moons (compact)}}
{{Solar System}}
{{DEFAULTSORT:Moons Of Mars}}

[[Category:Moons of Mars| ]]
[[Category:Lists of moons]]

{{Sourceattribution|Moons of Mars}}