The moon is named afterDeimos, a figure representingdread inGreek mythology.[17] The name was suggested by academicHenry Madan, who drew from Book XV of theIliad, whereAres (Greek counterpart of the Roman godMars) summons Dread (Deimos) and Fear (Phobos).[18]
Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of thedwarf planet symbols, proposed a Greekdelta (the initial of Deimos) combined with Mars's spear as the symbol of Deimos (). This symbol is not widely used.[19]
The origin of Mars's moons is unknown and the hypotheses are controversial.[20] The main hypotheses are that they formed either bycapture or byaccretion.
Because of the postulated similarity to the composition ofC- or D-type asteroids, one hypothesis is that the moons may be objects captured into Martian orbit from theasteroid belt, with orbits that have been circularized either byatmospheric drag ortidal forces,[21] as capture requires dissipation of energy. The currentMartian atmosphere is too thin to capture a Phobos-sized object byatmospheric braking.[20]Geoffrey Landis has pointed out that the capture could have occurred if the original body was abinary asteroid that separated due to tidal forces.[22] The main alternative hypothesis is that the moons accreted in the present position. 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 aplanetesimal.[23][24]
In 2021, Amirhossein Bagheri (ETH Zurich), Amir Khan (ETH Zurich),Michael Efroimsky (US Naval Observatory) and their colleagues proposed a new hypothesis on the origin of the moons. By analyzing the seismic and orbital data from theMars InSight Mission and other missions, they proposed that the moons were 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.[25]
Size comparison between Phobos, Deimos and theMoon (right)
Deimos is a gray-colored body. Like most bodies of its size, Deimos is highly non-spherical with triaxial dimensions of 16.1 km × 11.8 km × 10.2 km (10.0 mi × 7.3 mi × 6.3 mi), corresponding to a mean diameter of 12.5 km (7.8 mi) which makes it about 57% the size of Phobos.[7] Deimos is composed of rock rich in carbonaceous material, much like C-typeasteroids andcarbonaceous chondrite meteorites.[26] It iscratered, but the surface is noticeably smoother than that of Phobos, caused by the partial filling of craters withregolith.[citation needed] The regolith is highlyporous and has a radar-estimated density of only1.471 g/cm3.[27]
Only two geological features on Deimos have been given names. ThecratersSwift andVoltaire are named after writers who speculated on the existence of two Martian moons before Phobos and Deimos were discovered.[30]
Deimos'sorbit is nearly circular and is close to Mars'sequatorial plane. Deimos is possibly an asteroid that was perturbed byJupiter into anorbit that allowed it to be captured by Mars, though this hypothesis is still controversial and disputed.[20] Both Deimos and Phobos have very circular orbits which lie almost exactly in Mars's equatorial plane, and hence a capture origin requires a mechanism for circularizing the initially highlyeccentric orbit, and adjusting its inclination into the equatorial plane, most likely by a combination ofatmospheric drag andtidal forces;[21] it is not clear that sufficient time was available for this to have occurred for Deimos.[20]
As seen from Mars, Deimos would have anangular diameter of no more than 2.5 minutes (sixty minutes make one degree), one twelfth of the width of theMoon as seen fromEarth, and would therefore appear almost star-like to the naked eye.[32] At its brightest ("full moon") it would be about as bright asVenus is from Earth; at the first- or third-quarter phase it would be about as bright asVega. With a smalltelescope, a Martian observer could see Deimos's phases, which take 1.2648[33] days (Deimos'ssynodic period) to run their course.[32]
Unlike Phobos, which orbits so fast that it rises in the west and sets in the east, Deimos rises in the east and sets in the west, slower than Mars's rotation speed. The Sun-synodic orbital period of Deimos of about 30.4 hours exceeds the Martian solar day ("sol") of about 24.7 hours by such a small amount that 2.48 days (2.41 sols) elapse between its rising and setting for an equatorial observer. From Deimos-rise to Deimos-rise (or setting to setting), 5.466 days (5.320 sols) elapse.[citation needed]
Because Deimos's orbit is relatively close to Mars and has only a very small inclination to Mars's equator, it cannot be seen from Martian latitudes greater than 82.7°.[34]
Deimos's orbit is slowly getting larger, because it is far enough away from Mars and because oftidal acceleration. It is expected to eventually escape Mars's gravity.[35]
Deimos regularlypasses in front of the Sun as seen from Mars. It is too small to cause atotal eclipse, appearing only as a small black dot moving across the Sun. Its angular diameter is only about 2.5 times the angular diameter of Venus during atransit of Venus from Earth. On 4 March 2004 a transit of Deimos was photographed by Mars roverOpportunity, and on 13 March 2004 a transit was photographed by Mars roverSpirit.[citation needed]
Deimos and Phobos as seen from Mars, compared inapparent size to theMoon as seen from Earth. If they would be as far away from Mars as the Moon from Earth, they would appear as faint star-like features in the Martian sky.
Overall, its exploration history is similar to thoseof Mars andof Phobos.[36] Deimos has been photographed close-up by several spacecraft whose primary mission has been to photograph Mars, including in March 2023 during a rare close encounter by theEmirates Mars Mission.[37] No landings on Deimos have been made.
In 1997 and 1998, the proposedAladdin mission was selected as a finalist in the NASADiscovery 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 (~1 km/s).[38] These samples would be returned to Earth for study three years later.[39][40] The principal investigator wasCarle M. Pieters ofBrown University. The total mission cost, including launch vehicle and operations was $247.7 million.[41] Ultimately, the mission chosen to fly wasMESSENGER, a probe to the planetMercury.[42]
In 2008, NASAGlenn Research Center began studying a Phobos and Deimossample-return mission that would use solar electric propulsion. The study gave rise to the "Hall" mission concept, aNew Frontiers-class mission currently under further study.[43]
Also, the sample-return mission calledGulliver has been conceptualized and dedicated to Deimos,[44] in which 1 kilogram (2.2 pounds) of material from Deimos would be returned to Earth.[44]
Another concept of sample-return mission from Phobos and Deimos isOSIRIS-REx 2, which would use heritage from the firstOSIRIS-REx.[45]
In March 2014, a Discovery class mission was proposed to place an orbiter in Mars orbit by 2021 and study Phobos and Deimos. It was calledPhobos And Deimos & Mars Environment (PADME).[46][47]
Human exploration of Deimos could serve as a catalyst for the human exploration of Mars. Recently, it was proposed that the sands of Deimos or Phobos could serve as a valuable material foraerobraking in the colonization of Mars.[48] SeePhobos for more detail.
In April 2023, astronomers released close-up global images, for the first time, of Deimos that were taken by the MarsHope orbiter.[49][50] Observations reported by this mission contravene the captured asteroid hypothesis and indicatebasaltic planetary origin of Deimos.[51]
During itsgravity assist from Mars en route to65803 Didymos, theESA'sHera took observations of Deimos in March 2025, approaching at a distance of 300 km (190 mi).[52]
The JAXAMMX Missionto Phobos and Deimos is planned for launch in October 2026.[53] It will make flybys of Deimos to investigate its composition and structure, as well as performing a sample returnon Phobos and placing a rover on that moon.
^Blunck, Jürgen (2009). "The Satellites of Mars; Discovering and Naming the Satellites".Solar System Moons: Discovery and Mythology.Springer. p. 5.ISBN978-3-540-68852-5.
^Hall, A.;Names of the Satellites of Mars, Astronomische Nachrichten, Vol. 92, No. 2187 (14 March 1878, signed 7 February 1878), p. 47/48
^Bala, Gavin Jared; Miller, Kirk (7 March 2025)."Phobos and Deimos symbols"(PDF).unicode.org. The Unicode Consortium. Retrieved14 March 2025.
^abcdBurns, J. A., "Contradictory Clues as to the Origin of the Martian Moons," inMars, H. H. Kiefferet al., eds., U. Arizona Press, Tucson, 1992
^Landis, G. A., "Origin of Martian Moons from Binary Asteroid Dissociation," American Association for the Advancement of Science Annual Meeting; Boston, MA, 2001;abstract.
^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
^Mars Phobos and Deimos Survey (M-PADS)–A Martian Moons Orbiter and Phobos Lander (Ball, Andrew J.; Price, Michael E.; Walker, Roger J.; Dando, Glyn C.; Wells, Nigel S; and Zarnecki, John C. (2009). Mars Phobos and Deimos Survey (M-PADS)–A Martian Moons Orbiter and Phobos Lander. Advances in Space Research, 43(1), pp. 120–127.)
^Lee, P. et al. 2010. Hall: A Phobos and Deimos Sample Return Mission.44th Lunar Planet. Sci. Conf., The Woodlands, TX. 1–5 Mar 2010. [#1633]Bibcode:2010LPI....41.1633L.
^Arias, Francisco. J (2017).On the Use of the Sands of Phobos and Deimos as a Braking Technique for Landing Large Payloads on Mars. 53rd AIAA/SAE/ASEE Joint Propulsion Conference. Atlanta, GA.doi:10.2514/6.2017-4876.ISBN978-1-62410-511-1. AIAA 201–4876.
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