Enhanced color image ofPhobos (MRO, 23 March 2008)
Enhanced color image ofDeimos (MRO, 21 February 2009)
The twomoons ofMars arePhobos andDeimos.[1] They are irregular in shape.[2] Both were discovered by American astronomerAsaph Hall in August 1877[3] and are named after theGreek mythological twin charactersPhobos (fear and panic) andDeimos (terror and dread) who accompanied their fatherAres (Mars in Roman mythology, hence the name of the planet) into battle.
Compared to theEarth'sMoon, the moons Phobos and Deimos are very small. Phobos has a diameter of 22.2 km (13.8 mi) and a mass of 1.08×1016 kg, while Deimos measures 12.6 km (7.8 mi) across, with a mass of 1.5×1015 kg. Phobos orbits closer to Mars, with asemi-major axis of 9,377 km (5,827 mi) and an orbital period of 7.66 hours; while Deimos orbits farther with a semi-major axis of 23,460 km (14,580 mi) and an orbital period of 30.35 hours.
Two major hypotheses have emerged as to the origin of the moons: The first suggests that they originated from Mars itself, perhaps from a giantimpact event suggested to have created theMartian dichotomy and theBorealis Basin. The second suggests that they are captured asteroids. Both hypotheses are compatible with current data, though upcoming sample return missions may be able to distinguish which hypothesis is correct.[4]
Speculation about the existence of the moons ofMars had begun when the moons ofJupiter were discovered. WhenGalileo Galilei (1564–1642), as a hidden report about his having observed two bumps on the sides ofSaturn (later discovered to be its rings), used the anagramsmaismrmilmepoetaleumibunenugttauiras forAltissimum planetam tergeminum observavi ("I have observed the most distant planet to have a triple form"),Johannes Kepler (1571–1630) had misinterpreted it to meanSalve umbistineum geminatum Martia proles (Hello, furious twins, sons of Mars).[5]
Perhaps inspired by Kepler (and quotingKepler's third law of planetary motion),Jonathan Swift's satireGulliver's Travels (1726) refers to two moons in Part 3, Chapter 3 (the "Voyage toLaputa"), 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.[6][7] In the 20th century, V. G. Perminov, a spacecraft designer of early Soviet Mars andVenus spacecraft, speculated that Swift found and deciphered records that Martians left on Earth.[8] 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 mathematicianJohn Arbuthnot.[9]
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.[10][11] In recognition of these literary references, two craters on Deimos are namedSwift andVoltaire,[12][13] while on Phobos there is one namedregio,Laputa Regio, and one namedplanitia,Lagado Planitia, both of which are named after places inGulliver's Travels (the fictionalLaputa, a flying island, andLagado, imaginary capital of the fictional nationBalnibarbi).[14] Many of thecraters on Phobos are also named after characters inGulliver's Travels.[15]
Asaph Hall III, discoverer of Phobos and DeimosThe telescope used byAsaph Hall in the discovery of the Martian moons
Asaph Hall discovered Deimos on 12 August 1877 at about 07:48UTC and Phobos on 18 August 1877, at theUS Naval Observatory (theOld Naval Observatory in Foggy Bottom) inWashington, D.C., at about 09:14GMT (contemporary sources, using the pre-1925astronomical convention that began the day at noon,[16] give the time of discovery as 11 August 14:40 and 17 August 16:06Washington mean time respectively).[17][18][19] 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:[20]
"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 cm)refractor (telescope with a lens) then located at Foggy Bottom.[21] In 1893 the lens was remounted and put in a new dome, where it remains into the 21st century.[22]
The names, originally spelledPhobus andDeimus, respectively, were suggested byHenry Madan (1838–1901), Science Master ofEton, from Book XV of theIliad, whereAres summons Fear and Fright.[23] The granddaughter of Henry Madan's brotherFalconer Madan wasVenetia Burney, who first suggested the name ofPluto.
In 1959,Walter Scott Houston perpetrated a celebratedApril Fool's hoax in the April edition of theGreat 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,Iosif Shklovsky,[24] who, based on a later-disproven density estimate,suggested Phobos was a hollow metal shell.
Searches have been conducted for additional satellites. In 2003,Scott S. Sheppard andDavid C. Jewitt surveyed nearly the entireHill sphere of Mars forirregular 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 km using an albedo of 0.07.[25]
Apparent sizes of the moons of Mars,Deimos andPhobos, and theMoon as viewed from thesurface of their respective planets (Mars's moons imaged by theCuriosity rover, 1 August 2013)Size comparison between Phobos, Deimos and theMoon (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 anangular 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 howVenus 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 totalsolar eclipses on Mars as the moons are far too small to completely cover the Sun. On the other hand, totallunar eclipses of Phobos happen almost every night.[26]
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 outsidesynchronous 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 aretidally 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 theRoche limit, Phobos will be broken up by these tidal forces and either crash into Mars or form a ring.[27][28] 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.[29] Deimos, on the other hand, is far enough away that its orbit is being slowly boosted instead,[30] akin to Earth's Moon.
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.)
March 5, 2024: NASA released images of transits of the moonDeimos, the moonPhobos and the planetMercury as viewed by thePerseverance rover on the planet Mars.
Transits viewed from Mars by thePerseverance rover
Animation illustrating the asteroid-belt origin for the moons
The origin of the Martian moons is still controversial.[32] Phobos and Deimos both have much in common with carbonaceousC-type asteroids, withspectra,albedo, anddensity very similar to those of C- or D-type asteroids.[33] Based on their similarity, one hypothesis is that both moons may be capturedmain-belt asteroids.[7][34] Both moons have very circular orbits which lie almost exactly in Mars'sequatorial plane, and hence a capture origin requires a mechanism forcircularizing the initially highly eccentric orbit, and adjusting its inclination into the equatorial plane, most probably by a combination of atmospheric drag andtidal forces,[35] although it is not clear that sufficient time is available for this to occur for Deimos.[32] Capture also requires dissipation of energy. The currentatmosphere of Mars is too thin to capture a Phobos-sized object by atmospheric braking.[32]Geoffrey Landis has pointed out that the capture could have occurred if the original body was abinary asteroid that separated under tidal forces.[34]
Phobos could be a second-generation Solar System object thatcoalesced in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as Mars.[36]
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 largeplanetesimal.[37] The high porosity of the interior of Phobos (based on the density of 1.88 g/cm3, voids are estimated to comprise 25 to 35 percent of Phobos's volume) is inconsistent with an asteroidal origin.[38] Observations of Phobos in thethermal infrared suggest a composition containing mainlyphyllosilicates, which are well known from the surface of Mars. The spectra are distinct from those of all classes ofchondrite meteorites, again pointing away from an asteroidal origin.[39] Both sets of findings support an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit,[40] similar to theprevailing 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.[41][42] Simulations suggest the object colliding with Mars had to be within the size range ofCeres andVesta 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.[43]
Most recently, Amirhossein Bagheri and his colleagues fromETH Zurich andUS Naval Observatory, proposed a new hypothesis on the origin of the moons. By analyzing the seismic and orbital data fromMars 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.[44] But a recent paper suggests that it seems unlikely that Phobos and Deimos are split directly from a single ancestral moon.[45] 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 104 years.
Another suggestion is that Mars was hit by an object from beyond the orbit of Saturn or Neptune, about 3% the mass of the planet and consisting of at least 30% and up to 70% water ice. This would create a disc around the planet with large amounts of water that cooled it down and changed the chemical composition of the rocks, likely producing a type of minerals calledphyllosilicates.[46]
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 SovietPhobos program were successfully launched in 1988, but neither conducted the intended jumping landings on Phobos and Deimos due to failures (althoughPhobos 2 successfully photographed Phobos). The post-Soviet RussianFobos-Grunt probe was intended to be the firstsample 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 uncontrolledre-entry on 15 January 2012, over thePacific Ocean,west of Chile.[47][48][49]
In 1997 and 1998, the Aladdin 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. These samples would be returned to Earth for study three years later. Ultimately, NASA rejected this proposal in favor ofMESSENGER, a probe to Mercury.[50]
In 2007, theEuropean Space Agency andEADS Astrium proposed and developed a mission to Phobos in 2016 with a lander and sample return, but this mission was never flown.[citation needed]Canadian Space Agency has been considering the Phobos Reconnaissance and International Mars Exploration (PRIME) mission to Phobos with orbiter and lander since 2007.[citation needed] Since 2013 NASA developed thePhobos Surveyor mission concept with an orbiter and a small rover.[51][52] NASA'sPADME mission was designed to conduct multiple flybys of the Martian moons, but was not chosen for development.[53] Also, NASA assessed theOSIRIS-REx II, concept mission for a sample return from Phobos.[54] Another sample return mission from Deimos, calledGulliver, has been conceptualized.[55]
JAXA plans to launchMartian Moons eXploration (MMX) mission in 2026 to bring back the first samples from Phobos.[56][57] The spacecraft will enter orbit around Mars, then transfer to Phobos,[58] and land once or twice and gather sand-like regolith particles using a simplepneumatic system.[59] The lander mission aims to retrieve a minimum 10 g (0.35 oz) of samples.[60][61] The spacecraft will then take off from Phobos and make several flybys of the smaller moon Deimos before sending the Return Module back toEarth, arriving in July 2029.[58][56]
^Sheehan, William (1996)."Chapter 5: 1877".The Planet Mars: A History of Observation and Discovery. University of Arizona Press.Bibcode:1996pmho.book.....S. Archived fromthe original on 3 November 2017. Retrieved28 February 2013.
^ab"Close Inspection for Phobos".Archived from the original on 14 January 2012. Retrieved2 April 2011.One idea is that Phobos and Deimos, Mars's other moon, are captured asteroids.
^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)
^"Swift".Gazetteer of Planetary Nomenclature. USGS Astrogeology Research Program.
^"Voltaire".Gazetteer of Planetary Nomenclature. USGS Astrogeology Research Program.
^"Target: Phobos".Gazetteer of Planetary Nomenclature. USGS Astrogeology Research Program.
^"Phobos Craters".Gazetteer of Planetary Nomenclature. USGS Astrogeology Research Program. Retrieved16 January 2023.
^Morley, T. A. (1989). "A catalogue of ground-based astrometric observations of the Martian satellites, 1877-1982".Astronomy and Astrophysics Supplement Series.77 (2): 209.Bibcode:1989A&AS...77..209M.
^Nemiroff, R.; Bonnell, J., eds. (14 April 2008)."Phobos: Doomed Moon of Mars".Astronomy Picture of the Day.NASA.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.
^abLandis, G. A. (2002).Origin of Martian Moons from Binary Asteroid Dissociation. American Association for the Advancement of Science Annual Meeting. Boston, MA.arXiv:0903.3434.
^Martin Pätzold & Olivier Witasse (4 March 2010)."Phobos Flyby Success". ESA.Archived from the original on 7 March 2010. Retrieved4 March 2010.
^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
^Elifritz, T. L. (2012). "OSIRIS-REx II to Mars — Mars Sample Return from Phobos and Deimos".Concepts and Approaches for Mars Exploration.1679: 4017.Bibcode:2012LPICo1679.4017E.
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