Moons of Mars

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Phobos colour 2008.jpg
Color image of Phobos (MRO, 23 March 2008)
Color image of Deimos (MRO, 21 February 2009)

The two moons of Mars are Phobos and Deimos. [1] Both were discovered by Asaph Hall in August 1877 [2] and are named after the Greek mythological twin characters Phobos (panic/fear) and Deimos (terror/dread) who accompanied their father Ares into battle. Ares, god of war, was known to the Romans as Mars.

Mars Fourth planet from the Sun in the Solar System

Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System after Mercury. In English, Mars carries a name of the Roman god of war, and is often referred to as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance that is distinctive among the astronomical bodies visible to the naked eye. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth.

Phobos (moon) natural satellite of Mars

Phobos is the innermost and larger of the two natural satellites of Mars, the other being Deimos. Both moons were discovered in 1877 by American astronomer Asaph Hall.

Deimos (moon) Moon of Mars

Deimos is the smaller and outermost of the two natural satellites of the planet Mars, the other being Phobos. Deimos has a mean radius of 6.2 km (3.9 mi) and takes 30.3 hours to orbit Mars. Deimos is 23,460 km (14,580 mi) from Mars, much further than Mars's other moon, Phobos. It is named for Deimos who in Greek mythology is the twin brother of Phobos, and personifies terror.


Compared to the Earth's moon, Phobos and Deimos are small. Phobos has a diameter of 22.2 km (13.8 mi) and a mass of 1.08×1016 kg, while these measures for Deimos are 12.6 km (7.8 mi) and 2.0×1015 kg. Phobos orbits closer to Mars, with a semi-major axis of 9,377 km (5,827 mi) and an orbital period of 7.66 hours; Deimos's semi-major axis is 23,460 km (14,580 mi) with an orbital period of 30.35 hours. By way of contrast, Earth's moon has a diameter of 3475 km, approximately the distance from New York City to Salt Lake City, while the diameter of Mars's larger moon, Phobos, corresponds to a trip across town from one Salt Lake suburb to another.

Earth Third planet from the Sun in the Solar System

Earth is the third planet from the Sun and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth formed over 4.5 billion years ago. Earth's gravity interacts with other objects in space, especially the Sun and the Moon, Earth's only natural satellite. Earth revolves around the Sun in 365.26 days, a period known as an Earth year. During this time, Earth rotates about its axis about 366.26 times.

Moon Earths natural satellite

The Moon is an astronomical body that orbits planet Earth and is Earth's only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, and the largest among planetary satellites relative to the size of the planet that it orbits. The Moon is after Jupiter's satellite Io the second-densest satellite in the Solar System among those whose densities are known.


Early speculation

Curiosity's view of the Martian moons: Phobos passing in front of Deimos - in real-time (video-gif, 1 August 2013) PIA17352-MarsMoons-PhobosPassesDeimos-RealTime.gif
Curiosity's view of the Martian moons: Phobos passing in front of Deimos – in real-time (video-gif, 1 August 2013)

The existence of the moons of Mars had been speculated about since the moons of Jupiter were discovered. It is known that when Galileo, as a hidden report about him having observed two bumps on the sides of 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"), Johannes Kepler had misinterpreted it to mean Salve umbistineum geminatum Martia proles (Hello, furious twins, sons of Mars). [3]

Galileo Galilei Italian polymath (1564-1642)

Galileo Galilei was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Galileo has been called the "father of observational astronomy", the "father of modern physics", the "father of the scientific method", and the "father of modern science".

Johannes Kepler 17th-century German mathematician, astronomer and astrologer

Johannes Kepler was a German astronomer, mathematician, and astrologer. He is a key figure in the 17th-century scientific revolution, best known for his laws of planetary motion, and his books Astronomia nova, Harmonices Mundi, and Epitome Astronomiae Copernicanae. These works also provided one of the foundations for Newton's theory of universal gravitation.

Perhaps inspired by Kepler (and quoting 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 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. [4] [5] In the 20th century, V. G. Perminov, a spacecraft designer of early Soviet Mars and Venus spacecraft, speculated Swift found and deciphered records that Martians left on Earth. [6] 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 making 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. [7]

Keplers laws of planetary motion Scientific laws describing motion of planets around the Sun

In astronomy, Kepler's laws of planetary motion are three scientific laws describing the motion of planets around the Sun.

Jonathan Swift 17th/18th-century Anglo-Irish satirist, essayist, and poet

Jonathan Swift was an Anglo-Irish satirist, essayist, political pamphleteer, poet and cleric who became Dean of St Patrick's Cathedral, Dublin.

<i>Gullivers Travels</i> novel by Jonathan Swift

Gulliver's Travels, or Travels into Several Remote Nations of the World. In Four Parts. By Lemuel Gulliver, First a Surgeon, and then a Captain of Several Ships, is a prose satire by Irish writer and clergyman Jonathan Swift, that is both a satire on human nature and the "travellers' tales" literary subgenre. It is Swift's best known full-length work, and a classic of English literature. He himself claimed that he wrote Gulliver's Travels "to vex the world rather than divert it".

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. [8] [9] In recognition of these 'predictions', two craters on Deimos are named Swift and Voltaire, [10] [11] while on Phobos there is one named regio, Laputa Regio, and one named planitia, Lagado Planitia, both of which are named after places in Gulliver's Travels (the fictional Laputa, a flying island, and Lagado, imaginary capital of the fictional nation Balnibarbi). [12] Many of the craters on Phobos are also named after characters in Gulliver's Travels. [13]

Voltaire French writer, historian, and philosopher

François-Marie Arouet, known by his nom de plumeVoltaire, was a French Enlightenment writer, historian and philosopher famous for his wit, his criticism of Christianity, especially the Roman Catholic Church, and his advocacy of freedom of religion, freedom of speech, and separation of church and state.

Micromégas is a 1752 novella by the French philosopher and satirist Voltaire. Along with his story "Plato's Dream", it is an early example in the literary genre of science fiction, and is a significant development in the history of literature.

Swift (Deimian crater) crater on Deimos

Swift crater is a crater on Mars's moon Deimos. It is about 3 km (1.9 mi) in diameter. Swift crater is named after Jonathan Swift, whose 1726 book Gulliver's Travels predicted the existence of two moons of Mars. Swift crater is one of two named features on Deimos, the other being Voltaire crater. On 10 July 2006, Mars Global Surveyor took an image of Deimos from 22,985 km (14,282 mi) away showing Swift crater.


Asaph Hall discovered Deimos on 12 August 1877 at about 07:48 UTC and Phobos on 18 August 1877, at the US Naval Observatory in Washington, D.C., at about 09:14 GMT (contemporary sources, using the pre-1925 astronomical convention that began the day at noon, give the time of discovery as 11 August 14:40 and 17 August 16:06 Washington mean time respectively). [14] [15] [16] 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.

Coordinated Universal Time Primary time standard by which the world regulates clocks and time

Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is within about 1 second of mean solar time at 0° longitude, and is not adjusted for daylight saving time. In some countries where English is spoken, the term Greenwich Mean Time (GMT) is often used as a synonym for UTC and predates UTC by nearly 300 years.

Washington, D.C. Capital of the United States

Washington, D.C., formally the District of Columbia and commonly referred to as Washington or D.C., is the capital of the United States. Founded after the American Revolution as the seat of government of the newly independent country, Washington was named after George Washington, first President of the United States and Founding Father. As the seat of the United States federal government and several international organizations, Washington is an important world political capital. The city is also one of the most visited cities in the world, with more than 20 million tourists annually.

Hall recorded his discovery of Phobos in his notebook as follows: [17]

The telescope used to discover the Martian moons Usno-telescope-equalized-1.png
The telescope used to discover the Martian moons
"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. [18] In 1893 the lens was remounted and put in a new dome, where it remains into the 21st century. [19]

The names, originally spelled Phobus and Deimus, respectively, were suggested by Henry Madan (18381901), Science Master of Eton, from Book XV of the Iliad, where Ares summons Fear and Fright. [20]

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, Iosif Shklovsky, [21] who, based on a later-disproven density estimate, suggested Phobos was a hollow metal shell.

Recent surveys

Searches have been conducted for additional satellites. In 2003, Scott S. Sheppard and David C. Jewitt surveyed nearly the entire 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 km using an albedo of 0.07. [22]


Apparent sizes of the moons of Mars, Deimos and Phobos, and the Moon as viewed from the surface of their respective planets (Curiosity rover, 1 August 2013) PIA17351-ApparentSizes-MarsDeimosPhobos-EarthMoon.jpg
Apparent sizes of the moons of Mars, Deimos and Phobos, and the Moon as viewed from the surface of their respective planets (Curiosity rover, 1 August 2013)

If viewed from Mars's surface near its equator, full Phobos looks about one third as big as a full moon on Earth. It has an 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, and is below the horizon and thus invisible when viewed from Mars's polar ice caps. Deimos looks more like a bright star or planet for an observer on Mars, only slightly bigger than Venus looks from Earth; 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 solar eclipses on Mars, as the moons are far too small to completely cover the Sun. On the other hand, total lunar eclipses of Phobos are very common, happening almost every night. [23]

The motions of Phobos and Deimos would appear very different from that of our own Moon. Speedy Phobos rises in the west, sets in the east, and rises again in just eleven hours, while Deimos, being only just outside 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 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 approaches Mars closely enough (see Roche limit), Phobos will be broken up by these tidal forces. [24] 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. [25] Deimos, on the other hand, is far enough away that its orbit is being slowly boosted instead, [26] as in the case of our own Moon.

Orbital details

Name and pronunciationImageDiameter (km)Surface

Area (km)

Mass (kg)Semi-major
axis (km)
period (h)
Average moonrise
period (h, d)
Mars I Phobos /ˈfbəs/
Phobos moon (large).jpg
22.2 km (13.8 mi)(27×21.6×18.8 km)1,548 km1.08×10169,377 km (5,827 mi)7.6611.12 h (0.463 d)
Mars II Deimos /ˈdməs/
12.6 km (7.8 mi)
(10×12×16 km)
483 km2×101523,460 km (14,580 mi)30.35131 h (5.44 d)
Mars Moons Orbit distance.jpeg
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.)


Animation illustrating the asteroid-belt origin for the moons MoonsOfMarsImproved3.gif
Animation illustrating the asteroid-belt origin for the moons

The origin of the Martian moons is still controversial. [27] Phobos and Deimos both have much in common with carbonaceous C-type asteroids, with spectra, albedo, and density very similar to those of C- or D-type asteroids. [28] Based on their similarity, one hypothesis is that both moons may be captured main-belt asteroids. [5] [29] Both moons 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 highly eccentric orbit, and adjusting its inclination into the equatorial plane, most probably by a combination of atmospheric drag and tidal forces, [30] although it is not clear that sufficient time is available for this to occur for Deimos. [27] Capture also requires dissipation of energy. The current atmosphere of Mars is too thin to capture a Phobos-sized object by atmospheric braking. [27] Geoffrey Landis has pointed out that the capture could have occurred if the original body was a binary asteroid that separated under tidal forces. [29]

Phobos could be a second-generation Solar System object that coalesced in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as Mars. [31]

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 planetesimal. [32] 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' volume) is inconsistent with an asteroidal origin. [33] Observations of Phobos in the thermal infrared suggest a composition containing mainly phyllosilicates, which are well known from the surface of Mars. The spectra are distinct from those of all classes of chondrite meteorites, again pointing away from an asteroidal origin. [34] Both sets of findings support an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit, [35] 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. [36] [37]


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 Phobos program were successfully launched in 1988, but neither conducted the intended jumping landings on Phobos and Deimos due to failures (although Phobos 2 successfully photographed Phobos). The post-Soviet Russian 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.

In 1997 and 1998, the Aladdin mission was selected as a finalist in the NASA 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. These samples would be returned to Earth for study three years later. Ultimately, NASA rejected this proposal in favor of MESSENGER, a probe to Mercury.

In 2007, the 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. Since 2007 the Canadian Space Agency has been considering the PRIME mission to Phobos with orbiter and lander. Since 2008 NASA Glenn Research Center began studying a Phobos and Deimos sample return mission. Since 2013 NASA developed the Phobos Surveyor mission concept with an orbiter and a small rover that is proposed to launch sometime after 2023.[ citation needed ] Proposed NASA's PADME mission proposes to launch in 2020 and reach Mars orbit in 2021 to conduct multiple flybys of the Martian moons.[ citation needed ] Also, NASA is assessing the OSIRIS-REx II, concept mission for a sample return from Phobos. [38] Another sample return mission from Deimos, called Gulliver. has been conceptualized.[ citation needed ] Russia plans to repeat Fobos-Grunt mission around 2024.[ citation needed ]

Orbits of moons and spacecraft orbiting Mars. PIA19396-PlanetMars-OrbitsOfMoonsAndOrbiters-20150504.jpg
Orbits of moons and spacecraft orbiting Mars.

See also

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Further reading