SOLAR SYSTEM FAMILY-FACTS ABOUT THE MOONS OF MARS
Mars has got the most bizarre pair of natural satellites in our solar system. And for a long time, these extremely tiny moons were hidden from the eyes of man. As a result, many astronomers questioned their existence. But after a painstaking search, one astronomer called Asaph Hall finally discovered the moons in the month of August, 1877. The two Martian moons were named Phobos and Deimos, after the twin sons of Ares-the Greek god of war.
Discovery of the Martian moons: The search for the Martian moons began with famous Italian astronomer, Galileo Galilei while he observed the mighty Jupiter with his recently made telescope, on 17th January 1610. Upon careful observation he figured out that the planet was accompanied by four smaller bodies, later to be known as the famous Galilean Moons. After observing Jupiter, Galileo trained his telescope towards Saturn and took a note in Latin as he observed the most distant planet(known at that time). But Johannes Kepler misinterpreted this conclusion and got the idea that Galileo had predicted the existence of a pair of Martian moons!
The initial search for the Martian moons gained further momentum because of a satirical piece by the famous Jonathan Swift. In his satire, ''Gulliver's Travels(1726)'', Swift wrote that the astronomers of Laputa have discovered two Martian satellites, that revolve around the planet at a distance of 3 and 5 Martian diameters, and with an orbital period of 10 and 21.5 hours respectively. These estimates were later found to possess a rough similarity with observed values. During the early 18th century, it was known that the inner planets of our solar system i.e., Mercury and Venus did not possess any moon(s), Earth got one and Jupiter got four. So Jonathan Swift set the number of Martian moons at two based upon some common arguments and on Kepler's laws of planetary motion. Apart from Swift, Voltaire also wrote about the Martian moons in his short story, ''Micromegas(1752)''. Soon the world got excited about these little satellites, which were completely invisible to the unaided eye of man. But none of the attempts could yield a positive result. And no astronomer, including Sir William Herschel could detect any tell-tale sign of a Martian moon. Disheartened, he concluded that the red planet might be without a satellite.
Fortunately, there was one person who refused to give up the search. That man was Asaph Hall-an astronomer associated with the US Naval Observatory in Washington D.C., during the years from 1862. When Hall tried to look for the moons in the month of August 1877, he was greeted with failure and out of frustration, he decided to abandon his search. But his wife, Angelina urged him to try one more time. And on the night of 11th August he discovered a faint object around Mars. After more observations, the outer moon(to be named Deimos) was finally discovered on the night of 17th August, 1877, and the inner moon(to be named Phobos) on the night of 18th August, 1877. The names, originally spelled as Phobus and Deimus, the twin sons of the Greek god of war, Ares, were suggested by Henry Madan, a professor of the Eton College.
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| Phobos/Image Credits: Nasa, via Wikimedia Commons |
Characteristics of Phobos: Phobos, who symbolizes panic and phobia according to the Greek Mythology, is the inner satellite of Mars. This irregularly shaped object, with a mean radius of only 11 km orbits its primary(Mars) at a distance of 9376 km from the centre. Thereby, it is the only satellite in our solar system which is at a least distance of separation from its parent planet. Because of this proximity, Phobos has got a very high orbital velocity compared to the rotational velocity of Mars and completes an orbit once in every 8 hours.
Phobos is too small for gravity to give it a spherical shape, and due to the same reason, it is also devoid of an atmosphere. The surface is heavily cratered due to a large number of past impacts with the largest one being the Stickney Crater(diameter of 9 km), and is named after the maiden name of professor Hall's wife Angelina Stickney. Apart from Stickney, some craters have been named after the scientists who have studied them like D'Arrest, Hall, Opik, Roche, Shklovsky, and etc. Other craters like Gulliver, Grildrig, Skyresh, Drunlo, and etc., have been named after characters from the book Gulliver's travels.
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| Stickney Crater on Phobos/Image Credits: NASA, via Wikimedia Commons |
Characteristics of Deimos: Deimos, who symbolizes dread and terror, is the outermost and the smallest of the two Martian moons, with an average radius of 6.2 km only. This satellite orbits Mars at a distance of 23,460 km and has an orbital period of 30.4 hours. Like Phobos, Deimos is too small to maintain a spherical shape and eventually, each of the Martian moon looks more like a potato(obviously a very large one!). The surface of Deimos is comparatively smoother and the craters have been partially filled by a layer of regolith. Only two impact craters i.e., Swift(diameter of 1 km) and Voltaire(diameter of 1.9 km) have been named so far.
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| Deimos/Image Credits: NASA, via Wikimedia Commons |
Observation of Transists: The apparent size of the Martian moons are too less to effectively cover the Sun, and as a result instead of solar eclipses a hypothetical Martian observer would witness frequent events of transit. These transits have been observed and photographed by the Spirit rover, the Opportunity rover and the Curiosity rover.
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| Solar Eclipse by Phobos/Image Credits: NASA, via Wikimedia Commons |
Origin of the moons: Both the moons have a debatable origin, and the scientific community have not yet given a definite verdict. One hypothesis, based upon their structural and compositional similarities with that of carbonaceous asteroids suggests Phobos and Deimos to be captured objects from the main asteroid belt, a region of rocky bodies in between Mars and Jupiter. A second hypothesis suggests that Phobos formed after the formation of Mars and Deimos was hurled towards the red planet by the immense gravity of Jupiter. Another hypothesis states that once upon a time, Mars was surrounded by numerous Phobos and Deimos sized bodies, which probably formed from the collision of a planetesimal. However, the debate has not yet settled and it would require further investigation.
Explorations: There haven't been any dedicated or separate mission to study the Martian moons. However, all the missions intended for the study of Mars launched by the American space agency(NASA) viz., Mariner-7, Mariner-9, Viking-1, Mars Global Surveyor, Mars Reconnaissance Orbiter, Spirit Rover, Opportunity Rover and Curiosity Rover have observed these tiny moons. The Soviet Union on the other hand sent two probes i.e., Phobos-1 and Phobos-2 under their Phobos Program in 1988. Unfortunately, Phobos-1 failed and Phobos-2 could only provide a handful of data before permanent loss of communications. The Russian Space Agency(Roscosmos) launched a sample return mission called Phobos-Grunt in 2011, but it crashed back on Earth due to spacecraft failure. At present, Phobos has been considered as a possible target for future manned missions.
Future of the moons: The far future of these tiny moons are quite gloomy. Theoretical modelling have predicted that tidal forces have started to affect the orbit of Phobos, and within 30-50 million years from now, the same tidal forces would disintegrate this inner satellite and give the red planet a thin ring that might last for a period of 100 million years or so. While, Deimos is destined to escape the gravitational influence of Mars and fly away.
Some Quick Facts about Phobos:
Some Quick Facts about Deimos:
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