Instruct: Given a question, retrieve relevant passages that can help answer the question.
Query: How many moons does Mars have?
Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in resonance with each other or with smaller bodies (this is also common in satellite systems). All except Mercury and Venus have natural satellites, often called "moons". Earth has one, Mars has two, and the giant planets have numerous moons in complex planetary-type systems. Many moons of the giant planets have features similar to those on the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life (especially Europa).
Due to the small size of Phobos (about ) and its rapid orbital motion, an observer on the surface of Mars would never experience a solar eclipse for longer than about thirty seconds. Phobos also takes only 7 hours 39 minutes to orbit Mars, while a Martian day is 24 hours 37 minutes long, meaning that Phobos can create two eclipses per Martian day. These are annular eclipses, because Phobos is not quite large enough or close enough to Mars to create a total solar eclipse.
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. 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. 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. 
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. In recognition of these 'predictions', two craters on Deimos are named Swift and Voltaire, 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). Many of the craters on Phobos are also named after characters in "Gulliver's Travels".
Some of the earliest mentions of Mars's moons in fiction predate their discovery. In Jonathan Swift's famous satire "Gulliver's Travels" (1726), the astronomers of flying island Laputa are described as having discovered two satellites of Mars. Voltaire's short story "Micromégas" (1752), about alien visitors from Sirius and Saturn, also describes Mars as having two moons. Voltaire is thought to have been influenced by Swift. In recognition of these mentions, many geological features of Phobos and Deimos are named after characters and places from "Gulliver's Travels", including among others "Laputa Regio" and "Lagado Planitia" on Phobos, and craters Swift and Voltaire on Deimos.
Uranus has 27 named moons, five of which are massive enough to have achieved hydrostatic equilibrium. There are another 13 inner moons that orbit within Uranus's ring system, and another nine outer irregular moons. Unlike most planetary moons, which are named from antiquity, all the moons of Uranus are named after characters from the works of Shakespeare and Alexander Pope's work "The Rape of the Lock".
Fictional representations of Mars have been popular for over a century. Interest in Mars has been stimulated by the planet's dramatic red color, by early scientific speculations that its surface conditions might be capable of supporting life, and by the possibility that Mars could be colonized by humans in the future. Almost as popular as stories about Mars are stories about Martians engaging in activity (frequently invasions) away from their home planet.
The Moon's value is 0.0549, the most eccentric of the large moons of the Solar System. The four Galilean moons have eccentricity < 0.01. Neptune's largest moon Triton has an eccentricity of (), the smallest eccentricity of any known moon in the Solar System; its orbit is as close to a perfect circle as can be currently measured. However, smaller moons, particularly irregular moons, can have significant eccentricity, such as Neptune's third largest moon Nereid (0.75).
Many places on Mars show channels of different sizes. Many of these channels probably carried water, at least for a time. One study that used HiRISE pictures found over 17,000 km of ancient river valleys in Arabia Terra. Many ancient river valleys have been determined to be relatively recent, according to research published in 2016 in the Journal of Geophysical Research: Planets. These valleys carried water into lake basins. One lake, nicknamed "Heart Lake," had a volume similar to Lake Ontario. The climate of Mars may have been such in the past that water ran on its surface. It has been known for some time that Mars undergoes many large changes in its tilt or obliquity because its two small moons lack the gravity to stabilize it, as our moon stabilizes Earth; at times the tilt has even been greater than 80 degrees
The small inner moons constantly perturb each other. The system is chaotic and apparently unstable. Simulations show that the moons may perturb each other into crossing orbits, which may eventually result in collisions between the moons. Desdemona may collide with either Cressida or Juliet within the next 100 million years.
Uranus has five major moons: Miranda, Ariel, Umbriel, Titania, and Oberon. They range in diameter from 472 km for Miranda to 1578 km for Titania. All these moons are relatively dark objects: their geometrical albedo varies between 30 and 50%, whereas their Bond albedo is between 10 and 23%. Umbriel is the darkest moon and Ariel the brightest. The masses of the moons range from 6.7 kg (Miranda) to 3.5 kg (Titania). For comparison, the Moon has a mass of 7.5 kg. The major moons of Uranus are thought to have formed in the accretion disc, which existed around Uranus for some time after its formation or resulted from a large impact suffered by Uranus early in its history.
Mars 3 was an unmanned space probe of the Soviet Mars program which spanned the years between 1960 and 1973. Mars 3 was launched May 28, 1971, nine days after its twin spacecraft Mars 2. The probes were identical robotic spacecraft launched by Proton-K rockets with a Blok D upper stage, each consisting of an orbiter and an attached lander. After the Mars 2 lander crashed on the martian surface, Mars 3 lander became the first spacecraft to attain soft landing on Mars on 2 December 1971. It failed shortly after landing.The type of Orbiter was the Variant M-71 of the Bus/Obiter 4M-V. The primary purpose of the orbiter was to study the topography of the surface; analyze its soil composition; measure various properties of the atmosphere; monitor "solar radiation, the solar wind, and the interplanetary and martian magnetic fields". In addition, it served as a "communications relay to send signals from the lander to Earth".
There have been four successful robotically operated Mars rovers. The Jet Propulsion Laboratory managed the "Sojourner" rover, the "Opportunity" rover, "Spirit" rover, and, now the "Curiosity" rover. On January 24, 2016 NASA reported that current studies on Mars by the "Curiosity" and "Opportunity" rovers would be searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable. The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on Mars is now a primary NASA objective. Since June 2018, the "Opportunity" rover has been out of contact after going into hibernation mode in a dust storm. NASA has stated they are unsure if they will ever be able to regain contact.
Ganymede (Jupiter III) is the largest and most massive moon of Jupiter and in the Solar System. The ninth largest object in the Solar System, it is the largest without a substantial atmosphere. It has a diameter of and is 8% larger than the planet Mercury, although only 45% as massive. Possessing a metallic core, it has the lowest moment of inertia factor of any solid body in the Solar System and is the only moon known to have a magnetic field. Outward from Jupiter, it is the seventh satellite and the third of the Galilean moons, the first group of objects discovered orbiting another planet. Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively.
The Galilean moons are the four largest moons of Jupiter—Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in January 1610, and recognized by him as satellites of Jupiter in March 1610. They were the first objects found to orbit another planet. Their names derive from the lovers of Zeus. They are among the largest objects in the Solar System with the exception of the Sun and the eight planets, with a radius larger than any of the dwarf planets. Ganymede is the largest moon in the Solar System, and is even bigger than the planet Mercury, though only around half as massive. The three inner moons—Io, Europa, and Ganymede—are in a 4:2:1 orbital resonance with each other. Because of their much smaller size, and therefore weaker self-gravitation, all of Jupiter's remaining moons have irregular forms rather than a spherical shape.
Gravity on Mars is only about 38% as strong as it is on Earth, and the long-term effects of such reduced gravity are unknown. Astronauts who are weightless for long periods of time lose significant amounts of bone and muscle mass. It is unclear if the gravity on Mars is strong enough to avoid or minimize these health problems. The Mars Gravity Biosatellite was meant to provide data on how mammalian health is affected by long-term exposure to lower levels of gravity, focusing on bone loss, changes in bone structure, muscle atrophy, and changes in the inner ear. The results from the five-week mission experiment would have been compared against a variety of Earth-based controls, including vivarium, hindlimb suspension, partial weight suspension, flight habitat effects, and short-radius centrifuge testing.
Uranus was the last giant planet without any known irregular moons, but since 1997 nine distant irregular moons have been identified using ground-based telescopes. Two more small inner moons, Cupid and Mab, were discovered using the Hubble Space Telescope in 2003. As of 2016, the moon Margaret was the last Uranian moon discovered, and its characteristics were published in October 2003.
After Herschel discovered Titania and Oberon on January 11, 1787, he subsequently believed that he had observed four other moons: two on January 18 and February 9, 1790, and two more on February 28 and March 26, 1794. It was thus believed for many decades thereafter that Uranus had a system of six satellites, though the four latter moons were never confirmed by any other astronomer. Lassell's observations of 1851, in which he discovered Ariel and Umbriel, however, failed to support Herschel's observations; Ariel and Umbriel, which Herschel certainly ought to have seen if he had seen any satellites beside Titania and Oberon, did not correspond to any of Herschel's four additional satellites in orbital characteristics. Herschel's four spurious satellites were thought to have sidereal periods of 5.89 days (interior to Titania), 10.96 days (between Titania and Oberon), 38.08 days, and 107.69 days (exterior to Oberon). It was therefore concluded that Herschel's four satellites were spurious, probably arising from the misidentification of faint stars in the vicinity of Uranus as satellites, and the credit for the discovery of Ariel and Umbriel was given to Lassell.
The first two moons to be discovered were Titania and Oberon, which were spotted by Sir William Herschel on January 11, 1787, six years after he had discovered the planet itself. Later, Herschel thought he had discovered up to six moons (see below) and perhaps even a ring. For nearly 50 years, Herschel's instrument was the only one with which the moons had been seen. In the 1840s, better instruments and a more favorable position of Uranus in the sky led to sporadic indications of satellites additional to Titania and Oberon. Eventually, the next two moons, Ariel and Umbriel, were discovered by William Lassell in 1851. The Roman numbering scheme of Uranus's moons was in a state of flux for a considerable time, and publications hesitated between Herschel's designations (where Titania and Oberon are Uranus II and IV) and William Lassell's (where they are sometimes I and II). With the confirmation of Ariel and Umbriel, Lassell numbered the moons I through IV from Uranus outward, and this finally stuck. In 1852, Herschel's son John Herschel gave the four then-known moons their names.
Mars 2020 is a Mars rover mission by NASA's Mars Exploration Program with a planned launch in July or August 2020. It will investigate an astrobiologically relevant ancient environment on Mars, investigate its surface geological processes and history, including the assessment of its past habitability, the possibility of past life on Mars, and the potential for preservation of biosignatures within accessible geological materials. It will cache sample containers along its route for a potential future Mars sample-return mission.
The largest and most conspicuous volcanoes on Mars occur in Tharsis and Elysium regions. These volcanoes are strikingly similar to shield volcanoes on Earth. Both have shallow-sloping flanks and summit calderas. The main difference between Martian shield volcanoes and those on Earth is in size: Martian shield volcanoes are truly colossal. For example, the tallest volcano on Mars, Olympus Mons, is 550 km across and 21 km high. It is nearly 100 times greater in volume than Mauna Loa in Hawaii, the largest shield volcano on Earth. Geologists think one of the reasons that volcanoes on Mars are able to grow so large is because Mars lacks plate tectonics. The Martian lithosphere does not slide over the upper mantle (asthenosphere) as on Earth, so lava from a stationary hot spot is able to accumulate at one location on the surface for a billion years or longer.
The USSR intended to have the first artificial satellite of Mars beating the planned American Mariner 8 and Mariner 9 Mars orbiters. In May 1971, one day after Mariner 8 malfunctioned at launch and failed to reach orbit, Cosmos 419 (Mars 1971C), a heavy probe of the Soviet Mars program M-71, also failed to launch. This spacecraft was designed as an orbiter only, while the next two probes of project M-71, Mars 2 and Mars 3, were multipurpose combinations of an orbiter and a lander with small skis-walking rovers that would be the first planet rovers outside the Moon. They were successfully launched in mid-May 1971 and reached Mars about seven months later. On November 27, 1971 the lander of Mars 2 crash-landed due to an on-board computer malfunction and became the first man-made object to reach the surface of Mars. On 2 December 1971, the Mars 3 lander became the first spacecraft to achieve a soft landing, but its transmission was interrupted after 14.5 seconds.
Viking 1 was the first of two spacecraft (along with "Viking 2") sent to Mars as part of NASA's Viking program. On July 20, 1976, it became the second spacecraft to soft-land on Mars, and the first soft lander to successfully perform its mission. (The first spacecraft to soft-land on Mars was the Soviet Union's Mars 3 on December 2, 1971, which stopped transmitting after 14.5 seconds.) "Viking 1" held the record for the longest Mars surface mission of days (over 6¼ years) or Martian solar days, until that record was broken by "Opportunity" on May 19, 2010.
The timestamps printed on the photos in the NASA press release do not correspond to the actual time that the shadow was imaged, rather they represent the "image start time" of a vertically much larger original image. Mars Global Surveyor orbits Mars in a Sun-synchronous polar orbit with orbital period 117.65 minutes, moving from south pole to north pole, and continuously points its camera straight down. The result is an image in the form of a very long thin vertical strip, where the pixels in the top part of the image are imaged nearly one hour after those in the bottom part of the image. In principle the image could be as much as 43200 pixels high, but "downtrack summing" is used to merge adjacent lines. For instance, a downtrack summing factor of 27 causes every 27 lines to be merged into one, resulting in an image 1600 pixels high. Thus to determine the actual time that the Phobos shadow was imaged, it is necessary to locate the original image and measure how many pixels from the bottom of the image the shadow is found and add the corresponding offset to the image start time.
Many Solar System objects are known to possess satellite systems, though their origin is still unclear. Notable examples include the largest satellite system, the Jovian system, with 79 known moons (including the large Galilean moons) and the Saturnian System with 62 known moons (and the most visible ring system in the Solar System). Both satellite systems are large and diverse. In fact all of the giant planets of the Solar System possess large satellite systems as well as planetary rings, and it is inferred that this is a general pattern. Several objects farther from the Sun also have satellite systems consisting of multiple moons, including the complex Plutonian system where multiple objects orbit a common center of mass, as well as many asteroids and plutinos. Apart from the Earth-Moon system and Mars' system of two tiny natural satellites, the other terrestrial planets are generally not considered satellite systems, although some have been orbited by artificial satellites originating from Earth.
Mars is the only planet in the Solar System on which deserts have been identified. Despite its low surface atmospheric pressure (only 1/100 of that of the Earth), the patterns of atmospheric circulation on Mars have formed a sea of circumpolar sand more than 5 million km² (1.9 million sq mi) in the area, much larger than deserts on Earth. The Martian deserts principally consist of dunes in the form of half-moons in flat areas near the permanent polar ice caps in the north of the planet. The smaller dune fields occupy the bottom of many of the craters situated in the Martian polar regions. Examination of the surface of rocks by laser beamed from the Mars Exploration Rover have shown a surface film that resembles the desert varnish found on Earth although it might just be surface dust. The surface of Titan, a moon of Saturn, also has a desert-like surface with dune seas.
The origin of the Martian moons is still controversial. 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. Based on their similarity, one hypothesis is that both moons may be captured main-belt asteroids. 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, although it is not clear that sufficient time is available for this to occur for Deimos. Capture also requires dissipation of energy. The current atmosphere of Mars is too thin to capture a Phobos-sized object by atmospheric braking. Geoffrey Landis has pointed out that the capture could have occurred if the original body was a binary asteroid that separated under tidal forces.
The origin of Mars' moons, Phobos and Deimos (pictured right), is unknown and remains controversial. One theory is that the moons are captured asteroids. However, the moons' near circular orbits and low inclination relative to the Martian equator are not in agreement with the capture hypothesis. The detection of minerals on Phobos similar to those in the Martian lithosphere, and the unusually low density and high porosity of Phobos, such that the moon would not be expected to remain aggregate if dynamically captured, suggest that the moons could have formed via accretion in Martian orbit, similar to how Earth's Moon formed.