Instruct: Given a question, retrieve passages that can help answer the question.
Query: How many moons does Mars have?
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, god of war, into battle. Ares was known as Mars to the Romans.
Epistemological solipsists claim that realism begs the question: assuming there is a universe that is independent of the agent's mind, the agent can only ever know of this universe through the agent's senses. How is the existence of the independent universe to be scientifically studied? If a person sets up a camera to photograph the moon when they are not looking at it, then at best they determine that there is an image of the moon in the camera when they eventually look at it. Logically, this does not assure that the moon itself (or even the camera) existed at the time the photograph is supposed to have been taken. To establish that it is an image of an independent moon requires many other assumptions that amount to begging the question.
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
Crater counting is a method for estimating the age of a planet's surface. The method is based upon the assumptions that when a piece of planetary surface is new, then it has no impact craters; impact craters accumulate after that at a rate that is assumed known. Consequently, counting how many craters of various sizes there are in a given area allows determining how long they have accumulated and, consequently, how long ago the surface has formed. The method has been calibrated using the ages obtained by radiometric dating of samples returned from the Moon by the Luna and Apollo missions. It has been used to estimate the age of areas on Mars and other planets that were covered by lava flows, on the Moon of areas covered by giant mares, and how long ago areas on the icy moons of Jupiter and Saturn flooded with new ice.
There are probably a few thousand astronomical objects named after people. These include the names of a few thousand asteroids and hundreds of comets. Also, many topological features on solar system bodies have been named after people, including many hundreds of craters on the Moon, Mars and other planets and satellites. In addition to craters there are also various other topological features such as mountains, valleys, ridges on the Moon and other bodies which are also named after people. Finally, a very few stars are named after people (according to the IAU), such as Barnard's star. Thus this list is somewhat incomplete. (Star-registry companies keep lists of stars they claim to have named after people. The IAU does not recognize those claims.)
Moons of solid Solar System bodies have been created by both collisions and capture. Mars's two small moons, Deimos and Phobos, are thought to be captured asteroids.
The Earth's Moon is thought to have formed as a result of a single, large head-on collision.
The impacting object probably had a mass comparable to that of Mars, and the impact probably occurred near the end of the period of giant impacts. The collision kicked into orbit some of the impactor's mantle, which then coalesced into the Moon. The impact was probably the last in the series of mergers that formed the Earth.
It has been further hypothesized that the Mars-sized object may have formed at one of the stable Earth–Sun Lagrangian points (either or ) and drifted from its position. The moons of trans-Neptunian objects Pluto (Charon) and Orcus (Vanth) may also have formed by means of a large collision: the Pluto–Charon, Orcus–Vanth and Earth–Moon systems are unusual in the Solar System in that the satellite's mass is at least 1% that of the larger body.
It is now widely believed that ice accumulated in many areas of Mars, including Deuteronilus Mensae, when the planet's orbital tilt was very different from now (the axis of Mars has considerable "wobble", meaning its angle changes over time). A few million years ago, the tilt of the axis of Mars was 45 degrees instead of its present 25 degrees. Its tilt, also called obliquity, varies greatly because its two tiny moons cannot stabilize it, like our relatively large moon does the Earth.
Roman numerals are usually not assigned to satellites until they are named, so many satellites that have been discovered but only have provisional designations do not usually have Roman numerals assigned to them. (An exception is Saturn's moon Helene, which received the Roman numeral XII in 1982, but was not named until 1988.) While the International Astronomical Union was assigning names to all satellites from 1975 to 2013, the use of Roman numeral designations diminished, and some are very rarely used; Phobos and Deimos are rarely referred to as Mars I and Mars II, and the Moon is never referred to as "Earth I". However, some of the more recently discovered moons have not been named even after their orbital elements were known well enough to receive Roman numerals, and as such the only possible nomenclature for them is their Roman numeral designations; the first of these unnamed but numbered moons was Jupiter LI.
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.
In 1864, William Rutter Dawes observed "that the ruddy tint of the planet does not arise from any peculiarity of its atmosphere seems to be fully proved by the fact that the redness is always deepest near the centre, where the atmosphere is thinnest." Spectroscopic observations in the 1860s and 1870s led many to think the atmosphere of Mars is similar to Earth's. In 1894, though, spectral analysis and other qualitative observations by William Wallace Campbell suggested Mars resembles the Moon, which has no appreciable atmosphere, in many respects.
Many places on Mars show channels of different sizes. Many of these channels probably carried water, at least for a time. 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 of Mars has even been greater than 80 degrees
A woman becomes capable of bearing a child after appearance of the first of the menses recurring in a cycle found to be governed by the Moon and Mars. When the Moon is in an "anupchayasthana" i.e. in a "kendrasthana" or a "trikonasthana" or in the 2nd or the 12th from its natal position, the menses for conception occurs but if the Moon is in an "upachayasthana" i.e. in the 3rd, the 6th, the 10th or 11th,and receives the aspect of a masculine benefic planet the woman gets sexual union with man but does not conceive. If the Sun, the Moon, Venus and Mars occupy their own "navamsas" and Jupiter is in the "lagna-kendra" or in a "trikonasthana", conception takes place. Bhattotpala citing Saravali (Slokas VIII. 2-4) in his commentary adds that a potent woman can become pregnant when at the time of commencement of the regular menses the Moon aspected by Mars is not in an "upachayasthana" from the "Janam-rasi" i.e. from the natal position of the Moon; if the Moon is aspected by Mars or is in an "upachayasthana" then she will not conceive.
A ray system comprises radial streaks of fine "ejecta" thrown out during the formation of an impact crater, looking somewhat like many thin spokes coming from the hub of a wheel. The rays may extend for lengths up to several times the diameter of their originating crater, and are often accompanied by small secondary craters formed by larger chunks of ejecta. Ray systems have been identified on the Moon, Earth (Kamil Crater), Mercury, and some moons of the outer planets. Originally it was thought that they existed only on planets or moons lacking an atmosphere, but more recently they have been identified on Mars in infrared images taken from orbit by "2001 Mars Odyssey"s thermal imager.
The origin of the Martian moons is 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. Another hypothesis is that the moons of Mars may have formed after a huge collision with a protoplanet one third the mass of Mars that formed a ring around Mars. A third hypothesis is that they may have co-accreted with the parent planet. 
One of the pictures below shows branched channels. Many places on Mars show channels of different sizes. Many of these channels probably carried water, at least for a time. 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 of Mars has even been greater than 80 degrees
With the presence of various orbiters, landers, and rovers, it is possible to practice astronomy from Mars. Although Mars's moon Phobos appears about one-third the angular diameter of the full moon on Earth, Deimos appears more or less star-like, looking only slightly brighter than Venus does from Earth.
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.
Roach has said that she's always been interested in science, specifically topics relating to space and space travel itself; which is why the decision to write "Packing for Mars" was not a random adventure that Roach decided to embark on. "I had no idea until I started this book that when you're heading to the moon or to Mars, you're essentially coasting. I thought it was like a car where you'd have your foot on the gas the entire time, and I used to think, 'Jesus, that's a lot of gas.' " By the end of her book, she was able to vividly describe and make understandable the many nitty-gritty details that would normally be overlooked. She does this by addressing and answering the more practical and technical questions that an audience member may have, such as, how the astronauts go to the bathroom, eat, and sleep, as well as, question the effects of zero gravity on the bodies of the space travelers.
Missions dedicated to explore the two moons of Mars, Phobos and Deimos. Many missions to Mars have also included dedicated observations of the Moons, while this section is about missions focused solely on them. There have been three unsuccessful dedicated missions and many proposals. Because of the proximity of the Mars moons to Mars, any mission to them may also be considered a mission to Mars from some perspectives.
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]
Encounters with other embryos could destabilize a disk orbiting Mars reducing the mass of moons that form around Mars. After Mars is scattered from the annulus by encounters with other planets it continues to have encounters with other objects until the planets clear material from the inner Solar System. While these encounters enable the orbit of Mars to become decoupled from the other planets and remain on a stable orbit, they can also perturb the disk of material from which the moons of Mars form. These perturbations cause material to escape from the orbit of Mars or to impact on its surface reducing the mass of the disk resulting in the formation of smaller moons.
The oldest meteorite fragments found on Earth are about 4.54 billion years old; this, coupled primarily with the dating of ancient lead deposits, has put the estimated age of Earth at around that time. The Moon has the same composition as Earth's crust but does not contain an iron-rich core like the Earth's. Many scientists think that about 40 million years after the formation of Earth, it collided with a body the size of Mars, throwing into orbit crust material that formed the Moon. Another hypothesis is that the Earth and Moon started to coalesce at the same time but the Earth, having much stronger gravity than the early Moon, attracted almost all the iron particles in the area.
Many people have long advocated a human mission to Mars, perhaps eventually leading to the permanent colonization of Mars, as the next logical step for a human space program after lunar exploration. Aside from the prestige such a mission would bring, advocates argue that humans would easily be able to outperform robotic explorers, justifying the expense. Aerospace engineer Bob Zubrin is one of the proponents of such missions. Various asteroids, moons, and places on Mars can be mined while crops are initially being planted.[112] This method will save lives on Earth, add potentially trillions of dollars to the world economy annually, and provide a stable colony on Mars. Some have also stated that persons should stay on Earth, but get food and resources from Mars and surrounding asteroids, moons, greenhouses in space and other structures. Some critics contend unmanned robots can perform better than humans at a fraction of the expense. If life exists on Mars, a human mission could contaminate it by introducing earthly microbes, so robotic exploration would be preferable.[113]
Because of the many missions studying Mars since the 1960s, there is good coverage of its surface which contains large numbers of craters. Many of the craters on Mars differ from those on the Moon and other moons since Mars contains ice under the ground, especially in the higher latitudes. Some of the types of craters that have special shapes due to impact into ice-rich ground are pedestal craters, rampart craters, expanded craters, and LARLE craters.
Because Mars has some atmosphere a suit does has some pressure on it from the outside, but one of the design questions is if the suits will also be used in Mars orbit in the vacuum of space, or just on the Mars surface. Another consideration would be operations on a Martian moon, where there is extremely low gravity. On the surface, there is a risk of puncture from Mars rocks, which are known to have damaged the wheels on the Curiosity rover. An additional need is to deal with dust storms. In orbit, there is greater exposure to radiation and lower gravity than on the surface of Mars.