mars (page 2)

Atmosphere

The tenuous atmosphere of Mars, visible on the horizon in this low-orbit photo

Mars lost its magnetosphere 4 billion years ago,^[120] so the solar wind interacts directly with the Martian ionosphere, lowering the atmospheric density by stripping away atoms from the outer layer. Both Mars Global Surveyor and Mars Express have detected ionised atmospheric particles trailing off into space behind Mars,^[120][121] and this atmospheric loss will be studied by the upcoming MAVEN orbiter. Compared to Earth, the atmosphere of Mars is quite rarefied. Atmospheric pressure on the surface today ranges from a low of 30 Pa (0.030 kPa) on Olympus Mons to over 1,155 Pa (1.155 kPa) in Hellas Planitia, with a mean pressure at the surface level of 600 Pa (0.60 kPa).^[122] The highest atmospheric density on Mars is equal to the density found 35 km^[123] above the Earth's surface. The resulting mean surface pressure is only 0.6% of that of the Earth (101.3 kPa). The scale height of the atmosphere is about 10.8 km,^[124] which is higher than Earth's (6 km) because the surface gravity of Mars is only about 38% of Earth's, an effect offset by both the lower temperature and 50% higher average molecular weight of the atmosphere of Mars. The atmosphere of Mars consists of about 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen, carbon dioxide, and water.^[6][125] The atmosphere is quite dusty, containing particulates about 1.5 µm in diameter which give the Martian sky a tawny color when seen from the surface.^[126]

Methane on Mars -"potential sources and sinks" (November 2, 2012).

Methane has been detected in the Martian atmosphere with a mole fraction of about 30 ppb;^[12][127] it occurs in extended plumes, and the profiles imply that the methane was released from discrete regions. In northern midsummer, the principal plume contained 19,000 metric tons of methane, with an estimated source strength of 0.6 kilogram per second.^[128][129] The profiles suggest that there may be two local source regions, the first centered near 30°N 260°W and the second near 0°N 310°W.^[128] It is estimated that Mars must produce 270 ton/year of methane.^[128][130]

The implied methane destruction lifetime may be as long as about 4 Earth years and as short as about 0.6 Earth years.^[128][131] This rapid turnover would indicate an active source of the gas on the planet. Volcanic activity, cometary impacts, and the presence of methanogenic microbial life forms are among possible sources. Methane could also be produced by a non-biological process called serpentinization^[b] involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.^[132]

The Curiosity rover, which landed on Mars in August 2012, is able to make measurements that distinguish between different isotopologues of methane;^[133] but even if the mission is to determine that microscopic Martian life is the source of the methane, the life forms likely reside far below the surface, outside of the rover's reach.^[134] The first measurements with the Tunable Laser Spectrometer (TLS) indicated that there is less than 5 ppb of methane at the landing site at the point of the measurement.^{[135][136][137][138]} The Mars Trace Gas Mission orbiter planned to launch in 2016 would further study the methane,^[139][140] as well as its decomposition products such as formaldehyde and methanol.

Ammonia was also tentatively detected on Mars by the Mars Express satellite, but with its relatively short lifetime, it is not clear what produced it.^[141] Ammonia is not stable in the Martian atmosphere and breaks down after a few hours. One possible source is volcanic activity.^[141]

Climate

Dust Storm on Mars.

November 18, 2012

November 25, 2012

Locations of Opportunity and Curiosity rovers are noted (MRO).

Of all the planets in the Solar System, the seasons of Mars are the most Earth-like, due to the similar tilts of the two planets' rotational axes. The lengths of the Martian seasons are about twice those of Earth's, as Mars's greater distance from the Sun leads to the Martian year being about two Earth years long. Martian surface temperatures vary from lows of about −143 °C (at the winter polar caps)^[8] to highs of up to 35 °C (in equatorial summer).^[9] The wide range in temperatures is due to the thin atmosphere which cannot store much solar heat, the low atmospheric pressure, and the low thermal inertia of Martian soil.^[142] The planet is also 1.52 times as far from the Sun as Earth, resulting in just 43% of the amount of sunlight.^[143]

If Mars had an Earth-like orbit, its seasons would be similar to Earth's because its axial tilt is similar to Earth's. The comparatively large eccentricity of the Martian orbit has a significant effect. Mars is near perihelion when it is summer in the southern hemisphere and winter in the north, and near aphelion when it is winter in the southern hemisphere and summer in the north. As a result, the seasons in the southern hemisphere are more extreme and the seasons in the northern are milder than would otherwise be the case. The summer temperatures in the south can reach up to 30 kelvins warmer than the equivalent summer temperatures in the north.^[144]

Mars also has the largest dust storms in the Solar System. These can vary from a storm over a small area, to gigantic storms that cover the entire planet. They tend to occur when Mars is closest to the Sun, and have been shown to increase the global temperature.^[145]

Orbit and rotation

Mars's average distance from the Sun is roughly 230 million km (1.5 AU, or 143 million miles) and its orbital period is 687 (Earth) days as depicted by the red trail, with Earth's orbit shown in blue.(Animation)

Comparison of Orbits.

View from north ecliptic pole.

View from ascending node.

Planet Mars (red) and Dwarf Planet Ceres (yellow).

Mars's average distance from the Sun is roughly 230 million km (1.5 AU) and its orbital period is 687 (Earth) days. The solar day (or sol) on Mars is only slightly longer than an Earth day: 24 hours, 39 minutes, and 35.244 seconds. A Martian year is equal to 1.8809 Earth years, or 1 year, 320 days, and 18.2 hours.^[6]

The axial tilt of Mars is 25.19 degrees, which is similar to the axial tilt of the Earth.^[6] As a result, Mars has seasons like the Earth, though on Mars, they are nearly twice as long given its longer year. Currently, the orientation of the north pole of Mars is close to the star Deneb.^[13] Mars passed an aphelion in March 2010^[146] and its perihelion in March 2011.^[147] The next aphelion came in February 2012^[147] and the next perihelion came in January 2013.^[147]

Mars has a relatively pronounced orbital eccentricity of about 0.09; of the seven other planets in the Solar System, only Mercury shows greater eccentricity. It is known that in the past, Mars has had a much more circular orbit than it does currently. At one point, 1.35 million Earth years ago, Mars had an eccentricity of roughly 0.002, much less than that of Earth today.^[148] The Mars cycle of eccentricity is 96,000 Earth years compared to the Earth's cycle of 100,000 years.^[149] Mars also has a much longer cycle of eccentricity with a period of 2.2 million Earth years, and this overshadows the 96,000-year cycle in the eccentricity graphs. For the last 35,000 years, the orbit of Mars has been getting slightly more eccentric because of the gravitational effects of the other planets. The closest distance between the Earth and Mars will continue to mildly decrease for the next 25,000 years.^[150]

Search for life

The Viking 1 Lander sampling arm created a number of deep trenches as part of the surface composition and biology experiments. The digging tool on the sampling arm (lower center) could scoop up samples of material and deposit them into the appropriate experiment. Some holes were dug deeper to study soil which was not affected by solar radiation and weathering. (Chryse Planitia)

The current understanding of planetary habitability – the ability of a world to develop and sustain life – favors planets that have liquid water on their surface. This most often requires that the orbit of a planet lie within the habitable zone, which for the Sun currently extends from just beyond Venus to about the semi-major axis of Mars.^[151] During perihelion, Mars dips inside this region, but the planet's thin (low-pressure) atmosphere prevents liquid water from existing over large regions for extended periods. The past flow of liquid water demonstrates the planet's potential for habitability. Some recent evidence has suggested that any water on the Martian surface may have been too salty and acidic to support regular terrestrial life.^[152]

The lack of a magnetosphere and extremely thin atmosphere of Mars are a challenge: the planet has little heat transfer across its surface, poor insulation against bombardment of the solar wind and insufficient atmospheric pressure to retain water in a liquid form (water instead sublimates to a gaseous state). Mars is also nearly, or perhaps totally, geologically dead; the end of volcanic activity has apparently stopped the recycling of chemicals and minerals between the surface and interior of the planet.^[153]

Evidence suggests that the planet was once significantly more habitable than it is today, but whether living organisms ever existed there remains unknown. The Viking probes of the mid-1970s carried experiments designed to detect microorganisms in Martian soil at their respective landing sites and had positive results, including a temporary increase of CO₂ production on exposure to water and nutrients. This sign of life was later disputed by some scientists, resulting in a continuing debate, with NASA scientist Gilbert Levin asserting that Viking may have found life. A re-analysis of the Viking data, in light of modern knowledge of extremophile forms of life, has suggested that the Viking tests were not sophisticated enough to detect these forms of life. The tests could even have killed a (hypothetical) life form.^[154] Tests conducted by the Phoenix Mars lander have shown that the soil has a very alkaline pH and it contains magnesium, sodium, potassium and chloride.^[155] The soil nutrients may be able to support life, but life would still have to be shielded from the intense ultraviolet light.^[156]

Curiosity rover self-portrait at "Rocknest" (October 31, 2012), with the rim of Gale Crater and the slopes of Aeolis Mons in the distance.

At the Johnson Space Center lab, some fascinating shapes have been found in the meteorite ALH84001, which is thought to have originated from Mars. Some scientists propose that these geometric shapes could be fossilized microbes extant on Mars before the meteorite was blasted into space by a meteor strike and sent on a 15 million-year voyage to Earth. An exclusively inorganic origin for the shapes has also been proposed.^[157]

Small quantities of methane and formaldehyde recently detected by Mars orbiters are both claimed to be possible evidence for life, as these chemical compounds would quickly break down in the Martian atmosphere.^[158][159] Alternatively, these compounds may instead be replenished by volcanic or other geological means, such as serpentinization.^[132]

Habitability

The German space agency discovered Earth lichens do survive in simulated Mars conditions.^[160] The simulation based temperatures, atmospheric pressure, minerals, and light on data from Mars probes.^[160] An instrument called REMS is designed to provide new clues about the signature of the Martian general circulation, microscale weather systems, local hydrological cycle, destructive potential of UV radiation, and subsurface habitability based on ground-atmosphere interaction;^[161][162] and landed on Mars as part of Curiosity (MSL) in August 2012. Microorganisms make up 80% of Earth's biomass.^[160]

Exploration missions

Panorama of Gusev crater, where Spirit rover examined volcanic basalts

In addition to observation from Earth, some of the latest Mars information comes from five active probes on or in orbit around Mars including three orbiters and two rovers. This includes 2001 Mars Odyssey,^[163] Mars Express, Mars Reconnaissance Orbiter, Opportunity rover, and Curiosity rover.

Dozens of unmanned spacecraft, including orbiters, landers, and rovers, have been sent to Mars by the Soviet Union, the United States, Europe, and Japan to study the planet's surface, climate, and geology. The public can request images of Mars via the HiWish program.

The Mars Science Laboratory, named Curiosity, launched on November 26, 2011, reached Mars on August 6, 2012 UTC. It is larger and more advanced than the Mars Exploration Rovers, with a movement rate up to 90 m per hour.^[164] Experiments include a laser chemical sampler that can deduce the make-up of rocks at a distance of 7 m.^[165]

Astronomy on Mars

Phobos transits the Sun, as seen by Mars rover Opportunity on March 10, 2004

With the existence of various orbiters, landers, and rovers, it is now possible to study astronomy from the Martian skies. While Mars's moon Phobos appears about one third the angular diameter of the full Moon as it appears from Earth, Deimos appears more or less star-like, and appears only slightly brighter than Venus does from Earth.^[166]

There are various phenomena, well-known on Earth, that have been observed on Mars, such as meteors and auroras.^[167] A transit of the Earth as seen from Mars will occur on November 10, 2084.^[168] There are also transits of Mercury and transits of Venus, and the moons Phobos and Deimos are of sufficiently small angular diameter that their partial "eclipses" of the Sun are best considered transits (see Transit of Deimos from Mars).^[169][170]

Viewing

Animation of the apparent retrograde motion of Mars in 2003 as seen from Earth

Because the orbit of Mars is eccentric, its apparent magnitude at opposition from the Sun can range from −3.0 to −1.4. The minimum brightness is magnitude +1.6 when the planet is in conjunction with the Sun.^[7] Mars usually appears distinctly yellow, orange, or red; the actual color of Mars is closer to butterscotch, and the redness seen is just dust in the planet's atmosphere; considering this, NASA's Spirit rover has taken pictures of a greenish-brown, mud-colored landscape with blue-grey rocks and patches of light red sand.^[171] When farthest away from the Earth, it is more than seven times as far from the latter as when it is closest. When least favorably positioned, it can be lost in the Sun's glare for months at a time. At its most favorable times – at 15- or 17-year intervals, and always between late July and late September – Mars shows a wealth of surface detail to a telescope. Especially noticeable, even at low magnification, are the polar ice caps.^[172]

As Mars approaches opposition, it begins a period of retrograde motion, which means it will appear to move backwards in a looping motion with respect to the background stars. The duration of this retrograde motion lasts for about 72 days, and Mars reaches its peak luminosity in the middle of this motion.^[173]

Closest approaches

Relative

The point at which Mars's geocentric longitude is 180° different from the Sun's is known as opposition, which is near the time of closest approach to the Earth. The time of opposition can occur as much as 8½ days away from the closest approach. The distance at close approach varies between about 54^[174] and about 103 million km due to the planets' elliptical orbits, which causes comparable variation in angular size.^[175] The last Mars opposition occurred on March 3, 2012 at a distance of about 100 million km.^[176] The average time between the successive oppositions of Mars, its synodic period, is 780 days but the number of days between the dates of successive oppositions can range from 764 to 812.^[177]

As Mars approaches opposition it begins a period of retrograde motion, which makes it appear to move backwards in a looping motion relative to the background stars. The duration of this retrograde motion is about 72 days.

Absolute, around the present time

Mars oppositions from 2003–2018, viewed from above the ecliptic with the Earth centered

Mars made its closest approach to Earth and maximum apparent brightness in nearly 60,000 years, 55,758,006 km (0.372719 AU), magnitude −2.88, on 27 August 2003 at 9:51:13 UT. This occurred when Mars was one day from opposition and about three days from its perihelion, making Mars particularly easy to see from Earth. The last time it came so close is estimated to have been on September 12, 57 617 BC, the next time being in 2287.^[178] This record approach was only very slightly closer than other recent close approaches. For instance, the minimum distance on August 22, 1924 was 0.37285 AU, and the minimum distance on August 24, 2208 will be 0.37279 AU.^[149]

An email sent during the close approach in 2003 has, in succeeding years, repeatedly spawned hoax emails saying that Mars will look as big as the Moon.^[179]

Historical observations

The history of observations of Mars is marked by the oppositions of Mars, when the planet is closest to Earth and hence is most easily visible, which occur every couple of years. Even more notable are the perihelic oppositions of Mars which occur every 15 or 17 years, and are distinguished because Mars is close to perihelion, making it even closer to Earth.

Ancient and medieval observations

The existence of Mars as a wandering object in the night sky was recorded by the ancient Egyptian astronomers and by 1534 BCE they were familiar with the retrograde motion of the planet.^[180] By the period of the Neo-Babylonian Empire, the Babylonian astronomers were making regular records of the positions of the planets and systematic observations of their behavior. For Mars, they knew that the planet made 37 synodic periods, or 42 circuits of the zodiac, every 79 years. They also invented arithmetic methods for making minor corrections to the predicted positions of the planets.^[181][182]

In the fourth century BCE, Aristotle noted that Mars disappeared behind the Moon during an occultation, indicating the planet was farther away.^[183] Ptolemy, a Greek living in Alexandria,^[184] attempted to address the problem of the orbital motion of Mars. Ptolemy's model and his collective work on astronomy was presented in the multi-volume collection Almagest, which became the authoritative treatise on Western astronomy for the next fourteen centuries.^[185] Literature from ancient China confirms that Mars was known by Chinese astronomers by no later than the fourth century BCE.^[186] In the fifth century CE, the Indian astronomical text Surya Siddhanta estimated the diameter of Mars.^[187]

During the seventeenth century, Tycho Brahe measured the diurnal parallax of Mars that Johannes Kepler used to make a preliminary calculation of the relative distance to the planet.^[188] When the telescope became available, the diurnal parallax of Mars was again measured in an effort to determine the Sun-Earth distance. This was first performed by Giovanni Domenico Cassini in 1672. The early parallax measurements were hampered by the quality of the instruments.^[189] The only occultation of Mars by Venus observed was that of October 13, 1590, seen by Michael Maestlin at Heidelberg.^[190] In 1610, Mars was viewed by Galileo Galilei, who was first to see it via telescope.^[191] The first person to draw a map of Mars that displayed any terrain features was the Dutch astronomer Christiaan Huygens.^[192]

Martian "canals"

Map of Mars by Giovanni Schiaparelli

Mars sketched as observed by Lowell sometime before 1914. (South top)

Map of Mars from Hubble Space Telescope as seen near the 1999 opposition. (North top)

By the 19th century, the resolution of telescopes reached a level sufficient for surface features to be identified. A perihelic opposition of Mars occurred on September 5, 1877. In that year, Italian astronomer Giovanni Schiaparelli used a 22 cm (8.7 in) telescope in Milan to help produce the first detailed map of Mars. These maps notably contained features he called canali, which were later shown to be an optical illusion. These canali were supposedly long straight lines on the surface of Mars to which he gave names of famous rivers on Earth. His term, which means "channels" or "grooves", was popularly mistranslated in English as "canals".^[193][194]

Influenced by the observations, the orientalist Percival Lowell founded an observatory which had a 30 cm and 45 cm telescope (11.8 and 17.7 in). The observatory was used for the exploration of Mars during the last good opportunity in 1894 and the following less favorable oppositions. He published several books on Mars and life on the planet, which had a great influence on the public.^[195] The canali were also found by other astronomers, like Henri Joseph Perrotin and Louis Thollon in Nice, using one of the largest telescopes of that time.^[196][197]

The seasonal changes (consisting of the diminishing of the polar caps and the dark areas formed during Martian summer) in combination with the canals lead to speculation about life on Mars, and it was a long held belief that Mars contained vast seas and vegetation. The telescope never reached the resolution required to give proof to any speculations. As bigger telescopes were used, fewer long, straight canali were observed. During an observation in 1909 by Flammarion with a 84 cm (33 in) telescope, irregular patterns were observed, but no canali were seen.^[198]

Even in the 1960s articles were published on Martian biology, putting aside explanations other than life for the seasonal changes on Mars. Detailed scenarios for the metabolism and chemical cycles for a functional ecosystem have been published.^[199]

Spacecraft visitation

Foothills of Aeolis Mons ("Mount Sharp") (white balanced image).

Once spacecraft visited the planet during NASA's Mariner missions in the 1960s and 70s these concepts were radically broken. In addition, the results of the Viking life-detection experiments aided an intermission in which the hypothesis of a hostile, dead planet was generally accepted.^[200]

Mariner 9 and Viking allowed better maps of Mars to be made using the data from these missions, and another major leap forward was the Mars Global Surveyor mission, launched in 1996 and operated until late 2006, that allowed complete, extremely detailed maps of the Martian topography, magnetic field and surface minerals to be obtained.^[201] These maps are now available online, for example, at Google Mars. Mars Reconnaissance Orbiter and Mars Express continued exploring with new instruments, and supporting lander missions.

In culture

Mars is named after the Roman god of war. In different cultures, Mars represents masculinity and youth. Its symbol, a circle with an arrow pointing out to the upper right, is also used as a symbol for the male gender.

The many failures in Mars exploration probes resulted in a satirical counter-culture blaming the failures on an Earth-Mars "Bermuda Triangle", a "Mars Curse", or a "Great Galactic Ghoul" that feeds on Martian spacecraft.^[202]

Intelligent "Martians"

An 1893 soap ad playing on the popular idea that Mars was populated.

The popular idea that Mars was populated by intelligent Martians exploded in the late 19th century. Schiaparelli's "canali" observations combined with Percival Lowell's books on the subject put forward the standard notion of a planet that was a drying, cooling, dying world with ancient civilizations constructing irrigation works.^[203]

Many other observations and proclamations by notable personalities added to what has been termed "Mars Fever".^[204] In 1899 while investigating atmospheric radio noise using his receivers in his Colorado Springs lab, inventor Nikola Tesla observed repetitive signals that he later surmised might have been radio communications coming from another planet, possibly Mars. In a 1901 interview Tesla said:

It was some time afterward when the thought flashed upon my mind that the disturbances I had observed might be due to an intelligent control. Although I could not decipher their meaning, it was impossible for me to think of them as having been entirely accidental. The feeling is constantly growing on me that I had been the first to hear the greeting of one planet to another.^[205]

Tesla's theories gained support from Lord Kelvin who, while visiting the United States in 1902, was reported to have said that he thought Tesla had picked up Martian signals being sent to the United States.^[206] Kelvin "emphatically" denied this report shortly before departing America: "What I really said was that the inhabitants of Mars, if there are any, were doubtless able to see New York, particularly the glare of the electricity."^[207]

In a New York Times article in 1901, Edward Charles Pickering, director of the Harvard College Observatory, said that they had received a telegram from Lowell Observatory in Arizona that seemed to confirm that Mars was trying to communicate with the Earth.^[208]

Early in December 1900, we received from Lowell Observatory in Arizona a telegram that a shaft of light had been seen to project from Mars (the Lowell observatory makes a specialty of Mars) lasting seventy minutes. I wired these facts to Europe and sent out neostyle copies through this country. The observer there is a careful, reliable man and there is no reason to doubt that the light existed. It was given as from a well-known geographical point on Mars. That was all. Now the story has gone the world over. In Europe it is stated that I have been in communication with Mars, and all sorts of exaggerations have spring up. Whatever the light was, we have no means of knowing. Whether it had intelligence or not, no one can say. It is absolutely inexplicable.^[208]

Pickering later proposed creating a set of mirrors in Texas, intended to signal Martians.^[209]

In recent decades, the high-resolution mapping of the surface of Mars, culminating in Mars Global Surveyor, revealed no artifacts of habitation by 'intelligent' life, but pseudoscientific speculation about intelligent life on Mars continues from commentators such as Richard C. Hoagland. Reminiscent of the canali controversy, some speculations are based on small scale features perceived in the spacecraft images, such as 'pyramids' and the 'Face on Mars'. Planetary astronomer Carl Sagan wrote:

Mars has become a kind of mythic arena onto which we have projected our Earthly hopes and fears.^[194]

Martian tripod illustration from the 1906 French edition of The War of the Worlds by H.G. Wells.

The depiction of Mars in fiction has been stimulated by its dramatic red color and by nineteenth century scientific speculations that its surface conditions not only might support life, but intelligent life.^[210] Thus originated a large number of science fiction scenarios, among which is H. G. Wells' The War of the Worlds, published in 1898, in which Martians seek to escape their dying planet by invading Earth. A subsequent US radio adaptation of The War of the Worlds on October 30, 1938 by Orson Welles was presented as a live news broadcast, and became notorious for causing a public panic when many listeners mistook it for the truth.^[211]

Influential works included Ray Bradbury's The Martian Chronicles, in which human explorers accidentally destroy a Martian civilization, Edgar Rice Burroughs' Barsoom series, C. S. Lewis' novel Out of the Silent Planet (1938),^[212] and a number of Robert A. Heinlein stories before the mid-sixties.^[213]

Author Jonathan Swift made reference to the moons of Mars, about 150 years before their actual discovery by Asaph Hall, detailing reasonably accurate descriptions of their orbits, in the 19th chapter of his novel Gulliver's Travels.^[214]

A comic figure of an intelligent Martian, Marvin the Martian, appeared on television in 1948 as a character in the Looney Tunes animated cartoons of Warner Brothers, and has continued as part of popular culture to the present.^[215]

After the Mariner and Viking spacecraft had returned pictures of Mars as it really is, an apparently lifeless and canal-less world, these ideas about Mars had to be abandoned and a vogue for accurate, realist depictions of human colonies on Mars developed, the best known of which may be Kim Stanley Robinson's Mars trilogy. Pseudo-scientific speculations about the Face on Mars and other enigmatic landmarks spotted by space probes have meant that ancient civilizations continue to be a popular theme in science fiction, especially in film.^[216]

The theme of a Martian colony that fights for independence from Earth is a major plot element in the novels of Greg Bear as well as the movie Total Recall (based on a short story by Philip K. Dick) and the television series Babylon 5. Some video games also use this element, including Red Faction and the Zone of the Enders series. Mars (and its moons) were also the setting for the popular Doom video game franchise and the later Martian Gothic.

Mars surface details

Streaks on slopes in Acheron Fossae

Nanedi Valles inner channel

HiRISE view of the aftermath of an avalanche that cascaded down a steep 700 m slope at the edge of Mars's north polar layered terrain

Moons

Enhanced-color HiRISE image of Phobos, showing a series of mostly parallel grooves and crater chains, with its crater Stickney at right

Enhanced-color HiRISE image of Deimos (not to scale), showing its smooth blanket of regolith.

Mars has two relatively small natural moons, Phobos and Deimos, which orbit close to the planet. Asteroid capture is a long-favored theory, but their origin remains uncertain.^[217] Both satellites were discovered in 1877 by Asaph Hall, and are named after the characters Phobos (panic/fear) and Deimos (terror/dread) who, in Greek mythology, accompanied their father Ares, god of war, into battle. Mars was the Roman counterpart of Ares.^[218][219] In modern Greek, though, the planet retains its ancient name Ares (Aris: Άρης).^[220]

From the surface of Mars, the motions of Phobos and Deimos appear very different from that of our own moon. Phobos rises in the west, sets in the east, and rises again in just 11 hours. Deimos, being only just outside synchronous orbit – where the orbital period would match the planet's period of rotation – rises as expected in the east but very slowly. Despite the 30 hour orbit of Deimos, 2.7 days elapse between its rise and set for an equatorial observer, as it slowly falls behind the rotation of Mars.^[221]

Orbits of Phobos and Deimos (to scale)

Because the orbit of Phobos is below synchronous altitude, the tidal forces from the planet Mars are gradually lowering its orbit. In about 50 million years, it could either crash into Mars's surface or break up into a ring structure around the planet.^[221]

The origin of the two moons is not well understood. Their low albedo and carbonaceous chondrite composition have been regarded as similar to asteroids, supporting the capture theory. The unstable orbit of Phobos would seem to point towards a relatively recent capture. But both have circular orbits, very near the equator, which is very unusual for captured objects and the required capture dynamics are complex. Accretion early in the history of Mars is also plausible, but would not account for a composition resembling asteroids rather than Mars itself, if that is confirmed.

A third possibility is the involvement of a third body or some kind of impact disruption.^[222] More recent lines of evidence for Phobos having a highly porous interior^[223] and suggesting a composition containing mainly phyllosilicates and other minerals known from Mars,^[224] point toward an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit,^[225] similar to the prevailing theory for the origin of Earth's moon. While the VNIR spectra of the moons of Mars resemble those of outer belt asteroids, the thermal infrared spectra of Phobos are reported to be inconsistent with chondrites of any class.^[224]

Mars may have additional moons smaller than 50–100 meters, and a dust ring is predicted between Phobos and Deimos.^[226]

Views

Global mosaic of Mars from data gathered by the Viking 1 orbiter in 1980, showing Valles Marineris (center) and the Tharsis Montes (left).

Mars in 1999, by the Hubble Space Telescope's Wide Field and Planetary Camera 2. The view is centered on Syrtis Major, a basaltic plateau that is dark because reddish iron oxide dust is absent. To the right, clouds overlie the volcanoes of Elysium.

Hubble's ACS/HRC views Mars at its 2003 opposition, yielding the sharpest visible-light (RGB) photo yet taken from Earth. At about 8 km / pixel, various martian craters and markings are revealed. The ACS "Fastie finger" is blocking light on the left.[227]