Mars' Moons: Facts About Phobos & Deimos Mars' Moons: Facts About Phobos & Deimos By Nola Taylor Redd, Space.com Contributor | June 27, 2016 06:00pm ET MORE Of the four rocky, terrestrial planets, only Mars has more than one moon. The two small bodies that orbit the red planet are both smaller than Earth's moon, and raise a number of questions about the formation of the early solar system. Identity crisis Phobos and Deimos bear more resemblance to asteroids than to Earth's moon. Both are tiny — the larger, Phobos, is only 14 miles across (22 kilometers), while the smaller, Deimos, is only 8 miles (13 km), making them some of the smallest moons in the solar system. Both are also made up of material that resembles Type I or II carbonaceous chondrites, the substance that makes up asteroids . With their elongated shapes, they even look more like asteroids than moons. Even from Mars , the moons don't look like moons. The more distant moon, Deimos, appears more like a star in the night sky.

When it is full and shining at its brightest, it resembles Venus as seen on Earth. Phobos has the closest orbit to its primary of any moon in the solar system, but still only appears a third as wide as Earth's full moon . Phobos orbits only 3,700 miles (6,000 km) from the Martian ground. Its surface is marred by debris that may have come from impacts on Mars. It travels around the planet three times a day, zipping across the Martian sky approximately once every four hours. The fast-flying moon appears to travel from west to east. Deimos orbits much farther away, tending to stay 12,470 miles (20,069 km) from the red planet's surface. The moon takes about 30 hours, a little over a Martian day, to travel around its host. Lunar origins Because of their odd shapes and strange composition, scientists thought for a long time that both moons were born asteroids. Jupiter's gravity could have nudged them into orbit around Mars, allowing the red planet to capture them. But the orbits of the moons make such a birth appear unlikely. Both moons take stable, nearly circular paths around the red planet.

Captured bodies tend to move more erratically. An atmosphere could have slowed the pair down and settled them into their present-day orbits, but the air on the Martian planet is thin and insufficient for such a task. It is possible that the moons formed like the planet, from debris left over from the creation of Mars. Gravity could have drawn the remaining rocks into the two oddly shaped bodies. Or, the moons could have spawned from a violent birth, much like Earth's moon . A collision, common in the early solar system, could have blown chunks of the red planet into space, and gravity may have pulled them together into the moons. Similarly, an early moon of Mars could have been impacted by a large object, leaving Phobos and Deimos as the only remaining bits. Discovery and death For years, scientists thought that Mars had no moon. Johannes Kepler suggested the possibility of two moons around the red planet, but only from a numerical standpoint; Earth had one moon and Jupiter, at the time, was known to have four , so the middle planet would likely have two. It wasn't until American astronomer Asaph Hall made a thorough study of the planet in 1877 that the tiny, closely orbiting bodies were found.

Hall discovered Deimos on Aug. 12 and Phobos on Aug. 18. The two tiny bodies had been hidden in the glare from the planet. Hall named the two satellites for the sons of the Greek god of war, Ares (Mars to the Romans). The twin boys, Phobos (Fear) and Deimos (Dread or Panic), attended their father in battle. But the sons won't be in attendance around Mars forever. Phobos is slowly spiraling inward at a rate of 6 feet (1.8 meters) every century. Within 50 million years, the moon will either collide with Mars or become a ring of rubble around it; stretch marks on its surface are likely early signs that the moon is breaking apart , one 2015 study said, although others have postulated these marks are instead grooves from Mars ejecta . Deimos, on the other hand, is slowly drifting away from the planet. Exploration While no mission has explored Phobos and Deimos as its primary objective, several spacecraft have snapped pictures during flybys. The first was NASA's Mariner 9 spacecraft, which orbited the planet Mars starting in 1971.

It took pictures of Deimos and Phobos from afar and showed that these were small moons that looked like potatoes. Several other orbiting spacecraft have performed long-range observations, including NASA's Viking orbiters (1970s and 1980s), the Soviet Phobos 2 mission (1980s), NASA's Mars Global Surveyor (1990s and 2000s), the European Mars Express mission (2000s), and NASA's MAVEN (Mars Atmosphere and Volatile Evolution mission). Russia attempted to send a mission to Phobos called Phobos-Grunt, in 2011, but the spacecraft became stuck in Earth orbit and fell back to Earth in 2012. While rovers from the surface of Mars can't get close to the moon, they can take pictures from the surface of what is going on; NASA's Spirit, Opportunity and Curiosity have all taken images. In particular, the Curiosity rover has taken some great photos and videos of Phobos and Deimos, including footage of Phobos eclipsing the sun.

To see these images, check out the following stories: Phobos, moon of Mars - The Solar System on Sea and Sky Mars Global Surveyor image of Phobos showing light colored material (NASA/JPL) Chariot of Fear Phobos [FOH-bohs] is the innermost of Mars' moons and is the largest. It is closer to its planet than any other moon in the Solar System. It is also one of the smallest moons in the Solar System. Phobos was named after Fear, one of the charioteers of the Roman god, Mars. In Greek mythology, Phobos is one of the sons of Ares (Mars) and Aphrodite (Venus). Phobos and its cousin, Deimos, were discovered in 1877 by American astronomer Asaph Hall while observing Mars at the U.S. Naval Observatory in Washington. Viking spacecraft image of Phobos showing craters (NASA/JPL) Viking spacecraft image of Phobos showing craters (NASA/JPL) A Tiny Moon Phobos is a tiny moon. At only 17 miles in length, it is the larger of Mars' two moons.

At a distance of 5826 miles (9380 km) from Mars, Phobos appears as no more than a tiny, bright dot in the Martian sky. It is so close to Mars that it cannot be seen above the horizon from all points on the planet. Because it is so close, it takes a little more than 7 hours to orbit Mars. This moon has a highly irregular shape, looking more like an asteroid than a moon. In fact, some astronomers believe that it may have been a large asteroid that was captured by the gravitational pull of Mars. Others have found evidence that contradicts this theory. We may never know exactly where this moon came from, or why it has such a strange shape. An interesting feature of Phobos is the fact that it appears to revolve around Mars in the opposite direction as its cousin Deimos. Both moons revolve in the same direction, but Phobos revolves in less than eight hours, only 1/3 the time it takes Mars to rotate on its axis. Phobos orbits Mars more than twice during each Martian day! This is because it orbits below the synchronous orbit radius of Mars.

Because its orbit is so low, tidal forces are causing its orbit to get lower every year. In about 50 million years, Phobos will either crash into the surface of Mars or be broken up into a ring. Features of Phobos Phobos is a rather unremarkable object. It is essentially just a large rock potted with several craters. The largest is named Stickney, after discoverer Asaph Hall's wife. This huge gash is six miles wide, nearly 1/3 the size of the moon itself! Two other large craters are named Hall and Roche. These are about half the size of Stickney. Several other smaller craters dot the moon's surface with an average size of 500 feet (152 meters). Another interesting feature found on Phobos is the presence of surface fractures. These were probably caused by the giant impacts that formed the large craters, or from the strong gravitational effects of Mars. Phobos is believed to be composed of carbon-rich rock like that found in C-type asteroids that exist in the outer asteroid belt. Due to the moon's low density, however, it is unlikely to be pure rock. It is probably composed of part rock and part ice.

The Soviet spacecraft, Phobos 2, detected some type of gas being ejected from Phobos. Astronomers are not sure what is causing this, but the best theory suggests water may be escaping from the moon's interior. Phobos has no atmosphere and no magnetic field. An Uncertain Future Because Phobos orbits so close to Mars, gravity is continually pulling it closer to the planet. It is believed that it will one day crash into Mars, possibly in as little as 10 million years. Once Phobos gets down to an altitude of only 2,250 miles (3,620 km) above the surface of Mars, it will enter what's known as the Roche limit. At this point, the tidal forces of Mars will begin to tear the small moon apart. When this happens, it may eventually turn it into a small ring that will continue to spiral into the planet. The Moons of Mars THE MOONS OF MARS 1 OVERVIEW Mars has two moons, Phobos and Deimos, that are both very small and irregular in shape.

They orbit quite close to the planet, and Phobos, the inner and larger moon, has the distinction of being the only natural satellite in the solar system that orbits its planet in less time than it takes the planet to rotate about its axis. Phobos - NASA Viking Orbiter Because of their small size, they were discovered only a little over a century ago. They are both very dark objects, covered in craters and possibly many metres of fine dust. However, strangest of all is the apparent knowledge of their existence 150 years before they were discovered! 2 DISCOVERY The English Astronomer William Herschal made an unsuccessful search for satellites of Mars in 1783. His renown was such that many astronomers were thereby convinced that Mars had no moons. A very careful search was conducted in 1864 at the Copenhagen Observatory by H L d'Arrest. However, he was no more succesful than was Herschal. It was not until 1877 that the two moons of Mars were discovered by Asaph Hall at the US Naval Observatory in Washington DC, USA. The outer satellite was discovered on August 11th and the inner one on August 17th.

The telecope used by Hall for his discoveries was a 26- inch refractor. 3 NAMING In the words of Asaph Hall, "Of the various names that have been proposed for these satellites, I have chosen those suggested by Mr Madan of Eton, England, viz: Deimos for the outer satellite; Phobos for the inner satellite. These are generally the names of the horses that draw the chariot of Mars; but in the lines referred to [in the 15th book of the Iliad] they are personified by Homer, and mean the attendants, or sons of Mars {the god of war}." The Greek word phobos means fear. The English word phobia is derived from this. Deimos is translated terror, or sometimes flight. It is thus easy to remember the order of these moons. Phobos is the inner moon, Deimos the outer. When fear is close, flight takes you further away. 4 DESCRIPTION The moons of Mars are so small that no detail can be seen with an Earth based telescope. It was not until 1971 that the US Mariner 9 spacecraft obtained the first images to show surface detail.

Both moons are irregular in shape, and have surfaces that are pockmarked by many craters. Phobos is the larger of the two moons and is roughly an ellipsoid 19.2 x 21.4 x 27.0 km across its 3 axes. Deimos is about 11 x 12 x 15 km. Deimos Image from Russian Phobos 2 spacecraft The surface of Phobos shows many sharp, uneroded craters of all sizes together with a vast network of linear features. These grooves are many kilometres long and hundreds of metres wide. They appear to be surface fractures associated with a very large (in comparison to the moon) crater called Stickney. This crater is 10 km in diameter and it dominates the moon's topography. Both moons have crater densities comparable to the lunar uplands, and this suggests an age of a few billion years for each body. 5 ORIGIN The irregular shapes of the moons and their heavy cratering shows that they have had a history of collisions and possibly fragmentation with other bodies. Several theories have been proposed for their origin. One belief is that these moons are asteroids that have been gravitationally captured by Mars.

Another is that they are fragments of an original larger Martian satellite that was struck and fragmented by an asteroid, the other pieces having escaped the Martian gravitational field. It appears unlikely that they could have simply grown from an orderly accumulation of many small grains of an original solar nebulae. Sample return missions from both Mars and its moons are needed to resolve this issue. 6 ORBITS Both moons orbit quite close to Mars. Phobos, in fact, lies just outside the Roche limit (that radius below which a gravitational-adhesive bound body may break up due to tidal forces induced by the primary). It is believed that there may well be significant internal movements of Phobos caused by tidal forces exerted on the moon by Mars. These could be measured by a seismometer placed on the surface. The orbital parameters are given in the table below: Parameter 7.65 30.3 We might note that the very low orbital eccentricity means that both moons have almost circular orbits. They also orbit very close to the equatorial plane of Mars (by contrast, the Earth's moon has an inclination that varies from 18 to 29 degrees). In the table above, the mean altitude is the average distance of the moon above the Martian surface.

The sidereal period is the time that it takes the moon to complete a revolution of 360 degrees in space (to come back to the same point with respect to the stars). This is not the time between two successive moonrises (see section 8). It is interesting to examine a table showing the period (sidereal) of a Martian orbital satellite with orbital radius: *MARS Orbiting Satellites* 1.36 1807.9 [Orbit of Deimos] Areosynchronous orbit is the Martian analog of geosynchronous orbit. This is the orbit that a spacecraft would need to remain stationary over a given point on the Martian equator. Because this orbit is close to that of the moon Deimos, satellite controllers may have a more difficult job than their terrestrial counterparts in station keeping communication and other satellites in this orbit (due to gravitational perturbations from Deimos). Fortunately the small mass of the moon will minimise such effects. Both moons have rotation periods that are locked to their orbital motion. This means they continually present the same face to Mars (as our Moon does to the Earth). The long axis of each moon points toward Mars.

This is due to gravity gradient stabilisation (the same phenomenon is sometimes used to stabilize low Earth orbiting satellites - using a long "gravity gradient" boom). And finally, it is now known that the orbit of Phobos is slowly decaying - coming closer to Mars - and its expected lifetime is about 50 million years. Atmospheric drag should make the last part of its de-orbit fairly rapid. 7 PROPERTIES Not a great deal is known about the properties of these two small moons. For several decades the Viking Orbiters provided the best images of the martian moons. However, recent missions such as the US Mars Global Surveyor and the European Mars Express have added both images and data to our knowledge. Phobos as seen by the European Space Agency Mars Express mission. Craters cover the surface of the moon, some with diameters that are substantial fractions of the moon's size. The largest crater is called Stickney. ESA image The surfaces of both Phobos and Deimos are very dark. This is seen by examining the albedo of several planets and moons in the solar system. BODY Callisto 0.26 This (visual geometric) albedo is a measure of the surface reflectivity of the object.

A value of 0 would mean a totally black surface, and a value of 1 would indicate that all the light incident on the object is reflected back. Of all the planets, Venus has the highest albedo due to its cloud cover. Phobos and Deimos, on the other hand are the darkest bodies (with the exception of some asteroids) in the solar system. It is no wonder they were so hard to find! The low albedo is probably due both to the composition - it is believed that both moons are similar to carbonaceous chondrite meteorites - and the pulverisation of this material by countless surface meteoroid impacts. The regolith (surface covering rock) may consist of a good depth of fine carbon-like material. The table below gives estimated masses, and average surface gravities. Parameter 40 25 We can thus see that the surface gravity is less than 1/1000 that of Earth. The escape velocity, particularly on Deimos would be low enough for an athletic person to achieve, and even normal activities could propel you into space, and you would be a long time returning back to the moon!

8 THE VIEW FROM MARS It is not the sidereal period of a satellite, but its synodic period that is important when observing from the planet's surface. This is the time between two successive appearances of the moon at the same place in the sky (for example between successive moonrises or moonsets). For Phobos this time is 11.1 hours. Thus you would see two moonrises and two moonsets on most Martian days (sometimes you may see three). Because this moon is below Areosynchronous orbit, it will rise in the west and set in the east. Deimos moves much more slowly in the sky. Because it is just above Areosynchronous orbit, it has a period only a few hours longer than the rotational period of Mars. It will rise in the east, move slowly across the sky and set in the west a little over 2.5 days latter. It will rise again in 5.5 days, its synodic period. Even though the moons of Mars are much closer to their primary (ie Mars) than is our Moon to the Earth, their small size still makes them appear smaller in the Martian sky than does the Moon in our sky.

When overhead, Phobos subtends an angle of 11 minutes of arc which is only about one third of the Moon as seen from Earth. Another difference is that this angle varies as Phobos moves across the sky. When rising or setting, this moon subtends only 8 minutes of arc to an observer on the equator. This is a 30% reduction in the size it shows at the zenith. It is interesting to speculate that because of the pyschological illusion of increased size near the horizons (where the eye has a reference of comparison), the physical size reduction may go largely unnoticed. In this image taken by the Mars Exploration Rover 'Spirit' from the surface of Mars, Phobos and Deimos are seen in the night sky close to one another, illustrating their relative brightness. NASA/JPL image Deimos, which is not only smaller physically, but also further away, will appear as barely more than a bright star with an angular diameter of 2 minutes of arc when overhead at the equator. This is only twice the angle subtended by Venus at its closest approach to Earth. The variation in size as Deimos moves across the sky will be rather smaller than for Phobos.

The irregular shape of Phobos will be clearly apparent from Mars, but that of Deimos may be barely noticed. That is, Phobos will clearly be non-circular, but Deimos will barely be perceived as a disk with no visible structure. The fact that both moons have very low inclination orbits means that they will not be visible from higher latitudes. Phobos will not be visible at latitudes above about 70 degrees, nor Deimos above about 83 degrees. The low inclination of the moon orbits also means that they will often pass into the shadow of Mars. Such moon eclipses will occur about 1400 times for Phobos and 130 times for Deimos each Martian year (about 687 Earth days). There are also a similar number of solar transits each year. Neither moon subtends a large enough angle to eclipse the sun, even though the sun subtends a smaller angle from Mars than it does from the Earth. The sun appears about two thirds of the size it does from Earth (21 minutes of arc). Phobos appears about one half this size and Deimos about one tenth.

Two eclipse images taken by the Mars Exploration Rover 'Spirit' from the surface of Mars, show the size of the two moons relative to the Sun. NASA/JPL image 9 A MYSTERY? In 1726 the British author Jonathon Swift wrote a satirical book called 'Gulliver's Travels'. In this work, Swift wrote that the inhabitants of Laputa had discovered two satellites that revolve around Mars. Not content with this, Swift even specified the orbital radius and periods of these moons as follows: Inner Moon 21.5 30.3 Many people have been astounded by Swift's apparent knowledge of these Martian moons 150 years before they were discovered by Asaph Hall. It is particularly interesting why Swift should have placed the inner moon so close to the planet, with such an unusually fast period of revolution. However, as amazing as this may seem at first glance, a detailed analysis shows that Swift's choices are quite in accord with the available scientific knowledge of the day. In 1610, following Galileo's discovery of 4 moons around Jupiter, Kepler speculated that Mars would be found to have 2 moons.

He based this on observations that in proceeding out from the Sun, Venus had no moons, Earth one, Mars two, the "missing" planet three, and Jupiter four. This belief would have been strengthened in Swift's time, as by then five moons had been discovered around Saturn. If we look at the orbital radii of the moons of Jupiter, we might see why Swift chose the orbits he did for the Martian moons. *Orbital radii of Jovian Moons in Jovian diameters* Moon 1 (Io) NASA - Under the Moons of Mars Under the Moons of Mars 11.19.07 Editor's Note: This is the 27th in a series of essays on exploration by NASA's Chief Historian, Steven J. Dick. A recent conference on the moons of Mars reminded me of the wonders that await us even in our own solar system. The two tiny moons, Phobos and Deimos, have a storied history. Long before their discovery in 1877, Johannes Kepler speculated that since the Earth had one moon and Jupiter had four known in his time, Mars might have two moons since it orbits between Earth and Jupiter.

In 1726 Jonathan Swift, probably influenced by this speculation, wrote in Gulliver's Travels that the scientists of Laputa discovered "two lesser stars, or satellites, which revolve about Mars." In real life, however, all searches failed until the American astronomer Asaph Hall discovered them using the giant 26-inch refractor of the U. S. Naval Observatory in Washington, D.C. The discovery was made because Hall came to doubt the conventional wisdom that Mars had no moon, and because he searched much closer to the planet than others had. In the early morning of August 17, 1877, having been urged on in the search by his wife Chloe Angeline Stickney, Hall made the remarkable discovery: not one, but two moons that traveled so close to Mars that they were within the glare of the planet. The inner moon was brighter, and zipped around Mars in less than 8 hours, faster than the planet's own rotation – instead of taking a leisurely month as does our Moon. To a hypothetical Martian, the inner moon would rise in the west and set in the east, and pass through all its phases in a few hours.

The news took both the public and the astronomical world by storm. Hall chose to name the inner satellite Phobos (Flight) and the outer satellite Deimos (Fear), based on Homer's Illiad, where Phobos and Deimos are the attendants, or sons, of Mars. Phobos as imaged on Aug. 19, 1998, by the Mars Global Surveyor. The spacecraft was approximately 1,080 km (671 mi) from Phobos at closest approach. Over the next century, the moons were revealed to be even stranger. It was recognized almost immediately from their brightness that they were small, maybe six or seven miles in diameter. Then in the mid-20th century, observations showed that Phobos was slowly spiraling into Mars, and would crash into the planet in less than 100 million years, an eyeblink in astronomical terms. Reputable scientists even suggested this motion might be explained by hollow artificial satellites lofted by the Martians. Alas, it is now known to be due to solid tidal interaction with Mars. It took the space age to reveal the real nature of the moons.

In 1971 the Mariner 9 spacecraft – the same one that showed Mars to have river-like dry channels – revealed that the moons were potato-shaped, and indeed only a few miles in size along their longest axis. In the 1970s surface features were given names, with Phobos landmarks named after individuals involved with its discovery, and Deimos features named after literary and artistic allusions. Thus on Phobos we find Hall and Stickney (with the latter much the larger), while Deimos sports Swift and Voltaire. The two tiny objects were no longer mere points of light, but new worlds with their own "geography." Much greater resolution was achieved when the two Viking Orbiters flew within a few hundred kilometers of the surfaces of the moons in the summer and fall of 1976, then as close as 100 to 300 kilometers in February and May 1977, detecting features as small as 10 to 200 meters. In October 1977 Viking Orbiter II flew less than 30 kilometers from Deimos.

The low resolution photos were useful in determining the sizes, shapes and rotational properties of the Martian satellites, while the higher resolution photos revealed small scale features such as surface fractures, crater chains, and wall slumping of craters. It was during a close flyby of Phobos in late 1976 that numerous mysterious grooves were discovered on the surface. More study showed they were likely associated with the crater Stickney, and may come from deep fractures under the surface. Deimos has no such grooves. A major objective of Vikings observations of the moons was to determine their composition through color measurements and mass determination, a goal in which it was only partially successful. In 1977 scientists noted that the surface of Phobos was similar to Type I or II carbonaceous chondrites and to the asteroid Ceres. From this the researchers inferred that Phobos and Deimos may have originated in the asteroid belt and were captured by Mars. Almost exactly one century after Hall had discovered the moons, enough information had been gleaned to speculate on their origin.

Further observations were made by the Soviet Phobos 2 mission in 1989, Mars Global Surveyor in 1998 and 2003, and the European's Mars Express beginning in 2004. The latter continues to make sporadic close flybys of Phobos with high resolution imaging, and the Mars Reconnaissance Orbiter is also studying the spectral properties of the Martian moons as part of its larger Mars mission. Thanks to the Mars Exploration Rovers Spirit and Opportunity, we even know how the moons appear from the surface -- as tiny dots due to their small size. Unlike the Earth, the Martian surface will not be moonlit, despite having two moons instead of one. Taking advantage of extra solar energy collected during the day, NASA's Mars Exploration Rover Spirit settled in for an evening of stargazing, photographing the two moons of Mars as they crossed the night sky. Spirit took this succession of images at 150-second intervals from a perch atop "Husband Hill" in Gusev Crater on Martian day, or sol, 594 (Sept.

4, 2005), as the faster-moving Martian moon Phobos was passing Deimos in the night sky. Phobos is the brighter object on the left and Deimos is the dimmer object on the right. Image Credit: NASA/JPL/Cornell/Texas A&M Despite these advances, a great deal remains unknown about the moons of Mars, including even their basic composition and their origin. The attendees at the First International Conference on the Exploration on Phobos and Deimos discussed what was known, what was unknown, and how to find out more. Most continue to believe the moons are captured asteroids, but the capture mechanism is unknown and the scenarios unlikely. Some believe rings of Mars may exist – just as they do around Jupiter, Saturn, Uranus and Neptune. Surprisingly, the fact they have not yet been seen is no proof of their absence – gossamer dust rings might be so thin that even spacecraft instruments have not yet seen them. In any case, it seems the two moons may hold the secret to the evolution of small bodies in the solar system. Conferees discussed the possibility of both human and robotic missions to the moons, and their possible use as outposts for the study of Mars close-up.

As various studies have emphasized since 1981, landing humans on the moons of Mars would save a considerable amount of money because the liftoff for return to Earth is much easier from such small bodies with less gravity. The moons of Mars have often been featured in popular culture, beginning (aside from Jonathan Swift) with the Mars novels of Edgar Rice Burroughs at the beginning of the 20th century. In 1955 science fiction writer Donald Wollheim published his bracing adventure novel The Secret of the Martian Moons (I still have my copy!). In Kim Stanley Robinson's Mars trilogy the first colonists to Mars build a city within the crater Stickney. More recently novelist Tom Mallon set his story Two Moons in 1877 Washington, D.C., and Ty Drago wrote his debut novel Phobos. Some video games even feature Phobos. Meanwhile, NASA continues to study its options for exploration of the Mars system, including even a human mission to a near earth asteroid, which may resemble the moons of Mars. Today, 130 years after Asaph Hall's discovery, Phobos and Deimos still hold their secrets.

As missions to Mars continue in the 21st century, they are likely to receive increased attention from both science and the general public. And these are only two of the 169 known moons of the solar system – each with its own history and mystery, each awaiting exploration. Further Reading Dick, Steven. "Asaph Hall, the Great Refractor and the Moons of Mars," chapter 6 in Sky and Ocean Joined: The U. S. Naval Observatory, 1830-2000 (Cambridge University Press, 2003). Lunar and Planetary Institute, First International Conference on the Exploration of Phobos and Deimos, Nov. 5-7, 2007. Program and abstracts at . Mallon, Thomas. Two Moons (Pantheon, 2000). Portree, David S. F. Humans to Mars: Fifty Years of Mission Planning, 1950-2000 (NASA SP-2001-4521). Online at . Steven J.

Dick Martian Moons | Mars Exploration Program PHOBOS AND DEIMOS (moons of Mars) Mars has two small moons: Phobos and Deimos. Phobos (fear) and Deimos (panic) were named after the horses that pulled the chariot of the Greek war god Ares, the counterpart to the Roman war god Mars. Both Phobos and Deimos were discovered in 1877 by American astronomer Asaph Hall. The moons appear to have surface materials similar to many asteroids in the outer asteroid belt, which leads most scientists to believe that Phobos and Deimos are captured asteroids. Two Moons Passing ESA - Space for Kids - Our Universe - Phobos and Deimos Phobos Phobos and Deimos In 1877, the American astronomer Asaph Hall discovered two small moons circling the planet Mars. They were named Phobos (Fear) and Deimos (Panic). More than 130 years later, scientists are still trying to unravel the mysteries of these moons. They may be asteroids – small rocks left over from the birth of the planets – that were captured by Mars. Both moons are blacker than coal and look like battered potatoes.

Phobos is 27 km across at its widest point and has a large impact crater on one side. It flies around Mars three times in one Martian day. Deimos is even smaller – one of the smallest moons in the Solar System. It is further from Mars, so each orbit around the planet takes a little over one day. Their small size means that gravity on both moons is very weak. An astronaut standing on Phobos would be 1000 times slighter than on Earth. Any large jumps might result in the astronaut flying off into space! Close flybys by ESA’s Mars Express spacecraft have provided clues about what these planets are made of and where they came from. Mars Express is the only spacecraft presently operating around the Red Planet which is in a highly elliptical (egg-shaped) orbit. This path takes it across the orbit of Phobos, enabling it to see all sides of the little moon.

(Like Earth’s Moon, Phobos always keeps the same face towards the nearby planet.) Between 23 July and 15 September 2008 , Mars Express carried out eight flybys of Phobos, sweeping past at distances ranging from 4,500 km to only 93 km. Close-up images show that Phobos, the larger moon, is a potato-shaped space rock measuring just 27 km x 22 km x 19 km. It is very dark - blacker than coal – and heavily cratered. By carefully tracking the way Phobos pulls on Mars Express during each flyby, scientists have found that the satellite is not very dense. It is almost certainly a ‘rubble pile’ - made of many chunks of rock, held together by gravity - rather than a single, solid object. Mars Express studies suggest that Phobos and Deimos may once have been asteroids that orbited the Sun between Mars and Jupiter. However, no one can yet explain how they were captured by Mars and ended up in ‘normal’ orbits above its equator.

Phobos: Facts About the Doomed Martian Moon Phobos: Facts About the Doomed Martian Moon By Nola Taylor Redd, Space.com Contributor | June 21, 2016 11:52pm ET MORE NASA's Mars Reconnaissance Orbiter took this image of the larger of Mars' two moons, Phobos, from a distance of about 6,800 km (about 4,200 miles). Credit: NASA/JPL/University of Arizona The Martian moon Phobos orbits only a few thousand miles above the Red Planet's surface. Its proximity to its planet is one of the reasons astronomers were unable to see the satellite until the late 19th century. In fact, the moon is getting closer to Mars over the centuries, and eventually will either break up or be pulled into the Martian surface. Robotic probes on Mars' surface can see Phobos passing overhead: Discovery and name In the early 17th century, German astronomer Johannes Kepler proposed that Mars might host two moons, given that it lay between the Earth and Jupiter, which were known to have one and four satellites. No evidence of such moons could be found.

Although most people thought that Mars had no moons, American astronomer Asaph Hall performed a methodical study from the U.S. Naval Observatory in Washington, D.C., searching closer to the planet than previous surveys. After searching without success, a frustrated Hall was about to give up, but his wife, Angelina, urged him to continue. The next night, August 12, 1877, he discovered the moon that would later be known as Deimos. Six days later, he found Phobos as well. The two moons lay so close to the planet that they were hidden by the glare of Mars. Asteroid-size, they are also two of the smallest moons in the solar system, with the larger Phobos 7.24 times as massive as its companion, Deimos . In Greek mythology, Phobos was one of the sons of the god of war Ares (Mars to the Romans). The twin sons attended their father in battle. Phobos means fear (as in phobia), while Deimos means flight (especially in the sense of fleeing after a defeat), according to NASA.

Exploring the moons NASA's Mars rover Curiosity snapped this photo of the larger Martian moon Phobos during a Mars sky observing session. Phobos is Mars' larger moon, but only 14 miles across. Image released Sept. 26, 2012. Credit: NASA/JPL It took almost another century for scientists to begin to understand the two tiny Martian moons. In 1971, NASA's Mariner 9 spacecraft became the first manmade satellite to orbit another planet. Images from the craft revealed that both Phobos and Deimos have lumpy, potato-like shapes, rather than spherical like most moons. Observations of Phobos were limited by the tidal locking of the moon to the planet, resulting in the same side always facing outward. As the exploration of continued, scientists were able to learn more information about the satellites circling Mars. The Viking orbiters flew by in the late 1970s. The Soviet Phobos 2 mission, NASA's Mars Global Surveyor , and the European Mars Express all provided more clues about the two curious moons.

Rovers from the planet's surface even got in on the act, with Spirit, Opportunity, and Curiosity all providing images from the ground . NASA's MAVEN (Mars Atmosphere and Volatile Evolution mission) has also imaged the moon to try to learn more about Phobos' composition. In 2011, Russia attempted to send a mission to the Martian moon called Phobos-Grunt, which was supposed to come back to Earth in 2014 bearing a small sample of the moon. The spacecraft, however, was marooned in Earth orbit due to issues with its rocket. Phobos-Grunt fell back to Earth in early 2012 and crashed into the Pacific Ocean. NASA may not be finished with Phobos yet. The space agency is considering the Phobos Surveyor mission , which would deploy small, hedgehog-like probes to the surface of the moon. The mission is funded under NASA's Innovative Advanced Concepts program, which looks at concepts that could be decades away from flight. One of the competing concepts for NASA's next low-cost, Discovery-class mission selection (expected to be completed in 2016) would explore both Martian moons.

Called PADME (Phobos And Deimos and Mars Environment), the spacecraft would try to figure out where the moons came from — the asteroid belt, by forming around Mars, or other scenarios. If selected, this mission would launch in 2020 and arrive in 2021. Some human spaceflight proponents have also suggested NASA should land people on Phobos before Mars ; NASA hopes to have a human mission sent there by the 2030s. Formation and composition The examination of Phobos and its companion revealed more than their odd, non-spherical shapes. The two moons are dark gray in color, and heavily cratered. The moons are some of the darkest, least reflective objects in the solar system. After observing the pair, scientists concluded that they were made of material similar to Type I or II carbonaceous chondrites, the material that makes up asteroids and dwarf planets. The composition and odd shape led some scientists to conclude that Phobos and Deimos came from the asteroid belt, with Jupiter's gravity long ago nudging them into orbit around Mars. However, scientists aren't certain that the asteroid belt is the source of the moons' birth. Both have a nearly circular orbit unusual for captured objects.

The thin atmosphere of Mars would have a hard time providing the necessary braking to settle the pair into their present-day orbits. Similarly, the moons are not as dense as objects in the asteroid belt. Instead, the moons may have formed as satellites around Mars, with dust and rock drawn together by gravity. A more violent birth may have occurred by collision. A large impactor smashing into the red planet could have sent pieces flying into the air, where gravity may have drawn them together. An existing moon might also have been destroyed, creating the rubble that later formed Phobos and Deimos. Traveling only 3,700 miles (6,000 kilometers) above the Martian surface, Phobos flies around the Red Planet three times a day. Crossing the sky in about four hours, the moon appears to rise in the west and set in the east. Many science fiction films feature large moons dominating alien skies, but despite its proximity to its planet, Phobos is only a third as wide as the full moon seen from Earth. On the other hand, Mars dominates the horizon of Phobos, taking up a fourth of the sky. Doomed But Phobos won't zip around Mars forever.

The doomed moon is spiraling inward at a rate of 1.8 centimeters (seven-tenths of an inch) per year, or 1.8 meters (about 6 feet) each century. Within 50 million years, the moon will either collide with its parent planet or be torn into rubble and scattered as a ring around Mars. The unusual characteristics of Phobos, including its decaying orbit, led some scientists in the 1950s and 1960s to conclude that it was artificial. One prominent proponent was the science adviser to President Dwight D. Eisenhower. Further examination revealed that the moon bears stronger resemblance to a rubble pile than an artificial satellite, and images sent back from orbiting craft show that it formed in nature. On the surface The larger of the two moons, Phobos has a diameter of 10 by 14 by 11 miles (17 by 22 by 18 km). The surface is covered with a dusty powder 3 feet (1 meter) thick, likely caused by meteor bombardment. A large impact crater dominates the moon.

Stretching nearly 6 miles (9.5 km), Stickney Crater covers most of the surface. The impact that formed it likely caused many secondary impacts, as rocks flew up and fell back to Phobos. The crater bears the maiden name of Hall's encouraging wife. The moon is marred by long grooves. In 2015, a NASA-led study suggested that the grooves on Phobos are actually early signs of the moon breaking apart due to tidal stresses induced by Mars' gravity. Another theory stated that the grooves could be remnants of a huge impact that left behind the crater Stickney, but the grooves did not center on this crater. Another explanation is the moon's grooves could come from material displaced from Mars that hits Phobos' surface, which was espoused in an Open University study published in 2014. Temperatures vary on Phobos, reaching highs of 25 degrees Fahrenheit (minus 4 degrees Celsius) during the day, while nights can be as cold as minus 170 degrees F (minus 112 degrees C). The moon is so small that a 150-pound person standing on its surface would weigh only two ounces.

Facts about Phobos:

Question: Mars has two moons. Phobos is one, but what is the other one called?
Answer: Deimos