Phobos (moon)
Phobos (
IPA , Greek
Φόβος: "Fright"), is the larger and innermost of
Mars' two
moons, and is named after
Phobos, son of
Ares (Mars) from
Greek Mythology. Phobos orbits closer to a major planet than any other moon in the solar system, less than 6000 km above the surface of Mars, and is also one of the smallest known moons in the solar system. Its
systematic designation is
Mars I. The adjectival form of the name is
Phobian, and in fairly common usage.
 |
The telescope used to discover Phobos |
Phobos was discovered by American astronomer
Asaph Hall on
August 18,
1877, at the
US Naval Observatory in
Washington, D.C., at about 09:14
GMT (contemporary sources, using the pre-1925 astronomical convention that began the day at noon, give the time of discovery as
August 17 16:06
Washington mean time). [
1]. Asaph Hall also discovered
Deimos, Mars' other moon.
The notebook of the discovery of Phobos by Asaph Hall is as follows:
"I repeated the examination in the early part of the night of [August] 11th, and again found nothing, but trying again some hours later I found a faint object on the following side and a little north of the planet. I had barely time to secure an observation of its position when fog from the River stopped the work. This was at half past two o'clock on the night of the 11th. Cloudy weather intervened for several days."On 15 August the weather looking more promising, I slept at the Observatory. The sky cleared off with a thunderstorm at 11 o'clock and the search was resumed. The atmosphere however was in a very bad condition and Mars was so blazing and unsteady that nothing could be seen of the object, which we now know was at that time so near the planet as to be invisible."On August 16 the object was found again on the following side of the planet, and the observations of that night showed that it was moving with the planet, and if a satellite, was near one of its elongations. Until this time I had said nothing to anyone at the Observatory of my search for a satellite of Mars, but on leaving the observatory after these observations of the 16th, at about three o'clock in the morning, I told my assistant, George Anderson, to whom I had shown the object, that I thought I had discovered a satellite of Mars. I told him also to keep quiet as I did not wish anything said until the matter was beyond doubt. He said nothing, but the thing was too good to keep and I let it out myself. On 17 August between one and two o'clock, while I was reducing my observations, Professor Newcomb came into my room to eat his lunch and I showed him my measures of the faint object near Mars which proved that it was moving with the planet."On August 17 while waiting and watching for the outer moon, the inner one was discovered. The observations of the 17th and 18th put beyond doubt the character of these objects and the discovery was publicly announced by Admiral Rodgers."The names were suggested by
Henry Madan (1838–1901), Science Master of
Eton, from Book XV of the
Iliad, where
Ares summons Fear and Fright.
Phobos was photographed close-up by
Mariner 9 in
1971,
Viking 1 in
1977,
Phobos 2 in
1988,
Mars Global Surveyor in
1998 and
2003, and by
Mars Express in
2004 .
 |
Orbits of Phobos and Deimos (to scale), seen from above Mars' north pole |
Phobos orbits Mars below the synchronous orbit radius, meaning that it moves around Mars faster than Mars itself rotates. Therefore it rises in the west, moves comparatively rapidly across the sky (in 4 h 15 min or less) and sets in the east, approximately twice a day (every 11 h 6 min). It is so close to the surface (in a low-inclination equatorial orbit) that it cannot be seen above the horizon from latitudes greater than 70.4°.
This low orbit means that Phobos will eventually be destroyed:
tidal forces are lowering its orbit, currently at the rate of about 1.8 metres per century, and in about 50 million years it will either impact the surface of Mars or (more likely) break up into a
planetary ring. Given Phobos' irregular shape (intermediate between a prolate and oblate
spheroid) and modeling it as a pile of rubble (specifically a
Mohr-Coulomb body), it has been calculated that Phobos is stable with respect to tidal forces, but it is estimated that Phobos will pass the
Roche Limit for a rubble pile of its description when its orbital radius drops to about 8400 km, and will probably break up soon afterwards
[Holsapple K.A. (2001), Equilibrium Configurations of Solid Cohesionless Bodies, Icarus, v. 154, p. 432â€"448 [2]]. Because of its ellipsoidal shape alone, the gravity on Phobos' surface varies by about 210%; the tidal forces raised by Mars more than double this variation (to about 450%) because they compensate for a little more than half of Phobos' gravity at its sub- and anti-Mars poles.
As seen from Phobos, Mars would be 6400 times larger and 2500 times brighter than the full
Moon as seen from Earth, taking up a full 1/4 of the width of a celestial hemisphere.
As seen from Mars' equator, Phobos would be one-third the angular diameter of the full
Moon as seen from
Earth. Observers at higher Martian latitudes (less than the 70.4° latitude of invisibility) would see a smaller angular diameter because they would be farther away from Phobos. Phobos' apparent size would actually vary by up to 45% as it passed overhead, due to its proximity to Mars' surface. For an equatorial observer, for example, Phobos would be about 0.14° upon rising and swell to 0.20° by the time it reaches the zenith. By comparison, the
Sun would have an apparent size of about 0.35° in the Martian sky.
Phobos' phases, in as much as they could be observed from Mars, take 0.3191 days to run their course (Phobos'
synodic period), a mere 13 seconds longer than Phobos' sidereal period.
|
A montage of three separate images taken by Viking 1 on October 19, 1978. The large crater (mostly in darkness) on the upper left of the image is Stickney |
Phobos is a dark body that appears to be composed of
C-type surface materials. It is similar to the C-type (blackish
carbonaceous chondrite)
asteroids that exist in the outer asteroid belt. Phobos's density is too low to be pure rock, however, and it is probably composed of a mixture of rock and ice.
The
Soviet spacecraft
Phobos 2 detected a faint but steady outgassing from Phobos. Unfortunately Phobos 2 failed before it could determine the nature of the material, but it is most likely water. Recent images from
Mars Global Surveyor indicates that Phobos is covered with a layer of fine dust about a metre thick, similar to the
regolith on the
Earth's
Moon.
Phobos is highly nonspherical, with dimensions of 27 × 21.6 × 18.8 km. It is heavily
cratered, and the most prominent surface feature is the large crater named
Stickney, after the maiden name of Asaph Hall's wife
Chloe Angeline Stickney Hall. Like
Mimas's crater
Herschel on a smaller scale, the impact that created Stickney must have almost shattered Phobos. The grooves and streaks on the surface were probably also caused by the Stickney impact. The grooves are typically less than 30 m deep, 100 – 200 m wide, and up to 20 km in length.
The unique
Kaidun meteorite is claimed to be a piece of Phobos, but this has been difficult to verify since little is known about the composition of the moon.
Phobos and Deimos both have much in common with carbonaceous (C-type)
asteroids, with very similar
spectra,
albedo and
density to those seen in C-type asteroids. This has led to speculation that both moons could have been captured into Martian orbit from the main
asteroid belt. However, both moons have very circular orbits which lie almost exactly in Mars'
equatorial plane, while captured moons would be expected to have eccentric orbits in random inclinations. Some evidence suggests that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large
planetesimal [Craddock R.A. (1994), The Origin of Phobos and Deimos, Abstracts of the 25th Lunar and Planetary Science Conference, held in Houston, TX, 14-18 March 1994., p.293].
Around
1958, the distinguished Russian astrophysicist
Iosif Samuilovich Shklovsky, studying the secular acceleration of Phobos' orbital motion, suggested a "thin sheet metal" structure for Phobos, a suggestion which led to speculations on Phobos' artificial origin. Shklovsky based his analysis on estimates of the upper martian atmosphere's density, and deduced that for the weak braking effect to be able to account for the secular acceleration, Phobos had to be very light —one calculation yielded a hollow iron sphere 16 km across but less than 6 cm thick.
Competing explanations were based on the
land tides Phobos could raise on Mars. The reality of the secular acceleration itself (corresponding to an altitude loss of about 5 per revolution, about 5 cm per year) was later subjected to doubt, and the problem vanished on its own by
1969. In a February 1960 letter to the journal
Astronautics, however,
Siegfried Frederick Singer, then science advisor to
President Eisenhower, came out in support of Shklovsky's theory, going as far as stating that "[Phobos'] purpose would probably be to sweep up radiation in Mars' atmosphere, so that Martians could safely operate around their planet". A few years later, in
1963,
Raymond H. Wilson Jr., Chief of Applied Mathematics at
NASA, allegedly announced to the
Institute of Aerospace Sciences that "Phobos might be a colossal base orbiting Mars", and that NASA itself was considering the possibility.
Similar "hollow Moon" claims occurred at around the same time.
In part 3 chapter 3 (the "Voyage to Laputa") of
Jonathan Swift's famous satire
Gulliver's Travels, a fictional work written in
1726, the astronomers of Laputa are described as having discovered two satellites of Mars orbiting at distances of 3 and 5 Martian diameters, and periods of 10 and 21.5 hours, respectively. The actual orbital distances and periods of Phobos and Deimos are 1.4 and 3.5 Martian diameters, and 7.6 and 30.3 hours, respectively. This is regarded as a fascinating coincidence; no telescope in Swift's day would have been even remotely powerful enough to discover these satellites.For a discussion and a possible explanation see [
3].
* The first episode of the
computer and video game Doom takes place in a
UAC base on Phobos.
* Phobos also featured in
Kim Stanley Robinson's
Mars trilogy as having a base built by the first colonists to Mars. Later the moon is overtaken by multinational forces and is
de-orbited by the Martian rebels.
* In the
comic Sailor Moon, Phobos is the name of one of two crows belonging to
Sailor Mars.
* Phobos is the final mission in the video game
Armored Core 2 where the player must battle to the center to destroy the orbit control mechanism and prevent it from crashing into Mars and destroying the newly formed Martian civilization.
* In the video game Unreal Tournament and Unreal Tournament 2004 a map is featured, which is located on an
orbital station orbiting Phobos.
* Phobos was referenced in the Sealab episode "Der Deiv", in which Captain Murphey knighted Marcos "Sir Phobos".
* In
Greg Bear's
Moving Mars, quantum computers allow the Martian colonists to teleport Phobos and Deimos above Earth, and this threat forces the Earth military to stand down from its confrontation with Mars.
* The video games
Unreal Tournament and
Unreal Tournament 2004 contain a deathmatch arena located on Phobos.
* Parts of
Leather Goddesses of Phobos, the
interactive fiction game from
Infocom, take place in fanciful interpretations of the environments of Phobos and Mars.
*
The Observatory, 1 (1877) 181*
Astronomische Nachrichten, 91 (1878) 13/14*
Monthly Notices of the Royal Astronomical Society, 38 (1878) 205*
Astronomische Nachrichten, 92 (1878) 47/48*
Asaph Hall and the Moons of Mars* I. S. Shklovsky,
The Universe, Life, and Mind, Acad. of Sc. USSR, Moscow, 1962
*
Irish Astronomical Journal, Vol. 6, p. 40, March 1963
*
E. J. Öpik,
Irish Astronomical Journal, Vol. 6, p. 281, September 1964
* S. F. Singer,
Astronautics, February 1960
* S. F. Singer,
On the origin of the Martian satellites Phobos and Deimos, Seventh International Space Science Symposium held 10-
18 May 1966 in Vienna, North-Holland Publishing Company, 1967
*
Deimos, the other moon of
Mars*
List of features on Phobos and Deimos*
Transit of Phobos from Mars*
Shadow of Phobos on Mars*
Flight around Phobos (movie)
*
Animation of Phobos
