Phobos - Just the facts

Phobos - Just the facts (A article written in 2007)

Phobos, is the larger & innermost of Mars' two moons (the other being Deimos), & 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 (3728 miles) above the surface of Mars, & is also one of the smaller known moons in the solar system. Its systematic designation is Mars I. It is a Greek word meaning Fright.

The telescope used to discover Phobos was ysed when 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). Hall also discovered Deimos, Mars' other moon. At the time, he was deliberately searching for Martian moons. Hall had previously seen what appeared to be a Martian moon on August 10. Due to bad weather, he did not definitively identify them until later.

The notebook of the discovery of Phobos by Hall is as follows:

I repeated the examination in the early part of the night of [August] 11th, & again found nothing, but trying again some hours later I found a faint object on the following side & 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 & the search was resumed. The atmosphere however was in a very bad condition & Mars was so blazing & 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, & the observations of that night showed that it was moving with the planet, & 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 & I let it out myself. On 17 August between one & two o'clock, while I was reducing my observations, Professor Newcomb came into my room to eat his lunch & 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 & watching for the outer moon, the inner one was discovered. The observations of the 17th & 18th put beyond doubt the character of these objects & the discovery was publicly announced by Admiral Rodgers.
—Asaph Hall

The names were suggested by Henry Madan (1838–1901), Science Master of Eton, from Book XV of the Iliad, where Ares summons Fear & Fright.

Jonathan Swift's 'prediction'
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 & 5 Martian diameters, & periods of 10 & 21.5 hours, respectively. The actual orbital distances & periods of Phobos & Deimos are 1.4 & 3.5 Martian diameters, & 7.6 & 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.

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) & 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°.

As seen from Phobos, Mars would be 6400 times larger & 2500 times brighter than the full Moon as seen from Earth, taking up a ¼ 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 & 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, inasmuch 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.
Death of a moon
Phobos' 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, & in 30-80 million years it will either impact the surface of Mars or (more likely) break up into a planetary ring. Given Phobos' irregular shape & 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 when its orbital radius drops to about 7100 km, & will probably break up soon afterwards.

The Soviet spacecraft Phobos 2 reported a faint but steady release of dust particles from Phobos. Phobos 2 failed before it could determine the nature of the material. Recent images from Mars Global Surveyor indicate that Phobos is covered with a layer of fine-grained regolith at least 100 meters thick.

Phobos is highly nonspherical, with dimensions of 27 × 21.6 × 18.8 km. Because of its shape alone, the gravity on its 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- & anti-Mars poles.

Phobos is heavily cratered. The most prominent surface feature is the Stickney crater, named after Asaph Hall's wife's maiden name, Chloe Angeline Stickney Hall. Like Mimas's crater Herschel on a smaller scale, the impact that created Stickney must have almost shattered Phobos. Many grooves & streaks also cover the oddly shaped surface. The grooves are typically less than 30 m deep, 100 to 200 m wide, & up to 20 km in length, & were originally assumed to have been the result of the same impact that created Stickney. Analysis of results from the Mars Express spacecraft, however, revealed that the grooves are in fact independent of Stickney, & are deposits of material thrown into space by impacts on the Martian surface.

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 detailed composition of the moon.

Phobos & Deimos both have much in common with carbonaceous (C-type) asteroids, with very similar spectra, albedo & 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. Captured moons would be expected to have eccentric orbits in random inclinations. Some evidence suggests that Mars was once surrounded by many Phobos- & Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large planetesimal.

"Hollow Phobos" claims
Around 1958, 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 that Phobos was of artificial origin. Shklovsky based his analysis on estimates of the upper Martian atmosphere's density, & 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.

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", & that NASA itself was considering the possibility.

While various theories attempted to explain the secular acceleration of Phobos, the existence of the acceleration was later subjected to doubt,[25] & the problem was solved by 1969.

These claims had been made using an overestimated value of 5 cm/yr for the rate of altitude loss, which has been revised to 1.8 cm/yr. Tidal effects had not been considered, but are now believed to explain the secular acceleration. The density of Phobos is now measured to be 1.9 g/cm³, which is inconsistent with a hollow shell.

Similar "hollow Moon" & "hollow Earth" claims have been made.

Phobos has been photographed close-up by several spacecraft whose primary mission has been to photograph Mars. The first was Mariner 9 in 1971, Viking 1 in 1977, Mars Global Surveyor in 1998 & 2003, & by Mars Express in 2004. The Phobos 1 probe was lost en route to Mars, while its sister spacecraft, Phobos 2 was lost prior to beginning detailed examination of the satellite although some data had been returned from earlier parts of the mission.

The Russian Space Agency is planning on launching a joint sample return mission with China in 2009, called Phobos-Grunt.

Astrium in the UK is planning a sample return mission.

The European Space Agency ESA, had missions which in 2004 took great photos of the Moon Phobos

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