“They have likewise discovered two lesser stars, or satellites, which revolve around Mars, whereof the innermost is distant from the centre of the primary exactly three of his diameters, and the outermost five: the former revolves in the space of ten hours, and the latter in twenty-one and a half.”
So wrote Jonathon Swift in Gulliver’s Travels when Gulliver travels on from Lilliput to floating island of Laputa, a land inhabited by mathematicians and astronomers. Swift was writing in 1726, a century and a half before the two small Martian moons were actually discovered.
Swift’s description is surprisingly accurate. The innermost moon, Phobos, orbits at a mean distance of 9,376 km (2.76 Mars radii) from the Martian centre; the outermost, Deimos, orbits at a mean distance of 23,463 km (6.92 Mars radii) from the Martian centre. The orbital period of Phobos is 7 hrs 39 mins; that of Deimos is 30 hrs 18 min.
Inevitably, there has been speculation that Swift learned about the moons from visiting Martians. In fact, there is nothing particularly mysterious about the ‘discovery’. At the time, Jupiter was known to have four moons; Earth has one, and Mars could therefore have two. Any Martian moons had to be small and close to the planet, or they would already have been observed. Swift would have used Kepler’s laws of planetary motion to calculate the orbital periods. Voltaire, writing in 1752, also mentions two Martian moons. It is presumed that he was influenced by Swift.
The actual discovery came in August 1877. Asaph Hall was an astronomer at the United States National Observatory in Washington, DC. In 1875, he was put in charge of the Observatory’s 26-inch (66 cm) refracting telescope, then the largest refractor in the world (it would be surpassed by the 28-inch refractor at the Royal Greenwich Observatory in 1893). In 1877, Mars made a close approach to Earth, and Hall’s wife, mathematician Angeline Stickney, encouraged him to look for Martian moons. Hall himself had believed that the chance of finding any moons was so small that without Angeline’s encouragement he might have given up.
On 12 August, Hall sighted Deimos, but soon lost it due to fog rising from the Potomac River. Not until the 17th were weather condition again favourable, and he recovered Deimos on the other side of Mars to where he had first seen it. On the 18th, while waiting for Deimos to come into view, he found Phobos. Further observations confirmed the existence of the two satellites, and the discovery was announced by the USNO Superintendent, Admiral John Rogers the next day.
Hall named the moons Phobos (fear) and Deimos (terror) at the suggestion of Henry Madan, Science Master of Eton. Madan was inspired by Book XV of Homer’s Iliad in which Ares summons Fear and Fright.
Phobos has an apparent magnitude of +11.80 and Deimos +12.45, within the range of a good amateur telescope of 25 cm (10 inch) or more.
Although by no means the smallest moons in the Solar System, Phobos and Deimos are tiny. Phobos measures 27 x 22 x 18 km (17 x 14 x 11 miles) mean diameter 22.2 km (13.8 miles) and its mass is 1.08×1016 kg, and Deimos is 15 x 12 x 11 km (9 x 7.5 x 7 miles) mean diameter 12.6 km (7.8 miles) and a mass of 2.0×1015 kg. The surface gravity of Phobos is 0.0057 ms-2 or 5.8 x 10-4 times that of Earth and the escape velocity is 11.39 ms-1 or 41 km/hr (25 mph); for Deimos the surface gravity is 0.0030 ms-2 or 3.0 x 10-4 times that of Earth and the escape velocity is 5.56 ms-1 or 20 km/hr (12.5 mph). A high jump athlete could just about jump into space from Deimos, though not from Phobos.
Phobos orbits just 6,000 km (3,700 miles) above the Martian surface, closer to its primary than any other Solar System body, and it is only slightly further from Mars than London is from New York. It is so close to Mars that it is not visible south of 70.4°S or north of 70.4°N. The orbital period is far shorter than the Martian day of 24 hrs 37 mins, so as seen from the surface of Mars it rises in the west, moves across the sky in 4 hours and 15 minutes, and sets in the east.
The orbit of Phobos is decaying at a rate of 1.8 cm per year, meaning that it will eventually collide with Mars or be pulled apart by tidal forces. This could happen in 30 to 50 million years from now. In 1958, the Russian astrophysicist Iosif Samuilovich Shklovsky suggested that based on the braking effect of the Martian upper atmosphere and the observed rate of orbital decay, Phobos would have to be hollow – possibly a sphere with a diameter of 16 km (10 miles) but a thickness of only 6 cm (2.5 inch). The suggestion that Phobos was an alien space station cropped up in the science fiction of the time, for example Mission to the Heart Stars, by James Blish. Shklovsky was assuming a decay rate of 5 cm per years, which was later shown to be an overestimate. In fact, purely tidal effects can account for the orbital decay; because Phobos is orbiting faster than Mars rotates, these effects are pulling it down rather than pushing it further away, as is the case for Deimos. Both moons are tidally locked, keeping the same face to Mars at all times.
Phobos is heavily cratered. The largest crater, the 9 km (5.6 mile) diameter Stickney, is named for Asaph Hall’s wife Angeline Stickney. The crater takes up a substantial portion of the surface area of Phobos, and the impact that created it must have nearly shattered the moon. Hall has had to make do with a much smaller crater. Two other features, Laputa Regio and Lagado Planitia are named after places in Gulliver’s Travels. The surface also bears many grooves and streaks, typically less than 30 meters (98 ft) deep, 100 to 200 meters (330 to 660 ft) wide, and up to 20 km (12 miles) in length. The grooves were once thought to have been caused by the impact that formed Stickney, but they appear to be of different ages. One possibility is that they are ‘stretch marks’ caused by the tidal deformation of Phobos, but these are too weak to deform a solid body. The suggestion, therefore, is that Phobos is a ‘rubble pile’ surrounded by a layer of powdery regolith (loose material) about 100 m (330 ft) deep. If so, it will break up when it falls to within a distance of 2.1 Mars radii (6,800 km; 4,225 miles) of the centre at which point its feeble gravity will be overwhelmed by that of Mars. At all events, the density of Phobos is too low for it to be composed of solid rock.
Deimos is less heavily cratered than Phobos. Only two features have been given names: the craters Swift and Voltaire.
Phobos and Deimos both appear to be composed of C-type rock, similar to blackish carbonaceous chondrite asteroids. The traditional view is that they are captured asteroids, but the low eccentricity and inclination of their orbits argues against this. One possibility is that they were formed from ejecta produced a large asteroid collided with Mars.
Very little was known about the physical condition of either satellite prior to the space age. The first photographs were taken by the Mariner 7 fly-by probe in August 1969; two years later the first closeups were obtained by the Mariner 9 orbiter. The satellites have been extensively photographed since. Both have also been photographed by rovers on the Martian surface. Due to its low orbital inclination, Phobos regularly causes annular eclipses of the Sun, but as its apparent diameter from the Martian surface is only a third that of the moon, it is too small to cause a total eclipse. The eclipses last around thirty seconds.
No successful landings have yet been made on either, although the Russians have made two attempts to land probes on Phobos. Phobos 1 and Phobos 2 were launched in 1988. Phobos 1 was lost en route to Mars after a technician accidentally shut down the probe’s attitude thrusters. Phobos 2 reached Mars orbit successfully, and it returned images of both Mars and Phobos. It was then supposed to approach to within 50 m of Phobos and deploy a pair of landers, but during this phase a computer malfunction caused the probe to lose contact with Earth.
In November 2011, the Russians tried again. Fobos-Grunt (‘Phobos Ground’) was supposed to be a sample-return mission, but the spacecraft failed to leave orbit and eventually fell back to Earth. Since the failure of this mission, there have been a number of proposals for a sample return mission to Phobos, but none are likely to launch in the next few years.
Many proposals for human exploration of Mars call for landings on Phobos and Deimos as a first stage. Human missions to the Martian moons would result in the development and operation of new technologies, many of which would be required for an eventual landing on Mars, but without the attendant complexities and risks.