The Planet Mars:
A History of Observation and Discovery

William Sheehan



Chapter 14
The Hurtling Moons of Mars

Obviously the Viking missions were a watershed in the study of Mars. Since then, three more spacecraft have been to the planet. In July 1988, two Russian spacecraft, Phobos 1 and Phobos 2, were launched. Contact was lost with Phobos 1 on its way out from Earth, but Phobos 2 successfully entered Martian orbit in January 1989. During the next fifty-nine days it obtained enough photographs to map nearly the entire planet---unfortunately, the full results have not yet been published in the West. There was also the American Mars Observer, which, in a stunning setback, went dead in August 1993, just as it was entering the final phase of its approach to the planet---only three days from its destination!

The primary objective of the Russian Phobos mission had been not the planet itself, but Phobos, the larger of the two Martian satellites. Plans called for placing a small lander on the surface of Phobos, but unfortunately, contact was lost in March 1989, just as Phobos 2 was starting to image the small moon and approach it for the landing phase.

Tiny as they are, the moons are intriguing worlds in their own right. The events leading up to their discovery by Asaph Hall in 1877 have already been discussed, but, strangely, their existence had been guessed on several earlier occasions, including by Jonathan Swift in 1726.

That year Swift published Gulliver's Travels, which describes the imaginary exploits of Lemuel Gulliver. Though his visit among the tiny Lilliputians is perhaps the best known, Gulliver made other explorations. On his "Voyage to Laputa," Gulliver learns that the scientists there
have . . . discovered two lesser stars, or satellites, which revolve about Mars; whereof the innermost is distant from the center of the primary planet exactly three of its diameters, and the outermost five; the former revolves in the space of ten hours, and the latter in twenty one and a half; so that the squares of their periodical times are very near in the same proportion with the cubes of their distance from the center of Mars; which evidently shows them to be governed by the same law of gravitation that influences the other heavenly bodies.1
Swift's prediction is surprising in that he not only had the number of moons right, but he also placed them close to the planet---the distances of the actual Martian moons are 1.4 and 3.5 diameters of Mars, compared with 3 and 5 as given by Swift. One would almost be tempted to think that Swift obtained an actual glimpse of the moons through a telescope, were it not for the fact that there was no telescope at the time anywhere close to being powerful enough to show them. Voltaire, in his 1750 story Micromégas, which tells of the visit by an inhabitant of the star Sirius to the solar system, also credited Mars with two moons, but here, at least, there is no mystery; he must have been influenced by Swift's tale.

The idea that Mars might have two satellites harks back still earlier, however, to Kepler's misconstrual of the anagram in which Galileo announced the discovery of what we now know to be the ring of Saturn.2 Probably Swift had learned of Kepler's earlier surmise. Moreover, since at the time he wrote it was believed that Mercury and Venus were companionless, Earth had one satellite, Jupiter had four, and Saturn had five, Mars's place in this progression seemed to call for two moons. Since they remained hidden, the moons had to be very small, and if they were very close to the planet they would be lost in its glare. However Swift arrived at his prediction, there can be no doubt that it was simply a lucky guess.

After the proper discovery of the satellites by Asaph Hall in August 1877, it was immediately apparent that they are highly unusual objects. Phobos lies at a distance of 9,400 kilometers from the center of Mars, or only 6,000 kilometers from the Martian surface. Mars seen from its surface would be an astounding sight; its disk would subtend an angle of 43°, and it would fill nearly half the sky from horizon to zenith! The present period of revolution of Phobos around Mars is only seven hours and thirty-nine minutes. Thus it completes three full revolutions in the time that Mars takes to rotate once on its axis---a state of affairs so surprising that Hall at first thought there must be two or three inner moons! Owing to its rapid motion, Phobos rises in the west and sets in the east, and it remains above the horizon for only four and a half hours at a time.

Because its orbital inclination is only about 1°, Phobos, for all practical purposes, lies in the equatorial plane of the planet. It is eclipsed by the planet's shadow 1,330 times every Martian year, managing to escape only for brief periods around the times of the summer and winter solstices. Observers on the Martian surface above 70° north and south latitudes would never catch sight of it at all, since it would never clear the horizon.

Deimos lies 23,500 kilometers from the center of Mars, and its orbit, too, is nearly equatorial. The period of revolution is about thirty hours, and it remains above the Martian horizon for sixty hours at a time. It never rises above the horizon in the polar regions above 82° north or south latitude.

In 1945, after analyzing measures of the positions of the satellites made since their discovery in 1877, B. P. Sharpless announced that Phobos appeared to be rapidly spiraling inward toward Mars.3 Such an acceleration could only be produced by some sort of drag, and in 1959 a Russian astronomer, Iosif Shklovskii, concluded that the drag was due to friction with the outer atmosphere of Mars. This was reasonable enough; however, in order to explain the rapid rate of its acceleration, Shklovskii went further and proposed that Phobos must be hollow inside---and that it might even be an artificial space station!4 Subsequently, someone suggested that the reason the satellites were not discovered until 1877, despite careful searches by William Herschel and Heinrich d'Arrest, was that they did not yet exist!

Needless to say, Shklovskii's view was always regarded with considerable skepticism, and later studies have shown that although Phobos is indeed spiraling inward toward Mars, the rate of its acceleration is only about half that derived by Sharpless---about 15° in orbital longitude since 1877. This is a small enough quantity to be accounted for by frictional forces due to tides raised by Phobos in the solid body of Mars. The acceleration will continue for another 40 million years or so, until the moon immolates itself by crashing into the planet.5

Owing to similar tidal forces, Deimos, whose period of revolution is slower than the period of Mars's spin, is spiraling very slowly outward from Mars; however, the effect is very slight and actually produces very little change in its orbit.

Both Martian satellites are tiny, and this, together with their proximity to the bright planet, explains why they were not discovered earlier. In Earth-based telescopes they are mere glints of light, and only with the advent of the spacecraft era have we begun to find out what they are really like (appendix 4).

The first close-up pictures of Phobos and Deimos were obtained by the Mariner 9 spacecraft in 1972; since then, they have also been imaged by the Viking orbiter spacecraft and the Russian Phobos, which sent back some useful results from Martian orbit in March 1989 before suddenly losing contact. Phobos, which measures 27 by 19 kilometers, is shaped rather like a potato; Deimos too is oddly shaped, though less so than Phobos, and measures 15 by 11 kilometers.

Both moons have suffered heavy bombardment and have numerous impact craters to show for it. Phobos has a particularly large one, named Stickney (after the maiden name of Asaph Hall's wife, who encouraged him to continue his flagging search for the moons). It is 10 kilometers across, and the impact that formed it must have come close to smashing Phobos into pieces. Radiating in all directions from Stickney are a series of ridges and grooves. The grooves are widest (700 m) and deepest (90 m) close to the crater itself, and they converge again near the crater's antipode, which is nearly groove-free. Obviously these features are intimately associated with Stickney itself, and seem to be deep-seated fractures formed during the impact. After Stickney, the largest craters on Phobos are Hall, Roche, Todd, Sharpless, and d'Arrest.

Deimos's surface appears different because most of the craters are partially filled with debris; in many cases they can be identified only because of their bright rims. The two largest, Swift and Voltaire, measure about 3 kilometers across.

The surfaces of both satellites are quite dark, so they are not very effective for lighting up the lonely Martian nights. From Mars, Phobos would appear only about as bright as Venus does from Earth, and Deimos would resemble the bright stars Vega or Arcturus. The Martian moons are thought to be captured asteroids (or asteroid fragments), and in many ways they resemble the asteroids that have thus far been imaged at close range; 951 Gaspra and 243 Ida even have grooves like those around Stickney. There can be little doubt that they are related kinds of objects.

But if Phobos and Deimos are captured asteroids, the details of their capture remain rather murky. Most asteroids stay within the main asteroid belt, but at 2.5 astronomical units (a.u.) there is a clear zone; asteroids there are in a resonance position with Jupiter---that is, they complete exactly three revolutions for every revolution that Jupiter completes. They are, then, regularly disturbed, and as a result their orbits are chaotic. Their orbital eccentricities can become so great that they can even cross the orbits of the other planets---many cross the orbit of Mars, and a few, known as the Apollo group, veer inside that of the Earth.

Rarely, one of these asteroids might be captured by Mars, but if so, it would first have to lose energy, perhaps through aerodynamic drag. Soon after its formation, Mars may have been surrounded by a nebula; an asteroid passing through this nebula would have been slowed enough by friction for its orbit to decay, first into a closed elliptical path around Mars, and later into a more circular orbit. It would continue to spiral quickly in toward Mars until it reached the point where its period became synchronous with the rotation of the planet, after which there would have been little relative velocity between the captured object and the nebula. At this point it would have been stabilized. This would have occurred early in the history of the solar system, when space was still cluttered with rubble. An impact with a stray object later may have broken the synchronous moon apart---the fragment which then became Phobos landed inside the synchronous position, and owing to tidal forces has continued to spiral inward ever since, while that which became Deimos landed outside, close to its present position.6

This is plausible enough, but is it true? At the moment we simply do not know; it remains equally possible that the satellites are planetesimals left behind within Mars's gravitational sphere of influence after the planet itself was formed---examples of the kind of objects whose impacts on Mars created the Hellas and Argyre basins during the violent bombardment of the Noachian Age.

We still have a great deal to learn about the Martian moons, but it is sobering indeed to realize that we now have detailed maps of the surfaces of these objects, which for almost a century after their discovery appeared in even the largest telescopes as mere specks of light.

© 1996 The Arizona Board of Regents

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The University of Arizona Press, 2/2/97 2:16PM