February 20 1963 - The Satellites of Mars

Of all the satellites in the Solar System, the two dwarf attendants of Mars are the most peculiar. It has even been suggested that they are artificial space- stations launched by the Martians, though it is true that theories of this sort are difficult to take seriously. However, it seemed worth while to devote a programme to these curious little bodies, despite the fact that large telescopes are needed to show them.

In 1727 the great satirist Jonathan Swift published his story Voyage to Laputa, one of the remarkable episodes in the life of Dr Lemuel Gulliver. This time Gulliver visited the flying island of Laputa, and spent some time there. The people, he recorded, were assiduous astronomers, and paid great attention to the heavenly bodies:

They have made a catalogue of ten thousand fixed stars, whereas the largest of ours do not contain above one-third part of that number. They have likewise discovered two lesser stars, or satellites, which revolve about Mars, whereof the innermost is distant from the centre of the primary planet 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 that the squares of their periodical times are very near in the same proportion with the cubes of their distance from the centre of Mars, which evidently shows them to be governed by the same law of gravitation that influences the other heavenly bodies.

Thirty years later, the two Martian moons were also mentioned by Voltaire in his strange novel Micromegas. One part of his reasoning was amusing, even if not scientific. He said that since Mars is farther away from the Sun than we are, it receives less sunlight - and so how could it possibly manage with less than two moons?

However, both Gulliver and Micromegas were works of fiction, and there was no serious suggestion that Mars might be attended by satellites. Of "course, there was no theoretical objection; in 1783 a search was made by William Herschel, discoverer of the planet Uranus and possibly the greatest observer in astronomical history, while much later, between 1862 and 1864, another search was conducted by H. D'Arrest at the Copenhagen Observatory. But D'Arrest, like Herschel, was unsuccessful, and it was generally thought that Tennyson was right when he wrote of 'the snowy poles of moonless Mars'.

In 1877 Mars was excellently placed, and great attention was paid to it. This was the year in which G. V. Schiaparelli, in Milan, first drew attention to the features which have become famous (or, perhaps, notorious!) as the Martian canals. In addition, the search for satellites was recommenced, this time by Asaph Hall, with the aid of the large refractor at Washington. It was obvious that the satellite, or satellites, would be small and faint even if they existed at all, but the effort seemed to be worth while.

Left Astronomer  Asaph Hall using the main telescope at the United States Navel Observatory

Hall began his search in early August. For some time there was no result, but then, in Hall's own words:

The sweep round the planet was repeated several times on the night of the nth, and at 2.30 I found a faint object on the following side and a little north of the planet, which afterwards proved to be the outer satellite. I had hardly time to secure an observation of its position when fog from the Potomac River stopped the work. Cloudy weather intervened for several days. On the night of August 15, the sky cleared up at eleven o'clock and the search was resumed; but the atmosphere was in a very bad condition, and nothing was 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. On August 17, while waiting and watching for the outer satellite, I discovered the inner one. The observations of the 17th and 18th put beyond doubt the character of these objects... 7. Still, for several days the inner moon was a puzzle. It would appear on two different sides of the planet in the same night, and at first I thought there were two or three inner moons, since it seemed to me at that time very improbable that a satellite should revolve around its primary in less time than that in which the primary rotates. To decide this point I watched this moon throughout the nights of August 20 and 21 and saw that there was in fact but one inner moon, which made its revolution around its primary in less than one-third the time of the primary's rotation, a case unique in our Solar System.

At the suggestion of an Englishman, Hall named the two satellites Phobos and Deimos, after the two attendants of the mythological war-god.

The most remarkable fact concerned the period of the inner moon, Phobos, which proved to be 7 hours 39 minutes. The rotation period of Mars was known, with considerable accuracy, to be 24 hours 37 minutes, or roughly half an hour longer than that of the Earth. For Phobos, then, the 'month' was shorter than the 'day'; and as Hall remarked, the case was unique in the Solar System. So far as natural satellites are concerned, it still is.

Now let us return to Jonathan Swift, who had forecast the satellites a century and a half earlier. Swift knew that the rotation period of Mars was about 24 J hours, and he also knew that the planet's diameter was about 4,200 miles. It is of interest to compare the figures which he gave, against the actual values:

Swift gave the distance of Phobos as greater than it really is, and made the period too long, but the remarkable part of the whole episode is that in his day there was no telescope in the world capable of showing either satellite. Yet Swift correctly gave the period of the inner moon as much less than the rotation period of the primary planet. It can have been nothing more than a 'shot in the dark', but it was a strange prediction none the less.

Both Phobos and Deimos are extremely small. Their diameters are hard to determine with any precision, since their tiny disks are not measurable, and all that can be done is to make estimates according to their brightness, assuming normal albedo or reflecting power. It seems that Phobos may be ten miles across, and Deimos five. In any case, their diameters can hardly exceed fifteen and eight miles respectively, so that they are true dwarfs, and very different in nature from our own Moon or the large satellites of Jupiter and Saturn. Their gravitational pulls are very weak, and, obviously, neither body can retain any trace of atmosphere. It is not however true that a man could jump off either of them by sheer muscle-power, assuming that their density is comparable with that of the Earth or Mars.

The stellar magnitudes are about 10 for Phobos and n for Deimos, but the fact that the satellites are so close to Mars makes them unobservable with small telescopes; at its maximum elonga­tion, under the best possible conditions, Phobos is a mere 20" from the limb of Mars, while for Deimos the figure is 65". Even with the 33-inch refractor at Meudon Observatory, one of the most powerful telescopes in Europe, I have found them very elusive, and nothing can be discovered about their physical characteristics.

Both satellites move practically in the plane of the Martian equator, and since they are relatively so close in they would not be seen from the planet's poles; in fact Phobos would be permanently invisible from all Martian latitudes greater than 69 degrees, Deimos from latitudes greater than 82 degrees north or south. Even when at their best, they would be relatively feeble objects to a hypothetical Martian astronomer, and as a source of illumination at night they would be of little use. Phobos would rise in a westerly direction and set in the east; it would cross the sky in only four and a half hours, during which time it would go through more than half its cycle of phases from new to full and it would appear con­siderably larger when high up than when low down. The interval between successive risings would be just over eleven hours. Deimos would rise in the east and set in the west, in the conventional manner; but its revolution period is not a great deal longer than the rotation period of Mars, so that it would remain above the Martian horizon at any one place for more than sixty hours continuously. Of course, both satellites would suffer frequent eclipse by the shadow of Mars, and for a large part of the 687-day Martian 'year' they would be visible only in the twilight zones as they entered or left the shadow of the planet.

Neither satellite would appear large enough to produce a total eclipse of the Sun, and eclipse phenomena would be better termed 'satellite transits'. They would, of course, be frequent, but each would last for considerably less than three minutes - indeed, for less than one minute in the case of Phobos.

It has been suggested that in the future, when space-travel has been achieved, the journey to Mars may be done by using either Phobos or Deimos as a natural space-station. The basic idea is to carry out the journey to the satellite in a 'deep-space' craft, not designed to land on a large planet, and then drop down from the satellite to Mars in a different type of vehicle. Whether such a procedure will be necessary remains to be seen; on the whole it seems doubtful, and in any case it would be the reverse of easy, while it is not likely that any manned voyage to Mars will be attempted until techniques have advanced sufficiently to permit a direct landing on the planet. Meanwhile, something must be said about a really remarkable theory put forward by I. Shklovsky, of the USSR. According to Shklovsky, Phobos and Deimos are not normal satellites at all, but are space-stations which have been launched by the Martians for reasons of their own! It is probably true to say that such an idea would, under most circumstances have been dismissed lightly. However, Shklovsky is a world-famous astrophysicist, and has been responsible for some most important advances, so that his theories about Mars must be given due consideration, _even though few scientists will have any faith in them.

Phobos is very close to Mars; as we have seen, its distance from the planet's surface is only about 3,700 miles. Studies of secular changes in the orbital elements carried out by Sharpless in 1945 and by Kuiper in 1956, indicate that Phobos is slowly approaching Mars and that if such a trend continues a collision will take place in between 35,000,000 and 40,000,000 years from now. These figures have been challenged, but Shklovsky has used them as a basis.

As is well known, many of the artificial earth satellites launched since 1957 have been non-permanent, since they have been affected by atmospheric resistance and have been dragged down to destruction; in fact, a perigee distance of several hundreds of miles is necessary if an artificial satellite is to avoid this fate. Shklovsky supposes that Phobos is moving in a region where there is still a trace of the Martian atmosphere, and that it is being almost imperceptibly affected by the resistance, so that over an immensely long period it, too, will be brought down.

There is no valid reason why the Martian mantle should not linger on up to an altitude of 3,700 miles, but at such a height it must be extremely tenuous, and it could not possibly set up enough resistance to have any measurable effect upon a normal astronomical body about ten miles in diameter. Shklovsky, there­fore, goes on to suppose that Phobos must have virtually no mass. This would indicate that it must be hollow - and that it is artificial. The same would presumably apply to Deimos. Supporters of the idea also argue that the satellites were not seen before 1877 because they had not then been launched.

If the theory is accepted, it must also be conceded that the Martians are our technological superiors. Unfortunately, it seems that conditions on the planet are not favourable even for the existence of small animals, to say nothing of advanced life-forms.* Percival Lowell's picture of a highly-organized world, with a vast irrigation system to carry water from the polar ice-caps through to the arid regions near the equator, has long since been aban­doned; the canals have a basis of reality, but are not so narrow and artificial-looking as Lowell supposed. The Martian atmosphere, too, is thin and oxygen-poor. Primitive vegetation is as much as can be expected there.

All the evidence available to us indicates that there are no Martians, and it is very hard to take Shklovsky's theory of Phobos and Deimos at all seriously; I admit that I cannot pretend to do so. However, the satellites are certainly most peculiar objects, and their exact nature remains uncertain. Probably the most likely solution is that Phobos and Deimos are captured asteroids. They are of 'asteroid size', and they can hardly be of the same type as major satellites such as the Moon; they are too small. Moreover, Mars is not so very far outside the main asteroid belt, and there is no real reason to doubt that it could have captured two such bodies in the remote past. The small outer moons of Jupiter, as well as Phoebe in Saturn's system may also be ex-asteroids.

There is certainly a great deal about Mars that we still do not know, and many fascinating problems remain to be solved. It will be agreed, too, that the two dwarf satellites are by no means the least intriguing features of the Red Planet.

* Observation of Mars by means of balloon-carried instruments is now in progress and may produce interesting results in the near future.