Cosmic Connection (14 page)

I also hope that when the features on Phobos and Deimos are eventually named by the International Astronomical Union, one will be named after Mrs. Hall. But since another feature will surely be named after Asaph Hall, we have a problem–two craters named Hall would be confusing. In an astronomy talk I gave at Harvard University, I commented wistfully that the problem would be solved if only we knew Mrs. Hall’s maiden name. My friend Owen Gingerich, Professor of the History of Science at Harvard, instantly leaped to his feet with the words “Angelina Stickney” tripping off his lips. So when the time comes, I hope there will be a “Stickney” on one of the moons of Barsoom.

The subsequent study of Phobos and Deimos between 1877 and 1971 has a curious history. The moons of Mars are so tiny that they appear, even to the largest Earthbound telescopes, as dim points of light. They are too faint for the pre-1877 telescopes to have seen them at all. Their orbits can be calculated by noting their positions at various times. In 1944 at the U. S. Naval Observatory (where an understandably proprietary interest in Phobos and Deimos must have developed), B. P. Sharpless collected all the observations available in his day to determine the orbits to the best possible precision. He found–no doubt to his surprise–that the orbit of Phobos appeared to be decaying, what astronomers call a secular acceleration. Over longish periods of time the satellite seemed to be approaching more and more closely to Mars and moving more and more rapidly. This phenomenon is quite familiar to us today. The orbits of artificial satellites are decaying all the time in the Earth’s atmosphere. They are initially slowed by collisions with the diffuse upper atmosphere of the Earth, but by Kepler’s laws the net result is a more rapid motion.

Sharpless’ conclusion of a secular acceleration for Phobos remained an unexplained and almost unexamined curiosity until it was considered around 1960 by the Soviet astrophysicist I. S. Shklovskii. Shklovskii considered a wide range of alternative hypotheses for the secular acceleration, among them the influence of the Sun, the influence of a hypothetical magnetic field on Mars, and the tidal influence of the gravity of Mars. He found that none of these came close to working. He then reconsidered the possibility of atmospheric drag. The exact size of the Martian satellites was known poorly and indirectly in those days before spacecraft investigation of Mars, but it was known that Phobos was roughly ten miles in diameter. The altitude of Phobos above the surface of Mars was also known. Shklovskii and others before him found that the density of the atmosphere was far too low to produce the drag that Sharpless had deduced. It was at this point that Shklovskii made a brilliant and daring guess.

All the calculations showing atmospheric drag to be ineffectual had assumed that Phobos was an object of ordinary density. But what if its density were very low? Despite its enormous size, its mass would then be quite small, and its orbit could be appropriately affected by the thin upper atmosphere of Mars.

Shklovskii calculated the required density of Phobos, and found a value about one one-thousandth the density of water. There is no natural object or substance with such low density; balsa wood, for example, has about half the density of water. With such a low density, there was only one conclusion possible: Phobos had to be hollow. A vast hollow object ten miles across could not have arisen by natural processes. Shklovskii, therefore, concluded that it was produced by an advanced Martian civilization. Indeed, an artificial satellite ten miles across requires a technology far in advance of our own; it would also be far in advance of the technology imagined on Barsoom by Burroughs, which was a kind of sword and small spaceship technology.

Since there were no signs of such an advanced civilization on Mars today, Shklovskii concluded that Phobos–and possibly Deimos–had been launched in the distant past by a now extinct Martian civilization. (The interested reader may find more details of this remarkable argument of Shklovskii’s in the book
Intelligent Life in the Universe
, jointly authored by Shklovskii and myself. [San Francisco, Holden-Day, 1966; New York, Delta Books, 1967]). Subsequent to Shklovskii’s first work on the subject, the motions of the moons of Mars were reexamined in England by G. A. Wilkins, who found that possibly there was no secular acceleration. But he could not be sure.

Shklovskii’s extraordinary suggestion that the moons of Mars might be artificial is one of three hypotheses on their origin. The other two–certainly interesting in their own right, but naturally paling in comparison with the Shklovskii hypothesis–are (1) that the moons are captured asteroids, or (2) that they are debris left over from the origin of Mars itself.

Asteroids are hunks of rock and metal that go around the Sun between the orbits of Mars and Jupiter. There are unlikely, but theoretically possible, scenarios in which the gravitation of Mars can capture a close-passing asteroid.

In the Martian-debris hypothesis, it is imagined that pieces of rock of various sizes fell together to form Mars; that the last generation of such infalling pieces produced the large old impact craters on Mars (see page 131); and that Phobos and Deimos are by chance the only remnants still extant of the early catastrophic history of Mars.

It is clear that establishing any one of these three hypotheses on the origin of the moons of Mars would be a major scientific achievement.
The Mariner Mars mission of 1971, which I had the pleasure to work on, was originally to have involved two spacecraft,
Mariner 8
and
Mariner 9
. They were to be placed in different orbits for different purposes in the study of Mars itself. After these orbits were finally agreed upon, I noticed that they were not all that far from the orbits of Phobos and Deimos. It also seemed to me that television and other close-up observations of Phobos and Deimos by the Mariner spacecraft might permit us to determine something of their origin and nature.

I therefore approached officials of NASA, which organized and ran the mission, for permission to program observations of Phobos and Deimos. While the mission controllers at Jet Propulsion Laboratory, the actual operating organization, were not unsympathetic to this idea, some officials at NASA headquarters were against it. There was a mission plan, written in a large book, which stated what
Mariners 8
and
9
were about. Nowhere in the mission plan were Phobos and Deimos mentioned. Ergo, I could not look at Phobos and Deimos.

I pointed out that my proposal required only moving the scan platforms on the spacecraft so that the cameras could observe the Martian satellites. The response was negative again. A short time later, I advanced the argument that if Phobos and Deimos were indeed captured asteroids, examining them from
Mariner 9
was the equivalent of a free mission to the asteroid belt: The proposed scan platform maneuver would save NASA two hundred million dollars or so. This argument was judged, at least in some circles, to be more compelling. After about a year of my lobbying, a planning group on satellite astronomy was set up, and tentative plans were made for examining Phobos and Deimos. The satellite astronomy working group was, at my suggestion, chaired by Dr. James Pollack, a former student of mine; but it was a sign of NASA reluctance that the group was formed only after the launch of
Mariner
9, and only about two months before its arrival at Mars. (
Mariner 8
had, meanwhile, failed.)

When
Mariner 9
arrived at Mars, we found a planet almost entirely obscured by dust. Since there was little to look at on Mars, a great and previously undetectable enthusiasm for examining Phobos and Deimos dramatically materialized. The first step was to take wide-angle photographs from a distance in order to establish with some precision the orbits and locations of the moons. This task was accomplished in a preliminary way about two weeks after injection of the spacecraft into Martian orbit.
Mariner
9 has an orbital period of about twelve hours, so that it made close to two revolutions around Mars per day.

The television pictures from
Mariner 9
were radioed from Mars to Earth in much the same way that a newsprint wire-photo is transmitted on Earth. The picture is divided into a large number of small dots (for
Mariner
9, several hundred thousand dots), each dot with its own brightness, or shade of gray, running from black to white. After the picture is taken by the spacecraft and recorded there on magnetic tape, it is played back to Earth, dot by dot. The communication says, in effect: Dot number 3277, gray level 65; dot number 3278, gray level 62, and so on. The picture is reassembled by computer on the Earth–essentially by following the dots.

The first moderately close-up photograph of Phobos was obtained on revolution 31. Page 100 shows a Polaroid photo of the video-monitor image of Phobos on revolution 31, received on November 30, 1971. The image is much too indistinct to make any conclusions whatever.

Late that same night, Dr. Joseph Veverka, of Cornell, another former student of mine, and I worked into the small hours of the morning at the Image Processing Laboratory of JPL to bring out–by computer contrast-enhancement techniques–all of the detail present in the image. The result is shown on page 102. The shape is irregular. Are those blotches craters?

Our computer-enhanced photograph was constructed on the computer’s video monitor, line by line, from top to bottom. As the apparent large crater at the top gradually emerged, we saw a single bright spot at its center; for just a moment, I had the sense that we were seeing a star through an enormous hole in Phobos–or, even more chilling, that we were seeing an artificial light. But when we requested the computer to remove all single-bit errors, the bright spot went away.

On revolution 34,
Mariner 9
and Phobos came within less than four thousand miles of each other, one of the closest approaches in the entire mission. Late on the night of the receipt of that picture, Veverka and I were again computerenhancing. Our results were like those seen on the accompanying page 103. I am not sure what an artificial satellite ten miles across looks like, but this does not seem to be it. Phobos looks not so much like an artificial satellite as a diseased potato. It is, in fact, very heavily cratered. For it to have accumulated so many craters in that part of the Solar System, it must be very old, probably billions of years old.

Phobos appears to be an entirely natural fragment of a larger rock severely battered by repeated collisions; holes have been dug, pieces have been chipped off. It looks a little like the hand axes, chipped along natural fracture planes, made by our Pleistocene ancestors. There is no sign of technology on it. Phobos is not an artificial satellite. When pictures of Deimos were computer contrast-enhanced, the same conclusion applied to it.

Phobos and Deimos are the first satellites of another planet to have been photographed close-up. They were also observed by the ultraviolet spectrometer and the infrared radiometer on board
Mariner 9
. We have been able to determine their sizes and shapes and something of their color. They are extremely dark objects–darker than the darkest material that is likely to be in the room in which you are sitting right now.