Cosmic Connection (15 page)

Indeed, they are among the very darkest objects in the Solar System. Since there are so few objects this dark anywhere, we hope to be able to conclude something about their composition. They are both covered by at least thin layers of finely pulverized material. They provide important clues to collisional processes in the early Solar System. I believe we are looking at the end-product of a kind of collisional natural selection, in which fragments have been broken off from a larger parent body, and we are seeing only the two pieces, Phobos and Deimos, that remain. The moons of Mars are also important collision calibrators for Mars. Phobos, Deimos, and Mars have very likely been together in the same part of the Solar System for a very long period of time. The number of craters of a given size on Mars is much less, in general, than on Phobos and Deimos, providing important information on erosional processes that exist on Mars and that do not exist on airless and waterless Phobos and Deimos.

Because we now have the first good information on the size and shape of these objects, and because we now have good reason to think that they have typical densities of ordinary rock, we can calculate something about what it would be like to stand on, let’s say, Phobos. First of all, Mars, less than six thousand miles away, would fill about half the sky of Phobos. Marsrise would be a spectacular event. Eventual construction of an observatory on Phobos to examine Mars might not be such a bad idea. We know from
Mariner
9 that both Phobos and Deimos are rotating as our Moon does, always keeping the same face to their planet. When Phobos is above the day hemisphere of Mars, the reddish light of Mars would be enough to read by at night on Phobos.

Because of their small sizes, Phobos and Deimos have very low gravitational accelerations. Their gravities do not pull very hard. The pull on Phobos is only about one one-thousandth of that on Earth. If you can perform a standing high jump of two or three feet on Earth, you could perform a standing high jump of half a mile on Phobos. It would not take many such jumps to circumnavigate Phobos. They would be graceful, slow, arcing leaps, taking many minutes to reach the high point of the self-propelled trajectory and then to return gently to the ground.

Even more interesting would be a game like baseball on Phobos. The velocity necessary to launch an object into orbit about Phobos is only about twenty miles per hour. An amateur baseball pitcher could easily launch a baseball into orbit around Phobos. The escape velocity from Phobos is only about thirty miles per hour, a speed easily reached by professional baseball pitchers. A baseball that had escaped from Phobos would still be in orbit about Mars–a man-launched moonlet. If Phobos were perfectly spherical, a lonely astronaut with an interest in baseball could invent a curious but somewhat sluggish version of this already rather sluggish game. First, as pitcher, he could throw the ball sidearm–at the horizon at between twenty and thirty miles per hour. He could then go home for lunch, because it will take about two hours for the baseball to circumnavigate Phobos. After lunch, he can pick up a bat, face the other direction and await his pitch of two hours earlier. Apart from the fact that good pitchers are seldom good hitters, hitting this pitch would be pretty easy: About fifteen seconds elapse from the appearance of the baseball at the horizon to its arrival in the vicinity of our astronaut. If he swings and misses–or, more likely, if the ball is wide of the plate–he can then go home for a two-hour nap, returning with his catcher’s mitt to catch the ball. Alternatively, if he succeeds in hitting a fly ball at a velocity somewhere between twenty and thirty miles per hour, he can go home and take his nap, returning this time with a fielder’s mitt, awaiting the return of the ball from the opposite horizon two hours later. Because Phobos is gravitationally lumpy, the game would be more difficult than I have indicated. Since daylight on Phobos lasts only about four hours, lights would have to be erected, or the game modified so that all pitching, hitting, and catching events happen on the day side.

These sports possibilities may, one day a century or two hence, provide a tourist industry for Phobos and Deimos. But baseball on Phobos is no more an argument for going there than, to take a random example, golf is for going to the Moon. The scientific interest in the moons of Mars–whether captured asteroids or debris from the formation of the planet–is, however, immense. Sooner or later, certainly on a time scale of centuries, there will be instruments–and then men–on the surface of Phobos looking up with awe at an immense red planet that fills the sky from zenith to horizon.

And what about the opposite view? What do the moons of Barsoom look like from the surface of Mars? Because Phobos is so close to Mars, it would be seen as a clearly discernible disc, even though it is intrinsically such a tiny object. In fact, Phobos would appear as about half the apparent size of our Moon seen from the surface of Earth. We have found from
Mariner 9
that only one side of Phobos is visible from Mars, just as only one side of our Moon is visible from Earth. That face of Phobos is, more or less, the face on page 102. Until
Mariner 9
, no one–except Martians, if such there be–ever knew that face.

Because Phobos is so close to Mars, Kepler’s laws constrain it to move comparatively rapidly about the planet. It makes approximately 2½ revolutions about Mars in 24 hours. Deimos, on the other hand, takes 30 hours 18 minutes to revolve in its orbit once about Mars. Both moons revolve in their orbits in the same direction or sense as Mars rotates on its axis. Thus, Deimos rises in the east and sets in the west as–from terrestrial chauvinism–we believe a well-behaved satellite should. But Phobos makes it once around its orbit in less time than it takes for Mars to rotate. Accordingly, Phobos rises in the west and sets in the east, taking about 5½ hours to transit from horizon to horizon. This is not exactly “hurtling”–the motion would not be easily perceptible against the field of stars in a minute’s watching–but it’s not plodding, either. There will be some nights at the equator on Mars when Phobos sets in the east at sunset and then rises in the west well before dawn.

Phobos is so close to the equatorial plane of Mars that it is entirely invisible from the polar regions of the planet. If we were to imagine intelligent beings developing on Mars, astronomy might very well be the province of only the equatorial, and not the high-latitude, societies. I am not sure whether Helium was an equatorial kingdom.

Freud says somewhere that the only happy men are those whose boyhood dreams are realized. I cannot say that it has made my life carefree. But I will never forget those early-morning hours in a chilly California November when Joe Veverka, a JPL technician, and I were the first human beings ever to see the face of Phobos.

The State of California was kind enough to give me an automobile license plate marked “PHOBOS.” My car is not particularly sluggish, but it cannot circumnavigate our planet twice a day, either. The license plate pleases me. I would have preferred “BARSOOM,” but there is a strictly enforced limit of six letters per license plate.

T
he mountains of the Earth are the product of ages of geological catastrophes. The major folded mountain ranges are thought to be produced by the collision of enormous continental blocks during continental drift. The motion of continents toward and away from each other, at a rate of about an inch a year, seems terribly slow to us. But since the Earth is billions of years old, there has been plenty of time for continents to bang around all over our planet.

Lesser mountains were produced by volcanic events. Hot molten rock, called lava, upwells through tubes in the upper layers of the Earth–tubes of structural weakness through which the underlying pressure is relieved–and produces large surface piles of cooling volcanic slag. The resulting hole in the top of the volcanic mountain–the geologists call it a summit caldera–is the channel through which successive episodes of lava-upwelling occur. In the summit caldera of an active volcano, as, for example, in Hawaii, we can actually see molten lava. These individual volcanic mountains and mountain ranges, which are not really separate entities, are signs of a geologically vigorous and dynamic Earth.

What about Mars? It is a smaller planet than Earth; its central pressures and temperatures are less; it has a lower average density than Earth. These circumstances combine to suggest that Mars should be geologically less active than Earth, perhaps like the Moon. But even on the Moon, a much smaller object than Mars, with even lower anticipated interior temperatures than Mars, recent signs of volcanic activity have been uncovered by the Apollo missions. We do not even today understand the connection between the size and structure of a planet and the presence of volcanism and mountains, although we do know that there are no significant folded mountain ranges on the Moon.

Our present ignorance on this subject is exceeded by the ignorance of the early planetary astronomers, less than a century ago, as they peered through small telescopes and tried to guess what distant Mars was like. One of the earliest astronomers to commit himself on the question of mountains on Mars was Percival Lowell. Lowell believed (see Chapter 18) that he had found evidence of an extensive network of straight lines, crisscrossing the Martian surface with remarkable regularity and straightness, and that could only have been produced by a race of intelligent beings on that planet. He believed that these “canals” were truly canals carrying water. We now know that the problem was not so much with his logic as with his observations; none of the Mariner or other recent quantitative observations of Mars have shown any sign of the Lowellian canals.

In the 1890s Lowell argued that Mars must have no mountains, because mountains would be a severe impediment to the construction of a comprehensive network of canals. But surely a race that could construct a planet-wide network of canals should be able now and again to mow down an awkwardly placed mountain.

Nevertheless, Lowell was among the very first astronomers to apply an actual observational test to the question of mountains on Mars. He looked beyond the terminator. The terminator is the line–sharp or fuzzy, depending on the absence or presence of a planetary atmosphere–that separates the day from the night side of a planet. The terminator moves around the planet once a day–the local planetary day. But if there are mountains just on the night side of the terminator, the mountains will receive the rays of the setting Sun when their adjacent valleys are in darkness. Galileo first used this technique to discover what he called the mountains of the Moon–although the lunar mountains are mainly enormous pieces of rubble that fell out of the sky in the final phases of the formation of the Moon, rather than mountains of the terrestrial type, produced by a geologically active interior.

Lowell and his collaborators found cases of bright projections beyond the Martian terminator, illuminated by the rays of the setting Sun. But when they calculated their altitudes–an easy task for anyone grounded in high school geometry–the mountains were found to be many tens of miles high. Such elevations on Mars seemed to him absurd because of his canal argument. Moreover, the next day–the day on Mars is almost exactly the same length as a day on Earth–when the feature was seen again, its position had changed. This behavior is quite uncharacteristic of mountains of whatever origin, and Lowell correctly concluded that he had been seeing dust storms, in which fine particles from the Martian surface had been carried some tens of miles into the Martian atmosphere.

Such dust storms are also observed when we examine through the telescope the day side of Mars. We sometimes see that the characteristic configuration of bright and dark markings on the planet is temporarily obscured. There is an intrusion of bright-area material into the dark area, followed by a reappearance of the former configuration. These changes were interpreted in Lowell’s time as dust storms arising in the bright areas and obscuring the adjacent dark areas. The present interpretation, based on the full range of
Mariner 9
close-up observations, confirms this view (see Chapter 19).

Lowell and his contemporaries called the bright areas “deserts,” and this, too, seems to be an appropriate name. The Lowellians concerned themselves with the problem of whether bright areas tended to be higher or lower than dark areas, even though the elevation difference was expected to be extremely small. A dark area seen at the illuminated limb, or edge, of the planet seemed to be a notch or depression. But this could be understood merely in terms of the darkness of the dark area: If it were dark against a dark sky, we would not see it at all. We might gain the mistaken impression of a notch or depression. The prevailing opinion of most astronomers seems to have been that the dark areas were slightly lower than the bright, but the difference was estimated by Lowell as only half a mile or less.

In 1966, I re-examined this problem with Dr. James Pollack. We used two main arguments. Mars has in its winter hemisphere a large polar cap which, at various times, has been ascribed to frozen water or frozen carbon dioxide. Even at the present time its composition is unsettled; both substances are probably present. As the polar cap retreats in each hemisphere, once each year, there are regions where frost is left behind. Later, when the frost leaves these regions, they are found to be brighter than their surroundings. By analogy with the Earth, we might expect them to be high mountainous regions that remain frosted after the snows of the valleys have melted or evaporated. Indeed, one Martian polar region–the so-called Mountains of Mitchel–was identified as mountainous by this argument alone.