by Carl Sagan
Now, the reason that Venus has such an atmosphere and Earth does not seems to be a relatively small increment of sunlight. Were the Sun to grow brighter or Earth’s surface and clouds to grow darker, could Earth become a replica of the classical vision of Hell? Venus may be a cautionary tale for our technical civilization, which has the capability to alter profoundly the environment of Earth.
Despite the expectation of almost all planetary scientists, Mars turns out to be covered with thousands of sinuous tributaried channels probably several billion years old. Whether formed by running water or running CO2, many such channels probably could not be carved under present atmospheric conditions; they require much higher pressures and probably higher polar temperatures. Thus the channels-as well as the polar laminated terrain on Mars-may bear witness to at least one, and perhaps many, previous epochs of much more clement conditions, implying major climatic variations during the history of the planet. We do not know if such variations are internally or externally caused. If internally, it will be of interest to see whether the Earth might, through the activities of man, experience a Martian degree of climatic excursions-something much greater than the Earth seems to have experienced at least recently. If the Martian climatic variations are externally produced-for example, by variations in solar luminosity-then a correlation of Martian and terrestrial paleoclimatology would appear extremely promising.
Mariner 9 arrived at Mars in the midst of a great global dust storm, and the Mariner 9 data permit an observational test of whether such storms heat or cool a planetary surface. Any theory with pretensions to predicting the climatic consequences of increased aerosols in the Earth’s atmosphere had better be able to provide the correct answer for the global dust storm observed by Mariner 9. Drawing upon our Mariner 9 experience, James Pollack of NASA Ames Research Center, Brian Toon of Cornell and I have calculated the effects of single and multiple volcanic explosions on the Earth’s climate and have been able to reproduce, within experimental error, the observed climatic effects after major explosions on our planet. The perspective of planetary astronomy, which permits us to view a planet as a whole, seems to be very good training for studies of the Earth. As another example of this feedback from planetary studies on terrestrial observations, one of the major groups studying the effect on the Earth’s ozonosphere of the use of halocarbon propellants from aerosol cans is headed by M. B. McElroy at Harvard University-a group that cut its teeth for this problem on the aeronomy of the atmosphere of Venus.
We now know from space-vehicle observations something of the surface density of impact craters of different sizes for Mercury, the Moon, Mars and its satellites; radar studies are beginning to provide such information for Venus, and although it is heavily eroded by running water and tectonic activity, we have some information about craters on the surface of the Earth. If the population of objects producing such impacts were the same for all these planets, it might then be possible to establish both an absolute and a relative chronology of cratered surfaces. But we do not yet know whether the populations of impacting objects are common-all derived from the asteroid belt, for example-or local; for example, the sweeping up of rings of debris involved in the final stages of planetary accretion.
The heavily cratered lunar highlands speak to us of an early epoch in the history of the solar system when cratering was much more common than it is today; the present population of interplanetary debris fails by a large factor to account for the abundance of the highland craters. On the other hand, the lunar maria have a much lower crater abundance, which can be explained by the present population of interplanetary debris, largely asteroids and possibly dead comets. It is possible to determine, for planetary surfaces that are not so heavily cratered, something of the absolute age, a great deal about the relative age, and in some cases, even something about the distribution of sizes in the population of objects that produced the craters. On Mars, for example, we find the flanks of the large volcanic mountains are almost free of impact craters, implying their comparative youth; they were not around long enough to accumulate very much in the way of impact scars. This is the basis for the contention that volcanoes on Mars are a comparatively recent phenomenon.
The ultimate objective of comparative planetology is, I suppose, something like a vast computer program into which we put a few input parameters-perhaps the initial mass, composition, angular momentum and population of neighboring impacting objects-and out comes the time evolution of the planet. We are very far from having such a deep understanding of planetary evolution at the present time, but we are much closer than would have been thought possible only a few decades ago.
Every new set of discoveries raises a host of questions which we were never before wise enough even to ask. I will mention just a few of them. It is now becoming possible to compare the compositions of asteroids with the compositions of meteorites on Earth (see Chapter 15). Asteroids seem to divide neatly into silicate-rich and organic-matter-rich objects. One immediate consequence appears to be that the asteroid Ceres is apparently undifferentiated, while the less massive asteroid Vesta is differentiated. But our present understanding is that planetary differentiation occurs above a certain critical mass. Could Vesta be the remnant of a much larger parent body now gone from the solar system? The initial radar glimpse of the craters of Venus shows them to be extremely shallow. Yet there is no liquid water to erode the Venus surface, and the lower atmosphere of Venus seems to be so slow-moving that dust may not be able to fill the craters. Could the source of the filling of the craters of Venus be a slow molasseslike collapse of a very slightly molten surface?
The most popular theory on the generation of planetary magnetic fields invokes rotation-driven convection currents in a conducting planetary core. Mercury, which rotates once every fifty-nine days, was expected in this scheme to have no detectable magnetic field. Yet such a field is manifestly there, and a serious reappraisal of theories of planetary magnetism is in order. Only Saturn and Uranus have rings. Why? There is on Mars an exquisite array of longitudinal sand dunes nestling against the interior ramparts of a large eroded crater. There is in the Great Sand Dunes National Monument near Alamosa, Colorado, a very similar set of sand dunes nestling in the curve of the Sangre de Cristo mountains. The Martian and the terrestrial sand dunes have the same total extent, the same dune-to-dune spacing and the same dune heights. Yet the Martian atmospheric pressure is 1/200 that on Earth, the winds necessary to initiate the saltation of sand grains are ten times that for Earth, and the particle-size distribution may be different on the two planets. How, then, can the dune fields produced by windblown sand be so similar? What are the sources of the decameter radio emission on Jupiter, each less than 100 kilometers across, fixed on the Jovian surface, which intermittently radiate to space?
Mariner 9 observations imply that the winds on Mars at least occasionally exceed half the local speed of sound. Are the winds ever much larger? What is the nature of a transonic meteorology? There are pyramids on Mars about 3 kilometers across at the base and 1 kilometer high. They are unlikely to have been constructed by Martian pharaohs. The rate of sandblasting by wind-transported grains on Mars is at least 10,000 times that on Earth because of the greater speeds necessary to move particles in the thinner Martian atmosphere. Could the facets of the Martian pyramids have been eroded by millions of years of such sandblasting from more than one prevailing wind direction?
The moons in the outer solar system are almost certainly not replicas of our own, rather dull satellite. Many of them have such low densities that they must be composed largely of methane, ammonia or water ices. What will their surfaces look like close up? How will impact craters erode on an icy surface? Might there be volcanoes of solid ammonia with a lava of liquid NH3 trickling down the sides? Why is Io, the innermost large satellite of Jupiter, enveloped in a cloud of gaseous sodium? How does Io help to modulate the synchrotron emission from the Jovian radiation belt in which it lives? Why is one side of Iapetus, a moon of Saturn, si
x times brighter than the other? Because of a particlesize difference? A chemical difference? How did such differences become established? Why on Iapetus and nowhere else in the solar system in so symmetrical a way?
The gravity of the solar system’s largest moon, Titan, is so low and the temperature of its upper atmosphere sufficiently high that hydrogen should escape into space extremely rapidly in a process known as blow-off. But the spectroscopic evidence suggests that there is a substantial quantity of hydrogen on Titan. The atmosphere of Titan is a mystery. And if we go beyond the Saturnian system, we approach a region in the solar system about which we know almost nothing. Our feeble telescopes have not even reliably determined the periods of rotation of Uranus, Neptune and Pluto, much less the character of their clouds and atmospheres, and the nature of their satellite systems. The poet Diane Ackerman of Cornell University writes: “Neptune/is/elusive as a dappled horse in fog. Pulpy?/Belted? Vapory? Frost-bitten? What we know/wouldn’t/fill/a lemur’s fist.”
One of the most tantalizing issues that we are just beginning to approach seriously is the question of organic chemistry and biology elsewhere in the solar system. The Martian environment is by no means so hostile as to exclude life, nor do we know enough about the origin and evolution of life to guarantee its presence there or anywhere else. The question of organisms both large and small on Mars is entirely open, even after the Viking missions.
The hydrogen-rich atmospheres of places such as Jupiter, Saturn, Uranus and Titan are in significant respects similar to the atmosphere of the early Earth at the time of the origin of life. From laboratory simulation experiments we know that organic molecules are produced in high yield under such conditions. In the atmospheres of Jupiter and Saturn the molecules will be convected to pyrolytic depths. But even there the steady-state concentration of organic molecules can be significant. In all simulation experiments the application of energy to such atmospheres produces a brownish polymeric material, which in many significant respects resembles the brownish coloring material in their clouds. Titan may be completely covered with a brownish, organic material. It is possible that the next few years will witness major and unexpected discoveries in the infant science of exobiology.
The principal means for the continued exploration of the solar system over the next decade or two will surely be unmanned planetary missions. Scientific space vehicles have now been launched successfully to all the planets known to the ancients. There is a range of unapproved proposed missions that have been studied in some detail. (See Chapter 16.) If most of these missions are actually implemented, it is clear that the present age of planetary exploration will continue brilliantly. But it is by no means clear that these splendid voyages of discovery will be continued, at least by the United States. Only one major planetary mission, the Galileo project to Jupiter, has been approved in the last seven years-and even it is in jeopardy.
Even a preliminary reconnaissance of the entire solar system out to Pluto and a more detailed exploration of a few planets by, for example, Mars rovers and Jupiter entry probes will not solve the fundamental problem of solar system origins; what we need is the discovery of other solar systems. Advances in ground-based and spaceborne techniques in the next two decades might be capable of detecting dozens of planetary systems orbiting nearby single stars. Recent observational studies of multiple-star systems by Helmut Abt and Saul Levy, both of Kitt Peak National Observatory, suggest that as many as one-third of the stars in the sky may have planetary companions. We do not know whether such other planetary systems will be like ours or built on very different principles.
We have entered, almost without noticing, an age of exploration and discovery unparalleled since the Renaissance. It seems to me that the practical benefits of comparative planetology for Earthbound sciences; the sense of adventure imparted by the exploration of other worlds to a society that has almost lost the opportunity for adventure; the philosophical implications of the search for a cosmic perspective-these are what will in the long run mark our time. Centuries hence, when our very real political and social problems may be as remote as the very real problems of the War of the Austrian Succession seem to us, our time may be remembered chiefly for one fact: this was the age when the inhabitants of the Earth first made contact with the cosmos around them.
CHAPTER 11
A PLANET NAMED GEORGE
And teach me how
To name the bigger light, and how the less,
That burn by day and night…
WILLIAM SHAKESPEARE,
The Tempest, Act I, Scene 2
“Of course they answer to their names?” the Gnat remarked carelessly.
“I never knew them to do it,” [said Alice.]
“What’s the use of their having names,” said the Gnat, “if they won’t answer to them?”
LEWIS CARROLL,
Through the Looking Glass
THERE IS ON the Moon a small impact crater called Galilei. It is about 9 miles across, roughly the size of the Elizabeth, New Jersey, greater metropolitan area, and is so small that a fair-sized telescope is required to see it at all. Near the center of that side of the Moon which is perpetually turned toward the Earth is a splendid ancient battered ruin of a crater, 115 miles across, called Ptolemaeus; it is easily seen with an inexpensive set of field glasses and can even be made out, by persons of keen eyesight, with the naked eye.
Ptolemy (second century A.D.) was the principal advocate of the view that our planet is immovable and at the center of the universe; he imagined that the Sun and the planets circled the Earth once daily, imbedded in swift crystalline spheres. Galileo (1564-1642), on the other hand, was a leading supporter of the Copernican view that it is the Sun which is at the center of the solar system and that the Earth is one of many planets revolving around it. Moreover, it was Galileo who, by observing the crescent phase of Venus, provided the first convincing observational evidence in favor of the Copernican view. It was Galileo who first called attention to the existence of craters on our natural satellite. Why, then, is crater Ptolemaeus so much more prominent on the Moon than crater Galileo?
The convention of naming lunar craters was established by Johannes Höwelcke, known by his Latinized name of Hevelius. A brewer and town politician in Danzig, Hevelius devoted a great deal of time to lunar cartography, publishing a famous book, Selenographia, in 1647. Having hand-etched the copper plates used for printing his maps of the telescopic appearance of the Moon, Hevelius was faced with the question of what to name the features depicted. Some proposed naming them after Biblical personages; others advocated philosophers and scientists. Hevelius felt that there was no logical connection between the features on the Moon and the patriarchs and prophets of thousands of years earlier, and he was also concerned that there might be substantial controversy about which philosophers and scientists-particularly if they were still alive-to honor. Taking a more prudent course, he named the prominent lunar mountains and valleys after comparable terrestrial features: as a result we have lunar Apennines, Pyrenees, Caucasus, Juras and Atlas mountains and even an Alpine valley. These names are still in use.
Galileo’s impression was that the dark, flat areas on the moon were seas, real watery oceans, and that the bright and rougher regions densely studded with craters were continents. These maria (Latin for “seas”) were named primarily after states of mind or conditions of nature: Mare Frigoris (the Sea of Cold), Lacus Somniorum (the Lake of Dreams), Mare Crisium (the Sea of Crises), Sinus Iridum (the Bay of Rainbows), Mare Serenitatis (the Sea of Serenity), Oceanus Procellarum (the Ocean of Storms), Mare Nubium (the Sea of Clouds), Mare Fecunditatis (the Sea of Fertility), Sinus Aestuum (the Bay of Billows), Mare Imbrium (the Sea of Rains) and Mare Tranquillitatis (the Sea of Tranquillity)-a poetic and evocative collection of place names, particularly for so inhospitable an environment as the Moon. Unfortunately, the lunar maria are bone-dry, and samples returned from them by the U.S. Apollo and Soviet Luna missions imply that never in their past were they
filled with water. There never were seas, bays, lakes or rainbows on the Moon. These names have survived to the present. The first spacecraft to return data from the surface of the Moon, Luna 2, touched down in Mare Imbrium; and the first human beings to make landfall on our natural satellite, the astronauts of Apollo 11, did so, ten years later, in Mare Tranquillitatis. I think Galileo would have been surprised and pleased.
Despite Hevelius’ misgivings, the lunar craters were named after scientists and philosophers by Giovanni Battista Riccioli in a 1651 publication, Almagestum Novum. The title of the book means “The New Almagest,” the old Almagest having been the life’s work of Ptolemy. (“Almagest,” a modest title, means “The Greatest” in Arabic.) Riccioli simply published a map on which he placed his personal preferences for crater names, and the precedent and many of his choices have been followed without question ever since. Riccioli’s book came out nine years after the death of Galileo, and there has certainly been adequate opportunity to rename craters later. Nevertheless, astronomers have retained this embarrassingly ungenerous recognition of Galileo. Twice as large as crater Galileo is one called Hell after the Jesuit father Maximilian Hell.
One of the most striking of the lunar craters is Clavius, 142 miles in diameter and the site of a fictional lunar base in the movie 2001: A Space Odyssey. Clavius is the Latinized name of Christoffel Schlüssel (= “key” in German = Clavius), another member of the Jesuit order, and a supporter of Ptolemy. Galileo engaged in a protracted controversy on the priority of discovery and the nature of sunspots with yet another Jesuit priest, Christopher Scheiner, which developed into a bitter personal antagonism and which is thought by many historians of science to have contributed to the house arrest of Galileo, the proscription of his books, and his confession, extracted under threat of torture by the Inquisition, that his previous Copernican writings were heretical and that Earth did not move. Scheiner is commemorated by a lunar crater 70 miles across. And Hevelius, who objected altogether to the naming of lunar features after people, has a handsome crater named after himself.