by Carl Sagan
Beyond the asteroid belt, on the planets and large moons of the outer solar system, no nondescriptive names have so far been bestowed. Jupiter, for example, has a Great Red Spot and a North Equatorial Belt, but no feature called, say, Smedley. The reason is that when we see Jupiter we are looking at its clouds, and it would not be a very fitting or at least not a very long-lived memorial to Smedley to name a cloud after him. Instead, the present major question on nomenclature in the outer solar system is what to name the moons of Jupiter. The moons of Saturn, Uranus and Neptune have satisfying or at least obscure classical names (see Table 2). But the situation for the fourteen moons of Jupiter is different.
TABLE 2
NAMES OF THE SATELLITES OF THE OUTER PLANETS
Saturn :Neptune
Janus:Triton
Mimas:Nereid
Enceladus:
Tethys:Uranus
Dione:Miranda
Rhea:Ariel
Titan:Umbriel
Hyperion:Titania
Iapetus:Oberon
Phoebe:
:Pluto
:Charon
The four large moons of Jupiter were discovered by Galileo, whose theological contemporaries were convinced by a vague amalgam of Aristotelian and Biblical ideas that the other planets could have no moons. The contrary discovery by Galileo was disconcerting to fundamentalist churchmen of the time. Possibly in an effort to circumvent criticism, Galileo called the moons the Medicean satellites-after his funding agency. But posterity has been wiser: they are known instead as the Galilean satellites. In a similar vein, when William Herschel of England discovered the seventh planet he proposed calling it George. If wiser heads had not prevailed, we might today have a major planet named after George III. Instead we call it Uranus.
The Galilean satellites were assigned their Greek mythological names by Simon Marius (commemorated on the Moon by a crater 27 miles across), a contemporary of Galileo and a disputant with him for the priority of their discovery. Marius and Johannes Kepler felt that it would be extremely unwise to name celestial objects after real people and particularly after political personages. Marius wrote: “I want the thing done without superstition and with the sanction of theologians. Jupiter especially is charged by the poets with illicit loves. Especially well-known among these are three virgins, whose love Jupiter secretly coveted and obtained, namely: Io… Callisto… and Europa… Yet even more ardently did he love the beautiful boy Ganymede… and so I believe that I have not done badly in naming the first Io, the second Europa, the third, on account of the splendor of its light, Ganymede, and lastly the fourth Callisto.”
However, in 1892 E. E. Barnard discovered a fifth moon of Jupiter with an orbit interior to Io’s. Barnard resolutely insisted that this satellite should be called Jupiter 5 and by no other name. Since then, Barnard’s position has been maintained, and of the fourteen Jovian moons now known, only the Galilean satellites had, until recently, names officially sanctioned by the IAU. However unreasonable it may be, people show a strong preference for names over numbers. (This is clearly illustrated in the resistance of college students to being considered “only a number” by the college bursar; by the outrage of many citizens at being known to the government only by their social security number; and by the systematic attempts in jails and prison camps to demoralize and degrade the inmates by assigning them a numeral as their only identity.) Soon after Barnard’s discovery, Camille Flammarion suggested the name Amalthea for Jupiter 5 (Amalthea was in Greek legend the goat that suckled the infant Zeus). While being suckled by a goat is not precisely an act of illicit love, it must have seemed, to the Gallic astronomer, adequately close.
The IAU committee on Jovian nomenclature, chaired by Tobias Owen of the State University of New York at Stony Brook, has proposed a set of names for Jupiter 6 through 13. Two principles guided their selection: the name chosen should be that of “an illicit love” of Jupiter, but one so obscure as to have been missed by those indefatigable cullers of the classics who name asteroids, and must end with an a or an e depending on whether the moon goes around Jupiter clockwise or counterclockwise. But in the opinion of at least some classical scholars, these names are obscure to the point of bewilderment, and the result leaves many of the most prominent Jovian paramours unrepresented in the Jupiter system. The result is particularly poignant in that Hera (Juno), the wife so often scorned by Zeus (Jupiter), is not represented at all. Evidently, she was inadequately illicit. An alternative list of names, which includes most of the prominent paramours as well as Hera, is also shown in the table below. Were these names employed, it is true they would duplicate asteroid names. This is in any case already a fact for the four Galilean satellites, where the amount of confusion thus engendered has been negligible. On the other hand, there are those who support Barnard’s position that numbers are sufficient; prominent among these is Charles Kowal [10] of the California Institute of Technology, the discoverer of Jupiter 13 and Jupiter 14. There seems to be merit in all three positions and it will be interesting to see how the debate turns out. At least we do not yet have to judge the merits of contending suggestions for naming features on the Jovian satellites.
TABLE 3
PROPOSED NAMES FOR JOVIAN SATELLITES
Satellite -I.A.U. Committee Names-Alternative Names Suggested Here
J V-Amalthea-Amalthea
VI-Himalia-Maia
VII-Elara-Hera
VIII-Pasiphaë-Alcmene
IX-Sinope-Leto
X-Lysithea-Demeter
XI-Carme-Semele
XII-Anake-Danaë
XIII-Leda-Leda
XIV--
But that time is not long off. There are thirty-one known moons of Jupiter, Saturn, Uranus and Neptune. None has been photographed close up. The decision has recently been made to name features on the moons in the outer solar system after mythological figures from all cultures. However, very soon the Voyager mission will obtain high-resolution images of about ten of them, in addition to the rings of Saturn. The total surface area of the small objects in the outer solar system greatly exceeds the areas of Mercury, Venus, Earth, Moon, Mars, Phobos and Deimos together. There will be ample opportunity for all human occupations and cultures to be represented eventually, and I daresay provisions for nonhuman species can also be made. There are probably more professional astronomers alive today than in the total prior recorded history of mankind. I suppose that many of us will also be commemorated in the outer solar system-a crater on Callisto, a volcano on Titan, a ridge on Miranda, a glacier on Halley’s comet. (Comets, incidentally, are given the names of their discoverers.) I sometimes wonder what the arrangement will be-whether those who are bitter rivals will be separated by being placed on different worlds, and whether those whose discoveries were collaborative will nestle together, crater rampart to crater rampart. There have been objections that political philosophers are too controversial. I myself would be delighted to see two enormous, adjacent craters called Adam Smith and Karl Marx. There are even enough objects in the solar system for dead political and military leaders to be accommodated. There are those who have advocated supporting astronomy by selling crater names to the highest bidders, but I think this goes rather too far.
THERE IS A curious problem about names in the outer solar system. Many of the objects there have extremely low density, as if they were made of ice, great fluffy snowballs tens or hundreds of miles across. While objects impacting these bodies will certainly produce craters, craters in ice will not last very long. At least for some objects in the outer solar system, named features may be transient. Perhaps that is a good thing: it would give us a chance to revise our opinions of politicians and others, and will give eventual recourse if flushes of national or ideological fervor are reflected in solar system nomenclature. The history of astronomy shows that some suggestions for celestial nomenclature are better ignored. For example, in 1688 Erhard Weigel at Jena proposed a revision of the ordinary zodiacal constellations-the lion, virgin, fish
and water carrier that people have in mind when they ask you what “sign” you are. Weigel proposed instead a “heraldic sky” in which the royal families of Europe would be represented by their tutelary animals: a lion and a unicorn for England, for example. I hate to imagine descriptive stellar astronomy today had that idea been adopted in the seventeenth century. The sky would be carved into two hundred tiny patches, one for each nation-state existing at the time.
The naming of the solar system is fundamentally not a task for the exact sciences. It has historically encountered prejudice and jingoism and lack of foresight at every turn. However, while it may be a little early for self-congratulation, I think astronomers have recently taken some major steps to deprovincialize the nomenclature and make it representative of all of humanity. There are those who think it is a pointless, or at least thankless, task. But some of us are convinced it is important. Our remote descendants will be using our nomenclature for their homes: on the broiling surface of Mercury; by the banks of the Martian valleys; on the slopes of Titanian volcanoes; or on the frozen landscape of distant Pluto, where the Sun appears as a point of bright light in a sky of unremitting blackness. Their view of us, of what we cherish and hold dear, may be determined largely by how we name the moons and planets today.
CHAPTER 12
LIFE IN THE SOLAR SYSTEM
“I see nobody on the road,” said Alice.
“I only wish I had such eyes,” the King remarked in a fretful tone. “To be able to see Nobody! And at that distance too! Why, it’s as much as I can do to see real people, by this light!”
LEWIS CARROLL,
Through the Looking Glass
MORE THAN three hundred years ago, Anton van Leeuwenhoek of Delft explored a new world. With the first microscope he viewed a stagnant infusion of hay and was astounded to find it swarming with small creatures:
On April 24th, 1676, observing this water by chance, I saw therein with great wonder unbelievably very many small animalcules of various sorts; among others, some that were three to four times as long as broad. Their entire thickness was, in my judgement, not much thicker than one of the little hairs that cover the body of a louse. These creatures had very short, thin legs in front of the head (although I can recognize no head, I speak of the head for the reason that this part always went forward during movement)… Close to the hindmost part lay a clear globule; and I judged that the very hindmost part was slightly cleft. These animalcules are very cute while moving about, oftentimes tumbling all over.
These tiny “animalcules” had never before been seen by any human being. Yet Leeuwenhoek had no difficulty in recognizing them as alive.
Two centuries later Louis Pasteur developed the germ theory of disease from Leeuwenhoek’s discovery and laid the foundation for much of modern medicine. Leeuwenhoek’s objectives were not practical at all, but exploratory and adventuresome. He himself never guessed the future practical applications of his work.
In May of 1974 the Royal Society of Great Britain held a discussion meeting on “The Recognition of Alien Life.” Life on Earth has developed by a slow, tortuous step-by-step progression known as evolution by natural selection. Random factors play a critical role in this process-as, for example, which gene at what time will be mutated or changed by an ultraviolet photon or a cosmic ray from space. All the organisms on Earth are exquisitely adapted to the vagaries of their natural environments. On some other planet, with different random factors operating and extremely exotic environments, life may have evolved very differently. If we landed a spacecraft on the planet Mars, for example, would we even be able to recognize the local life forms as alive?
One theme which was stressed at the Royal Society discussion was that life elsewhere should be recognizable by its improbability. Take trees, for example. Trees are long skinny structures, above ground fatter at the top than at the bottom. It is easy to see that after millennia of rubbing by wind and water, most trees should have fallen down. They are in mechanical disequilibrium. They are unlikely structures. Not all top-heavy structures are produced by biology. There are, for example, pedestal rocks in deserts. But were we to see a great many top-heavy structures, all closely similar, we could make a reasonable guess that they were of biological origin. Likewise for Leeuwenhoek’s animalcules. There are many of them, closely similar, highly complex and improbable in the extreme. Without ever having seen them before, we correctly guess they are biological.
There have been elaborate debates on the nature and definition of life. The most successful definitions invoke the evolutionary process. But we do not land on another planet and wait to see if any nearby objects evolve. We do not have the time. The search for life then takes on a much more practical aspect. This point was brought out with some finesse at the Royal Society discussion when, after an exchange remarkable for its rambling metaphysical vagueness, Sir Peter Medawar rose to his feet and said, “Gentlemen, everyone in this room knows the difference between a live horse and a dead horse. Pray, therefore, let us cease flogging the latter.” Medawar and Leeuwenhoek would have seen eye to eye.
But are there trees or animalcules on the other worlds of our solar system? The simple answer is that no one yet knows. From the vantage point of the nearest planets, it would be impossible to detect photographically the presence of life on our own planet. Even from the closest orbital observations of Mars made to date, from the American spacecraft Mariner 9 and Viking 1 and 2, details on Mars much smaller than 100 meters across have remained invisible. Since even the most ardent enthusiasts of extraterrestrial life do not anticipate Martian elephants 100 meters long, many important tests have not yet been performed.
At the present time we can only assess the physical environments of the other planets, determine whether they are so severe as to exclude life-even forms rather different from those we know on Earth-and in the case of the more clement environments perhaps speculate on the life forms that might be present. The one exception is the Viking lander results, briefly discussed below.
A place may be too hot or too cold for life. If the temperatures are very high-say, several thousands of degrees Centigrade-then the molecules that make up the organism will fall to pieces. Thus it is customary to exclude the Sun as an abode of life. On the other hand, if the temperatures are too low, then the chemical reactions that drive the internal metabolism of the organism will proceed at too ponderous a pace. For this reason the frigid wastes of Pluto are customarily excluded as an abode of life. However, there may be chemical reactions which proceed at respectable rates at low temperatures but which are unexplored here on Earth, where chemists dislike working in laboratories at −230°C. We must be careful not to take too chauvinistic a view of the matter.
The giant outer planets of the solar system, Jupiter, Saturn, Uranus, and Neptune, are sometimes excluded from biological considerations because their temperatures are very low. But these temperatures are the temperatures of their upper clouds. Deeper down in the atmospheres of such planets, as in the atmosphere of the Earth, much more clement conditions are to be encountered. And they appear to be rich in organic molecules. By no means can they be excluded.
While we human beings enjoy oxygen, this is hardly a recommendation for it, since there are many organisms that are poisoned by it. If the thin protective ozone layer in our atmosphere, made by sunlight from oxygen, did not exist, we would rapidly be fried by ultraviolet light from the Sun. But on other worlds, ultraviolet sunshades or biological molecules impervious to near-ultraviolet radiation can readily be imagined. Such considerations merely underline our ignorance.
An important distinction among the other worlds of our solar system is the thickness of their atmospheres. In the total absence of an atmosphere it is very difficult to conceive of life. As on Earth, the biology on other planets must, we think, be driven by sunlight. On our planet, the plants eat the sunlight and the animals eat the plants. Were all the organisms on Earth forced (by some unimaginable catastrophe) into a subterranean existence, li
fe would cease as soon as accumulated food stores were exhausted. The plants, the fundamental organisms on any planet, must see the Sun. But if a planet has no atmosphere, not only ultraviolet radiation but X-rays and gamma rays and charged particles from the solar wind will fall unimpeded on the planetary surface and frizzle the plants.
Furthermore, an atmosphere is necessary for exchange of materials so that the basic molecules for biology are not all used up. On Earth, for example, green plants give off oxygen-a waste product-into the atmosphere. Many respiring animals, like human beings, breathe the oxygen and give off carbon dioxide, which the plants in turn imbibe. Without this clever (and painfully evolved) equilibrium between plants and animals, we would rapidly run out of oxygen or carbon dioxide. For these two reasons-radiation protection and molecular exchange-an atmosphere seems required for life.
Some of the worlds in our solar system have exceedingly thin atmospheres. Our Moon, for example, has at its surface less than one million millionth the atmospheric pressure on Earth. Six places on the near side of the Moon were examined by Apollo astronauts. No top-heavy structures, no lumbering beasts were found. Nearly four hundred kilograms of samples have been returned from the Moon and meticulously examined in terrestrial laboratories. There were no animalcules, no microbes, almost no organic chemicals, or even any water. We expected the Moon to be lifeless, and apparently it is. Mercury, the closest planet to the Sun, resembles the Moon. Its atmosphere is exceedingly thin, and it ought not to support life. In the outer solar system there are many large satellites the size of Mercury or our own Moon, composed of some mix of rock (like the Moon and Mercury) and ices. Io, the second moon of Jupiter, falls into this category. Its surface seems to be covered with a kind of reddish salt deposit. We are very ignorant about it. But because of its very low atmospheric pressure, we do not expect life on it.