Bully for Brontosaurus (51 page)

Frequency distribution for the number of chromosome pairs in monocot plants. Note that all peaks are for even numbers of chromosomes. This occurs because so many plant species are produced by polyploidy, or doubling of chromosome number, and a doubling of any number, odd or even, produces an even number.
FROM VERNE GRANT,
THE ORIGIN OF ADAPTATIONS
, 1963.

The very idea of a nucleus with 1,260 chromosomes, all obeying the rules of precise alignment and division as cells proliferate, inspired G. Ledyard Stebbins, our greatest living evolutionary botanist, to a rare emotion for a scientific paper—rapture (since Ledyard and I share a passion for Gilbert and Sullivan, I will write, for his sake, “modified rapture”—and he will know the reference and meaning): “At meiosis, these chromosomes pair regularly to form about 630 bivalents, a feat which to cytologists is as remarkable a wonder of nature as are the fantastic elaborations of form exhibited by orchids, insectivorous plants, and many animals” (see Stebbins, 1966, in the bibliography).

In fifteen years of writing these monthly essays, I have specialized in trying to draw general messages from particulars. But this time, I am stumped. I don’t know what deep truth of nature emerges from the documentation of minimal and maximal chromosome numbers. Oh, I can cite some clichés and platitudes: Quantity is not quality; good things come in small packages. I can also state the obvious conclusion that inheritance and development do not depend primarily upon the number of distinct rods holding hereditary information—but this fact has been featured in textbooks of genetics for more than seventy years.

No, I think that every once in a while, we must simply let a fact stand by itself, for its own absolutely unvarnished fascination. Has your day not been brightened just a bit by learning that a plant can orchestrate the division of its cells by splitting 630 pairs of chromosomes with unerring accuracy—or that an ant, looking much like others, can gallivant about with an absolute minimum of one chromosome per cell? If so, I have earned my keep, and can go cultivate my garden. I think I’ll try growing some ferns. Then I might take some colchicine, which often induces polyploidy, and maybe, just maybe….

The
Voyager
expedition represents the greatest technological and intellectual triumph of our century. The fact that this tiny, relatively inexpensive machine could explore and photograph every outer planet except Pluto (but including Neptune, now the most distant planet, as Pluto temporarily moves closer to the sun on its eccentric orbit) is not only, as the cliché goes, a triumph of the human spirit (not to mention good old American tinkering and know-how), but also a living proof that billions of bucks, bureaucratic immuring, and hush-hush military spin-offs need not power our space program—and that knowledge and wonder really could be the main motivation and reward.

Such a triumph must be celebrated by any writer in natural history. I have chosen my own idiosyncratic mode.
Voyager
’s results convey many messages. These two essays, with their common theme, embody my reading of the main lesson from the standpoint of an evolutionary biologist: Planets are like organisms in that they have irreducible individuality and must therefore be explained by methods of historical analysis; they are not like molecules in a chemical equation. Planets therefore affirm the larger goal of unity among sciences by showing that methods of one approach (biological-historical) apply to cardinal objects of another mode often viewed as disparate or even opposed (physical-experimental).

WHEN MIRANDA
, confined for all her conscious life on Prospero’s magic island, saw a group of men for the first time, she exclaimed, “O, wonder! How many goodly creatures are there here! How beauteous mankind is! O brave new world, that has such people in’t” (the source, of course, for Aldous Huxley’s more sardonic citation). Now, almost 400 years after Miranda spoke through Shakespeare, we have returned the favor, gazed for the first time upon Miranda and found her every bit as wonderful—“so perfect and so peerless…she will outstrip all praise and make it halt behind her.”

Prospero used all his magic to import his visitors by tempest. We have seen Miranda, the innermost large moon of Uranus, through the most stunning feat of technical precision in all our history. Ariel himself, Prospero’s agent of magic (and also another moon of Uranus), would have been astounded. For we have sent a small probe hurtling though space for nine years, boosting it with the gravitational slings of both Jupiter and Saturn toward distant Uranus, there to transmit a signal across 2 billion miles and three light-hours, showing the face of Miranda with the same clarity that Prospero beheld when he gazed upon his daughter’s beauty and exclaimed, “Thou didst smile, infused with a fortitude from heaven.”

It is easy to wax poetic about this feat (especially with a little help from the Bard himself).
Voyager
’s data from Jupiter, Saturn, and now Uranus have supplied more scientific return for expended output than anything else that space exploration ever dared or dreamed. In the chorus of praise, however, we have not always recognized how much this new information has transcended the visually dazzling—how deeply our
ideas
about the formation and history of the solar system have been changed. This confluence of aesthetics and intellect must be celebrated above all—and I should like to record my delight by thoroughly repudiating an early essay in this series (March 1977) as an illustration both of our new understanding and of the vital generalization so obtained.

My story is the tale of an old and eminently reasonable hypothesis, proposed long ago and beautifully affirmed by the first explorations of other worlds—our moon, then Mercury, and finally Mars. Then, at the height of its triumph, the theory begins to unravel, first at the moons of Jupiter, then at the surface of Venus, and finally and irretrievably, in the face of Miranda.

The initial hypothesis sought to explain the surfaces (and inferred histories) of rocky planets and moons as simple consequences of their differences in size. Why, in particular, is the earth so different from the moon? Our moon is a dead world, covered with impact craters that have not eroded away since their formation, often billions of years ago. The earth, by contrast, is a dynamic world of relative smoothness.

This difference, we assume, is a result of historical divergence, not initial disparity. Billions of years ago, when the planets were young and our portion of space still abounded with debris not yet swept up in planets and moons, the earth must have been as intensely cratered as the moon. The current difference must therefore be a result of the moon’s retention, and the earth’s obliteration, of their early histories. Why the difference?

On earth, both internal and external “machines” recycle the landscape on a scale of millions of years. The atmosphere (external machine) generates agents of erosion—running water, wind, and ice—that quickly obliterate the topography of any crater. Yet even without rain and wind, the earth’s internal activity of volcanism, earthquakes, and ultimately of plate tectonics itself would eventually disaggregate and erase any old topography. Surfaces do not last for billions of years on an active planet. But neither machine works on the moon. With no atmosphere, erosion proceeds (even in geological time) at a snail’s pace. Likewise, the moon is a rigid body with a crust 600 miles thick. Moonquakes do not fracture the lunar surface and volcanoes do not rise from the tiny molten core.

The earth’s activity and moon’s silence are consequences of a single factor—size. Large bodies have much lower ratios of surface to volume than small bodies of the same shape, since surfaces (length × length) grow so much more slowly than volumes (length×length×length) as size increases. Our planet powers its two machines by low surface-to-volume ratios. The earth generates heat (by radioactivity) over its relatively large volume and then loses this heat through its relatively small surface—thus remaining hot and active enough to propel plate tectonics. The moon, by contrast, and by virtue of its higher surface-to-volume ratio, lost most of its internal heat long ago, and solidified nearly throughout. Likewise, the earth’s large mass generates enough gravity to hold an atmosphere and power its external machine, while the moon lost any gases once produced.

As planetary exploration began, this “size-dependent” theory of planetary surfaces and their histories received its first tests and passed elegantly. The first photos of Mercury showed nothing but craters—as expected for a body about the same size as our moon.

Mars posed a clearer and more crucial test. As a planet about midway in size between the earth and moon, it should preserve some of its early topography, but also display the action of weak internal and external machines. The
Surveyor
flyby and
Viking
landings affirmed this prediction. The surface of Mars is about 50 percent cratered. The remaining areas show abundant signs of erosion, primarily by winds today (dune fields and etched boulders) and by running water in the past (now frozen), and internal churning more limited than on the earth. Most intriguing are signs of incipient (but unrealized) plate tectonics—as though the Martian crust remains pliant enough to fracture, but too rigid to move.

At this point in space exploration, I felt confident enough to write an essay extolling the size hypothesis as a sufficient and elegantly simple explanation of planetary surfaces and their histories. Contrasting the earth with the smaller bodies then known, I wrote (in March 1977) that “the difference arises from a disarmingly simple fact
—size itself, and nothing else:
the earth is a good deal larger than its neighbors.”

The first test after my essay appeared would be
Voyager
’s photographic survey of the Galilean satellites of Jupiter—the four moon-sized rocky bodies that, by the size hypothesis, would surely be intensely cratered worlds, cold and dead. Thus, I waited with confidence as
Voyager
approached Io, the innermost moon of Jupiter. The first photos, distant and fuzzy, revealed some circular structures initially read as craters. Well and good. But the next day brought sharp photos, and evoked both wonder and surprise. The circles were not craters, but giant volcanoes, spewing forth lakes of sulfur. In fact, not a single crater could be found on Io, the most active satellite in the solar system. Yet, as a body smaller than our moon, Io should have been cold and cratered.

The explanation now offered for Io’s intense activity had been predicted just a few days before the photos arrived. Io is so close to giant Jupiter that the interplay between Jupiter’s gravitational tug and the reverse pull of the three other large satellites from the opposite side keeps the interior of Io fluid enough to resist rigidification.

As this information arrived, I could only stand by in awe and reflect that Io had been misnamed. The four Galilean satellites honor some of Jove’s many lovers—an ecumenical assortment including his homosexual partner Ganymede; the nymph Callisto; and Europa, the mother of King Minos. Io, the fallen priestess, was changed to a heifer by jealous Hera, afflicted with a gadfly, and sent to roam Europe, where she forded (and indirectly named) the Bosporus (literally, the cow crossing) and finally emerged, human again, in Egypt. I thought that this innermost moon should be renamed Semele, to honor another lover who made the mistake of demanding that Jove appear to her in his true, rather than his disguised (and muted) human, form—and was immediately burned to a crisp!

Is the size hypothesis therefore wrong because Io violated its prediction? The principle of surfaces and volumes, as a basic law of physics and the geometry of space, is surely correct. Io does not challenge its validity, but only its scope. The size hypothesis does not merely claim that the surface-to-volume principle operates—for this we can scarcely doubt. The hypothesis insists, rather, that the surface-to-volume principle so dominates all other potential forces that we need invoke nothing further to understand the history and topography of rocky planetary surfaces. Io does not refute the principle. But Io does prove dramatically that other circumstances—in this case proximity to opposing gravitational sources—can so override the surface-to-volume rule that its predictions fail or, in this case (even worse), are diametrically refuted.

Planetary surfaces lie in the domain of complex historical sciences, where modes of explanation differ from the stereotypes of simple and well-controlled laboratory experiments. We are not trying to demonstrate the validity of physical laws. Rather, we must try to assess the relative importance of several complex and interacting forces. The validity of the surface-to-volume principle was never at issue, only its relative importance—and Io has challenged its domination.

We must therefore know, in order to judge the size hypothesis, whether Io is a lone exception in a singular circumstance or a general reminder that the surface-to-volume principle ranks as only one among many competing influences—and therefore not as
the
determinant of planetary surfaces and their history. The test will not center upon arguments about the laws of physics, but upon observations of other bodies, for we must establish, empirically, the relative importance of a hypothesis that worked until
Voyager
photographed Io.

Venus was the next candidate. Our sister planet, although closer to us than any other, had remained shrouded (literally) in mystery by its dense cover of clouds. But Russian and American probes have now mapped the Venusian surface with radio waves that can penetrate the clouds, as wavelengths in the visible spectrum cannot. Results are ambiguous and still under analysis, but proponents of the size hypothesis can scarcely react with unalloyed pleasure. Venus and Earth are just about the same size and Venus should, by the surface-to-volume hypothesis, be as active as our planet. Our sister world is, to be sure, no dead body. We have seen high mountains, giant rifts, and other signs of extensive tectonic activity. But Venus also seems to maintain too much old and cratered terrain for a body of its size, according to the principle of surfaces and volumes alone.

Scientists have advanced many explanations for the difference between Venus and Earth. Perhaps tidal forces generated by the moon’s gravity keep Earth in its high state of geological flux. Venus has no satellite. Perhaps the high surface temperature of Venus, generated by a greenhouse effect under its dense cover of clouds, keeps the surface too pliable to form the thin and rigid plates that, in their constant motion, keep Earth’s surface so active.

Voyager
then moved toward Uranus and a final test. By this time, buffeted by Io and Venus, I was holding out little hope for the size hypothesis (and also wishing that the
Rubáiyát
had not spoken so truly about the moving finger, and that my publishers might deep-six all unsold copies of
The Panda’s Thumb
, with its reprint of my original 1977 essay). In fact, anticipating final defeat for that elegantly simple proposal of earlier years, I actually managed to turn disappointment into a modest professorial coup. I have long believed that examinations have little intellectual value, existing only to fulfill, and ever so imperfectly at that, any large institution’s need for assessment by number. Yet, for the first time, the moons of Uranus allowed me to ask an examination question with some intellectual interest and integrity.

I realized that the final examination for my large undergraduate course had been set for the morning of January 24, at the very hour that
Voyager
would be relaying photographs of Uranian moons to earth. I therefore predicted that Miranda, although the smallest of five major moons, would be most active among them, and asked the students to justify (or reject) such a speculation—though the conjecture itself is absurd under the size hypothesis with its evident prediction that Miranda, as the smallest moon with the highest surface-to-volume ratio, should be cratered and devoid of internal activity. I asked the students:

(When I first wrote the exam in early January, I couldn’t even provide my students with the moons’ diameters, for they had not yet been measured precisely, though we knew that Miranda was smallest. Between writing and administering the exam,
Voyager
measured the diameters, and we rushed to the printers with an insert. Science, at its best, moves very quickly indeed.)

So I was ready for the final undoing of the size hypothesis, but not for the actual result of Miranda’s countenance. The conjecture of my question turned out to be quite wrong. I had been thinking of Io and the gravity of a nearby giant planet. Since Miranda is closest to Uranus, I supposed that it might be lit with modern volcanoes. But no volcanoes are belching forth sulfur, or anything else, on Miranda. The actual observations, however, spoke even more strongly against the size hypothesis and its prediction of a cold, cratered world.

I had made one good prediction, probably for the wrong reason: Miranda is the most geologically active of Uranian moons, despite its small diameter of but 300 miles. (The moons of Uranus, outdoing even the mythic splendor of Jupiter’s satellites, bear lovely Shakespearean names—in order from Uranus out: Miranda, Ariel, Umbriel, Titania, and Oberon. In addition,
Voyager
has discovered at least ten additional and much smaller moons between Miranda and the planet’s surface.) The first photos of Miranda stunned and delighted the boys in Pasadena even more than her namesake had mesmerized Ferdinand on Prospero’s island. Laurence Soderblom, speaking for the
Voyager
imaging team, exclaimed: “It’s just mind boggling…. You name it, we have it…. Miranda is what you would get if you can imagine taking all the bizarre geological features in the solar system and putting them on one object.” A brave new world, indeed. So much for the size hypothesis and its uniform blanket of craters.