Who discovered that CFCs posed a threat to the ozone layer? Was it the principal manufacturer, the DuPont Corporation, exercising corporate responsibility? Was it the Environmental Protection Agency protecting us? Was it the Department of Defense defending us? No, it was two Ivory-tower, white-coated university scientists working on something else—Sherwood Rowland and Mario Molina of the University of California, Irvine. Not even an Ivy League university. No one instructed them to look for dangers to the environment. They were pursuing fundamental research. They were scientists following their own interests. Their names should be known to every schoolchild.
In their original calculations, Rowland and Molina used rate constants of chemical reactions involving chlorine and other halogens that had been measured in part with NASA support. Why NASA? Because Venus has chlorine and fluorine molecules in its atmosphere, and planetary aeronomers had wanted to understand what's happening there.
Confirming theoretical work on the role of CFCs in ozone depletion was soon done by a group led by Michael McElroy at Harvard How is it they had all these branching networks of halogen chemical kinetics in their computer ready to go? Because they were working on the chlorine and fluorine chemistry of the atmosphere of Venus. Venus helped make and helped confirm the discovery that the Earth's ozone layer is in danger. An entirely unexpected connection was found between the atmospheric photochemistries of the two planets. A result of importance to everyone on Earth emerged from what might well have seemed the most blue-sky, abstract, impractical kind of work, understanding the chemistry of minor constituents in the upper atmosphere of another world.
There's also a Mars connection. With Viking we found the surface of Mars to be apparently lifeless and remarkably deficient even in simple organic molecules. But simple organic molecules ought to be there, because of the impact of organic-rich meteorites from the nearby asteroid belt. This deficiency is widely attributed to the lack of ozone on Mars. The Viking microbiology experiments found that organic matter carried from Earth to Mars and sprinkled on Martian surface dust is quickly oxidized arid destroyed. The materials in the dust that do the destruction are molecules something like hydrogen peroxide—which we use as an antiseptic because it kills microbes by oxidizing there, Ultraviolet light from the Sun strikes the surface of Mars unimpeded by an ozone layer; if any organic matter were there, it would be quickly destroyed by the ultraviolet light itself and its oxidation products. Thus part of the reason the topmost layers of Martian soil are antiseptic is that Mars has an ozone hole of planetary dimensions—by itself a useful cautionary tale for us, who are busily thinning and puncturing our ozone layer.
(2) Global warming is predicted to follow from the increasing greenhouse effect caused largely by carbon dioxide generated in the burning of fossil fuels—but also from the buildup of other infrared-absorbing gases (oxides of nitrogen, methane, those same CFCs, and other molecules).
Suppose that we have a three-dimensional general circulation computer model of the Earth's climate. Its programmers claim it's able to predict what the Earth will be like if there's more of one atmospheric constituent or less of another. The model does very well at "predicting" the present climate. But there is a nagging worry: The model has been "tuned" so it will come out right—that is, certain adjustable parameters are chosen, not from first principles of physics, but to get the right answer. This is not exactly cheating, but if we apply the same computer model to rather different climatic regimes—deep global warming, for instance—the tuning might then be inappropriate. The model might be valid for today's climate, but not extrapolatable to others.
One way to test this program is to apply it to the very different climates of other planets. Can it predict the structure of the atmosphere on Mars and the climate there? The weather? What about Venus? If it were to fail these test cases, we would be right in mistrusting it when it makes predictions for our own planet. In fact, climate models now in use do very well in predicting from first principles of physics the climates on Venus and Mars.
On Earth, huge upwellings of molten lava are known and attributed to superplumes convecting up from the deep mantle and generating vast plateaus of frozen basalt. A spectacular example occurred about a hundred million years ago, and added perhaps ten times the present carbon dioxide content to the atmosphere, inducing substantial global warming. These plumes, it is thought, occur episodically throughout Earth's history. Similar mantle upwelling seem to have occurred on Mars and Venus. There are sound practical reasons for us to want to understand how a major change to the Earth's surface and climate could suddenly arrive unannounced from hundreds of kilometers beneath our feet.
Some of the most important recent work on global warming has been done by James Hansen and his colleagues at the Goddard Institute for Space Sciences, a NASA facility in New York City. Hansen developed one of the major computer climate models and employed it to predict what will happen to our climate as the greenhouse gases continue to build up. He has been in the forefront of testing these models against ancient climates of the Earth. (During the last ice ages, it is of interest to note, more carbon dioxide and methane are strikingly correlated with higher temperatures.) Hansen collected a wide range of weather data from this century and last, to see what actually happened to the global temperature, and then compared it to the computer model's predictions of what should have happened. The two agree to within the errors of measurement and calculation, respectively. He courageously testified before Congress in the face of a politically generated order from the White House Office of Management and Budget (this was in the Reagan years) to exaggerate the uncertainties and minimize the dangers. His calculation on the explosion of the Philippine volcano Mt. Pinatubo and his prediction of the resulting temporary decline in the Earth's temperature (about half a degree Celsius) were right on the money. He has been a force in convincing governments worldwide that global warming is something to be taken seriously.
How did Hansen get interested in the greenhouse effect in the first place? His doctoral thesis (at the University of Iowa in 1967) was about Venus. He agreed that the high radio brightness of Venus is due to a very hot surface, agreed that greenhouse gases keep the heat in, but proposed that heat from the interior rather than sunlight was the principal energy source. The Pioneer 12 mission to Venus in 1978 dropped entry probes into the atmosphere; they showed directly that the ordinary greenhouse effect—the surface heated by the Sun and the heat retained by the blanket of air—was the operative cause. But it's Venus that got Hansen thinking about the greenhouse effect.
Radio astronomers, you note, find Venus to be an intense source of radio waves. Other explanations of the radio emission fail. You conclude that the surface must be ridiculously hot. You try to understand where the high temperatures come from and are led inexorably to one or another kind of greenhouse effect. Decades later you find that this training has prepared you to understand and help predict an unexpected threat to our global civilization. I know many other instances where scientists who first tried to puzzle out the atmospheres of other worlds are making important and highly practical discoveries about this one. The other planets are a superb training ground for students of the Earth. They require both breadth and depth of knowledge, and they challenge the imagination.
Those who are skeptical about carbon dioxide greenhouse warming might profitably note the massive greenhouse effect on Venus. No one proposes that Venus's greenhouse effect derives froth imprudent Venusians who burned too much coal, drove fuel-inefficient autos, and cut down their forests. My point is different. The climatological history of our planetary neighbor, an otherwise Earthlike planet on which the surface became hot enough to melt tin or lead, is worth considering—especially by those who say that the increasing greenhouse effect on Earth will be self-correcting, that we don't really have to worry about it, or (you can see this in the publications of some groups that call themselves conservative) that the greenhouse effect itself is a "hoax."
(3) Nuclear winter is the predicted darkening and cooling of the Earth—mainly from fine smoke particles injected into the atmosphere from the burning of cities and petroleum facilities—that is predicted to follow a global thermonuclear war. A vigorous scientific debate ensued on just how serious nuclear winter might be. The various opinions have now converged. All three-dimensional general circulation computer models predict that the global temperatures resulting from a worldwide thermonuclear war would be colder than those in the Pleistocene ice ages. The implications for our planetary civilization—especially through the collapse of agriculture—are very dire. It is a consequence of nuclear war that was somehow overlooked by the civil and military authorities of the United States, the Soviet Union, Britain, France, and China when they decided to accumulate well over 60,000 nuclear weapons. Although it's hard to be certain about such things, a case can be made that nuclear Winter played a constructive role (there were other causes, of course) in convincing the nuclear-armed nations, especially the Soviet Union, of the futility of nuclear war.
Nuclear winter was first calculated and named in 1982/83 by a group of five scientists, to which I'm proud to belong. This team was given the acronym TTAPS (for Richard P. Turco, (even B. Toon, Thomas Ackerman, James Pollack, and myself). Of the five TTAPS scientists, two were planetary scientists, and the other three had published many papers in planetary science, The earliest intimation of nuclear winter came during that same Mariner 9 mission to Mars, when there was a global dust storm and we were unable to see the surface of the planet; the infrared spectrometer on the spacecraft found the high atmosphere to be warmer and the surface colder than they ought to have been. Jim Pollack and I sat down and tried to calculate how that could come about. Over the subsequent twelve years, this line of inquiry led from dust storms on Mars to volcanic aerosols on Earth to the possible extinction of the dinosaurs by impact dust to nuclear winter. You never know where science will take you.
PLANETARY SCIENCE fosters a broad interdisciplinary point of view that proves enormously helpful in discovering and attempting to defuse these looming environmental catastrophes. When you cut your teeth on other worlds, you gain a perspective about the fragility of planetary environments and about what other, quite different, environments are possible. There may well be potential global catastrophes still to be uncovered. If there are, I bet planetary scientists will play a central role in understanding them.
Of all the fields of mathematics, technology, and science, the one with the greatest international cooperation (as determined by how often the co-authors of research papers hail from two or more countries) is the field called "Earth and space sciences." Studying this world and others, by its very nature, tends to be non-local, non-nationalist, non-chauvinist. Very rarely do people go into these fields because they are internationalists. Almost always, they enter for other reasons, and then discover that splendid work, work that complements their own, is being done by researchers in other nations; or that to solve a problem, you need data or a perspective (access to the southern sky, for example) that is unavailable in your country. And once you experience such cooperation—humans from different parts of the planet working in a mutually intelligible scientific language as partners on matters of common concern—it's hard not to imagine it happening on other, nonscientific matters. I myself consider this aspect of Earth and space sciences as a healing and unifying force in world politics; but, beneficial or not, it is inescapable.
When I look at the evidence, it seems to me that planetary exploration is of the most practical and urgent utility for us here on Earth. Even if we were not roused by the prospect of exploring other worlds, even if we didn't have a nanogram of adventuresome spirit in us, even if we were only concerned for ourselves and in the narrowest sense, planetary exploration would still constitute a superb investment.
CHAPTER 15 THE GATES OF THE WONDER WORLD OPEN
The great floodgates of the wonder-world swung open.
—HERMAN MELVILLE, MOBY DICK, CHAPTER 1 (1851)
Sometime coming up, perhaps just around the corner, there will be a nation—more likely, a consortium of nations—that will work the next major step in the human venture into space. Perhaps it will be brought about by circumventing bureaucracies and making efficient use of present technologies. Perhaps it will require new technologies, transcending the great blunderbuss chemical rockets. The crews of these ships will set foot on new worlds. The first baby will be born somewhere up there. Early steps toward living off the land will be made. We will be on our way. And the future will remember.
TANTALIZING AND MAJESTIC, Mars is the world next door, the nearest planet on which an astronaut or cosmonaut could safely land. Although it is sometimes as warm as a New England October, Mars is a chilly place, so cold that some of its thin carbon dioxide atmosphere freezes out as dry ice at the winter pole.
It is the nearest planet whose surface we can see with a small telescope. In all the Solar System, it is the planet most like Earth. Apart from flybys, there have been only two fully successful missions to Mars: Mariner 9 in 1971, and Vikings 1 and 2 in 1976. They revealed a deep rift valley that would stretch from New York to San Francisco; immense volcanic mountains, the largest of which towers 80,000 feet above the average altitude of the Martian surface, almost three times the height of Mount Everest; an intricate layered structure in and among the polar ices, resembling a pile of discarded poker chips, and probably a record of past climatic change; bright and dark streaks painted down on the surface by windblown dust, providing high-speed wind maps of Mars over the past decades and centuries; vast globe-girdling dust storms; and enigmatic surface features.
Hundreds of sinuous channels and valley networks dating back several billion years can be found, mainly in the cratered southern highlands. They suggest a previous epoch of more benign and Earthlike conditions—very different from what we find beneath the tenuous and frigid atmosphere of our time. Some ancient channels seem to have been carved by rainfall, some by underground sapping and collapse, and some by great floods that gushed up out of the ground. Rivers were pouring into and filling great thousand-kilometer-diameter impact basins that today are dry as dust. Waterfalls dwarfing any on Earth today cascaded into the lakes of ancient Mars. Vast oceans, hundreds of meters, perhaps even a kilometer, deep may have gently lapped shorelines barely discernible today. That would have been a world to explore. We are four billion years late.*
