For me personally, the kind of science that I do is utterly unthinkable in any other age. I find myself engaged in the spacecraft exploration of nearby worlds, something that would have been considered the most rank fantasy just two generations ago, when the Moon was the paradigm of the unobtainable. Some of you will remember those poems and popular songs-'Fly Me to the Moon,' meaning asking for the impossible. And yet in our time a dozen human beings have walked on the surface of the Moon. And as I will stress in tomorrow's talk, that same technology that permits us to travel to other planets and stars also permits us to destroy ourselves-on a global scale, on a scale unprecedented in all of human history, and the mere knowledge that this is possible, even if we are lucky enough for it never to come about, must powerfully influence the lives of everybody who grows up in our time in a way that was not true for any other generation in human history.
I've spent much of my time over the last twenty years in the exploration of the solar system. Our robot emissaries have left the Earth, have visited every planet known to the ancients, from Mercury to Saturn, and reconnoitered some forty attendant smaller worlds, the satellites of those planets. We have flown by all those worlds, we have orbited and landed on three of them: the Moon, Venus, and Mars. There are something approaching a million close-up pictures of other worlds in our libraries. And it is a remarkable experience. Here's a world never before known by human beings, and then, for the first time, it is explored. This is a continuation of the spirit of adventure that I think has been a propelling force in human history. The worlds are lovely. They're exquisite. It is a kind of aesthetic experience to see them.
In the case of Mars, because of the Viking missions, we have been on the surface of that planet for some years, at least in two locales, and have essentially every day examined our surroundings. I personally spent in a certain sense a year on Mars in the course of that mission. I spent at least a great deal of my waking moments thinking about Mars. Now, at the end of such an experience, I feel something I hadn't planned on. And it is that these worlds, as exquisite and instructive as they are, are, as far as we can tell at this point, lifeless. There is in that lovely Martian landscape not a footprint, not an artifact, not even an old beer can, not a blade of grass, not a kangaroo rat, not even, so far as we can tell, a microbe. Mars and the Moon and Venus, as far as we can tell-the only planets we've landed on-are utterly lifeless. Maybe there's life in some places we haven't looked on those worlds. Maybe there used to be life and it is no longer. Maybe there one day will be life. But as far as we can tell here and now, there is none.
After that sort of experience, you then look back on your own world and you begin to have a kind of special feeling for it. You recognize that what we have here is in some sense rare. As I've argued previously, I suspect life and intelligence are a cosmic commonplace. But not so common that they're on every world. And in fact in our solar system we may discover that there is life only on this world.
This says that life is not guaranteed, that life requires something special, something improbable. I'm not for a moment suggesting it requires miraculous, divine, mystical intervention. But in a natural world, you can have probable events and you can have improbable events. And I'm sure this depends on the nature of the environments of the other planets. But there isn't any other planet that's just like the Earth, and, so far as we know so far, there isn't any other planet that has life on it. There are certainly premonitions and stirrings of life, the kind of organic chemistry on Titan, the big moon of Saturn that I referred to earlier. But that's still not the same as life. And so, by performing a first cursory inspection of our solar system, one realizes something important about where we come from.
When you investigate the vistas of time, you find something very similar. Because it is clear from the fossil record that almost every species that has ever existed is extinct; extinction is the rule, survival is the exception. And no species is guaranteed its tenure on this planet. I would like to describe to you one event that I've already referred to as central to the origin of the human species, because it is connected with the main topic of this talk. This is the worldwide extinction event that happened 65 million years ago, at the boundary between the Cretaceous and Tertiary periods of geological time, which also corresponds to the end of the Mesozoic age and the beginning of more recent times.
This is a close-up of a cliff base on a roadside near Gubbio in northern Italy. You can make out the scale of the image from the edge of a five-hundred-lire piece right up at the top. The surface crust has been scraped away a little bit, and the white material is calcium carbonate, essentially chalk, similar to the composition of the White Cliffs of Dover. These are the remains of countless small microorganisms that lived in the Cretaceous seas, forming little calcium carbonate shells that slowly fell through the warm waters of those seas and built up, during
Cretaceous time, for many millions of years. This deposit, as you can see, comes to an abrupt end. Time is increasing toward upper left. A layer of reddish brown rock lies above the older white carbonate, separated by a sharp boundary. And it's below this boundary that you find the last dinosaurs, and above the boundary you find an astonishing rate of proliferation of the small mammals into larger mammals, the events that are prerequisite for our own origins.
The sharpness of this boundary worldwide suggests some quite recent catastrophic event. The boundary is that thin layer of gray clay running diagonally across the picture. The clay- this is also true worldwide-has a quite high concentration, an anomalously high concentration, of a chemical element called iridium and other elements like it in the platinum group of metals. It is known that asteroids, and presumably cometary nuclei as well, have much higher abundances of iridium than do ordinary rocks on the Earth. And this iridium anomaly, now supported by a wide range of other data, is generally taken to be evidence for what happened to extinguish the dinosaurs and most of the other species of life on the Earth 65 million years ago.
This is an artist's conception of an object, maybe an asteroid, maybe a cometary nucleus, impacting the Cretaceous oceans. It's about ten kilometers across. It is bigger than the thickness of the ocean, so it is the same as impacting on land. The net result is to carve out in the ocean floor an immense crater and propel the fine particles thus generated into high orbit, making a vast
cloud of pulverized ocean bottom and pulverized impacting object that takes some years to settle out from the Earth's high atmosphere. During that period of time, sunlight is impeded from reaching the surface of the Earth, and the net result is a darkened and cold surface worldwide, which led, because of the differences in mammalian and reptilian physiology, to the extinction of the dinosaurs and many other kinds of life.
That is what happened to the dinosaurs. They were powerless to anticipate it and certainly to prevent it. What I would like now to describe is a catastrophe that in some respects is quite similar, one that endangers the future of our own species. It is very different in one respect: Unlike the dinosaurs, we ourselves, at enormous cost in treasure, have created this danger. We are solely responsible for its existence, and we have the means of preventing it, if we are sufficiently courageous and sufficiently willing to reconsider the conventional wisdom. That problem is nuclear war.
The bombs that destroyed Hiroshima and Nagasaki-everybody's read about them, we know something about what they did-killed some quarter of a million people, making no distinctions according to age, sex, class, occupation, or anything else. The planet Earth today has fifty-five thousand nuclear weapons, almost all of which are more powerful than the bombs that destroyed Hiroshima and Nagasaki and some of which are, each of them, a thousand times more powerful. [7] Some twenty to twenty-two thousand of these weapons are called strategic weapons, and they are poised for as rapid delivery as possible, essentially halfway across the world to someone else's homeland.
The ballistic missiles are sufficiently capable that typical transit times are less than half an hour. Twenty thousand strategic weapons in the world is a very large number. For example, let's ask how many cities there are on the planet Earth. If you define a city as having more than one hundred thousand people in it, there are twenty- three hundred cities on the Earth. So the United States and the Soviet Union could, if they wished, destroy every city on the Earth and have eighteen thousand strategic weapons left over to do something else with.
