by Carl Sagan
This is a photograph of Iapetus, one of the outer moons of Saturn. The gray area is not in shadow. There is actually a remarkable division of one hemispheric surface into dark material and the other hemisphere into bright material. And the clear spectral signature of water ice is present in the bright areas.
We did not fly very close with either Voyager 1 or Voyager 2 to Iapetus. We think this is organic matter. It is very dark. At the center of this dark stuff, the albedo, the reflectivity, is something like 5 percent. I can't be sure, but I suspect that there is nothing in the room you are sitting in as dark as 5 percent albedo. Also, it is reddish. That is, it does not reflect much light, but it reflects more light in the red than in the blue part of the visible spectrum. And the values of the albedo and color are inconsistent with a wide range of other materials that you might offhand guess it might be-various of the salts, for example. They are very consistent with complex organic matter of various sorts. We know there is complex organic matter out there. I gave you one argument from the comets. Another argument is a category of meteorites called carbonaceous meteorites that fall to Earth, and they have several percent to as much as 10 percent of complex organic matter in them.
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This is a family portrait of some of the small moons of Saturn. All of them were discovered by the Voyager spacecraft. None of these were known before. The smallest ones are maybe ten kilometers across. The biggest one may be a hundred kilometers. They're little worlds, and all of them are dark and red like Iapetus.
These are rings of Uranus. You may not think it's a very good picture, but it took an awful lot of work to make it. The picture was taken at 2.2 microns, in the infrared part of the spectrum. The rings are known to be quite different from the rings of Saturn. They are thinner, they are wispier, and they are black, again suggesting the prevalence of dark, reddish, presumably organic matter in the outer solar system.
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Now, this is not in the outer solar system. This is Phobos, the innermost moon of Mars, which may or may not be a captured asteroid from farther out in the solar system, and it too has this dark, reddish composition. Its mean density is known, and it is consistent with organic matter.
Deimos is the outermost Martian moon. Despite its different appearance from Phobos, it is likewise very dark, very red, same story.
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And I should mention that Mars itself, around which Pho-bos and Deimos are orbiting (all that rocky stuff is Mars, and the foreground instrumentation is the Viking 1 Lander), at least in the two places that we landed with Viking 1 and Viking 2, shows not a hint of organic matter. I will return to Martian exploration later, but I want to stress that the limits to the presence of organic matter on Mars are very low. There is not one part in a million of simple organic molecules and not one part in a billion of complex organic molecules. Mars is very dry, denuded in organic matter, and yet there are these two moons that may be made entirely of organic matter orbiting it. It's an interesting dilemma. These are two trenches that were dug by this sample arm in the Martian soil. So we gathered material from the subsurface and withdrew it back into the spacecraft and examined it with a gas chromatograph/mass spectrometer for organic matter, of which there was none.
I want to continue the story about organic matter in the outer solar system. And the best story by far, the one that we have the most information on, although it is still quite limited, is for Titan. Titan is the largest moon in the Saturn system. It is remarkable for many reasons, the most striking of which is that it is the only moon in the solar system with a significant atmosphere. The surface pressure on Titan (we know from Voyager 1) is about 1.6 bars, that is, about 1.6 times what it is in the room I am in as I write this. Since the acceleration due to gravity is about one-sixth on Titan what it is here on Earth, there is ten times more gas in the Titanian atmosphere than in the terrestrial atmosphere, which is a substantial atmosphere.
The organic molecules found in the gas phase in the atmosphere of Titan by the Voyager 1 and 2 spacecraft include hydrogen cyanide (HCN, which we've talked about before), cyanoacetylene, butadiene, cyanogen (which is two CNs glued together), propylene, propane (which we know), acetylene, ethane, ethylene (these are all components of natural gas). Methane, likewise. And the principal constituent of the atmosphere, there as here, is molecular nitrogen.
It is, I think, very interesting that we have a world in the outer solar system that is loaded with the stuff of life. And we can calculate, at the present rate at which these materials are being formed on Titan, how much of this stuff has accumulated during the history of the solar system. The answer is the equivalent of a layer at least hundreds of meters thick all over Titan, and possibly kilometers thick. The difference depends on how long a wavelength of ultraviolet light can be used for such synthetic experiments. And, incidentally, there is also a range of entertaining evidence that there is a surface ocean of liquid
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hydrocarbon at Titan. [3] So just think of that environment. There's land; probably there's ocean. The land is covered with this organic muck that falls from the skies. There is a submarine deposit underneath this ocean of liquid ethane and methane of more of this complex stuff, and then down deep is frozen methane and frozen water and so on.
Now, that's a world worth visiting. What's happened to that stuff over the last 4.6 billion years? How far along has it gotten? How complex are the molecules there? What happens when occasionally there is an external or an internal event that heats things locally and melts some ice and makes some liquid water? Titan is a world crying out for detailed exploration, and it seems to be a planetary-scale experiment on the early steps that here on Earth led to the origin of life but there on Titan were very likely frozen, literally, at the early stages because of the general unavailability of liquid water.
Likewise, there is a very stunning range of studies-mainly in the last two decades-of interstellar organic matter: not just a multiplicity of worlds in our solar system but the cold, dark spaces between the stars are also loaded with organic molecules.
we are looking toward the center of the galaxy in the direction of the constellation Sagittarius. You can see a set of dark clouds, some quite extensive, some much smaller. It is in these giant molecular clouds that well upwards of 50 different kinds of molecules have been found, most of which are organic. And it is precisely in such dark clouds that the collapse of solar nebulae is expected to happen, and therefore the forming solar systems should be composed, in part, of complex organic matter. The conclusion is that complex organic materials are everywhere.
Now let's return to the question of the origin of life on Earth. The organic stuff could have fallen in during the formation of the Earth, or it could have been generated in situ from simpler materials on the Earth in the same way as on Titan. At the present time there is no way of assessing the relative contributions from these two sources. What seems clear is that either source would be sufficient-adequate.
The Earth formed from the collapse of lumps of matter of the sort we talked about earlier, condensing from the solar nebula. Therefore in its final stages of formation, it was collecting objects that collided at high velocity and produced a set of catastrophic events, including the melting of much of the surface. This, it turns out, was not a good environment for the origin of life, as you might have suspected. But after a while, when the final sweeping up of the debris in the solar system was more or less completed, water delivered from the outside or outgassed from the inside started forming on the surface, filling in the ancient impact craters. And a trickle of material was still falling in from space. At the same time, electrical discharges and ultraviolet light from the Sun and other energy sources produced in-
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digenous organic matter. The amount of organic matter that could have been produced in the first few hundred million years of Earth history was suf
ficient to have produced in the present ocean a several-percent solution of organic matter. That is just about the dilution of Knorr's chicken soup, and not all that different from the composition either. And chicken soup is widely known to be good for life. In fact, it is just this warm, dilute soup, in the words of J. B. S. Haldane, who was one of the first two people to realize that this sequence of events was likely, in which the standard scenario for the origin of life occurs.
In the laboratory -we can take molecules of water, ammonia, and methane-rather like the ones we've been talking about for Titan-and dissociate them by ultraviolet light. The fragments make a set of precursor molecules, including hydrogen cyanide, which then combine and, in water, form the amino acids. In such experiments not just the building blocks of the proteins but the building blocks of the nucleic acids are routinely produced. There is a range of subsequent experiments, in which the smaller molecular building blocks join together to form large and complex molecules.
If we look at the fossil record, we find that there is a range of evidence for microfossils dating back not just to the beginning of the Cambrian but dating back to as much as 3,500 million years ago.
Now, just think about these numbers. The Earth itself forms about 4,600 million years ago. Because of the final stages of accretion, we know that the Earth environment was not suitable for the origin of life back then. From studies of the late crater-ing on the Moon, it looks-since the Earth and the Moon were presumably in the same part of the solar system then as now- as if the Earth was not in a suitable state for the origin of life until perhaps 4,000 million years ago. So if the Earth is not appropriate to the origin of life until 4,000 million years ago and the first fossils are around 3,500 million years ago, then there are only about 500 million years for the origin of life. But those earliest fossils are by no means extremely simple organisms. They are, in fact, colonial algal stromatolites, and a great deal of evolution had to precede them. And that therefore says that the origin of life happened in significantly less than 500 million years. We don't know how much less. Six days was once a popular hypothesis. It's not excluded by these data, but at least it cannot be as long as 500 million years. It must have happened very fast. A process that happens quickly is a process that in some sense is likely. The faster it happens, the more likely it is. There is a difficulty in extrapolating from a single case; nevertheless this evidence suggests that the origin of life was in some sense easy, in some sense sitting in the laws of physics and chemistry. And if that's true, that is a very important fact for the consideration of extraterrestrial life.
There is a classic objection to this kind of argument about the origin of life. As far as I know, this objection was first posed by Pierre Lecompte du Noiiy in a 1947 book called Human Destiny and is regularly rediscovered about once every half decade. It goes something like this: Consider some biological molecules. Not all of them. We'll give the evolutionists the benefit of the doubt. Let's just take a small, simple one, not something thousands of amino acids long. Let's pick an enzyme with a hundred amino acids. That's a very modest enzyme. Now, a way to think of it is as a kind of necklace on which there are a hundred beads. There are twenty different kinds of beads, any one of which could be in any one of these positions. To reproduce the molecule precisely, you have to put all the right beads-all the right amino acids-in the molecule in the right order. If you were blindfolded while assembling a necklace from equally abundant beads, the chance of getting the right bead in the first slot is 1 chance in 20. The chance of getting the right bead in the second slot is also 1 chance in 20, so the chance of getting the right bead in the first and second slots simultaneously is 1 chance in 202. Getting the first three correct is 1 chance in 203, and getting all hundred correct is 1 chance in 20100. Well, you can see 20100 is 2100 x 10100. And since 210 is a thousand, which is 103, then 2100 is 1030, so this is the same as 10130. One chance in 10130 of assembling the right molecules the first time. Ten to the hundred-thirtieth power, or 1 followed by 130 zeros, is vastly more than the total number of elementary particles in the entire universe, which is only about ten to the eightieth (1080).
So let's imagine that every star in the universe has a planetary system like ours. Let's say one planet has oceans. Let's suppose that the oceans are just as thick as ours. Let us suppose that there is a few-percent solution of organic matter in every one of those oceans and that in every tiny volume of the ocean that has enough molecules there is an experiment happening once every microsecond to construct this particular hundred-amino-acid-long protein. So in the ocean every microsecond an enormous number of these little experiments are going on. And identical things are happening in the next star system and the next star system, filling an entire galaxy. And then not just in that galaxy but in every galaxy in the universe. It turns out that if that sequence of experiments had gone on for the entire history of the universe, you could never produce one enzyme molecule of predetermined structure. And in fact it's much worse than that.
If you did that same experiment once every Planck time, the shortest unit of time that is permissible in physics, you still couldn't generate a single hemoglobin molecule, from which many people have decided that God exists, because how else do you make these molecules? If you haven't heard this before, doesn't this seem like a pretty compelling argument? Strong argument, right? A whole universe of experiments once every Planck time. Can't beat that.
Now let's take another look. Does it matter if I have a hemoglobin molecule here and I pull out this aspartic acid and I put in a glutamic? Does that make the molecule function less well? In most cases it doesn't. In most cases an enzyme has a so-called active site, which is generally about five amino acids long. And it's the active site that does the stuff. And the rest of the molecule is involved in folding and turning the molecule on or turning it off. And it's not a hundred places you have to explain, it's only five to get going. And 205 is an absurdly small number, only about 3 million. Those experiments are done in one ocean between now and next Tuesday. Now, remember what it is we're trying to do: We're not trying to make a human being from scratch, to have all the molecules of a human being fall simultaneously together in a primitive ocean and then have someone swim out of the water. That's not what we're asking for. What we're asking for is something that gets life going, so this enormously powerful sieve of Darwinian natural selection can start pulling out the natural experiments that work and encouraging them, and neglecting the cases that don't work.
So it turns out here, as in some of the arguments I was talking about yesterday, there is an important point that is left out in these apparent deductions of divine intervention by looking at the natural world. A very dramatic, strong statement of this sort has been made by the astronomers Fred Hoyle and N. C. Wick-ramasinghe. And their phrase, after a calculation in this spirit, goes something like this:
They say it is no more likely that the origin of life could occur spontaneously by molecular interaction in the primitive ocean than that a Boeing 747 would be spontaneously assembled when a whirlwind passed over a junkyard. That's a vivid image. It's also a very useful image, because, of course, the Boeing 747 did not spring full-blown into the world of aviation; it is the end product of a long evolutionary sequence, which, as you know, goes back to the DC-3 and so on until you get to the Wright biplane. Now, the Wright biplane does look as if it were spontaneously assembled by a whirlwind in a junkyard. And while I don't mean to criticize the brilliant achievement of the Wright brothers, as long as you remember that there is this evolutionary history, it's a lot easier to understand the origin of the first example.
I want to close on a beautiful little piece of poetry written by a woman in rural Arkansas. Her name is Lillie Emery, and she is not a professional poet, but she writes for herself and she has written to me. And one of her poems has the following lines in it:
My kind didn't really slither out of a tidal pool, did we? God, I need to believe you created me: we are so small down here.
I
think there is a very general truth that Lillie Emery expresses in this poem. I believe everyone on some level recognizes that feeling. And yet, and yet, if we are merely matter intricately assembled, is this really demeaning? If there's nothing in here but atoms, does that make us less or does that make matter more?
Four
EXTRATERRESTRIAL INTELLIGENCE
There was a time when angels walked the Earth, Now they cannot even be found in Heaven.
• Yiddish proverb •
If there is as a continuum from self-reproducing molecules, such as DNA, to microbes, and an evolutionary sequence continuum from microbes to humans, why should we imagine that continuum to stop at humans? Why should there be an open-ended gap in the spectrum of beings? And isn't it a little suspicious that the gap would begin with us?
It's of interest to me that our language has not really any appropriate terms for such beings. The theological languages have terms like angels and demigods and seraphim and so on. Even here it's interesting that the theological expectations of beings superior to humans generally represent a hierarchy of power but not of intelligence. And here again I think it is clear that we have imposed human values onto the universe. Certainly on this planet it is not apparent that there are beings more intelligent than humans, although a case can be made for dolphins and whales, and in fact if humans succeed in destroying themselves with nuclear weapons, a case could be made that all the other animals are smarter than humans.