The Varieties of Scientific Experience: A Personal View of the Search for God

the cosmos, and what this says is that in the early stages of the formation of the solar nebula there should have been a preferential condensation of methane in the outer parts of the solar system, but not in the inner parts. And if that is generally true, then we ought to expect more organic matter in the outer parts and much less in our neck of the cosmic woods.

Well, there is certainly not a huge amount of methane on the Moon or Mercury. But when we do go out to the orbit of Saturn we start finding not only evidence for methane-the planets Jupiter, Saturn, Uranus, and Neptune have lots of methane in their spectra-but we find a set of data that strongly implies the presence of complex organic molecules in the outer solar system.

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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