Pale Blue Dot - Carl Sagan [42]
A variety of simple organic molecules was found, present as gases, mainly hydrocarbons and nitriles. The most complex of them have four “heavy” (carbon and/or nitrogen) atoms. Hydrocarbons are molecules composed of carbon and hydrogen atoms only, and are familiar to us as natural gas, petroleum, and waxes. (They’re quite different from carbohydrates, such as sugars and starch, which also have oxygen atoms.) Nitriles are molecules with a carbon and nitrogen atom attached in a particular way. The best known nitrile is HCN, hydrogen cyanide, a deadly gas for humans. But hydrogen cyanide is implicated in the steps that on Earth led to the origin of life.
Finding these simple organic molecules in Titan’s upper atmosphere—even if present only in a part per million or a part per billion—is tantalizing. Could the atmosphere of the primeval Earth have been similar? There’s about ten times more air on Titan than there is on Earth today, but the early Earth may well have had a denser atmosphere.
Moreover, Voyager discovered an extensive region of energetic electrons and protons surrounding Saturn, trapped by the planet’s magnetic field. During the course of its orbital motion around Saturn, Titan bobs in and out of this magnetosphere. Beams of electrons (plus solar ultraviolet light) fall on the upper air of Titan, just as charged particles (plus solar ultraviolet light) were intercepted by the atmosphere of the primitive Earth.
So it’s a natural thought to irradiate the appropriate mixture of nitrogen and methane with ultraviolet light or electrons at very low pressures, and find out what more complex molecules can be made. Can we simulate what’s going on in Titan’s high atmosphere? In our laboratory at Cornell—with my colleague W. Reid Thompson playing a key role—we’ve replicated some of Titan’s manufacture of organic gases. The simplest hydrocarbons on Titan are manufactured by ultraviolet light from the Sun. But for all the other gas products, those made most readily by electrons in the laboratory correspond to those discovered by Voyager on Titan, and in the same proportions. The correspondence is one to one. The next most abundant gases that we’ve found in the laboratory will be looked for in future studies of Titan. The most complex organic gases we make have six or seven carbon and/or nitrogen atoms. These product molecules are on their way to forming tholins.
WE HAD HOPED FOR A BREAK in the weather as Voyager 1 approached Titan. A long distance away, it appeared as a tiny disk; at closest approach, our camera’s field of view was filled by a small province of Titan. If there had been a break in the haze and clouds, even only a few miles across, as we scanned the disk we would have seen something of its hidden surface. But there was no hint of a break. This world is socked in. No one on Earth knows what’s on Titan’s surface. And an observer there, looking up in ordinary visible light, would have no idea of the glories that await upon ascending through the haze and beholding Saturn and its magnificent rings.
From measurements by Voyager, by the International Ultraviolet Explorer observatory in Earth orbit, and by ground-based telescopes, we know a fair amount about the orange-brown haze particles that obscure the surface: which colors of light they like to absorb, which colors they pretty much let pass through them, how much they bend the light that does pass through them, and how big they are. (They’re mostly the size of the particles in cigarette smoke.) The “optical properties” will depend, of course, on the composition of the haze particles.
In collaboration with Edward Arakawa of Oak Ridge National Laboratory in Tennessee, Khare and I have measured the optical properties of Titan tholin. It turns out to be a dead ringer for the real Titan haze. No other candidate material, mineral or organic, matches the optical constants of Titan. So we can fairly claim to have bottled the haze of Titan—formed high in its atmosphere, slowly falling out, and accumulating in copious