Pale Blue Dot - Carl Sagan [41]
In a way, I grew up with Titan. I did my doctoral dissertation at the University of Chicago under the guidance of Gerard P. Kuiper, the astronomer who made the definitive discovery that Titan has an atmosphere. Kuiper was Dutch and in a direct line of intellectual descent from Christianus Huygens. In 1944, while making a spectroscopic examination of Titan, Kuiper was astonished to find the characteristic spectral features of the gas methane. When he pointed the telescope at Titan, there was the signature of methane.* When he pointed it away, not a hint of methane. But moons were not supposed to hold onto sizable atmospheres, and the Earth’s Moon certainly doesn’t. Titan could retain an atmosphere, Kuiper realized, even though its gravity was less than Earth’s, because its upper atmosphere is very cold. The molecules simply aren’t moving fast enough for significant numbers to achieve escape velocity and trickle away to space.
Daniel Harris, a student of Kuiper’s, showed definitively that Titan is red. Maybe we were looking at a rusty surface, like that of Mars. If you wanted to learn more about Titan, you could also measure the polarization of sunlight reflected off it. Ordinary sunlight is unpolarized. Joseph Veverka, now a fellow faculty member at Cornell University, was my graduate student at Harvard University, and therefore, so to speak, a grandstudent of Kuiper’s. In his doctoral work, around 1970, he measured the polarization of Titan and found that it changed as the relative positions of Titan, the Sun, and the Earth changed. But the change was very different from that exhibited by, say, the Moon. Veverka concluded that the character of this variation was consistent with extensive clouds or haze on Titan. When we looked at it through the telescope, we weren’t seeing its surface. We knew nothing about what the surface was like. We had no idea how far below the clouds the surface was.
So, by the early 1970s, as a kind of legacy from Huygens and his line of intellectual descent, we knew at least that Titan has a dense methane-rich atmosphere, and that it’s probably enveloped by a reddish cloud veil or aerosol haze. But what kind of cloud is red? By the early 1970s my colleague Bishun Khare and I had been doing experiments at Cornell in which we irradiated various methane-rich atmospheres with ultraviolet light or electrons and were generating reddish or brownish solids; the stuff would coat the interiors of our reaction vessels. It seemed to me that, if methane-rich Titan had red-brown clouds, those clouds might very well be similar to what we were making in the laboratory. We called this material tholin, after a Greek word for “muddy.” At the beginning we had very little idea what it was made of. It was some organic stew made by breaking apart our starting molecules, and allowing the atoms—carbon, hydrogen, nitrogen—and molecular fragments to recombine.
The word “organic” carries no imputation of biological origin; following long-standing chemical usage dating back more than a century, it merely describes molecules built out of carbon atoms (excluding a few very simple ones such as carbon monoxide, CO, and carbon dioxide, CO2). Since life on Earth is based on organic molecules, and since there was a time before there was life on Earth, some process must have made organic molecules on our planet before the time of the first organism. Something similar, I proposed, might be happening on Titan today.
The epochal event in our understanding of Titan was the arrival in 1980 and 1981 of the Voyager 1 and 2 spacecraft in the Saturn system. The ultraviolet, infrared, and radio instruments revealed the pressure and temperature through the atmosphere—from the hidden surface to the edge of space. We learned how high the cloud tops are. We found that the air on Titan is composed mainly of nitrogen, N2, as on the Earth today. The other principal constituent is, as Kuiper found, methane, CH4, the starting material from which