Pale Blue Dot: A Vision of the Human Future in Space - Carl Sagan, Ann Druyan (1997)
Chapter 7. AMONG THE MOONS OF SATURN
Seat thyself sultanically among the moons of Saturn.
—HERMAN MELVILLE, MOBY DICK, CHAPTER 107 (1851)
There is a world, midway in size between the Moon and Mars, where the upper air is rippling with electricity—streaming in from the archetypical ringed planet next door, where the perpetual brown overcast is tinged with an odd burnt orange, and where the very stuff of life falls out of the skies onto the unknown surface below. It is so far away that light takes more than an hour to get there from the Sun. Spacecraft take years. Much about it is still a mystery—including whether it holds great oceans. We know just enough, though, to recognize that within reach may be a place where certain processes are today working themselves out that aeons ago on Earth led to the origin of life.
On our own world a long-standing—and in some respects quite promising—experiment has been under way on the evolution of matter. The oldest known fossils are about 3.6 billion years old. Of course, the origin of life had to have happened well before that. But 4.2 or 4.3 billion years ago the Earth was being so ravaged by the final stages of its formation that life could not yet have come into being: Massive collisions were melting the surface, turning the oceans into steam and driving any atmosphere that had accumulated since the last impact off into space. So around 4 billion years ago, there was a fairly narrow window—perhaps only a hundred million years wide—in which our most distant ancestors came to be. Once conditions permitted, life arose fast. Somehow.
The first living things very likely were inept, far less capable than the most humble microbe alive today—perhaps just barely able to make crude copies of themselves. But natural selection, the key process first coherently described by Charles Darwin, is an instrument of such enormous power that from the most modest beginnings there can emerge all the richness and beauty of the biological world.
Those first living things were made of pieces, parts, building blocks which had to come into being on their own—that is, driven by the laws of physics and chemistry on a lifeless Earth. The building blocks of all terrestrial life are called organic molecules, molecules based on carbon. Of the stupendous number of possible organic molecules, very few are used at the heart of life. The two most important classes are the amino acids, the building blocks of proteins, and the nucleotide bases, the building blocks of the nucleic acids.
Just before the origin of life, where did these molecules come from? There are only two possibilities: from the outside or from the inside. We know that vastly more comets and asteroids were hitting the Earth than do so today, that these small worlds are rich storehouses of complex organic molecules, and that some of these molecules escaped being fried on impact. Here I’m describing homemade, not imported, goods: the organic molecules generated in the air and waters of the primitive Earth.
Unfortunately, we don’t know very much about the composition of the early air, and organic molecules are far easier to make in some atmospheres than in others. There couldn’t have been much oxygen, because oxygen is generated by green plants and there weren’t any green plants yet. There was probably more hydrogen, because hydrogen is very abundant in the Universe and escapes from the upper atmosphere of the Earth into space better than any other atom (because it’s so light). If we can imagine various possible early atmospheres, we can duplicate them in the laboratory, supply some energy, and see which organic molecules are made and in what amounts. Such experiments have over the years proved provocative and promising. But our ignorance of initial conditions limits their relevance.
What we need is a real world whose atmosphere still retains some of those hydrogen-rich gases, a world in other respects something like the Earth, a world in which the organic building blocks of life are being massively generated in our own time, a world we can go to to seek our own beginnings. There is only one such world in the Solar System.* That world is Titan, the big moon of Saturn. It’s about 5,150 kilometers (3,200 miles) in diameter, a little less than half the size of the Earth. It takes 16 of our days to complete one orbit of Saturn.
No world is a perfect replica of any other, and in at least one important respect Titan is very different from the primitive Earth: Being so far from the Sun, its surface is extremely cold, far below the freezing point of water, around 180° below zero Celsius. So while the Earth at the time of the origin of life was, as now, mainly ocean-covered, plainly there can be no oceans of liquid water on Titan. (Oceans made of other stuff are a different story, as we shall see.) The low temperatures provide an advantage, though, because once molecules are synthesized on Titan, they tend to stick around: The higher the temperature, the faster molecules fall to pieces. On Titan the molecules that have been raining down like manna from heaven for the last 4 billion years might still be there, largely unaltered, deep-frozen, awaiting the chemists from Earth.
THE INVENTION OF THE TELESCOPE in the seventeenth century led to the discovery of many new worlds. In 1610 Galileo first spied the four large satellites of Jupiter. It looked like a miniature solar system, the little moons racing around Jupiter as the planets were thought by Copernicus to orbit the Sun. It was another blow to the geocentrists. Forty-five years later, the celebrated Dutch physicist Christianus Huygens discovered a moon moving about the planet Saturn and named it Titan.* It was a dot of light a billion miles away, gleaming in reflected sunlight. From the time of its discovery, when European men wore long curly wigs, to World War II, when American men cut their hair down to stubble, almost nothing more was discovered about Titan except the fact it had a curious, tawny color. Ground-based telescopes could, even in principle, barely make out some enigmatic detail. The Spanish astronomer J. Comas Solá reported at the turn of the twentieth century some faint and indirect evidence of an atmosphere.
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 carbon-based organic molecules are generated there.
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 amounts on its surface. What is this stuff made of?
It’s very hard to know the exact composition of a complex organic solid. For example, the chemistry of coal is still not fully understood, despite a long-standing economic incentive. But we’ve found out some things about Titan tholin. It contains many of the essential building blocks of life on Earth. Indeed, if you drop Titan tholin into water you make a large number of amino acids, the fundamental constituents of proteins, and nucleotide bases also, the building blocks of DNA and RNA. Some of the amino acids so formed are widespread in living things on Earth. Others are of a completely different sort. A rich array of other organic molecules is present also, some relevant to life, some not. During the past four billion years, immense quantities of organic molecules sedimented out of the atmosphere onto the surface of Titan. If it’s all deep-frozen and unchanged in the intervening aeons, the amount accumulated should be at least tens of meters (a hundred feet) thick; outside estimates put it at a kilometer deep.
But at 180°C below the freezing point of water, you might very well think that amino acids will never be made. Dropping tholins into water may be relevant to the early Earth, but not, it would seem, to Titan. However, comets and asteroids must on occasion come crashing into the surface of Titan. (The other nearby moons of Saturn show abundant impact craters, and the atmosphere of Titan isn’t thick enough to prevent large, high-speed objects from reaching the surface.) Although we’ve never seen the surface of Titan, planetary scientists nevertheless know something about its composition. The average density of Titan lies between the density of ice and the density of rock. Plausibly it contains both. Ice and rock are abundant on nearby worlds, some of which are made of nearly pure ice. If the surface of Titan is icy, a high-speed cometary impact will temporarily melt the ice. Thompson and I estimate that any given spot on Titan’s surface has a better than 50–50 chance of having once been melted, with an average lifetime of the impact melt and slurry of almost a thousand years.
This makes for a very different story. The origin of life on Earth seems to have occurred in oceans and shallow tidepools. Life on Earth is made mainly of water, which plays an essential physical and chemical role. Indeed, it’s hard for us water-besotted creatures to imagine life without water. If on our planet the origin of life took less than a hundred million years, is there any chance that on Titan it took a thousand? With tholins mixed into liquid water—even for only a thousand years—the surface of Titan may be much further along toward the origin of life than we thought.
DESPITE ALL THIS, we understand pitifully little about Titan. This was brought home forcefully to me at a scientific symposium on Titan held in Toulouse, France, and sponsored by the European Space Agency (ESA). While oceans of liquid water are impossible on Titan, oceans of liquid hydrocarbons are not. Clouds of methane (CH4), the most abundant hydrocarbon, are expected not far above the surface. Ethane (C2H6), the next most abundant hydrocarbon, must condense out at the surface in the same way that water vapor becomes a liquid near the surface of the Earth, where the temperature is generally between the freezing and melting points. Vast oceans of liquid hydrocarbons should have accumulated over the lifetime of Titan. They would lie far beneath the haze and clouds. But that doesn’t mean they would be wholly inaccessible to us—because radio waves readily penetrate the atmosphere of Titan and its suspended, slowly falling fine particles.
In Toulouse, Duane O. Muhleman of the California Institute of Technology described to us the very difficult technical feat of transmitting a set of radio pulses from a radio telescope in California’s Mojave Desert, so they reach Titan, penetrate through the haze and clouds to its surface, are reflected back into space, and then returned to Earth. Here, the greatly enfeebled signal is picked up by an array of radio telescopes near Socorro, New Mexico. Great. If Titan has a rocky or icy surface, a radar pulse reflected off its surface should be detectable on Earth. But if Titan were covered with hydrocarbon oceans, Muhleman shouldn’t see a thing: Liquid hydrocarbons are black to these radio waves, and no echo would have been returned to Earth. In fact, Muhleman’s giant radar system sees a reflection when some longitudes of Titan are turned toward Earth, and not at other longitudes. All right, you might say, so Titan has oceans and continents, and it was a continent that reflected the signals back to Earth. But if Titan is in this respect like the Earth—for some meridians (through Europe and Africa, say) mainly continent, and for others (through the central Pacific, say) mainly ocean—then we must confront another problem:
The orbit of Titan around Saturn is not a perfect circle. It’s noticeably squashed out, or elliptical. If Titan has extensive oceans, though, the giant planet Saturn around which it orbits will raise substantial tides on Titan, and the resulting tidal friction will circularize Titan’s orbit in much less than the age of the Solar System. In a 1982 scientific paper called “The Tide in the Seas of Titan,” Stanley Dermott, now at the University of Florida, and I argued that for this reason Titan must be either an all-ocean or an all-land world. Otherwise the tidal friction in places where the ocean is shallow would have taken its toll. Lakes and islands might be permitted, but anything more and Titan would have a very different orbit than the one we see.
We have, then, three scientific arguments—one concluding that this world is almost entirely covered with hydrocarbon oceans, another that it’s a mix of continents and oceans, and a third requiring us to choose, counseling that Titan can’t have extensive oceans and extensive continents at the same time. It will be interesting to see what the answer turns out to be.
What I’ve just told you is a kind of scientific progress report. Tomorrow there might be a new finding that clears up these mysteries and contradictions. Maybe there’s something wrong with Muhleman’s radar results, although it’s hard to see what it might be: His system tells him he’s seeing Titan when it’s nearest, when he ought to be seeing Titan. Maybe there’s something wrong with Dermott’s and my calculation about the tidal evolution of the orbit of Titan, but no one has been able to find any errors so far. And it’s hard to see how ethane can avoid condensing out at the surface of Titan. Maybe, despite the low temperatures, over billions of years there’s been a change in the chemistry; maybe some combination of comets impacting from the sky and volcanoes and other tectonic events, helped along by cosmic rays, can congeal liquid hydrocarbons, turning them into some complex organic solid that reflects radio waves back to space. Or maybe something reflective to radio waves is floating on the ocean surface. But liquid hydrocarbons are very under-dense: Every known organic solid, unless extremely frothy, would sink like a stone in the seas of Titan.
Dermott and I now wonder whether, when we imagined continents and oceans on Titan, we were too transfixed by our experience on our own world, too Earth-chauvinist in our thinking. Battered, cratered terrain and abundant impact basins cover other moons in the Saturn system. If we pictured liquid hydrocarbons slowly accumulating on one of those worlds, we would wind up not with global oceans, but with isolated large craters filled, although not to the brim, with liquid hydrocarbons. Many circular seas of petroleum, some over a hundred miles across, would be splattered across the surface—but no perceptible waves would be stimulated by distant Saturn and, it is conventional to think, no ships, no swimmers, no surfers, and no fishing. Tidal friction should, we calculate, be negligible in such a case, and Titan’s stretched-out, elliptical orbit would not have become so circular. We can’t know for sure until we start getting radar or near-infrared images of the surface. But perhaps this is the resolution of our dilemma: Titan as a world of large circular hydrocarbon lakes, more of them in some longitudes than in others.
Should we expect an icy surface covered with deep tholin sediments, a hydrocarbon ocean with at most a few organic-encrusted islands poking up here and there, a world of crater lakes, or something more subtle that we haven’t yet figured out? This isn’t just an academic question, because there’s a real spacecraft being designed to go to Titan. In a joint NASA/ESA program, a spacecraft called Cassini will be launched in October 1997—if all goes well. With two flybys of Venus, one of Earth, and one of Jupiter for gravitational assists, the ship will, after a seven-year voyage, be injected into orbit around Saturn. Each time the spacecraft comes close to Titan, the moon will be examined by an array of instruments, including radar. Because Cassini will be so much closer to Titan, it will be able to resolve many details on Titan’s surface indetectable to Muhleman’s pioneering Earth-based system. It’s also likely that the surface can be viewed in the near infrared. Maps of the hidden surface of Titan may be in our hands sometime in the summer of 2004.
Cassini is also carrying an entry probe, fittingly called Huygens, which will detach itself from the main spacecraft and plummet into Titan’s atmosphere. A great parachute will be deployed. The instrument package will slowly settle through the organic haze down into the lower atmosphere, through the methane clouds. It will examine organic chemistry as it descends, and—if it survives the landing—on the surface of this world as well.
Nothing is guaranteed. But the mission is technically feasible, hardware is being built, an impressive coterie of specialists, including many young European scientists, are hard at work on it, and all the nations responsible seem committed to the project. Perhaps it will actually come about. Perhaps winging across the billion miles of intervening interplanetary space will be, in the not too distant future, news about how far along the path to life Titan has come.
*There could have been none. We’re very lucky that there is such a world to study. The others all have too much hydrogen, or not enough, or no atmosphere at all.
*Not because he thought it remarkably large, but because in Greek mythology members of the generation preceding the Olympian gods—Saturn, his siblings, and his cousins—were called Titans.
*Titan’s atmosphere has no detectable oxygen, so methane is not wildly out of chemical equilibrium—as it is on Earth—and its presence is in no way a sign of life.