Astronomy — Titan, Saturn’s large moon
Titan, Saturn’s large moon, by Carl Sagan
I know a world, midway in size between the Moon and Mars, where the upper air is crackling with electricity, where the perpetual brown overcast is tinged an odd burnt orange, and were the stuff of life falls out of the skies like manna from heaven onto the unknown surface below. This world is so far away that light takes more than an hour to get there from the Sun. A spacecraft would take years. We have examined it in reflected light, probed its surface with radar, and investigated it close-up with robot spacecraft.
Yet much about this world is still enshrouded in mystery — including whether mighty oceans surge beneath its dusky haze. We know just enough, though, to recognize that within our reach is a place where some of the same processes are working themselves out today that long ago, on our world, led to the origins of life.
The beginnings of life: Our most distant one-celled ancestors of around 4 billion years ago were far less capable than the humblest microbe alive today, perhaps just barely able to make copies of themselves. Those first organisms were, as we are, made of pieces, parts, and building blocks based on carbon and called organic molecules. Of the stupendous number of possible organic molecules, only a very few are used at the heart of life. The two most important kinds are the amino acids, the building blocks of proteins, and the nucleotide bases, the building blocks of the nucleic acids DNA and RNA.
Just before the origins 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 then than now and that some of these small worlds are rich storehouses of complex organic molecules. Consider the organic molecules generated in the air and waters of the early Earth. We don’t know very much about the composition of the early atmosphere. There couldn’t have been much molecular oxygen, because our present atmospheric oxygen is generated by green plants, and there weren’t any green plants on the Earth yet. There was probably more hydrogen, because the material from which the Earth formed was rich in hydrogen, and because hydrogen escapes from the upper atmosphere into space. We can guess what the early atmosphere was, duplicate it in the laboratory, supply some energy and see what organic molecules are made. Indeed, over the years, such experiments have been very provocative and promising. But our ignorance of initial conditions limits their relevance. What we need is a real world, a world something like the Earth, a world whose atmosphere still retains some hydrogen-rich gases, a world in which the organic building blocks of life are being generated in our own time, a world where we could go to seek our own beginnings.
Titan, a world like ours?: There is only one such world in the Solar System. This world is Titan, the big moon of Saturn. The atmosphere of Titan is composed mainly of nitrogen, as is the Earth’s today The other principle constituent is methane (CH4), the starting material from which carbon-based organic molecules are generated there. There is no oxygen. Ultraviolet light from the Sun is falling on Titan as it did on the primitive Earth. Beams of electrons trapped in the magnetic field of giant Saturn fall on the upper air of Titan, just as charged particles from the solar wind fell on the atmosphere of the primitive Earth.
But no world is a perfect replica of any other, and there is at least one important respect in which Titan is very different from the primitive Earth – being so far from the Sun, its surface is extremely cold (about 180 degrees below zero Centigrade). This temperature is far below the freezing point of water. Thus, while the Earth at the time of the origins of life was mainly ocean-covered, Titan has no oceans of liquid water. The low temperatures give scientists an advantage, however. Once molecules are synthesized on Titan they stick around. The higher the temperature, the faster the molecules fall to pieces. On Titan, the molecules that have been raining down for the last 4 billion years might still be largely preserved, deep-frozen in their earliest state, awaiting discovery by scientists on Earth.
Early findings: Titan was discovered by the Dutch physicist Christiaan Huygens in 1655. He saw it through one of the first astronomical telescopes as a dot of light gleaming in reflected sunlight from a billion miles away. From the time of its discovery to World War 22, almost nothing more was known about Titan except that it had a curious tawny color. No features of Titan could be discerned; even with the largest telescopes, it was still no more than a point of light. Kuiper discovered the gas methane on Titan in 1944, and Titan remains the only known moon in the solar system with a substantial atmosphere. By the late 1960’s, it became clear that scientists were not seeing all the way to Titan’s surface. There was an intervening layer of clouds or haze.
Breakthrough: The epochal event in our understanding of Titan was the arrival in 1980 and 1981 of the two Voyager spacecraft in the the Saturn system. Threading its way past moon after moon, skirting the edge of the magnificent ring system, Voyager 2 left the Saturn system on a trajectory that would take it triumphantly to Uranus and Neptune. But to make a close flyby of Titan, Voyager 1 had to forgo the option of a rendezvous with the worlds beyond. Titan was considered so important that we were willing to sacrifice much to study it.
The ultraviolet, infrared and radio instruments on Voyager gathered in a treasure-trove of data. We found that nitrogen was the chief constituent of the atmosphere. We ascertained the pressure and temperature from the surface to great heights. We discovered simple organic molecules present as gases, mainly hydrocarbons and nitriles. Hydrocarbons are molecules composed of carbon and hydrogen atoms only; they are familiar to us as natural gas, petroleum, and waxes. Nitriles have a carbon and nitrogen atom attached to each other in a particular way. The best know is HCN, hydrogen cyanide, a deadly gas for humans. But it is possible that HCN was involved in steps that led to the origins of life here on Earth. Finding these simple organic molecules is tantalizing. Recently chemists have simulated Titan’s manufacture of these gases. High-energy electrons are used to irradiate a mixture of nitrogen and methane at very low pressures, simulating the altitude where the electrons are stopped by gases in Titan’s upper atmosphere as they are emitted by Saturn. This process makes a large variety of organic gases, the most complex with 7 or 8 carbon and/or nitrogen atoms. These product gases seem to be on their way to forming tholins and other organic gases found on Titan.
A life-giving haze? We had hoped for a break in the weather as Voyager 1 approached Titan. A long distance away, the giant moon appeared only as a tiny circle; at closest approach, our camera’s field of view was filled by the haze above a small province of Titan. If there had been a break in the clouds only a few miles across, we might have seen details on it enigmatic surface. Sadly, there was no break. No one yet knows what its surface looks like. But from Voyager and other measurements we know a fair amount about the orange-brown haze particles: which colors of light they like to absorb, which colors they pretty much let pass through, how much they bend the light that does pass through them. These optical properties will depend, of course, on their composition. Chemists have measured the optical properties of tholin, the brownish haze surrounding Titan. Scientists feel that organic solids form high in the atmosphere of Titan and then slowly fall and accumulate on the surface of Titan. What is this ‘stuff’ made of? It is very hard to know the exact chemical composition of a complex organic solid. For example, the chemistry of coal is still not well understood. But there are some interesting things we do know about Titan’s tholin. If we drop it in liquid water, we make amino acids. Some of these amino acids are common in living things on Earth. Others are of a completely different sort. There is also a hint that nucleotide bases are present, plus a very rich array of other organic molecules, some relevant to life, some not. Temperatures on Titan are so low that there cannot be oceans of liquid water. There may be wet times and/or places because of volcanic activity or cometary collisions. During the last 4 billion years, immense quantities of organic tholins must have sedimented out on Titan. If it is all deep-frozen and unchanged in the intervening eons, the amount accumulated might be tens of meters deep.
A hydrocarbon ocean? While oceans of liquid water are impossible on Titan, oceans of liquid hydrocarbons are expected. They must condense out at the surface in the same way water vapor becomes a liquid near the surface of the Earth. Vast oceans of liquid hydrocarbons should have accumulated over the lifetime of Titan. They would lie far beneath the haze and clouds. That doesn’t mean they would be inaccessible to us because radio waves readily penetrate through the atmosphere of Titan. Scientists have sent radio waves to Titan and had them reflect back to Earth. The waves appear to reflect back from certain parts of Titan’s surface but not from others. It is postulated that Titan has continents and oceans.
The orbit of Titan around Saturn is not a perfect circle. It is elliptical. If Titan has extensive hydrocarbon oceans, the giant planet Saturn, around which it orbits, will raise substantial tides on Titan. If there are continents as well, the resulting tidal friction would have turned Titan’s orbit into a circle in much less time than the age of Titan. Scientists thus have postulated that Titan must be all ocean or all land. A few lakes or islands might be permitted, but anything more and Titan would have a different orbit than the one we see. Thus we have a paradox – Titan should have oceans, but it can’t have oceans.
Unanswered questions: Maybe the radar results are inaccurate, maybe the orbital calculations are incorrect, maybe there has been a congealing of the liquid hydrocarbons (turning them into some complex organic solid that reflects radio waves), maybe there is something floating on the oceans that reflects radio waves, or maybe there is an icy surface covered with a deep layer of tholins.
Here is another brief article about Titan, entitled “Titan’s antigreenhouse effect”
Much has been written about the greenhouse effect that warms the surfaces of Venus and Earth. For the first time a major antigreenhouse effect has been discovered in the solar system. This mechanism cools the surface of Saturn’s largest moon, Titan, the only moon that possess an appreciable atmosphere in our solar system. A thick high-altitude haze of organic particles causes the antigreenhouse cooling. This is the same layer that hid Titan’s surface from the Voyager space probe’s view. The haze absorbs sunlight, preventing heat from reaching the surface. At the same time, it allows thermal infrared radiation from the surface to escape into space. This effect lowers Titan’s surface temperature by 9 degrees Kelvin. However, methane and hydrogen gas in Titan’s lower atmosphere perform the same function that carbon dioxide and water vapor do on Earth – they trap infrared radiation via the greenhouse effect and warm Titan’s surface by about 21 degrees Kelvin. This overcompensates for the antigreenhouse cooling, leading to a net warming of 12 degrees Kelvin. The temperature of Titan’s surface is thus 94 degrees Kelvin rather than the 82 degrees Kelvin that would be expected in the absence of an atmosphere.
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