Background Science
Why Titan?
Imagine a world somewhat smaller than Mars and bigger than Mercury, where the air is denser than that in your living room, and the pressure is about the same as at the bottom of a swimming pool. Yet the distant sun is never seen, and high noon no brighter than twilight on Earth. The cold is so great that water is always frozen out of the atmosphere; yet the simplest organic molecule methane takes its place as cloud-former and rain maker - perhaps even the stuff of lakes or seas. Methane, wafted hundreds of miles above the surface of this world, is cracked open by sunlight and cosmic rays; a menagerie of more complicated organics are produced, and these float down to the surface to accumulate over time.

• Volcanism and impacts shape the surface of this world and provide energy to make ever more complex organic molecules, in a planet-wide tapestry that is an organic chemist's dream.

• This is Titan, Saturn's largest moon, partly revealed to us by Voyager 1 in 1980. Through its many instruments, Voyager discovered and characterized a dense atmosphere around this cold world. Yet its cameras could not penetrate the organic haze and we still do not know what awaits the Huygens Probe at the end of its journey to Titan's surface.

• We only have hints of the surface from Earth-based radar and from images taken by Hubble and specially adapted telescopes on the ground. By looking beyond the wavelengths of visible light, to the infrared, Hubble could see through the haze and glimpse the surface. The resulting map of bright and dark terrains is crude, because Titan lies a billion miles beyond Hubble. We do not know what the bright and dark areas mean - are the bright parts water ice plateaux thrust above lowlands darkened by solid and liquid organic molecules?

• Only ESA's Huygens Probe can tell us. Huygens will land on what looks to be a very interesting boundary between the light and the dark regions.
• What we know about Titan :
What we do know about Titan is that it has a very dense and interesting atmosphere. Titan has a greenhouse-warmed climate, as does the Earth. Titan's greenhouse is powered by sunlight, like Earth's, but sustained by different gases - methane, hydrogen and nitrogen. Because these gases are part of the cycle of organic chemistry, the stability of Titan's climate is tied to this chemistry. In particular, methane is being steadily depleted over time. If it is not replenished, or replenished only irregularly, Titan's atmosphere may occasionally thin and cool down as methane's greenhouse contribution is lost. Cassini/Huygens will look for evidence of past episodes of climate collapse in the surface geology, for example, by finding small impact craters which could not have formed under the current very thick atmosphere. Most intriguing is that the response of Titan's atmosphere to methane depletion may have been much stronger early in its history, IF the sun was fainter back then than it is today - as astrophysical models predict. The so-called 'faint early sun' seems discordant with geological evidence for liquid water on Mars and Earth early in their histories, and so anything Titan can tell us of this ancient time is potentially quite exciting.
• What about life?
For all the superb research done in laboratories, we do not yet know how life on Earth came to be. To find other worlds where the events of life's origins may be playing out today, on a natural tapestry much larger and longer-lived than any Earth-bound laboratory, is a goal of planetary exploration.

• And Titan is certainly a target. So cold that liquid water is only a transient product of volcanism or impacts. Titan is almost certainly not the home of life today. But its organic chemical cycles may constitute a natural laboratory for replaying some of the steps leading to life. Titan is in some ways the closest analogue to the Earth's environment before life began. We know life is abundant on Earth, and has played key roles in our planet's evolution. We suspect that the outermost solar system probably retains the original inventory of organics from the beginning. Then there are three objects - Mars, Europa, Titan - which may have undergone some amount of organic chemical evolution: almost to the threshold of life, perhaps, on Titan, possibly beyond it on Mars and maybe Europa. These are speculations. But if all goes well, Europe and the United States will together peer beneath the dense smog of Titan to see what this world can tell us of how we came to be.

Courtesy Jonathan I. Lunine
Taken from a press briefing,
3 September 1997, Washington DC.

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