Beginning with Pioneers 10 and 11, Voyager 1 and Voyager 2 returned a wealth of data, highlighting the extraordinary variety manifested by the satellites of Jupiter and Saturn. Ranging from the sulfur volcanoes on Io to the possibility that Europa has a liquid water interior to the cold methane and nitrogen atmosphere of Titan.

Building upon the exciting discoveries of these first explorers, NASA sent the Galileo orbiter (and descent probe) to Jupiter, which has perhaps revolutionized our thinking on the possibility of life outside of the habitable zone.

The next target for deep space exploration is the Cassini/Huygens mission to Saturn and Titan respectively. Sadly perhaps, this represents the last of NASA�s big budget space missions.

However, ESA has a large stake in this mission, being in the most part, responsible for the Huygens entry probe. What is so exciting about Titan and why is it important to devote a large fraction of resources of a mission to study it?

Titan in the pre-space age
In 1655 the Dutch astronomer Christiaan Huygens (1629-95) discovered the rings of Saturn through a telescope of his own design. (In his lifetime he also invented the pendulum clock and the first accurate time-keeping device, his contributions to mathematics, astronomy, time measurement and the theory of light are considered fundamental). In the same year, Huygens also discovered Titan and determined its period of revolution.

However, the moon wasn�t named until almost two centuries later when Sir John Herschel (discoverer of Uranus) assigned names to the seven moons of Saturn that were known at that time.

The name Titan was well chosen, as Herschel knew only that Titan was the brightest moon of Saturn. Since then it has proved to be the largest. Indeed, the moon is larger than Mercury, but smaller than Mars.

However, measurements by Voyager 1 show that it does not quite hold the record as the solar system�s largest moon. Jupiter�s moon Ganymede has a radius of 2,640 kilometers, whereas the solid body of Titan (excluding the atmosphere) has a radius of 2,575 kilometers.

About the time Percival Lowell was proposing canals on Mars, the Catalan astronomer Jos� Comas Sol�, inferred the presence of an atmosphere on Titan. Sol� reported that he observed that Titan was darker at its limb than it was at its center.

He suggested that the mechanism for this was sunlight reflected toward the Earth by Titan�s limb must pass through more of Titan�s atmosphere than sunlight reflected by the center.

Sol�s observations led Sir James Jeans to include Titan and giant moons of Jupiter in his theoretical study of the escape of atmospheres from the bodies of the solar system. Jeans hypothesized that even though the gravity on Titan was weak compared to the Earth. Titan nevertheless probably retained an atmosphere due to its low temperature.

This lead to the prediction that the temperature of Titan must be between 60 and 100 Kelvin (273 K = 0oC), which would imply that a gaseous substance whose molecular weight is 16 or more should not have escaped from Titan over the history of the solar system. (Molecular hydrogen (H2) has a molecular weight of two; molecular oxygen (O2) has a molecular weight of 32]. Several substances satisfy Jean�s limit on weight. These include argon, neon, molecular nitrogen (N2) and methane (CH4).

In 1944 the legendary planetary scientist Gerald P. Kuiper, of the University of Chicago, identified methane in the spectrum of Titan � the first strong evidence that Titan had an atmosphere. Subsequent observations by radar, telescopes and laboratory modeling painted varied pictures of Titan.

For example, based upon previous work, John J. Caldwell of Princeton University, suggested that the atmosphere of Titan was 90% methane with a surface pressure of 20 millibars (1000 millibars is roughly the atmospheric pressure at sea level on the Earth) and a surface temperature of 86 K.

Alternatively, based upon radio measurements, Donald M. Hunten of the University of Arizona proposed that Titan could have an atmosphere of molecular hydrogen at 20 bars and a surface temperature of 200 K.

A middle ground was reached when Walter J. Jaffe and Tobias Owen observed Titan with the Very Large Array of radio telescopes situated in New Mexico. They found that Titan had a surface temperature of 87 K and that it could have a significant atmosphere with the caveat that nitrogen provided no more than two bars.

Titan - post Voyager
On 12 November 1980, Voyager 1 passed within 7,000 kilometers of Titan. The encounter was made at a cost--Voyager 1 would not be able to use Saturn�s gravity to sling shot it on to Uranus and Neptune. However, the gamble was worth it. Data were gathered and interpreted from a myriad of different instruments, including the cameras, and an ultraviolet spectrophotometer.

The combination of this data revealed a moon that had an atmosphere like the early Earth--rich in nitrogen, argon, methane, and hydrogen, at a pressure of 1.5 bars. Not only that, but the surface gravity of Titan is only 0.14 times as strong as the surface gravity of the Earth.

This implies that there is 10 times as much gas at the surface of Titan than the Earth. Several more exotic gases were identified, including hydrogen cyanide (HCN) and several hydrocarbons, such as propane (C3H8) and cyanoacetylene (HC3N).

Like the Earth, Titan has a greenhouse-warmed climate. 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.

In contrast, little is known about the surface of Titan. The camera of Voyager 1 could not penetrate the organic haze. We only have hints of the surface from Earth-based radar and from images taken by Hubble and specially adapted telescopes on the ground. Titan may have a methane sea.

However such methane, transported hundreds of miles above the surface of this world, might be cracked open by sunlight and cosmic rays. A menagerie of more complicated organics would be produced, and these would float down to the surface and accumulate over time, a thick carbon soup might have formed.

By looking beyond the wavelengths of visible light, to the infrared, the Hubble space telescope can see through the haze and glimpse the surface. The resulting map of bright and dark terrain 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 plateaus thrust above lowlands darkened by solid and liquid organic molecules?

The next step
ESA's Huygens, a robotic probe, is part of a joint NASA/ESA mission to Saturn and its largest moon Titan. Launched 15 October 1997, from Cape Canaveral, Space launch Complex 40, aboard a Titan IVB-Centaur, the probe will require four gravity assists to research its final destination.

In 2004 NASA's Cassini orbiter will deliver Huygens to Titan, and relay the Probe's signals to the Earth. The Orbiter will go on to spend the next four years examining Saturn and its rings and moons, including thirty passes over Titan.

The mission is international in nature. On-the-spot information from Huygens, providing a �ground truth� will help in interpreting the Orbiter's more global observations of Titan, to be made with five different remote-sensing instruments using visible, ultraviolet and infrared light, as well as radar.

Titan and life
We have no clear understanding of the composition of the atmosphere on early Earth--which is important to understanding the origin of life. What we need is a world that retained some of those hydrogen-rich gases, and where the organic building blocks of life are being synthesised in our own era.

These processes may occur on Titan. However, being so far from the Sun, the temperature of the moon is around 93 K. Although, such low temperatures are an advantage, because when compounds are made in the atmosphere, they are preserved for a long time.

Carl Sagan and his colleagues at Cornell University have shown that when gases thought to compose the atmosphere of primitive Earth are �sparked� after 10 minutes a strange brown pigment is formed. Gradually the interior of the vessel became covered by a thick brown tar-like substance, which they called tholin.

Analysis of tholin showed that it consisted of a rich collection of organic molecules, including the constituents of proteins (such as amino acids) and nucleic acids. Sagan used this apparatus to demonstrate that tholins may also be present in comets and also on Titan.

A number of years ago, Carol Stoker, a planetary scientist at NASA Ames Laboratory, showed that bacteria could grow on such material. However, Titan is almost certainly not the home of terrestrial life today. The temperature is way too cold.

But its organic chemical cycles may constitute a natural laboratory for replaying some of the steps leading to the origin of life. Titan is in some ways the closest analogue to the Earth's environment before life began. This is one of the main reasons for sending the Huygens Probe to Titan.

The examination of comets, as in ESA's Giotto and Rosetta missions, reveals quite complicated carbon molecules available in cosmic space. Delivery by comets is thought to be one of the major mechanisms for seeding the primordial Earth both with the building blocks of life, but also with water.

By identifying the compounds that are synthesized on Titan, the Huygens Probe may go along way to helping to provide answers, or lead to better defined questions, for solving the origin of life on Earth.

Dr. Julian A. Hiscox School of Animal and Microbial Sciences University of Reading can be contacted via j.a.hiscox@[email protected] - remove @nospam@ and replace with a single @ sign.

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