With this in mind, Titan - as important a scientific subject for Cassini's studies as Saturn itself - will be the subject of this next report.

Titan is an utterly unique member of the Solar System: an icy moon only slightly bigger than the planet Mercury, but possessing an atmosphere fully four times denser than Earth's at its surface.

Because of the orange organic smog that fills its atmosphere and shields its surface from visible-light viewing, it's probably the most mysterious world left to explore in our the Solar System - even more so than Pluto be it a planet or not.

Until recently, we didn't even known whether its surface was mostly solid or liquid. But even with most evidence pointing to a solid surface, there remains a whole raft of major scientific questions to be answered.

First is the question of how it got such a dense atmosphere that is mostly nitrogen, but with some methane, along with other trace constituents. After all, Jupiter's comparably big icy moons Ganymede and Callisto have virtually no atmosphere. And because Titan is half ice and half rock and therefore much lower in density than our Moon - so that its surface is much farther away from its center of mass - its surface gravity is actually slightly weaker: one seventh of Earth's gravity compared to one sixth for the Moon. Yet Titan has so much gas piled on to it that - even in that weak gravity - its surface air pressure is 1.6 times that of Earth.

How did it get and keep all that gas?

The favored theory is that because Saturn is smaller and formed further away from the Sun than Jupiter, its initial was much less than that of Jupiter. As such the nebula swirling around Saturn during its formation was much colder than that around Jupiter, thus allowing not only water ice but also ammonia to condense into ice crystals which were then incorporated into the solid substance of Saturn's moons.

Then, as Titan itself formed out of material continuing to fall into it in a snowballing process, the heat it gave off boiled the ammonia to create an initial dense ammonia atmosphere, which was then quickly broken down into nitrogen by the Sun's increasing UV light. As for the methane, the water-ice crystals in the Saturnian nebula would probably have been cold enough to carry a good deal of the methane trapped within the water ice crystals.

Alternatively, Titan may have received its nitrogen directly during its formation - either trapped in the ices of the Saturnian nebula or imported from outside, as swarms of comets forming in the even colder outer Solar System wandered into the inner System and crashed into it to add their nitrogen and methane to the now fast forming Titan.

If so, this didn't happen to Jupiter's moons because Jupiter's stronger gravity caused comets to crash so fast that virtually all their vaporized material was blasted completely away from those moons, leaving them still airless.

Since any ices cold enough to contain nitrogen in its original form would also carry rather large amounts of argon and deuterium, Huygens may be able to settle this question by measuring those two gases in Titan's atmosphere.

Then there's the matter of that strange orange "smog", and the complex organic compounds that formed very slowly over the eons out of the methane and nitrogen in Titan's air - broken down by the Sun's UV light and the charged particles in Saturn's radiation belts.

This smog is actually highly rarified, and a person could see through it for dozens of kilometers; the only reason it obscures Titan's surface from us is that the smog layer is so thick. Because of that unique combination of weak gravity and dense atmosphere, Titan's atmosphere towers above its surface to an awesome height with the top of its haze fully 200 km above its surface.

It should be kept in mind that, by all indications, Titan has few if any actual clouds - just that smog.

But, rarified or not, it's extremely important scientifically, because it's a rich natural laboratory for the synthesis of complex organic molecules of the sort that must have served for the beginning of life's evolution on ancient Earth.

The appearance of life on Earth is still one of the great scientific mysteries: we are still nowhere near able to simulate the process in labs, and there is considerable puzzlement as to how it actually occurred so fast by sheer chance in nature.

Even today we are essentially completely in the dark as to what the individual steps in that process might have been. And, unfortunately, all the chemical evidence of what the intermediate "pre-biotic" steps might have been were "eaten" billions of years ago by Earth's early organisms themselves.

For this reason, NASA considers the search for preserved "pre-biotic" chemical remains on other worlds in the Solar System to be just as important scientifically as the search for actual life on those worlds.

If the process by which complex organic compounds evolve into life occurred only partway on Mars and Europa before the growing cold or dryness of those worlds aborted it, that same cold or dryness may well have preserved a large part of those complex pre-biotic compounds intact - giving us perhaps our only chance to genuinely understand the process by which life appeared on our own planet.

Titan also plays an important part in that search. Its surface is a cryogenically cold -180 deg C - far too cold for liquid water to exist - and has always been about that cold, so the chance of life actually having ever evolved there is virtually nonexistent; and without liquid water to help form them, the vast variety of complex organic compounds which contain oxygen will also be almost nonexistent there.

But that still leaves a vast collection of other complex hydrocarbons and of "nitriles" - organic compounds that contain nitrogen and which may be largely responsible for the orange color of the haze.

And since the radiation-triggered natural synthesis processes which produce these compounds out of Titan's air have been going on very slowly but continuously for over 4 billion years, there should be many types of organic compounds there - in detectable amounts - that simply have not yet been produced by simulations of natural pre-biotic chemical synthesis in Earth laboratories, due to simple limitations on the amount of time and resources scientists have had to run such experiments.

So, despite its chemical limitations, the huge reservoir of organic material, made up of smoke-sized solid particles, that have very slowly snowed out of Titan's atmosphere - perhaps 100 to 400 meters of it over the past 4 billion years - is certain to provide chemists with a great deal of new knowledge about the different kinds of complex organic substances that may have been produced on the ancient Earth during its first few hundred million years.

Moreover, as was first pointed out by Carl Sagan in the early 1990s, there are many individual patches of Titan's surface, around its impact craters, where gigantic meteorite impacts certainly DID produce enough temporary heat to melt the water ice in Titan's surface - and, in fact, to keep some of that water liquid for periods that may sometimes have lasted for thousands of years.

Again, this is probably far too short a time for life to have ever evolved on Titan - but it is long enough for a great many additional organic compounds which do contain oxygen to form there, providing us with still more insight into the chemical synthesis processes that must have occurred on ancient Earth.

Although the chances that the Huygens probe will happen to land at such a spot are small, they will be the single most important target for the next Titan probes, which will actually be able to travel across and survey its surface by driving or flying. And Huygens may still be able to find some such compounds mixed in with the general surface.

Finally, there's the related question of just what Titan's surface is actually like: solid or liquid? The Sun's and Saturn's radiation should also, over the eons, slowly turn its methane into the slightly more complex hydrocarbon ethane - enough to form a layer of liquid ethane several hundred meters deep over Titan's surface.

Moreover we know that this conversion process occurs at a fast enough rate to completely destroy Titan's current atmospheric supply of methane in only a few tens of millions of years. As such its methane must be replenished from some other source either on or under its surface - either by evaporation from a huge surface supply of liquid methane mixed in with the liquid ethane, or as a result of venting from underneath Titan's icy surface.

If this methane venting is spasmodic due to ice "volcanoes" or giant meteor impacts rather than gradual and continuous, then Titan's atmosphere may keep changing in nature over time. This may then cause Titan to periodically lose all its methane and the associated greenhouse effect caused by that methane, with the result that its surface actually becomes cold enough for most of its atmospheric nitrogen to liquify on the surface until the next big methane outburst warms the satellite up again.

At any rate, since methane and ethane are the two components of natural gas, most planetologists thought it likely that Titan was covered with an ocean of liquid natural gas - making it the only other world besides Earth to have an ocean on its surface albeit a thoroughly unearthly one.

It was speculated that this methane-ethane ocean might even be kilometers deep, covering most or all of Titan's surface features.

But in 1990, we were finally able to start bouncing radar echoes off Titan - and, at about the same time, it was also confirmed that the orange organic haze which is so impenetrable in visible light is actually almost completely transparent in many near-infrared wavelengths, making it possible for Earth-based telescopes and the Hubble Telescope to start viewing Titan's surface in those wavelengths.

So we began obtaining our first fuzzy views of Titan's surface - and where upon we discovered to our surprise that most of its surface is both much too highly radar-reflective, and much too light-colored in near-IR wavelengths, to be liquid ethane or methane which, like most liquid hydrocarbons, are as black as oil.

Indeed, fuzzy but definite surface features are starting to become apparent - the most spectacular by far being an Australia-sized patch on one side of Titan which is far more radar-reflective and considerably lighter in color than the rest of the moon.

Only a few small patches have been discovered so far which are dark enough that they may be isolated lakes of liquid hydrocarbons - and none are more than a few hundred km across. The rest of Titan's surface is sufficiently light in color, and radar-bright enough, that it seems to resemble very dirty water ice.

So where is Titan's ocean?

There are several theories: some researchers suggest that as suggested above its methane is really replenished from underground vents, and that the liquid ethane-methane mix on its surface is really in the form of a scattering of isolated lakes and small seas - perhaps filling Titan's craters.

Dr. David Stevenson at the California Institute of Technology suggests, that Titan may instead have an "aquifer": with its upper kilometer or so of surface being highly fragmented and ground up from eons of meteor impacts and as a result serving as a reservoir fully capable of storing all of Titan's liquid natural gas as the underground equivalent of a "water table".

As for those "water-ice volcanoes" mentioned above: there is a very real chance that Titan occasionally vents a liquid mixture of ammonia and water from its somewhat warmer interior - after all, such a mixture can stay liquid at temperatures as low as -97 deg C.

There is even growing speculation, on theoretical grounds, that Titan may have a whole underground ocean layer of an ammonia-water mixture - similar to the subterranean oceans that may exist on Europa, Ganymede and Callisto - buried 30-50 km below its supercold icy surface, and perhaps as much as 200-300 km deep.

While life is certainly impossible on Titan's surface, a few scientists have speculated that this deeply buried ocean - like those on Ganymede and Callisto - might conceivably serve as a place where microbial life could evolve and survive.

However, the relative lack of chemicals to serve as an energy source for such microbes in these oceans makes this far less likely than the already questionable possibility that life exists in Europa's subsurface sea.

Finally, there's the question of Titan's meteorology. As I said, the fuzzy images we have so far suggest that while Titan has translucent smog galore, it has little and perhaps no actual cloud cover.

Some recent observations do hint, however, that it may have a sprinkling of small and short-lived cloud patches of liquid methane, and furthermore that they may tend to disappear with a speed which suggests that they may be storm clouds pouring down drops of liquid methane rather than water.

These would be very strange rainfalls: in that weak gravity, the drops would fall as slowly as snowflakes - and, since Titan's air temperature does rise slightly toward its surface, they might evaporate back into gas before reaching the surface in Titan's lowlands.

However, they may reach the surface in the highlands, carving out rivers there in a strange parody of Earth's water cycle.

It has also been speculated that the huge lighter-colored patch on Titan is such a highland, and that such methane rains wash it clean of the dark-orange organic gunk that is slowly and permanently accumulating on the rest of Titan's surface.

However, one problem with this theory is that clean water ice is actually almost as dark in color in the 2-micron IR band as hydrocarbons are, whereas the Titanian "continent" is also brighter than its surroundings in that color.

An alternative theory, therefore, is that this Titanian continent is high enough that its mountains are covered with a bright frozen methane snow.

Clearly, what we have here is a world about which we know very little, except that it is extremely strange and full of many surprises yet.

We can only hazard at the vaguest of guesses as to what even the basic physical consistency and features of its surface are. Cassini and the Huygens probe that it carries will therefore provide us with our first decent understanding of Titan.

In part two of this series we look at how the Huygens probe will do that - and how the European Space Agency hopes to conquer a serious design with the communications system Huygens' will use to relay data back to the Cassini mothership.

Related Links
Main Cassini Portal NASA/ESA
Titan and the Origin of Life on Earth
Some Speculations on Titan's Past, Present and Future
Weather on Titan
Beneath the Clouds of Titan
Hubble Titan Surface Photos and Map
SpaceDaily
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