Arizona
SATURN鈥檚 giant moon, Titan, is a mysterious world. Hidden beneath an impenetrable mantle of dusty clouds, its surface is the largest unexplored area in the Solar System. But over the past few years, using Earth-based observations and the powerful Hubble Space Telescope, astronomers have started to catch glimpses of Titan through its smoggy veil. And it looks like an exotic world with oceans of methane and a landscape that may bear the scars of a turbulent history.
All very tantalising, which is why this October a billion-pound international space mission, Cassini/Huygens, will be launched to explore Saturn and Titan up close. The spacecraft will reach its target in 2004. While Cassini orbits Saturn and takes images of the planet and its moon, the Huygens probe will parachute down through the moon鈥檚 clouds to give us our first pictures of the strange land beneath (see 鈥淒esign for the unknown鈥�).
Titan is the largest of Saturn鈥檚 18 known satellites, and the second largest satellite in the Solar System, after Jupiter鈥檚 Ganymede. However, since its discovery in 1655 by the Dutch physicist Christiaan Huygens, Titan has yielded
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few of its secrets. The first clues about Titan鈥檚 surface came in 1980 from measurements of its atmosphere made by the Voyager I spacecraft. Its dense atmosphere is unique 鈥� the pressure at the surface is 1.5 times that at the Earth鈥檚 surface. No other moon in the Solar System has an atmosphere even a thousandth as thick. Like the air on Earth, most of Titan鈥檚 air is made of nitrogen. However, like the atmospheres of all the outer planets, it also contains much more methane than the Earth 鈥� a few per cent. This large amount of methane puzzles astronomers. Methane is converted into heavier hydrocarbons by the action of ultraviolet light from the Sun. These compounds make up the opaque haze that obscures Titan鈥檚 surface. At the present conversion rate, all the methane in the moon鈥檚 atmosphere should have been
destroyed in just a few million years, much less than the 4.5 billion years that Titan has been around. So unless methane has just happened to appear at this point in Titan鈥檚 history, which seems unlikely, something must be continuously feeding methane into the atmosphere.
One possibility is that the methane comes from volcanic eruptions. But unless the supply rate is magically fine-tuned, the methane should have run out by now, or it should start to accumulate as a liquid ocean on the surface. 杏吧原创s
were intrigued by the idea of a methane ocean soon after the Voyager encounter, but subsequent data showed that the atmosphere just above the surface was not humid, as would be expected above a pure methane ocean.
Why was this? Jonathan Lunine of the University of Arizona was a student at the California Institute of Technology in Pasadena when he came up with a possible answer in the early 1980s. When sunlight breaks down methane the main product is ethane. Lunine noted that at the temperature found at Titan鈥檚 surface (94 K), ethane is also a liquid. And a mixture of methane and ethane would produce a low methane humidity. Such an ocean would slowly become richer in ethane while
keeping the atmosphere supplied with methane.
Yuk Yung, professor of planetary science at Caltech, showed that if the photolytic breakdown of methane had been going ever since the birth of Titan, it would have created enough liquid ethane to cover the globe in an ocean several hundred metres deep. Add in the methane and the ocean would be kilometres deep.
Titan would be a liquid-covered world. In 1989, Duane Muhleman and colleagues at Caltech tested this idea. He managed the impressive technological feat of bouncing radio waves off Titan. The idea was that a hydrocarbon ocean would reflect radio waves very poorly but an icy surface would be very reflective. Using the powerful NASA Goldstone dish in the Mojave Desert, California, as a transmitter, and the sensitive Very Large Array radio telescope as a receiver, Muhleman managed to pick up a noisy but distinctive signal. The data revealed that there was too much reflection for Titan to be completely covered by oceans. At about the same time, astronomers began to realise that Titan鈥檚 hazy atmosphere was not quite as impenetrable as they had thought. Voyager鈥檚 cameras operated in visible wavelengths, which are blocked by tiny haze particles. But by using longer wavelengths, in the near-infrared, astronomers realised that they might be able to see through the haze. The precise wavelengths for Titan had to be chosen carefully because methane absorbs some infrared wavelengths very strongly. But there are a few 鈥渨indow鈥� regions at which Titan鈥檚 atmosphere is relatively clear.
Mark Lemmon, a student at the University of Arizona, observed Titan through these windows in July 1992. When he looked again that September, he knew immediately that something was different. While Titan鈥檚 brightness had remained the same at most wavelengths, in the window regions the brightness varied with the moon鈥檚 orbital position. Titan always shows Saturn the same face just as the Moon does to the Earth. This means we can work out which part of the Titan鈥檚 surface we are looking at from its orbital position. If the variations in brightness on Titan were random, they might be due to clouds. But Lemmon found that the variations correlated with orbital position, clearly showing that parts of the moon鈥檚 surface were brighter than the rest. This result was soon confirmed by other researchers 鈥� Caitlin Griffith, then at NASA Ames Research Centre near San Francisco, and Athena Coustenis of the Meudon Observatory in Paris 鈥� using much more powerful telescopes in Hawaii, spurring observations with the Hubble Space Telescope. The early results were tantalising, but inconclusive. There seemed to be some bright spots, but the images had to be processed extensively because they were taken before Hubble鈥檚 first repair. What鈥檚 more, the Hubble images did not cover the whole of Titan鈥檚 orbit, so there was no way of knowing whether a bright spot was due to something on the surface or just a cloud.
In 1994, Hubble took a set of images with its new sharper camera, and this time the pictures covered a complete orbit of Titan. Peter Smith from the University of Arizona, Lemmon and I used these images to make maps of Titan鈥檚 surface in three different infrared windows. Most striking was a bright region, just south of the equator, about the size of Australia. It was at the same longitude as Lemmon鈥檚 bright measurements, and was clear in all three maps. We don鈥檛 know what this region is, but one explanation is that rain falling on high ground is washing the area clean of dark sludge.
Two years earlier, I had calculated that methane raindrops, if they exist on Titan, could be larger than raindrops on Earth. Raindrops on Earth are never larger than around 6 millimetres 鈥� above that size, aerodynamic forces overcome the surface tension that holds the drops together. On Titan, a methane raindrop could be 10 millimetres across but it would fall at only 1.6 metres per second, or six times slower than a typical drop on Earth. Falling so slowly through the thick bottom layers of Titan鈥檚 atmosphere gives the drops time to evaporate, so it is only on mountains that raindrops can reach the ground and wash away any surface sludge.
The liquids and sludge would presumably accumulate in the low places. But what would these be? On Earth, movement of plates creates large ocean basins where most of the planet鈥檚 water is stored. If Titan doesn鈥檛 have similar tectonics, most liquids and sludge may end up in crater basins. Craters on other icy satellites often have domed centres, where the ice has rebounded up in the centre of the crater cavity. If this also happened on Titan, most crater lakes may be ring-shaped, with islands in the centre. And the islands may have smaller craters in them, producing bull鈥檚-eye lakes that should be easy for Cassini to spot.
Sludge mix
Taken together, the data show that Titan鈥檚 surface is not pure ice like that of Jupiter鈥檚 satellite Europa. Nor is it like the dark organic material of the dark side of Iapetus, another of Saturn鈥檚 moons. Rock, ice and organic material are probably all there, but whether as rocky patches and hydrocarbon lakes on an icy background, or a more intimately mixed sludge, will remain a mystery until the Huygens probe arrives to take measurements from beneath the haze. So Titan may have a bizarre and exotic landscape, a far cry from a serene global ocean. But if there is no global ocean, where is the methane and ethane coming from? Perhaps it is lurks in a porous subsurface, between grains of ice or in caverns, with only some occurring as lakes and seas on the surface. But there is another explanation: perhaps methane has not always been abundant on
Titan.
Last year Lunine, Chris McKay from NASA鈥檚 Ames Research Centre and I decided to look at what might have happened if there were times in the moon鈥檚 history when the methane ran out. Using a climate model developed by McKay, we found that the consequences would have been cataclysmic (Science, 31 January, p 642). Methane is a greenhouse gas and without it Titan鈥檚 atmosphere would cool down. Eventually, it would be cold enough for the nitrogen in the atmosphere to
form clouds and rain down onto the surface, washing away dark material and making the surface appear brighter. But a more gleaming landscape would reflect more sunlight and make the surface even cooler. In extreme cases, for example, 3 billion years ago when the Sun was 20 per cent fainter, the nitrogen could start to freeze. If this happened, the atmosphere would be 10 to 100 times thinner than it is now, and Titan would probably look like Neptune鈥檚 frigid moon Triton, which is covered in bright nitrogen frost.
Gradually, as the Sun became brighter, melted methane may have erupted onto the surface. Or the impact from a comet may have exposed the methane. Either way, the surface would have warmed a little because of the injection of greenhouse gas and more of the methane would have evaporated into the atmosphere. Such a positive feedback system could explain why there is so much methane in Titan鈥檚 atmosphere today.
The big squeeze
The possibility of such cataclysmic ice ages is intriguing but how can we find out if they ever happened? One clue comes from the shape of surface volcanoes, which depend on atmospheric pressure. Atmospheric pressure controls the behaviour of gas bubbles in lava. In a high-pressure atmosphere, like the present one, the bubbles are tightly squeezed and the lava trickles out gently. But a low-pressure atmosphere allows the bubbles to expand, so that lava sprays out and forms a cinder cone. If we find cinder cones (or rather, snow cones 鈥� the 鈥渓ava鈥� on Titan is probably molten ice), this suggests that they formed at a time when the moon鈥檚 atmosphere had collapsed.
Another clue about Titan鈥檚 atmosphere could come from the crater record. Large comets and asteroids will punch through the atmosphere to make large craters, whereas smaller ones break up in the atmosphere, like the Tunguska explosion over Siberia in 1908. So the number of small craters relative to the number of large ones should tell us how thick the atmosphere has been in Titan鈥檚 past.
As Cassini orbits Saturn, it will be able to map the distribution of craters using cameras and radar, and identify volcanic features on Titan. With luck, as the Huygens probe makes its two-hour descent by parachute to Titan鈥檚 surface,
its cameras will take in details of the landscape. The probe should also measure the abundance of argon isotopes in the atmosphere, which will help to determine how much volcanic outgassing from the interior there has been in the past. We will have to wait almost seven years for answers about Titan鈥檚 turbulent history. But our best guesses so far suggest that the wait will be well worthwhile.
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Design for the unknown
THE 320-kilogram Huygens probe, built by European engineers, will hitch a 2-billion-mile ride on the NASA Cassini orbiter. Cassini is scheduled for launch on 6 October. After six and a half years, the 2-tonne Cassini will go into orbit round Saturn. On 6 November 2004, explosive bolts will release Huygens on springs. The probe will coast along in a sleep mode for 21 days, before plunging into Titan鈥檚 atmosphere at around 6 kilometres per second. When the probe slows to around 500 metres per second, at an altitude of about 170 kilometres, Huygens will deploy a parachute, its shield will drop away, and the scientific measurements will begin in earnest.
Data will be sent by radio link to the Cassini orbiter, some tens of thousands of kilometres away. The information from instruments on board Huygens will be collected by two computers and transmitted by two separate radio links.
The probe carries a 50-kilogram payload of six instruments, and these will generate about 20 megabits of data that should answer our most pressing questions about Titan. The probe鈥檚 camera will take high-resolution panoramic images of the surface, as well as spot clouds. The camera will also measure light levels in the hazy atmosphere, and the optical properties of the haze particles. And just before impact, a lamp attached to the outside of the probe will switch on to provide enough light for the on board instruments to obtain a spectrum of ices and liquids on the surface. This will reveal their constituent compounds.
A gas chromatograph/mass spectrometer will unravel the complex chemical and isotopic composition of the atmosphere. It will also analyse products from the Aerosol Collector/Pyrolyser, which will collect aerosol particles on a small filter, and then heat them to test their composition.
Huygens will hit the surface after 135 minutes of descent, at 5 metres per second 鈥� the speed a calculator knocked off a desk hits the floor on Earth. My calculations suggest the probe should continue operating for up to an hour if it lands on anything softer than solid ice. A force sensor, or penetrometer, attached to the bottom of the probe will record the impact, and will tell us whether the surface is like sand or gravel, or sticky because of organic material. If the probe lands in a hydrocarbon sea, it will float and accelerometers will measure the wave height. A small sonar will measure the depth of the sea, and other instruments will analyse the liquid鈥檚 composition.