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A plot of the transparency of Titan's atmosphere as a function of wavelength in the near-infrared region of the spectrum. There are 'windows' at 0.83, 0.94, 1.075, 1.28 and 1.59 microns through which it is possible to see tropospheric clouds and measure the surface reflectivity. (Based on data in 'Windows through Titan's atmosphere?', W. Grundy, M. Lemmon, U. Fink, P. Smith and M. Tomasko. Bull. Amer. Astron. Soc., vol. 23. p. 1186, 1992.)

clouds to enable them to track their motions. ''Try as we might, looking at the data we couldn't see anything that was a significant brightening on the surface and moved. It looked like there were no clouds or, if clouds were there, they were right at the noise level and we couldn't tell the difference,'' Smith reflected. He had looked for clouds precisely because there was no guarantee of being able to see the surface. ''When I wrote my proposal, I said that we intended to map the surface features, but the reviewers said: 'That's impossible.' Of course, we saw no clouds, and we mapped the surface!'' The result was crude, but it was the first glimpse of the mysterious surface.71 The entire disk was barely 20 pixels across, which gave a resolution of 300 kilometres per pixel. Geometrical constraints meant that only the equatorial and mid-latitudes could be studied in this way. In fact, variation in reflectance can arise from compositional, textural or topographical differences. A large bright patch on the leading hemisphere explained an earlier study which had shown a variation in brightness that was correlated with the moon's rotation. The data was sufficient to create a preliminary map with a prominent infrared-bright feature comparable in size to Australia stretching one-sixth of the way around the equatorial zone.

Although this feature was dubbed a 'continent', it could not be confirmed to be a topographic structure - let alone that it rose from an ocean. In fact, there was no definitive proof of any liquid hydrocarbons on the surface, and terrestrial microwave radar studies were inconclusive on this question.72 Infrared observations using 'adaptive optics' on the Canada-France-Hawaii Telescope noted several even

By repeatedly imaging Titan during its 16-day axial rotation using the Hubble Space Telescope the near-infrared reflectivity of the moon's surface was able to be charted. The low-albedo areas might be seas of ethane. The prominent bright area on Titan's leading hemisphere may be a 'continent'. (Courtesy of P.H. Smith, M.T. Lemmon, R.D. Lorenz, J.J. Caldwell, L.A. Sromovsky and M.D. Allison in STScI-PR94-55, December 1994.)

By repeatedly imaging Titan during its 16-day axial rotation using the Hubble Space Telescope the near-infrared reflectivity of the moon's surface was able to be charted. The low-albedo areas might be seas of ethane. The prominent bright area on Titan's leading hemisphere may be a 'continent'. (Courtesy of P.H. Smith, M.T. Lemmon, R.D. Lorenz, J.J. Caldwell, L.A. Sromovsky and M.D. Allison in STScI-PR94-55, December 1994.)

brighter spots dotted along its length.73,74,75 ''I think this could be a plateau with peaks,'' speculated Athena Coustenis of the Meudon Observatory in Paris, venturing that the brightest features might represent a frost of methane ice on their summits. ''I'm a big fan of mountains on Titan,'' admitted Ralph Lorenz of the University of Arizona, but he dismissed the possibility of their being tall enough to have become coated with frost. Indeed, by analogy with Ganymede, it has been argued that the range of elevation on Titan should rise no more than 3,000 metres from the mean;76,77 considerably lower than Coustenis presumed. Lorenz argued that the peaks might appear bright because they stimulate rainfall that has washed them 'clean' of a mantling deposit of dark material.78 As Peter Smith summed up: ''I think what we see is a very large range of ice mountains. You've got a constant wind that blows the wet air from the methane ocean. This freezes out and clouds form on the top of these mountains. The methane rain erodes these hills and exposes fresh ice.'' But why is the mountainous feature so expansive? ''There has to be something different about this place,'' agreed Smith. ''People have suggested that Titan was hit by a gigantic asteroid which exposed fresh ice, but that would have had to have happened very recently, in light of the rate at which the haze falls out from the atmosphere. And presently an impact by a body large enough to leave a crater of that size is very unlikely.''

What of the weather system? After years of fruitless observations by terrestrial telescopes, in September 1995 the UK Infrared Telescope on Hawaii made a lucky

The first images of Titan at 1. 3 microns were secured using 'adaptive optics' on the Canada-France-Hawaii Telescope in 1997. They showed several particularly bright spots on the 'continent'. (Courtesy 'Adaptive optics images of Titan at 1.3 and 1.6 microns at the CFHT', A. Coustenis, E. Gendron, O. Lai, J.-P. Veran, M. Combes, J. Woillez, Th. Fusca and L. Mugnier. Icarus, vol. 154, p. 501, 2002.)

The first images of Titan at 1. 3 microns were secured using 'adaptive optics' on the Canada-France-Hawaii Telescope in 1997. They showed several particularly bright spots on the 'continent'. (Courtesy 'Adaptive optics images of Titan at 1.3 and 1.6 microns at the CFHT', A. Coustenis, E. Gendron, O. Lai, J.-P. Veran, M. Combes, J. Woillez, Th. Fusca and L. Mugnier. Icarus, vol. 154, p. 501, 2002.)

sighting of tropospheric methane clouds. The first clue was a rapid increase in the 2-micron brightness. Caitlin Griffith of the Northern Arizona University in Flagstaff, leading the team, explained this as a thick methane cloud at an altitude of about 15 kilometres which gradually rotated over the limb into view during the two days available for their observations, eventually covering 10 per cent of the disk.79 It was strongly suggestive of a hurricane-sized system. Unfortunately, an immediate follow-on study was impracticable, and when this site was inspected in 1997 this feature was absent. Later observations, using more sensitive instruments, detected small clouds, each covering no more than 1 per cent of the disk, with rapid temporal variations that offered the prospect of using their variability to infer the tropospheric winds.80 ''Standing on the surface of Titan, we would see a very dimly lit world, as bright as Earth under a 'full' Moon,'' Griffith predicted. ''The Sun would appear as a diffuse light source through Titan's high smog. At night, we wouldn't see stars through this veil. On the ground, the atmosphere would be clear and the visibility unobscured, temperatures would be uniform and winds quiescent. Every week, sparse clouds would appear below the orangey haze, high in the sky, barely visible. They would quickly produce rain and disappear. Perhaps once a year, clouds would blanket the sky for a day or two.''

As the resolution of mapping improved, the case for a global ocean diminished. Infrared 'speckle interferometry' by the 10-metre Keck telescope in Hawaii was able to produce a sharper image than that of the Hubble Space Telescope.81 This showed a complex surface comprising bright areas that might be ice-and-rock continents and a large number of extremely dark areas, each several hundreds of kilometres across. This material is ''one of the darkest things in the Solar system'', pointed out Bruce Macintosh of the Lawrence Livermore National Laboratory, a member of the team. A thick hydrocarbon would have an albedo of 2 per cent.82 The rain of photolytic

This map of Titan depicts the 'continent' in terms of surface reflectance at 2.1 microns. (Adapted from 'Titan: high-resolution speckle images from the Keck Telescope', S.G. Gibbard, B. Macintosh, D. Gavel, C.E. Max, I. de Pater, A.M. Ghez, E.F. Young and C.P. McKay. Icarus, vol. 139, p. 189, 1999. Courtesy of Seran Gibbard of the Institute of Geophysics and Planetary Physics at the Lawrence Livermore National Laboratory.)

This map of Titan depicts the 'continent' in terms of surface reflectance at 2.1 microns. (Adapted from 'Titan: high-resolution speckle images from the Keck Telescope', S.G. Gibbard, B. Macintosh, D. Gavel, C.E. Max, I. de Pater, A.M. Ghez, E.F. Young and C.P. McKay. Icarus, vol. 139, p. 189, 1999. Courtesy of Seran Gibbard of the Institute of Geophysics and Planetary Physics at the Lawrence Livermore National Laboratory.)

particulates formed in the upper atmosphere must be accumulating on the surface. It has been estimated that over the moon's history this 'fall out' would correspond to a global blanket 1 kilometre thick. The organics may form of a semi-solid sludge.83 Their areal extent would seem to indicate that they have pooled in impact craters.84 Later observations in the windows from 1 to 5 microns gave a crude spectrum of the surface and, surprisingly, implied that water ice is predominant on a global basis,85 in turn suggesting that, contrary to expectation, much of Titan's surface is free of hydrocarbons.

Musings about Titan's surface have therefore undergone a turnaround since the model of a global ocean was prompted by the Voyager atmospheric data, with the result that many researchers now expect there to be no more than shallow seas in craters. The action of the tides in isolated seas would not affect the ellipticity of the moon's orbit.86 In their early analysis, Sagan and Dermott had argued for either a deep ocean or no ocean, and it would now seem that there is no ocean. If so, where is the reservoir that replenishes the atmospheric methane? Perhaps the icy lithosphere is porous and the fluid is underground in an 'oil field',87 or perhaps it is soaked into the regolith88 and methane is slowly diffusing into the atmosphere.

Life on Titan?

The Infrared Space Observatory that was launched in late 1995 made spectroscopic observations of Titan. In addition to verifying and refining the abundances reported by Voyager 1, its wider wavelength coverage enabled it to penetrate more deeply into the atmosphere. Its vertical profiles of the abundances of some of the more complex hydrocarbons enabled the models developed from the Voyager data to be refined. It also established the presence of carbon dioxide. Observing in the far-infrared in late 1997, the ISO's Short Wavelength Spectrometer discovered two lines at 40 microns indicating water vapour in the upper stratosphere.89,90 ''Water vapour makes Titan much richer,'' pointed out Athena Coustenis, the leader of the team. ''We knew there was carbon monoxide and carbon dioxide, so we expected water vapour. Now that we've found evidence for it, we can better understand the organic chemistry taking place on Titan, and also the sources of oxygen in the Saturnian system.'' In fact, the surface is too cold for ice to release water vapour. One suggestion is that the water comes from the rain of interplanetary dust, which in turn contains grains released by comets. ''We are seeing a mix of elaborate organic molecules closely resembling the chemical soup out of which life emerged, so it will help us to understand the organic chemistry that took place also in the young Earth,'' Coustenis observed.

H.C. Urey, the Nobel prize-winning chemist, argued that the Earth formed with a gaseous envelope drawn directly from the solar nebula. If so, this must have been dominated by hydrogen-containing gases. Any molecular hydrogen would promptly have been lost to space, but prodigious amounts would have remained in the form of methane, ammonia and water vapour. In such an environment, the free oxygen would have been 'reduced' by being bound up chemically. The chemical reactions that gave rise to life could not have occurred in an oxygenated environment, as the

. 39 4 microns

In 1997 the Infrared Space Observatory's Short Wavelength Spectrometer (SWS) detected emission lines at 39.37 and 43.89 microns corresponding to pure rotational transitions of water vapour that is evidently present in Titan's upper stratosphere. (Adapted from 'Evidence for water vapor in Titan's atmosphere from ISO/SWS data', A. Coustenis, E. Lellouch, Th. Encrenaz and A. Salama. Astron. & Astrophys., vol. 336, L.85, 1998.)

. 39 4 microns

In 1997 the Infrared Space Observatory's Short Wavelength Spectrometer (SWS) detected emission lines at 39.37 and 43.89 microns corresponding to pure rotational transitions of water vapour that is evidently present in Titan's upper stratosphere. (Adapted from 'Evidence for water vapor in Titan's atmosphere from ISO/SWS data', A. Coustenis, E. Lellouch, Th. Encrenaz and A. Salama. Astron. & Astrophys., vol. 336, L.85, 1998.)

extremely reactive oxygen would have readily broken down the organic molecules. Voyager 1's IRIS measured the abundance of molecular hydrogen in Titan's troposphere as being 0.2 per cent, and it is therefore a reducing atmosphere in which there can be no free oxygen or oxygen-rich compounds - the oxygen Titan inherited from the solar nebula is bound up in the refractory oxides in the rock in its interior and in the water ice in its crust. Titan's atmosphere is therefore believed to be similar to the oxygen-free envelope of early Earth. On the fair assumption that intense volcanism on the early Earth would have generated powerful lightning discharges, S.L. Miller, one of Urey's graduate students, performed an experiment in 1952 in which he put a spark generator into a jar containing water, ammonia, methane and hydrogen. After only a week he found that a significant fraction of the methane had been drawn from the atmosphere by reactions which had produced surprisingly large amounts of various amino acids, the 'building blocks' of life.91 Wilhelm Groth and H. von Weyssenhoff in Germany re-ran the experiment in 1959 by illuminating the jar with ultraviolet light rather than simulated lightning, with the same result. The Earth's surface is currently shielded from solar ultraviolet by the ozone layer in the stratosphere, but the ozone could not have formed until the atmosphere became oxygenated, and this is believed to have occurred subsequent to - and as a side-effect of - the development of life, so in the earliest of times the surface would have been irradiated by solar ultraviolet. Although Titan's low temperature means that water and ammonia must be frozen, and form part of its surface, the atmosphere is similar to that of these experiments. Could life develop on Titan?

The IRIS's detection of hydrogen cyanide was significant because it was the first molecule to be confirmed to be present that is not a straightforward C-H bond. It is formed from radicals left by the dissociation by ultraviolet of nitrogen and methane molecules in the upper atmosphere. It is a precursor to biotic chemistry.92 It plays a critical role in the chemical synthesis of amino acids and the bases present in nucleic acids.93 Apart from the Earth, Titan is unique in the Solar System in its abundance of the building blocks of complex organic chemistry.94 If there are pools of 'primordial soup' on its surface, it could be thought of as a cryogenic version of the prebiotic Earth.95 Although Carol Stoker of the Ames Research Center established that bacteria can grow on tholin material in the laboratory, conditions on Titan are -probably - too chilly for bacteria to survive. Nevertheless, as Carl Sagan and Reid Thompson at Cornell suggested in 1992, meteor impacts could melt the crust and create temporary pools of liquid water.96 Although the atmosphere lacks oxygen, it was calculated that for several million years organics in the crust around an impact site might be able to react with the oxygen in liquid water; sufficient time perhaps for simple amino acids to develop. If it could be established that self-replicating nucleic acid molecules exist on the moon's surface, it would have profound implications for the mystery of the origin of life. Finding the basis of life elsewhere, even if it has not yet developed a resilient cellular structure, would be a discovery of the first magnitude because, as Philip Morrison of the Massachusetts Institute of Technology once observed, this would ''transform the origin of life from the status of a miracle to that of a statistic''.

It has been speculated that indigenous biology may develop on Titan in 6 billion years, when the Sun evolves into a 'red giant'.97 Initially, the increased insolation will be absorbed by the haze-laden stratosphere, which will 'inflate', so the temperature at the surface will rise only to about 200K, but the enhanced greenhouse may raise this sufficiently to melt the icy surface. Arthur C. Clarke has pointed out that Titan will act as a convenient 'lifeboat' for our descendants as, fleeing the destruction of the inner Solar System, they migrate outwards with the 'habitable zone'. After a few hundred million years, however, Titan's atmosphere will leak away, so it will end up as an inhospitable rock.

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