Glimpse Of Titan

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Titan is the only member of Saturn's retinue to show a disk. Early efforts to directly measure its apparent diameter were subjective, and produced a wide range of values. E.E. Barnard and Percival Lowell independently measured it using micrometers, and calculated its diameter as 4,150 kilometres. As recently as the dawning of the Space Age, its diameter was given as 5,680 kilometres. In this range, it was clearly a close rival to Ganymede, the largest of Jupiter's Galileans.

Given an estimate of Titan's mass from studies of the orbital motions, its escape velocity would be only a little greater than the 2.4 kilometres per second of our own satellite. However, whereas the Moon is airless, the fact that Titan is so much colder would make any molecules of gas so sluggish that they might be unable to escape to space. In 1908, Catalan observer Jose Comas-Sola reported seeing 'limb darkening', implying the presence of an appreciable atmosphere. However, the fact that he made similar reports about each of the Galileans prompted considerable scepticism and, in all likelihood, his observations were flawed. Nevertheless, his reports stirred James Jeans in the UK to undertake a mathematical study of 'escape processes' on these satellites, concluding in 1925 in the case of Titan that, in spite of its relatively small size, as long as its temperature was in the range 60-100K it could have retained a primordial atmosphere acquired from the solar nebula. Measured on a fundamental scale, the weight of molecular hydrogen (H2) is 2 units, and that of molecular oxygen (O2) is 32. Jeans calculated that a gas exceeding 16 units would not have been able to exceed the escape velocity. Given reasonable assumptions for the composition of the solar nebula at Saturn's distance from the Sun, Titan would have acquired significant amounts of ammonia (NH3), molecular nitrogen (N2), methane (CH4) and the noble elements argon and neon. Ammonia would freeze at such temperatures, so Titan's internal composition probably includes ammonia ice. Although all the other candidates would remain in the gaseous phase, the fact that molecular nitrogen, argon and neon are inert meant that they were not readily detected spectroscopically. Only methane seemed likely to be detectable. After the University of Chicago teamed up with the University of Texas to operate the 82-inch reflector of the McDonald Observatory on Mount Locke, Texas, G.P. Kuiper turned the newly commissioned instrument to Titan in 1942 and observed distinctive limb-darkening on the yellowish-orange disk. In the winter of 1943-1944, infrared observations using an emulsion that was sensitive out to 0.7 micron revealed a spectrum similar to that of Saturn, with distinctive absorption bands for methane and a hint of bands for ammonia, so the moon did indeed possess an atmosphere of the primordial type inferred by Jeans.15

Despite the tiny size of Titan's disk, skilled observers using large telescopes at times of excellent seeing often reported albedo variations. On nights of exceptional seeing throughout the 1940s, B.F. Lyot and Pic du Midi colleagues made a series of drawings of the moon using the 24-inch refractor at very high magnification, and recorded a variety of splotches and light and dark bands. As Titan was believed to rotate synchronously, the fact that these markings were clearly not correlated with the 16-day orbital period meant that they could not be surface markings. Lyot

Between 1943 and 1950, B.F. Lyot and colleagues at the Pic du Midi Observatory viewed Titan using the 24-inch refractor at high magnification showing a variety of 'splotches' and 'bands'. Because the detail did not integrate into a consistent map, they concluded that it represented atmospheric structure (and in 1944 G.P. Kuiper established spectrographically that the moon does possess an atmosphere).

proposed that they were transient atmospheric features and, if so, this implied not only that the atmosphere was sufficiently 'optically thick' to hide albedo variations on the surface, but also thick enough to support an active weather system.

In 1946, Kuiper developed a sophisticated electronic detector that was sensitive farther into the infrared than contemporary emulsions for faint objects (2.5 microns as opposed to 0.7 micron for film) and mounted it in a spectrograph. After testing it at the Yerkes Observatory, he installed it on the McDonald Observatory's 82-inch reflector. His rich results highlighted the requirement for further laboratory testing to chart the chemicals producing absorption in this newly opened spectral window. In 1948, after considering all the evidence, he published his conclusion that Titan has an atmosphere of methane with a density comparable to that of Mars's atmosphere, which at that time was believed to have a surface pressure of 0.1 bar. In 1952 Kuiper established that Titan is unique in Saturn's retinue in having methane absorption in its spectrum. In the 1950s, most professional astronomers were obsessed with the large-scale structure of the Universe and paid scant attention to the Solar System, so Kuiper and Audouin Dollfus, Lyot's protege, had the planetary field more or less to themselves.

In 1972 L.M. Trafton of the University of Texas renewed infrared studies in the 1-to 2-micron range and found surprisingly strong absorption indicating either that the methane abundance was at least an order of magnitude greater than that observed by Kuiper, or that a large amount of an as-yet-unidentified gas was present.16,17 If the methane molecules were static they would all absorb light at precisely the same wavelength. However, they actually move around and collide with one another, so the Doppler effect from the distribution of speeds 'broadens' a line and the amount of absorption as a function of wavelength is dependent upon the ambient pressure. The profile of the spectral band therefore provides a pressure measure. If Kuiper had been right about the methane pressure, then the 'collisional broadening' of the 1.1-micron absorption feature implied that the methane molecules were being jostled by another gas which was present in considerably greater concentration. This was consistent with Lyot's observations of atmospheric structures. A polarisation study in 1973 confirmed the presence of cloud particles, which lent further support to the 'dense atmosphere' thesis.18,19 Also in 1973, J.J. Caldwell of Princeton began to develop a model in which methane was the primary constituent. He predicted that conditions at the surface were 86K and 20 millibars.20 Measurements of the surface temperature had proved to be complicated, and radio and infrared techniques in the mid-1960s had produced a variety of figures in the range 165-200K, which were not supportive of this model. F.J. Low's discovery in 1965 of an infrared excess21,22 prompted Caldwell's colleague, G.E. Danielson, to propose a temperature inversion in the upper atmosphere.23 However, D.M. Hunten of the University of Arizona countered that Trafton's data probably meant that considerably more gas was present than could be in the form of methane, and that methane is actually a minority constituent in a dense atmosphere. The majority constituent could clearly not be ammonia, as this would long since either have frozen on the surface or have been dissociated by solar ultraviolet, liberating the nitrogen and hydrogen.24 Molecular nitrogen is difficult to detect spectroscopically from Earth, but it could be the majority constituent. He predicted that the conditions on the surface could be an astonishing 20 bars at 200K.25,26

A compromise was achieved in 1980 when W.J. Jaffe and T.C. Owen, using the National Radio Astronomy Observatory's Very Large Array of radio telescopes in Socorro, New Mexico, measured the 'emission temperature' of Titan's surface at a wavelength of 6 centimetres and found it to be 87K.27 They inferred that nitrogen contributed no more than 2 bars to the atmospheric pressure, and further speculated that although much of the hydrogen that had been liberated by photodissociation of ammonia must have been lost, the hydrogen that remained in the lower atmosphere would create a 'greenhouse effect' in which sunlight absorbed at visible wavelengths was re-emitted in a cascade of longer-wavelength photons, which would act to warm the surface.28,29

On 2 September 1979, as Pioneer 11 withdrew from Saturn, its trajectory took it within 363,000 kilometres of Titan. A spin-scan image of the northern hemisphere showed a mottled orangey disk so optically thick that it hid the surface completely.30 It was an enticing glimpse, but the spacecraft's instruments were not designed for remote-sensing planetary satellites. The particles and fields instruments noted Titan's wake in the planet's magnetosphere, but there was no indication of an intrinsic magnetic field for the moon.31

The departing spacecraft encountered the magnetopause early on 3 September, but the solar wind was still gusting and the bow shock washed back and forth several times before it finally exited on 8 September, by which time the planet was so close to the Sun in the sky that the radio signal was very difficult to receive.

Although Pioneer 11 reconnoitred the route through the ring system that the JPL mission planners hoped to employ, the alignment of the outer planets was not yet conducive to the Grand Tour, so the exit trajectory sent it out of the Solar System in the opposite direction to Pioneer 10.

Ames's hardy Pioneers were therefore the first to pass through the asteroid belt, the first to investigate the Jovian magnetosphere, and the first to reach Saturn. Their

As Pioneer 11 withdrew from Saturn, it passed within 363,000 kilometres of Titan. This 'spin-scan' image of Titan's northern hemisphere displays a mottled orangey disk, but no sign of large-scale atmospheric structure or surface detail.

achievements, however, were soon overshadowed by the more sophisticated robotic explorers that followed.32

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