Voyager 1 's view of the fully-illuminated trailing hemisphere of Tethys from a range of 2 million kilometres (left) showed a strange circular albedo feature. A view of the Saturn-facing hemisphere from 1.2 million kilometres revealed a tremendous canyon system, and later views of the terminator from half that range revealed the cratered terrain immediately to the west of the canyon (whose southern section is visible on the terminator). North is towards the top in all cases.
kilometres wide near the centre of the disk. Although this looked just as the rim of a crater ought to when obliquely lit, the illumination was actually face-on. This had to be an unusual albedo feature. Unfortunately, given the rate at which the moon travelled around its orbit, rotating synchronously as it did so, this feature was in darkness when the geometry was best for imaging, at which time the terminator from pole to pole was shown to be heavily cratered terrain.
Voyager 1 closed within 2 million kilometres of Saturn as 11 November dawned, and its remote-sensing instruments were regularly inspecting Titan. The moon's disk now exceeded the narrow-angle camera's field of view, so 2-by-2 (and soon 3-by-3) mosaics were being assembled. ''We cannot see the surface of Titan,'' reported Hal Masursky as the resolution passed 35 kilometres per pixel without showing any sign of a gap in the haze. ''If we are going to learn what this extraordinary and fascinating body is like, we will need a Titan-orbiting imaging radar.'' At 15:00, as the spacecraft closed to within 500,000 kilometres of Titan, it switched from its 'far encounter' to its 'near encounter' sequence, which would start with the Titan fly-by. The bow shock was finally encountered at 16:50.50 It washed to and fro several times until 20:13, when the spacecraft crossed the magnetopause at 22.9 radii, at which time Titan was barely three hours distant. A high-resolution mosaic had shown the presence of a thin layer of haze beyond the limb, about 100 kilometres above the optically-thick cover. This extended all the way around the disk, and near the north pole it merged with an opaque dark hood. The absence of such a feature in the south was another aspect of the dichotomy between the two hemispheres. As the range diminished, the IRIS was able to secure infrared spectra of increasingly higher resolution. At 21:41, Voyager 1 was at its closest point to the moon, passing 4,000 kilometres above the main haze at a relative speed of 17 kilometres per second. At 22:25, the scan platform slewed onto the limb to enable the IRIS to measure the composition of the 'detached' layer of haze.51
With Titan inside the magnetosphere, Voyager 1's magnetometer had a better chance of determining whether the moon has its own magnetic field. At 22:45, the spacecraft prepared for being occulted by the moon by boosting the strength of its radio signal and ceasing to modulate this with data. At 23:00 the particles and fields instruments noted the spacecraft's flight through Titan's magnetospheric wake, but there was no indication of an intrinsic magnetic field; if the moon possesses one, its strength is less than one-thousandth of the Earth's field. At 23:11 Voyager 1 flew into the moon's shadow. A minute later it passed beyond the moon's limb as seen from the Earth, and the manner in which its radio signal was refracted profiled the physical properties of the atmosphere down to the surface. At 23:22 it crossed through the plane of Titan's orbit, travelling from north to south at an angle of 8.7 degrees. During its approach, the spacecraft had observed the illuminated face of the ring system, but in passing through the plane of the moon's orbit it also crossed the projection of the ring plane. At 23:24 it re-emerged into sunlight and at 23:27 it re-emerged from the far limb. The fact that the occultation ended 43 seconds earlier than expected meant that the spacecraft's trajectory was 'off by 200 kilometres. This prompted a reprogramming of the scan platform to aim it directly at the smaller targets that were scheduled for inspection closer to the
planet. Looking back, the Ultraviolet Spectrometer noted emissions from several detached layers of haze, the highest of which was some 400 kilometres above the top of the optically-thick haze. It was now evident why telescopic observers had had so much difficulty in attempting to measure the diameter of the moon's disk - the limb-darkening and the high altitude haze had made the limb indistinct.
One immediate result from the radio occultation was an accurate measurement of Titan's diameter: the solid body inside the obscuring haze is 5,150 kilometres in width. Given an estimate of its mass from how it deflected Voyager 1's trajectory, it was finally possible to calculate an accurate bulk density of 1.88 g/cm3, which implied 45 per cent ices. To a first approximation, in terms of diameter and density, this placed Titan between Callisto (4,806 kilometres and 1.86 g/cm3) and Ganymede (5,286 kilometres and 1.94 g/cm3). However, each of these three bodies is highly distinctive. As the Galileo spacecraft established during its extended tour of the Jovian system in the late 1990s, Ganymede strongly resembles the Earth in having a differentiated interior and a magnetic field, whereas Callisto is essentially undifferentiated. Having condensed out of the chillier nebula farther from the Sun, Titan ought to have acquired more methane (carbon) and ammonia (nitrogen) ices,52 and the dissipation of accretional heat would have driven these exotic ices to the surface, whereupon they would have evaporated to create an atmosphere.53 Dissociation of ammonia would have released nitrogen, some of which would have combined with methane to create a variety of organic molecules. However, telescopic spectroscopy had positively identified only methane. The radio occultation data measured how the temperature, pressure and composition of the atmosphere varied with altitude.54 The surface pressure clearly exceeded the 20 millibars predicted for a methane-dominated atmosphere. Given the measured diameter, the atmosphere is evidently dense and deep. The 1.5-bar surface pressure is all the more striking because a much deeper column of gas is required to create such a pressure on a body whose surface gravity is just 14 per cent of that of the Earth. In fact, Titan's atmosphere contains 10 times as much gas as the Earth's considerably shallower envelope. Actually, methane is present in more or less the expected amount - the atmosphere has been 'pumped up' by molecular nitrogen. The Ultraviolet Spectrometer had made the first detection of nitrogen by strong emission from the upper atmosphere where the molecules are excited by the flux of magnetospheric electrons. This shed light on the nature of the photochemical reactions that occur in the upper atmosphere. The IRIS confirmed molecular nitrogen (N2) as the main constituent, and then methane (CH4) with some acetylene (C2H2), ethylene (C2H4), ethane (C2H6) and propane (C3H8), cyanogen (C2N2) and cyanoacetylene (HC3N), hydrogen cyanide (HCN), molecular hydrogen (H2) and a whiff of helium.55,56 The complex hydrocarbons derive from photodissociation of methane, and the nitriles are the result of the C2H radical's action on unsaturated hydrocarbons and the dissociation of molecular nitrogen in the upper atmosphere by magnetospheric electrons.57
Temperature inversions are common in planetary atmospheres. The temperature of the Earth's atmosphere initially declines with increasing altitude, then increases again. In the troposphere, the tendency of warm air to rise causes convection, but this ceases at the tropopause, above which is the stratosphere. It was the discovery of this boundary that led to the realisation that the atmosphere is stratified. In fact, the temperature inversion that occurs in the stratosphere is caused by the absorption of solar ultraviolet by the ozone (O3) form of oxygen. Above the stratosphere is the mesosphere, and then the thermosphere, in which the temperature of the rarefied gas exceeds that at the surface. The thermal profile derived from Voyager 1's occultation confirmed that there is an inversion in Titan's upper atmosphere. The minimum temperature of 72K occurs at an altitude of 45 kilometres, but this is too warm for molecular nitrogen to condense. Above this, the temperature rapidly soars to 150K at 100 kilometres, and then increases somewhat more slowly to 175K by the top of the optically-thick haze at 200 kilometres. This heating results from the particles of the orangey smog absorbing shorter wavelength insolation. Titan's atmosphere is distinctly stratified, with the tropopause at 45 kilometres, the stratopause at 280 kilometres, just above the main haze, and the mesopause at 600 kilometres, in among the detached layers of thin haze. The visible surface is more properly called haze than cloud because it is composed of small particles that are well spaced. It is optically thick because it is so deep, but it is likely to be transparent over shorter distances. In the mesosphere, the dissociation of methane by solar ultraviolet has created a photochemical smog which is most evident in a concentration of aerosol in the 340-to 360-kilometre altitude range that was viewed on the limb as a distinct layer. The irradiation of the atmosphere by magnetospheric electrons when inside Saturn's magnetosphere, and energetic protons when exposed to the solar wind, will enhance the range of dissociation products and increase the number of molecular species. Methane readily polymerises, combining with itself to form chains. These complex molecules will form solid aerosols ranging in size from 0.2 to 0.8 micron. Polarisation measurements indicated that these are irregular conglomerates of smaller particles fused together by collisions, and are responsible for the atmosphere's orangey hue. Although the precise composition of the aerosols was not determined, Carl Sagan of Cornell University introduced the term 'tholins' to describe the range of solid, oily and tarry substances.58,59 The aerosols will initially remain in suspension, but will sink as they grow. As they penetrate the chilly troposphere they will be coated by ambient moisture in the form of liquid methane, accumulate into droplets and fall as rain, suggesting a surface coated with sludges and ices of hydrocarbons. The limb observations at closest approach enabled the IRIS to determine the composition with altitude. Nitrogen forms 90 per cent of the lower atmosphere by number, and 98 per cent of the upper atmosphere. Although methane is being dissociated in the upper atmosphere, it is evidently being renewed by diffusion from the troposphere, where it is concentrated. However, the diffusion rate is controlled by the 'cold trap' at the tropopause, where most of the methane condenses and rains out.
Titan cannot hold the hydrogen released by the photochemical reactions.60 As the rapidly rotating magnetic field sweeps past the moon, the hydrogen leaks away into the 'cavity' of the magnetospheric wake in a manner reminiscent of a plume from a smokestack streaming 'downwind'. Because it cannot escape Saturn's gravitational field, the hydrogen has formed the broad torus of neutral hydrogen that pervades the vicinity of the moon's orbit.61,62 The magnetospheric wind is compressed against the moon's trailing hemisphere. This turbulent shock wave induces an ultraviolet glow in the thermosphere of the moon's atmosphere, extending 1,000 kilometres into space. Because the hydrogen continuously leaks away, the polymerisation reactions are irreversible on a macroscopic scale, and the falling hydrocarbons accumulate on the surface. In the 20-micron band, a wavelength at which the atmosphere is more or less transparent, the IRIS was able to measure the surface temperature as 92K - a value that applied globally to within 3 degrees. As this was near the temperature at which methane can exist in both liquid and solid phases, it was speculated that in addition to sludges and ices of hydrocarbons there might be an ocean of liquid ethane diluted with 25 per cent liquid methane and 5 per cent liquid nitrogen.63 As methane melts at 91K and boils at 118K, it was further speculated that it might serve a similar role in Titan's weather system as water does on the Earth, following a cycle of evaporating from the sea, rising in the atmosphere on air currents, condensing, falling as rain and flowing over the surface back into the sea.
''The Titan data are very exciting,'' enthused R.A. Hanel, leader of the IRIS team, ''and much more important than we thought they would be.''
Having successfully achieved one of its main objectives in the Saturnian system, Voyager 1 now directed its instruments to the other members of the retinue.
Mimas was expected to be very heavily cratered due to its proximity to Saturn and as a result of the planet's gravitational field drawing in and accelerating material that strays too close. An early view presented the astounding sight of a crater whose 130-kilometre-wide raised rim spanned fully one-third of the moon's spheroidal form. Upon seeing it, one wag whimsically exclaimed that they had found the 'Death Star' of George Lucas's Star Wars movie. It also prompted gasps when later shown to the reporters thronging the von Karman Auditorium. It was remarkable that the icy moon had survived such a proportionately massive impact: the projectile must have been at least 10 kilometres across. Although initially dubbed Arthur, after King Arthur of legend, this crater was eventually named Herschel by the International Astronomical Union. As Voyager 1 penetrated more deeply into the system, it was able to document the crater from 400,000 kilometres at 4 kilometres per pixel, but it soon rotated over the terminator and the later sequence was of the intensely cratered southern terrain. Grooves 10 kilometres in width, 100 kilometres in length, and up to several kilometres in depth were more probably opened by the shock of large impacts than by endogenic processes. Although its bulk density implies that Mimas is mostly ice, the craterforms have not been softened because the low surface gravity is evidently insufficient to overcome the integrity of the ice which, so far from the Sun, possesses the strength of rock. Still bearing the scars of its early bombardment, it lives up to the stereotype of a moon in the style of Callisto. As L.A. Soderblom put it, ''Mimas is your basic unprocessed ice moon.''
Imagery of Tethys from a range of 1.2 million kilometres, with a resolution of 11 kilometres per pixel, showed a curvilinear feature 100 kilometres wide on the Saturn-facing hemisphere that was suggestive of an active geological past. Unfortunately, by the time that the range had been halved, most of this feature had slipped beyond the terminator. However, the improved resolution showed the leading hemisphere to be
Voyager 1's view of Mimas. The top set were taken at ranges between 800,000 and 400,000 kilometres, and show the giant crater in the centre of the leading hemisphere. North is towards the top. The bottom set were taken at ranges between 300,000 and 127,000 kilometres as the spacecraft passed the moon, showing the heavily cratered south polar region. North is to the left in this sequence, and the south pole is on the terminator on the rightmost image.
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