First Close Flyby Of Titan

At an altitude of 1,174 kilometres above the surface of Titan, the Ta fly-by on 26 October 2004 was 300 times closer than the T0 encounter in July. In fact, it was so close that Cassini would skim the topmost layers of haze. The highest priority was to perform the first direct sampling of the atmosphere to measure the thermal structure and the densities of molecular nitrogen and methane - the two principal atmospheric constituents. This data was required to plan the Huygens mission. The Composite Infrared Spectrometer was to measure the stratospheric temperature versus pressure (and hence density) to contribute to the validation of the altitudes at which Huygens would be programmed to deploy its parachutes. The extent to which Cassini had to work to maintain its attitude would provide 'drag' data, to review the planned 950-kilometre minimum fly-by altitude. Electron and ion observations were to be made by the Plasma Spectrometer, both while skimming the atmosphere and while passing through the 'wake' that the slowly orbiting moon left in the planet's rapidly rotating magnetosphere. The Magnetospheric Imaging Instrument was to characterise the ion composition and charge state of the outer atmosphere. The Radio and Plasma Wave Spectrometer was to monitor Titan's ionosphere, to establish how it interacted with Saturn's magnetosphere. The Dual-Technique Magnetometer was to study how the moon interacted with the magnetosphere to determine whether the body had its own

A full-disk mosaic of Titan centred on 15°S, 156°W, taken as Cassini approached on 26 October 2004. The surface detail was clearest where the line of sight was directly down through the atmosphere, because the contrast was progressively reduced as the viewing angle passed through haze towards the limb. The bright area to the right of centre was the 'continent'. The intended Huygens landing site was left of centre. There were bright clouds near the south pole.

A full-disk mosaic of Titan centred on 15°S, 156°W, taken as Cassini approached on 26 October 2004. The surface detail was clearest where the line of sight was directly down through the atmosphere, because the contrast was progressively reduced as the viewing angle passed through haze towards the limb. The bright area to the right of centre was the 'continent'. The intended Huygens landing site was left of centre. There were bright clouds near the south pole.

magnetic field. During the approach, Cassini was to image Titan every 15 minutes to make a movie of clouds. Next would be a 4-colour full-disk mosaic at 2 kilometres per pixel. Closer imagery at resolutions between 200 to 600 metres per pixel would provide a regional view of the western part of the 'continent' (recently nicknamed Xanadu) and the dark area to its west, where the Huygens probe would be targeted. The Ultraviolet Imaging Spectrograph was to determine the composition and distribution of haze aerosols. The Visual and Infrared Mapping Spectrometer was to study aerosols, cloud structures, the recently discovered methane fluorescence, and seek insight into the composition of the surface. The radar would take microwave radiometry to seek organics, and scatterometry to measure surface roughness in the hope of proving the presence of liquid on the surface.

The speed of the fly-by was 6 kilometres per second. In the 15 minutes prior to and following closest approach, the Ion and Neutral Mass Spectrometer analysed the outermost haze. Terrestrial observations had identified only 19 types of molecule but laboratory experiments implied that there should be more. The bulk composition and thermal structure of the atmosphere was much as it had been in 1980-1981, but the instrument detected many more 'heavy' organics than expected - including propane, benzene and diacetylene - indicating either that the in situ rate of creation of these species was faster than expected or that mixing was more vigorous than the models predicted.185 The Magnetometer established that if Titan had an intrinsic magnetic field, this must be extremely weak.186 The Plasma Spectrometer determined that the magnetospheric electrons that irradiate the upper atmosphere supplement the weak sunlight in promoting the chemical processes. The Ultraviolet Imaging Spectrograph revealed that the upper atmosphere glows due to magnetospheric electrons exciting nitrogen atoms, molecules and ions. (Note that this is a different glow to that in the infrared by methane lower in the atmosphere.) A fault resulted in the loss of most of the Composite Infrared Spectrometer's data, but the information that was received identified gaseous hydrocarbons in the 'hood' over the northern (winter) pole that were absent in the southern hemisphere - undoubtedly the result of a seasonal effect. The Ion and Neutral Mass Spectrometer measured the 14N/15N and 12C/13C isotopic ratios. The nitrogen ratio suggested that as much as 75 per cent of the nitrogen that was originally present in the atmosphere had been lost to space. Against expectation, however, the Plasma Spectrometer saw no sign of nitrogen escaping to form a torus matching Titan's orbit. The nitrogen ions that it did detect were located in the inner magnetosphere, and were associated with the 'E' ring.187 As water-ice is an efficient carrier of primordial noble gases, their absence suggested that the nitrogen in Titan's atmosphere derived from the volatisation of ammonia-ice by the heat of accretion as the moon formed, with the ammonia later being dissociated by solar ultraviolet. This indicated that the largest Jovian moon, Ganymede, which was comparable in size to Titan, accreted in a part of the solar nebula that was too warm for ammonia-ice, and hence is airless, whereas the nebula from which Titan formed, being further from the Sun, was cool enough to contain ammonia-ice and methane-ice and yet too warm for a nitrogen clathrate containing trapped primordial noble gases.

A press conference was held at JPL on 27 October to give the 'first impressions' of the imagery. One surprise was that the view of the surface was better in the near-infrared, in part due to the low contrast at visible wavelengths. There was a complex interplay between the bright and dark albedos. Intriguingly, there were 'streaks' that were suggestive of the movement of (dark) material across the surface, and from the fact that these consistently pointed in the direction of the prevailing zonal circulation it was inferred that this was windblown material. It had initially been presumed that the organic fall-out of particulates from the haze would be 'sticky', but recently V.I. Dimitrov and A. Bar-Nun had suggested that on their long, slow descent through the atmosphere the particulates would 'harden' and, being non-sticky, would pile up on

^ The snail' ^ Bright knobs | "j Dark patches rn Dark nwtttiKl

On 26 October 2004 Cassini took this mosaic of Titan's surface at a wavelength of 2 microns showing a possible cryovolcanic site dubbed 'the snail'. Also shown are views of this feature at various wavelengths across the near-infrared (bottom) and an interpretation.

^ The snail' ^ Bright knobs | "j Dark patches rn Dark nwtttiKl

On 26 October 2004 Cassini took this mosaic of Titan's surface at a wavelength of 2 microns showing a possible cryovolcanic site dubbed 'the snail'. Also shown are views of this feature at various wavelengths across the near-infrared (bottom) and an interpretation.

the surface like very fine dust.188,189 It had also been thought that since the energy of insolation was too weak to drive winds in Titan's lower atmosphere the surface must be stagnant, but then it was realised that gravitational tides induced by Saturn would generate 'atmospheric tides'.190 In fact, Saturn's tidal influence on Titan is 400 times greater than that of our Moon on Earth. ''Tides apparently dominate the near-surface winds because they are so strong throughout the atmosphere, top to bottom. Solar-driven winds are strong only high up,'' pointed out R.D. Lorenz.191 In Titan's dense atmosphere and weak gravity, even a surface wind averaging a speed of 1 kilometre per hour could readily 'bounce' grains along the ground in a process called saltation.

While circular albedo features were apparent, none was unambiguously an impact crater. In view of a statistical case for the occurrence of impacts, this was surprising. Nevertheless, imagery by the Visual and Near-Infrared Mapping Spectrometer of the dark area west of Xanadu taken just prior to closest approach with a resolution of 2 kilometres per pixel showed an infrared-bright circular feature 30 kilometres across at 8°N and 142°W. At its centre was a dark feature interpreted as a caldera on the summit of a dome that rose several hundred metres above its surroundings, with two elongated wings extending to the west that seemed to be overlapping sheets from separate flows. The dome could be a diapir that broached the surface. In view of its appearance, the feature was dubbed 'the snail'. ''We all thought that volcanoes must exist on Titan,'' said B.J. Buratti, a member of the team at JPL, ''and this is the most convincing evidence to date.'' The significance of this, as Christophe Sotin of the University of Nantes in France pointed out, was that cryovolcanism could replenish the methane content of the atmosphere.192

The initial results from the radar were announced on 28 October. The imaging

The first part of Cassini's synthetic-aperture radar track taken on 26 October 2004 to be made public was of an area of 478 by 250 kilometres centred at 54°W, 50°N. The fact that this area had not yet been observed optically made the interpretation of the radar albedo speculative.

track ran east to west at mid-northern latitudes, forming a thin 'noodle' some 4,500 kilometres long that covered an area equivalent to a mere 1 per cent of the moon's surface. Since a synthetic-aperture radar provides its own illumination, the geometry of a surface influences the degree to which there is a reflection; it is not like taking a picture in sunlight. Viewing angle is a factor because a surface that is tilted towards the spacecraft will appear brighter than one that is tilted away. Also, a surface that is rough at a scale comparable to the radar wavelength will scatter the beam and thus appear brighter owing to the number of favourably oriented reflectors. In addition, different materials have different reflectance: rock is more reflective than ice, which is more reflective than organics. It therefore requires a 'practiced eye' to interpret the albedo variation of a 'radar image'.

The first view published covered 478 by 250 kilometres, which was only a small fraction of the overall track. The interconnected dark spots on the generally bright albedo were consistent with a very smooth or highly absorbant solid, or conceivably a fluid. Another view showed a complex geological surface with a variety of terrain types, including sinuous bright features that cut across the darker albedos. However, because these areas had not yet been viewed by the optical instruments there was no infrared albedo information to factor into an interpretation of the radar imagery. The surprise was the absence of impact craters. A study by R.D. Lorenz had predicted that there ought to be 200 craters exceeding 20 kilometres in diameter per million square kilometres of surface.193 There would be very few craters less than about 10 kilometres in diameter, as the objects that would excavate such small features would burn up in the dense atmosphere without reaching the ground. Statistically speaking, it was reasonable to expect 40 craters with diameters larger than 20 kilometres to be on the imaging track, and at a spatial resolution of 300 metres per pixel these should have been visible. As impacts must have occurred, the absence of craters indicated that the moon had an 'active' surface. And, as J.I. Lunine told reporters, ''there are perhaps hints of features that might be craters that've been eroded or buried; perhaps by organics''. On the other hand, as Lorenz freely admitted, ''We see impact craters all

o

Was this article helpful?

0 0
Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment