Phoebe Flyby

Phoebe is some 220 kilometres in diameter, and slightly irregular. It has an elliptical retrograde orbit of 18 months that is more or less in the plane of the ecliptic. A study during December 2003 and January 2004 in support of Cassini precisely measured the repeating 0.12-magnitude variation to determine the rotational period as 9.2735 hours ( + 0.0015), which was an improvement over the previous measures by a factor of 10.156 Although Cassini would have only one opportunity to inspect Phoebe, the moon's rotation would offer most of its surface to view during the fly-by. An albedo of 0.06 indicated that there was dark material on the surface. On the basis of the Voyager data, it had been classified as a captured C-type (carbonaceous) asteroid, but this analysis predated the surprising finding that the nucleus of Halley's comet was black and the discovery of the Kuiper Belt (or Trans-Neptunian) Objects which accreted far from the Sun and, as such, were 'pristine' material of the solar nebula, thus raising the possibility that Phoebe was either a captured comet or Kuiper Belt Object. Cassini's remote-sensing instruments were to study the surface composition. ''This moon has a little ice and a lot of black material on its surface, but other than that we know very little,'' noted D.P. Cruikshank of the Ames Research Center and a member of the Visual and Infrared Mapping Spectrometer team. ''This is a unique opportunity. We've never had a close-up look at an irregular low-reflectivity moon of any planet before, so we are prepared to be surprised.''

The Phoebe fly-by marked the introduction of a 'live updating' facility, whereby a set of scientific observations were specified in terms of best-knowledge instrument pointing and timing, which was updated once the early imaging had been processed. The optical navigators worked around the clock in the days following the trajectory correction on 27 May to refine the fly-by geometry, and on 6 June issued this to the science teams, which gave the updates required to make their planned observations. An image taken on 7 June from a distance of 2.5 million kilometres with a resolution of 15 kilometres per pixel exceeded the best Voyager view of Phoebe. An image on 10 June from 658,000 kilometres with a resolution of 4 kilometres per pixel showed a dramatic sight looming.

Although an image taken from a range of 32,500 kilometres with the resolution of 190 metres per pixel showed Phoebe to have a heavily cratered surface with a great deal of albedo variation, it was just the 'taster', because the moon did not yet fill the narrow-angle camera's field of view. The morphologies of the overlapping craters of various sizes suggested an ancient surface. ''It is very interesting that a lot of craters smaller than 1 kilometre in size are visible,'' observed, Gerhard Neukum, a member of the imaging team based at the Frei University in Berlin, Germany. ''It means that, in addition to the large ones, many projectiles smaller than 100 metres have hit.'' It was open to question, however, whether these projectiles originated inside or outside the Saturnian system - that is, whether they were fragments of shattered Saturnian satellites or were of asteroidal or cometary origin. A part of the answer would relate to when Phoebe was captured by Saturn, and where it was previously. Although the Voyager imagery had shown bright areas on the generally dark surface, these proved to be as much as three times brighter than expected; in fact, sufficient to saturate the image at the exposure based on the mean albedo. The presence of a small crater with bright 'rays' implied that the dark material was a thin 'lag' layer that accumulated as the brighter water-ice was eroded by micrometeoroids, in which case the rays would darken as the lag regenerated.157 There are small moonlets in distant irregular orbits around Saturn, some of which, as in the case of Phoebe, are retrograde and near the ecliptic. While these may have been captured independently, it was possible that they were related to Phoebe, and the presence of 50-kilometre-diameter craters on Phoebe prompted speculation that these moonlets were impact ejecta. The International Astronomical Union had named a bright feature in the Voyager imagery Leto Regio. When it was realised that this corresponded to the bright streaks in the interior wall of the largest crater, the crater was initially referred to as Leto, then officially named Jason.158

The fly-by on 11 June 2004 was at an altitude of 2,071 kilometres. The imagery at

Views of Phoebe as Cassini made its approach. The non-synchronous way in which the moon rotates enabled the spacecraft to inspect most of its surface (bottom).

closest approach had a resolution of 12.3 metres per pixel, but was slightly smeared owing to the relative speed of 6.4 kilometres per second. The Visual and Infrared Mapping Spectrometer observed across its full spectral range from 0.5 to 5 microns to map surface composition at a resolution of 500 metres per pixel. The Ultraviolet Imaging Spectrograph measured surface reflectance in order to map the distribution of volatiles. The relative warmth of Phoebe's surface (in comparison to 'clean' icy satellites) enabled the Composite Infrared Spectrometer to attain a high signal-to-noise ratio in making compositional and thermal observations of both the dayside and nightside. The radar was expected to be able to penetrate to a depth of 20 centimetres, to characterise the physical properties of this material. The Visual and Infrared Mapping Spectrometer detected iron, bound water, trapped carbon dioxide, phyllosilicates, organics, nitriles and cyanide compounds consistent with a surface of cometary origin incorporating primitive materials of the outer Solar System.159 In fact, Phoebe was one of the most compositionally diverse objects yet studied. The temperature map by the Composite Infrared Spectrometer showed the surface to be 110K, with day/night characteristics establishing the surficial material to be fluffy and porous.160 The radar results were consistent with a dirty, rocky, icy surface. The presence of carbon dioxide was a clear indication that Phoebe was not a C-type asteroid. It must, in fact, have formed outside the orbit of Jupiter.161 Bodies having a 'primitive' mixture of rock, ice and carbonaceous material would have been plentiful in the outer reaches of the solar nebula. However, Phoebe's actual point of origin is not clear, in part because so little is known of the Kuiper Belt Objects. Monitoring of the spacecraft's trajectory during the fly-by indicated the moon's density to be 1.63 g/cm3, somewhat higher than expected, suggesting it has a significant amount of rock in its interior, but not as much as the best-studied Kuiper Belt Objects - for example, Pluto, its satellite Charon, and Triton (which may have been captured by Neptune) all have densities in the 1.9-2.1 g/cm3 range.162 As D.L. Matson of JPL said, one thing was certain: Phoebe was ''a geologist's delight'', and as an interloper in the Saturnian system it was a welcome bonus for the Cassini mission.

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