Iapetus Revealed

A meeting at JPL on 20 December 2004 reviewed the science plan for the fly-by of Iapetus. The sequence was uplinked on 30 December for execution the next day, and as Cassini passed the moon at a range of 123,400 kilometres it took imagery at a resolution of 1 kilometre per pixel. The dark linear feature detected in October was not only seen to span the width of Cassini Regio but, because it was visible on the limb, it was also identifiable as a ridge.206 It was basically continuous, and simply rose from the adjacent dark terrain without 'foothills', although it was formed in some parts by individual mountains. Where an impact had demolished a part of the ridge the bright rim of the crater suggested that it was made of ice.207 It was subsequently determined that the ridge ran precisely along the equator, and extended beyond Cassini Regio. An analysis of the Voyager imagery had shown Iapetus to be an imprecise sphere. While it could be described as an ellipsoid whose half-axes were 767 x 742 x 713 kilometres, its shape was irregular, with substantial depressions and bulges, which was unusual for a body of such a mass.208,209,210 If the ridge was a compression structure created as the moon's interior shrank, then this would be expected to form along the circumference of the widest diameter (as was the case), after which tidal effects would have tipped the spin axis perpendicular to its orbital plane and synchronised its rotation. The field of view included the northern polar region, where dark 'feathery' streaks on the bright terrain adjacent to Cassini Regio suggested that the dark material had been delivered to the surface ballistically. Indeed, to space artist Don E. Davis the streaks appeared "as if they had been airbrushed". The craters in this area shared a pattern of albedo that suggested that the interior walls facing Cassini Regio had been exposed to 'fall out' that had come from the centre of the leading hemisphere, leaving their 'shielded' walls bright. But, of course, opinion differed as to whether this material was of endogenic or exogenic origin.

An Arecibo study at a wavelength of 12.6 centimetres of the bright hemisphere in January 2002 and of the dark hemisphere in January 2003 gave the surprising result that the radar could not tell the two hemispheres apart.211 Furthermore, even the bright side was unusual. As team member G.J. Black of the University of Virginia explained, ''It is known that the bright side is mostly water-ice, but we find it does

A mosaic of images taken on 31 December 2004 showing the leading hemisphere of Iapetus, and considerable detail on Cassini Regio.
A crescent view over the north polar region of Iapetus taken on 31 December 2004, showing the 'feathery' albedo features.

not reflect the radar like any other icy satellite that we've studied with the radar; it appears much less reflective.'' If there was ammonia present, the ice would be bright optically, but would not reflect microwaves as strongly as 'pure' water-ice. The fact that Cassini Regio was indistinguishable ''could mean that on the dark side there is merely a thin coat of some darkening material on the ammonia-laden water-ice, just like an inch of dirt over clean snow. Such a coat would not have much effect on the radar reflection, which sees the underlying ice.'' If this dark material is such a thin veneer, then to preclude excavation of the bright substrate it must either have been applied very recently, or its deposition must be ongoing and at a sufficient rate to mask new exposures of ice. Although the summits of the peaks on western Cassini Regio were bright, they were exceptional. A massive landslide from the 15-kilometre-

A close up of Iapetus taken on 31 December 2004 showing a landslide covering half of a 120-kilometre-diameter flat-floored crater located adjacent to the 15-kilometre-tall scarp that forms the rim of a 600-kilometre impact basin.

tall scarp that formed the rim of a large basin in eastern Cassini Regio had spilled across the floor of a crater contained within the basin, and the exposed face of the scarp was dark.

The resolution of the Visual and Infrared Mapping Spectrometer was such that its image of the moon spanned only four pixels, but the fact that one pixel was wholly on the bright area and another was wholly on Cassini Regio facilitated a basic comparison of their compositions. The bright terrain appeared to be water-ice contaminated by a small amount of organic (tholin) material, and Cassini Regio was best modelled as a mixture of tholin, a hydrogen cyanide polymer, a small amount of water-ice and ferric oxide.212 This matched fairly well a model based on telescopic data.213 The fact that Cassini Regio was also laced with carbon dioxide (probably a photochemical product that was trapped in either water-ice, a mineral or a complex organic solid) implied that the carbon-bearing material was 'swept up' by the moon after its rotation had become synchronised. The Composite Infrared Spectrometer indicated that the dark material was loose and fluffy.214,215 The Ultraviolet Imaging Spectrograph determined that a distinctive far-ultraviolet absorption feature for water-ice was strong for both Phoebe and the bright side of Iapetus, but extremely weak for Cassini Regio.216 Nevertheless, this did not reject Phoebe as the source of the dark material on Iapetus, because the volatiles could have been liberated at emplacement, thereby concentrating the non-ice material. If Phoebe is a captured body from the frigid outer Solar System, then on becoming trapped in the warmer Saturnian environment it might have undergone a period of outgassing and supplied the material which has disfigured Iapetus. Intriguingly, although Cassini Regio is centred precisely on the leading hemisphere, it does not reach the poles, and in the equatorial zone it extends beyond both limbs onto the trailing hemisphere. Also, the spectrum of the central area is somewhat 'redder' than the equatorial extensions, whose spectrum is 'flatter', suggesting a progressive transition.217 If the dark material is indeed of exogenic origin, then it was deposited after the ridge formed, and after the moon adopted its current spin axis and synchronised its rotation with its orbital period - a timescale that might shed light on when Phoebe was captured.

Outstanding questions include:

• Why is Iapetus in an inclined orbit?

• Why is it so large for an 'outlier' in the system?

• Why is it so different from similarly sized Rhea?

On 10 September 2007 Cassini will fly within 1,300 kilometres of Iapetus to image it at a resolution 100 times better than that achieved so far, and hopefully this will provide insights into these mysteries.

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