At various times during its early orbits, Cassini imaged Hyperion from distances ranging between 1.3 and 1.6 million kilometres at spatial resolutions exceeding that of the Voyagers, but these were merely tasters. Imagery taken between 9 and 11 June 2005 as the range decreased from 815,000 to 168,000 kilometres made a movie that strikingly demonstrated the moon's irregular shape. There were craters
suggestive of a hydrocarbon lake. It was subsequently named 'Lake Ontario'.
apparent to the limit of resolution of about 1 kilometre per pixel, many of which were sharply defined, with intriguing dark material on their floors.
Hyperion posed a challenge to the imaging team, whose automated software for mosaicking imagery worked best with a sphere, and therefore had difficulty with this irregular body. A variety of bulk density estimates had been made over the years, but Cassini provided an opportunity to determine this with a fair degree of accuracy. The fly-by provided a measure of its mass and (its shape notwithstanding) its volume, from which a density of 0.6 g/cm3 was calculated. Solid water-ice is 0.93 g/cm3, and if Hyperion was an icy body then it was so porous that 40 per cent of its volume was vacant. Its irregular shape would seem to derive from its being a loose accretion of icy fragments, rather than a solid body that had been so battered as to have had large chunks smashed off it, or indeed, was itself a solid irregularly shaped chip off a larger object. That Hyperion is just below the size limit at which internal pressure will crush loosely packed ice and close the pore spaces to form solid ice is evident from the fact that it is somewhat larger than the irregular moonlets Epimetheus and Janus, and smaller than the spherical moons Mimas and Enceladus. Precisely why there should be a reaccreted object at this location in the Saturnian system is a mystery.
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