Wednesday, July 07, 2004
A mystery solved?
After T-0, Cassini took distant images of all the major icy moons. The inspection was cursory, with the exception of Iapetus, which was imaged repeatedly with multiple polarizations and exposures from a distance of 3 million km. Iapetus, discovered by the original Cassini, is a solar system oddity. It is a cosmic heliograph, more than ten times brighter on one side of its orbit than the other. Arthur C. Clarke would use the apparent hemispheric difference as a major plot point in the novelization of 2001: A Space Odyssey. The Voyagers would reveal that the leading hemisphere (in terms of its orbit around Saturn) of Iapetus is smeared with dark red material - similar to that which comprises the surface of Phoebe and (possibly) the sparser rings of Saturn. Establishing the formation of this material is one of the primary goals of the Cassini mission. While endogenic (volcanic) origins for this material, named Cassini Regio, have been proposed, exogenic mechanisms (where the stuff is dumped onto the surface from somewhere else in the Saturn system - probably Phoebe) seem more likely (e. g. Bell and others, 1985).
This image, the perhaps auspiciously designated N00006667, is the first image to show detail in Cassini Regio. An unfiltered short exposure, it plainly reveals a large multiring basin, almost directly on the Saturn facing point, within the eastern margin of the darkened area. Some speculations: if you dump a Phoebe equivalent at high speed into the 'near' side of Iapetus, most of the ejecta will head inward, and accelerate longitudinal with respect to the moon due to Kepler's Laws (not much will directly stick, as Iapetus' gravity is too low). But Iapetus' gravity will be sufficient to stretch out the orbits of ejected material, so a cloud of PhoebeJunk will end up in orbit ahead of Iapetus. This will then congeal on Iapetus' leading edge.
Cassini is scheduled for a close encounter with Iapetus, examining the edge of Cassini Regio in detail, during mid 2007, which will test this, and other hypothesis. But I suspect monolith-building aliens are now unnecessary.
This image, the perhaps auspiciously designated N00006667, is the first image to show detail in Cassini Regio. An unfiltered short exposure, it plainly reveals a large multiring basin, almost directly on the Saturn facing point, within the eastern margin of the darkened area. Some speculations: if you dump a Phoebe equivalent at high speed into the 'near' side of Iapetus, most of the ejecta will head inward, and accelerate longitudinal with respect to the moon due to Kepler's Laws (not much will directly stick, as Iapetus' gravity is too low). But Iapetus' gravity will be sufficient to stretch out the orbits of ejected material, so a cloud of PhoebeJunk will end up in orbit ahead of Iapetus. This will then congeal on Iapetus' leading edge.
Cassini is scheduled for a close encounter with Iapetus, examining the edge of Cassini Regio in detail, during mid 2007, which will test this, and other hypothesis. But I suspect monolith-building aliens are now unnecessary.
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12:19 PM
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