Slashdot Mirror
Phoebe Pictures Released
EccentricAnomaly writes "NASA has begun to release some pictures from Cassini's Phoebe flyby last Friday. If you look at the thumbnail of this image or if you look at the right of these images, you can see a group of craters that look like a skull just south of the equator and something that looks like George Washington (wearing his wig) near the north pole. Come up with some good names for features, and you can submit your ideas to the IAU. There's a process for naming a newly discovered crater, fossa, or sulcus. By the way, the naming convention for Phoebe is people associated with Phoebe or the Greek islands."
Aparently these moons occupy two of the most chaotic orbits in the solar system so it's no surprize that they should end up on the front page of slashdot.
May the Maths Be with you!
Really, I mean it: this is an ancient bit of rock whirling about in space, and we can study it from the comfort of home. Perversely, images like this always remind me that life is so short; doesn't anyone else see things like this and feel disheartened at mortality ? There is so much I will never have time to know, still less understand.
Life immitated art. The unusual colouration was known about for a long while - Cassini himself observed it when the moon seemed to "disappear" - and is a reason why Clark chose Iapetus for the location of the monolith. However, while the novel was written in the late 1960s, the anomaly in the pale surface was only detected by the Voyager flybys. There is a comment about the "black dot" in the foreward of one of the sequels, I forget which one, but Clark received a photo of the anomaly from NASA with the comment "Thinking of you..." or words to that effect.
UNIX? They're not even circumcised! Savages!
The theory that Iapetus was darkened by dust from Phoebe is alluring, but doesn't really hold up to scrutiny. It looks logical when you see a drawing of the Saturn system, with the half-dark moon sitting next to the dark moon, but that's about where the plausibility ends.
First, the moons are far, far apart. Phoebe orbits 5x farther from Saturn as Iapetus does - a difference on the order of 10 million km. Phoebe would have to be emitting a tremendous amount of dust for Iapetus to pick up any remotely visible amount of it.
Second, their orbits are inclined approximately 160 degrees apart. Iapetus orbits almost in Saturn's equatorial plane; Phoebe is nowhere near it. There's no astrophysical reason for the dust to get into Saturn's equatorial plane and stay there waiting for Iapetus. (Saturn's rings remain compressed into the equatorial plane by tidal forces near the planet, but those forces become extremely weak that far out. Iapetus orbits 30x farther from Saturn than the outermost large ring, and tidal forces decrease with at least the square of distance.) The volume of space that would have to be dusted by Phoebe to visibly darken Iapetus is simply prohibitively large.
Third, if Iapetus is darkened by dust, why not any of Saturn's other moons? OK, we don't yet know if Titan is, but the other moons should show some evidence of the same process, and they don't.
Fourth, Iapetus isn't half-and-half black and white like a chessboard or that race in Star Trek. Voyager showed that the dark area is a roughly circular area, roughly centered on the leading orbital hemisphere, with a highly irregular border. And there are light spots within the dark area - not a single dot in the center, but a few separated irregular areas. It's a surface feature of the moon with lower albedo, not "this half is black."
The dark area occupies a proportion of Iapetus's sphere similar to the proportion of the Pacific Ocean compared to Earth's sphere. Discounting the Velikovskyists, we're fairly sure that the Pacific Ocean was formed by Earthbound processes on our planet, so Iapetus's geological history could well have had something of similar scale.
I doubt we'll ever know for sure until an Iapetus lander spacecraft, which isn't even remotely in NASA's future plans yet. Yes, something weird happened on Iapetus, but it wasn't dust from little Phoebe.