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Science Frontiers ONLINE No. 18: Nov-Dec 1981 |
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Phoebe Not Locked To Saturn
When Voyager 2 passed through the Saturn system a few months ago, it snapped pictures of Phoebe, Saturn's outermost moon. Phoebe is nicely rounded, 200 kilometers in diameter, and swings around Saturn in a retrograde orbit 550 days long -- nothing anomalous so far. Phoebe, however, turns out to be the only solar-system satellite whose axial period of rotation is not about equal to its period of rotation about its parent planet. All other moons, including our own, are gravitationally "locked" so that they always point the same hemisphere at the parent planet.
(Anonymous; "Voyager's Fleeting Glimpse of Phoebe," New Scientist, 91:779, 1981.)
Comment. One inference here is that Phoebe is a relatively recent addition -- and a good-sized one -- to the Saturn system, and just hasn't been there long enough to become "locked."