Idiosyncratic Iapetus
Saturnian moon puts a time stamp on the outer solar system
By Ron Cowen
Iapetus, the third-largest and second-farthest-out of Saturn’s satellites, is the weirdest moon in the solar system. One half of it is as bright as snow, the other as black as charcoal. Neither spherical nor ellipsoidal, as most moons are, Iapetus looks like a walnut, with a bulging waistline and squashed poles. Accentuating its nutty appearance is a narrow, 20-kilometer-high ridge that girdles most of the moon’s equator, like the brim of a hat. No other moon in the solar system has such a ridge.
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Now astronomers think that they may have cracked the mystery of this walnut moon. Iapetus’ odd shape and isolated location, they say, suggest that its evolution came to an abrupt halt just a few hundred million years after it came into being. If that’s so, Iapetus may serve as a well-preserved relic from the early days of the solar system, not long after the planets were born.