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THE WOODLANDS, TEXAS — The dark dunes of Titan, Saturn’s largest moon, could have fallen from space. More than enough cometary material may have struck Titan to have formed its vast dune fields, planetary scientist William Bottke reported March 12 at the Lunar and Planetary Science Conference. Computer simulations suggest that the enigmatic drifts formed from objects hailing from the primordial Kuiper Belt, a modern source of comets beyond the orbit of Neptune. The proposed sc...