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<b>Have We Cracked Saturn's Walnut?</b><br /><br />LINK<br /><br />TWO oddities on Saturn's moon Iapetus could have a single cause. One hemisphere is much darker than the other, and a huge ridge on the equator on the darker side makes the moon look rather like a giant walnut. Both these mysterious features could be the result of an ancient dust-up between the moon and one of Saturn's primordial rings. At least, that's the conclusion radio astronomer Paulo Freire came to after looking at pictures of Iapetus taken by the Cassini spacecraft at the end of last year.<br /><br />When Italian astronomer Giovanni Cassini discovered the 1400-kilometre-wide moon in 1672, he noticed the hemisphere that faces forward as Iapetus orbits Saturn is much darker than the other half.<br /><br />And when NASA's Cassini flew by Iapetus on 31 December 2004 it spotted another intriguing feature: a ridge 1300 kilometres long, occasionally 20 kilometres high and stretching a third of the way round the equator. Carolyn Porco, leader of Cassini's imaging team at the Space Science Institute in Boulder, Colorado, says, "We don't yet have a viable theory for the equatorial ridge."<br /><br />Freire, of the Arecibo Observatory in Puerto Rico, thinks otherwise. Because both the ridge and the dark coating are on the same side of the moon, he thinks that they are linked. Freire argues that both features formed when the moon collided with the edge of one of Saturn's rings a long time ago. "I was looking at the Cassini pictures, and the idea suggested itself to me," he says.<br /><br />According to Freire, debris from the ring smashed into a narrow region along the moon's equator, piling up to create the ridge. Consistent with his claim that the moon only grazed the ring is the fact that the ridge does not extend over the entire hemisphere. If the moon had fully entered the ring, a ridge would have formed over a 180-degree arc of