Why This Picture Of Three Moons Looks Like A Picture Of Two

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Why This Picture Of Three Moons Looks Like A Picture Of Two

What looks like a pair of Saturnian satellites is actually a trio upon close inspection.

Saturn's moon Atlas is a tiny place. It's just 19 miles across at its widest point. But the little moon carries the weight of the world on its shoulders - or the weight of a very impressive ring system, anyway. Atlas was named for its position between Saturn's A and F rings, which makes it appear to sit nestled within them in photos.

If you can make the little guy out, anyway. In the Cassini orbiter image, Atlas takes up just two pixels.

Atlas appears to the left, a little speck butting up against Saturn's outermost ring in the lower half of the picture. Enceladus, a 313-mile wide moon now famous for containing a global, potentially habitable subsurface ocean, sits above the rings. Rhea, comparatively gigantic at 949 miles across, makes an appearance as well.

As tiny as it is, Atlas is pretty interesting. It has a big equatorial ridge, which is basically a feature that looks like a giant seam in the middle of the moon. Picture the raised edge of a walnut shell, or even that distinctive ridge in the middle of a bath bomb where its two halves come together. The ridges themselves might not seem that special, but they've been found only on Saturn's moons - nowhere else. And of Saturn's 62 confirmed moons, only Atlas, Pan and Iapetus are known to have this strange feature.

It would make sense for Atlas and Pan to have built up these ridges from dust particles they accumulated, because they live within Saturn's rings where particles of ice, dust and rock are plentiful. But Iapetus, the third-largest moon in the Saturnian system, sits far outside the rings. Unless it was somehow expelled into a wider orbit of its host planet, Iapetus's mysterious ridge must have formed by a different mechanism than Atlas's and Pan's.

Or perhaps the three ridges share a common origin that scientists have yet to decipher.

© 2016 The Washington Post 

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