Cassini spots a moon, solves a mystery

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March 3 2009 11:16 PM

Cassini spots a moon, solves a mystery

The folks working with the fantabulous Cassini Saturn probe just released a very cool image indicating a very cool discovery: a moonlet embedded in Saturn's G ring.


Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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Saturn's G ring is the second outermost ring (one more ring, E, is outside G, but very wide and diffuse). It's about 8000 km (5000 miles) wide, orbiting Saturn about 165,000 km (100,000 miles) out from the planet. It's mostly dust, which was a mystery: there was no moon associated with it to replenish the dust, the only ring like it around Saturn without such a source. These new images (click to emringen) clearly show there is a moon there indeed, though it hardly qualifies with such a grandiose name: it's only about 500 meters (600 yards) across.

These images were taken over about ten minutes, with each image being a 46 second exposure. The camera was tracking on the ring, so the stars appear as streaks. The moonlet can be seen to be embedded in the ring, and moving with it. That makes it clearly part of the G ring, and the obvious long-sought source of dust.

Incidentally, the left image is in visible light, the middle in red, and the right in near-infrared. Taking images at different wavelengths (colors) like that gives astronomers insight in the composition of the ring material -- ice and dust reflect light very differently, so looking at the relative brightnesses of the ring in different wavelengths can distinguish what material is in it.

This is a very cool thing: scientists predicted the moonlet was present because they had studied other rings and knew there had to be something in there making the dust particles, so they went looking for it, and bang! They found it.

That's science. Observation, hypothesis, test, result. A billion kilometers from Earth, and science still works. That's one of the many reasons I like it so much. That, plus it's simply so cool.

Credit: NASA/JPL/Space Science Institute