Madsen Pirie

It fills me with wonderment that we can take pictures like these.  This one, chosen recently as Wired’s “Space photo of the day,” is of Saturn’s moon Phoebe, taken during a Cassini fly-by.  When I learned, as a boy, the names of Saturn’s (then) nine moons, Phoebe came last because it was the outermost, at four times the distance of Iapetus, the second most distant.  Because its orbit is eccentric, unlike the others, it was thought that it might have been captured.  It has a low albedo, meaning it is very dark, and was thought to have perhaps been an asteroid captured by Saturn.

Examination of its surface now suggests a more exciting explanation.  It seems to be made mostly of ice with a thin black covering, disturbed in places by meteor strikes.  This suggests it might be a Kuiper Belt object thrown out of its orbit, and then captured by Saturn.  This in turn means we can potentially study the original material of the solar system from relatively close quarters.

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