Check this out: |Click to embiggenate. Original image here.
That’s a Hubble image of Jupiter and its moon Ganymede, just
before the satellite dips behind the planet’s disk. It was taken
in April of 2007 but just released today (which is good, because
I would’ve been ticked it missed my Top Ten this year). Look at
the detail you can see on Ganymede! What kills me here is the
scale of this: Ganymede is about the same size as Mercury! If
Jupiter weren’t there, Ganymede might be considered a planet on
its own. It would be visible to the naked eye, too. There’s
science lurking here as well. As Ganymede goes behind Jupiter,
we see it through more and more of the giant planet’s
atmosphere. We know how Ganymede normally looks when it’s not
being obscured by Jupiter, so we can observe it as it goes
behind the planet to get a measurement of the atmospheric
profile of Jupiter. This is especially handy to observe
Jupiter’s very high, very thin haze layer that exists way above
the visible cloud tops. If you look at the hi