Bad Astronomy
The entire universe in blog form
Dec. 18 2008 12:00 PM


Check this out:

Click to embiggenate. Original image here.

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!  


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 higher-resolution picture, you can just see how the shadow of Jupiter on Ganymede gets redder near the shadow edge, which gives a clue about the makeup of the planet's atmosphere.

But even without all that, I would approve of taking images like this. Wow!

Image credit: NASA, ESA, and E. Karkoschka (University of Arizona)