Peek-a-moon!

By Phil Plait | December 18, 2008 12:00 pm

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 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)

CATEGORIZED UNDER: Astronomy, Pretty pictures, Science

Comments (20)

Links to this Post

  1. Peek-a-movie! | Bad Astronomy | Discover Magazine | December 19, 2008
  1. Sorry about comments being turned off; they’re on again now.

  2. Mang

    Oooooooo …. and I don’t say that often.

  3. jdporter

    FAKE! Shooped!

    And the word is “embiggen”.
    ;-)

  4. Sili

    Presumably Ganymede was one of the moons that let Rømer to discover that the speed of light is finite.

  5. American Voyager

    That’s incredible! To think we had to wait for the Voyager flyby to get any kind of detail at all on these moons. Now we can pull it in from Earth using Hubble. WOW!! I remember “far encounter” pictures that weren’t that good! Does anybody know how close Voyager had to be to top this kind of detail? Another question: if we are so good at taking objects like Ganymede, why can’t we get closeups of the asteroids like this? The best ones I’ve seen are fuzzy. Still, it’s an awesome picture! One of those jaw droppers. Wouldn’t Galileo have been impressed…………..

  6. Chris A.

    @Phil:
    Umm, I’m not convinced the red color is due to Jupiter’s atmosphere. Remember that Hubble can’t take R, G, and B images simultaneously, and Ganymede moves a wee bit between shots while the filter is being changed.

  7. sean hogge

    A layperson’s spectroscopy question:

    If we were to sample the light filtering through the Jovian atmosphere, Ganymede’s spectrum would contribute, correct? I assume we correct this by “subtracting” in some form the Ganymedian spectrum from the Jovian spectrum for light from that source. Or is that a foolish guess?

  8. Oh, my… This is absolutely *wicked*! One of the best planetary images Hubble ever captured!

  9. YoJimbo

    That’s no Moon, That’s a space station!

  10. Gary Ansorge

    Sean: You’re right on,,,that’s exactly what we do.

    So, GAnymede has a significant mag field? Wouldn’t that imply a very low rate of H2 loss due to energetic particle bombardment?
    Granted, at that distance from Sol, it would likely be particles(electrons, mainly) from interaction with the Jupiter ionosphere. Too far from Sol for the Solar wind effect?
    Well, maybe not.

    GAry 7

  11. IVAN3MAN

    I can’t believe nobody has bothered to “Digg” this article already! Well, I just “dugg” it several minutes ago.

  12. The Mad LOLScientist, FCD

    =sw000000000000n= i has n00 wallpaparz! =^..^=

  13. Blizno

    “Wouldn’t Galileo have been impressed…………..”

    I can imagine Galileo passing his terminal house arrest with delighted hour after hour of studying the images we’ve obtained of the solar system that he could barely see through his tiny telescope.

  14. Autumn

    Has the Great Red Spot shrunk recently? I thought that the GRS was about two or three Earth diameters across, but if Ganymede is behind Jupiter in this image, Ganymede appears to me to be nearly Earth-sized. I am just remembering stuff off the top of my head, so I’m probably wrong about this, but I assumed that the moon was in front of Jupiter when I initially looked at the picture. Seems too big.

  15. My-Name-is-Kenneth

    What’s that growing black spot in Jupiter’s atmosphere? Is the planet losing its chemical stain?

  16. swan

    Like Autumn, I’m a bit befuddled by the apparent size of Ganymede compared to Jupiter. I even scaled this to a stock picture to make sure this wasn’t the Less Spot of Jupiter. ;)

  17. Autumn, swan: the GRS changes its size over time a bit. A quick look at wikipedia has a comparison of it and the Earth that makes the scale of this image look about right: http://en.wikipedia.org/wiki/Great_Red_Spot#Great_Red_Spot

  18. Autumn

    Thanks, Phil. I guess I was just making basic mistakes in trying to scale objects according to my preconceptions. After so many years of math, I still overestimate the relative diameters of spheres with regard to their volume.

  19. quasidog

    Unreal. The detail in Ganymede (not that I was aware it was) was the first thing that I noticed. An amazing shot.

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