Hiding Ganymede and an extrasolar connection

jupiterhubble1Jupiter & Ganymede. Credit: NASA, ESA, and E. Karkoschka (Univ. of Arizona)

Jupiter might not exactly be well placed for observing from the Northern Hemisphere this year, due to its relatively low altitude, but all is not lost. NASA have just released this beautiful, “close to natural” colour, image from the Hubble Space Telescope. It shows the Galilean moon Ganymede beginning to disappear behind the southwestern part of Jupiter’s enormous disc. You can watch a video of Ganymede disappearing behind Jupiter (over the course of two hours) on the NASA Hubble site. The images were taken in April 2007 and, as can be seen in the crop of the main image below, they show the huge rocky and icy moon in quite some detail.

ganymedehubbleGanymede disappearing. Credit: NASA, ESA, and E. Karkoschka (Univ. of Arizona)

Compare this new Hubble image of Ganymede to the one (below) from the New Horizons mission en-route to Pluto and you get a feel for just how good Hubble’s resolution is! The bright splodge towards the upper middle of Ganymede’s terminator in the New Horizons image is a huge crater known as Tros and it’s clearly visible, as a bright white spot, in Hubble’s shot as well. There are features smaller than Tros visible in the Hubble image too. That’s not to say that the New Horizons image isn’t any good. Far from it, it’s amazing — it’s just that Hubble is one impressive instrument!

newhorizonsganymedeA view of Ganymede from the New Horizons spacecraft as it whisked by.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Anyway I’m moving a little away from my point. Not only is Hubble’s new shot a wonderful picture, full of intriguing details on both Jupiter and Ganymede (especially the high resolution versions), it also illustrates nicely how astronomers have been able to study Jupiter’s atmosphere. The method astronomers have been using to do this is also similar to how astronomers ‘probe’ the atmospheres of distant exoplanets. Ganymede is reflecting light from the Sun back towards us (that’s how we can see it). That light passes through the upper reaches of Jupiter’s atmosphere, towards us, as Ganymede disappears behind the limb of the huge planet. By spectroscopically studying the chemical fingerprints stamped on this light, by Jupiter’s atmosphere, astronomers can then work out important properties and compositions of the gases in this part of the Jovian atmosphere.

exoplanetlimbAn illustration showing light passing through the atmosphere of an exoplanet.
Credit: ESA, NASA and Frederic Pont (Geneva University Observatory)

In a very similar way, when studying exoplanets, astronomers have been able to detect the chemical signatures of interesting molecules in exoplanetary atmospheres, that have been stamped on the light from the exoplanet’s parent star. So it isn’t just in studying the gas giants of our own Solar System that this versatile astronomical technique is used. It will be interesting to see what these observations from Hubble tell us about Jupiter’s atmosphere. If they are anything like the results seen from studying the atmospheres of distant exoplanets, they will be very interesting indeed.


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