Friday, 24 May 2013

Jupiter in the Infrared from VLT

Every now and then, you go back to look at some old data and discover something new.  I'm helping some colleagues with a proposal at the moment to study wave activity on Jupiter, and went back to some thermal infrared imaging of Jupiter obtained by the Very Large Telescope (VLT) down in Chile using the VISIR instrument.  Although we'd published it already as part of a study of the Great Red Spot (Fletcher et al., 2010, Thermal Structure and Composition of Jupiter’s Great Red Spot from High-Resolution Thermal Imaging, Icarus 208, p306-328), there's a lot more to this dataset than I first realised!  

I re-processed northern and southern hemisphere images at 8.6, 10.7 and 13.0 µm from August 15th 2007.  Jupiter was close to opposition at that time, and so large that it can't fit completely onto the detector array.  Furthermore, we have to use a technique called nodding in the infrared, switching between two slightly different views of the target and then subtracting them, meaning that we detect Jupiter differentially on top of a very bright background (our own warm atmosphere).  Three images for the southern hemisphere are shown below.

Jupiter at 8.6 µm, where bright emission means a gap in the clouds, dark tones represent cloudier regions.

Jupiter at 13 µm, sensitive to the temperatures of the hydrogen and helium gas, so bright colours indicate warmer regions.  Imperfect correction for the nodded image can be seen as a bright arc at the top of the frame.
Jupiter at 10.7 µm, sensitive to a combination of tropospheric temperatures and ammonia gas.  So bright colours can either mean an absence of ammonia, or warmer temperatures.
The real magic happens when we combine these three filters together to make a false colour image.  Now I'm a novice when it comes to this sort of thing, but I used Adobe Photoshop, shifting the three Jupiter images to register them properly (they were taken a few minutes apart, and Jupiter rotates rather quickly!) and adding them to the R, G and B layers.  I then tweaked the colour balance slightly, and voila.

A crude false colour image of Jupiter, using red=8.6 µm, green=13 µm and blue=10.7 µm.
Credit:  L.N. Fletcher/University of Oxford/ESO
I'm pretty happy with these results, and certainly make nice images for a proposal, even if the corrections for the nodded images aren't perfect (particularly for the southern hemisphere, where auroral emission at 13 µm makes a well-defined south pole that's hard to correct for).  Where you see red, it means an absence of tropospheric cloud/haze.  Green means warmer temperatures.  Blue means an absence of ammonia or warmer temperatures.  The warm/cloud-free/ammonia-free southern edge of the Great Red Spot shows as white, as does the northern edge of the SEB.  The turbulent wake to the northeast of the GRS is cloud-free and warm.  You can see cloud-free rifts near the southern edge of the NEB, and a whole host of vortices in both hemispheres.  In the southern hemisphere image, you can even see the glow of Jupiter's auroral hotspot (this is acetylene emission from high over the southern pole).  If I were more of an expert at photoshop, I might be able to clean these even more, but I suppose I should get back to doing some real work!!

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