I've been getting notes (like from my friend Melissa) asking me whether a picture of Jupiter and its moon Io is real or not. The answer is: kinda! And of course I'll explain. First, here's the picture in question:
[Click to enjovianate.]
First, let say that when I saw the picture I recognized it, but couldn't place it. Google to the rescue! I did an image search on "Jupiter Io" -- I recognized the moon as Io from the obvious volcanic plume and features on its surface -- and quickly got my answer.
Basically, it's a montage of two images, both of which are real but were taken separately. They were taken by NASA's New Horizons probe, which passed Jupiter in 2007. New Horizons is on its way to Pluto, and flew past Jupiter to steal some of its energy and boost the speed of the probe.
The shot of Jupiter is actually a composite of three images taken in the infrared, well past what the human eye can see. That big blue spot is actually the Great Red Spot! But the scientists applied a false color to the infrared images for this picture. The different colors more or less show cloud height: high-altitude clouds are blue, and clouds deeper in the Jovian atmosphere are red.
Io is actually depicted as more true to what your eye would see -- it's shown in visible light, not infrared. However, that image of the moon was taken nearly a day after the Jupiter pictures were taken! The two pictures were stitched together later. The red spot is an active volcano on Io, and the blue swoosh is the plume of ejecta reaching well above the moon's surface.
[Update (March 20, 2012): Apropos of this, and coincidentally, the USGS just released a global map of Io. On Google+, I posted a the Io map highlighting Tvashtar Paterae, the volcanic region erupting in this picture.]
I'll note that the phase of Jupiter -- almost half full or so -- isn't real, though. The infrared images of the planet were taken many minutes apart, and Jupiter rotates so quickly that they would've been blurred, smeared, if simply shown as they were taken. To compensate, the person putting the image together mathematically projected the pictures onto a sphere seen obliquely like this. In reality, the space probe was nearly directly above Jupiter's lit side when it took those images. Weird!
Anyway, apparently this picture was posted on reddit linking to an anonymous image hosting site that didn't have much by the way of explanation, and got out into the wild of the internet. People loved it -- I mean, come on, it's an awesome shot -- but weren't sure if it was even real, or where it came from. I'll note that way down in the comments on that post the true nature of the picture is revealed, but not before the picture got spread far and wide. That happens on the 'net, and I'm just glad I got people asking me about it. One of the big points of putting together pictures like this for the public is to pique their curiosity!
And I'm happy to oblige that desire to learn. Think about it: this picture isn't a fake, it's real, more or less, and it's from a small spacecraft on its way to Pluto, and then out of the solar system forever. And as usual, reality is way, way cooler than fakery.
Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Goddard Space Flight Center