That’s No Moon

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Oh wait now, it is. In a way.

This dark spot on the surface of Jupiter is the shadow of its most volcanic moon, Io. To wit:

Since Jupiter shines predominantly by reflected sunlight, anything that blocks that light leaves a shadow. If you could somehow be in that shadow, you would see a total eclipse of the Sun by IoIo‘s shadow is about 3600 kilometres across, roughly the same size as Io itself — and only slightly larger than Earth’s Moon. The featuredimage was taken last month by NASA’s robotic Juno spacecraft currently orbiting Jupiter. About every two months, Juno swoops close by Jupiter, takes a lot of data and snaps a series of images — some of which are made intoa video. Among many other things, Juno has been measuring Jupiter’s gravitational field, finding surprising evidence that Jupiter may be mostly a liquid. Under unexpectedly thick clouds, the Jovian giant may house a massive liquid hydrogen region that extends all the way to the centre.

Full sized (original horizontal) image here.

(ImageNASA/JPL-Caltech/SwRI/MSSS; Processing: Kevin M. Gill)

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