Io Eclipse Shadow on Jupiter from Juno.

What's that dark spot on Jupiter? It's the shadow of Jupiter 's most volcanic moon Io . 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 Io . Io 's shadow is about 3600 kilometers across, roughly the same size as Io itself -- and only slightly larger than Earth's Moon . The featured image 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 into a 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 center.