Moon Io in front of Jupiter

Jupiter's four largest satellites, including Io, the golden
ornament in front of Jupiter in this image from NASA's Cassini
spacecraft, have fascinated Earthlings ever since Galileo Galilei discovered them in 1610 in one of his first astronomical uses of the telescope.

This true-colour composite frame, made from narrow angle images
taken on 12 December 2000, captures Io and its shadow in transit against the disk of Jupiter. The distance of the spacecraft from Jupiter was 19.5 million kilometres. The
image scale is 117 kilometres per pixel.

The entire body of Io, about the size of Earth's Moon, is
periodically flexed as it speeds around Jupiter. As a
result of its non-circular orbit, it feels the periodically changing gravitational pull of the planet. The heat arising in Io's interior from this continual flexure makes it the most
volcanically active body in the solar system, with more than 100 active volcanoes. The white and reddish colors on its surface are due to the presence of different sulfurous materials. The black areas are silicate rocks.