The moon Io is home to over 400 active volcanoes, making it the most geologically active place in the solar system. And in this image created from the Galileo probe, you can see two huge volcanoes erupting from the surface.
Io's volcanism is the result of its position in space, sandwiched between Jupiter and the other three major (or Galilean) moons that orbit the planet. Being in the middle of all that mass creates huge friction within the Moon's interior. As for what's going on in this particular image, a NASA astronomer explains:
Two sulfurous eruptions are visible on Jupiter's volcanic moon Io in this color composite image from the robotic Galileo spacecraft that orbited Jupiter from 1995 to 2003. At the image top, over Io's limb, a bluish plume rises about 140 kilometers above the surface of a volcanic caldera known as Pillan Patera.
In the image middle, near the night/day shadow line, the ring shaped Prometheus plume is seen rising about 75 kilometers above Io while casting a shadow below the volcanic vent. Named for the Greek god who gave mortals fire, the Prometheus plume is visible in every image ever made of the region dating back to the Voyager flybys of 1979 - presenting the possibility that this plume has been continuously active for at least 18 years. The above digitally sharpened image of Io was originally recorded in 1997 from a distance of about 600,000 kilometers. Recent analyses of Galileo data has uncovered evidence of a magma ocean beneath Io's surface.
For more on Io and its intense surface, check out this recent post.