Answer: Io

Volcanic activity in our solar system is actually a pretty rare thing. Among all the inner and outer planets, the dwarf planets, and the moons scattered among them, the only bodies with confirmed volcanic activity are Earth, Triton (a moon of Neptune), Enceladus (a moon of Saturn), and Io (a moon of Jupiter)—though there is evidence of ancient volcanic activity on other bodies like Mars and suspected activity on Saturn’s moon Titan.

Among the currently active bodies, Io is far and away the most active. While volcanic eruptions on Earth are infrequent but terrifying, volcanic eruptions on Io are so fierce and numerous that the moon would surely appear as some sort of post apocalyptic hellscape. Thanks to the powerful gravitational pull of its host planet Jupiter as well as the nearby moons, Io is continuously deformed by the strong movement of internal tides, which create massive amounts of friction within the mantle of the moon and a correspondingly large amount of heat. The moon is covered with hundreds of volcanic vents that erupt with fearsome intensity.

When the New Horizons spacecraft passed by Io, for example, it recorded a volcano, seen in the photo here, ejecting huge plumes of material around 180 miles above the surface. The flyby observations correspond with observations on Earth that indicate huge eruptions that eject massive volumes of material (frozen vapor, volcanic snow, silicate rock, or molten sulfur) into the air are a common occurrence there.