The Jovian moon Io
is fascinating from a planetary science perspective--it's the most volcanically active place in our solar system, and its surface is pockmarked with volcanic craters. But it wouldn't be much fun to visit. Baker and Ratcliff write that "Jupiter's moon Io smells like a jumbo rotten egg." The stink is due to hydrogen sulfide on Io's surface and in its upper atmosphere, and the moon owes its distinctive yellow and red coloration to sulfur compounds.
Volcanic eruptions are quite common on Io, and they constantly refresh the atmosphere's supply of sulfur gas. The moon is highly active because it travels around Jupiter in a slightly elliptical orbit. As the moon repeatedly dances closer to and farther from the giant planet, Jupiter's gravity produces tidal flexing in the moon's interior that heats its mantle and causes violent explosions. In 2007 NASA's New Horizons
spacecraft flew by Io and observed a volcanic eruption with sulfur plumes that stretched 180 miles above the surface. The largest volcanic eruptions on Earth reach about 12 miles high.