Volcanic activity is caused by heat, below the surface, trying to escape. In earth, this heat builds up slowly from radioactive decay of elements like uranium. This process occurs on our moon as well, but since the moon is so much smaller than earth, the heat doesn't build up enough to cause any volcanic activity. Io also gets squeezed in the sideways direction, because of the converging gravitational pulls toward Jupiter's center. As Io's eccentric orbit carries it toward and away from Jupiter about every two days, the tidal forces become stronger and weaker, creating friction from the constantly varying forces.
Titan is the only moon in the solar system with a significant atmosphere.
Mercury, which is about the same size, has no atmosphere because it is so hot that any gases would have escaped its gravitational pull long ago. Life on earth is made of large organic molecules, so scientists have speculated that life might also have arisen on Titan. However, the extremely cold environment on Titan would not support any of earth's life forms, in part because any water would freeze into ice. So we don't even know whether life could exist on Titan, much less whether it actually does exist. Three billion dollars, spread among the population of the U.S., would be about ten dollars per person.