Several bodies are of interest, bacause of our understanding of the tolerances of life. Mars is one, primarily because it seemed to have held life once, and there is the possibility of buried Martian water, or microorganisms living beneath the surface. The other two bodies of interest are Io, as you mentioned, and Europa. Europa is of significant interest, because geologic evidence supports that the ocean of Europa may be molten beneath the surface, and if temperatures and conditions are suitable, like may exist. Io, is another target, not because there may be an abundance of water, but due to its heave volcanic activity.
Volcanoes are a source of significant chemical energy, both as heat, and elements.
They provide a rich environment for complicated chemistries to occur, which may include those associated with life. Certainly in the history of the Earth, volcanoes have had a significant contribution to the development and distribution of life on the planet. All rights reserved.