I have always been fascinated by volcanoes, and especially by Io, a tiny moon that beyond any expectation turned out to be the most volcanically active body in the Solar System. Now that the NASA Galileo mission is over and initial data analyses have been completed, this is an appropriate time to assess the “state of the satellite” and review what has been learned about Io over the past few decades.

A fascination with volcanoes is understandable, but I am also inspired to understand, through modeling of volcanic processes, how volcanoes work. Such motivation was instilled in me as a post-graduate student by Lionel Wilson and Harry Pinkerton at Lancaster University in the UK.

In this book, therefore, I have endeavored not only to describe what Galileo saw, but also to provide the necessary background for understanding the physical, volcanological processes taking place on Io, and to demonstrate how remote-sensing data of volcanic activity can be used to peel back the layers of a planet to reveal interior processes and structure. To put the majestic scale of volcanism on Io into proper context, comparison is made wherever possible with volcanic activity on Earth.

It has taken nearly two years to write this book. Along the way, I have had a great deal of help from friends, family, and colleagues.