It was with a mixture of incredulity and excitement that I heard on the BBC radio morning news that the NASA Voyager 1 mission had observed active, explosive volcanism on Io, one of the four Galilean satellites orbiting Jupiter. This was March 1979 and at that time we had seen no evidence of recent volcanic activity anywhere else in the Solar System other than Earth.
It should not have been a surprise. That Io, a Moon-sized planetary body, could be volcanically active had been predicted by a California-based team and their work was published a few days before Voyager returned the now-famous picture of a volcanic plume silhouetted against the black sky. They calculated that the net result of the gravitational forces from the giant planet Jupiter and the other Galilean satellites would be enough to induce tidal heating in the interior of Io, and that this would be sufficient to cause extensive volcanic activity. In all nine active plumes were discovered on Io by Voyagers 1 and 2. The surface is mainly covered by numerous flows and volcanic constructs, and, significantly, there are no observed impact craters, implying that the surface is young.
Just over 16 years later the NASA Galileo mission arrived at Jupiter to continue where Voyager left off. The mission's observations of Io provided outstanding data with which to study the volcanoes of Io and their activity. Comparisons could be made between Voyager observations and the state of the planet during Galileo flybys of the satellite. Some images had resolutions as good as 6 m per pixel. In addition to images, temperatures could be derived and volcanic thermal sources identified.
Ashley Davies describes and interprets these observations in considerable detail. He does so with authority, having been a member of the Galileo Near-Infrared Mapping Spectrometer Team, working directly with data as the mission progressed. He also has a background in physical volcanology which is fundamental to interpreting volcanism on other planets. There are six sections in the book. The first describes the missions to Io, the spacecraft instrumentation and the capabilities as well as Earth-based astronomical observations before and after Voyager. The second section compares Earth and Io and provides an introduction to what is discussed in the rest of the book The next section focuses on how we study volcanism on other planets by remote sensing, and the use of mathematical models to interpret volcanism on Io. This section provides a good introduction to some of the fundamentals of understanding the physics of volcanic processes.
Section four gives a detailed description of what we observe of Io's volcanoes. Separate chapters are devoted to each of the main volcanic centres. There follows a section dealing with different volcanic features such as shield volcanoes, lava flow morphology and explosive plumes, comparing them with those of Earth. Hot spots are also discussed. Finally, the author summarizes the present understanding of volcanism on Io, and raises questions for the future.
We have here an excellent text for both undergraduates and those involved in volcanic–planetary research.
