“Davies’s maps don’t match up with either the shallow or deep heating cases”
The astonishing level of volcanic activity on Jupiter’s moon Io was discovered by Voyager 1 as it flew past in 1979. Since then, we’ve been able to examine Io much more closely over a protracted period with the Galileo orbiter, and have calculated that this moon has an average global heat flow 30 times higher than Earth. Io is a hot, violent little world.
All of this activity is driven by tidal heating as Io orbits within the powerful gravitational influence of Jupiter. But what we don’t fully understand yet is whether this heating occurs just beneath Io’s crust or much deeper in the mantle.
Unlike Earth, where most volcanoes are concentrated along spreading centres or plate subduction zones (although some, like those in Hawaii, represent hot spots in the middle of a plate), Io shows no evidence of plate tectonics and its volcanoes are distributed all over its immobile surface. So what Ashley Davies and his colleagues at NASA’s Jet Propulsion Laboratory think is that by examining the pattern of volcanic activity across the face of Io they might be able to work out where the heating is taking place.
is an astrobiologist at the University of Leicester and the author of The Knowledge: How to Rebuild our World from Scratch (www. the-knowledge.org)
Davies has taken thermal imaging from the Galileo probe along with more recent ground-based observations using telescopes with adaptive optics and created a map of volcanic heat flow on Io’s surface. What you’d expect to see if the tidal heating were relatively shallow are hot spots nearer the equator and, in particular, at the point facing Jupiter and the corresponding point on the opposite side of Io (in the same way tides in our oceans form bulges both immediately below the Moon and also on the opposite side of the Earth). On the other hand, with deeper heating, you’d expect higher heat flow towards the poles. But puzzlingly, Davies’s maps don’t match up with either the shallow or deep heating cases, or indeed any combination of the two.
The first problem is that the observed volcanoes can only account for 54 per cent of the heat that must be flowing from Io’s crust. The rest of the thermal energy could be being lost by large areas of the surface that are only slightly warmer – this would have been difficult for the Galileo instruments to spot. And in fact, this might be the reason behind Io’s surprisingly warm poles. But the second mystery is that the heat flow map seems shifted around the moon’s face: clusters of volcanoes aren’t situated in line with Jupiter, but further around to the east.
Davies has some ideas as to what might be going on. Perhaps Io’s surface is rotating slightly relative to Jupiter, shifting the volcanic hotspots clockwise. Or maybe disturbances of convection in the mantle, or variation in thickness of the crust cause the offset in the volcanism. Whatever is going on, we are unlikely to resolve the mystery until we get a new mission to the Solar System’s most volcanic body. But besides the Io Volcano Observer that was proposed in 2015, there are currently no missions planned to visit this intriguing moon.