Figure 1: Io and Jupiter. Io has a diameter of 3,642 km, making it slightly larger than Earth’s Moon.
Io, a moon of Jupiter, is the most volcanically active object in the Solar System. This is due to the large amount of tidal heating being generated within the moon’s interior as it is pulled between Jupiter and the other Galilean satellites - Europa, Ganymede and Callisto. The extensive volcanism and high temperature lavas on Io suggest the presence of a global layer of magma beneath its crust.
The existence of such a subsurface global ocean of magma was determined by using Jupiter’s powerful magnetic field as a probe. At Io, induction caused by Jupiter’s magnetic field can be used to infer the conductivities and hence the properties of its subsurface layers. This is because the conductivity of rock material depends on its temperature and melt state. For instance, in comparison to solid rocks, fully or partially molten rocks have dramatically higher conductivities.
Figure 2: This cross-sectional visualization shows the internal structure of Jupiter’s moon Io as revealed by data from NASA’s Galileo spacecraft. A global magma ocean that is believed to be more than 50 km thick (shown in red-brown) underlies a low-density crust about 30 to 50 km thick (shown in gray). Io’s core, measuring about 1200 to 1800 km in diameter, is composed of iron and iron sulphide (shown in a metallic silver hue). Credit: NASA/JPL/University of Michigan/UCLA.
Data from magnetic observations carried out by the Galileo spacecraft during its flybys near Io in October 1999 and February 2000 show the presence of electromagnetic induction from a highly conductive global layer beneath the surface of Io. This global conducting layer is consistent with a subsurface magma ocean with a thickness of over 50 km and a rock melt fraction of 20 percent or more. The global magma ocean has an estimated temperature exceeding 1200°C and it exists beneath a low density crust 30 to 50 km thick.
“The hot magma in Io’s ocean is millions of times better at conducting electricity than rocks typically found on the Earth’s surface” said the study’s lead author, Krishan Khurana, a former co-investigator on the Galileo magnetometer team and a research geophysicist with UCLA’s Institute of Geophysics and Planetary Physics. “Just like the waves beamed from an airport metal detector bounce off metallic coins in your pocket, betraying their presence to the detector, Jupiter’s rotating magnetic field continually bounces off the molten rocks in Io’s interior. The bounced signal can be detected by a magnetometer on a passing spacecraft.”
The high conductivity of Io’s global magma ocean shields the interior of the moon from Jupiter’s powerfully magnetic field. As a result, the magnetic field inside Io maintains a vertical orientation despite the varying orientations of the external magnetic field (i.e. Jupiter’s magnetic field). This study was published in 2011 in the journal Science and it explains why Io is the most volcanic object known in the solar system.
KK Khurana et al., “Evidence of a Global Magma Ocean in Io’s Interior”, Science 3 June 1011: Vol. 332 pp. 1186-1189