Jupiter’s moon Io is the most volcanically active body in the Solar System, with roughly 400 active volcanoes regularly ejecting magma into space. This activity arises from Io’s eccentric orbit around Jupiter, which produces incredibly powerful tidal interactions in the interior. In addition to powering Io’s volcanism, this tidal energy is believed to support a global subsurface magma ocean. However, the extent and depth of this ocean remains the subject of debate, with some supporting the idea of a shallow magma ocean while others believe Io has a more rigid, mostly solid interior.
In a recent NASA-supported study, an international team of researchers combined data from multiple missions to measure Io’s tidal deformation. According to their findings, Io does not possess a magma ocean and likely has a mostly solid mantle. Their findings further suggest that tidal forces do not necessarily lead to global magma oceans on moons or planetary bodies. This could have implications for the study of exoplanets that experience tidal heating, including Super-Earths and exomoons similar to Io that orbit massive gas giants.
The study was led by Ryan Park, a Senior Research Scientist and Principal Engineer at NASA’s Jet Propulsion Laboratory (JPL). He was joined by multiple colleagues from NASA JPL, the Centro Interdipartimentale di Ricerca Industriale Aerospaziale (CIRI) at the Università di Bologna, the National Institute for Astrophysics (NIAF), the Sapienza Università di Roma, the Southwest Research Institute (SwRI), and NASA’s Goddard Space Flight Center, and multiple universities. Their findings were described in a paper that appeared in the journal Nature.
As they explain in their paper, two types of analysis have predicted the existence of a global magma ocean. On the one hand, magnetic induction measurements conducted by the Galileo mission suggested the existence of a magma ocean within Io, approximately 50 km [~30 mi] thick and located near the surface. These results also implied that about 20% of the material in Io’s mantle is melted. However, these results were subjected to debate for many years. In recent years, NASA’s Juno mission conducted multiple flybys of Io and the other Jovian moons and obtained data that supported this conclusion.
In particular, the Juno probe conducted a global mapping campaign of Io’s volcanoes, which suggested that the distribution of volcanic heat flow is consistent with the presence of a global magma ocean. However, these discoveries have led to considerable debate about these techniques and whether they can be used to distinguish whether a shallow global magma ocean drives Io’s volcanic activity. This is the question Park and his colleagues sought to address in their study:
“In our study, Io’s tidal deformation is modeled using the gravitational tidal Love number k2, which is defined as the ratio of the imposed gravitational potential from Jupiter to the induced potential from the deformation of Io. In short, if k2 is large, there is a global magma ocean, and if k2 is small, there is no global magma ocean. Our result shows that the recovered value of k2 is small, consistent with Io not having a global magma ocean.”
The significance of these findings goes far beyond the study of Io and other potentially volcanic moons. Beyond the Solar System, astronomers have discovered countless bodies that (according to current planetary models) experience intense tidal heating. This includes rocky exoplanets that are several times the size and mass of Earth (Super-Earths) and in the case of tidally-locked planets like the TRAPPIST-1 system. These findings are also relevant for the study of exomoons that also experience intense tidal heating (similar to the Jovian moons). As Park explained:
“Although it is commonly assumed among the exoplanet community that intense tidal heating may lead to magma oceans, the example of Io shows that this need not be the case. Our results indicate that tidal forces do not universally create global magma oceans, which may be prevented from forming due to rapid melt ascent, intrusion, and eruption, so even strong tidal heating – like that expected on several known exoplanets and super-Earths – may not guarantee the formation of magma oceans on moons or planetary bodies.”
Further Reading: Nature