THRILLING DISCOVERIES ABOUT THE TIDALLY TORTURED VOLCANIC WONDERLAND IO

In 1771, Pierre Simon Laplace described the 4:2:1 resonance of the orbital periods of Io, Europa, and Ganymede around Jupiter. The significance of this relation was realized over 200 years later by Peale et al. (1979), who pointed out that the Laplace resonance creates significant forced eccentricity in the orbits of Io and Europa, such that they are periodically deformed by massive Jupiter, generating internal heating. They predicted runaway melting of the interior of Io, thinning the lithosphere so that it could conduct away the heat, and that the Voyager spacecraft might observe widespread active volcanism. That latter prediction was dramatically confirmed a few months later by Voyager 1, which captured erupting volcanic plumes and hot spots on Io. Although Peale et al. predicted a thin lithosphere over a magma ocean, Voyager 1 revealed mountains >10 km high, suggesting that Io must have a thick, cold lithosphere, built up by rapid volcanic resurfacing and subsidence of crustal layers (O’Reilly and Davies, 1981). Considerable debate persists about whether or not Io has a magma ocean (see Keszthelyi, this conference).

My personal connection to Io began in 1981 with “discovery” of the eruption at Aten Patera between the Voyager 1 and 2 encounters. Really, this was a Voyager mission discovery, and I was the lucky student who first recognized it. This result lead to publication of “Two Classes of Volcanic Plumes on Io” (McEwen and Soderblom 1983). Earth-based telescopic and Galileo mission results showed that Io’s active volcanism is dominated by silicate rather than exotic sulfur volcanism favored by Voyager scientists. This meant that Io is highly relevant to the early evolution of the terrestrial planets (and some present-day exoplanets). During the Galileo mission we may have discovered extremely high-temperature ultramafic volcanism (McEwen et al. 1998), an even stronger link to early volcanism on Earth and other terrestrial planets. Widespread ultramafic volcanism suggests a large degree of mantle melting, perhaps consistent with a magma ocean. However, the Galileo cameras were not designed to measure such high temperatures and failed to acquire convincing spectral compositional data on the silicate lavas, and the mission did not acquire the best geophysical data on Io's interior, so uncertainty persists. A future Discovery mission called Io Volcano Observer may finally resolve these mysteries about Io, which in turn can advance understanding of the evolution of other planets and Moons (like Europa and Enceladus) with silicate mantles.