IO: FROM VOYAGER TO NEW HORIZONS

The discovery of active volcanism on Jupiter’s moon Io in 1979 by the Voyager spacecraft, following theoretical predictions, was a major turning point in our understanding of planetary geology and how planets and moons evolve. Io, about the same size as the Earth’s moon, still has rampant volcanic activity due to tidal dissipation which results from eccentricity in its orbit. The geology of Io was first revealed by the two Voyager spacecraft in 1979, followed by the Galileo spacecraft from 1996 to 2001, by Cassini on its way to Saturn in 2000 and, in 2007, by the New Horizons spacecraft which flew by the Jupiter system on its way to Pluto. Galileo’s temporal coverage and high spatial resolution observations from its close flybys of Io were particularly valuable in advancing our understanding of Io’s geology. We now know of at least 166 active volcanic centers and that three major eruption styles exist: long lava flows, violent fire fountaining and explosive events, and lava lakes and flows confined within Io’s many patera. Magma on Io is thought to be silicate (basaltic or possibly ultramafic) but, since there is no water, explosive volcanism occurs in the form of gases (mostly sulfur and sulfur dioxide) being released into the near vacuum, and in the form of powerfully gas-driven fire fountains. Explosive release of sulfur dioxide f f rom subsurface deposits due to contact with magma is also thought to be happening. A few large, dark deposits suggest large depositions of ash, but columns of ash have not been directly observed, possible due to the very small solid volume fractions in plumes. The distribution of volcanic centers on Io suggests that tidal dissipation occurs mainly in the asthenosphere. In addition to volcanism, Io also supports some of the largest mountains in the solar system, though the majority appears to be tectonic rather than volcanic structures. Io’s unique colorful appearance is created by volcanic plumes depositing sulfur and sulfur dioxide on the surface. In this paper we will review the major discoveries from spacecraft exploration and highlight the outstanding questions to be answered by future missions.