Active volcanism: Effusive eruptions

Abstract

Io, the volcanically active innermost large moon of Jupiter, was a\rtarget of intense study during the recently completed NASA Galileo\rmission to Jupiter (1989-2003). Galileo's suite of instruments\robtained unprecedented observations of Io, including high spatial\rresolution imaging in the visible and infrared. This paper reviews the\rinsights gained about Io's surface, atmosphere and space environment\rduring the Galileo mission. Io is thought to have a large Fe-FeS core,\rwhose radius is slightly less than half the radius of Io and whose\rmass is 20% of the moon. The lack of an intrinsic magnetic field\rimplies that the core is either completely solid or completely liquid.\rThe mantle of Io appears to undergo a high degree of partial melting\r(20-50% molten) that produces ultramafic lavas dominated by Mg-rich\rorthopyroxene in an apparent 'mushy magma ocean', suggesting an\rundifferentiated mantle. The crust of Io is thought to be rigid, 20-30\rkm thick, cold away from volcanic heat sources and composed of mafic\rto ultramafic silicates. Tidal flexing due to Io's orbital resonance\rproduces [?]100 m tides at the surface, generating heat that powers\rIo's volcanism. Silicate volcanism appears to be dominant at most hot\rspots, although secondary sulfur volcanism may be important in some\rareas. The key discoveries of the Galileo era at Io were: (1) the\rdetection of high-temperature volcanism (ultramafic, superheated mafic\ror 'ceramic'); (2) the detection of both S2 and SO2 gas in Ionian\rplumes; (3) the distinction between eruption styles, including between\rPelean plumes (originating from central vents) and Promethean plumes\r(originating from silicate lava flow fronts); (4) the relationship\rbetween mountains and paterae, which indicates that many paterae are\rformed as magma preferentially ascends along tectonic faults\rassociated with mountain building; (5) the lack of detection of an\rintrinsic magnetic field; (6) a new estimate of global heat flow; and\r(7) increased understanding of the relationship between Io, its plasma\rtorus and Jupiter's magnetic field. There is an apparent paradox\rbetween Io's potentially ultramafic volcanism (suggestive of a\rprimitive, undifferentiated mantle) and the widespread intensity of\rthe volcanism on Io (which should have produced a volume of lava\r[?]140 times the volume of Io over the last 4.5 Ga, resulting in more\rsilicic materials). The resolution of this paradox requires either an\rIo that only recently (geologically) entered its tidal resonance and\rbecame volcanically active or a response of Io's lithosphere-mantle to\rtidal heating that has in some way prevented extreme differentiation.\rUnderstanding this problem is one of many important issues about Io\rthat remain unresolved. We conclude this paper with a discussion of\rthe types of future observations, from the ground and from space, that\rwill be needed to address these issues.