What are the Compositions of Magmas Erupting on Io?

A.S. McEwen, L.P. Keszthelyi (LPL), Galileo SSI Team

Recent Earth-based and Galileo observations have shown that high- temperature hot spots are common on Io. The highest temperatures are well above 500 K, too hot for elemental sulfur alone, so Io's volcanism is probably driven by silicate eruptions. Two short-lived events observed from the ground (Veeder et al., 1994, JGR 99, 17,095; Spencer et al., GRL, in press) require temperatures of 1500 K or hotter. Because of rapid radiative cooling, even within a vigorous fire fountain, remote temperature measurements are usually at least 200 K lower than the actual eruption temperature. A magma temperature exceeding 1700 K is consistent with ultramafic melts, not basalts which are typically erupted at less than  1400 K. Were these high-temperature events fortuitous observations of unusual magma compositions, or are many eruptions on Io driven by very high-temperature magma? Galileo SSI has observed Io during 5 eclipses in the first 8 orbits, including observations through the broad-band clear filter ( 0.4-1.0 microns) and color filters with effective wavelengths near 0.76, 0.89, and 0.99 microns. Preliminary analyses of these data indicate that temperatures commonly exceed 1300 K, and sometimes exceed 1700 K. Hence, it now appears possible that much of Io's volcanism is driven by ultramafic magmas. Most ultramafic melts on Earth are primitive, undifferentiated magmas, but Io is likely to be very highly differentiated (Keszthelyi and McEwen, submitted). The differentiation of Io may have produced a depleted Mg-rich mantle which evolves toward a composition of pure forsterite ( tex2html_wrap_inline13 ), with a liquidus temperature near 2160 K. Could Mg-rich melts reach Io's surface? A dense melt can rise through a low density differentiated crust given deep magma chambers (> 100 km), a source of volatiles, or some other process to produce overpressurization. Other observations from SSI that may support this hypothesis are that: (1) the hot spots are invariably confined to relatively low elevations on Io, especially caldera floors; and (2) the low-albedo materials (closely associated with hot spots) do not have a detectable 1-micron absorption band, expected from Fe-rich silicates but not Fe-poor silicates such as forsterite.