IMPROVED ESTIMATES FOR IO ERUPTION TEMPERATURES: IMPLICATIONS FOR THE INTERIOR

The estimates of lava tempertures greater than 1600 C require that the interior of Io either (1) has an extremely unusual chemistry or (2) be almost completely molten. Neither of these possibilities fits within our current understanding of the evolution of the Jovian system. Recent tidal heating models suggest that there should be no more than about 20% partial melting of the interior. If Io has a broadly chondritic composition, then this suggests a maximum magma temperature of about 1200 C.

We have completed a re-analysis of the uncertainties in key observations from the Galileo spacecraft that led to reports of very high temperature eruptions at Pillan, Pele, and Tvashtar. We no longer find that temperatures above about 1300 C are required (though very high temperatures are still allowed by the data). At Pillan, there is more uncertainty in the timing of this erroneous double exposure than had been previously recognized. At Pele, the full implications of the non-Gaussian nature of the errors in both the SSI images and the NIMS spectra were not previously understood. In all three cases, but most distinctly at Tvashtar, the uncertainties introduced by applying thermal models developed for lava flows to lava fountains were not fully appreciated. Finally, we have found that the unique stress state of the Ionian lithosphere allows ascending magmas to be heated by as much as 200 C, as a result of viscous dissipation. Thus we now find that the observational and theoretical limits on Ionian magma temperatures are in accord.