JOVIAN SYSTEM DATA ANALYSIS PROGRAM PROPOSAL SUMMARY

The Galileo NIMS and SSI experiments provide unique high spatial resolution and high sensitivity measurements at wavelengths shortward of 5 micrometers of the hottest components of the thermal emission from Io's volcanos. But the peak of the thermal emission spectrum for all eruptions that last longer than a few days lies in the mid-IR near 10 micrometers wavelength, in a spectral gap between the longest wavelength detected from NIMS (5 micrometers) and the shortest discrete filter in the PPR (17 micrometers). This proposal is focussed on combining the Galileo measurements with earth-based near- and mid-IR measurements, especially the series of satellite occultations of Io acquired by the PI during 1997 under a Planetary Astronomy Program grant, to quantify the full thermal emission spectrum for each active eruption observed on Io during the Galileo primary mission. Recent advances in modelling of Io's volcanic thermal emission (e.g. Howell, 1997 features only 2 parameters, a resurfacing rate and an "age") show that at wavelengths shorter than 5 micrometers, the thermal emission spectrum reaches an "equilibrium" steady state after the eruption is only a few days old. The spectrum in this near-IR region then depends only on the eruption rate at which new hot surface is created. In contrast, the flux in the 10 micrometer mid-IR spectral region increases steadily with the duration of the eruption for hundreds of days and provides a sensitive measurement of the total area of accumulated, cooling flows that radiate the lion's share of energy and indicate the total volume of material erupted, critical information for estimating the amount of gas liberated or the total energy available for radiation brought to the surface by emplacement of hot mass. We have a unique opportunity to fill in the missing mid-IR component of the spectra for the known active volcanos detected during the Galileo prime mission by combining the ground-based mid-IR satellite occultation measurements acquired between March and December 1997 during the prime Galileo mission. The satellite occultation data cover most of Io's surface with spatial resolution that in many cases exceeds that of the NIMS instrument pixels allowing the 10 micrometer region flux from individual volcanos to be measured. Some of the questions that will be addressed include: 1) How do the spectra of the active eruptions compare with models (e.g. Howell, 1997)? Howell found that Voyager IRIS spectra of Loki and several other hot spots could be analyzed with his model, but that Pele showed a large deficiency in the mid-IR flux expected in comparison the the near-IR flux. 2) Is there evidence for different "classes" of eruptions based on differences in the emission spectrum, like the Loki/Pele contrast described in 1, and which eruptions can be assigned to which class? 3) How do these classes relate to the morphology and albedo changes of the erupting region, e.g. pyroclastic halos, etc.? 4) Galileo detects numerous small eruptions that are bright at the short wavelengths of SSI and NIMS. The occultations will be used to measure the mid-IR flux from many of these, or at least determine a firm upper limit, that can be directly interpreted as a maximum duration and hence a total flow area in terms of the Howell model. How much material is being contributed by these small eruptions and how does the integrated total compare to the massive 1997 Loki eruption which is very well quantified by the occultation and Galileo data? 5) For many individual eruptions, the combined Galileo and ground-based data sets will provide a time sequence of measurements at multiple wavelengths. By putting these together within the context of a model that predicts the spectral evolution of an eruption, we will learn in unprecedented quantitative detail about Io's active volcanos at the Galileo epoch, their characteristics, their energetics, their importance to other phenomena in the Jupiter system, and their comparison to terrestrial volcanos.