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A.G. Davies, R.M.C. Lopes-Gautier, W.D. Smythe, R.W. Carlson (JPL-Caltech), J.R. Spencer (Lowell Observatory), Galileo NIMS Team
The Galileo spacecraft has been observing Io throughout the Prime and Europa missions. This has allowed the evolution of individual hot spots to be examined. Of all of the many hot spots on Io, Loki has been the most energetic over the last 20 years. It has been observed by NIMS on a number of orbits, with observations including Loki in darkness being collected during orbits G7, C9 and E16. Here, we present preliminary results of analyses of the NIMS night-side and eclipse observations of Loki. The 'best-fit' temperatures given below are +/- 25 K. Areas are +/- 10%. The NIMS G7 observation caught Loki at the peak of a major brightening that began soon after the E6 encounter. A two-temperature fit to the NIMS G7 data produced temperatures and areas of approximately 990 K (covering 3 km2) and 460 K (covering 2320 km2). NIMS observed a less energetic Loki during orbit C9, with temperatures of 962 K (5 km2) and 373 K (11700 km2). The G7-C9 trend might be indicative of a spreading, cooling flow. Loki brightened again in May 1998, and was observed by NIMS in June during the E16 encounter. Temperatures were similar to those obtained during C9, at 345 K (3830 km2) and 612 K (27 km2), although the area of the cooler component was very much reduced. The low-temperature component may have been stronger during orbit C9 than E16 in part because the 1997 eruption had been under way for four months by C9, whereas the 1998 eruption had been active for two months by E16. The 3.5 micron brightnesses for these three events are 80 GW/mic/str during G7, 60 GW/mic/str during C9 (both consistent with ground-based observations), and 13 GW/mic/str during E16.