We have obtained frequent 1.7--4.8 mu m observations of Io's volcanic thermal emission in 1995 and 1996, from the NASA IRTF on Mauna Kea and from Lowell Observatory. In 1995 there were several dramatic volcanic events, including major outbursts on the leading hemisphere in March and September 1995; one of Loki's periodic brightenings during the Fall of 1995, in the months before the Galileo Io flyby; and three high-temperature events of a few weeks' duration (in late March, July, and August) on the Jupiter-facing hemisphere. In contrast, intensive monitoring in 1996 has shown no bright volcanic events at all between early February and mid-August. High-quality IRTF observations in June 1996, near the time of the first Galileo images at the "G1" encounter, provided fluxes and locations for up to 11 faint hot spots on the Jupiter-facing hemisphere. Due to the loss of Galileo G1 NIMS and PPR Io observations, these and other ground-based observations provided our only information on Io's volcanic thermal emission at the time that the Galileo images were taken. Notable features of the volcanic emission at the G1 encounter included the following: (i) Loki's thermal emission was at the faint end of its normal range. Its 3.5 mu m flux was about 6 GW mu m(-1) str(-1) , compared to about 34 GW mu m(-1) str(-1) at the time of the Voyager 1 flyby (Pearl and Sinton 1982), and about 70 GW mu m(-1) str(-1) during the winter 1991 Loki brightening (Spencer et al. 1994). (ii) No 3.5 mu m emission was seen from Ra Patera, the site of a plume seen by Galileo, with an upper flux limit of about 1 GW mu m(-1) str(-1) . This suggests that the current Ra plume eruption is from a low-temperature source: cooler than 370 K for a source diameter of 20 km, for example. (iii) A small burst of thermal emission from Surt, with a 3.5 mu m flux of 5 GW mu m(-1) str(-1) , was seen in early and late June. Surt is not normally a site of detectable emission in groundbased observations, though it may have been the site of the much brighter eruption seen in late March 1995.
AAS/Division for Planetary Sciences Meeting Abstracts #28
- Pub Date:
- September 1996