Volcanic Activity on Io during 1987{1992: 4.8-m Photometry

Over the past 6 years we have conducted an extensive monitoring program of Io at near infrared wavelengths, in order to determine the location, frequency, and duration of the volcanic activity on this satellite of Jupiter. The observations were obtained at the Wyoming 2.3-m telescope using a near-infrared speckle interferometry system that provides standard photometry and in addition can locate bright hotspots on the disk of the satellite. Most observations were made at a wavelength of 4.8 m. In this paper we concentrate primarily on analysis of the photometric data. The photometry provides a clear record of the variable activity at Loki. This source seems to brighten every couple years, and remains in the active state for a few months. We have seen no activity of comparable brightness at other longitudes. Preliminary models of the activity at Loki suggest that these brightenings correspond to eruption rates and durations similar to that during emplacement of the individual ows which make up the Columbia River Flood Basalts. Although we do see \brightenings" of Loki, in the 96 nights with reliable observations we have observed no \outbursts" where the brightness of Io would increase by a factor of two or more. When we combine our lack of detected outbursts with the number of outbursts seen by other observers we obtain the value of 2% for the fraction of nights on which outbursts are visible. However, this high of a frequency is diicult to reconcile with our lack of detections. We have also determined more conventional photometric properties of the satellite. The solar phase coeecient for the leading side is 0.018 magnitudes/degree. We detect a smooth rotational lightcurve for the leading hemisphere which peaks very close to longitude 90 0. It may be related to albedo patterns on the surface. The close coincidence of the peak to the apex of the motion suggests that external innuences may be responsible for that pattern.