Active Volcanoes on Io: Putting Ground-Based Observations of Jupiter Occultations into the PDS

Introduction: Most studies of the temporal and spatial variability of Ionian volcanoes have been forced to rely exclusively on spacecraft data, primarily Galileo [1-4], because only spacecraft data is generally available on the PDS in physical units and with relevant location information. However, ground-based observations, while at a lower spatial resolution, give the longest temporal baseline and often the highest temporal resolution of any data set. It was, for example, only the ground-based data that allowed the discovery of the semi-periodic nature of Loki, the most powerful volcano on Io [5]. Many fundamental questions about Io’s volcanoes have not been addressed because the ground-based data have not been available in an accessible public archive. Ground-based data: For more than two decades we have been observing Io from the Infrared Telescope Facility (IRTF) on Mauna Kea in Hawaii. We have observed more than one hundred occultations of Io’s Jupiter-facing hemisphere, while in eclipse, at 3.5 microns. During each approximately 5 minute long occultation event, we take as many images as the instrument allows (generally into the hundreds). During the period of June 1997 through the end of 2005, groundbased data were obtained on dates clustered around Galileo flybys of Io. In late 2006 and early 2007, we resumed our observations in conjunction with the New Horizons flyby of Jupiter [6] and in 2013 in conjunction with observations of the Jupiter system by JAXA’s EXCEED and NASA’s Juno [7-8]. Figure 1 shows the occultation lightcurves obtained on five dates in 2007 near the time of the New Horizons flyby. Each night’s occultation is shown in a different color and is offset for clarity. An occultation phase of 0.0 is the time at which the occultation begins and an occultation phase of 1.0 is the time at which the occultation ends. Each step in the lightcurve indicates a volcano or volcanoes disappearing behind Jupiter. The height of the step indicates the volcanic output and its timing gives a one-dimensional location. The solid line is a synthetic lightcurve created using observations of active volcanoes using New Horizons LEISA [9]. In the weeks leading up to and including the flyby, Io’s Jupiter-facing hemisphere was particularly dim without much activity. An eruption began sometime between the flyby and the March 31 observations. The dashed vertical lines indicate the time at which two of Io’s most active volcanoes would be observed: Kanehekilli/Janus and Loki [10]. Kanehekilli and Janus are close enough to each other that these observations cannot distinguish between them. Loki: Loki is the most continuously active and powerful volcano on Io. It is observed in nearly every one of our occultation lightcurves. During most of its history, Loki has erupted periodically (figure 2) and we have developed a quantitative model of Loki as an overturning lava lake [5, 7, 11]. More recent observations suggest that the period of Loki’s brightenings has shortened from 540 days to 420-470 days [12]. If this periodicity remains consistent for the next brightening, it should begin in late May 2018. Occultation Disappearance curves from 2007