Today, a new paper was published "in press" (accepted and revised, but not yet in a paper issue) in the journal Icarus titled, "Ground-based observations of time-variability in multiple active volcanoes on Io" by Julie Rathbun and John Spencer. In this paper, the two authors summarize the results they obtained by observing Io using NASA's Infrared Telescope Facility on more than 100 occasions between June 1997 and the end of 2005. They focus on variations in the thermal output of three volcanoes: Loki, Kanehekili, and Janus, as well as output from smaller volcanic centers like Grian Patera.
For their analysis, Rathbun and Spencer observed Io in the near-infrared at 2.26, 3.5, and 4.68 microns both in disk-resolved images while Io was in Jupiter's shadow and in sunlight. An example of an image taken while Io was in sunlight is shown at left. It was taken in November 1999 when the volcano Tvashtar Paterae erupted (seen much closer up by Galileo). In both cases (in eclipse and in sunlight), the spatial resolution of the observation is generally too low to pick up any but the brightest hotspots. The authors also measured the brightness at 3.5 microns of an eclipsed Io as it passed behind the dark limb of Jupiter. By noting the times when dips in the occultation light curve occurred, caused by Jupiter occulting a volcanic hotspot, the authors were able to constrain the location and intensity of an erupting volcano. Unfortunately, these would be one-dimensional fits of Jupiter's limb projected on the surface of Io. This method is also limited to finding hotspots on Io's Jupiter-facing hemisphere.
a very powerful eruption at Janus in January 2003. Combined with the observations of variations in the brightness of Janus and Kanehekili at shorter wavelengths by Galileo SSI and NIMS noted by Rathbun and Spencer, this indicates that the high-temperature component of the eruptions at these two volcanoes can vary greatly, even if the lower-temperature one stays comparatively consistent.
Finally, the authors examined shorter-lived volcanic eruptions from other sources they found in their data. These sources show significant variations in 3.5 micron brightness from near the background brightness to some of the brightest events seen in their decade of observing, such as an eruption of Grian Patera in June 1999. The observed variations are consistent with non-persistent volcanic activity creating fresh, cooling lava that emits light in the near-infrared. The authors noted weaker variations were observed in the mid-infrared by the PPR instrument on Galileo, which was sensitive to cooler, older lava flows.
Link: Ground-based observations of time-variability in multiple active volcanoes on Io [dx.doi.org]