Paper: Ground-based observations of the variability of Io's volcanoes

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.

Three of the most persistent hotspots on the sub-Jupiter hemisphere are Loki, Kanehekili, and Janus.  Rathbun and Spencer used their eight-year span of ground-based observations to chart variations in the amount of energy (in terms of Gigawatts) output by these volcanoes.  Loki, Io's most powerful volcano, experienced periodic increases in power output between 1990 and 2001.  In 2002, Rathbun and her colleagues suggested that this periodicity was due to a crust over a large lava lake foundering after becoming too thick, starting a wave of overturning crust that spreads counter-clockwise around the patera starting from the southwest corner of the volcano.  However, the authors note in this paper that this pattern ended after 2001 (around the time Rathbun published her paper describing the periodicity) as Loki's power output leveled out in 2001-2002 a bit below the average between the earlier active and inactive episodes, before weakening between 2005 and 2007.  Their extended history of Loki observations suggests that there have been no brightening events since 2001.  The authors concluded that the measured brightness of Loki at 3.5 microns, and the derived brightness at 2.26 and 4.68 microns (taken by subtracting the total power output of Io in eclipse when Loki is shown by the occultation data to be inactive from the power output of Io when Loki is active) is consistent with the author's thermal model of Loki.

Kanehekili and Janus are two volcanoes on Io's leading hemisphere located within Media Regio.  Ground-based observations by Rathbun and Spencer were unable to distinguish activity between these volcanoes are their proximity and Galileo observations of both of them as persistently-active volcanoes. The authors found that the 3.5 micron brightness of the region containing Janus and Kanehekili remained fairly consistent between 1996 and 1998 at a level similar to that of Loki in 2003 and 2004, before trending downward.  A significant increase was observed early in 2002, though the authors couldn't distinguish between an increase in activity at either volcano, or another volcano at that longitude.  I will point out that Marchis et al. 2005 observed a fairly bright hotspot at Janus in December 2001 using the Keck telescope, a few months prior to the Rathtbun and Spencer observations, and 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]