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]

LPSC 2008: Io Eclipse Observations

Julie Rathbun and John Spencer have a poster a this week's Lunar and Planetary Sciences Conference entitled, "Io Eclipse Observations: Does Loki Dominate Io's Infrared Flux?"

Rathbun has published a few papers over the last few years on the episodic brightening experienced at Loki and what these brightenings might tell us about the Loki's eruption style. The conclusion she reached in her 2006 paper "Loki, Io: New ground-based observations and a model describing the change from periodic overturn" is that Loki Patera is a periodically overturning lava lake. According to the model presented in that paper, the surface of the Loki lava lake founders when it has cooled and thickened to the point that is negatively buoyant compared to the lava below. The time between this episodic overturning of the lava lake crust varies depending on the amount of vesicles (basically gas bubbles) within the lava crust. Essentially, the more vesicles within the lava crust, the less dense it becomes, and thus the longer it takes since the last brightening before the crust overturns again. As I mentioned in a previous blog post, Rathbun's overturning lava lake model isn't the only one published to explain Loki's behavior.

This attention focused on Loki is the result of its apparent dominance of Io's thermal flux. Rathbun and Spencer examine data taken by NASA's Infrared Telescope Facility atop Mauna Kea in Hawaii to see if Loki does in fact dominate Io's thermal flux when it is observed in eclipse. When researchers observe Io at this facility, they observe it when Io is in eclipse so they can separate flux from reflected sunlight and thermal flux from the satellite's volcanoes. If they observe Io during an occultation just as it is going behind Jupiter, or just egresses from behind Jupiter (depending on when it is in eclipse), they can measure the moon's thermal flux as it more or less of the satellite is hidden behind Jupiter. This method can help pinpoint the location of hotspots on Io's surface. Measurements were also taken of Io's full-disk while the satellite was in eclipse. Such measurements were performed at more wavelengths but their low resolution makes it difficult to pinpoint the location of individual hotspots.

Using a model Rathbun et al. developed in 2006, the authors predict Loki's brightness at 2.2 and 4.8 micron based on the volcano's brightness at 3.5 microns and the duration of the eruption episode. They determined that the 2.2 microns brightness is a close match, but they assumed that the entire thermal flux from Io at 2.2 microns came from Loki. The authors indicate that they will look at the occultation data to determine the percentage of the flux from other volcanic eruptions in order to see just how much Loki dominates Io's thermal flux.

Link: Io Eclipse Observations: Does Loki Dominate Io's Infrared Flux? [www.lpi.usra.edu]

LPSC 2008: Detailed Analysis of the Tvashtar Plume Spectral Behavior

In the second LPSC abstract highlighted on this blog, Kandis Lea Jessup and John Spencer present the work they have done on Hubble images of Io taken during last year's New Horizons encounter. In particular, they are using the images they acquired at different wavelengths with Hubble's Wide Field and Planetary Camera 2 to study the spectral behavior of the Tvashtar plume.

While the images acquired by Hubble have a lower spatial resolution than those taken by New Horizons' LORRI camera (180 km per pixel for the WFPC2 versus at top resolution of 11.2 km per pixel for LORRI), the WFPC2 has a higher spectral resolution than New Horizons' MVIC instrument, particularly at ultraviolet wavelengths which is particularly important for identifying gases within Io's plumes. Jessup and Spencer observed Io and the Tvashtar plume on multiple occassions last February, allowing the authors to examine the plume's reflectance spectra (i.e. looking at how much light reflects off the plume, which can depend on composition, particle size, and phase angle) and absorption spectra (i.e. looking at how much light from the background Jupiter passes through the plume to Hubble).

The authors found the plume to be most noticeable in both sets of observation in the ultraviolet F255W filter, indicative of S2 gas in the plume. The authors had a similar result at Pele in 2000. They do note that the Tvashtar plume has a much higher optical depth in the F255W filter than Pele.

Interesting work. They do promise to present more work on how optical depth varies by wavelength for both Pele and Tvashtar in their poster. It is so interesting to see just how similar the Pele and Tvashtar plumes despite the apparent difference in volcanic styles: Pele being a vigorously erupting lava lake and Tvashtar being a fissure eruption. Must have to do with the magmas at both locations having a higher volatile content, allowing the formation of a bright lava fountain at Tvashtar and a constantly overturning lava lake at Pele. Note the fact that as far as I know, Pele and Tvashtar are the only two volcanoes where using relatively short exposures, it is easy to see their hotspots in the daylight, at wavelengths less than 1 micron, and at relatively low resolutions.

Link: Detailed Analysis of the Tvashtar Plume Spectral Behavior [www.lpi.usra.edu]