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 2009: Ground-based Observations of Io

Next up in our coverage of the Io abstracts submitted for next month's Lunar and Planetary Science Conference is "Ground-based Observations of Io in Support of the New Horizons Flyby" by Julie Rathbun and John Spencer. The authors examine data they acquired at the Infrared Telescope Facility (IRTF) in late 2006 through mid-2007 as part of a campaign to take complimentary data with the New Horizons spacecraft.

Rathbun and Spencer acquired observations during 21 night between August 2006 and June 2007. These observations consisted of sunlit images, like the one at right acquired in November 1999, eclipse observations (see the post right below this one for similar observations taken in 2000 by Cassini), and occultation lightcurves. Data from these nights were taken in the near infrared at 2.2, 3.5, and 4.8 microns, which can be useful for look for thermal emission as well as measure color temperatures (though with a low-end wavelength of 2.2 microns, it would miss the hottest components).

For the sun-lit observations, both reflected sunlight and thermal emission were observed, but these images were useful for expanding longitude coverage. Super-resolution processing of this data also improved the spatial resolution of the sunlit data. Rathbun and Spencer found a hotspot at Tvashtar, showing that the eruption there so vividly seen by New Horizons was ongoing as far back as January 18, 2007 (Tvashtar had been known to be active as early as mid-February 2007 based on HST data).

The eclipse and occultation curve observations have the benefit of excluding the contribution of reflected sunlight, but they are limited to the pro-jovian hemisphere. Even so, several additional active volcanoes were detected, such as Masubi, Kanehekili, and Dazhbog (perhaps east Dazhbog, a surface change seen by New Horizons). Occultation curves are disk-integrated brightness measurements taken as Jupiter passes between Io and the observer. As more and more of Io is covered by Io (or less and less in the case where Io is passing from behind Jupiter), thermal emission from individual volcanoes blink out (or blink in depending on whether you are looking at ingress or egress), which appears as a "step" in the disappearance/reappearance light curve. With accurate knowledge of the position of Jupiter and Io at the time of those steps, the location of Jupiter's limb at the time of that step can be plotted as a line on a map of Io. The active volcano making that "step" should then be on that line. Having both reapparance and disappearance light curves can further improve hotspot identification because the respective mapped curves should cross at the position of the active volcano. Rathbun and Spencer don't seem to have performed that analysis (or they don't have both disappearance and reappearance light curves from the same occultation).

By extending our knowledge of Io's active volcanism during the New Horizons epoch in time, we can place New Horizons results in better context. For example, we can tell that the Tvashtar eruption NH observed have been going on for a month and a half by the time the spacecraft observe the large plume there. The authors hope to compile similar data for the three volcanoes identified in the occultation curves and eclipse data and see how activity at those volcanoes varied during the New Horizons epoch.

Link: Ground-based Observations of Io in Support of the New Horizons Flyby [www.lpi.usra.edu]

Io DPS Talks

The Galilean Satellites session at the DPS meeting was held this morning in Ithaca, New York. The talks were also online as a webcast, allowing me to view (and all of you) to view the talks despite not being at the conference. The talks mostly focused on the icy satellites of Jupiter, particularly Europa and Ganymede, but two talks covered Io specifically. The first was given by Julie Rathbun (with co-author John Spencer) and was titled, "Loki, Io: Fitting a lava lake model to Eclipse Observations" (link takes you the abstract). The second was given by Nick Schneider (with coauthors C. Grava and C. Barbieri) and was titled, "Unusual Velocity Structures of Neutral Sodium Near Io's Wake."

Rathbun presented ground-based data of Io at multiple wavelengths in the near-infrared portion of the spectrum. This was done to see if the lava lake crust floundering model for Loki's eruption behavior was supported using multi-wavelength observations.

Ground-based observers have been monitoring activity at Loki Patera, the largest volcanic depression on Io, since 1988. This observation campaign has revealed that Loki goes through a cycle of activity, with periods of high-thermal emission (also called brightenings) and low emission. The Rathbun model suggests that this cycle is related to the style of activity at Loki. She (and her co-authors) propose that Loki Patera is a large lava lake, a depression filled with molten lava and covered by a thin crust of porous, solidified lava. Over time, this crust thickens to the point where the crust starts to collapse. This collapse occurs as a wave, moving from the southwest margin of the patera then counter-clockwise around the interior "island" to the northwest margin. A new thin crust forms behind this collapse wave, and is allowed to thicken until it is no longer bouyant over the molten lava below.

To test to see if this model is supported at multiple wavelengths, Rathbun examined disk-resolved Io images taken at NASA's Infrared Telescope Facility (IRTF) at 2.26 μm and 4.78 μm (similar to the image at right), to go along with the 3.5 μm observations used to develop their lava lake model. Using the model, which takes into account the average duration of a brightening event and the average peak 3.5 μm brightness during these events, they can predict the brightness of Loki at the other two wavelengths and the amount of power output in Gigawatts per micron per steradian. For the 3.5 μm observations, Rathbun and Spencer used occultation light curves, disk-integrated measurements of Io's brightness as it either leaves or enters Jupiter's shadow. Knowing the position of Io and the timing of these measurements, the authors can extract a position for any thermal emission source seen in the lightcurves.

For the disk-resolved images at the other two wavelengths, Rathbun and Spencer had to subtract the contribution from the other volcanoes on the sub-Jovian hemisphere to obtain an estimate for the brightness of Loki. Rathbun accomplished this by comparing global brightness measurements derived from the IRTF images between periods when Loki was active and when it was inactive. By subtracting the average global brightness between those two periods, she could get an estimate of Loki's average brightness during a brightening at 2.26 μm and 4.78 μm. The estimates had pretty large error bars, but the estimates seem to fit the predicted values from her lava lake model. This technique was also performed with IRTF observations at 3.5 μm, and they fit the occultation light curve measurements.

Rathbun and Spencer plan to compare the 2.26 μm estimates to a couple of lightcurve measurements at 2.2 μm accomplished during the Galileo mission. They also plan to look at the individual observations from the Galileo era when they had great temporal resolution.

The other talk, by Nick Schnieder, covered "Unusual Velocity Structures near Io's Wake." Io's atmosphere (and ultimately its volcanoes) supplies material for various structures in Jupiter's magnetosphere. Schneider used a spectrograph at the Telescopio Nationale Galileo in the Canary Islands to observe the various escape features for sodium in the banana-shaped neutral cloud that surrounds Io as Io went into and out of Jupiter's shadow. These include streams of fast moving sodium atoms from the neutral cloud and jets of sodium from Io's ionosphere. Schneider's observations revealed an additional escape mechanism. In this case, sodium jets away from Io toward Jupiter at only 15 km/sec. This suggests the sodium originates on the Jupiter-facing hemisphere and is perhaps limited to the leading hemisphere. How these jets are generated has not been determined. However, this new sodium features may provide a new way to study Io's volcanism, atmosphere, and plasma environment from Earth.

That finishes up the Io talks for DPS. Hopefully, AGU and next LPSC will provide more geology ;)

Io at DPS

The abstracts for next month's Division of Planetary Sciences Meeting are now online. Several talks and posters are about everyone's favorite moon. The abstracts for DPS (and the upcoming AGU meeting in December) are much shorter than those submitted for LPSC, which would be better described as mini-papers. So there is less to say about the abstracts themselves. A few abstracts do stand out:

• Julie Rathbun and John Spencer have a talk scheduled for the morning of October 15 titled, "Io Eclipse Observations: Determining the History of Loki’s Flux at Multiple Wavelengths." The abstract just summarizes Loki's importance to Io's total heat flux and Rathbun and Spencer's theory for Loki's observed activity. The talk will cover their work on separating Loki's contribution to the observed heat flux from the other volcanoes on the sub-Jovian hemisphere. They will then discuss Loki's extracted brightness at three wavelengths in the near-infrared.
• Erinna Chen et al. will present a poster covering her group's summer school Team X project, a New Frontiers-class Io mission concept. The mission, which they call Argus, would use a high-inclination orbit around Jupiter and would flyby Io at least 40 times. The mission would use Advanced Stirling Radioisotope Generators (ASRG) as a power source, pushing the mission to after Discovery 2013. The payload included in the study would consist of a narrow-angle camera, a thermal imager, an NIR spectrometer, a UV spectrometer, and an INMS-like instrument, similar to the IVO Discovery mission concept, but with more advanced instrumentation and 4-10 times the number of Io flybys, so presumably the increased cost between the Discovery mission concept and this New Frontiers mission would be taken up in increased payload size and radiation shielding.
  

While I will not be at the conference, I am a co-author on three abstracts: "Evidence for Past Lake-Level Change in Titan's Ontario Lacus" by Jason Barnes et al., "Tiger Stripes and Cassini ISS High-Resolution Imaging of Enceladus" by Paul Helfenstein et al., and "Cassini Imaging Observations of Titan’s High-Latitude Lakes" by Elizabeth Turtle et al. Of these, I spent the most time working on the last one, and includes the coolest discovery we have made at Titan from Cassini images:

Differences between the two ISS observations may be due to changes on the surface as a result of precipitation from a large cloud system observed in Fall 2004 (Schaller et al., 2006), although diffuse clouds or atmospheric scattering could also play a role.

The meeting will occur between October 10-15 in Ithaca, New York on the Cornell University campus.

Link: Division of Planetary Sciences Meeting 2008 [dps08.astro.cornell.edu]

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]