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 ;)