Tuesday, June 16, 2009

1979 Outburst Eruption of Surt: 30 Years Ago

On June 11, 1979, astronomer William M. Sinton observed Io and the other Galilean satellites using an infrared photometer at the University of Hawaii's 2.2-meter telescope at the summit of Mauna Kea. These observations were made as part of a three-day campaign to confirm Io's excessive brightness at 5 microns observed by Witteborn et al. 1979 by making observations of Io and the other Galileans using the photometer's M filter. Sinton published the results of these observations in the Astrophysics Journal the next year. While Sinton's June 10 observations didn't reveal excessive emission from Io at 5 microns, his observations the next day revealed a nearly 4x increase in the near-infrared flux of Io compared to the previous day, an increase he attributed in another paper from 1980 to a vapor explosion on Io's sub-jovian hemisphere that unroofed a liquid sulfur lake with a surface temperature of 600 K. Currently, such an observation would be reinterpreted as the either the sudden breakup of the crust of a silicate lava lake or the high-flow rate effusive eruption. Based on cooling models created by Ashley Davies et al. in 2005, the 600 K reported by Sinton 1980 would represent lava that erupted onto the surface (or was unroofed by a lava lake overturn event) an average of six hours prior to the observation (though with a single wavelength, we probably can't even say that much as this is a brightness temperature). Keep in mind that the eruption was still likely ongoing, but with a small area of very hot and fresh lava near the vent (or flow front) and larger areas of cooler lava.

As this eruption occurred between the two Voyager encounters in 1979, the location of the eruption could be surmised by looking for surface changes in the hemisphere. Looking at the region observed by Sinton on June 11, 1979, one surface change stands out, at the volcano Surt on Io's northern sub-Jupiter hemisphere. These changes, shown at left in the left-hand pair of images, included a darkening of the western half of the 75-kilometer wide Surt volcanic pit (shown above in an image taken by Voyager 1) and a faint, Pele-class plume deposit circling the volcano (marked by the purple arrows in the image at left). This suggests that the eruption at Surt in June 1979 involved a substantial outpouring of lava (or the violent overturning of an otherwise quiescent lava lake) and a Pele-type plume, the larger of the two primary plume types on Io. The fact that the plume deposit appears so faint compared to more prominent red rings seen at other Pele-type plumes like Pele, Tvashtar, and Dazhbog suggests either a low amount of sulfur and sulfur dioxide was exolved from the erupting magma or the eruption had a very short duration (a few days), or a combination of both. The rapid decay of Surt's 5-micron emission, as observed by Sinton, definitely suggests that the eruption was short-lived. Either way, very little sulfur and sulfur dioxide was deposited as a result of the 1979 Surt eruption compared to other eruptions with Pele-type plumes. Similar changes were observed at Surt following a bright eruption in February 2001 (the most powerful eruption in terms of power output ever observed), suggesting short duration and/or low volatile content eruptions are typical for Surt.

The Surt eruption in 1979 was the first Ionian volcanic eruption to be observed from Earth following the phenomenon's discovery in March 1979 by Voyager 1. It showed that the occasionally near-infrared brightenings first reported shortly before the Voyager 1 encounter are the result of massive volcanic eruptions on Io and provided astronomers on Earth the encouragment to monitor Io for similar eruptions while scientists waited for the next spacecraft to arrive at Jupiter. Such observations during the 1980s provided the data needed for a paradigm shift from the idea of Ionian volcanism being primarily sulfurous in composition to primarily silicate.

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