Davies et al. 2001). SSI images of Io's trailing hemisphere in July 1999, shown at left, showed slight changes in the Pillan pyroclastic deposit surrounding the eruption site. Most of the western half of the deposit had been subsequently covered over by reddish sulfur from Pele and the northern half was covered up by bright sulfur dioxide frost, likely from sapping from Pillan Mons. Finally, portions of the eastern half of the deposit were covered up by dark pyroclasic and bright plume deposits from the nearby Kami-Nari volcano. These changes further showed that Pillan was much quieter in terms of activity than it was during the eruption in 1997.
|13-frame mosaic covering the Pillan flow field. Full-resolution version also available.|
The rough texture of the lava surface may have resulted from a number of factors, including the interaction between the hot lava and the cold, volatile-rich surface it flowed over, turbulent flow, and the disconnect between the high effusion rate (the volume of lava flow for each meter of the vent fissure) and the speed of the flow front. In the first case, gas created by the heating of sulfur dioxide frost by encroaching lava flows would burst through the cooled crust of the lava flow. This action would disrupt the cooled lava crust and may form rootless vents that provide gas for the plume seen over Pillan in 1997. One such put can be seen in the left side of the mosaic. Turbulent flow within the lava flow would cause any cooled crust that may have formed to break up into blocks or rafts. These rafts can also disrupt the cooled crust as it is moved downstream by the still molten lava beneath. This can cause gouges to form in the flow field, like the one seen below and to the left of the dark pit on the left side of the mosaic. In the final case, the high lava effusion rate would lead to a crumpling of the lava crust as the flow front advanced at a speed of "only" 0.3 to 1 kilometer per day, the result of the leading edge giving up heat to mobilize surface frost. This would create the rubbly surface texture seen in the October 1999 images.
Marchis et al. on June 28, 2010. This last observation suggests that Pillan may currently be coming down from a recent eruption.
All was quiet at Pillan during the New Horizons encounter in 2007. The fading of the pyroclastic deposit from 1997 was complete by that point, and the floor of Pillan was once again covered in a thin coat of sulfur frost from the Pele plume, making it show up bright in the LORRI camera images. Only the still dark lava flow north of Pillan Patera suggested that anything had happened at Pillan between 1979 and 2007. No thermal hotspot was observed at Pillan by either LORRI during two eclipses of Io by Jupiter or by the LEISA near-infrared spectrometer.
In 1979, Pillan was a quiet volcano that was not even worthy of a name. But in 1997, a major eruption there allowed scientists to track the changes it created on Io's surface at a close range. The eruption became the archetype for fissure-fed eruptions on Io, producing large-scale surface changes, areas of incandescent lava, and lava fountains at the source vent. Similar, even more powerful eruptions would later be seen at Surt and Tvashtar. Potentially, the East Girru eruption seen by New Horizons may be most similar to what happened at Pillan in 1997, though the short duration of the encounter prevented follow-up observations of East Girru.
Thanks for reading this week's premiere of my "Io Volcano of the Week" series! If you haven't already, I encourage you to read Parts One and Two of my profile of Pillan. Next week I will be profiling a volcano that should only require one post to discuss: Zal Patera.