Today I thought I would write a post on Io's class of giant, volcanic plumes. These shield-shaped clouds of gas and dust are generated at sites of vigorous volcanic activity reaching upwards of 500 km above Io's surface. These plumes are one of two basic classes of volcanic debris clouds observed at Io. The smaller, Prometheus-type plumes are generated as lava flows burn off volatiles such sulfur dioxide.
While Prometheus-type plumes are comparatively common, Three volcanoes have had observed giant plumes: Pele, Tvashtar Paterae, and Grian Patera. These three plumes share a number of characteristics. The first obviously is their large size. They reach altitudes in excess of 350 km above the surface and can even reach altitudes as high as 500 km. The observed plumes are generally 1100 km in width, or 550 km from the source vent. Secondly, these plumes rarely exhibit a central column, like Prometheus-type plumes. So the plumes appear more like a shield, rather than an umbrella. In Spencer et al. 2007, this difference in shape was interpreted as being caused by different sources for the particulates in these plumes. In Prometheus-type plumes, the dust is ejected along with the gas from the source vent(s). In giant plumes, the dust is condensed from the gas when it reaches the shock canopy, the point when the ejected material stops ascending and begins to fall back to the surface. As you can seen in the movie above of Tvashtar's plume, this dust can form large clumps, likely due to static electricity (resulting from the Io-Jupiter flux tube). High-resolution images of Pele's plume from Voyager also show these clumps at that site. The amount of condensed dust is generally a function of the amount of gas in the plume, which can vary. Less dense plumes, like Pele and Grian, can make these plumes appear quite faint at visible wavelengths, to the point where the plume is considered stealthy. Observations at high phase angles or at ultraviolet wavelengths enhance our ability to see these types of plumes. A third characteristic of these plumes is their sulfur-enriched composition. These plumes have much higher S2 contents compared to Prometheus-type plumes, whose gaseous composition is almost entirely sulfur dioxide. Spencer et al. 2000 showed that the plume at Pele is composed of about seven parts SO2 and one part S2. However, the S2 to SO2 ratio can vary at these plumes from between 0.01 to 0.3. Tvashtar in 2007, for example, had a ratio closer to 0.01-0.02.
Giant plumes are often associated with outburst volcanic eruptions. These eruptions have vigorous outpourings of lava and usually have a large lava fountain or curtain at the vent. Giant plumes are thought to form from gas dissolved from these lava fountains. This was particularly the case for Tvashtar in 2000 and 2007 and at Grian in 1999. Pele is an exception to this, however. The source for Pele's plume is a vigorously overturning lava lake. The lava crust is broken up in a number of places at any given time and gas is dissolved from the lava from these cracks.
A final characteristic of Io's giant plumes is the large, red plume deposits they form. Unlike the deposits surrounding Prometheus-type plumes, these giant red rings are often oval in shape, with greater north-south radii. The plume deposit from Grian Patera can be seen as a red-orange ring near the limb in the image at right from near the tail end of that eruption. Their distinctive appearance also allows scientists to identify additional giant plume sites even if the plume itself was not observed. This includes Aten Patera and Surt by Voyager 2 in 1979 and Dazhbog Patera, Surt, and a deposit near 70 North, 55 West by Galileo.
A factor in the small number of detection is their transience (though again, because of the difference in eruption style, Pele is the exception). The deposit at Grian is a perfect example. The 1999 eruption of Grian Patera began shortly after June 8 of that year (as reported in Howell et al. 2001). It reached a peak brightness on June 22 and was observed by Galileo both in daylight and in eclipse on July 3. It was during these observations that Galileo observed a hotspot at Grian, as well as a giant plume and a red ring deposit surrounding Grian. A dark deposit also surrounded the source patera. During the next perijove in mid-August, the deposit had faded completely as the S2 in the deposit had recombined into the more stable, S8 form of sulfur.
I hope you all enjoyed this little discussion of giant plumes on Io. I should probably write more blog posts like this.