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By Jason Perry
January 4, 2001
According to data obtained by Franck Marchis, Renée Prangé and Thierry Fusco, November 2000 was busy month for volcanism on Io. As part of an observing campaign in conjunction with the Cassini flyby of Jupiter, these astronomers observed Io using the ADONIS Adaptive Optics system on the 3.8 m telescope at the European Southern Observatory in La Silla, Chile on November 15, 2000. In five hours of observation time that night, the team saw several hot spots, two of which had never been seen before. Three usuable images came out of these observations and they can be seen below. They were taken in the L' band which is centered around 3.8 µm. Figure 1 shows these three images. The multiple images on one night allowed the astronomers to measure each hot spots center-to-limb variation. This variation is due to line of sight issues as a volcano nears the limb as seen from Earth. The team also used these images as an opportunity to increase their spatial converage which includes previous observations runs in 1996, 1998, and 1999. The November 2000 observations also add to data already released from December 2000 showing a new eruption at Tvasthar Catena.
The most recognizable hot spotin these images was Loki with a vertical brightness (flux of the volcano if it were at the center of the disk) of 85 ±10 GW/Sr/µm. This is similar to Loki's activity in September 1998 when Loki was in the waning stages of a major eruption. This brightness is higher than that seen by John Spencer et al. in late December 2000. By repeatedly measuring Io's brightness at 3.5 um as it emerged from behind Jupiter, they were able to measure Loki's brightness as that volcano emerged from behind Jupiter. They found its brightness to be 32 GW/sr/µm, slightly lower than in November. Other known hotspots are seen including Daedalus, Janus-Kanehekili (actually three seperate volcanoes but due to the spatial resolution of the ADONIS images, they appear as one hotspot), Hi'iaka, and Gish Bar. The Loki hot spot may also be a conglomerate hot spot. The hot spot does not follow a typical cosine theta law meaning that another volcano's flux may be included in the Loki hot spot. This may explain the discrepancy between the November 2000 brightness data and the December 2000 brightness data. Janus-Kanehekili was found to have a flux of 46 ±10 GW/sr/µm.
Two previously unknown volcanoes were also discovered. 0011A, the first new hotspot discovered in November 2000, is located at 27 ±10 W, 20 ±7 N. As seen in figure 2 below, three dark spots are seen in that area with one being very close to the central coordinates of this area. A known hot spot, Ukko Patera, is just north of the uncertainty area. The other new hot spot, 0011B, is located in Io far north at 55 ±15 W, 58 ±15 N. Figure 3 shows this area in visible light. A previously known is seen in this area, 9610A. This lava flow saw a very bright eruption in October 1996. Due to the uncertainties of the 0011B position, it is not known if this is the site of the eruption.
The data published on the web today gives the ADONIS team additional spatial coverage in addition to more hot spot discoveries. The 0-70 W longitude region had previously only been views near the limb. Comparisons for this region will thus have to wait until additional data can be taken. More observations from the November 2000 observation run are being processed and will be released soon. These images could help to determine the nature and temperature of lava seen at these eruptions.
More information on these observations can be found at http://astron.berkeley.edu/~fmarchis/Io_OA/Run2000/ . Additional images can be found there.
Figure 1
Images from November 2000 observing run. The major bright spot in the first and third images is Loki and the major bright spot in the middle image is Janus-Kanehekili. 0011A is just north of the Janus-Kanehekili hot spot in the middle image. Io geometry for the first image can be found here. Io geometry for the middle image can be found here. Io geometry for the last image can be found here.
Figure 2
0011A uncertainty area in visible light. Three dark calderas are seen in this region. The one in the center is the most likely to be the origin for the eruption.
Figure 3
0011B uncertainty area in the visible light. The dark region in the upper right corner had not yet been imaged at the time this map was made. One known hot spot is seen in this region in the upper right. This dark lava flow may have been active in October 1996 in a ver bright eruption known as 9610A.
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