G29i31

During orbit 29 in late December 2OOO, Galileo and Cassini obtained complimentary coverage of Io, with Cassini providing better spectral and temporal coverage thanks to its more sophisticated imaging system and higher bandwidth and Galileo providing

Figure 3.7. Highlights from the I27 fly-by. (A) False-color view of Tvashtar Paterae from the TVASHT01 observation. (B) NIMS observation of the Pele caldera overlain on a false-color image from Voyager 1. (C) Map of night-time temperatures of Io's trailing hemisphere taken by the PPR instrument. (D) CAMAXT01 observation merged with color from orbit 21. (E) Partial frames from the CHAAC01 observation. Frames showing the north-east margin of Chaac Patera are seen at the top while frames showing the floor and the south-west margin are seen at the bottom. (F) PR0MTH01 observation. A dark flow with two spots of incandescent lava is highlighted to the right (NASA press release images PIA02550, PIA02560, PIA02548, PIA02566, PIA02551, PIA02564). (See also color section.)

Figure 3.7. Highlights from the I27 fly-by. (A) False-color view of Tvashtar Paterae from the TVASHT01 observation. (B) NIMS observation of the Pele caldera overlain on a false-color image from Voyager 1. (C) Map of night-time temperatures of Io's trailing hemisphere taken by the PPR instrument. (D) CAMAXT01 observation merged with color from orbit 21. (E) Partial frames from the CHAAC01 observation. Frames showing the north-east margin of Chaac Patera are seen at the top while frames showing the floor and the south-west margin are seen at the bottom. (F) PR0MTH01 observation. A dark flow with two spots of incandescent lava is highlighted to the right (NASA press release images PIA02550, PIA02560, PIA02548, PIA02566, PIA02551, PIA02564). (See also color section.)

better spatial coverage since it observed Io from a much closer distance. Cassini observations, shown in Figure 3.8(A), revealed a new plume at Tvashtar Paterae, one of the few found over a polar region (Porco et al., 2003). Low-phase Galileo color observations, with resolutions between 11 and 17 km per pixel (shown in Figure 3.8(B)), revealed a red plume deposit, similar to that seen around Pele, surrounding Tvashtar, forming a ring 1,440km across (Turtle et al., 2004).

Following orbit 29, the spacecraft continued to perform well, except for an anomaly that cropped up, starting in orbit 28, which caused the loss of a number of SSI images. A further extension mission was granted, including a fly-by of Callisto in May 2001, three fly-bys of Io in August and October 2001 and January 2002, and a

Figure 3.8. Highlights from orbit 29 and I32. Both (A) and (B) highlight a new eruption at Tvashtar observed during late 2000. The two figures in (A), enhanced images from the Cassini spacecraft, show a 385 km tall plume over Tvashtar as well as the plume over Pele. As seen in (B) from Galileo, both plumes have formed large red ring deposits. Panels (C-F) show highlights from the I32 fly-by. Both (C) and (D) show a new eruption at Thor, first seen by NIMS during I31 and in distant observations from the same orbit. Image (C) is taken from the TERMIN02 observation while (D) is a 13-16 km per pixel observation from NIMS. (E) Color observation of Tupan Patera, from the observation TUPAN_01. (F) Frame from the observation GSHBAR01, revealing fresh lava flows on its surface. (See also color section.)

Figure 3.8. Highlights from orbit 29 and I32. Both (A) and (B) highlight a new eruption at Tvashtar observed during late 2000. The two figures in (A), enhanced images from the Cassini spacecraft, show a 385 km tall plume over Tvashtar as well as the plume over Pele. As seen in (B) from Galileo, both plumes have formed large red ring deposits. Panels (C-F) show highlights from the I32 fly-by. Both (C) and (D) show a new eruption at Thor, first seen by NIMS during I31 and in distant observations from the same orbit. Image (C) is taken from the TERMIN02 observation while (D) is a 13-16 km per pixel observation from NIMS. (E) Color observation of Tupan Patera, from the observation TUPAN_01. (F) Frame from the observation GSHBAR01, revealing fresh lava flows on its surface. (See also color section.)

fly-by of Amalthea in November 2002. Following the Amalthea fly-by, Galileo would then be put on a collision course with Jupiter, to prevent contamination of Europa from potential microbial stowaways on Galileo. 0bservations during the first orbit of this new extension mission, orbit 30, were designed to look at the anti-Jovian and leading hemispheres at low-phase to search for changes, examine the new Tvashtar plume deposit found during 0rbit 29, and to characterize the volcanic features seen near 50° west longitude which had not been well observed by Galileo (or Voyager for that matter). Unfortunately, the SSI anomaly that started in orbit 28 occurred again during orbit 30, causing the loss of all orbit 30 images of Io.

I31 was a north polar fly-by of Io that took place on 6 August 2001. Closest approach occurred at a distance of 194 km above the surface of Io near 78 ° north latitude, 172° west longitude. Magnetometer measurements were performed and the results were particularly important, as they demonstrated that there is no internally generated magnetic field at Io. At a fly-by altitude of only 194 km, the spacecraft trajectory took it directly over Tvashtar, which had been recently active. Images and spectra showed that Tvashtar was probably quiescent at the time of Galileo's overflight. Nevertheless, detections by the PLS instrument of tenuous gases in the area indicated that another plume was active, and represent the first in situ detection of its kind of emissions from an active region not on Earth.

Imaging plans called for high-resolution observations of the I25 Tvashtar eruption site and other features (Turtle et al., 2004). Unfortunately, despite commands designed to fix the SSI anomaly seen since orbit 28, the problem appeared again, causing the loss of all SSI images except the low-resolution observations. These observations, combined with those taken by NIMS, revealed a major new eruption at a volcano later named Thor. SSI observations at 18 km per pixel prior to the encounter revealed a 500 km tall plume over Thor, the tallest plume ever seen at Io in reflected sunlight (Turtle et al., 2004). It quickly became clear that this was the plume whose gases were sampled by the PLS instrument. Low-resolution images of Thor during this fly-by revealed a new dark deposit surrounded by a white plume deposit, where no major activity or dark material was seen previously (Geissler et al., 2004). NIMS regional observations revealed a strong thermal emission source at Thor during I31 (Lopes et al., 2004) which pinpointed the location of the erupting plume. NIMS observations also revealed nine additional, previously undetected hot spots. Despite not seeing a plume or new plume deposit in the polar regions of Io during much of the Galileo mission (save perhaps one possible north polar detection in 1997), SSI low-resolution observations also revealed a new red ring plume deposit around Dazhbog Patera and additional red plume deposits at Surt, in addition to the deposits surrounding Tvashtar first seen on orbit 29 (Geissler et al., 2004). This brought the total of plume deposits north of 40° north latitude to four in only a matter of a few months, where maybe only one had been seen during the rest of the mission. PPR obtained global and regional maps of nightside thermal emission from the volcanoes and passive surface, and its best-ever map of daytime thermal emission and surface temperatures (Rathbun et al., 2004).

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