Today, we finish up our look back at Galileo's I24 flyby of Io that occurred 10 years ago last Sunday. I had hoped to get this done a few days ago but present day encounters of Titan and Tethys had taken priority. While last week's Cassini encounters felt almost routine, though you can never presume that just because you've seen an area many times before that you won't learn something new, the flyby we've been looking back at here in this blog was definitely not so. In our last installment, we saw how Galileo's engineers and scientists had to overcome the spacecraft going into safing hours before the flyby was to occur, scrambled images, and a stuck spectrometer grating to pull off a fairly successful flyby.
In this final installment, we take a look at some of the data that was returned by Galileo and what it taught us about Io. To put this data in a kind of video timeline, I created a Youtube video using Celestia and put together in Adobe Premiere Pro that I posted last week. Check it out if you haven't done so already.
Despite the degradation of the majority of images acquired by Galileo during orbit I24, many of these images were still usable following a reconstruction effort at JPL using an alorithm in National Instrument's LabVIEW software. For example, cooled lava flows, pits, and channels are visible in this 13-frame mosaic covering portions of the Pillan lava flow. The eruption that formed much of the terrain visible here occurred two years earlier in a massive event that produced not only a 3100 sq. km lava flow, but also a huge pyroclastic eruption. Lava from the eruption would then cascade over the edge of Pillan Patera (to the southwest of the area seen in the mosaic) to cover the floor of that depression with dark basaltic lava. The rough texture, pits, and channels seen across portions of the flow field suggest potentially turbulent flow of Pillan's lavas, as well as a strong interaction been the emplaced lavas and the SO2 frost that coated the ground prior to the lava flowing over the surface.
Observations at other flow fields, such as the mosaic covering Prometheus shown above, revealed a different story. Rather than a rough textured surface, SSI images revealed a patchwork of bright, sulfur dioxide frost, fresh dark flows, and older flows. The irregular margins of this flow field and the similar Zamama and Amirani fields suggested much thinner flow lobes (~1 meter versus 8-10 meters) than what was seen at the more rapidly emplaced Pillan flows. These flow fields, Amirani, Prometheus, and Zamama, are interpreted to compound flood basalt flow fields. Rather than being formed in one giant eruption, lava at these volcanoes is emplaced in the form of "small" breakouts on top of previously erupted lava. When lava from a breakout interacts with sulfur dioxide frost on top of the older lava, the sulfur is heated up and blasts its way through the meter-thick flow lobe, helping to form part of the plumes at these volcanoes.
Several of SSI's observations were dedicated to the other major landform type on Io, its many tall mountains. This includes the high resolution mosaic of Ot Mons, an older mountain in the middle of Colchis Regio. This mosaic and other lower resolution observations of Io's mountains closer to the terminator to the east revealed that nearly all of these structures are at some stage of degradation. This is despite the fact that these mountains are all less than one million years old due to Io's high resurfacing rate. The mosaic of Ot Mons revealed a structure covered in 100-meter tall, rounded hills with a mix of bright and dark material, with the dark material located mainly in topographic lows. The lower resolution mosaics of mountains such as Skythia Mons, Gish Bar Mons, and Monan Mons, revealed evidence for large landslides and significant faulting across the top of these mountains. In addition to evidence for various stages of degradation, evidence for layering was observed at several mountains, including one west of Donar Fluctus (seen near the center of the ZAMAMA02 mosaic).
These SSI observations provided further evidence that Io's mountains generally consisted of tilted crustal blocks. The observed layering would then be old lava flows, pryoclastic material, and sulfur and sulfur dioxide ices that form layers in Io's upper crust. The observed degradation shows that these mountains begin to fall apart soon after they form as a result of sulfur dioxide sapping from those ice layers within the mountains and Io-quakes that must be fairly common.
While NIMS, Galileo's Near-Infrared Spectrometer, was in ride-along mode for most of its observations and it lost much of its spectral resolution due to the stuck grating, it was able to make a number of exciting observations. One of its first observations of the flyby covered portions of Loki Patera (shown at right) while the volcano was on Io's nightside. This observation revealed warm material within fissures in the bright island on the patera floor as well as within the darker, lava lake surface to the east. This data revealed dark material with color temperatures ranging from 305 K on the patera floor to 350 K in the island cracks. The NIMS team determined that assuming a starting temperature of 1475 K, the temperatures correspond to surface ages of about 127 days and 39 days, respectively, suggesting that the patera floor dates back to a prior Loki eruption, while the lava in the island cracks may date from the early stages of a Loki eruption at that started in September 1999. The uniformity of the surface temperature of the caldera floor along with PPR data taken a few minutes earlier has been used as evidence that the dark patera floor of Loki consists of one large lava lake that overturns episodically.
As Galileo passed Io, it was able to turn its cameras toward Io's dayside. This allowed NIMS to examine the distribution of sulfur dioxide frost across Io's anti-Jupiter hemisphere as well as search for hotspots and examine the fine scale thermal features at several Ionian volcanoes, including Prometheus and Amirani. For example, the image at right shows the region surrounding the volcano Prometheus. In this data from the REGION01 observation, two hotspots are seen at Prometheus, one associated with the vent in the east and the current breakout region to the west. Based on this and higher resolution observation, as well as co-analysis with SSI data, it has been suggested that lava at Prometheus erupts along a fissure that bounds the eastern end of the flow field. This lava then travels west via covered lava tubes until they reach a local topographic low where the lavas then erupt again onto the surface in the form of dark breakouts. Higher resolution data also revealed a third hotspot between the two, from an area of activity in the center of the lava flow. The dark ring around Prometheus in the 4.2 micron data, in the middle of an SO2 absorption band, results from plume fallout, not only of the present day plume, but also from the previous eastern location of the plume, as seen by Voyager in 1979. NIMS also used its first opportunity to image Io at high resolution to search for small and faint hotspots and observations like the one above and below revealed several of these thermal features, including the first hotspots observed at volcanoes like Tien Mu Patera and Steropes Patera.
PPR observed Loki near the start of a new overturning event at the lava lake there, as evidenced by increased thermal emission near the southwestern end of the patera, where these events typically start before moving around the patera in a counter-clockwise motion. PPR also observed older flows at Zamama and Pillan, providing estimates for the age of these flows.
I hope you have all enjoyed this look back at Galileo's I24 flyby, which occurred 10 years ago on October 11, 1999. Galileo would go on to encounter Io five more time as it wound down its mission at Jupiter, helping to revolutionize our knowledge of this exciting satellite.