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By Jason Perry
Planetary scientists from around the country and around the world are gathering this week for the 32nd Annual Meeting of the DPS in Pasadena, California. The DPS or Division of Planetary Science is a part of the American Astronomical Society. Among the many results discussed are the findings of the Galileo spacecraft's flyby of Io in February 2000. Some of the many Io findings include the discover of an active lava lake at Pele, refinement of Prometheus' plume' origin site, possible impurities in Io's sulfur, and possible sulfur dioxide flooding at a caldera near Chaac.
The Session 28 on October 25, 2000 covered invited papers from the various instrument teams to present results from the I27 flyby. They included a talk on SSI's observations of Io's many landforms, two talks on NIMS's observations of active volcanism and spectral observations, a paper on PPR observations, and finally a talk on Io magnetic observations. Several other sessions on Io would follow, including a poster session.
 Prometheus |
During the I27 flyby, Galileo's SSI camera observed various unusual features on Io, from large, long-lived, active lava flows, to snow fields, mountains, and cliffs. Two of the main volcanoes on Io, Prometheus and Amirani, were imaged at medium-to-high resolution during this flyby, going over areas that were imaged at 100-500 m/pixel during the I24 flyby that took place one year ago. These repeat observations show lava erupting onto large flow fields from many distinct breakouts. This shows that the lava flows, which range from 95 km long at Prometheus to 250 km long at Amirani, consist of compound flows growing by inflation of pahoehoe flows (McEwen et al. 2000). |
Two mosaics of Prometheus were taken during the I27 flyby, an eight image, 12 m/pixel mosaic showing the lava flow front and a two-image, three-color, 165 m/pixel mosaic of the entire Prometheus lava flow, plus caldera and parts of a mesa east of the flow. In these images, several bright streaks were observed near new lava break outs. At two of these streaks, as seen in the color image, a blue "cloud" can be seen, made of airborne particles entrained in plumes.
| In the high-resolution mosaic, the bright streaks on the ridged plains look like snow on a Spanish tile roof. The snow is made of sulfur dioxide from the surrounding, volatile-rich plains, recently vaporized by the advancing, Promethean lava flow. The cause of the ridges seen in the plains is unknown. Scientists were hoping that the mosaic would also show the vent for the plume, but none was found. Instead, it is now believed that the plume is formed by the vaporization of sulfur dioxide at the many outbreaks seen on the lava flow. These then combine to create the Prometheus plume. |
 Prometheus Upclose |
 Amirani |
Amirani was also imaged, partly in color. This mosaic shows a very similar lava flow at Amirani. Multiple oubreaks of lava can be seen on the flows surface, signified by their darker color. The new lava flows are darker because they are too warm or too young for sulfur dioxide frost to cover them. At least eight outbreaks can be seen in the northern half of the lava flow. However, the lava seen at these outbreaks do not orginate there. The lava first comes to the surface in the southern part of the lava flow, then flow through insolated lava channels before erupting to the surface farther to the north (or to the west at Maui, which is not seen in this image). There are two possibilities for where the lava comes to the surface. One is at a crack south of the lava flow. This crack is marked by a dark line surrounded by red diffuse deposits. The lava then flows both to the northeast, where the lava then spreads out to the main Amirani lava flow, and to the southwest, into the caldera there. Long, narrow channels can be seen flowing away from the southern end of the lava flow. In addition to the lava, the source of the Amirani plume can be seen as a purple blob near a dark spot in the southern part of the lava flow. Other purplish areas can be seen on the lava flow, associated with vaporized sulfur dioxide and active lava flows. Another mosaic released from the I27 flyby, shows a chain of calderas from Chaac in the west and Camaxtli in the east. Camaxtli appears to be a caldera surrounded by green diffuse deposits created by droplets of lava formed during major eruptions at Camaxtli. Also seen at Camaxtli are a couple of lava flows on the caldera floor, which appear to originate from cracks at the bottom of the caldera walls, and bright deposits formed when the lava vaporized the pre-existing sulfur dioxide. |
| There appears to be a chain of seven paterae, with a right angle bend, in this mosaic. The straight edges of these calderas also appear to follow this pattern. This mosaic exemplifies the difference between Ionian calderas and terrestrial calderas. Ionian calderas don't reside on shields like most terrestrial calderas (with the exception of ash-flow calderas) and Ionian paterae have irregular and/or angular walls. A small caldera northwest of Chaac (the green caldera in the far left part of the image) is associated with the many bright yellow and black flows seen there. |
 Chain of Calderas on Io |
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The flows are similar to ones seen at Culann, where a small caldera is surrounded by black, green, red, and orange flows. The black flows are likely made of magnesium-rich, silicate lava while the bright yellow flows are likely made of sulfur. Much of the terrain in this mosaic is bumpy, possibly due to the sublimation of Sulfur dioxide. In the last image released, a 400-m tall promontory with a 28 deg. slope can be seen surrounded by frosted plains. This mosaic has the highest resolution of any image taken of Io with a resolution of 5.5 m/pixel. The promontory is surrounded by a frost ring, which is surrounded by a flat, darker plain, which is then surrounded by a rougher, even darker layered plain. Within the flat, frost-less plain, several narrow (10-20 m), 100-200 m long gullies can be seen. These may be evidence of sulfur and/or sulfur dioxide springs on Io. |
Many other talks were given about Io at the 2000 DPS conference. They include talks on Pele, Io's sulfur, heat flow, internal structure, and surface properties.
Jeffery Kargel, T. MacIntyre, B. Dalton, and R. Clark presented two talks/posters on the observational and laboratory evidence for sulfur impurities on Io. Sulfur impurities are known to exist in volcanic sulfur on Earth. These impurities cause the color of sulfur to change colors, like green or red. The authors note that the color and spectra of sulfur on Io varies from location to location, sometimes forming rings around a volcano. The green color seen at several calderas, including Chaac and Culann, could be due to iron impurities such as iron in the form of FeS mixed in the sulfur. The authors also state that the spectral absorption edge of sulfur at Pele and Culann was shifted to longer wavelengths. They state that this could be due to impurities of Arsenic (As), Tellurium (Te), and Selenium (Se). Only 1% of the sulfur needs to be impure for the color to change. For the red coloring, impurities are not the only possibility. One possibility is that the red results from the polymerization of S2, that has been seen by HST in plumes on Io, forming S3 and S4. Another possibility is radiation-caused disassociation of S8. There is no other known cause for the green color on Io.
The Near-Infrared Mapping Spectrometer (NIMS) and Photopolarimeter-Radiometer (PPR) instruments also imaged Io during I27. The NIMS found ultramafic lava flows at Pele and Tvashtar. The NIMS also found a caldera east of Chaac covered in pure sulfur dioxide, likely caused by the catastrophic flooding of SO2 in the caldera as it froze on the surface. The PPR instrument is designed to find warm lava flows and heat flow. In PPR images of Loki, much of the caldera had been resurfaced from the most recent eruption in late 1999. The temperature peak from I24 was not seen. At Pele, low temperatures were seen. NIMS had seen temperatures as high as 1500 K at Pele. Why the discrepancy? PPR can not "see" small hot areas, only large, relatively cool areas. Also, the area of Pele that PPR saw may have been covered in pyroclastics from the main vent. Other images of Io from PPR show Pillan and Zamama, where cooling flows with temperatures of 200 K were seen, and Daedalus, where an extensive area of warm lava flows were seen surrounding the central caldera [J.R. Spencer, personal communication]. D.L. Matson and other authors reported that the average night time temperature of 90-95 K is too high for the sun alone to explain the temperature. These scientists postulated that high heat flow maybe due to the entire surface of Io being covered in lavas at various stages of cooling.
Two reports were given on the mountains on Io. One, by Paul Schenk, R. Wilson, and H. Hargitai, described the morphology of mountains on Io. The other, by Elizabeth Turtle, Windy Jaeger, Alfred McEwen, and Laszlo Keszthelyi, described modeling of Ionian mountain formation. P. Schenk cataloged 105 mountains by dimensions, locations, morphological type, heights, and associated structures. The tallest mountains is Boosaule Montes at 16±2 km and the longest is 570 km. Schenk also determined that there are five basic types of mountains: massifs, ridges, peaks, plateaus, and mesas. Massifs, ridges, and peaks appear to be tilted crustal blocks. Debris fields from mass movement processes (like landslides) are seen at 10% of mountains, indicating the possibility of evolution between mountain types. Mountains demonstrate no pattern with respect to latitude but mountains do appear to be concentrated near longitude 70 and 270. This pattern is consistent with models of crustal compression. Finally, the other authors of "The Mountains on Io" state that the apparent relationship between volcanoes and mountains maybe due to lack of space on the surface than an actual relationship. The authors of "Numerical Modeling of mountain formation on Io" modeled several mountain formation scenarios. None of their models form mountains similar to those seen on Io. However, models that include a rising material from the mantle impinging on the crust produce isolated mountains like those on Io.
Other results showed the very interesting surface properties of Io. Cynthia Phillips and five other authors reported on Io's resurfacing rate and the changes seen by Galileo. The changes seen by Galileo come from Io's two main resurfacing mechanisms, plume deposits and lava flows. In addition color changes were found at several places on Io, some occurring in less than 3 months time. Most notable of these color changes occurred at Pillan where the caldera changed from black to green, likely due to the interactions between Pele's plume and the warm lava on the caldera floor. The authors also preformed calculations on Io's resurfacing rate. Based on the kinds of lavas, the lack of craters, and the changes seen, the likely resurfacing rate is between 0.02 cm/year and 14 cm/year, if the lava flows are made of sulfur. Resurfacing due to magnesium-rich silicate flows (komatiitic) would be 0.7 cm/year.
Many other findings were announced at the DPS meeting. Cl2SO2 was tentatively found on Io's surface, consistent with the molecule being mixed in SO2 ice. A long-term active lava lake with lava temperatures as high as 1600 K was identified at Pele. Loki appears to go through periodic eruptions. Finally, Io's radius was refined to 1820 km and Io's density was refined to 3527.8±2.9 kg/cubic meters, based on radio Doppler data taken during Galileo's 4 flybys of Io.
This conference does not represent the end of Galileo and its human attendants' interaction with Io. Sometime next year, JGR-Planets, a publication of the American Geophysical Union, will dedicate a special issue to Io and Galileo's results from Io flybys. Also, Galileo is tentatively scheduled to flyby Io three times in 2001 and 2002, possibly providing context for the results from the 1999/2000 flybys and provide new information on Io's projovian hemisphere. Io will also be one of the main targets for observation by the Saturn-bound Cassini spacecraft and the Hubble Space Telescope. So, though the excitement of the Io flybys maybe over and the real work begun, Ionian exploration is far from over.
References:
Davies, A.G., L.P. Keszthelyi, D.A. Williams, A.J.L. Harris 2000. The Lava Lake at Pele: A Comparison With Terrestrial Lava Lakes. DPS 32.
Kargel, J.S., P. Delmelle, and D.B. Nash 1999. Volcanogenic sulfur on Earth and Io: Composition and spectroscopy. Icarus 142, 249-280.
Kargel, J.S., T. MacIntyre, B. Dalton, R. Clark 2000. Io's Sulfur: Surface Distrubution and Chemical Nature of Impurities. DPS 32.
Lopes-Gautier, R. and 10 others 2000. Io's Volcanic Activity as Seen by the Near-Infrared Mapping Spectrometer on Galileo. DPS 32.
MacIntyre, T. J.S. Kargel, B. Dalton, and R. Clark. Reflection Spectra of Impure Sulfur: Io and New Lab Results. DPS 32.
Matson, D.L., T.V. Johnson, G.J. Veeder, D.L. Blaney, and A.G. Davies 2000. Upper Bound on Io's Heat Flow. DPS 32.
McEwen, A.S. 2000. High-Resolution Images of Active Volcanoes, Landforms, and Surface Textures. DPS 32.
Milazzo, M.P., L.P. Keszthelyi, A.S. McEwen 2000. Are Prometheus-Type Plumes on Io Produced by Lava-SO2 Interactions at the Flow Fronts? DPS 32.
Phillips, C.B., A.S. McEwen, L.P. Keszthelyi, P.E. Geissler, D.P. Simonelli, and M. Milazzo 2000. Volcanic Resurfacing Rates and Styles on Io. DPS 32.
Schenk, P., R. Wilson, H. Hargitai 2000. The Mountains of Io. DPS 32.
Schmitt, B., S. Rodriguez 2000. Tentative identification of a Chlorine molecule at Io's surface. DPS 32.
Schubert, G., W.B. Moore, J.D. Anderson, R.A. Jacobson, E.L. Lau 2000. Io's Gravity Field and Interior Structure. DPS 32.
Smythe, W.D. and 6 others 2000. Evidence for Massive Sulfur Dioxide Deposit on Io. DPS 32.
Spencer, J.R., J.E. Rathbun, L.D. Travis, L.K. Tamppari, L. Barnard, and T.Z. Martin 2000. A Closeup Look at Io's Volcanos with Galileo PPR. DPS 32.
Turtle, E.P., W.L. Jaeger, A.S. McEwen, L. Keszthelyi 2000. Numerical modeling of mountain formation on Io. DPS 32.
