HIGH-TEMPERATURE ULTRABASIC ERUPTIONS ON JUPITER’S MOON IO: LATEST RESULTS FROM THE GALILEO MISSION AND ASSESSMENT OF ERUPTION STYLES

After 6 years of study, NASA’s Galileo spacecraft has completed remote sensing observations in the Jovian System. Arguably the most surprising result to emerge from Galileo observations of Jupiter’s volcanic moon Io was the identification of apparently high-temperature ultrabasic (i.e., ultramafic) silicate eruptions. Study of Ionian ultramafic volcanism could provide insight into the eruption of terrestrial ultramafic lavas (komatiites) during the Earth’s Precambrian. The first step in this study has been an analysis of Galileo data to assess the variation in ultramafic eruption styles on Io. First, the July 1997 eruption at Pillan Patera, which lasted between 52-167 days and produced temperatures of at least 1600 C, included: 1) a 140-km tall plume eruption that emplaced a 400 km diameter dark, apparently orthopyroxene-rich pyroclastic deposit, and 2) a field of 8-10 m thick lava flows covering >3100 km². With an estimated flow rate of 2000-7000 m³/s and a jumbled, pitted surface morphology, these flows may have fragmented during rapid flow or through explosive interaction with a volatile-rich substrate. Second, activity at Tvashtar Catena has included at least three different eruption styles: 1) ~1.5 km tall lava fountains that produced dark pyroclastic deposits and flows (November 1999), 2) an apparent lava lake with temperatures of at least 1630 C (February 2000), and 3) a ~400 km tall plume eruption that produced a ~1400 km diameter ring of red sulfur pyroclastics (December 2000). Third, the long-lived hotspot at Pele appears to be an ultramafic lava lake (temperatures of at least 1530 C), that occassionally produces plume eruptions hundreds of km tall and extensive rings of sulfurous pyroclastics. Io’s environmental differences (e.g., low-gravity, low-pressure atmosphere) and abundant volatiles (as indicated by ubiquitous plumes and sulfurous pyroclastics) suggest that magma-volatile interactions and explosive eruptions play a greater role in ultramafic volcanism on Io than occurred during komatiitic eruptions in the Earth’s Precambrian. Komatiitic eruptions were dominated by subaqueous turbulent and inflationary emplacement of flow fields with little explosive activity, variable but low volatile contents, and formation of magmatic sulfide ore deposits by thermal erosion.