Volcanism On Io - Composition

Composition

Analysis of Voyager images led scientists to believe that the lava flows on Io were composed mostly of various forms of molten elemental sulfur. The colouration of the flows was found to be similar to its various allotropes. Differences in the lava colour and brightness are a function of the temperature of polyatomic sulfur and the packing and bonding of its atoms. An analysis of the flows that radiate out from Ra Patera revealed differently colored materials, all associated with liquid sulfur, at different distances from the vent: dark albedo material close to the vent at 525 K (252 °C; 485 °F), red material in the central part of each flow at 450 K (177 °C; 350 °F), and orange material at the farthest ends of each flow at 425 K (152 °C; 305 °F). This colour pattern corresponds to flows radiating out from a central vent, cooling as the lava travels away from it. In addition, temperature measurements of thermal emission at Loki Patera taken by Voyager 1's Infrared Interferometer Spectrometer and Radiometer (IRIS) instrument were consistent with sulfur volcanism. However, the IRIS instrument was not capable of detecting wavelengths that are indicative of higher temperatures. This meant that temperatures consistent with silicate volcanism were not discovered by Voyager. Despite this, Voyager scientists deduced that silicates must play a role in Io's youthful appearance, from the moon's high density and the need for silicates to support the steep slopes along patera walls. The contradiction between the structural evidence and the spectral and temperature data following the Voyager flybys led to a debate in the planetary science community regarding the composition of Io's lava flows, whether they were composed of silicate or sulfurous materials.

Earth-based infrared studies in the 1980s and 1990s shifted the paradigm from one of primarily sulfur volcanism to one where silicate volcanism dominates, and sulfur acts in a secondary role. In 1986, measurements of a bright eruption on Io's leading hemisphere revealed temperatures of at least 900 K (600 °C; 1,200 °F). This is higher than the boiling point of sulfur (715 K or 442 °C; 827 °F), indicating a silicate composition for at least some of Io's lava flows. Similar temperatures were also observed at the Surt eruption in 1979 between the two Voyager encounters, and at the eruption observed by Witteborn and colleagues in 1978. In addition, modeling of silicate lava flows on Io suggested that they cooled rapidly, causing their thermal emission to be dominated by lower temperature components, such as solidified flows, as opposed to the small areas covered by still molten lava near the actual eruption temperature.

Silicate volcanism, involving basaltic lava with mafic to ultramafic (magnesium-rich) compositions, was confirmed by the Galileo spacecraft in the 1990s and 2000s from temperature measurements of Io's numerous hot spots, locations where thermal emission is detected, and from spectral measurements of Io's dark material. Temperature measurements from Galileo's Solid-State Imager (SSI) and Near-Infrared Mapping Spectrometer (NIMS) revealed numerous hot spots with high-temperature components ranging from at least 1,200 K (900 °C; 1,700 °F) to a maximum of 1,600 K (1,300 °C; 2,400 °F), like at the Pillan Patera eruption in 1997. Initial estimates during the course of the Galileo mission suggesting eruption temperatures approaching 2,000 K (1,700 °C; 3,100 °F) have since proven to be overestimates since the wrong thermal models were used to calculate the temperatures. Spectral observations of Io's dark material suggested the presence of orthopyroxenes, such as enstatite, magnesium-rich silicate minerals common in mafic and ultramafic basalt. This dark material is seen in volcanic pits, fresh lava flows, and pyroclastic deposits surrounding recent, explosive volcanic eruptions. Based on the measured temperature of the lava and the spectral measurements, some of the lava may be analogous to terrestrial komatiites. Compressional superheating, which could increase the temperature of magma during ascent to the surface during an eruption, may also be a factor in some of the higher temperature eruptions.

While temperature measurements of Io's volcanoes settled the sulfur-versus-silicates debate that persisted between the Voyager and Galileo missions at Jupiter, sulfur and sulfur dioxide still play a significant role in the phenomena observed on Io. Both materials have been detected in the plumes generated at Io's volcanoes, with sulfur being a primary constituent of Pele-type plumes. Bright flows have been identified on Io, at Tsũi Goab Fluctus, Emakong Patera, and Balder Patera for example, that are suggestive of effusive sulfur or sulfur dioxide volcanism.