Io in the near infrared: Near-Infrared Mapping Spectrometer (NIMS) results from the Galileo flybys in 1999 and 2000

Galileo's Near-Infrared Mapping Spectrometer (NIMS) observed Io during the spacecraft's three flybys in October 1999, November 1999, and February 2000. The observations, which are summarized here, were used to map the detailed thermal structure of active volcanic regions and the surface distribution of SO₂ and to investigate the origin of a yet unidentified compound showing an absorption feature at ∼1 μm. We present a summary of the observations and results, focusing on the distribution of thermal emission and of SO₂ deposits. We find high eruption temperatures, consistent with ultramafic volcanism, at Pele. Such temperatures may be present at other hot spots, but the hottest areas may be too small for those temperatures to be detected at the spatial resolution of our observations. Loki is the site of frequent eruptions, and the low thermal emission may represent lavas cooling on the caldera's surface or the cooling crust of a lava lake. High-resolution spectral observations of Emakong caldera show thermal emission and SO₂ within the same pixels, implying that patches of SO₂ frost and patches of cooling lavas or sulfur flows are present within a few kilometers from one another. Thermal maps of Prometheus and Amirani show that these two hot spots are characterized by long lava flows. The thermal profiles of flows at both locations are consistent with insulated flows, with the Amirani flow field having more breakouts of fresh lava along its length. Prometheus and Amirani each show a white ring at visible wavelengths, while SO₂ distribution maps show that the highest concentration of SO₂ in both ring deposits lies outside the white portion. Visible measurements at high phase angles show that the white deposit around Prometheus extends into the SO₂ ring. This suggests that the deposits are thin and that compositional or grain size variations may occur in the radial direction. SO₂ mapping of the Chaac region shows that the interior of a caldera adjacent to Chaac has almost pure SO₂. The deposit appears to be topographically controlled, suggesting a possible origin by liquid flow.