Io's Hot Spots: Observations by Galileo's Near-Infrared Mapping Spectrometer (NIMS)
R.M.C. Lopes-Gautier, A.G. Davies, W.D. Smythe, R.W. Carlson (JPL-Caltech), L.A. Soderblom (USGS-Flagstaff), Galileo NIMS Team
The NIMS instrument is monitoring Io's volcanic activity during each orbit of the Galileo mission. NIMS has observed Io at least twice per orbit, obtaining global and part-global maps that are being used to analyze the composition of the surface (Carlson et al., 1997, submitted to GRL), the thermal activity of the hot spots (Lopes-Gautier et al., 1997, submitted to GRL) and hot spot thermal structure (Davies et al., 1997, submitted to GRL). During the first seven orbits, NIMS detected 30 hot spots, 16 of which were not previously known to be active areas. The spatial resolution of NIMS observations is higher for the anti-Jovian hemisphere and this is where most of the previously unknown hot spots were found. NIMS has not yet detected the Voyager hot spots at Creidne and near Nemea Planum, but this could be due to the relatively poor spatial resolution of the NIMS observations at these longitudes.
Our observations show that most hot spots appear to be persistently active from orbit to orbit, and 11 of those detected by NIMS were active in 1979 during the Voyager encounters. Single-temperature fits to the nightside spectra of hot spots yield temperatures ranging from about 400 K to 650 K, but many show evidence of a smaller, hotter component, thus being consistent with silicate volcanism (Davies et al, 1997, submitted to GRL).
The longitudinal coverage of Io by NIMS observations will be completed by acquiring the strip between longitudes 0 and 30 W during the ninth orbit (C9). The coverage to date, up to the seventh orbit (G7), shows that the distribution of hot spots on Io's surface may not be random. Hot spots surround the bright, SO -rich regions of Emakong Patera and Colchis Regio. This may reflect a tendency for volcanic centers to be concentrated around the edges of these basins. Further work, including verification and refinement of the topography defining these basins, is needed to verify if this "ring of fire" distribution is indeed significant.