Journal of Geophysical Research: Planets

A survey of Io's volcanism by adaptive optics observations in the 3.8-μm thermal band (1996–1999)


  • Franck Marchis,

  • Renée Prangé,

  • Thierry Fusco


In this study, we analyze a series of images of Io obtained with the European Southern Observatory adaptive optics system (Adaptive Optics Near Infrared System, ADONIS) at 3.8 μm from 1996 to 1999, with particular emphasis on the observations carried out in late 1999 in support of the Galileo flybys of Io. Use of a new myopic deconvolution method, Myopic Iterative Step Preserving Algorithm (MISTRAL), especially designed for planetary objects, significantly improves the quality and the reliability of the reconstructed images. Using a simulation of artificial images of Io, we estimate the extent to which this algorithm is able to prevent noise amplification, to better restore sharp edges, and to preserve the initial photometry. Once this algorithm has been applied to our data and the solar-reflected background has been subtracted, we have 89 images of Io available for search and temporal survey of the hot spots over 4 years. In most cases, the data, acquired during two consecutive nights, provide an almost global view of the surface of Io. We identify 20 hot spots for which we determine the coordinates and the brightness at 3.8 μm (as a function of time). More than half of the hot spots were detected on all the images and were considered as persistent. Particular emphasis has been put on the brightest two hot spots, Loki and Pele, for which we obtain a center-to-limb variation curve (a cosine curve for Loki and a curve decreasing slightly faster than a cosine law for Pele with possible physical interpretations). The temporal variations of their activity have been derived over these 4 years and, for Loki, compared with the results of other ground-based surveys. The variability of the other, fainter, sources has also been derived but with less accuracy. We have also identified a few new hot spots, some of which are transient, such as a diffuse emission seen in September and November 1999 at the location of the upcoming Tvashtar outburst. The data are based on observations (58.F-1003, 63.S-0260, and 64.S-0661) collected at the European Southern Observatory in La Silla, Chile.