Discovery of hotspots on Io using disk-resolved infrared imaging

Abstract

ACTIVE, tidally driven volcanism on Jupiter's satellite Io1,2 was discovered in Voyager images in 1979. Voyager also detected the thermal emission from the volcanoes3,4, and this emission has since been monitored from Earth using a variety of techniques employing single-element infrared detectors5–10. Because of Io's small angular size (no more than 1.2 arcsec), however, it has normally been possible to resolve and locate only the brightest individual hotspot, Loki, although additional hotspots can be located at five-year intervals during occultations of Io by the other galilean satellites11,12. Here we present first results using two new techniques for ground-based observation of Io's hotspots. First, use of an infrared array camera allowed us to exploit the Mauna Kea Observatory's excellent atmospheric stability to obtain direct infrared images of Io with resolution better than 0.5 arcsec. This allowed us to see more than one hotspot on Io in Jupiter eclipse (in Jupiter's shadow). Second, we used the camera to make the first observations of the Jupiter occultation (disappearance behind Jupiter) of the hotspots. These new techniques have revealed and located at least three hotspots, and will now permit routine ground-based monitoring of the locations, temperatures and sizes of multiple hotspots on Io.