Using the UH adaptive optics system on CFHT we recorded infrared images of the Jupiter facing hemisphere of Io during an eclipse by Jupiter. In this note we present some preliminary results that demonstrate the performances that can be obtained using adaptive optics to spatially resolved volcanoes on Io.
Io's volcanic activity was first predicted by Peale et al. (1979, Science 203, 892) as a consequence of tidal stress on Io by the Jupiter's gravitationnal field. The images returned by the Voyager spacecraft confirmed the existence of a very active volcanism on the innermost galilean satellite. Io's volcanoes are best imaged from the ground during eclipses by Jupiter (Spencer et al. 1990, Nature 348, 618). During such events, the only visible light at near-infrared wavelengths, is produced by the thermal emission of the volcanoes. Two types of vulcanism exist on Io: vulcanism driven by SO2 lava whose temperature do not exceed 700 K (Sinton et al. 1980, Science 210, 1015) and a more violent and hotter vulcanism that ejects silicates. Such high-temperature hot spots (temperature greater than 1000 K) were first observed at 5.0 microns by Veeder et al. (1994, J. Geophys. Res. 99, 17095) and more recently at shorter wavelength by Spencer et al. (1997, Geophys. Res. Letters 24, 2451) from the ground and by Belton et al. (1996, Science 274, 377) with the Galileo spacecraft.
We observed Io on July 16, 1997 from CFHT using the University of Hawaii adaptive optics system (13 actuators) and the near-infrared QUIRC camera. At that time, the configuration was very favorable to observe Io's volcanoes using an adaptive optic system since Io was in eclipse with Europa in sunlight located inside the Io's isoplanatic patch (distance less than 20 arcseconds). This configuration allowed the wavefront analysis to be done on Europa while Io's hot-spots were the science target. We imaged Io's volcanoes through two medium band filters at 1.7 and 2.3 microns. These wavelengths correspond to some absorption bands of methane which is present in the upper atmosphere of Jupiter. By imaging Io at these two wavelengths, we can strongly diminish the flux coming from the giant planet, which is almost invisible in our images. The integration time was limited to 4 seconds to avoid the drifting of Io during an exposure. Indeed the actual AO systems do not allow compensation of a differential motion between the reference source (e.g. Europa) and the science object (e.g. Io). Such a real time compensation should be soon implemented to the upgrade version of UHAO (36 actuators) and Pueo, allowing higher S/N ratio to be reached.
Figure 31 shows at top (A) an image which is the result of the addition of 40 images of 4 sec each recorded at 2.3 microns. The image at bottom (B) is in false color and logarithmic scale. It corresponds to the same image than (A) after deconvolution (40 iteration of Richardson-Lucy) and re-orientation to get the Io's polar axis vertical. Several hot-spots are visible, the most predominant beeing Loki and Kanehekili. The diffraction limit is reached at this wavelength and the resulting resolution on Io is about 400 km for one resolution element. Diffraction rings are visible on the top image (A) around Loki and Kanehekili. The other active regions corresponds to various known calderas on Io except for one region that is still unamed.
By imaging Io in eclipse with AO during close approach with another galilean satellite in sunlight, we can reach the highest resolution that can be currently obtained from the ground at these wavelengths by direct imaging techniques. With the actual instrumentation available, we are limited to the [1.0-2.5] micron range, which prevents us from detecting widespread cooling lava flows, but this spectral region is the best to detect bright outbursts. Such image returns very high constraints on the spatial extent of the active hot spots that require lava at silicate temperatures, attributing these events either to the presence of lava lakes or to fire fountains (Stansberry et al., 1997 Geophys. Res. Letters 24, 2455).
Although adequate configuration between the Galilean satellites do not occur as often as Io's eclipses by Jupiter (1.7 days), monitoring Io's volcanic activity from the ground with this technique allows us to obtain a follow up of the observations obtained by Galileo/NIMS with an equivalent spatial resolution. Other similar events will be observed in the future and the next opportunity will be to use Pueo/Kir on June 1998.