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

Abstract

Abstract. In this study, we analyze a series of images of Io obtained with theEuropean Southern Observatory adaptive optics system (Adaptive Optics Near Infrared System, ADONIS) at 3.8 tzrn from 1996 to 1999, with particular emphasis on the observations carried out in late 1999 in support of the Galileo fiybys 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 a•lgorithm 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 tzrn (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).

Keck AO survey of Io global volcanic activity between 2 and 5 μm

“…In this case, the values of the regularization parameters are too important, and the inversion process remains limited. Despite its complexity, MISTRAL has provided excellent results for various planetary images recorded at different telescopes (Marchis et al, 2001aHestroffer et al, 2002). We have intensively tested the method on real and artificial images.…”

“…The main improvement of this technique over more classical methods is that it avoids both noise amplification and creation of sharp-edged artifacts or "ringing effects" and it better restores the initial photometry. This algorithm has been extensively used on simulated and real AO observations (Conan et al, 2000;Marchis et al, 2001a).…”

“…Observations taken between 1996 and 2000 at 3.8 µm with the ESO 3.6-m AO-equipped telescope revealed the distribution and evolution of more than 20 low-temperature volcanic centers (Marchis et al, , 2001a. More recently, observations at shorter wavelengths (1-2.5 µm) performed in February, 2001, with the Keck 10-m telescope indicate the presence of two high temperature (>800 K) eruptions coinciding with the Amirani and Tvashtar geographical areas.…”

Tvashtar awakening detected in April 2006 with OSIRIS at the W.M. Keck Observatory

“…Subsequent images of the eruption in February 2000, show the activity had moved to the darker area in the top left of the image . Tvashtar was also observed to be active in ground based observations by Marchis et al (2001Marchis et al ( , 2002.…”

Adaptive Optics Mapping of Io's Volcanism in the Thermal IR (3.8 μm)

“…The radiated fluxes depend critically on the absolute photometric calibration, which unfortunately was not good for these observations. We have thus used data of much better photometric quality (estimated to within ±20%) obtained at the same CMLs in 1999 as an intermediate calibration (Marchis et al 2000). The solar reflected flux, which is not expected to vary much with time (Howell et al 1984), has merely been corrected for the variations of the distance of Io to the sun, and of the solar phase angle.…”