Io hot spots: Infrared photometry of satellite occultations

Infrared photometry of occultations of Io by the other Galilean satellites is used to map Io's active hot spots. Excluding a 15° wide longitude strip centered near 215°W, each point on Io's surface was occulted during at least one of seven occultations observed during the latter half of 1985. The greatest spatial resolution and sensitivity to hot spots occurs near the sub-Earth point, which always lies near the equator, and both spatial resolution and sensitivity decrease away from the sub-Earth point due to foreshortening. Most of the measurements were made with some combination of the UH 2.2 m, UKIRT, IRTF, and CFHT telescopes on Mauna Kea, Hawaii, or with the AAT in Australia, usually with one broad-bandpass infrared filter d centered at 3.8 μm (L′), 4.8 μm (M), or 8.7 μm used at each telescope. For several of the oscillations, data in more than one bandpass were acquired. A model for the occultation lightcurves is developed and fit to the data to determine the apparent path of the occulting satellite relative to Io. The astrometric results are compared to existing Galilean satellite ephemerides and are used to improve on them. The mean error in the apparent relative position of the occulting satellite is estimated to be 178 km. An occultation of Loki, Io's largest hot spot, measured at 3.8 μm wavelength, shows the region of thermal emission spatially resolved and approximately 200 km in diameter. The location of the emitting area is 308 ± 1°W longitude and 20 ± 3°N latitude, near Loki Patera and the fissure that was the source of two eruptive plumes during the Voyager encounters. A new hot spot is discovered on Io's leading hemisphere at 79 ± 5°W longitude and 22 ± 5°S latitude in the 3.8- and 4.8-μm data from 10 July UT. The region of emission is smaller than ∼20 km in diameter.