Occultation of In-Eclipse Io by Jupiter.

July 21, 1998 10.3 microns

(JPL-Mirlin Camera at NASA-IRTF)

On July 21, 1998 Glenn Orton, Brendan Fisher, and Charles Kaminski, observing at the NASA Infrared Telescope Facility, used JPL's MIRLIN camera to obtain a "movie" of the in-eclipse Io as it disappeared behind Jupiter. The observations were obtained at a wavelength of 10.3 microns. At this wavelength and under these conditions, the flux from Io comes entirely from volcanic hot spots. The occultation technique allows us to determine the brightness of the individual hot spots, as one-by-one they disappear behind the planet. (For a discussion of the occultation technique as applied at near-infrared wavelengths, look here..)

At long wavelengths the technique is more difficult because Jupiter is much brighter -- but the long wavelength measurements are essential if one is to obtain the total energy output from the hot spots.

Shown below is an image of Jupiter and Io shortly before the occultation disappearance begins. Io is already in eclipse and can just barely be seen in this image adjusted to show Jupiter. However if the display is adjusted to emphasize Io it is clearly evident just to the right of Jupiter. It is also evident when one subtract from each frame an image of Jupiter obtained after Io has disappeared.

When one processes the series of images obtained as Io disappears, then measures the brightness of Io, one obtains the following curve. It clearly shows the disappearance of one major hotspot (Loki) which contributes 2/3 of the total flux. The disappearance of the remaining flux can be modeled by a smooth decrease which would imply many faint hotspots distributed across the disk. However there is some slight evidence for a two-step character for that remaining flux, which would be consistent with the shorter-wavelength occultation lightcurve obtained from the Wyoming Infrared Observatory (WIRO) two days earlier by Robert Howell and Justin Gregg.

The location of the three possible hotspots on Io can be obtained by drawing on a map of Io the location of the Jupiter limb at the times of the three steps. When that is done the location of the large step clearly corresponds to Loki, which was undergoing a major eruption during the summer of 1998. The first step may correspond to Kanehekili – a persistent hotspot marked by the small square.

The noise in the 10.3 micron occultation is caused by several factors. The primary sources are imperfect registration and subtraction of the Jupiter image and missing flux caused by "bad pixels" in the camera. One of the 16 readout channels was not functioning, causing the pattern of bad pixels seen in the images below. Because Io occupies several pixels it is possible to interpolate across the missing ones to obtain a reasonable estimate of the flux, which was done in the above images and for the above lightcurve. However the interpolation is imperfect, especially if the peak Io pixel falls on or adjacent to one of the bad pixels. That effect is probably the cause of several discrepant points in the above lightcurve.

In the following movie the first frame is shown without Jupiter subtraction, and with the bad pixels "fixed" by interpolation. In subsequent frames a Jupiter image has been centered and subtracted, and the bad pixels are shown as black to enable one to judge possible interpolation problems. The Loki disappearance occurs at frame 71, 184 seconds after the occultation disappearance began. The frames are numbered and also labeled with time after event start.