IRTF Galileo Support Monitoring of Jupiter and Io

The NASA Infrared Telescope Facility (IRTF), on Mauna Kea, Hawaii has agreed to set aside time at the beginning or end of every night that the NSFCAM infrared camera is mounted on the IRTF, and Jupiter is observable, for a standardized set of observations of Io and Jupiter in support of the Galileo mission. This program observed Io on 14 nights in September and October 1995 (sample results shown below), and on about 40 nights from April to November 1996, and on 33 nights between February and December 1997. The Io program was then suspended with the end of the Galileo nominal mission, but was reinstated in the 1999 observing season to support Galileo's flybys of Io. The Io observations are coordinated by John Spencer, and executed by the telescope operators: Dave Griep, Bill Golisch, Charlie Kaminski, and Paul Fukumura-Sawada. Jupiter observations are coordinated by the IJW Atmospheres Discipline leader, Glenn Orton (go@orton.jpl.nasa.gov)

 The Io observations are designed to locate and track thermal emission from volcanic eruptions, using disk-resolved images. All images are taken at a plate scale of 0.056 arcsec/pixel, and consist of two "plain" L' (3.8 micron) Io/sky pairs, followed by some short Io images and a corresponding sky, three sets of real-time "Shift and Add" images with different bin positions, to freeze seeing, then five M (4.8 micron) Io/Sky pairs, which have rather low S/N but will be useful when we see very bright eruptions ("outbursts"). There is also a series of similar exposures of a local standard for photometric and PSF calibration. The 1999 local standard is SAO 92536, whose brightness is currently being calibrated.

Here is a 1998 summary of results of the monitoring program
 

Some IRTF Monitoring Results So Far: Shift&Add 3.8 Micron Io Images

Io without bright hot spots
6:01 UT, 95/09/08, central longitude 135 degrees. North is at the top. Io's angular diameter was 0.94 arcsec.

A major outburst !
5:14 UT, 95/09/27, central longitude 32 degrees. Seeing is poor: the disk of Io is the smooth region to the upper right of the outburst (the bright spot), and the fuzz below the spot is due to diffraction rings and seeing artifacts. Outburst location is not well determined, but may be consistent with the site of the 1995/03/02 outburst (98 W, 42 S, +/- 5 degrees): if true, this would be the first known "repeating" outburst site. Photometry of these images shows that the outburst increased Io's disk-integrated 3.8 micron brightness by a factor of at least 2.5.

A previously-unknown hot spot
4:55 UT, 95/10/05, central longitude 216 degrees. Io's disk was only 0.88 arcsec across when this image was taken. Approximate hot spot location is 220 W, 25 N, +/- 15 degrees, in north-west Colchis Regio. The spot was also seen on IRTF Galileo monitoring images taken two days earlier, and may correspond to a hot spot seen by Voyager 1 in 1979, and by Galileo in 1996 (now named Isum), in the same general area.

First 1996 image
16:20 UT, 96/05/29, central longitude 265 degrees. Io's diameter was a relatively huge 1.15 arcsec. No hot spots are visible, though Loki and Pele are on the disk, so neither of these volcanoes was very active at this time.

Leading hemisphere during Galileo G1 encounter
12:00 UT, 96/06/29, central longitude 61 degrees. Io's diameter was 1.19 arcsec. No hot spots are visible on the sunlit disk at this wavelength here or at any other longitude during G1, though Loki was faintly visible at 4.8 microns.

The images from each night are archived as compressed tar files (with Io and Jupiter images in separate archives for each night along with their respective stellar calibrations), and are available to anyone who wants them by anonymous ftp from irtf.ifa.hawaii.edu, directory pub/galileo, or directly from here. The archive files have names like io960530.tar.Z, where io is the target and 960530 is the date the archive was made. More recent files have names like io960613.16.tar.Z, where io is the target, 960613 is the UT date of the observations, and 16 is the UT hour that the observations were started.

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