__________________________________________________________________________ T H E O L E R O E M E R M E S S E N G E R _______________________________________________ JJJJ C G JJJJJJ I E JJJJ __________________________________________________________________________ Newsletter of the International Jupiter Watch Satellite Discipline E-mail issue 8 April 7, 1994 Editor and Discipline Leader: John Spencer Voice: (602) 774-3358 Lowell Observatory Fax: (602) 774-6296 1400 W. Mars Hill Rd. Internet: spencer@lowell.edu Flagstaff, AZ 86001 ------------------------------------------------------------------------------ EDITORIAL COMMENTS As always, I'd like more information about publications and observational results and plans. You're probably getting tired of reading about my observing programs, and the best way to shut me up is to crowd me out with your own contributions! ------------------------------------------------------------------------------ HST IMAGING OF IO: PROGRESS REPORT We (Spencer, McGrath, McEwen, Sartoretti, and Johnson) have now obtained all our HST Io images for our "Io's Surface and Volcanic Activity" program. We have imaged all Io longitudes at 0.34, 0.41, 0.57, 0.79, and 1.05 microns with WFPC2, with a resolution of 170 km and excellent image quality. We also took Faint Object Camera images at 0.34 microns at three longitudes, and a pair of long-exposure 1 micron images in Jupiter eclipse. Some intial results are summarised below: 1) Changes in the 15 years since Voyager are common but mostly subtle, best seen by "blinking" between HST and degraded-resolution Voyager images. The overall appearance is remarkably similar. The pyroclastic deposits of Pele still dominate their quadrant of Io, Loki and Babbar Paterae are still visible as a dark spots in essentially their Voyager locations, and most bright SO2 deposits, even small ones, are largely unchanged. The most dramatic change noted so far is the darkening of southern Colchis Regio near lat -10, long 170 at 0.35 and 0.41 microns, first noticed by Paola Sartoretti in Cycle 3 images of Io. Several changes are near the sites of known Voyager-era plumes or groundbased hot spots (Loki, Masubi, Kanehekili). 2) There were no previous disk-resolved images of Io's albedo patterns longward of 0.6 microns, the limit of the Voyager cameras. There was thus no information on the spatial distribution of the 0.6-0.7 micron absorption edge seen in ground-based disk-integrated Io spectra. The long wavelength HST images show that the distribution of this absorption edge is different from the 0.4-0.5 micron edge (possibly due to sulfur allotropes) that controls the albedo patterns seen in the Voyyager images. Most strikingly, the region with the highest 0.79/0.57 micron reflectance ratio co-incides exactly with the pyroclastic deposits of the Pele volcano. 3) We saw no obvious plumes off Io's limb, though the brightest plumes seen by Voyager would have been visible. Possibly more detailed analysis will reveal some. 4) A pair of 7-minute 1-micron images in Jupiter eclipse showed faint residual sunlight on Io but no obvious volcanic thermal emission. The lack of emission, while disappointing, will at least let us place stringent upper limits on the amount of exposed silicate magma on Io at the time of the observation. 5) As a bonus, we caught Jupiter in the wide-field camera on two occasions, though only the 1-micron images are not overexposed. The GRS is conspicuous in one frame, and the south polar haze hood is well seen (The 1 micron filter is in a CH4 absorption band). ----------------------------------------------------------------------- GROUNDBASED RADIOMETRY OF IO Bob Howell (rhowell@uwyo.edu) observed Io in the 3-5 micron region from the WIRO telesope in Wyoming on 1994/03/03 and 1994/03/04: on 3/4 he obtained photometry of Io in eclipse and during Jupiter occultation one Io orbit before the 3/6 HST 1-micron eclipse observations. He writes: Here are the preliminary numbers for M, L', and the 3.4um filter which I call S02C. (That stands for SO2 Continuum.) We also observed an occultation by Jupiter in that later filter, and saw something disappear at 10:54:58 UT. M L' SO2C Out of Eclipse Magnitude 3.518 4.011 3.82 In Eclipse Magnitude 4.685 6.879 7.90 In / Out ratio 0.341 0.071 0.023 I need to warn about a couple complications in the above numbers. First, my reduction of the standards and the extinction is crude, so the absolute magnitudes could easily by off by about 0.05 to 0.10, but the relative numbers are better. Second, there could be a significant amount of scattered Jupiter light contamination in the M and SO2 eclipse values. The L' result should be good, because I did get to make measurements on each side of Io before it got too close to Jupiter. I did not get to do this for M and SO2C, so there is some unknown contamination. I do not yet have a good feeling for the M contamination, although I hope it is small. I'll see if I can extract it indirectly from other measurements we made. For now, treat the M eclipse flux as an upper limit. I do have leading and trailing side photometry, and I may be able to get an independent estimate that way. The L' magnitude is similar to values observed previously when Loki was quiescent. The M magnitude, however, is at the high end of the normal range and is similar to values during the 1991 Loki brightening, but this may include a contribution from Jupiter scattered light, as Bob points out. The occultation lightcurve showed that at least half the 3.4 micron eclipse flux came from the point source which disappeared at 10:54:58 UT. My calculations place this point source at Loki, but Bob's preliminary calculations put the location of Jupiter's limb at the time of the disappearance about 350 km west of Loki. We haven't resolved this discrepancy yet. On balance, it seems that Loki was relatively quiescent on March 4th, and the brightening seen in January had faded, though possibly with residual brightening remaining at the longer wavelengths. Even though Loki was faint it still contributed at least half the total volcanic flux at 3.4 microns- an indication that activity elsewhere on Io's Jupiter-facing hemisphere is also low at present. ------------------------------------------------------------------------ OTHER RECENT OBSERVATIONAL RESULTS CCD photometry of Io in support of the HST observations was obtained by Beth Clark (beth@astro.as.utexas.edu) on 1994/03/07 and Orlando Naranjo (naranjo@ciens.ula.ve) on 1994/03/06, but this has not yet been reduced. Spencer just obtained new CCD spectra of the Galilean satellites at the Lowell Observatory 72" on March 29 - April 5th. The Ganymede 5770 A absorption band is still prominent on Ganymede's trailing side, and shows no obvious fine structure at 1 Angstrom resolution. Callisto's trailing side, previously unobserved, shows no similar feature. ------------------------------------------------------------------------ OBSERVING PLANS Bob Howell will be continuing 3-5 micron speckle photometry of Io in Wyoming on the following dates: Local dates (evening of ...) UT dates March 28 - April 7 29-08 April 30 - May 5 01-06 June 18 - June 24 18-24 Lonne Lane (alane@jplal2.DNET.NASA.GOV) provides the following information about his upcoming UV observations at Mauna Kea, along with an offer to support HST observations: Deborah Domingue and I finally have established the MKO observing period with their staff. We have the nights of 14-17 May. The UVSI is presently being refurbished for this activity, and we are trying to add a video camera to the system for a good record of exactly where we are pointed for each spectrum acquired. What do you know about planned HST observations of planetary objects...we might be able to provide some 300-340 nm supporting data when we are not on our principal program. John Clarke (clarke@sunshine.sprl.umich.edu) will be obtaining FUV and visible-wavelength torus imaging of Io with HST in May 1994- see the last newsletter for details. Jay Goguen (jdg@scn5.jpl.nasa.gov), Dennis Matson, and Torrence Johnson have IRTF time April 28-29 to monitor Io's IR lightcurve. Nick Schneider (nick@pele.colorado.edu), Linda Sauter, and John Spencer will use CSHELL on the IRTF on May 27 and 29 to look for 1-micron emission from neutral sulfur in Io's atmosphere. Spencer will again use NSFCAM on the IRTF to monitor the Io volcanism in Jupiter eclipse on June 19 and 21. Tony Mallama (tmallama@ccmail.stx.com) and Douglas Caprette in Maryland, Donald Collins and Michal Vojticek-Lom in North Carolina, and James Park and Peter Nelson in Australia, will once again make CCD timings of eclipses of all the Galilean satellites. Precise astrometry of the Galilean satellites is being obtained by the U.S. Naval Observatory Flagstaff Station by Alice Monet of the USNO (alice@nofs.navy.mil), in collaboration with Bill Owen of JPL, in support of the Galileo mission. Residuals are down to the 60 km level so it looks like the c. 300 km positional errors in published ephemerides will soon be cleared up. Eric Becklin at UCLA (becklin@eggneb.astro.ucla.edu) would like to start a program of monitoring Io volcanic activity, mostly at 2.1 microns in Jupiter eclipse, using the 24-inch telescope on top of the UCLA astronomy building, but has been temporarily thwarted by the LA earthquake, which has jammed the telescope dome. ------------------------------------------------------------------------- UPCOMING CONFERENCES The Spring AGU meeting in Baltimore will have a special session on HST solar system observations, many of which concern the Jupiter system. The session will be held on Wednesday 25 May from 8:15 am - 12:00 noon. ------------------------------------------------------------------------- PUBLICATIONS IN (OR EMERGED FROM) THE PIPELINE This list is contains a few new articles. If you have an article to publicise, let me know and I'll include it here. R Vasundhara (1994). Mutual Phenomena of the Galilean Satellites - An Analysis of the 1991 Observations from Vbo. Astronomy and Astrophysics 281: 565-575. A. Mallama et al. (1994) Precise timings of Galilean Satellite eclipses: an assessment of the E3 Ephemeris. Icarus 107, 212-214. Spencer, J. R., B. E. Clark, L. M. Woodney, W. M. Sinton, and D. Toomey (1993). Io hot spots in 1991: Results from Europa occultation photometry and infrared imaging. Icarus 107, 195-208. -----------------------------------------------------------------------