__________________________________________________________________________ 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 3 August 17, 1993 Editor: Discipline Leader: John Spencer Jay Goguen Lowell Observatory Jet Propulsion Lab, M.S. 183-501 1400 W. Mars Hill Rd. 4800 Oak Grove Dr. Flagstaff, AZ 86001 Pasadena, CA 91109 Voice: (602) 774-3358 Voice: (818) 354-8748 Fax: (602) 774-6296 Fax: (818) 354-0966 Internet: spencer@lowell.edu Internet: jdg@scn2.jpl.nasa.gov ------------------------------------------------------------------------------ EDITORIAL COMMENTS It has proved difficult to find time to both observe Jupiter and prepare newsletters about it: at least, that's my excuse for the lack of a full-scale "Ole Roemer Messenger" since last January. E-mail subscribers have received several "Io Volcano News" reports in the meantime, giving updates on Io's volcanic activity, and I suspect that brief, informal, e-mail reports may become the main means of communication in future, as they take much less work and are more likely to happen in a timely manner. If I missed important news from this issue, let me know- I can issue an addendum in a week or two. ------------------------------------------------------------------------------ VOLCANIC ACTIVITY UPDATE Infrared observations of Io by Spencer and co-workers; Howell; Blaney; Bezard; and Goguen and co-workers were made in January, February, March, May, and July of 1993. Loki was in its quiescent state whenever it was observed, and no other unusual activity was seen. The hot spots "Kanehekili" and "Hi`iaka" continued to be conspicuous on Io's leading hemisphere, as they have been since 1990. ------------------------------------------------------------------------------ NEW ABSORPTION FEATURE ON GANYMEDE As a change from Io news, I can report the discovery of a shallow (2% deep) but well-defined absorption feature on Ganymede's trailing side in CCD spectra taken by myself, Wendy Calvin, and Mark Wagner at the Lowell Observatory 72" telescope in March and July 1993. The feature is deepest at 5770 A, is about 100 A wide, and is much sharper on the long-wavelength side. We don't see it on Io or the leading sides of Ganymede, Callisto or Europa. It passes all tests for reality that we have been able to think of. It's not due to water ice and so far we haven't been able to identify it. Suggestions are welcome! See our poster at DPS. ------------------------------------------------------------------------------ SHOEMAKER-LEVY 9 IMPACT WITH JUPITER As most of you will know by now, it is now almost certain that the >17 nuclei of this multiple comet will hit the night side of Jupiter within a day or two of 1994/07/21. There's a great deal of activity going on in planning for this unique event, with workshops in Tucson on August 23rd and in the Washington DC area in January 1994. The comet will approach Jupiter over the south pole and will thus not pass close to any of the major satellites, so most of the action is in the Jupiter atmosphere and torus (?) disciplines of IJW. However, because the impact will be hidden from direct view from Earth, the only hope for ground-based observations of the impact itself is to look at its reflection on the satellites, so satellite photometric (and spectroscopic?) expertise in this discipline may be very useful. Kevin Zahnle estimates that reflected light from the bolide produced by atmospheric entry of a 5km comet nucleus, and the subsequent fireball, might increase the total brightness of Io in sunlight in the visible by about 10% for several seconds. Current best estimates of the sizes of the comet are nuclei are closer to 1km, but it will still be well worth doing satellite photometry, especially if we are lucky enough to have a satellite in Jupiter eclipse at the time of an impact. Current predictions of the impact times of individual nuclei are still uncertain by several days so it's too early to say what satellites might be suitably placed. However, to assist in advance planning, satellites will be directly behind Jupiter at the following times (and will be visible in Jupiter eclipse a few hours later): Io Europa Ganymede Day Hour Day Hour Day Hour ----------- ----------- ----------- 7/16 21.7 7/19 8.3 7/22 11.3 7/18 16.2 7/22 21.6 7/20 10.7 7/22 5.2 7/23 23.6 7/25 18.1 Callisto reaches eastern elongation on July 19th, but in any case is likely to be too far off to be useful. For those with large telescopes Amalthea may also be a feasible reflector, but I don't have an Amalthea ephemeris for 1994 yet... For more information, check out Mike A'Hearn's bulletin board: telnet to "pdssbn.astro.umd.edu" and log on as "c1993e" ----------------------------------------------------------------------------- IO CONFERENCE, SAN JUAN CAPISTRANO, JUNE 1993 This was a very successful conference and only a brief summary is possible here. New results on Io's surface and volcanism included the KAO discovery of a 25% deep absorption band on Io at 2.788 microns by Salama et al., possibly due to H2O in SO2 but also co-inciding with an SO2 frost band in new laboratory reflectance spectra by Nash; the suggestion by Nash that an areal mixture including 16% Na2S and 8% S2O (on the leading side) can explain the kink in the visible spectrum near 5500 A, clearly seen in new CCD spectra by Spencer; possible identification by Khanna et al. of Na2SO3, NaHSO4, SO3, NaOH, and S8 in 15-40 micron Voyager thermal emission spectra of Io; new HST images of Io from 2300 A to 7100 A presented by Sartoretti and Secosky, showing little obvious change since Voyager but apparent regional changes in only 1 hour (!) at 2300 A; new HST 2250-3300 A spectra of Io, showing structure that is hard to fit with SO2 frost, presented by Clarke; a summary by Ostro of radar observations of Io (Io is distinctive, with quite high radar albedo and circular polarisation ratio, but good disk resolution will not be possible till the late 1990s); a discussion by McEwen of the apparent correlation of SO2 distribution with low elevations on Io; spectacular Voyager stereo imagery of the surface by Schenk; comparisons of dayside and nightside Voyager images of Io by Buratti and Simonelli, with conflicting conclusions; a discussion by Matson of the previously unrecognised importance of solar heating of low-temperature hot spots in understanding Io's heat flow, and evidence for the lack of large (> 30%) temporal and longitudinal variations in global heat flow since 1983, presented by Veeder; and new 2-14 micron spectra of Io's surface and hot spots by Blaney and Hammel. My apologies if I left you out of the above list! There was also much discussion of Galileo observations of Io. McEwen said that about 130 400x400 high-resolution CCD images of Io would be returned during the December 1995 Io flyby, but that the highest resolution (c. 100m) images of Io might be heavily degraded by radiation effects. Carlson showed that NIMS observations would also be compromised by radiation, though less severely. 25 km resolution spectral mapping from 0.7 to 5.2 microns will be possible. Spencer described plans for mapping Io's 17-50 micron thermal emission with the PPR instrument: global maps with about 200 km resolution will be possible as well as targeted observations of individual hot spots. Ground-based support of Galileo observations was deemed essential, especially monitoring of infrared volcanic emission with highest possible time resolution to complement Galileo observations. In particular, Goguen pointed out that neither NIMS nor PPR covers the peak in volcanic thermal emission near 10 microns, and only groundbased observations can thus monitor variations in Io's total heat flow. ------------------------------------------------------------------------------ ICY GALILEAN SATELLITE CONFERENCE, FEBRUARY 1-3 1994 There will be a complementary conference on the other Galilean satellites, also at the San Juan Institute, on February 1-3 1994. The aim is to discuss progress in our understanding of Europa, Ganymede, and Callisto since the last Galilean Satellite conference in 1980, and provide input on the planning of Galileo observations of these bodies. A special journal issue and NASA Conference Publication may result from this conference. Abstract deadline is December 3rd 1993. Contact Paul Schenk at LPI (schenk@lpi.jsc.nasa.gov, (713) 486-2175) for more details. ------------------------------------------------------------------------------ ECLIPSES AND MUTUAL EVENTS OF THE FIRST EIGHT SATURNIAN SATELLITES DURING THE 1993-1996 PERIOD This Spring Jay Goguen received a pre-print of a prediction paper with the above title by J.-E. Arlot and W. Thuillot. You should be able to get a copy directly from the authors by contacting them at: Bureau des Longitudes 77, avenue Denfert-Rochereau 75014 - Paris 40 51 22 21 (telephone) (33) 1 46 33 28 34 (fax) We should consider whether the Saturnian satellites fall within the aegis of the IJW, but certainly skills and personnel overlap. Observations of these upcoming events will help to refine the ephemerides of the Saturnian satellites and will be important for the Cassini mission. ------------------------------------------------------------------------------ PUBLICATIONS IN (OR EMERGED FROM) THE PIPELINE This list, of papers on the surfaces of the Jovian satellites published or submitted since the last Ole Roemer in February 1993, is bound to be incomplete. If you have or know of other papers to include here, please let me know and I'll put them in an addendum to this newsletter. M.L. Johnson, D.S. Burnett (1993), SO2-Rock Interaction on Io - Reaction Under Highly Oxidizing Conditions. Journal of Geophysical Research - Planets 98, 1223-1230 RM Nelson, AL Lane, ME Morrill, BD Wallis, J Gibson, WD Smythe, LJ Horn, BJ Buratti (1993), The Brightness of Jupiter's Satellite Io Following Emergence from Eclipse - Selected Observations, 1981-1989. Icarus 101, 223-233. AL Whipple, PJ Shelus, GF Benedict (1993), Astrometric Observations of the Faint Outer Satellites of Jupiter During the 1990-1991 and 1991-1992 Oppositions. Astronomical Journal 105, 347-349 J.R. Spencer, 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. Submitted to Icarus, May 1993. G.J. Veeder, D.L. Matson, T.V. Johnson, D.L. Blaney, and J.D. Goguen (1993), Io's Heat Flow from Infrared Radiometry: 1983-1993. Submitted to Icarus, June 1993. N.J. Sack, R.A. Baragiola, and R.E. Johnson (1993), Effect of Plasma Bombardment of Io's Leading and Trailing Hemispheres, Icarus 104, 152-154. W.M. Calvin, R. N. Clark (1993), Spectral Distinctions between the Leading and Trailing Hemispheres of Callisto: New Observations. Icarus, 104, 69-78. Abstract: An analysis of recent telescopic observations of Callisto results in new insights regarding spectral variations from the leading to the trailing hemisphere of Callisto. Examination of data in the wavelength range from 2.0--2.5 um indicates that previous suggestions of spectral differences are most likely the result of experimental uncertainty or error. Slight variations in the slope of this wavelength range are consistent with larger ice grain sizes on the trailing hemisphere. The new observations confirm the presence of an absorption feature centered on 3.4 um in the spectrum of the leading hemisphere. Theoretical spectral modeling indicates this feature is caused by small amounts of fine-grained water ice. Finally, an absorption feature near 3.1 um, is indicated but cannot be confirmed due to the strong variation in the spectrum of water ice in this region. If this feature is real, rather than an artifact of the reflectance modeling, it is similar in location and bandwidth to a feature seen in the spectrum of Ceres, attributed to NH4-bearing clays. D. P. Simonelli, J. Boucher, P. Helfenstein, J. Veverka, and M. O'Shaughnessy (1993), Search for temperature-related albedo changes in nightside and posteclipse images of Io, submitted to Icarus April 1993. Abstract: Using an image-summing process that increases the visibility of Jupiterlit surface features in Voyager images, we have produced the best-ever violet- filter image of the nightside of Io, and the best-ever nightside/dayside brightness ratio map of this Jovian moon. The ratio map shows no convincing evidence, on either global or regional scales, of diurnal temperature-dependent albedo variations. We have also taken an image-ratioing technique developed by O'Shaughnessy et al (1989, Lunar Planet. Sci. 20, 812-813), which those authors applied to Voyager violet-filter observations of one Io eclipse reappearance, and extended it to two other, higher-resolution Voyager posteclipse imaging sequences. In none of the three imaging sequences do we find any isolated surface regions that convincingly exhibit posteclipse temperature-related albedo variations. These negative results suggest that on Io, pure S8 sulfur, and transient nighttime or in-eclipse deposits of SO2 frost, are at best limited to isolated areas smaller than the resolution of the images in use (i.e., smaller than a few tens of kilometers in size). Such limits are consistent with (1) the negative results reported by the majority of telescopic observers who have searched for posteclipse brightening of Io; (2) indications that physical processes in the Ionian surface environment will change any S8 into other allotropes of sulfur; and (3) suggestions that Io's atmosphere is too thin to allow the deposition of transient, optically-thick SO2 frost layers at nighttime or during eclipse. ------------------------------------------------------------------------------ UPCOMING OBSERVING RUNS It's a bit late for this section- Jupiter season is virtually over (solar conjunction occurs on 18th October 1993). However, I will be attempting to observe Io's hot spots with the new IRTF 1-5 micron Facility Camera on September 3, 5, and 12 UT in order to practice observations near to the sun in preparation for the Galileo Io encounter in December 1995. For 1994, quadrature occurs on February 3rd and opposition on May 1st. IRTF proposal deadline for January - June 1994 is October 1st. ------------------------------------------------------------------------------