__________________________________________________________________________ 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 Hardcopy issue number 1 September 8, 1992 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 This document begins the revival of the Ole Roemer Messenger, the newsletter of the International Jupiter Watch Satellite Discipline. The Satellite Discipline covers observational work on the solid surfaces of the Galilean satellites, with an emphasis on time-variable phenomena and therefore a strong bias towards Io. I intend to send out an issue whenever newsworthy events occur here or at Jupiter, but at least 2 or 3 times per year. Contributions are needed! I would like to include dates and topics of upcoming observing runs, titles and/or abstracts of submitted papers, news of upcoming conferences, reports on observational results, and anything else you think is newsworthy. I'd also appreciate feedback on format and/or content. DISTRIBUTION Unlike the old Ole Roemer Messenger, e-mail will be the dominant publishing medium, but I will mail hardcopies to people who don't have access to e-mail or whose e-mail addresses I don't know: these will be plain printouts of the e-mail version. Please tell me if you received hardcopy but would prefer e-mail: e-mail is faster, cheaper, and less hassle. Let me know if you want to be added to or removed from the mailing list. Also, if you have an Internet address but got this via another network, please let me know and I'll send to Internet instead, in the interests of efficiency. If you got it twice through some error of mine, let me know also. The Messenger will also be available by anonymous ftp over the Internet. Type "ftp lowell.edu" or "ftp 192.103.11.2", log in as "anonymous", use your name as a password, and type "cd pub/ijw". In the ijw directory, a "dir" command will show issues of the Messenger in the form of ASCII files with names like "roemer1.doc". Typing "get roemer1.doc" will transfer a copy to your own machine, and "bye" will terminate the ftp session. Because e-mail and hardcopy versions may not always go out simultaneously there may be slight differences between them. But I will try to make sure that everyone gets all the information. I will also send out periodic brief e-mail bulletins describing the state of Io volcanic activity as deduced from infrared observations, usually on the day of the observations. Let me know if you _don't_ want to receive these bulletins. __________________________________________________________________________ REPORT ON THE TIME-VARIABLE PHENOMENA IN JOVIAN PLANETARY SYSTEMS MEETING, ANNAPOLIS, JULY 13-16 1992. POSTER SUMMARIES This successful meeting featured eight poster talks primarily related to the satellite discipline. Below is a summary: more detail on many of these topics can be found in the DPS abstracts later in this newsletter. Four posters presented results from monitoring of the thermal emission from Io's volcanic hot spots by various techniques. Jay Goguen et al. showed that simultaneous 3.8 and 10 micron photometry of the 1991 occultations by Europa requires that Loki progressively increased in area and decreased in temperature during Jan. - Mar. indicating that the occultation measurements witnessed the evolution of a major eruption of Loki. Torrence Johnson et al. (represented by Diana Blaney) presented ten years of 4.8 - 20 micron photometry of Io's lightcurve, indicating variability at Loki and a newly-reported major outburst on the Loki hemisphere on January 9, 1990, which required surface temperatures of about 1500 K (a new record for Io). Bob Howell and David Klassen presented higher time resolution lightcurve photometry at 4.8 microns, and photometry of Europa and Jupiter occultations of Io also showing large variations in Loki's thermal emission. John Spencer et al. showed 2.2 - 4.8 micron resolved images and photometry of Io in Jupiter eclipse and during Jupiter occultations, revealing several variable hot spots, including Loki, on the Jupiter facing hemisphere. The remaining satellite-discipline posters were more varied. Paola Sartoretti et al. showed HST images of Io at 2900 A and 4000 A, showing no change in the 4000 A appearance of the trailing hemisphere since Voyager time at the c. 250 km resolution of the HST images, but indicating strong 2900 A absorption, probably by SO2, in regions of Io that are bright in visible-wavelength images. Anthony Mallama showed the importance of albedo patterns and photometric shading on the astrometric interpretation of occultation and eclipse photometry of the Galilean satellites. Keith Noll reported a possible 4.8 micron hemispheric albedo dichotomy on Ganymede, the trailing hemisphere being brighter. Finally, Fredrick West considered the possiblity of solid or gaseous CO2 on the icy Galilean satellites. CONCLUSIONS Combining infrared photometry of Io from all observers over the past three years, the satellite discipline was able to chart the course of two major brightenings of Loki, one lasting from mid-fall of 1989 to mid-winter 1990, and the second, more tightly constrained, lasting from December 1990 till April 1991. Loki was very faint the rest of the time, in particular remaining quiet throughout the 1991-1992 apparition. No correlation between Loki activity and the brightness of the Io torus or sodium emissions was found in a joint discussion with the atmosphere / torus discipline, and there was a consensus that the atmosphere was the "missing link" in the causal chain between volcanic eruption of materials from Io's interior and their appearance in the magnetosphere, and that more atmospheric observations were needed. We noted the importance of continued observations in the 8 - 10 micron region, which have been sparser than shorter wavelength observations in recent years, and which sample the low-temperature volcanic emission which carries most of the heat flow. We also noted the importance of real-time first-order reduction of photometry, so that outbursts or other significant events can be detected as they occur, enabling follow-up observations. There are continuing problems with the ephemerides for the Galilean satellites: the relative positions of Io and Europa during the 1991 occultation series were up to several hundred km different from ephemeris predictions. Jay Goguen has generated working ephemerides for the Europa occultations (only) by adjusting orbital parameters to simultaneously fit the best occultation lightcurves, but the source of the discrepancy with the standard JPL ephemeris is still unknown. The satellite working group agreed that there has been a problem in the past comparing the differing volcanic emission flux units in different people's publications, and we are working on a comprehensive description of units, with conversion factors, to be included in a future publication. By the way, note that we have decided to refer to the Loki events as "brightenings" rather than "outbursts", in order to distinguish them from the much briefer and hotter outbursts that have been observed, such as the January 1990 event. _________________________________________________________________________ UPCOMING OBSERVING PLANS Next Jupiter Opposition: March 31, 1993. I look forward to contributions to this section from other observers, both ground-based and HST. Please let me know when you have been granted observing time, and supply the following kinds of details if possible. Who: J. Spencer and B. Clark What: Jupiter eclipse and occultation imaging of Io, 1.6 - 4.8 microns. Why: Continued monitoring of locations, temperatures, and areas of multiple hotspots on the Jupiter-facing hemisphere. Where: IRTF, Mauna Kea When (UT): 1992/12/04; 1992/12/11. ________________________________________________________________________ PUBLICATIONS IN THE PIPELINE Likewise, please let me know if there are submitted, accepted, or recently published publications related to the surfaces of the Galilean Satellites that you would like to publicise, including an abstract if you wish. Spencer, J., R. Howell, B. Clark, D. Classen, and D. O'Connor, "Volcanic Activity on Io at the Time of the Ulysses Encounter", Science, in press (Ulysses Jupiter encounter special issue, to appear in early Septemeber). Abstract: We report ground-based infrared observations of Io's volcanic activity between October 1991 and March 1992, contemporaneous with the February 8 1992 Ulysses observations of the Io torus, whose population of heavy ions is ultimately derived from Io volcanism. Volcanic thermal emission was at the low end of the normal range at all Io longitudes during this period: in particular the dominant hot spot "Loki" was quiescent. Resolved images showed at least four hot spots on Io's Jupiter-facing hemisphere, including Loki and a long-lived spot on the leading hemisphere (`"Kanehekili"), of comparable 3.5 micron brightness but higher temperature. "Europa, Ganymede, And Callisto: New Radar Results From Arecibo And Goldstone" S. J. Ostro (300-233, Jet Propulsion Laboratory, Pasadena, CA 91109), D. B. Campbell, R. A. Simpson, R. S. Hudson, J. F. Chandler, K. D. Rosema, I. I. Shapiro, E. M. Standish, R. Winkler, D. K. Yeomans, R. Velez, R. M. Goldstein "The Structure Stability And Global Distribution Of Io's Atmosphere", E. Lellouch, M. Belton, I. De-Pater, G. Paubert, S. Gulkis, and T. Encrenaz, Icarus 98, 271-295. The following titles emerged from a search of recent A & A Monthly Indexes: On The Gas Dynamic Structure Of Volcanic Plumes On Io (B. Zhang) Acta Astrophysica Sinica V.11:1, P. 72, 1991 Discovery Of A Second Narrow Absorption Feature In The Near Infrared Spectrum Of Io (D.F. Lester, L.M. Trafton, T.F. Ramseyer, N.I. Gaffney) Preprint. Mcdonald Obs.: Univ.Texas, Austin No. 179, 1992 Tracking Io's Hot-Spots, Sky And Telescope V. 82, No. 6/Dec, P.585, 1991 ______________________ PUBLICATION DEADLINES: Results of the 1991 Europa occultations of Io: deadline for submission to Icarus is October 1st 1992. Papers arising from the IJW Annapolis meeting: deadline for submission to JGR Planets is now October 5th 1992. ________________________________________________________________________ UPCOMING MEETINGS/WORKSHOPS DPS 1992: OCTOBER 12-16, MUNICH Here are the preliminary program and abstracts for the Io-surface-related papers at the 1992 DPS meeting, as obtained from the LPI on-line conference program search system: Monday, October 12, 1992 SESSION 05 ....... Outer Planet Satellites I 2:10 - 4:00 p.m. Room B Chair(s): E. Grun D. L. Matson 05.01 Matson D. L.* Veeder G. J. Johnson T. V. Blaney D. L. Goguen J. D. Io's Volcanic Emission During Ulysses' Jovian Encounter In this report we present observations of Io obtained at the time of Ulysses' closest approach to Jupiter. The observations are then compared with data for 1983, 1986 (which shows a "typical" leading hemisphere outburst), and with two models. A "Voyager" model was constructed by McEwen et al. (1985, J. Geophys. Res., 90, 12345) to fit the Voyager IRIS observations. The Ulysses model was derived from the Voyager model, with the proviso that a minimum number of changes be made to the model parameters in order to approximate the 1992 groundbased data. The simplest change that provides a good match to the 1992 data is removal of the hot, 450 K component of Loki. Thus, at the time of Ulysses' encounter with Jupiter, the volcanic activity on Io was somewhat lower, but yet comparable to the levels seen by Voyager. This fact is of interest because the measurements made by Ulysses and Voyager in the magnetosphere and particularly in the Io torus are thought to depend in some way on the level of volcanic activity exhibited by lo. This follows because Io is the source of the ions in the torus, which, in turn, is a major source for the magnetosphere. The rate of supply of new material to the torus may be related to the rate at which volcanic activity supplies hot magma and gases to Io's surface and atmosphere. Our observations were made with NASA's 3-m Infrared Telescope Facility (IRTF). This work was carried out under contract to NASA. 05.02 McEwen A. S.* Isbell N. R. Edwards K. E. Pearl J. C. New Voyager 1 Hot Spot Identifications and the Heat Flow of Io The infrared interferometer spectrometer (IRIS) on the Voyager 1 spacecraft observed individual thermal anomalies on Io from about 4- to 55-micrometer wavelength (180-2500 cm^-1). The circular field-of-view (FOV) ranges from 50 to 700 km in diameter at Io. Nine major hot spots were identified from IRIS data by Pearl and Sinton [1982, Satellites of Jupiter, D. Morrison, ed., 724], and two additional hot spots have been identified by Pearl [1985, BAAS 17, 691; 1988, BAAS 20, 818]. We have derived new models for each of the ~400 IRIS spectra of Io by minimizing the chi-squared statistic over each spectrum from 200 to 2400 cm^-1. For almost every spectrum the best fit was achieved with a graybody emissivity of one. We have identified six "new" volcanic hot spots (at least 170 K) that can be matched to specific volcanic features seen in the images: Daedalus Patera (lat. 20 degrees, long. 275 degrees; 4 x 10^4 km^2 at 180 K), Mazda Catena (lat. -8 degrees, long. 314 degrees; 60 km^2 at 420 K), Aten Patera (lat. -48 degrees, long. 311 degrees; 4 x 10^3 km^2 at 191 K), Viracocha Patera (lat. -62 degrees, long. 282 degrees; 7 x 10^3 km2 at 195 K), Mbali Patera (lat. -32 degrees, long. 8 degrees; 2 km^2 at 574 K), and an unnamed volcanic center (lat. 10 degrees, long. 335 degrees; 7.0 x 10^3 km^2 at 186 K). There is also evidence in eight regions for other "new" hot spots for which we have not yet identified corresponding volcanic features. The total power emanating from Io must include contributions from conducted heat and low-temperature geothermal anomalies in addition to the obvious hot spots. The great majority of the IRIS observations, away from obvious volcanic hot spots, require a two-temperature model to adequately fit the data. Typically, about 70-99% of the FOV is modeled with a temperature from about 80 to 120 K and about 1-30% of the FOV requires a temperature about 50 to 90 K higher than the first temperature. Local temperature contrasts of up to 20 K can be produced by a variety of "passive" mechanisms, but we believe that ubiquitous geothermal anomalies explain the 50 to 90 K enhancements. By using a subset of the spectra with FOVs that cover the major hot spots once and only once, we computed a total power of 1013 W (2.4 W m^-2 if spread globally), which nearly equals the upper limit to the steady-state energy dissipation from tidal heating. The IRIS spectra analyzed here cover only ~50% of Io's surface (including the most energetic hemisphere) and does not include estimates for conducted heat or low-temperature anomalies. Therefore, Io's global heat flow during the Voyager 1 flyby probably substantially exceeded that possible from steady-state tidal heating; provided this time period was not highly anomalous, time-variable tidal heating, either periodic or episodic, is required. 05.03 Spencer J. R.* Clark B. E. Woodney L. Io Hot Spot Imaging in 1991/1992: It's Been a Quiet Year at Loki Patera We are continuing to monitor volcanic activity on the Jupiter-facing hemisphere of Io by means of disk-resolved imaging and Jupiter occultation photometry with the 1-5-micrometer infrared camera "ProtoCAM" at the IRTF. We observed Io in sunlight and Jupiter eclipse on 10 occasions between October 1991 and May 1992, a period that included the Ulysses encounter with Jupiter in February 1992. Loki, the largest hot spot on the Jupiter-facing hemisphere, is strongly variable: it was bright in December 1989, very faint in the Spring and Fall of 1990, and bright throughout the winter of 1991. Volcanic thermal emission from Loki was at a very low level throughout the 1991/1992 season, though still detectable in images and Jupiter occultations. Loki's 3.8-micrometer flux in December 1991 was a factor of 16 lower than in December 1989, for instance. Disk-resolved multiwavelength imaging showed that the quiescent Loki was cooler and larger than other hot spots visible on the disk, and cooler than during its active phase: its 3.8: 4.8-micrometer color temperature was 355 + 20 K on 1992/01/24, while Io's diskintegrated 3.8:4.8-micrometer color temperature on 1991/01/24, when Loki was active and dominated the thermal emission, was 482 +- 41 K. The data analyzed so far is consistent with the idea that Loki's flux changes are due mostly to a change in the temperature rather than the area of the exposed volcanic materials. Kanehekili, a hot spot near 10 degrees S, 40 degrees W first seen in December 1989, had similar flux to the quiescent Loki at 3.8 micrometers during the 1991/1992 season, but was smaller and hotter, with a 3.8:4.8-micrometer brightness temperature of 465:40 on 1992/01/24. Other hot spots are detectable in the data but are more difficult to isolate and locate, though persistent emission in the southern hemisphere is seen in the general neighborhood of Creidne Patera, a hot spot detected by Voyager in 1979. 05.04 Johnson T. V.* Matson D. L. Veeder G. J. Blaney D. L. Goguen J. D. A High Temperature Io Event in 1990 Flux at both 4.8 and 8.7 micrometers in the Loki hemisphere was significantly higher in late 1989 and early 1990 than in earlier or later (1992) apparitions. On January 9, 1990, 4.8-micrometer flux from these longitudes was further enhanced by a factor of ~2.5 over that observed just two days earlier; 8.7-micrometer flux was up only ~20% and little change was seen at 20 micrometers during the same time. The characteristics of a new source (colocated at Loki) required to match these flux levels were determined using the modeling approach of Johnson et al. (Science, 242, 1280, 1988). A hot spot with radius 4.2 km and T = 1500 K provides a good match to the data; temperatures below 1000 K cannot match both the 8.7- and 4.8-micrometer flux levels simultaneously. Following the arguments in Johnson et al., such high temperatures imply that the source of the emission was primarily molten silicate and not sulfur. These observations were performed at NASA's IRTF in Hawaii; the research was carried out Caltech/JPL under contract to NASA. 05.05 Goguen J. D.* Veeder G. J. Matson D. L. Johnson T. V. Brown R. H. Toomey D. Sinton W. M. Nair H. A. Wang S. Gradie J. Spatial Resolution of Io's Hot Spots: Simultaneous 3.8 and 10 Micrometer Photometry of Occultations of Io by Europa We report the results of simultaneous L' (3.8 micrometer) and N (10 micrometer) filter photometry of the series of occultations of Io by Europa obtained with the NASA/IRTF during January March, 1991. During a typical occultation, Europa's limb occults Io's surface with an apparent velocity of <20 km/s. Because thermal emission from a few bright hot spots contributes a signficant portion of Io's total flux at these wavelengths, fast photometry during occultations of a hot spot can be used to map its emitting region at a spatial resolution of ~10 km. We selected the 2 wavelengths to sample the hot (~600 K, L') and warm (~300 K, N) components of the thermal emission that are needed to match the shape of the hot spot infrared spectrum. Successful observations were acquired on 1/1, 1/8, 1/15, 1/26, 2/2, 2/9, 2/27, and 3/6/91 UT with 2, one-dimensional scans acquired across each occulted hot spot on each date. Both Loki and Pele were active and they were occulted repeatedly with different limb scan orientations. E-W scans in the L' filter on 2/2 and 2/9 resolve the Loki region into 2 narrow (20-30 km wide) N-S oriented "fissures" ~100 km apart. The L' flux from both Loki and Pele varied markedly during this period with Loki's flux increasing by ~50% during February. This work represents one phase of research carried out at Caltech's Jet Propulsion Laboratory under a contract with NASA. 05.07 Sartoretti P.* McGrath M. A. Paresce F. Imaging of Io with the Faint Object Camera Aboard HST Abstract not available. 05.14-P Howell R. R. Klassen D. R. Infrared Speckle Observations of Io: 1987-1992 During the past five years we have used a scanning speckle interferometer to regularly measure the volcanic activity on Io. The data, primarily at a wavelength of 5 micrometers, can be used to obtain rotational lightcurves which indicate the general level of activity as a function of longitude. They can also be used to provide locations for the brighter hot spots. The lightcurves for the most part show little variation for the leading side, but show systematic changes in the trailing (Loki) hemisphere. The Loki hemisphere appears to undergo brightenings every year or two, which last for several months. A few short term brightenings have been detected at other longitudes, but as yet no clear pattern of repetition has been established. The 1991 series of mutual events provides additional information concerning the structure of the Loki source. The events occurred during a time of enhanced activity at Loki, and the data obtained in 1991 provides a detailed history of the eruption. The duration and the level of activity in 1991 were typical of the brightenings seen in past years. ______________________ SODIUM ATMOSPHERES WORKSHOP San Juan Institute, San Juan Capistrano, late Jan. or early Feb. 1993 This is intended to be a small meeting covering the "sodium atmospheres" of the Moon, Mercury, and Io. Contact Doug Nash, (714) 240-2010; 240-0482 (fax). ______________________ IO CONFERENCE San Juan Institute, San Juan Capistrano, June 1993. This is planned to be a comprehensive 1-week, single-session conference covering all aspects of Io, from the interior to the magnetospheric plasma, to set the stage for the upcoming Galileo encounter. Some type of follow-up publication is envisaged. The exact date is not yet determined but it will not clash with the ACM III meeting in mid-June. Again, contact Doug Nash, (714) 240-2010; 240-0482 (fax), for more details. _________________________________________________________________________