From spencer Thu Sep 21 11:46:05 1995 From spencer Thu Sep 21 11:46:05 1995 To: oleroemer Subject: Io Volcano News X-Sun-Charset: US-ASCII Content-Length: 5218 X-Lines: 101 Status: RO I have the following detailed report on the JPL group's 1995 results from Jay Goguen (jdg@kea.jpl.nasa.gov), followed by a brief update on other recent IRTF observations, including the Io monitoring program: ----------------- MID-IR RADIOMETRY OF IO WITH IRTF DURING 1995 We used IRTF to measure Io's mid-IR (4.8-20 um) flux on 4 observing runs during June-August 1995. The ensuing discussion of preliminary results of those observations refers to an associated post-script plot of Io's thermal emission at 4.8 (M), 8.7, 12.5 um as a function of Io's orbital longitude, available via anonymous ftp from kea.jpl.nasa.gov in directory /pub/io_rad/ in file io_1995.ps. This description is in file io_1995.txt. The current Io volcanic eruptions (summer 1995) can be placed within the context of its historical activity by comparing this to similar plots for observations from 1983-1993 published in Veeder et al. (1994), JGR Planets 99, 17095-17162, which includes a complete description of the observations, reduction and analysis techniques. Our 1995 data includes 2 complete orbital lightcurves obtained during 11-16 June 1995 UT (open squares), a period when all observers report very low activity, and 17-24 August 1995 UT (solid circles), about 1 month into the eruption of the new hot spot first reported on 20 July UT by M. Silverstone (UCLA). The 17-24 August lightcurve includes radiometry between 310-350 degrees longitude acquired on 24 August UT, just 1 orbit prior to the 26 August UT observations reported by Spencer and Dumas who identified 6 active eruptions on Io's Jupiter-facing hemisphere. Our data also include eclipse radiometry on 2 July (X symbols) and 18 July UT (* symbols). The low flux points at < 30 degrees longitude are eclipse reappearance measurements. The difference between our 4.8 um radiometry from June and August shows the net effect of the summer eruptions as an increase of about a factor of 2 in the hot spot component of the 4.8 um flux at longitudes > 270 and < 40 degrees (the ~75% reflected sunlight component has been modelled and subtracted from our 4.8 um lightcurves to obtain the spectral emittance). The contribution of the active eruptions is only ~20% of the emittance at 8.7 um and it is negligible at 12.5 um. In summary the net effects of the 6 active eruptions on the mid-IR radiometric lightcurves is well within the range of normal variation seen over the past decade or so and published in Veeder et al. (1994). A strong signal at 2-3 um and a weak signal at 8.7-12.5 um requires a small and hot source region, for example, an active silicate eruption with a limited area covered by the warm flows that increase the 8.7-12.5 um flux. If the eruption lasts long enough and the effusion rate is large enough, the 8.7-12.5 um flux will increase with time as the area covered by warm flows increases. In this scenario, the 2-3 um in-eclipse imaging is a sensitive detector of active vents while the 10 um region radiometry measures the total area covered by warm flows. A possible explanation is that the in-eclipse imaging at 2-3 um is now sensitive enough and frequent enough to begin to detect Io's widespread "normal" activity that must be occurring nearly continuously to support the relatively steady level of the mid-IR flux and heat flow documented in Veeder et al. (1994). The detection of the 9 active eruptions in Voyager images at a time when Io was not unusally IR-bright suggests that small eruptions are frequent and widespread. Perhaps we have now achieved the instrumental sensitivity to detect them and Galileo has engendered the additional observational stamina to look long and hard enough and across sufficient spectral range to recognize them! Our 3 in-eclipse emission measurements tell a similar story. The table below gives the in-eclipse fluxes observed at 4.8 and 8.7 um. We have fit these fluxes with a single temperature circular gray body using an emissivity of 0.9 to provide a simple physical model for the integrated in-eclipse hemisphere hot spot emission and its variation during July and August. Date 4.8 Flux 8.7 Flux Single Gray Body (UT) W/m2/um W/m2/um T(K) r(km) 07/02/95 0.013 0.028 356 39 07/18/95 0.017 0.035 363 40 08/19/95 0.030 0.051 385 43 Jay Goguen Diana Blaney Dennis Matson Glenn Veeder Torrence Johnson JPL ----------------- I (Spencer) just observed the 9/18 and 9/20 occultations and eclipses of Io by Jupiter. Conditions weren't great but it appears that Loki is still bright, and is probably the brightest hot spot visible when Io is in eclipse. The daily Galileo-support monitoring program of Io and Jupiter at the IRTF now has most of the bugs out, and should now be executed every evening that weather is good and NSFCAM, the IRTF 1-5 micron camera, is on the telescope. Image archives are publicly available by anonymous ftp to irtf.ifa.hawaii.edu, directory pub/galileo. Note that due to software limitations, the name of each archive corresponds to the HST date, one day earlier than the UT date.