Report from the workshop "Global Change on Triton" held during DPS 2000 meeting on Thursday, October 26, organized by Leslie Young (SwRI) and Mike Hicks (JPL). ======== Overview ======== The workshop was centered around four summary presentations on what evidence exists for global change on Triton. 1. Atmosphere and occultations (Jim Elliot) 2. Visible-wavelength spectroscopy and photometry (Mike Hicks) 3. Near infrared-wavelength spectroscopy (Dale Cruikshank) 4. Ultraviolet-wavelength spectroscopy and photometry (Leslie Young) Throughout, vigorous discussion was encouraged. What follows is a transcription of notes made by Leslie Young during the workshop. References to the literature are given in abbreviated form, usually as simply the first author and the year of publication. In most cases, the references are easily found by referring to the Neptune and Triton U of Arizona book. For obscure or recent references, a more complete citation is given. ========= Attendees ========= Bonnie Buratti Bonnie.J.Buratti@jpl.nasa.gov Catherine de Bergh debergh@obspm.fr Dale Cruikshank dale@ssa1.arc.nasa.gov Mona Delitsky mld@scn5.jpl.nasa.gov Candy Hansen cj@frostrus.jpl.nasa.gov Jim Elliot jle@mit.edu Will Grundy grundy@lowell.edu Mike Hicks hicksm@galah.jpl.nasa.gov Susan Kern susank@as.arizona.edu Mike Person mjperson@mit.edu Martha Schaefer mschaefer@astro.as.utexas.edu John Spencer spencer@lowell.edu Larry Sromovsky sro@calypso.ssec.wisc.edu Larry Trafton lmt@astro.as.utexas.edu Leslie Young layoung@boulder.swri.edu ================================================================= Summary presentation #1: Atmosphere and occultations (Jim Elliot) ================================================================= Pre-Voyager atmosphere: See absorption from ices (N2, CH4) in near-IR spectroscopy. Because of vapor-pressure equilibrium, we expected an atmosphere. Voyager detected an atmosphere: RSS (Radio science) 16 microbar (Tyler et al) revised to 14 microbar (Gurrola thesis) UVS (Ultraviolet spectrometer) Measured N2 density (450-700 km), CH4 density (25-50 km). ISS (Imaging subsystem) Streaks, clouds, haze, plumes = evidence of atmosphere. Post-Voyager models: Troposphere: Yelle 1991, Stansberry 1992 Radiative-convective: Stevens 1992, Lellouch 1992, Krasnopolsky 1993, Strobel and Summers 1995 Frost transport: Spencer and Moore 1992, Hansen and Paige 1992 Circulation: Ingersoll 1989, Forget 1999 (Pluto/Triton Flagstaff workshop). Occultations observed: Date Star Comment 1993 July 10 Tr60 1995 Aug 9 Tr148A,B Lots of chords, central flash 1997 July 18 Tr176 Portable telescopes 1997 Nov 4 Tr180 HST/FGS -- wonderful dataset Occultation thermal profiles and radiative-conductive profiles: Pressure at 1400 km radius has changed: 1.8 microbar to 2.15 microbar. Extrapolation to surface suggests pressure increase from 14 to 18 microbar. Thermal profile has also changed. The "post-Voyager" model disagrees with the Tr180 inversion. Issues: The thermal profile in the middle atmosphere (0-50 km) is different from post-Voyager models, and hard to reproduce with models. Changing pressure. Non-spherical shape => too-large winds. [ DISCUSSION: Can we get the surface pressure from the frost temperature, as measured by the line shape of the N2 feature? Will Grundy replies: Need to constrain grain size and temperature both. You can do this some if you have multiple bands, but with N2 you only have the 2.15 micron feature. However, there is a side lobe to the 2.15 micron feature that only appears when the temperature is below 42 K. One problem is that this side lobe coincides with a solar line. We need to be careful with solar correction. Take standard spectra of spectrally-neutral solar-system objects. Spex at IRTF, a wonderful new instrument, can get R=2000 spectra over a large wavelength range (e.g., 2-5 micron). The previous spectra (e.g., UKIRT) have R=850 only. Reasonable spectra can be taken with Spex in one hour. Service observing? Maybe -- tricky spectra, can't be taken by just anyone. On the other hand, Bobby Bus is an experienced spectroscopist and just joined IRTF. ] ========================================================= Summary presentation 2: visible observations (Mike Hicks) ========================================================= Pre-Voyager: Harris 1961 and others. Lightcurve amplitude at 0.89 < 0.02 Voyager albedo maps: lightcurve amplitude changes with wavelength. Changes in albedo with time: Voyager (1989) vs. Bell 1979, Cruikshank 1979 (data from 1977) Buratti - historical paper, looking at albedos 1950-1990 (Buratti et al 1994) Colors changes more than can be accounted for by viewing geometry. Geometric albedo shows no detectable change. [ Question/discussion What about that first point with no error bar? (Harris 1961) Yes, that skews the linear fit to color changes. Open question: does the color still have detectable change if you remove that first point? ] Spectra: Grundy & Fink 1991. Flat spectra, with weak CH4 absorption (but not at .72 micron) Much less CH4 than on Pluto. Bosh and Tryka 1995, HST (part of occultation attempt). Visible Spectrum showing "red Triton" [ Discussion: since V is already bright, it's likely that most of the reddening is Triton getting fainter shortward of 0.5 micron, rather getting brighter at V ] [ Discussion: differential refraction. Mike convinces several skeptics that differential refraction is not a problem. First of all, they rotate the slit in the proper direction. Second of all, they are more recently using a 5.0 arcsec slit. The 1995 (?) spectrum shows a bump at ~0.5 microns. Will expresses concern that this is a sign of poor Neptune subtraction. ] 0.72 micron CH4 band comes and goes. No discernible longitude dependence. [ Question: telluric water correction? No, standards all correct well. ] Photometry: Hicks and Buratti have some V-R photometry, but this doesn't show the reddening yet. [ Discussion on V-R: How diagnostic is V-R anyway? Within the group of "normal" spectra, the V-R changes; same for "reddened" spectra. Note that Voyager ISS albedos plotted in the 90-day Science issue have been revised in Nelson et al., and these show different V-R colors ] [ Discussion on Neptune subtraction for photometry: Several groups (Grundy, Hicks, Young) have unreduced photometry. Why? Because Neptune subtraction is such a problem. Hicks is relatively happy with his "ring mask" technique. Find the center of Neptune. Make a radial profile around Neptune (leaving out pixels with Triton). ] [ Discussion on untapped photometric data: Jim Elliot mentioned that there exist periods of concentrated observations of Triton for the purpose of occultation prediction. These have been only reduced for astrometry so far. ] ========================================================== Summary presentation #3: IR observations (Dale Cruikshank) ========================================================== Spectra by Anderson 1974 --- flat! Spectra by Cruikshank (1979, Icarus 40), specifically specifically their spectra KC635 and KC645, show blueish Triton. NIR spectra: CH4 and N2 frost absorption 1980 deep absorptions 1981 not so deep 1982-1986 Neptune near galactic center. Nearly impossible to get clean spectra. 1987-1992 Subdued strength of CH4 bands. A snowfall? 1995-1998 No change seen on Triton. Best spectral analysis is Quirico et al 1999. One good model has two units. 55% of surface covered with unit 1: N2 + 0.11% CH4 + 0.05% CO 45% of surface covered with H2O + CO2. ======================================================= Summary presentation #4: UV observations (Leslie Young) ======================================================= Advantages & disadvantages of UV: Many species have absorption edges 2000-4000 A In particular, ice stays bright farther than minerals In general, solid bodies have higher spatial contrast in UV, and Triton's no exception. But: less diagnostic "fingerprints" than in IR. Sunlight diminishing, especially shortward of 2600 A, so lower SNR. Earth's atmosphere opaque in UV. Need spacecraft or space telescopes. Few observations. Voyager 2 UV observations of Triton: Imaging subsystem (ISS) maps at 3500 A Photopolarimeter (PPS) photometry at 2500 A. UVS: Emission lines N2 (958 A, 981 A), NII (1085 A), HI (1216 A) CO, Ar, Ne not seen. UVS occultation of the sun and beta Canis Majoris=>N2, CH4 absorption Summary of Triton UV observations from space telescopes: IUE, August 1988. Spectra, 2600-3200 A HST/FOS, May 1992 & September 1993. Spectra, 1900-3300 A HST/FOC, September 1995. Images, 2780 and 4100 A HST/STIS, August-September 1999. Images, 2710 and 3740 A. Spectra, 2200-3200 A [ Discussion: Larry Sromovsky has HST WFPCII data from 1994 and 1995. They were taken as part of a Neptune imaging campaign. The problem is that they are at the edge of the field. There is filter distortion problems. Also variable CTE, so that the brightness of Triton is only good to a few percent. Leslie says that a few percent is very useful, since Triton's albedo at 2710 changes by ~20 percent between 1993 and 1999. Larry says that the albedos are more similar to the 1999 colors than 1993 ] UV albedo vs. location: Triton's lightcurve increases w/ decreasing wavelength; spatial contrast increases in the UV (Hillier et al. 1991, Stern et al 1995). A comparison Voyager maps at 3500 A (UV filter) and 5600 A (Clear filter) shows a large region of UV absorption near +20 long, -25 lat (Flynn et al. 1996). 1995 FOC images show essentially no change since Voyager. UV albedo vs. wavelength IUE (1988): red slope from 2500-3200 A (but Neptune scattering was a problem). Voyager ISS+PPS (1989): slightly red from 2500-3600 A (but different instruments, large error bars). HST/FOC (1993): 1900-3280 A. Red slope ceases for l < 2750 A (blue albedo upturn or broad, solid-state absorption feature). Broad absorption features 2000-2100 A. HST/STIS (1999): 2200-3200 A. Red slope 2200-3200 A. Possible absorption edge near 2550 A? UV albedo vs. time Albedo 2630-3100 A brighter by ~20 % in 1999 vs. 1993. Albedo 2230-3100 A is redder in 1999 vs. 1993, with a possible absorption edge near 2550 A. Lightcurve amplitude is similar in 1993 and 1989. But, the observed amplitude is ~35% smaller in 1999 than in 1989. Contrast decreasing with time? [ Discussion: Leslie asks Larry Trafton (one of the IUE observers) if it is worth trying to clean up the IUE data, now that there's evidence for change. Dale comes out strongly in favor of starting to get good data on a regular basis, starting now, instead of readdressing older, perhaps compromised, data. This is especially true given the frequency of the transient events as seen by Buratti and Hicks. If we start looking now, we shouldn't have to wait long to see some real change. ] ================== General Discussion ================== Leslie agreed to host a web page and mailing list for those interested in coordinated observations. Look for it under http://www.boulder.swri.edu/~layoung