Discovery of Mg-rich pyroxenes in comet C/1995 O1 (Hale-Bopp): Pristine grains revealed at perihelion

摘要: The NASA Ames HIFOGS spectrometer observed comet C/1995 O1 (Hale-Bopp) at epochs including 96 Oct 7–14 UT (2.8 AU), 97 Feb 14–15 (1.2 Apr 11 (0.93 and Jun 22, 25 (1.7 AU). 7.5–13.5 μm spectrophotometry (R = 360 - 180) of the silicate feature 2.8 AU is identical in shape to ISO SWS spectra Hale-Bopp (Crovisier et al., 1997); strong 11.2 peak structured identified as olivine. Upon close passage sun, 1.2 0.93 reveals peaks 9.3 10.0 μm. post-perihelion 10 1.7 weaker but has nearly same pre-perihelion AU, reverting its shape: there no change dust chemistry by sun. appearance rh ≲ attributed rise contribution pryoxenes (clino-pyroxene orthopyroxene crystals) feature, leads hypothesis that pyroxenes are significantly cooler than olivines. radiating on Wien side blackbody transition Rayleigh-Jeans tail upon closer approach Sun. Composite fits features using IDPs laboratory minerals shows a good empirical fit obtained when about 150 K pyroxenes, because they contribute signficantly perihelion, more abundant perihelion temperature implies Mg-rich other olivines, amorphous pyroxenes. PUMA-1 flyby measurements P/Halley also indicated an overabundance compared Comet Hale-Bopp's similar pyroxere from ’Spray‘ class, known for their D-richness unaltered morphologies: may be pristine relic ISM grains.