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The Compositional Evolution of C/2012 S1 (ISON) from Ground-Based High-Resolution Infrared Spectroscopy as Part of a Worldwide Observing CampaignVolatile production rates, relative abundances, rotational temperatures, and spatial distributions in the coma were measured in C/2012 S1 (ISON) using long-slit high-dispersion (lambda/delta lambda approximately 2.5 times 10 (sup 4)) infrared spectroscopy as part of a worldwide observing campaign. Spectra were obtained on Universal Time 2013 October 26 and 28 with NIRSPEC (Near Infrared Spectrometer) at the W.M. Keck Observatory, and Universal Time 2013 November 19 and 20 with CSHELL (Cryogenic Echelle Spectrograph) at the NASA IRTF (Infrared Telescope Facility). H2O was detected on all dates, with production rates increasing markedly from (8.7 plus or minus 1.5) times 10 (sup 27) molecules per second on October 26 (Heliocentric Distance = 1.12 Astronomical Units) to (3.7 plus or minus 0.4) times 10 (sup 29) molecules per second on November 20 (Heliocentric Distance = 0.43 Astronomical Units). Short-term variability of H2O production is also seen as observations on November 19 show an increase in H2O production rate of nearly a factor of two over a period of about 6 hours. C2H6, CH3OH and CH4 abundances in ISON (International Scientific Optical Network) are slightly depleted relative to H2O when compared to mean values for comets measured at infrared wavelengths. On the November dates, C2H2, HCN and OCS abundances relative to H2O appear to be within the range of mean values, whereas H2CO and NH3 were significantly enhanced. There is evidence that the abundances with respect to H2O increased for some species but not others between October 28 (Heliocentric Distance = 1.07 Astronomical Units) and November 19 (Heliocentric Distance = 0.46 Astronomical Units). The high mixing ratios of H2CO to CH3OH and C2H2 to C2H6 on November 19, and changes in the mixing ratios of some species with respect to H2O between October 28 to November 19, indicates compositional changes that may be the result of a transition from sampling radiation-processed outer layers in this dynamically new comet to sampling more pristine natal material as the outer processed layer was increasingly eroded and the thermal wave propagated into the nucleus as the comet approached perihelion for the first time. On November 19 and 20, the spatial distribution for dust appears asymmetric and enhanced in the antisolar direction, whereas spatial distributions for volatiles (excepting CN) appear symmetric with their peaks slightly offset in the sunward direction compared to the dust. Spatial distributions for H2O, HCN, C2H6, C2H2, and H2CO on November 19 show no definitive evidence for significant contributions from extended sources; however, broader spatial distributions for NH3 and OCS may be consistent with extended sources for these species. Abundances of HCN and C2H2 on November 19 and 20 are insufficient to account for reported abundances of CN and C2 in ISON near this time. Differences in HCN and CN spatial distributions are also consistent with HCN as only a minor source of CN in ISON on November 19 as the spatial distribution of CN in the coma suggests a dominant distributed source that is correlated with dust and not volatile release. The spatial distributions for NH3 and NH2 are similar, suggesting that NH3 is the primary source of NH2 with no evidence of a significant dust source of NH2; however, the higher production rates derived for NH3 compared to NH2 on November 19 and 20 remain unexplained. This suggests a more complete analysis that treats NH2 as a distributed source and accounts for its emission mechanism is needed for future work.
Document ID
20170002356
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in journal)
Authors
Russo, N. Dello
(Johns Hopkins Univ. Laurel, MD, United States)
Vervack, R. J., Jr.
(Johns Hopkins Univ. Laurel, MD, United States)
Kawakita, H.
(Kyoto Sangyo Univ. Japan)
Cochran, A.
(McDonald Observatory Austin, TX, United States)
McKay, A. J.
(McDonald Observatory Austin, TX, United States)
Harris, W. M.
(Arizona Univ. Tucson, AZ, United States)
Weaver, H.A.
(Johns Hopkins Univ. Laurel, MD, United States)
Lisse, C. M.
(Johns Hopkins Univ. Laurel, MD, United States)
DiSanti, M. A.
(NASA Goddard Space Flight Center Greenbelt, MD United States)
Kobayashi, H.
(Kyoto Sangyo Univ. Japan)
Date Acquired
March 17, 2017
Publication Date
December 11, 2015
Publication Information
Publication: Icarus
Publisher: Elsevier
Volume: 266
ISSN: 0019-1035
e-ISSN: 1090-2643
Subject Category
Astronomy
Lunar And Planetary Science And Exploration
Report/Patent Number
GSFC-E-DAA-TN40118
Funding Number(s)
CONTRACT_GRANT: NNX15AH29G
CONTRACT_GRANT: NNX08AE38A
Distribution Limits
Public
Copyright
Other

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