Ammonia abundances in comets

S. Wyckoff, S. Tegler, L. Engel

Research output: Contribution to journalArticlepeer-review

6 Scopus citations


Spectra of comets Halley, Hartley-Good, Thiele and Borrelly were obtained with the 4-m telescopes at Cerro Tololo and Kitt Peak Observatories and with the Multiple Mirror Telescope. The emission band strengths of the NH2 bands were measured to determine NH2 column densities in all four comets. Production rates which were determined using two independent models (Haser and vectorial) agree within the observational errors and indicate that a simple two-step decay model approximates well the distribution of NH2 in the comae of these four comets. If the observed NH2 derives from a single parent with a point-source origin, the most likely parent is NH3 which for the photolytic process, NH3 + hν → NH2 + H, has a branching ratio ∼95%. This branching ratio leads to ammonia-to-water abundance ratios ranging from 0.01% to 0.4% in the four comets, indicative of significant differences. Three independent determinations of the ratio in comet Halley give Q(NH3)/Q(H2O) = 0.002 ± 0.001. No significant difference in the ammonia abundance was found before and after perihelion in comet Halley, which indicates that the ice component of the nucleus is chemically homogeneous within the observational uncertainties. The ammonia abundance ratio determined for comet Halley 14 March 1986 is an order of magnitude smaller than that inferred from the spacecraft ion mass spectrometer data.

Original languageEnglish (US)
Pages (from-to)169-176
Number of pages8
JournalAdvances in Space Research
Issue number3
StatePublished - 1989

ASJC Scopus subject areas

  • Aerospace Engineering
  • Astronomy and Astrophysics
  • Geophysics
  • Atmospheric Science
  • Space and Planetary Science
  • General Earth and Planetary Sciences


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