Engel, Lisa | Engineering Papers

NASA NTRS1991

Ammonia abundances in four comets

NH2 emission band strengths were measured in four comets and the NH2 column densities were determined in order to measure the ammonia content of the comets. The mean ammonia/water abundance ratio derived for the four comets is found to be 0.13 + or - 0.06 percent, with no significant variation among the comets. The uniformity of this abundance attests to a remarkable degree of chemical homogeneity over large scales in the comet-forming region of the primordial solar nebula, and contrasts with the CO abundance variations found previously in comets. The N2 and NH3 abundances indicate a condensation temperature in the range 20-160 K, consistent with virtually all comet formation hypotheses.

Wickoff, Susan↗

NASA NTRS1991

Nitrogen abundance in Comet Halley

Data on the nitrogen-containing compounds that observed spectroscopically in the coma of Comet Halley are summarized, and the elemental abundance of nitrogen in the Comet Halley nucleus is derived. It is found that 90 percent of elemental nitrogen is in the dust fraction of the coma, while in the gas fraction, most of the nitrogen is contained in NH3 and CN. The elemental nitrogen abundance in the ice component of the nucleus was found to be deficient by a factor of about 75, relative to the solar photosphere, indicating that the chemical partitioning of N2 into NH3 and other nitrogen compounds during the evolution of the solar nebula cannot account completely for the low abundance ratio N2/NH3 = 0.1, observed in the comet. It is suggested that the low N2/NH3 ratio in Comet Halley may be explained simply by physical fractionation and/or thermal diffusion.

Wyckoff, Susan↗

NASA NTRS1990

Unidentified ions in comets

Optical spectra were taken of comets Halley, West, and Brorsen-Metcalf in the wavelength region 3200 to 6400 A. These spectra were offset approximately 10(exp -5) to 10(exp -6) km tailward from the nucleus so that the features detected were all ions with the exception of a very small residual C2 emission at 5165 A. The full labeled tail spectrum of comet Halley is given. While most of the features detected are attributable to CO(+), H2O(+), CO2(+), CH(+), and OH(+), there are three moderately strong bands in the spectra of comets Halley and Brorsen-Metcalf which remain unidentified. These features were not detected in comet West. All three spectra in the 4800 to 5400 A region are presented, Brorsen-Metcalf is the gassiest of the three. The central wavelengths of the unidentified features are 4930, 5300, and 6000 A. In an effort to identify the ions responsible for these features as well as to confirm previous identifications, laboratory spectra of ions were compared with the comet spectra. H2O(+) and CO(+) are known to have extensive emission in the 4800 to 6200 A region. Therefore, the possibility that these unidentified features are due to H2O(+) and CO(+) was investigated. A search for the following bands was conducted: H2O(+) (11-0), H2O(+) (12-0), H2O(+) (13-1), CO(+) (1-1), and CO(+) (0-0). CO(+) (1-1) and CO(+) (0-0) were previously identified and are present in all the spectra examined. The H2O(+) (11-0) band was identified. Comparison of the H2O(+) (11-0) synthetic spectrum with comet Brorsen-Metcalf data is presented. The relative fluxes of the blended H2O(+) lines in the synthetic spectrum match those of the cometary data. After eliminating H2O(+) and CO(+) as significant contributors to the stronger unidentified features, spectra of ions which are not yet identified in the optical region of comets were compared to the data. These ions are NH(+), CS(+), and C2(+). None of these ions appears to be a significant contributor to the optical spectra of these three comets. In conclusion, H2O(+) (11-0) was unabiguously identified in cometary optical spectra. No other molecular ion for which there are laboratory spectra can explain the unidentified features found in the ion tails of comets Halley and Brorsen-Metcalf.

Engel, Lisa↗