Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.207b&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 207
Astronomy and Astrophysics
Astrophysics
1
Scientific paper
High resolution infrared spectroscopy of the H2O upsilon(sub)3 band in Comets P/Halley and Wilson has permitted measurement of individual rotational line intensities (Larson et al. 1989). Mumma et al. (1988) suggested that cosmogonic information is preserved in the relative abundances of the nuclear spin species. The H2O molecule is organized into ortho and para species. In order that the total wave function remain symmetric with respect to H atom exchange, o-species have only asymmetric rotational levels, while p-species have only symmetric levels. The lowest ortho level lies 24 cm^-1 (34 degrees K) above the lowest para level, so the ortho/para ratio will be temperature dependent. Above 60 degrees K, o/p achieves the constant statistical equilibrium value of 3/1. Spin species conversion is prohibited by collisional and radiative processes, and requires a strong nonuniform magnetic field. It is hypothesized that cometary water began as a thin layer of ice condensed on grains in cold interstellar molecular clouds. This ice was subject to UV radiation <200 nm, which initially set o/p at ~25 degrees K or below. The grains subsequently accreted to form comets and this o/p was carried into the nucleus and remained unaltered. When the comet becomes active, outbursts emanate from the interior, where the overburden has provided protection from cosmic ray damage. While in the Oort cloud, the outer layers of a comet experience cosmic ray processing that is capable of resetting o/p to the high temperature limit. Thus, a dynamically new comet, such as Wilson, might be expected to show a statistically equilibrated o/p, whereas a short-period comet, such as P/Halley, might show o/p characteristic of its formation. It also appears that D/H in cometary ice was established in interstellar cloud cores, and did not later equilibrate with nebular gas. D/H for Comet P/Halley lies in the range 6-48 x 10^-5 (Eberhardt et al. 1987), much higher than the diffuse ISM, protosolar, Jupiter, and Saturn values (0.5-3.6 x 10^-5). The cometary range is comparable to D/H for Earth (16 x 10^-5) as well as Uranus, Neptune, and Titan, indicating that these bodies acquired their hydrogen in the form of ices as opposed to nebular H2. Similarly, the D/H range for carbonaceous and ordinary chondrites is high (8-105 x 10^-5), and is thought to reflect incorporation of insterstellar material into meteorites (Zinner, 1988). We have developed a novel experimental apparatus to study nuclear spin conversion during formation and processing of thin mantles of water-based ices. Vapor is condensed in a specially designed cryogenic spectroscopy cell that has no paramagnetic surfaces. The cell is attached to the cold head of a closed-cycle He refrigerator, and the entire refrigerator shroud is placed inside an evacuable FTIR spectrometer. Ice samples are held at ~10 degrees K, and irradiated with VUV, thus simulating conditions in the intermediate density regions of interstellar clouds. The ice is then rapidly heated to the sublimation point; temperature during warm up and cool down is controllable to within 1 degree K. The resultant vapor is captured in the cell and the infrared spectrum collected with the FTS. First results from the experimental program will be presented. References: Eberhardt P. et al. (1987) Astron. Astrophys. 187, 481-184. Larson H.P. et al. (1989) Ap. J. 338, 1106-1114. Mumma et al. (1988) Proc. STScI Conference on Origins and Evolution of Planetary Systems. Zinner E. (1988) in Meteorites in the Early Solar System (eds. J.F. Kerridge and M.S. Matthews) pp. 956-983. Univ. Arizona Press, Tucson. AZ.
Broadhurst Leigh C.
Mumma Michael J.
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