Trapping of N 2, CO and Ar in amorphous ice—Application to comets

Abstract

Recent attempts using high resolution spectra to detect N + 2 in several comets were unsuccessful [Cochran, A.L., Cochran, W.D., Baker, E.S., 2000. Icarus 146, 583–593; Cochran, A.L., 2002. Astrophys. J. 576, L165–L168]. The upper limits on N + 2 in comparison with the positively detected CO + for Comets C/1995 O1 Hale–Bopp, 122P/1995 S1 de Vico and 153P/2002 C1 Ikeya–Zhang range between N 2 + /CO + < ( 0.65 – 5.4 ) × 10 −4 . Ar was not detected in three recent comets [Weaver, H.A., Feldman, P.D., Combi, M.R., Krasnopolsky, V., Lisse, C.M., Shemansky, D.E., 2002. Astrophys. J. 576, L95–L98], with upper limits of Ar/CO < ( 3.4 – 7.8 ) × 10 −2 for Comets C/1999 T1 McNaught–Hartley, C/2001 A2 LINEAR and C/2000 WM1 LINEAR. The Ar detected by Stern et al. [Stern, S.A., Slater, D.C., Festou, M.C., Parker, J.Wm., Gladstone, G.R., A'Hearn, M.F., Wilkinson, E., 2000. Astrophys. J. 544, L169–L172] for Comet C/1995 O1 Hale–Bopp, gives a ratio Ar/CO = 7.25 × 10 −2 , which was not confirmed by Cosmovici et al. [Cosmovici, C.B., Bratina, V., Schwarz, G., Tozzi, G., Mumma, M.J., Stalio, R., 2006. Astrophys. Space Sci. 301, 135–143]. Trying to solve the two problems, we studied experimentally the trapping of N 2 + CO + Ar in amorphous water ice, at 24–30 K. CO was found to be trapped in the ice 20–70 times more efficiently than N 2 and with the same efficiency as Ar. The resulting Ar/CO ratio of 1.2 × 10 −2 is consistent with Weaver et al.'s [Weaver, H.A., Feldman, P.D., Combi, M.R., Krasnopolsky, V., Lisse, C.M., Shemansky, D.E., 2002. Astrophys. J. 576, L95–L98] non-detection of Ar. However, with an extreme starting value for N 2/CO = 0.22 in the region where the ice grains which agglomerated to produce comet nuclei were formed, the expected N 2/CO ratio in the cometary ice should be 6.6 × 10 −3 , much higher than its non-detection limit.