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., AʹHearn, 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 N2 and with the same efficiency as Ar. The resulting Ar/CO ratio of 1.2 × 10 −2 is consistent with Weaver et al.ʹs [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 N2/CO = 0.22 in the region where the ice grains which agglomerated to produce comet nuclei were formed, the expected N2/CO ratio in the cometary ice should be 6.6 × 10 −3 , much higher than its non-detection limit.
