An interpretation of the nitrogen deficiency in comets

We propose an interpretation of the composition of volatiles observed in comets based on their trapping in the form of clathrate hydrates in the solar nebula. The formation of clathrates is calculated from the statistical thermodynamics of Lunine and Stevenson (1985, Astrophys. J. Suppl. 58, 493–531), and occurs in an evolutionary turbulent solar nebula described by the model of Hersant et al. (2001, Astrophys. J. 554, 391–407). It is assumed that clathrate hydrates were incorporated into the icy grains that formed cometesimals. The strong depletion of the N2 molecule with respect to CO observed in some comets is explained by the fact that CO forms clathrate hydrates much more easily than does N2. The efficiency of this depletion, as well as the amount of trapped CO, depends upon the amount of water ice available in the region where the clathration took place. This might explain the diversity of CO abundances observed in comets. The same theory, applied to the trapping of volatiles around 5 AU, explains the enrichments in Ar, Kr, Xe, C, and N with respect to the solar abundance measured in the deep troposphere of Jupiter (Gautier et al., 2001a,b).