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We propose a scenario that explains the apparent nitrogen deficiency incomets in a way consistent with the fact that the surfaces of Pluto and Tritonare dominated by nitrogen-rich ice. We use a statistical thermodynamic model toinvestigate the composition of the successive multiple guest clathrates thatmay have formed during the cooling of the primordial nebula from the mostabundant volatiles present in the gas phase. These clathrates agglomerated withthe other ices (pure condensates or stoichiometric hydrates) and formed thebuilding blocks of comets. We report that molecular nitrogen is a poorclathrate former, when we consider a plausible gas phase composition of theprimordial nebula. This implies that its trapping into cometesimals requires alow disk temperature ($\sim$20 K) in order to allow the formation of its purecondensate. We find that it is possible to explain the lack of molecularnitrogen in comets as a consequence of their postformation internal heatingengendered by the decay of short-lived radiogenic nuclides. This scenario isfound consistent with the presence of nitrogen-rich ice covers on Pluto andTriton. Our model predicts that comets should present xenon-to-water andkrypton-to-water ratios close to solar xenon-to-oxygen and krypton-to-oxygenratios, respectively. In contrast, the argon-to-water ratio is predicted to bedepleted by a factor of $\sim$300 in comets compared to solar argon-to-oxygen,as a consequence of poor trapping efficiency and radiogenic heating.

The dual origin of the nitrogen deficiency in comets: selective volatile trapping in the nebula and postaccretion radiogenic heating