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

TL;DR

This paper explains nitrogen deficiency in comets through selective volatile trapping during nebula cooling and postaccretion heating, aligning with nitrogen-rich surfaces on Pluto and Triton, and predicts specific noble gas ratios in comets.

Contribution

It introduces a thermodynamic model showing nitrogen's poor trapping in clathrates and the role of radiogenic heating in depleting nitrogen in comets, providing a comprehensive explanation for observed compositions.

Findings

Nitrogen is poorly trapped in clathrates at typical nebula temperatures.

Postformation radiogenic heating can deplete nitrogen in comets.

Comets are predicted to have near-solar xenon and krypton ratios, but depleted argon ratios.

Abstract

We propose a scenario that explains the apparent nitrogen deficiency in comets in a way consistent with the fact that the surfaces of Pluto and Triton are dominated by nitrogen-rich ice. We use a statistical thermodynamic model to investigate the composition of the successive multiple guest clathrates that may have formed during the cooling of the primordial nebula from the most abundant volatiles present in the gas phase. These clathrates agglomerated with the other ices (pure condensates or stoichiometric hydrates) and formed the building blocks of comets. We report that molecular nitrogen is a poor clathrate former, when we consider a plausible gas phase composition of the primordial nebula. This implies that its trapping into cometesimals requires a low disk temperature ($\sim$20 K) in order to allow the formation of its pure condensate. We find that it is possible to explain the lack of molecular nitrogen in comets as a consequence of their postformation internal heating engendered by the decay of short-lived radiogenic nuclides. This scenario is found consistent with the presence of nitrogen-rich ice covers on Pluto and Triton. Our model predicts that comets should present xenon-to-water and krypton-to-water ratios close to solar xenon-to-oxygen and krypton-to-oxygen ratios, respectively. In contrast, the argon-to-water ratio is predicted to be depleted by a factor of $\sim$300 in comets compared to solar argon-to-oxygen, as a consequence of poor trapping efficiency and radiogenic heating.