Synthesis of molecular oxygen via irradiation of ice grains in the protosolar nebula

TL;DR

This study models the irradiation of ice grains in the protosolar nebula to explain molecular oxygen presence in comets, concluding that in-situ irradiation is insufficient and pre-solar cloud formation is more likely.

Contribution

It introduces a coupled disk-transport-irradiation model to evaluate oxygen production in the protosolar nebula, finding it inadequate to explain observed levels.

Findings

Irradiation in the nebula produces less oxygen than observed in comets.

Transport cycles in the disk do not significantly increase oxygen production.

Pre-solar cloud formation is the most plausible origin of cometary molecular oxygen.

Abstract

Molecular oxygen has been detected in the coma of comet 67P/Churyumov--Gerasimenko with a mean abundance of 3.80 $\pm$ 0.85\% by the ROSINA mass spectrometer on board the Rosetta spacecraft. To account for the presence of this species in comet 67P/Churyumov--Gerasimenko, it has been shown that the radiolysis of ice grains precursors of comets is a viable mechanism in low-density environments, such as molecular clouds. Here, we investigate the alternative possibility that the icy grains present in the midplane of the protosolar nebula were irradiated during their vertical transport between the midplane and the upper layers over a large number of cycles, as a result of turbulent mixing. Consequently, these grains spent a non-negligible fraction of their lifetime in the disk's upper regions, where the irradiation by cosmic rays was strong. To do so, we used a coupled disk-transport-irradiation model to calculate the time evolution of the molecular oxygen abundance radiolytically produced in ice grains. Our computations show that, even if a significant fraction of the icy particles have followed a back and forth cycle towards the upper layers of the disk over 10 million of years, a timespan far exceeding the formation timescale of comet 67P/Churyumov--Gerasimenko, the amount of produced molecular oxygen is at least two orders of magnitude lower than the Rosetta observations. We conclude that the most likely scenario remains the formation of molecular oxygen in low-density environments, such as the presolar cloud, prior to the genesis of the protosolar nebula.