Formation of cometary O₂ ice and related ice species on grain surfaces in the midplane of the pre-solar nebula

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: eistrup@strw.leidenuniv.nl
² School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
e-mail: c.walsh1@leeds.ac.uk

Received: 7 May 2018
Accepted: 20 July 2018

Abstract

Context. Detection of abundant O₂ at 1–10% relative to H₂O ice in the comae of comets 1P/Halley and 67P/Churyumov-Gerasimenko has motivated attempts to explain the origin of the high O₂ ice abundance. Recent chemical modelling of the outer, colder regions of a protoplanetary disk midplane has shown production of O₂ ice at the same abundance as that measured in the comet.

Aims. We aim to carry out a thorough investigation to constrain the conditions under which O₂ ice could have been produced through kinetic chemistry in the pre-solar nebula midplane.

Methods. We have utilised an updated chemical kinetics code to evolve chemistry under pre-solar nebula midplane conditions. Four different chemical starting conditions and the effects of various chemical parameters have been tested.

Results. Using the fiducial network, and for either reset conditions (atomic initial abundances) or atomic oxygen only conditions, the abundance level of O₂ ice measured in the comets can be reproduced at an intermediate time, after 0.1–2 Myr of evolution, depending on ionisation level. When including O₃ chemistry, the abundance of O₂ ice is much lower than the cometary abundance (by several orders of magnitude). We find that H₂O₂ and O₃ ices are abundantly produced (at around the level of O₂ ice) in disagreement with their respective abundances or upper limits from observations of comet 67P. Upon closer investigation of the parameter space, and varying parameters for grain–surface chemistry, it is found that for temperatures 15–25 K, densities of 10⁹−10¹⁰ cm⁻³, and a barrier for quantum tunnelling set to 2 Å, the measured level of O₂ ice can be reproduced with the new chemical network, including an updated binding energy for atomic oxygen (1660 K). However, the abundances of H₂O₂ and O₃ ices still disagree with the observations. A larger activation energy for the O + O₂ → O₃ reaction (E_act > 1000 K) helps to reproduce the non-detection of O₃ ice in the comet, as well as reproducing the observed abundances of H₂O₂ and O₂ ices. The only other case in which the O₂ ice matches the observed abundance, and O₃ and H₂O₂ ice are lower, is the case when starting with an appreciable amount of oxygen locked in O₂.

Conclusions. The parameter space investigation revealed a sweet spot for production of O₂ ice at an abundance matching those in 67P and 1P, and O₃ and H₂O₂ ice abundances matching those in 67P. This means that there is a radial region in the pre-solar nebula from 120–150 AU, within which O₂ could have been produced in situ via ice chemistry on grain surfaces. However, it is apparent that there is a high degree of sensitivity of the chemistry to the assumed chemical parameters (e.g. binding energy, activation barrier width, and quantum tunnelling barrier). Hence, because the more likely scenario starting with a percentage of elemental oxygen locked in O₂ also reproduces the O₂ ice abundance in 67P at early stages, this supports previous suggestions that the cometary O₂ ice could have a primordial origin.