Constraining the Origins of Neptune's Carbon Monoxide Abundance with CARMA Millimeter-wave Observations

TL;DR

This study uses CARMA millimeter-wave observations and radiative transfer analysis to constrain Neptune's CO vertical profile, suggesting a significant external or internal source of CO and implications for Neptune's atmospheric composition.

Contribution

It provides new constraints on Neptune's CO abundance profile and evaluates potential external and internal sources of CO using observational data and impact rate models.

Findings

Neptune's tropospheric CO mole fraction is about 0.1 ppm.

Stratospheric CO mole fraction is about 1.1 ppm.

Impacts of comets could account for observed CO levels if diffusion is low.

Abstract

We present observations of Neptune's 1- and 3-mm spectrum from the Combined Array for Research in Millimeter-wave Astronomy (CARMA). Radiative transfer analysis of the CO (2-1) and (1-0) rotation lines was performed to constrain the CO vertical abundance profile. We find that the data are well matched by a CO mole fraction of 0.1^+0.2_-0.1 parts per million (ppm) in the troposphere, and 1.1^+0.2_-0.3 ppm in the stratosphere. A flux of 0.5-20 times 10^8 CO molecules cm-2 s-1 to the upper stratosphere is implied. Using the Zahnle et al. (2003) estimate for cometary impact rates at Neptune, we calculate the CO flux that could be formed from (sub)kilometer-sized comets; we find that if the diffusion rate near the tropopause is small (200 cm2 s-1), these impacts could produce a flux as high as 0.5^+0.8_-0.4 times 10^8 CO molecules cm-2 s-1. We also revisit the calculation of Neptune's internal CO contribution using revised calculations for the CO ->CH4 conversion timescale in the deep atmosphere (Visscher et al. 2011). We find that an upwelled CO mole fraction of 0.1 ppm implies a global O/H enrichment of at least 400, and likely more than 650, times the protosolar value.