Surveying exogenous species in Saturn with ALMA I. Detecting and Mapping CO

TL;DR

This study uses ALMA observations to map CO in Saturn's atmosphere, finding a narrow vertical layer with homogeneous distribution, suggesting a recent comet impact as the primary source of CO.

Contribution

First detailed ALMA-based vertical and meridional mapping of Saturn's stratospheric CO, constraining its sources and distribution with radiative transfer modeling.

Findings

CO confined between 0.1 and 1 mbar with a negative vertical gradient.

Homogeneous meridional distribution with a marginal N-hemisphere enhancement.

Evidence favors a recent comet impact over ring infall as CO source.

Abstract

The origin of carbon monoxide (CO) in Saturn's stratosphere remains uncertain, with proposed sources including internal thermochemical production, cometary impacts, and exogenic material from the rings and icy moons (i.e. Enceladus). We aim to constrain the vertical and meridional distribution of stratospheric CO and assess the relative contributions of these potential sources. Here, we analysed high-spectral-resolution ALMA observations of the CO (J=3-2) line obtained on 25 May 2018, sampling Saturn's limb from 20{\deg}S to 69{\deg}N. CO vertical profiles were retrieved using a line-by-line radiative transfer model combined with spectral inversion techniques, testing multiple prior scenarios representative of different source hypotheses. CO is confined to a narrow layer between 0.1 and 1 mbar, with a robust negative vertical gradient and mean abundances of (3.7+/- 0.8) x 10$^{-8}$ at 0.1 mbar and (7.2 +/- 0.9) x 10$^{-8}$ at 1 mbar. The meridional distribution is statistically homogeneous, with a marginal enhancement near 60{\deg} N plausibly related to Enceladus. No significant equatorial enhancement is detected. The absence of a strong equatorial enhancement rules out a long-lived steady source associated with ring infall. The observations are most consistent with a relatively recent ($\approx$200-year-old or younger) cometary impact whose material has since been horizontally mixed, while any Cassini Grand Finale ring influx was either too recent or inefficient to affect CO abundances at the probed pressure levels.