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

TL;DR

The paper investigates the origins of Saturn's stratospheric CO by mapping its vertical and meridional distribution using ALMA CO $J=3\!\rightarrow\!2$ limb observations and a line-by-line radiative transfer inversion. Four CO priors representing external/internal source scenarios are tested, with a temperature field from Cassini/CIRS guiding the retrieval. They find CO confined to a thin $0.1$–$1$ mbar layer, a negative vertical gradient, and a largely homogeneous meridional distribution, with a marginal peak near $60^{\circ}$N, and no strong equatorial enhancement. These results argue against a steady ring- or Enceladus-derived source at the equator and support a relatively recent cometary delivery (~$2\times10^{2}$ years) that has since been horizontally mixed. The work constrains external source contributions to Saturn's CO budget and motivates further multi-hemisphere, time-resolved ALMA studies and in-situ cross-checks.

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°S to 69°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° 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.

Figures (18)

• Figure 1: Map of the CO (J=3-2) line area, as observed with ALMA on May 25$^\mathrm{th}$, 2018, after reduction, correction, calibration and subtraction of the continuum image (see Benm:22 for method details). The 1-bar level is represented by the black ellipse and the central meridian is shown with a black dashed line. Isolatitudes are displayed with grey solid lines. The position of the A and B rings are highlighted with a grey-filled region and the beam is shown with a white-filled ellipse.
• Figure 2: Continuum-subtracted CO spectra extracted from the image presented in Fig. \ref{['fig:CO_line_intensity_map']} at eastern and western limbs for different latitudes (red to blue lines), with the CO (J=3-2) rest frequency indicated by the vertical black dashed line (345.796 GHz). From the northernmost latitude of 69$^{\circ}$N to the equator, the intensity and the width of the CO line varies depending on latitude, implying variation on the vertical information content.
• Figure 3: Temperature field used as input by the line-by-line radiative transfer model. This is a temperature map created from a combination of Cassini/CIRS observations in limb view from 15$^\circ$S to 10$^\circ$N, and in nadir view out of this equatorial region.
• Figure 4: CO prior profiles used in this study: Prior 1 (dark orange) is a constant low abundance profile set at a volume mixing ratio of 10$^{-8}$, Prior 2 (orange) is a constant high abundance profile set at a volume mixing ratio of 10$^{-7}$, Prior 3 (dark blue, dash-dotted line) is a two-layered profile describing an external steady source scenario, and Prior 4 (light blue, dotted line) is a two-layered profile corresponding to a steady source of CO coming from the troposphere.
• Figure 5: (Top) Example of the CO line fitting at 45$^{\circ}$N-213$^\circ$W (western limb) for the four different retrieval calculations carried in this study. (Bottom) Residual flux density (observed spectrum - synthetic spectrum) showing that all CO prior profiles are fitting the observed spectrum within the noise of the observations. The black dashed line indicates the frequency of the peak, and the dotted lines depict different values of frequency offset from the peak, for which we have calculated the contribution function.