Saturn’s stratospheric carbon monoxide abundance observed with ALMA

In giant planet atmospheres, carbon monoxide (CO) can originate either from internal sources, such as thermochemical processes, or from external sources, such as meteorite impacts.

A dual origin of CO has already been inferred on Jupiter through infrared spectroscopy [1], and on Neptune via submillimeter spectroscopy [2,3]. For both planets, these studies concluded that stratospheric CO primarily originated from external sources, namely large cometary impacts—the most recent being the collision of comet Shoemaker-Levy 9 (SL9) with Jupiter in 1994 [4–8], along with an internal source. In contrast, the origin of CO in Saturn's atmosphere remains uncertain. Observations of the vertical distribution of CO in Saturn’s atmosphere are consistent with a cometary source approximately 220 years ago [9–11], although an internal origin cannot be ruled out, particularly given the potential influence of H₂O photochemistry on the vertical CO profile. The specific contribution of material infalling from the rings, both in the equatorial and mid-latitude regions remain also to be estimated.

Observed on May 25, 2018 using ALMA (project 2017.1.00636.S), maps of HCN (5-4) and CO (3-2) lines have been obtained with a combination of seven pointings of the main array in configuration C43.2, complemented by three pointings of the ACA to mosaic the full disk of the planet. Together with the HCN line, the CO line observed by ALMA is detected at the limb of the planet only, from the equator up to the south and north polar regions, with a latitudinal resolution about 2°, and both were initially used to provide the first absolute wind measurements in Saturn's stratosphere [12]. These analyses have revealed variations in the width of the CO line across the entire limb, suggesting that information on the vertical distribution of Saturn’s CO is accessible—potentially shedding light on its external or internal origin. Here, we will present the retrieved CO stratospheric abundances from 25°S to the northern polar regions resulting from different initial condition setups (preliminary results with an apriori CO profile as constant for a volume mixing ratio of 10^-8 are shown in the figure, and at the observation date, Saturn southern hemisphere was masked by its rings). We test the sensibility of the retrieved abundances to the CO prior and temperature profiles. We will also discuss the retrieved abundances in the context of the ring material infalling from the rings detected in situ by Cassini [13,14], and compare our results with photochemical model calculations taking into account ring material influx in Saturn’s neutral and ionized atmosphere [15].

 

 

 

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