Comets and Asteroids

A Survey Of CO, CO2, And H2O In Comets And Centaurs

By Keith Cowing
Press Release
September 21, 2022
Filed under , , ,
A Survey Of CO, CO2, And H2O In Comets And Centaurs
Figure 5. CO/H2O data from 45 comae. Comets from the previous figures are plotted using the same color scheme. Comets that did not have simultaneous CO and CO2 detections are plotted with new colors: added OCCs are magenta, added JFCs are cyan, and one R added interstellar comet is orange (2I/Borisov). The dashed horizontal line shows the median of all data (except Borisov) obtained within Helio < 2.5 au: (QCO/Q H2O)median = 3.3 ± 1.3%, with a higher QCO/Q H2O value for most OCCs compared with JFCs. This plot also beyond the CO snow line, while most other comets have low CO/H shows how anomalous C/2016 R2’s composition was compared to all other comets and is consistent with a very distant formation region 2O ratios consistent with formation within the CO snow line. Outside 2.5 au, the fractional abundance of CO to H2O steadily increases, which is likely due to the decreased efficiency of water-ice sublimation at larger heliocentric distances. Overall, a two-slope fit to this data results in QCO/Q H2O ∼ R1.6 Helio between 0.5 au to 2.5 au, and QCO/Q H2O ∼ R3.6 Helio from 2.5 au to 4.6 au. The fits were calculated using detections only and did not include upper limits, nor were they weighted by uncertainties. Beyond 2.5 au, the slope for CO/H2O is noticeably steeper than that for CO2’s/H2O (see Figure 6). Accounting for the upper limits would further increase the CO/H2O slope for the large RHelio comets, but also increase the scatter in the fit. At larger distances there are fewer measurements, and the fit from 2.5 au to 4.6 au should be considered with caution. The data are listed in Table 4 and Table 5.

CO and CO2 are the two dominant carbon-bearing molecules in comae and have major roles in driving activity. Their relative abundances also provide strong observational constraints to models of solar system formation and evolution but have never been studied together in a large sample of comets.

We carefully compiled and analyzed published measurements of simultaneous CO and CO2 production rates for 25 comets. Approximately half of the comae have substantially more CO2 than CO, about a third are CO-dominated and about a tenth produce a comparable amount of both. There may be a heliocentric dependence to this ratio with CO dominating comae beyond 3.5 au.

Eight out of nine of the Jupiter Family Comets in our study produce more CO2 than CO. The six dynamically new comets produce more CO2 relative to CO than the eight Oort Cloud comets that have made multiple passes through the inner solar system. This may be explained by long-term cosmic ray processing of a comet nucleus’s outer layers.

We find (QCO/QH2O)median = 3 ± 1\% and (QCO2/QH2O)median = 12 ± 2\%. The inorganic volatile carbon budget was estimated to be QCO+QCO2)/QH2O ∼ 18\% for most comets. Between 0.7 to 4.6 au, CO2 outgassing appears to be more intimately tied to the water production in a way that the CO is not. The volatile carbon/oxygen ratio for 18 comets is C/Omedian ∼ 13\%, which is consistent with a comet formation environment that is well within the CO snow line.

Olga Harrington Pinto, Maria Womack, Yanga R. Fernandez, James Bauer

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2209.09985 [astro-ph.EP] (or arXiv:2209.09985v1 [astro-ph.EP] for this version)
Submission history
From: Olga Harrington Pinto
[v1] Tue, 20 Sep 2022 20:42:53 UTC (3,824 KB)
Astrobiology, Astrochemistry,

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