CO+CO2 Production in Comet C/2020 F3 (NEOWISE) using NEOWISE detections.

Abstract

The NEOWISE mission utilizes the Wide-Field Infrared Survey Explorer (WISE) spacecraft to detect and characterize Near-Earth Objects (NEOs) (Wright et al. 2010). The prime WISE mission was part of NASA’s Explorer Program, with the goal to map the entire sky using an infrared telescope with four infrared wavelength bands (3.4, 4.6, 12, and 22 mm). In December 2013, the WISE spacecraft was re-activated with two infrared wavelength bands W1 and W2 (respectively at 3.4 and 4.6 mm) to continue its search for NEOs under the NEOWISE program (Mainzer et al. 2014) as a NASA Planetary Defense asset. As the NEOWISE mission continued to take images, over 270 active comets were observed during the NEOWISE mission phase. The mission has also discovered 24 comets during the reactivation phase from 2013 to the present, 45 comets overall (including all phases starting from December, 2009),  including comet C/2020 F3 (NEOWISE). This is a long-period comet having an orbital semi-major axis of 270 AU (Inbound) and 358 AU (Outbound), with eccentricity = 0.9992, and an Inclination of 129°. This comet was discovered on 2020 March 27 by NEOWISE.   One of the advantages of the NEOWISE data is that we can constrain CO+CO2 production in comets, while ground-based observations do not provide signals detecting especially CO2 due to the Earth's atmospheric effect. In this study, we describe four visits of comet C/2020 F3 observed by NEOWISE in 2020-2021 and provide analyses of the CO+CO2 production rates indicating the coma gas activity of the comet at heliocentric distances Rh > 3 au. Since NEOWISE is a survey mission that samples the sky at near-90° solar elongations, we revisited the data collected when the NEOWISE survey covered the region of the sky where Comet C/2020 F3 was predicted to be located. The four visits that we analyzed were observed on 2020 Jan 17 (visit A), 2020 Mar 28 (visit B), 2021 Mar 01 (visit C), and 2021 Jul 13 (visit D). We mainly use the 4.6 mm W2 signals to constrain the CO+CO2 production rate because the presence of the strong gas (CO+CO2) emission is noticeable in the 4.6 mm wavelength bandpass; the W1 band (3.4 mm bandpass) is mostly dominated by dust signal. We follow the CO+CO2 extraction routine that has been used for calibrating a few hundred comets observed by NEOWISE (Bauer et al., 2015; 2017, Gicquel et al., 2023).   Figure 1 NEOWISE observations of comet C/2020 F3 at 3.1 au (Visit A), 2.1 au (Visit B), 4.0 au (Visit C), and 4.9 au (Visit D). Given the uncertainty, visit B (Rh = 2.1 AU) and visit C (Rh = 4.0 AU) show more significant signals in both W1 and W2 bands than the other two visits. We found that CO+CO2 production rates of Comet C/2020F3 were 8.7 [0.24] E+26 and 2.6 [0.3] E+26 mol s-1 for visit B and C, respectively, while visits A and D had no detection. The measured gas rates yielded a comparable rate consistent with the preliminary result for the detected SPCs and LPCs, primarily observed in 2013-2015, a heliocentric distance between 2 to 4 AU (Bauer et al., 2021). If CO2 emission dominated W2 flux, this result supports that CO2 could be a strong contributor to inducing comet activity at relatively large heliocentric distances. We also employed a nucleus extraction technique (Bauer et al. 2017) to separate the coma and nucleus signals and constrained the size of the nucleus. We will present an analysis of the maximum active area on the nucleus corresponding to each measured gas species at the time of the comet’s discovery. As a final note, the existing numerical model used for the CO+CO2 extraction routine was developed in IDL. The revised implementation used in the current work has been translated to a more user-friendly interface written in Python, and can be adapted for similar analysis using the NEO Surveyor (NEOS) mission’s data (Mainzer et al. 2023) in the future.