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We report production rates of H₂O and nine trace molecules (C₂H₆, CH₄, H₂CO, CH₃OH, HCN, NH₃, C₂H₂, OCS, and CO) in long-period comet C/2020 S3 (Erasmus) using the high-resolution, cross-dispersed infrared spectrograph (iSHELL) at the NASA Infrared Telescope Facility, on two pre-perihelion dates at heliocentric distancesR_h= 0.49 and 0.52 au. Our molecular abundances with respect to simultaneously or contemporaneously measured H₂O indicate that S3 is depleted in CH₃OH compared to its mean abundance relative to H₂O among the overall comet population (Oort Cloud comets and Jupiter-family comets combined), whereas the eight other measured species have near-average abundances relative to H₂O. In addition, compared to comets observed atR_h< 0.80 au at near-infrared wavelengths, S3 showed enhancement in the abundances of volatile species H₂CO, NH₃, and C₂H₂, indicating possible additional (distributed) sources in the coma for these volatile species. The spatial profiles of volatile species in S3 in different instrumental settings are dramatically different, which might suggest temporal variability in comet outgassing behavior between the nonsimultaneous measurements. The spatial distributions of simultaneously measured volatile species C₂H₆and CH₄are nearly symmetric and closely track each other, while those of CO and HCN co-measured with H₂O (using different instrument settings) are similar to each other and are asymmetric in the antisunward direction.

 

High-resolution near-infrared ground-based spectroscopic observations of comet 67P/Churyumov–Gerasimenko near its maximum activity in 2021 were conducted from the W. M. Keck Observatory, using the facility spectrograph NIRSPEC. 67P is the best-studied comet to date because of the unprecedented detail and insights provided by the Rosetta mission during 2014–2016. Because 67P is the only comet where the detailed abundances of many coma volatiles were measured in situ, determining its composition from the ground provides a unique opportunity to interpret Rosetta results within the context of the large database of ground-based compositional measurements of comets. However, previous apparitions, including in 2015, have been unfavorable for in-depth ground-based studies of parent volatiles in 67P. The 2021 apparition of 67P was thus the first-ever opportunity for such observations. We report gas spatial distributions, rotational temperatures, production rates, and relative abundances (or stringent upper limits) among seven volatile species: C₂H₂, C₂H₆, HCN, NH₃, CH₃OH, H₂CO, and H₂O. The measured abundances of trace species relative to water reveal near average or below average values compared to previous comets studied at infrared wavelengths. Both gas rotational temperatures and the spatial distributions of H₂O, C₂H₆, and HCN measured with Keck-NIRSPEC in 2021 are consistent with the outgassing patterns revealed by Rosetta in 2015 at very similar heliocentric distance  (post-perihelion). These results can be integrated with both Rosetta mission findings and ground-based cometary studies of the overall comet population, for which we encourage a wide-scale collaboration across measurement techniques.

 

Near-Earth asteroids (NEAs) are a key test bed for investigations into planet formation, asteroid dynamics, and planetary defense initiatives. These studies rely on understanding NEA sizes, albedo distributions, and regolith properties. Simple thermal models are a commonly used method for determining these properties; however, they have inherent limitations owing to the simplifying assumptions they make about asteroid shapes and properties. With the recent collapse of the Arecibo Telescope and a decrease of direct size measurements, as well as future facilities such as LSST and NEO Surveyor coming online soon, these models will play an increasingly important role in our knowledge of the NEA population. Therefore, it is key to understand the limits of these models. In this work we constrain the limitations of simple thermal models by comparing model results to more complex thermophysical models, radar data, and other existing analyses. Furthermore, we present a method for placing tighter constraints on inferred NEA properties using simple thermal models. These comparisons and constraints are explored using the NEA (285263) 1998 QE2 as a case study. We analyze QE2 with a simple thermal model and data from both the NASA IRTF SpeX instrument and NEOWISE mission. We determine an albedo between 0.05 and 0.10 and thermal inertia between 0 and 425J m⁻²s^−1/2K⁻¹. We find that overall the simple thermal model is able to well constrain the properties of QE2; however, we find that model uncertainties can be influenced by topography, viewing geometry, and the wavelength range of data used.

 

We present a comprehensive analysis of the chemical composition of the Jupiter-family comet and potential spacecraft target 46P/Wirtanen, in the near-IR wavelength range. We used iSHELL at the NASA Infrared Telescope Facility to observe the comet on 11 pre-, near-, and postperihelion dates in 2018 December and 2019 January and February during its historic apparition. We report rotational temperatures, production rates, and mixing ratios with respect to H₂O and C₂H₆or 3σupper limits of the primary volatiles H₂O, HCN, CH₄, C₂H₆, CH₃OH, H₂CO, NH₃, CO, C₂H₂, and HC₃N. We also discuss the spatial outgassing of the primary volatiles, to understand their sources and the spatial associations between them. The spatial profiles of H₂O in 46P/Wirtanen suggest the presence of extended H₂O outgassing sources in the coma, similar to the EPOXI target comet 103P/Hartley 2. 46P/Wirtanen is among the few known hyperactive comets, and we note that its composition and outgassing behavior are similar to those of other hyperactive comets in many ways. We note that the analyzed parent volatiles showed different variations (relative mixing ratios) during the apparition. We compared the chemical composition of 46P/Wirtanen with the mean abundances in Jupiter-family comets and the comet population as measured with ground-based near-IR facilities to date. The molecular abundances in 46P/Wirtanen suggest that although they were changing, the variations were small compared to the range in the comet population, with CH₃OH showing notably more variation as compared to the other molecules.

 

Abstract Comets provide a valuable window into the chemical and physical conditions at the time of their formation in the young solar system. We seek insights into where and when these objects formed by comparing the range of abundances observed for nine molecules and their average values across a sample of 29 comets to the predicted midplane ice abundances from models of the protosolar nebula. Our fiducial model, where ices are inherited from the interstellar medium, can account for the observed mixing ratio ranges of each molecule considered, but no single location or time reproduces the abundances of all molecules simultaneously. This suggests that each comet consists of material processed under a range of conditions. In contrast, a model where the initial composition of disk material is “reset,” wiping out any previous chemical history, cannot account for the complete range of abundances observed in comets. Using toy models that combine material processed under different thermal conditions, we find that a combination of warm (CO-poor) and cold (CO-rich) material is required to account for both the average properties of the Jupiter-family and Oort cloud comets, and the individual comets we consider. This could occur by the transport (either radial or vertical) of ice-coated dust grains in the early solar system. Comparison of the models to the average Jupiter-family and Oort cloud comet compositions suggests the two families formed in overlapping regions of the disk, in agreement with the findings of A’Hearn et al. and with the predictions of the Nice model.