John W. Noonan

Context. Electron impact reactions with molecules occur in a variety of astrophysical environments, including planetary and small body atmospheres. The resulting emission could be used to probe local plasma conditions and to reveal the presence of neutral molecular oxygen, which has no dipole moment and, hence, no strong emission features on its own. However, there is a serious lack of data on emission cross-sections at various energies. Aims. The study aims to improve the understanding of the electron induced fluorescence of O ₂ and its fragments by investigating the spectra of O ₂ at different electron energies. The research also emphasises the importance of comprehending the O ₂ ⁺ cation to interpret the ultraviolet-visible (UV-VIS) spectra accurately, filling a gap in the modelling of such emission spectra. Methods. We experimentally determined absolute emission cross-sections at near-UV and near-infrared (NUV-NIR) wavelengths (200 - 930 nm). Quantum chemical calculations were performed to obtain the potential energy curves of O ₂ ⁺ states with internuclear distance in the range of 0.85-10.0 Å. The resulting energy levels were used for spectrum simulations, considering the X ² Π _g , A ² Π _u , and a ⁴ Π _u states and incorporating vibrational effects through numerical integration of the 1D nuclear Schrodinger equation based on SO-X2C-MRCISD+Q/aug-ccpCV6Z-X2C potential energy curves. Simple models were also constructed to analyse the optical emission from molecular reaction products. Results. The main result of the work is the rich data set of emission cross-sections for energies og 30 eV, 40 eV, 50 eV, and 100 eV. The experimental spectra of O ₂ ⁺ (A ² Π _u -X ² Π _g ) were compared to the ab initio calculated spectra. Conclusions. Even though electron-induced oxygen emission has been studied relatively extensively, this work reports a wider range of experimentally determined emission cross-sections at four different electron impact energies. The data agree well with the two largest previously published datasets: one in the UV-VIS range and one in the VIS-IR range.

Near-Earth asteroid (469219) Kamo‘oalewa is a uniquely stable quasi-satellite of the Earth and a target of the Tianwen-2 spacecraft mission. Here we report observations taken with JWST’s NIRSpec instrument in integral field unit (IFU) mode in 2026 February. The JWST reflectance spectrum is notably less red (more neutral) from 1.0 to 2.5 μ m than previous ground-based spectrophotometric observations. New observations made with the Large Binocular Telescope (LBT) in 2026 April, observed and processed similarly to the 2021 observations, find zJ colors in agreement with JWST. Kamo‘oalewa’s infrared colors appear more similar to typical silicate-rich asteroids and unlike the reddened, space-weathered lunar-like silicates suggested by previous observations. In agreement with the ground-based spectrum, we detect a faint silicate absorption feature at 0.93 ± 0.01 μ m. We do not detect a 2.0 μ m silicate absorption. Models of Kamo‘oalewa’s faint thermal emission (beginning near 4.5 μ m) find a mean diameter of D = 18 ± 2 m and best-fit visible albedo p V = 0.5 9 − 0.17 + 0.25 , with models as low as p _V = 0.36 providing adequate model fits. This combination of color, albedo, and absorption bands is similar to oldhamite-bearing enstatite-rich compositions. Kamo‘oalewa’s brightness variations over the course of the JWST program provide independent confirmation of its rotation period of 27.9 minutes, with an axis ratio ∼1.4 ( D ∼ 15−21 m).

Constraining the sulfur reservoirs of interstellar objects provides essential clues to the nature of sulfur species in protoplanetary disks and interstellar molecular clouds and a potential solution to the cosmic sulfur depletion problem. Observations of interstellar objects, chemical messengers from these systems, allow us sensitive probes of their volatile and refractory content and, when placed in context with recent developments in laboratory astrophysics, allow us to propose hypotheses for their formation and initial conditions. We present Hubble Space Telescope Observations of the interstellar object 3I/ATLAS, taken in 2025 December and 2026 January, that show fluorescence emissions from the CO Fourth Positive Group that are well fit by production rates of 3.8 × 10 ²⁷ and 1.1 × 10 ²⁷ molec. s ⁻¹ , respectively. We also place stringent upper limits on the atomic sulfur column and show that 3I/ATLAS’s comae is significantly depleted in sulfur, with S/O 3 σ upper limits of 0.4% and 0.1% for the 2025 December and 2026 January epochs, respectively. We hypothesize that this sulfur depletion is the result of a physical and chemical formation environment that was extremely cold, UV-shielded, and/or sulfur-depleted and therefore not conducive to the formation of H ₂ S or SO/SO ₂ while still effectively forming OCS and depleting the H ₂ S reservoir, with substantial material inherited from the dense interstellar cloud. That same process would also sequester sulfur into FeS and S ₆ –S ₈ allotropes, which would be refractory at the observed heliocentric distances and would not contribute to the atomic sulfur coma. These species, which are often highlighted as potential solutions to the cosmic sulfur depletion problem, are also theorized to be the source of excess atomic sulfur seen in solar system comets.

We report a survey of molecular emission from cometary volatiles using the James Webb Space Telescope (JWST) toward interstellar object 3I/ATLAS carried out on UT 2025 December 22 and 23 at a heliocentric distance ( r _H ) of 2.37–2.41 au. These measurements of CO, CO ₂ , H ₂ O, CH ₃ OH, and CH ₄ sampled molecular chemistry in 3I/ATLAS as it receded from its encounter with our Sun and entered the vicinity of the H ₂ O ice line—the region between r _H = 2 and 3 au where the temperature becomes too low for H ₂ O to vigorously sublime, and CO and CO ₂ begin to control the overall activity. CO was the most abundant molecule, followed by H ₂ O and CO ₂ , whose molecular abundances with respect to CO were (40.5 ± 3.1)% and (41.6 ± 0.3)%, respectively. This work presents spatial–spectral maps of column density and rotational temperature as a function of distance from the nucleus for all detected species. The spatial distributions of both quantities were highly anisotropic for the apolar species in the coma of 3I/ATLAS, yet were more nearly symmetric for the polar molecules. These results demonstrate how volatiles were segregated in the nucleus ices of 3I/ATLAS and reveal heating and cooling mechanisms in its coma. Derived maps of the ortho-to-para ratio (OPR) for H ₂ O were flat with increasing distance from the nucleus and consistent with a coma-averaged value OPR = 2.7 ± 0.2, slightly less than the expected equilibrium value of 3.

Interstellar objects provide the only directly observable samples of icy planetesimals formed around other stars, and can therefore provide insight into the diversity of physical and chemical conditions occurring during exoplanet formation^1-3. Here we report isotopic measurements of the interstellar comet 3I/ATLAS, which reveal an elemental composition unlike any Solar System body. The water in 3I/ATLAS is enriched in deuterium, at a level of D/H = (0.98 ± 0.06)%, which is more than an order of magnitude higher than in known comets, while its range of ¹²C/¹³C ratios (141-191 for CO₂ and 123-172 for CO) exceeds typical values found in the Solar System, as well as nearby interstellar clouds and protoplanetary disks. Such extreme isotopic signatures indicate formation at temperatures ≲ 30 K in a relatively metal-poor environment. When interpreted with respect to models for Galactic chemical evolution, the carbon isotopic composition implies that 3I/ATLAS may have accreted as long ago as 12 billion years, following a period of intense, early star formation. 3I/ATLAS thus represents a preserved fragment of an ancient planetary system.

As the Centaurs of the outer solar system have become better studied, their relationship with the Jupiter-family comets (JFCs) and their connection to the primordial protoplanetary disk have come under deeper scrutiny: which properties of the comets observed closer to the Sun are original, and which were modified significantly en route during their Centaur phase? The active Jupiter co-orbital comet P/2019 LD2 (ATLAS) attracted significant attention after its discovery when it was realized that the object would transition from a Centaur orbit to a JFC orbit in 2063, the first time this change will be able to be observed and monitored in real time. In this study, we present new ground-based and Hubble Space Telescope (HST) imaging of the single other Jupiter co-orbital comet, P/2023 V6 (PANSTARRS), throughout late 2024 and early 2025. V6 was not detected in ground-based imaging after early October, and deep imaging with HST taken in early December is consistent with an m _V = 28.06 ± 0.03 point source at its expected location. This implies both that the object ceased activity sometime in late 2024 and is thus in a different thermal state than LD2, despite both having large active fractions, and that V6’s diameter is D ∼ 340 m. This is likely the smallest object directly observed by HST beyond the main belt, and certainly the smallest Centaur yet discovered, highlighting the ability to detect active subkilometer objects beyond Jupiter with current surveys and potentially probing smaller sizes with the Legacy Survey of Space and Time.

We report results from an analysis of the volatile composition and evolution of the main-belt comet (MBC) 133P/Elst–Pizarro using JWST NIRSpec and NIRCam observations and ground-based observations during its 2024 active apparition, and also assess the body of JWST MBC observations acquired to date. Using NIRSpec, we measure water vapor outgassing rates at two points in 133P’s orbit, finding Q H 2 O = ( 1.9 ± 0.6 ) × 1 0 25 molecules s ⁻¹ on UT 2024 June 12 (at a true anomaly of ν = 8° and heliocentric distance of r _h = 2.674 au), and Q H 2 O = ( 1.4 ± 0.4 ) × 1 0 25 molecules s ⁻¹ on UT 2024 October 14 (at ν = 37 . ° 4 and r _h = 2.747 au). These measurements nominally represent a decline of ∼25% in Q H 2 O between the visits, although they are also consistent with no change within uncertainties. We do not detect CO, CO ₂ , or CH ₃ OH, placing 133P’s hypervolatile depletion ( Q CO 2 / Q H 2 O < 0.009 ) at a similar level found for previously observed MBCs. We find log ( A f ρ / Q H 2 O ) values for the three MBCs for which water vapor outgassing has been successfully detected that are consistent within uncertainties with an average value of log ( A f ρ / Q H 2 O ) = − 24.6 ± 0.2 . Lastly, we find no clear correlations of water production rates with nucleus size, semimajor axis, or heliocentric distance among MBCs observed by JWST so far, but would particularly encourage future JWST observations of additional MBCs interior to the 5A:2J mean-motion resonance with Jupiter and at high inclinations, as well as multiple observations of MBCs during single active apparitions to further investigate areas of interest identified from the current sample of JWST-observed MBCs.

We present JWST NIRSpec and NIRCam observations of 457P/Lemmon-PANSTARRS, a main-belt comet that displayed activity around its 2020 perihelion and that was observed to regain activity during its 2024 perihelion by a ground-based observing campaign. The previous successful measurements of water production from two main-belt comets by the JWST NIRSpec instrument confirmed the hypothesis that H ₂ O reservoirs are responsible for activity in dynamically stable main-belt comets. However, the main-belt comets observed with JWST thus far, 238P/Read and 358P/PANSTARRS, occupy orbits in the outer main belt, with main-belt comets with smaller semimajor axes not yet sensitively tested for H ₂ O. We find that, despite clearly displaying dust activity in both ground-based and JWST imaging over a broad period, there were no corresponding H ₂ O, CO, CO ₂ , or CH ₃ OH emissions within sensitive upper limits—notable given that 457P is the first main-belt comet with a semimajor axis within the 5:2 mean-motion resonance with Jupiter. We show that we were sensitive to production rates of gas predicted by the dust/gas ratios of 238P and 358P, and we hypothesize that 457P may be more depleted than its companions; Q _H2O must be less than 2 × 10 ²⁴ molecules s ⁻¹ , or 0.035 kg s ⁻¹ . Further surveying of main-belt comets across the parameter space of semimajor axis and eccentricity will shed light on whether 457P represents an edge member of a spectrum or a distinct subclass of main-belt comets.