Emerging Diversity among the Main-belt Comets: Insights from JWST and Ground-based Observations of 457P/Lemmon-PANSTARRS

Abstract 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.