Emerging Diversity Among the Main Belt Comets: JWST Observations of 457P/Lemmon-PanSTARRS

The recent detection of water vapor sublimation from objects in the asteroid belt, a region that should be devoid of any natal ice, challenges existing models of thermal evolution in the inner solar system. Prior to the launch of JWST, the assessment of cometary activity for the main belt comets (MBCs) was driven by observations of the surrounding dust coma, where water production rates were estimated, but never directly detected, despite numerous attempts to do so using the world’s most powerful telescopes. The exceptional sensitivity of the JWST NIRSpec instrument has now made it possible to measure gas-phase volatiles from MBCs, providing the first definitive evidence of ongoing sublimation in this population. However, the sample size of MBCs with measured water production rates remains small, and each additional measurement (or constraint) on the production rates for additional objects will have a large effect on the interpretation of the uniformity of H₂O as a source.

Following the water production rate measurements and CO,CO₂, and CH₃OH upper limits for 238P/Read (Kelley et al. 2023) and 358P/PANSTARRS (Hsieh et al. 2025) with JWST NIRCam and NIRSpec, we obtained observations of another MBC, 457P/Lemmon-PANSTARRS, on September 20, 2024. Unlike 238P and 358P, 457P is not associated with the Themis family of asteroids, but may have its own family (Xin et al. 2024), suggesting it could be compositionally different. Notably, we do not find evidence of H₂O, or any other volatile emission, from 457P at the time of observation despite a clear sunward tail in the NIRCam images. We derive a 3σ H₂O production rate upper limit well below that detected for 238P (<10²⁵ molec./s), indicating a significant difference in activity state during the observations. In this talk we will discuss the hypotheses for the observed state: recent global shutoffs, shadowing of an active region, mass wasting, and continuous low global activity levels. We will analyze how each mechanism would shape the observed activity and evaluate how well the dust abundance and morphology and volatile upper limits are fit by each unique hypothesis. Finally,  we will place our results in context with the detections from 238P and 358P to discuss the diversity of activity drivers among MBCs.