The Ensemble Volatile Compositional Properties of 133P/Elst-Pizarro and Other JWST-Observed Main-Belt Comets

Main-belt comets (MBCs) are small solar system objects that display comet-like activity with characteristics indicative of the sublimation of volatile ices yet have dynamically stable orbits in the main asteroid belt.  Until recently, sublimation has only been inferred as the primary activity driver for MBCs, largely based on observations of recurrent activity near perihelion and the inability of other activity drivers to plausibly account for such behavior over a wide range of objects.  Starting in 2022, however, JWST observations of four active MBCs — 238P/Read, 358P/PANSTARRS, 133P/Elst-Pizarro, and 457P/Lemmon-PANSTARRS — have provided unambiguous spectroscopic evidence of the presence of water vapor sublimation in at least three of these objects: 238P (JWST GTO program 1252; Kelley et al., 2023, Nature, 619, 720), 358P (JWST GO program 4250; Hsieh et al., 2025, PSJ, 6, 3), and 133P (JWST GO programs 4250 and 5551; this work).  These observations have also shown that these MBCs have a striking relative absence of CO₂, which is commonly found in other comets with similar water production rates, pointing to MBCs comprising a distinct volatile inventory compared to other classical comets.  The enhanced sensitivity of JWST’s NIRSpec instrument has significantly advanced the study of main-belt comets, shifting the focus from the long-standing question of whether volatile sublimation could be directly detected to now characterizing its properties and examining how they vary across different objects and observing conditions.

We will present and discuss JWST observations of 133P, the archetype of the MBC population, obtained at two positions in its orbit. These observations confirm that the activity is driven by water ice sublimation and continue to show the relative absence of more volatile species, extending the emerging pattern established by 238P and 358P.  JWST NIRCam and NIRSpec observations of 133P were obtained on UT 2024 June 12 and UT 2024 October 14 and 28 when the object was at a true anomaly of 8° and a heliocentric distance of 2.67 au, and a true anomaly of ~40° and a heliocentric distance of ~2.75 au, respectively.  Preliminary measurements show water sublimation rates of Q(H₂O)~10²⁵ molecules/s during both sets of observations, while only upper limits were obtained for CO, CO₂, and CH₃OH sublimation rates.  

We will also present results from optical imaging campaigns conducted for all four JWST-observed MBCs over the courses of their entire corresponding active apparitions from a range of ground-based facilities giving us information on the photometric and morphological evolution of each target over many months before and after their JWST observations.  We will specifically describe efforts to correlate measured water sublimation rates with estimated dust production rates (as parameterized by the Afρ parameter) for comparison to other types of comets, ascertain the feasibility of estimating water sublimation rates for non-JWST-observed comets from Afρ measurements alone, and examine how Afρ/Q(H₂O) varies for objects with different physical and orbital properties, and different observational circumstances when observed by JWST.