Main-Belt Comet Yields Water Vapor Clues: New Near-Infrared and Infrared Discoveries from the James Webb Space Telescope


Researchers using the James Webb Space Telescope (JWST) have observed a water vapor coma around main-belt comet 238P/Read. The study, led by Michael S. P. Kelley and Henry H. Hsieh, provides insights into the comet's activity driven by water-ice sublimation and distinguishes it from other comets due to the absence of significant carbon dioxide gas.

A breathtaking digital illustration of a comet streaking across a starry sky © tashechka -

A breathtaking digital illustration of a comet streaking across a starry sky © tashechka -

The discovery of water vapor around main-belt comet 238P/Read marks a significant milestone in understanding the volatile inventory of the early Solar System. Researchers from the University of Maryland have used the James Webb Space Telescope (JWST) to observe this phenomenon, revealing key differences between main-belt comets and the general cometary population. The study was conducted by Michael S. P. Kelley, Henry H. Hsieh, Dennis Bodewits, and others from the Department of Astronomy at the University of Maryland in College Park, Maryland, and published in the journal Nature (1).

Details and Findings

Main-belt comets, located within the asteroid belt, exhibit comet-like activity through dust comae or tails during perihelion passages, indicating the presence of ice sublimation. Historically, despite extensive observation efforts, no gas has been detected around these comets. However, recent observations using JWST have changed this narrative.

On September 8, 2022, JWST captured images and spectra of comet 238P/Read using its NIRCam and NIRSpec instruments. These observations, taken 95 days post-perihelion, revealed a cometary coma and tail, along with a prominent 2.7 μm (2700 nm) emission feature. This feature corresponds to water vapor emission with a production rate of approximately 9.9 x 1024 molecules per second, equivalent to 0.30 kg per second (1). The near-infrared region is often used for analysis of comets and other celestial bodies (1–2).

Comparative Analysis

Comparing the JWST spectrum of comet Read to that of comet 103P/Hartley 2, obtained by the Deep Impact spacecraft, highlights critical differences. While Hartley 2 displayed both water vapor and CO2 gas emission bands at 2.7 μm (2700 nm) and 4.3 μm (4300 nm), respectively, Read's spectrum lacked the CO2 band. The upper limit for CO2 production in comet Read was calculated at 7 x 1022 molecules per second, or at most 5 grams per second. This resulted in a CO2/H2O ratio of less than 0.7%, significantly lower than in other comets (1).


The absence of CO2 in Read's coma suggests a fundamental difference between main-belt comets and the broader comet population. The findings imply that main-belt comets could represent a previously unobserved sample of volatile material, crucial for understanding the Solar System's early volatile inventory and subsequent evolution (1).

Active Surface Area Estimates

To further understand the mass-loss process, researchers estimated the sublimating surface area of comet Read. Using a water-ice sublimation model, they calculated an active area between 0.03 and 0.11 km², with a lower active area being more appropriate given the sunward asymmetry of the water coma. This active fraction aligns with typical comets, reflecting Read's small size and surface characteristics (1).

Impact Hypothesis

An alternative hypothesis for the observed water vapor involves a localized source, possibly resulting from a small impactor uncovering buried ice. However, simulations indicate that an impactor large enough to create the required active area might catastrophically disrupt the comet's nucleus, making this scenario less likely (1).

Spectral Features

The infrared spectrum of Read showed a broad absorption feature from 2.8 (2800 nm) to 3.7 μm (3700 nm), similar to features in comets Hartley 2, Churyumov–Gerasimenko, and asteroid (24) Themis. However, the lack of short-wavelength water-ice features at 1.5 (1500 nm) and 2.0 μm (2000 nm) suggests small particle sizes or a different surface composition (1).

Dynamical Context

Dynamically, comet Read is associated with outer main-belt asteroids, differing from classical comet populations. Its orbit, stable for about 20 million years, and low inclination indicate it is not a recent interloper from the outer Solar System. This aligns with the observed CO2 depletion, supporting the theory of long-term residence in the asteroid belt (1).


The study concludes that comet Read's activity is driven by water-ice sublimation, with its current activity possibly declining over time. The findings highlight the unique characteristics of main-belt comets and their importance in understanding the early Solar System's volatile inventory. Future research will aim to provide more detailed insights into the formation and evolutionary history of these intriguing celestial objects (1).


Kelley, M. S. P.; Hsieh, H. H.; Bodewits, D.; et al. Spectroscopic Identification of Water Emission From a Main-Belt Comet. Nature 2023, 619, 720–723. DOI: 10.1038/s41586-023-06152-y

Bonev, B. P.; Russo, N. D.; Kawakita, H.; et al. The Return of the Rosetta Target: Keck Near-infrared Observations of Comet 67P/Churyumov–Gerasimenko in 2021. Astron. J. 2023, 166 (6), 233. DOI: 10.3847/1538-3881/acee59

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