Spectroscopic identification of water emission from a main-belt comet.

Main-belt comets are small solar system bodies located in the asteroid belt that repeatedly exhibit comet-like activity (i.e., dust comae or tails) during their perihelion passages, strongly suggesting ice sublimation. 1,2 Although the existence of main-belt comets implies the presence of extant water ice in the asteroid belt, no gas has been detected around these objects despite intense scrutiny with the world's largest telescopes. 3 Here, we present JWST observations which clearly show that main-belt comet 238P/Read has a coma of water vapour, but lacks a significant CO 2 gas coma. Our findings demonstrate that the activity of comet Read is driven by water-ice sublimation, and implies that main-belt comets are fundamentally different from the general cometary population. Whether comet Read experienced different formation circumstances or evolutionary history, it is unlikely to be a recent asteroid belt interloper from the outer solar system. Based on these results, main-belt comets appear to represent a sample of volatile material that is currently unrepresented in observations of classical comets and the meteoritic record, making them important for understanding the early solar system's volatile inventory and its subsequent evolution.