When astronomers peer into the solar system’s asteroid belt, they expect to find millions of small, rocky bodies. Yet in 1996, researchers spotted an object in the main belt sporting a coma and tail made from vaporizing volatiles. Today fewer than 20 of the appropriately named main-belt comets are known, compared with thousands of comets whose orbits stretch into the outer solar system. Main-belt comets are still a mystery, as their origins are unknown, and they lead to questions about what distinguishes comets from asteroids.
The second main-belt comet ever discovered, comet 238P/Read, was recently observed using the Near Infrared Spectrograph on the James Webb Space Telescope (JWST). The observations revealed water vapor on comet Read, meaning water ice is present in the comet’s nucleus. It’s the only detection of water vapor within the asteroid belt, aside from on the dwarf planet Ceres. The other twist? The comet has hardly any carbon dioxide, a common volatile known to be present in many comets.
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