Starship Asterisk*

Comet 238 P/Read and Comet 109 P/Hartley 2 Spectral Data Comparison

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Illustration Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)

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NASA’s James Webb Space Telescope has enabled another long-sought scientific breakthrough, this time for solar system scientists studying the origins of Earth’s abundant water. Using Webb’s NIRSpec (Near-Infrared Spectrograph) instrument, astronomers have confirmed gas – specifically water vapor – around a comet in the main asteroid belt for the first time, indicating that water ice from the primordial solar system can be preserved in that region. However, the successful detection of water comes with a new puzzle: unlike other comets, Comet 238P/Read had no detectable carbon dioxide. ...

Comet Read is a main belt comet – an object that resides in the main asteroid belt but which periodically displays a halo, or coma, and tail like a comet. Main belt comets themselves are a fairly new classification, and Comet Read was one of the original three comets used to establish the category. Before that, comets were understood to reside in the Kuiper Belt and Oort Cloud, beyond the orbit of Neptune, where their ices could be preserved farther from the Sun. Frozen material that vaporizes as they approach the Sun is what gives comets their distinctive coma and streaming tail, differentiating them from asteroids. Scientists have long speculated that water ice could be preserved in the warmer asteroid belt, inside the orbit of Jupiter, but definitive proof was elusive – until Webb. ...

The missing carbon dioxide was a bigger surprise. Typically, carbon dioxide makes up about 10 percent of the volatile material in a comet that can be easily vaporized by the Sun’s heat. The science team presents two possible explanations for the lack of carbon dioxide. One possibility is that Comet Read had carbon dioxide when it formed but has lost that because of warm temperatures. ...

• Nature (online 15 May 2023) DOI: 10.1038/s41586-023-06152-y