UMD Astronomer Leads Study that Finds Water, and a New Mystery, in Rare Main Belt Comet

Research scientist Michael Kelley offered a theory that could explain Comet 238P/Read’s conspicuous lack of carbon dioxide.

Using infrared technology on NASA’s James Webb Space Telescope, astronomers confirmed gas—specifically water vapor—around a comet in the main asteroid belt for the first time.

Their study, published May 15, 2023, in the journal Nature, centered on Comet 238P/Read. It’s a main belt comet, which means it resides in the main asteroid belt between Mars and Jupiter and periodically displays a halo—or coma—and a tail like a comet. 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 the advent of Webb.

Comet Read
This illustration of Comet 238P/Read shows the main belt comet sublimating—which occurs when its water ice vaporizes—as its orbit approaches the sun. The James Webb Space Telescope’s detection of water vapor at Comet Read is a major benchmark in the study of main belt comets and, more broadly, the origin of Earth’s abundant water. Credit: NASA, ESA.

“In the past, we’ve seen objects in the main belt with all the characteristics of comets, but only with this precise spectral data from Webb can we say yes, it’s definitely water ice that is creating that effect,” said the study’s lead author Michael Kelley, a research scientist in the University of Maryland’s Department of Astronomy. “With Webb’s observations of Comet Read, we can now demonstrate that water ice from the early solar system can be preserved in the asteroid belt.”

Their findings also present a new puzzle: Unlike other comets, Comet Read had no detectable carbon dioxide. Typically, carbon dioxide makes up about 10% of the volatile material in a comet that can be easily vaporized by the sun’s heat.

Kelley said it is possible that Comet Read had carbon dioxide when it formed but lost it because of warm temperatures.

“Being in the asteroid belt for a long time could do it—carbon dioxide vaporizes more easily than water ice and could percolate out over billions of years,” Kelley said.

Alternatively, he said, Comet Read may have formed in a particularly warm pocket of the solar system where no carbon dioxide was available.

Main belt comets are a fairly new classification, and Comet Read was one of the three original 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 materials that vaporize as they approach the sun are what give comets their distinctive coma and streaming tail, differentiating them from asteroids.

Beyond shedding light on main belt comets, these findings bring researchers a step closer to understanding the origins of Earth’s abundant water.

“Our water-soaked world, teeming with life and unique in the universe as far as we know, is something of a mystery—we’re not sure how all this water got here,” said Stefanie Milam, Webb deputy project scientist for planetary science and co-author of the Nature paper. “Understanding the history of water distribution in the solar system will help us to understand other planetary systems, and if they could be on their way to hosting an Earth-like planet.”

The next step for researchers will be seeing how other main belt comets compare with Comet Read, explained astronomer Heidi Hammel of the Association of Universities for Research in Astronomy, lead for Webb’s Guaranteed Time Observations for solar system objects and co-author of the study.

“These objects in the asteroid belt are small and faint, and with Webb we can finally see what is going on with them and draw some conclusions,” Hammel said. “Do other main belt comets also lack carbon dioxide? Either way, it will be exciting to find out.”

Milam imagines the possibilities of bringing the research even closer to home.

“Now that Webb has confirmed there is water preserved as close as the asteroid belt, it would be fascinating to follow up on this discovery with a sample collection mission and learn what else the main belt comets can tell us,” Milam said.


This release was adapted from text provided by the Space Telescope Science Institute.

The research paper, “Spectroscopic identification of water emission from a main-belt comet,” was published in the journal Nature on May 15, 2023.

This research was supported by NASA (Contract No. NAS 5-03127 and Award No. 21-SMDSS21-0013). This story does not necessarily reflect the views of this organization.

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