Science and Exploration

NASA’s Webb Telescope Finds Elusive Water Vapor in Rare Main Belt Comet

By John Williams
May 25, 2023
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NASA’s Webb Telescope Finds Elusive Water Vapor in Rare Main Belt Comet
Spectral data gathered on Comet 238P/Read by the NIRSpec (Near-Infrared Spectrograph) instrument on the NASA/ESA/CSA James Webb Space Telescope in 2022.
Image credit: NASA, ESA, CSA, and J. Olmsted (STScI).

Solar system researchers using NASA’s James Webb Space Telescope spotted a long-sought-after signature for water vapor for the first time in a rare main belt comet. The study, published this month in the journal Nature, offers insight into the origins of Earth’s abundant water, and for the first time indicates the presence of water from the primordial solar system in the region of the solar system between Mars and Jupiter.

“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,” study coauthor Stefanie Milam, Webb deputy project scientist for planetary science, said in a press release. “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,” she added.

The finding comes with a mystery, however, study coauthor Henry Hsieh, a comet researcher from the Planetary Science Institute in Hawai’i, tells SpaceRef. Unlike other comets, the comet in the study (dubbed Comet 238P/Read) had no detectable carbon dioxide.

Comets in the outer solar system typically are a mixture of volatile ices, such as water, carbon dioxide, carbon monoxide, and more complex molecules, Hsieh explains. The ices sublimate to gas once the comet reaches certain temperatures. Scientists also think that the makeup of a comet reflects the conditions in the region of the solar system where it formed. For example, if a comet contains carbon dioxide ice, it must have formed in a part of the early solar system that was far enough from the sun for CO2 ice to be present. In most comets, water requires the most heating to sublimate from ice to gas. Gases, such as CO2 and carbon monoxide bake out much faster.

“Given these considerations, the lack of CO2 in Comet 238P suggests that it formed in a part of the early solar system where no CO2 ice was present, meaning that it probably formed much closer to the Sun during the early part of our solar system’s history than other comets,” said Hsieh.

Or, Hsieh added, it lost its CO2 ice because it has been in the warmer inner solar system far longer than other comets.

“Even though I along with my collaborators over the years collected a substantial body of indirect evidence that water sublimation was the likely cause of main-belt comet activity, the question of whether there was any direct, concrete spectroscopic evidence of water outgassing would continually come up, and so it is personally exciting to me to finally have an answer to this question I’ve been asking over all these years,” Hsieh said.

Conducting this sort of research with the James Webb Space Telescope offers significant advantages over doing so from terrestrial observatories. From Earth, water in comets is tough to detect, Hsieh explains. Water vapor dominates the terrestrial atmosphere, making ground-based detection of outgassing in comets much harder. So, astronomers focus on searching for other gases, such as cyanide gas. Scientists commonly observe the gas alongside water in comets from the outer solar system.

This technique has weaknesses, however, if proportions of cyanide gas, water, and carbon dioxide are not found in the same amounts between outer and inner solar system comets.

“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,” University of Maryland astronomer Michael Kelley, who led the research team behind the discovery, said in a NASA press release.

“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,” Kelley added.

The next step is to see how other main belt comets compare to Comet Read, study coauthor and Heidi Hammel, an astronomer at the Association of Universities for Research in Astronomy (AURA), said in the NASA release. She’s also the lead for Webb’s Guaranteed Time Observations for solar system objects.

“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. Do other main belt comets also lack carbon dioxide? Either way it will be exciting to find out,” Hammel said.

John Williams

John is a Colorado-based science writer, astrophotographer, science outreach enthusiast, and creative technologist. He is the author of award-winning Hubble Star Cards and a few children's books.