The powerful James Webb Space Telescope has for the first time detected the presence of water around a comet in the main asteroid belt.
Using Webb’s NIRSpec (Near-Infrared Spectrograph) instrument, astronomers confirmed the presence of gas — precisely water vapor — around Comet 238P/Read, indicating that water ice from the early solar system may have been preserved in this region.
“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 the Webb can we say yes, it’s definitely water ice that is creating that effect,” said lead author and University of Maryland astronomer Michael Kelley.
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“With Webb’s observations of Comet Read, it is now possible to demonstrate that water ice from the early solar system can be preserved in the asteroid belt,” Kelley said. The study was published in Nature magazine.
Comet Read is a main belt comet, which is an object that resides in the main asteroid belt but intermittently exhibits a coma and tail like a comet. Comet Read was one of the three original comets used to establish the category of main belt comets, which is a relatively novel classification.
Before that, it was believed that comets resided in the Kuiper Belt and Oort Cloud, beyond Neptune’s orbit, where their ices could be conserved at greater distances from the Sun. First Infrared Image of Three Asteroid Belts Outside the Solar System Obtained by NASA’s James Webb Space Telescope (See Images).
Comets are distinguished from asteroids by their coma and streaming tail, which are caused by frozen material that vaporizes as they approach the Sun. Before Webb, scientists had long hypothesized that water ice could be conserved in the heated asteroid belt, within Jupiter’s orbit, but definitive proof was lacking.
In contrast to other comets, however, Comet 238P/Read lacked detectable carbon dioxide (CO2), which makes up about 10% of the volatile material in a comet that is readily vaporized by the Sun’s radiation. The team hypothesized that comet Read originally contained CO2 but has since lost it due to rising temperatures.
CO2 vaporizes more readily than water ice and could percolate out over billions of years, according to Kelley. Alternately, he said, Comet Read could have formed in a particularly heated region of the solar system where there was no carbon dioxide.
The next stage involves extending the investigation beyond Comet Read to determine how it compares to other main belt comets. “Now that Webb has confirmed that water is preserved as close as the asteroid belt, it would be fascinating to follow up this discovery with a sample collection mission and learn more about what the main belt comets can teach us,” said Stefanie Milam, Webb’s deputy project scientist for planetary science.