Webb Detects Water Vapor around Main-Belt Comet 238P/Read
This artist’s concept of shows the main-belt comet 238P/Read sublimating — its water ice vaporizing as its orbit approaches the Sun. Image credit: NASA / ESA.
Also known as P/2005 U1, it resides in the main asteroid belt but periodically displays a coma and a tail.
It has an orbital period of 5.63 years and a diameter of 600 m (2,000 feet).
“Main-belt comets themselves are a fairly new classification, and 238P/Read was one of the original three comets used to establish the category,” said Webb deputy project scientist for planetary science Dr. Stefanie Milam, an astronomer at NASA’s Goddard Space Flight Center, and her colleagues.
“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.”
This graphic presentation of spectral data highlights a key similarity and difference between observations of comet 238P/Read by Webb’s NIRSpec instrument in 2022 and observations of comet 103P/Hartley 2 by NASA’s Deep Impact mission in 2010. Image credit: NASA / ESA / CSA / J. Olmsted, STScI.
Astronomers 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.
“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 Dr. Michael Kelley, an astronomer at the University of Maryland.
“With Webb’s observations of 238P/Read, we can now demonstrate that water ice from the early Solar System can be preserved in the asteroid belt.”
According to the team, the missing carbon dioxide was a bigger surprise.
Typically, carbon dioxide makes up about 10% of the volatile material in a comet that can be easily vaporized by the Sun’s heat.
The authors present two possible explanations for the lack of carbon dioxide.
“One possibility is that 238P/Read had carbon dioxide when it formed but has lost that because of warm temperatures,” Dr. Kelley said.
“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.”
“Alternatively, 238P/Read may have formed in a particularly warm pocket of the Solar System, where no carbon dioxide was available.”
Source: Sci News