Water has been found in the Solar System's main asteroid belt for the first time, giving scientists an insight into where Earth's oceans may have come from, NASA says.
Astronomers using the James Webb Space Telescope's (JWST) NIRSpec (near-infrared spectrograph) confirmed water vapour around Comet 238P/Read, circulating between Jupiter and Mars.
In a statement, NASA said the discovery confirmed longstanding speculation that water ice could be preserved in the warmer asteroid belt inside Jupiter's orbit, and could have been delivered to a young Earth.
"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 a co-author on the study reporting the finding.
How planets get water is a major research topic for the JWST: the telescope's website explains that water close to a star would be in a gaseous state and drift away from a forming rocky planet like the Earth.
This has led some astronomers to look to comets as a source of supply for our own water.
"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," Dr Milam said.
Comet Read is a main belt comet: a classification introduced in 2006, based on Read and two others, for objects that reside in the asteroid belt but periodically display a halo — called a coma — and a tail like a comet.
Conventional comets are understood to reside beyond the orbit of Neptune, where ice can be preserved far from solar radiation. Their distinctive tails are formed by frozen material vaporising as they approach the Sun.
"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," explained astronomer Michael Kelley of the University of Maryland, lead author of the study.
Lack of carbon dioxide creates new mystery
The JWST's measurements, however, uncovered a new mystery: Comet 238P/Read had no detectable carbon dioxide.
The gas normally makes up about 10 per cent of a comet's volatile material, so researchers say its absence was more surprising than the presence of water.
The research team suggests two possible explanations.
"Being in the asteroid belt for a long time could do it — carbon dioxide vaporises more easily than water ice, and could percolate out over billions of years," Dr Kelley said.
The other option, Dr Kelley said, was that Comet Read may have formed in a particularly warm pocket of the solar system, where no carbon dioxide was available.
Researchers next hope to examine how other main belt comets compare to Comet Read.
"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," said co-author Heidi Hammel of the Association of Universities for Research in Astronomy.
Dr Milam is more ambitious: "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," she said.
The study is published in the journal Nature.
