The James Webb Space Telescope has detected water vapor on an object in the asteroid belt, a region of the solar system where it wasn’t known to survive. The new observations also reveal something missing unexpectedly.

Asteroids and comets may seem very different – asteroids are dusty, dry balls of rock that orbit in a belt between Mars and Jupiter, while comets are icy, tailed and hail from beyond Neptune. But the line between them is getting fuzzier all the time, as some objects in the main asteroid belt have been seen to occasionally grow tails and haloes like comets. These are called active asteroids, or main-belt comets, which really only serves to confuse the issue further.

While it’s long been assumed that the gas in the tails and haloes of these objects was made of water vapor, Webb has now confirmed it. The telescope’s Near-Infrared Spectrograph (NIRSpec) instrument analyzed the emission spectrum coming off a main-belt object called Comet 238P/Read, and found the clear signature of water.

The implications of that find are bigger than you might expect. In the distant reaches of the solar system, it’s cold enough for water to freeze on these objects. It’s only when they venture closer to the Sun that this ice vaporizes, forming the clear tails and haloes that characterizes comets. But it wasn’t clear whether water would survive long-term in the relatively warm main asteroid belt.

“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 Michael Kelley, lead author of the study. “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.”

This lends weight to the leading hypothesis about where Earth’s water originally came from – asteroids and comets regularly striking the planet in the distant past. But the observations of Comet Read also raised new questions.

To the team’s surprise, the comet had no detectable carbon dioxide. Usually, the gas makes up around 10% of a comet’s volatile material, the stuff that’s easily vaporized by the Sun to form the halo and tail. The researchers suggested two main possibilities for how it can lack CO₂ completely – either it was born in a warm region of the solar system where there was none available, or it used to have some but it boiled off long ago.

“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,” said Kelley.

Future observations will check whether Comet Read is an anomaly or if other main-belt comets are also lacking carbon dioxide.

The research was published in the journal Nature.

Source: ESA