There’s a lot of water in the solar system. Several moons and planets are loaded with it, while comets are remotely loaded with it. Where it warms in the sun’s heat, it sublimates into a gas that drifts into the vacuum of space.
Strangely enough, while water ice has been discovered in the main belt of the asteroids, its vapor has so far been absent. Thanks to the James Webb Space Telescope, we know there’s water in it asteroid Belt.
The water is being ejected from one of the rare comets in the main belt, an object called Comet 238P/Read, showing that water has been preserved there from the time the solar system formed. Scientists had assumed that conditions might be too close to the sun for much ice to remain.
The discovery also confirms that objects in the asteroid belt may have helped transport water to Earth when the solar system was young, and that comets in the main belt have enough ice for the outgassing that occurs when that ice is subjected to the heat of the planet sun sublimated. So far, only dust, not outgassing of steam, has been observed from comets in the main belt.
“Since the discovery of main-belt comets, we have accumulated ample evidence that their activity is generated by sublimation, but until now it has all been indirect.” says planetary scientist Henry Hsieh from the Planetary Science Institute in the USA.
“This new result from JWST represents the first direct evidence of sublimation in the form of water outgassing — or outgassing of any kind — from a main-belt comet, following studies dating back to 2008 to detect outgassing in main-belt comets using some of the largest ground-based ones.” telescopes in the world.
Not surprisingly, most of the objects in the asteroid belt are asteroids, and they’re relatively inert chunks of rock just hanging around in space. In contrast, comets are defined by their activity, which is also largely influenced by their icy, dusty composition.
Normally, they orbit the sun in large elliptical orbits, which they transport from the outer solar system. The ice within them sublimates as they approach the Sun (called perihelion), creating a dusty, gassy atmosphere and long tails streaming away from the Sun.
There aren’t many comets that we’ve spotted in the Main Belt, but because they’re so close to the Sun, relatively speaking, scientists weren’t sure if they had enough frozen material to produce the sublimation seen in coming comets observed from greater distances.
Although Comet Read’s orbit is entirely within the asteroid belt, it’s still a relatively wide swath of the Solar System to circle, and the object is still at perihelion. During this portion of its orbit, a team led by University of Maryland astronomer Michael Kelley used JWST to examine it closely for signs of outgassing.
Using the telescope’s near-infrared spectrograph, the researchers captured and analyzed the light spectrum of the fuzzy haze that appeared around the comet during perihelion. In fact, the peaks in the spectrum showed not only outgassing, but also outgassing of water.
“In the past we have seen objects with all the characteristics of comets in the main belt, but only with this precise spectral data from JWST can we say: Yes, it is definitely water ice that is producing this effect.” Kelly explains.
“With the JWST observations of Comet Read, we can now show that water ice from the early Solar System can be preserved in the asteroid belt.”
Oddly enough, something was missing. In a stark and puzzling contrast to other comets in the Solar System, which typically make up about 10 to 20 percent of their volatiles with carbon dioxide, the researchers found no carbon dioxide on Comet Read.
There are two possible explanations for this compositional anomaly, since carbon dioxide ice sublimes more readily than water ice. One explanation is that while the comet contained carbon dioxide, it lost it entirely, while some water ice remained. Another reason is that the place in the solar system where Comet Read formed was too warm for carbon dioxide, so it simply never existed to begin with.
Future work may be required to investigate the likelihood of these possibilities. But Comet Read’s answers have also given astronomers much food for thought.
“Water in main-belt comets is important because objects from the asteroid main-belt have been suggested as a potential source of water for Earth in the early Solar System, where modern-day human-belt comets appear to provide an opportunity to test this hypothesis.” says Hsieh.
“However, this only works if they actually contain water ice. The confirmation of water outgassing in at least one main-belt comet confirms that there is a realistic possibility of learning about the origin of water on Earth from main-belt comets.”
The results were published in Nature.
