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It is well known that Earth is unique among the rocky planets in our solar system because of the vast oceans of liquid water on its surface.
Exactly how all this water ended up on Earth is more of a mystery, although scientists have recently ruled out molten meteorites as a source.
Now data from NASA’s James Webb telescope reveal for the first time that water vapor is present around a comet in our solar system’s main belt — the rocky ring between the orbits of Mars and Jupiter.
The comet, called 238P/Read, has a water vapor cloud around it known as a “coma,” due to ice melting as it gets closer to the sun in its orbit.
The presence of water in a comet this close to Earth is unprecedented and supports the theory that water was carried to Earth by comets early in our planet’s history.
Astronomers have confirmed gas – specifically water vapor – around a comet in the asteroid belt for the first time. This artist’s impression of the comet, called Comet 238P/Read, shows it sublimating — its water ice evaporating as it orbits near the sun. This sublimation distinguishes comets from asteroids by creating their distinctive tail and hazy halo or coma.
238P/Read is located in the asteroid belt – the ring-shaped region in the solar system, located between the orbits of Jupiter and Mars. This is unusual, since most of the orbits are in the Kuiper Belt and the even more distant Oort Cloud
The discovery is described in a new study led by NASA and researchers at the University of Maryland, published in the journal Nature.
The study confirms for the first time the presence of water vapor around a comet in the main belt.
“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 study author Stefanie Milam of the Goddard Space Flight Center. from NASA. .
“Understanding the history of water distribution in the solar system will help us understand other planetary systems, and whether they are on track to host an Earth-like planet.”
Comets are made of ice, dust and rocky material and differ from asteroids, which are made of metallic and rocky material.
When a comet gets close to the sun, its ice and dust content begins to evaporate (known as sublimation), giving it a distinctive tail and a hazy or cloudy halo, known as a coma.
Asteroids, meanwhile, don’t do this because of their lack of ice.
The researchers studied data from Webb’s NIRSpec (Near-Infrared Spectrograph) instrument, which can measure the near-infrared spectrum of more than 100 objects at once.
The 238P/Read results showed “a clear peak” in the region of the spectrum associated with water, indicating that it is present around the comet.
This image shows new observations of 238P/Read by James Webb and observations of a Jupiter-family comet, 103P/Hartley 2, by NASA’s Deep Impact mission in 2010. Both show a clear peak in the region of the spectrum associated with becomes with water. It is the first time that a gas has been confirmed in such a comet in the main belt. However, unlike 103P/Hartley 2, comet 238P/Read did not show a hump indicating the presence of CO2
This image of comet 238P/Read was captured on September 8, 2022 by the NIRCam (Near-Infrared Camera) instrument on NASA’s James Webb Space Telescope. It shows the hazy halo, called the coma, and tail that are characteristic of comets, but not asteroids
While the observations “clearly show” that the comet has a coma of water vapor in its main belt, it has no detectable carbon dioxide (CO2), the team said.
This is an unusual finding, as CO2 typically makes up one-tenth of the volatile material in a comet, which is easily vaporized by the sun’s heat.
Possible explanations for this are that the comet had CO2 when it formed, but it was lost due to warm temperatures, or that it formed in a warm pocket of the solar system where CO2 was not available.
Comets mainly originate from two regions – the Kuiper Belt and the Oort Cloud, both of which extend well beyond the main belt – beyond Neptune’s orbit.
The Kuiper Belt is 30 astronomical units (AU) to about 50 AU from the Sun, while the Oort Cloud is much further – between about 2,000 and 100,000 AU.
One AU — a unit of length equal to the average distance between the Earth and the Sun — is 93 million miles, so these are seriously big distances.
At some point, a star passing near the solar system may have disrupted the motions of comets in the Oort cloud, sending them into the solar system.
Asteroid belt comets are a fairly new classification, with the first main belt comet being discovered in 1996.
Before then, it was believed that comets were simply in the Kuiper Belt and Oort Cloud, but the fact that comets reach further into the solar system towards Earth supports all comet-based theories about why our oceans exist.
The James Webb Space Telescope (pictured here in space) sees the universe in light invisible to human eyes
Scientists have long speculated that water ice could be preserved in the warmer asteroid belt, within Jupiter’s orbit, but definitive evidence was elusive – until Webb.
“In the past, we’ve seen objects in the main belt with all the characteristics of comets,” said Michael Kelley of the University of Maryland, lead author of the study.
“But only with this precise spectral data from Webb can we say yes, it is definitely water ice that creates that effect.”
Theories have long suggested that water was supplied to Earth later in the history of the solar system, possibly via comets.
More recent research indicates that hydrogen in the Earth played a role in the formation of the oceans.
However, these two theories are not mutually exclusive, and a combination of the two could explain why such a large portion of the Earth’s surface – 71 percent – is made up of water.