It’s well known that Earth is unique among the rocky planets in our solar system due to its vast oceans of liquid water on its surface.
How exactly all this water got to Earth is more of a mystery, although scientists recently ruled out melted meteorites as the source.
Now, data from NASA’s James Webb telescope reveals for the first time that water vapour is present around a comet in our solar system’s main belt – the rocky ring located between the orbits of Mars and Jupiter.
The comet, called 238P/Read, has a water vapour cloud surrounding it known as a ‘coma’, resulting from ice melting as it gets closer to the sun in its orbit.
The presence of water in a comet so close to Earth is unprecedented and supports the theory that water was transported to Earth by comets early in our planet’s history.
Astronomers have confirmed gas – specifically water vapour – around a comet in the main asteroid belt for the first time. This artist’s impression of the comet, called Comet 238P/Read, shows it sublimating – its water ice vaporising as its orbit approaches the sun. This sublimation is what distinguishes comets from asteroids, 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, as most orbit in the Kuiper Belt and even more distant Oort Cloud
The discovery has been detailed in a new study led by NASA and University of Maryland researchers, published in the journal Nature.
Comet Read: Basic facts
Official name: 238P/Read
Discovered: October 24, 2005
Location: Main belt
Orbital duration: 5.64 years
Distance from the sun: 2.37 AU (min), 3.96 AU (max)
The study confirms the presence of water vapour around a comet in the main belt for the first time.
‘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 author Stefanie Milam at NASA’s Goddard Space Flight Center.
‘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.’
Comets are made up of ice, dust and rocky material, and are different from asteroids, which are made up of metals and rocky material.
When a comet gets close to the sun, its ice and dust content start to vaporise (known as sublimation) which gives it a distinctive tail and a hazy or cloudy halo, known as a coma.
Asteroids, meanwhile, don’t do this due to 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.
Results from 238P/Read showed ‘a distinct peak’ in the region of the spectrum associated with water, indicating it is present around the comet.
This graphic 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 distinct peak in the region of spectrum associated with water. It marks the first time that a gas has been confirmed in such a main belt comet. However, unlike 103P/Hartley 2, Comet 238P/Read did not show a bump indicating the presence of CO2
This image of Comet 238P/Read was captured by the NIRCam (Near-Infrared Camera) instrument on NASA’s James Webb Space Telescope on September 8, 2022. It displays the hazy halo, called the coma, and tail that are characteristic of comets, but not asteroids
While the observations ‘clearly show’ that the main-belt comet has a coma of water vapour, it has no detectable carbon dioxide (CO2), the team say.
This is an unusual finding, seeing as CO2 typically makes up a tenth of the volatile material in a comet that can be easily vaporised by the sun’s heat.
Potential explanations for this are 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 no CO2 was available.
Comets primarily originate from two regions – the Kuiper Belt and the Oort Cloud, which both extend much further than the main belt – beyond the orbit of Neptune.
The Kuiper Belt is 30 astronomical units (AU) to approximately 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 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 disturbed the motions of comets in the Oort Cloud, sending them into the solar system.
Comets in the main asteroid belt a fairly new classification, with the first main-belt comet discovered in 1996.
Before that, comets were understood to just reside in the Kuiper Belt and Oort Cloud, but the fact that comets reached further into the solar system towards Earth bolsters any comet-based theories for why our oceans exist.
The James Webb Space Telescope (depicted here in space) sees the universe in light that is invisible to human eyes
Scientists 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,’ said Michael Kelley at the University of Maryland, lead author of the study.
‘But only with this precise spectral data from Webb can we say yes, it’s definitely water ice that is creating that effect.’
Theories have long suggested that water was delivered to Earth from the outer Solar System later in its history, potentially via comets.
More recent research indicates that hydrogen inside the Earth played a role in the formation of the oceans.
These two theories are not mutually exclusive, however, and a combination of both could explain why so much of the Earth’s surface – 71 per cent – is water.
Explained: The difference between an asteroid, meteorite and other space rocks
An asteroid is a large chunk of rock left over from collisions or the early solar system. Most are located between Mars and Jupiter in the Main Belt.
A comet is a rock covered in ice, methane and other compounds. Their orbits take them much further out of the solar system.
A meteor is what astronomers call a flash of light in the atmosphere when debris burns up.
This debris itself is known as a meteoroid. Most are so small they are vapourised in the atmosphere.
If any of this meteoroid makes it to Earth, it is called a meteorite.
Meteors, meteoroids and meteorites normally originate from asteroids and comets.
For example, if Earth passes through the tail of a comet, much of the debris burns up in the atmosphere, forming a meteor shower.