Nav: Home

A family of comets reopens the debate about the origin of Earth's water

May 23, 2019

Where did the Earth's water come from? Although comets, with their icy nuclei, seem like ideal candidates, analyses have so far shown that their water differs from that in our oceans. Now, however, an international team, bringing together CNRS researchers at the Laboratory for Studies of Radiation and Matter in Astrophysics and Atmospheres (Paris Observatory - PSL/CNRS/ Sorbonne University/University of Cergy-Pontoise) and the Laboratory of Space Studies and Instrumentation in Astrophysics (Paris Observatory - PSL/CNRS/Sorbonne University/University of Paris), has found that one family of comets, the hyperactive comets, contains water similar to terrestrial water. The study, published in the journal Astronomy & Astrophysics on May 20, 2019, is based in particular on measurements of comet 46P/Wirtanen carried out by SOFIA, NASA's Stratospheric Observatory for Infrared Astronomy.

According to the standard theory, the Earth is thought to have formed from the collision of small celestial bodies known as planetesimals. Since such bodies were poor in water, Earth's water must have been delivered either by a larger planetesimal or by a shower of smaller objects such as asteroids or comets.

To trace the source of terrestrial water, researchers study isotopic ratios (1), and in particular the ratio in water of deuterium to hydrogen, known as the D/H ratio (deuterium is a heavier form of hydrogen). As a comet approaches the Sun, its ice sublimes (2), forming an atmosphere of water vapour that can be analysed remotely. However, the D/H ratios of comets measured so far have generally been twice to three times that of ocean water, which implies that comets only delivered around 10% of the Earth's water.

When comet 46P/Wirtanen approached the Earth in December 2018 it was analysed using the SOFIA airborne observatory, carried aboard a Boeing aircraft. This was the third comet found to exhibit the same D/H ratio as terrestrial water. Like the two previous comets, it belongs to the category of hyperactive comets which, as they approach the Sun, release more water than the surface area of their nucleus should allow. The excess is produced by ice-rich particles present in their atmosphere.

Intrigued, the researchers determined the active fraction (i.e. the fraction of the nucleus surface area required to produce the amount of water present in their atmosphere) of all comets with a known D/H ratio. They found that there was an inverse correlation between the active fraction and the D/H ratio of the water vapour: the more a comet tends towards hyperactivity (i.e. an active fraction exceeding 1), the more its D/H ratio decreases and approaches that of the Earth.

Hyperactive comets, whose water vapour is partially derived from icy grains expelled into their atmosphere, thus have a D/H ratio similar to that of terrestrial water, unlike comets whose gas halo is produced only by surface ice. The researchers suggest that the D/H ratios measured in the atmosphere of the latter are not necessarily indicative of the ice present in their nucleus. If this hypothesis is correct, the water in all cometary nuclei may in fact be very similar to terrestrial water, reopening the debate on the origin of Earth's oceans.
(1) The isotopic ratio is the ratio, within the same sample, between two isotopes (two forms with a different mass) of a chemical element. This can be used both to date a sample and determine its source.
(2) Sublimation is the direct transition from a solid (in this case, ice) to a gas (water vapour).


Related Comet Articles:

Hubble observes 1st confirmed interstellar comet
Hubble has given astronomers their best look yet at an interstellar visitor -- comet 2I/Borisov -- whose speed and trajectory indicate it has come from beyond our solar system.
Interstellar Comet with a Familiar Look
A new comet discovered by amateur astronomer Gennady Borisov is an outcast from another star system, yet its properties determined so far are surprisingly familiar -- a new study led by JU researchers shows.
NASA telescopes take a close look at the brightest comet of 2018
As the brilliant comet 46P/Wirtanen streaked across the sky, NASA telescopes caught it on camera from multiple angles.
New insights on comet tails are blowing in the solar wind
Combined observations of Comet McNaught -- one of the brightest comets visible from Earth in the past 50 years -- have revealed new insights on the nature of comets and their relationship with the Sun.
Molecular oxygen in comet's atmosphere not created on its surface
Scientists have found that molecular oxygen around comet 67P is not produced on its surface, as some suggested, but may be from its body.
Is the interstellar asteroid really a comet?
The interstellar object Oumuamua was discovered back on Oct. 19, 2017, but the puzzle of its true nature has taken months to unravel, and may never be fully solved.
Comet Chury formed by a catastrophic collision
Comets made up of two lobes, such as Chury, visited by the Rosetta spacecraft, are produced when the debris resulting from a destructive collision between two comets clumps together again.
NASA telescope studies quirky comet 45P
When comet 45P zipped past Earth early in 2017, researchers observing from NASA's Infrared Telescope Facility, or IRTF, in Hawai'i gave the long-time trekker a thorough astronomical checkup.
The return of the comet-like exoplanet
Astronomers from UNIGE), also members of the PlanetS, focused the Hubble Space Telescope on an exoplanet that had already been seen losing its atmosphere, which forms an enormous cloud of hydrogen, giving the planet the appearance of a giant comet.
Comet 67P is constantly undergoing a facelift
Changes that the Rosetta spacecraft discovered on the surface of Comet 67P/Churyumov-Gerasimenko, including the collapse of entire cliffs, were likely driven by seasonal events, according to a new study.
More Comet News and Comet Current Events

Top Science Podcasts

We have hand picked the top science podcasts of 2019.
Now Playing: TED Radio Hour

Why do we revere risk-takers, even when their actions terrify us? Why are some better at taking risks than others? This hour, TED speakers explore the alluring, dangerous, and calculated sides of risk. Guests include professional rock climber Alex Honnold, economist Mariana Mazzucato, psychology researcher Kashfia Rahman, structural engineer and bridge designer Ian Firth, and risk intelligence expert Dylan Evans.
Now Playing: Science for the People

#540 Specialize? Or Generalize?
Ever been called a "jack of all trades, master of none"? The world loves to elevate specialists, people who drill deep into a single topic. Those people are great. But there's a place for generalists too, argues David Epstein. Jacks of all trades are often more successful than specialists. And he's got science to back it up. We talk with Epstein about his latest book, "Range: Why Generalists Triumph in a Specialized World".
Now Playing: Radiolab

Dolly Parton's America: Neon Moss
Today on Radiolab, we're bringing you the fourth episode of Jad's special series, Dolly Parton's America. In this episode, Jad goes back up the mountain to visit Dolly's actual Tennessee mountain home, where she tells stories about her first trips out of the holler. Back on the mountaintop, standing under the rain by the Little Pigeon River, the trip triggers memories of Jad's first visit to his father's childhood home, and opens the gateway to dizzying stories of music and migration. Support Radiolab today at