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

A family of comets reopens the debate about the origin of Earth's water
The comet 46P/Wirtanen on January 3, 2019. Credit: Nicolas Biver

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 , researchers study isotopic ratios1, 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, 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.  

A family of comets reopens the debate about the origin of Earth's water
Scientists at work aboard a Boeing 747 SOFIA. Credit: Nicolas Baker/IRAP/NASA/CNRS Photothèque 

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 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 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 , reopening the debate on the origin of Earth's oceans.


Explore further

NASA telescopes take a close look at the brightest comet of 2018

More information: Dariusz C. Lis et al. Terrestrial deuterium-to-hydrogen ratio in water in hyperactive comets, Astronomy & Astrophysics (2019). DOI: 10.1051/0004-6361/201935554
Journal information: Astronomy & Astrophysics

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Citation: A family of comets reopens the debate about the origin of Earth's water (2019, May 23) retrieved 18 September 2019 from https://phys.org/news/2019-05-family-comets-reopens-debate-earth.html
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May 23, 2019
I fail to understand the need for one absolute, exact cause/event for the Earth's water supply.

To insist for any subject of such complexity, that my hypothesis is only correct if I deny conflicting hypothesis.

What if they are all correct?
A multitude of differing water sources? Supplies from separate events across hundreds of millions or more of years?

Starting with the planetesmal disk sorting out materials like a centrifuge? That might explain why the Proto-Earth got the majority of Inner System water/
While Venus & Mars were short-changed.

Then, the Theia collision bringing in a large supply as the Hadean Earth was stripping the still-forming Luna of it's volatiles.

Then came the Great Bombardment. Another addition to the Earth's hydrosphere>?

Then, the outlying comets.
Slung in-system by Proto-Jupiter.
Over billions of years a randomized series of unpredictable cometary events.
That, all in all, added a substantial percentage to the Earth's water supply.

May 24, 2019
Recent research from The University of Münster shows that the bulk of Earth's water came from the Theia collision. They show that Theia came from the water rich outer solar system. There is probably too much water on Earth to have been delivered by asteroids and comets during the late heavy bombardment. No doubt however, so much water did come from multiple sources.

May 25, 2019
Interestingly (or not!), 46P/Wirtanen was the original target for the Rosetta mission. Problems with the Ariane launcher caused a delay, and the target was changed to 67P/C-G. 67P's D/H ratio was the highest ever measured for a comet;

http://sci.esa.in...r-ratio/

May 26, 2019
We live on a water world. Not only the water on the earth...there are more
than 3 times the amount in the earth. How did the water get here?

Gen 1:2 And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.

May 26, 2019
We live on a water world. Not only the water on the earth...there are more
than 3 times the amount in the earth. How did the water get here?

Gen 1:2 And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.


And what has that crap got to do with science?

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