Mystery of where Earth's water came from deepens: Comet water is different

December 10, 2014 bySeth Borenstein
The image comprised of four images taken with the navigation camera on Rosetta and released by the European Space Agency ESA on Thursday, Nov. 20, 2014 shows comet 67P/Churyumov-Gerasimenko on Nov. 17, 2014 from a distance of 42 km (26 miles) from the center of the comet. The mystery of where Earth's water came from has gotten murkier, with astronomers essentially eliminating one of the chief suspects: comets. The European Space Agency's Rosetta space probe closely examined the type of comet that some scientists theorized could have brought water to our planet 4 billion years ago. It found water, but the wrong kind.(AP Photo/ESA)

The mystery of where Earth's water came from got murkier Wednesday when some astronomers essentially eliminated one of the chief suspects: comets.

Over the past few months, the European Space Agency's Rosetta space probe closely examined the type of comet that some scientists theorized could have brought to our planet 4 billion years ago. It found water, but the wrong kind.

It was too heavy. One of the first scientific studies from the Rosetta mission found that the comet's water contains more of a hydrogen isotope called deuterium than water on Earth does.

"The question is who brought this water: Was it comets or was it something else?" asked Kathrin Altwegg of the University of Bern in Switzerland, lead author of a study published in the journal Science.

Something else, probably asteroids, Altwegg concluded. But others disagree.

Many scientists have long believed that Earth had water when it first formed, but that it boiled off, so that the water on the planet now had to have come from an outside source.

The findings from Rosetta's mission to the duck-shaped comet 67P/Churyumov-Gerasimenko complicate not just the question of the origin of Earth's water but our understanding of comets.

Until now, scientists pretty much sorted comets into two types: near and far. The near ones, sometimes called the Jupiter family, originally come from the Kuiper Belt outside Neptune and Pluto. The far ones hail from the Oort Cloud, which is much farther out.

In 1986, a spacecraft came within about 400 miles (640 kilometers) of Halley's comet, an Oort Cloud comet, and analyzed its water. It proved to be heavier than Earth's water. But three years ago, scientists examined the water in a Kuiper Belt comet, Hartley 2, and it was a perfect match for Earth's water, so the was back, stronger than ever, Altwegg said.

The comet visited by Rosetta is a Kuiper Belt comet, but its water was even heavier than Halley's, Altwegg said. That shows that Kuiper Belt comets aren't as uniform as thought, and it once again complicates the issue of Earth's water.

"That probably rules out Kuiper Belt comets from bringing water to Earth," she said.

University of Maryland astronomer Michael A'Hearn, who wasn't part of the research, called the results startling but said they don't eliminate comets altogether. The water could have come from other types of Kuiper Belt comets, he said.

NASA Near Earth Object program manager Donald Yeomans, however, thinks it does pretty much rule out comets.

While asteroids are a good suspect—they probably had more water on them 4 billion years ago than they do now—another possibility is that Earth kept some of its original water in its crust or in ice at the poles, Altwegg said.

Explore further: First comet found with ocean-like water: New clues to creation of Earth's oceans

More information: 67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio, Science DOI: 10.1126/science.1261952. www.sciencemag.org/content/early/2014/12/09/science.1261952

ABSTRACT
The provenance of water and organic compounds on the Earth and other terrestrial planets has been discussed for a long time without reaching a consensus. One of the best means to distinguish between different scenarios is by determining the D/H ratios in the reservoirs for comets and the Earth's oceans. Here we report the direct in situ measurement of the D/H ratio in the Jupiter family comet 67P/Churyumov-Gerasimenko by the ROSINA mass spectrometer aboard ESA's Rosetta spacecraft, which is found to be (5.3 ± 0.7) × 10−4, that is, ~3 times the terrestrial value. Previous cometary measurements and our new finding suggest a wide range of D/H ratios in the water within Jupiter family objects and preclude the idea that this reservoir is solely composed of Earth ocean-like water.

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28 comments

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PsycheOne
1.9 / 5 (14) Dec 10, 2014
Leaving aside the type of water on a comet, has anyone ever calculated how many comets would have to hit the Earth to deliver several ocean's worth of water. Quick intuitive calculation: way, way too many.
dogbert
3.8 / 5 (11) Dec 10, 2014
We know that planet forming disks of dust around new stars have lots of water in them. It should not be surprising to think that a planet formed worth lots of water at the time of its formation.
big-ben-not-the-bell
2.4 / 5 (7) Dec 10, 2014
Geez guys...Our primordial planet was one giant volcano, spewing hydrogen compounds and oxides into the super heated atmosphere, on a global scale. The H2O condensed on the inside of our atmosphere like condensation on the inside of a window in winter in a cold climate. Torrential rains, for probably millions of years, cooled the surface of our planet forming the crust and filling our oceans. It's not rocket science.
baudrunner
2.3 / 5 (6) Dec 10, 2014
They wonder why Earth has so much water, but do not question where the water that makes up most of a comet's mass comes from.
foolspoo
3.7 / 5 (6) Dec 10, 2014
too many? youve got a long way to go psyche
EyeNStein
2.6 / 5 (5) Dec 10, 2014
Have they factored in the billions of years of exposure to solar wind?
They recently found that the 'H' (hydrogen/deuterium) in Moon surface water compounds was from constant bombardment. http://www.space....ind.html
KBK
3.7 / 5 (3) Dec 10, 2014
Have they added in the recent findings that approx 5x times the ocean water, is hiding beneath the surface of the earth?

If so, would that alter their premise?
cantdrive85
1.9 / 5 (14) Dec 10, 2014
The "mystery" only "deepens" for those who were convinced they already knew the answer.
h20dr
1.5 / 5 (8) Dec 10, 2014
Gee O Dee.
Egleton
1.6 / 5 (7) Dec 11, 2014
Fish wee.
Rustybolts
5 / 5 (5) Dec 11, 2014
Just imagine what we won't know tomorrow!
verkle
Dec 11, 2014
This comment has been removed by a moderator.
vlaaing peerd
5 / 5 (7) Dec 11, 2014
I would like to read what makes them think this particular comet is representative for all comets hydrogen/water contents. It seems a bit early to dismiss this based on just one comet.
PetePassword
5 / 5 (4) Dec 11, 2014
I would like to read what makes them think this particular comet is representative for all comets hydrogen/water contents. It seems a bit early to dismiss this based on just one comet.

Me too. My first thought was 'One comet and you apply that to every comet?' Did all comets originate from the same source? Do they appear by the same process even? Does any of it matter?
jsdarkdestruction
5 / 5 (14) Dec 11, 2014
Maybe someone else placed it there.

Who? How? using what laws of physics? be specific please. how do you propose we go about verifying that?
Maybe we still know so little about really what went on thousands of years ago.

this occurred billions of years ago.what happened thousands of years ago is irrelevant.
weathervane
3.9 / 5 (7) Dec 11, 2014
Wake me up when they have a sample size more than one
Vietvet
4.7 / 5 (12) Dec 11, 2014
The NY Times has a more detailed article (with links to studies) covering this story.

http://www.nytime...amp;_r=0
Eikka
5 / 5 (3) Dec 11, 2014
Wouldn't a comet in space be exposed to more cosmic radiation than water on earth, essentially converting H2O into D2O at a different rate over the 4.5 billions years since the formation of the earth?

So the difference could be explained by the fact that the comet is simply more irradiated over time. Much of earth's water is protected from radiation and energetic particles by the atmosphere and the magnetic field.
foolspoo
5 / 5 (10) Dec 11, 2014
verkle, you poor shlub
gkam
4.6 / 5 (9) Dec 11, 2014
"Wouldn't a comet in space be exposed to more cosmic radiation than water on earth, essentially converting H2O into D2O at a different rate over the 4.5 billions years since the formation of the earth?"
--------------------------------------------------------
How would a cosmic ray add a neutron to a hydrogen atom?

Does that happen?
saposjoint
4.4 / 5 (7) Dec 11, 2014
It might, but not likely. Maybe by a secondary process? Ask Andrea Rossi. :D
allworld212
5 / 5 (3) Dec 11, 2014
If Hartley 2 was a perfect match for Earth water than how is Kuiper belt comets completely eliminated?
Urgelt
5 / 5 (1) Dec 12, 2014
weathervane wrote, "Wake me up when they have a sample size more than one."

Yep, that was my feeling. One comet sample a solar system does not make.
rogerbcooper
4 / 5 (2) Dec 12, 2014
Cometary isotopic enrichment: the lighter hydrogen isotope after being dissociated from water by solar radiation is selectively swept off into space.
imido
Dec 15, 2014
This comment has been removed by a moderator.
Lex Talonis
1 / 5 (3) Dec 15, 2014
On the first day I created the heavens and the earths....

etc.

Torbjorn_Larsson_OM
3 / 5 (2) Dec 15, 2014
Asteroids and comets overlap in most traits, such as pore spaces and volatile content. But they can see a qualitative difference in nitrogen isotopes. This D/H ratio, which is used to check many spectroscopic measurements of it (I see a standing question in this thread), is not as good. No, they can't exclude Kuiper belt objects, just say that they wouldn't suffice. (Too many with higher D/H than Earth's water.)
Eikka
5 / 5 (1) Dec 15, 2014
How would a cosmic ray add a neutron to a hydrogen atom?

Does that happen?


Perhaps by the ray being a neutron in the first place? That's how heavy water is generated in CANDU reactors on earth.

The sun is a source of neutron radiation, and presumably the comet passes quite close to the sun from time to time.

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