Rosetta results: Comets ‘did not bring water to Earth’

Dec 16, 2014

By Rebecca Morelle

Scientists have dealt a blow to the theory that most water on Earth came from comets.

Results from Europe’s Rosetta mission, which made history by landing on Comet 67P in November, shows the water on the icy mass is unlike that on our planet.

The results are published in the journal Science.

The authors conclude it is more likely that the water came from asteroids, but other scientists say more data is needed before comets can be ruled out.

Since August, the Rosetta probe has been orbiting Comet 67P/Churyumov-Gerasimenko, and on 12 November its lander, Philae, made a historic touchdown on the object’s surface.


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9 comments on “Rosetta results: Comets ‘did not bring water to Earth’

  • @OP – The authors conclude it is more likely that the water came from asteroids, but other scientists say more data is needed before comets can be ruled out.

    It would appear that the isotopic analysis of THIS comet does not match that of Earth’s oceans.

    This could mean that Earth’s water came from volcanism gradually bringing water to the surface, following from water being included in the accretion of the planet from planetesimals at an earlier stage, – or it could have come from asteroids. There is also the possibility, that the Rosetta comet is not typical of outer Solar-System small bodies.



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  • Alan. Could there be some property of deuterium that means that water from comets like Comet 67P, that is high in deuterium, when they arrive on planet earth, give a neutron. I’ve no idea how this would happen. Our proximity to the sun with no ozone? Could the radiation from the sun punch out a neutron giving us today’s ratio, that is now stable due to the creation of the ozone layer later in the earth’s history. Could the recycling of water through the earth’s crust shed a neutron?

    I just can’t see how Earth could have been selective in which comets it allowed to hit its surface during the bombardment.

    Earth. “Okay listen up Solar System. Planet Earth will only accept Hartley 2 class comets, but all you 67P’s can P off.”

    The earth would have been bombarded with all of the solar system objects. That’s why I ask the question, can something happen to water, once it is on our planet, that gives us this unique ratio.



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  • I would say the conclusion needs more data. More landings on commets and analysis what they are made of. And it leaves the question where the water on earth came from as in the early stage the temperatures were too high to keep water on the protoplanet.



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  • I have a problem with demanding too close an isotopic match between our oceans and comets. There would have been an exact match during the Late Heavy Bombardment, but since then the comets that have not accreted into the sun would have made quite a few close passes to the sun. With each pass the ice vaporizes and forms a cloud around the comet with non-isotopic hydrogen at the farthest eat extent of the cloud and isotopic hydrogen being gravitationally retained closer in. The lighter hydrogen would tend to get swept away wthereby skewing the isotopic ratio in the direction of isotopic hydrogen. Meanwhile Earth’s isotopic ratios have remained the same since Earth’s orbit is not prone to frequent close passes with the sun.



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  • David R Allen Dec 17, 2014 at 5:29 pm

    Alan. Could there be some property of deuterium that means that water from comets like Comet 67P, that is high in deuterium, when they arrive on planet earth, give a neutron. I’ve no idea how this would happen.

    I don’t know, but there is of course this other factor concerning water erupted by volcanism and what part of the oceans have been supplied by this over billions of years.

    http://www.bbc.co.uk/news/science-environment-30527357

    The researchers dated some of the deep water to between one and 2.5bn years old

    The world’s oldest water, which is locked deep within the Earth’s crust, is present at a far greater volume than was thought, scientists report.

    The liquid, some of which is billions of years old, is found many kilometres beneath the ground.

    Researchers estimate there is about 11m cubic kilometres (2.5m cu miles) of it – more water than all the world’s rivers, swamps and lakes put together.

    The study was presented at the American Geophysical Union Fall Meeting.

    It has also been published in the journal Nature.

    The team found that the water was reacting with the rock to release hydrogen: a potential food source.



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  • Ejner Dec 19, 2014 at 7:43 am

    The lighter hydrogen would tend to get swept away wthereby skewing the isotopic ratio in the direction of isotopic hydrogen. Meanwhile Earth’s isotopic ratios have remained the same since Earth’s orbit is not prone to frequent close passes with the sun.

    http://scitechdaily.com/earth-loses-50000-tonnes-of-mass-every-year/
    According to some calculations, the Earth is losing 50,000 tonnes of mass every single year, even though an extra 40,000 tonnes of space dust converge onto the Earth’s gravity well, it’s still losing weight.

    Chris Smith, a microbiologist, and Dave Ansel, a Cambridge University physicist provided the answer in BBC Radio 4’s More or Less program. The 40,000 tonnes of mass that accumulates comes from space dust, remnants of the formation of the solar system.

    The biggest mass loss comes from escaped hydrogen and helium, which escape with 95,000 tonnes of mass and 1,600 tonnes respectively. These elements are too light to stay permanently in the gravity well, so they tend to escape into space.

    The net loss is about 0.000000000000001% every year, so it doesn’t account for much when compared to the total mass of the Earth, which is 5,972,000,000,000,000,000,000 tonnes. It will take trillions of years for all of the hydrogen to be depleted. Helium represents 0.00052% of the atmosphere and it’s a scarcer element.

    I am not sure to what extent this affects the long term isotopic balance in the oceans, but there is a popular misconception that planetary masses are fixed entities.



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  • As Jacobsen explains, the chance discovery of a diamond containing a
    lump of ringwoodite that had been spewed out of a volcanic vent
    millions of years ago showed that the mineral held as much water as
    the examples he had reformed in the lab. Ringwoodite could hold 10
    times as much water as previously thought – meaning there could be
    oceans of water still sitting in the mantle rocks beneath us.

    http://www.bbc.com/future/story/20141029-are-oceans-hiding-inside-earth



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  • The effects of the Solar heating of the mixture of ices and dust is becoming evident as Rosetta takes more pictures of the comet’s structures!

    http://www.theguardian.com/science/2015/jul/01/rosetta-spacecraft-enormous-sinkholes-comet-67p?CMP=twt_gu
    Cameras on the Rosetta spacecraft have spotted a series of enormous pits on comet 67P/Churyumov-Gerasimenko that plunge hundreds of metres down into the body’s cold interior.

    Scientists on the mission believe the pits formed in the same way as sinkholes on Earth, which appear out of the blue when the natural ceilings above underground caverns suddenly collapse under their own weight.

    The comet pits are typically 200 metres across and reach down about the same distance into the comet’s body. Some of the holes may have formed in the distant past, but the walls of others are jagged and show no signs of erosion, suggesting they formed very recently.

    The discovery of sinkholes shows that comets have substantial variations in their internal structure, a finding that rules out some theories of how the bodies form in the first place, from rubble left over from the birth of the solar system.



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