Rosetta: Earth’s Water Didn’t Come from Comets

By Carl Engelking | December 11, 2014 1:29 pm

Comet 67P as seen by the Rosetta probe. (ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0)

Europe’s Rosetta mission has only been in orbit around Comet 67P/Churyumov-Gerasimenko for a few months but it’s already shed light on one of the central questions it was launched, ten years ago, to answer: Where did Earth’s water come from?

Based on evidence of the comet’s water, Rosetta scientists announced this week that it adds evidence that, contrary to prevailing theory, comets didn’t supply Earth with its water. The new findings instead suggest that asteroids were the likely bearers of life-giving H2O on our planet.

The Magic Ratio

Since August, the Rosetta probe has been orbiting Comet 67P/Churyumov-Gerasimenko, and on Nov. 12, as you may already know, its Philae lander touched down on the surface — a first for mankind. But back aboard Rosetta, an instrument called Rosina has been taking measurements of the gasses spewing out of 67P.

Using two mass spectrometers, Rosina can sift through the comet’s gas emissions and identify a unique signature of its water: the ratio of deuterium to hydrogen. On Earth, you can find three deuterium atoms for every 10,000 water molecules. So, the thinking goes, if you can find objects in space with the same deuterium to hydrogen ratio as water on Earth, those objects become prime candidates for sources of Earth’s water.

Asteroids and comets are ideal subjects to study because they are essentially time capsules of what happened during the formation of our solar system and our planet. By examining comets today, we get a glimpse billions of years into the past.

No Match

According to data from Rosina, the deuterium to hydrogen ratio on 67P is roughly three times higher than that of Earth’s oceans. The findings build upon previous measurements from other comets, and the story is getting clearer: comets weren’t the primary source of water on Earth.

According to the European Space Agency, of the 11 comets we have deuterium to hydrogen measurements for, only two matched the unique signature of Earth’s water. However, meteorites originating from asteroids in the Asteroid Belt have matched the Earth’s water signature.

Although asteroids carry less water, a high overall number of impacts could supply enough water to fill the oceans, according to the ESA. They published their findings Wednesday in the journal Science. 

With the recent revelation that we’ve only identified 10,000 of an estimated 1 million asteroids that could hit Earth, Rosetta’s findings are another reminder of Earth’s love-hate relationship with these cosmic drifters.

CATEGORIZED UNDER: Space & Physics, top posts
  • Jake Bob

    Something I don’t quite understand here- if we take the proxy of Earth’s age to be asteroids, that means we believe them to have formed at the same time as Earth. If so, why do we need an extraterrestrial water source? Isn’t this a chicken or the egg rationale? Is the composition of an asteroid so different than the internal Earth? Is it a matter of scale?

    • Dave Thompson

      My understanding is that the newly formed earth, congealed from the space dust and rocks of the early solar system, was hot and molten and contained very little water. Once the earth cooled down, the subsequent bombardment of asteroids, comets and space dust allowed liquid water to form and stay on the earth.

      • Patrick Rusk

        But that early water wouldn’t disappear, right? It would become vapor. If it escaped as a gas, it couldn’t go far and would eventually come back to the surface. Isn’t that plausible?

        • b_c

          No, because of solar wind that swept away early atmospheres.

          • Brian_Balke

            I have to admit to being confused still on this point. I’m assuming that asteroids form through “cold” accretion, so they retain their water despite the fact that they have even less gravity than the moon. My question is how the early earth got hot. If it got hot by collisions, then I could see that the water would be released from the planetoids and swept up in the solar wind. If it got hot due to gravitational compression after planetoid accretion, then water could be locked in the mantle, where it might take a long time to move to the surface of the Earth, and would be retained as an atmosphere formed.

          • Marcelo Cabrera

            Earth got very hot after some specific events, like the giant impact that created the Moon. During that time, Earth kept almost all the iron of the colliding object, hence the Moon has no iron core and our planet was a giant molten ball of lava, no water there.

        • Jim Snyder

          It is a process. Near the beginning, earth’s mass would also be low, but cooling and gaining mass would capture the vapor better as time went on.

  • DoctorMcZ

    I’d also note that Deuterium is a stable isotope of Hydrogen and hence not susceptible to radioactive decay. Had this been the case, it would have required a reasonable estimate of the time when water arrived on earth in order to meaningfully compare its composition to that of water in heavenly bodies.

    • Randy Wester

      Comets outgas and condense, which exposes their their hydrogen atoms and water molecules to the solar wind. Couldn’t this result in differential sorting by molecular weight, or preferential collection of heavy hydrogen atoms? Much of Earth’s water has bee protected from the solar wind for eons, perhaps in the mean time the comets’ “Contents may have settled during shipping”.

  • curious

    why isn’t the moon covered with 2/3rds water???
    why isn’t mars covered with 2/3rds water???

    • b_c

      The moon’s gravity is simply too weak to retain any liquid water. Mars is weak also and its core is pretty much cold, but it probably still has enough water for future human settlements. Earth’s internal heat releasing water from original solar system material and gravity strong enough to keep an atmosphere are some of the reasons why there is water on earth and an atmosphere, plus the fact we’re right in the middle of the Goldilocks zone.

      • fatman45

        And don’t forget the magnetic field to keep the solar wind from stripping away the atmosphere.

  • JoO

    Well, if you have to different comets with their unique composition of H and O atoms, and if they mix you don’t know!
    Beside that I have figure our how spider pater gets created on inpakt place!

    • Charlie Erickson

      Nice one, JoO

      And if by chance, comets may do, well hence asteroids. Water and Earth plus Mars just makes Deuterium seem oblivious to the composition spoken here, in perfekt form.

  • larrymonske

    this particular comet may not have the same ice as others so you cant place the moniker that comets didnt bring water. This one is over being a comet and its ice is blown away by the sun. Captured ice ball see Hyperion it is ultimate in a ice bound comet just full of ice.

    • Guest

      Sure, illiterates know more than scientists; yep.


Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!


Briefing you on the must-know news and trending topics in science and technology today.

See More

Collapse bottom bar