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Comet water muddies the question of where Earth’s oceans came from

Rosetta image of Comet 67P, processed to give it the same color as human eyes might see it. (Most probes and telescopes filter light differently than our eyes.) Note that the comet is very dark gray, which is why comets in generally are known as "dirty snowballs". [Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA ]

Rosetta image of Comet 67P, processed to give it the same color as human eyes might see it. (Most probes and telescopes filter light differently than our eyes.) Note that the comet is very dark gray, which is why comets in generally are known as “dirty snowballs”. [Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA ]

Comets are an obvious culprit if you want to understand how water gets from one part of the Solar System to another. They’re famously known as “dirty snowballs”, for their mixture of water ice, ices made of other molecules, and a few organic molecules thrown in to give them a dark gray color. Since very early Earth was hot enough to boil away any surface water, one popular idea is that comets brought enough water to fill the oceans during a period of time known as the Late Heavy Bombardment.

That hypothesis is nice and simple, but it’s not universally accepted among planetary scientists, because comets don’t seem to be playing nicely. For instance, the most recent measurement taken by the Rosetta probe in orbit around Comet 67P/Churyumov-Gerasimenko shows that its water contains roughly three times the abundance of deuterium — the isotope of hydrogen with one neutron attached — than we see on Earth. I covered this story for The Daily Beast:

However, it’s wrong to say the new Rosetta results rule out comets as the origin of the oceans. That’s partly because we’ve only gotten this kind of measurement for a small number of comets, and only one of those—Comet 103P/Hartley 2—had the same relative deuterium content as Earth. Both 103P and 67P are Jupiter-family comets, which are widely thought to have formed in the outer Solar System and plunged into smaller orbits thanks to Jupiter’s gravity. The difference in their chemistry makes the story even messier, so it’s likely we’ll be talking about the origin of Earth’s oceans for many years to come. [Read more…]

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2 Responses to “Comet water muddies the question of where Earth’s oceans came from”


  1. 1 Brett December 14, 2014 at 14:57

    If it turns out that asteroids and meteorites (same thing obviously) were the main contributors instead of comets, would that possibly be proof for the theory of Jupiter swinging inwards and launching a ton of main belt asteroids into the inner solar system?

    • 2 Matthew R. Francis December 15, 2014 at 10:27

      I’d hesitate to say anything about “proof”, but the biggest problem (as I understand it) with asteroids supplying water is that they are comparatively dry, requiring a lot more of them. Also, the one study I know about asteroid deuterium abundance shows too little compared with the amount in saltwater on Earth. That’s not definitive (and actual planetary scientists would have a lot more to say about this than I can!), but it’s suggestive that we’re still a little ways from understanding exactly how everything worked in the early Solar System.


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