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Earth Science

Earth May Have Kept Its Own Water Rather Than Getting It From Asteroids (sciencemag.org) 45

sciencehabit writes: Carl Sagan famously dubbed Earth the 'pale blue dot' for our planet's abundant water. But where this water came from—and when it arrived—has been a longstanding debate. Many scientists argue that Earth formed as a dry planet, and gained its water millions of years later through the impact of water-bearing asteroids or comets. But now, scientists say that Earth may have had water from the start, inheriting it directly from the swirling nebula that gave birth to the solar system. If true, the results suggest that water-rich planets may abound in the universe.
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Earth May Have Kept Its Own Water Rather Than Getting It From Asteroids

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  • Other compounds contain Hydrogen
    and the early earth had plenty of compounds with oxygen

  • why not? (Score:3, Interesting)

    by vtcodger ( 957785 ) on Saturday November 14, 2015 @08:12AM (#50929499)

    Thanks to plate tectonics, We don't have much direct information on the early stages of the the Earth's history. But I've never understood why it was assumed that (much of) the Earth's water hasn't been there since our planet coalesced. Of course, I've also never understood the necessity to invoke an improbable planetary collision to explain the moon. It's not like binary pairs of large objects are rare in the universe.

    And even if the water did come entirely from cometary impacts after formation, why would that preclude lots of other watery planets? Are comets assumed to be rare in other planetary systems? Why?

    • by ljw1004 ( 764174 )

      I've also never understood the necessity to invoke an improbable planetary collision to explain the moon. It's not like binary pairs of large objects are rare in the universe.

      Earth and the Moon have different compositions. So they can't both have been formed in the same way.

      • "Earth and the Moon have different compositions"

        As I understand it, the composition of the two bodies is quite similar. In fact that's the primary reason that the collision theory was formulated. In the collision theory, they moon is composed primarily of material blasted from the Earth by the collision. see https://en.wikipedia.org/wiki/... [wikipedia.org]

      • Different in the sense that the Earth consists of corey stuff and crusty stuff and the moon only has crusty stuff?

        I mean it's not possible that the impact only whacked the outside off, or that whichever fragment got biggest fastest snagged all the heavy shit, is it?

  • by Viol8 ( 599362 ) on Saturday November 14, 2015 @08:35AM (#50929545) Homepage

    Why couldn't it have been a combination of water from the proto earth and asteroids and comets later on? There's little doubt the earth was bombarded for millions of years after it formed so it seems silly to pretend that didn't deliver any water to the planet.

    • "Why couldn't it have been a combination of water from the proto earth and asteroids and comets later on?"

      Seems like substantial water contributions from comets would almost certainly have to be the case unless we somehow totally misunderstand the nature and composition of comets or assume that they somehow avoid ever impacting the Earth.

  • ... throughout the solar system, and, by extension, abundant everywhere. Earth formed containing plenty, as every celestial body will. It would have all been fine crystals initially. Now I'll just whip back in my TARDIS and grab some early samples to prove my hypothesis.

  • Earth clearly has water, and we think Mars was much, much wetter billions of years ago, and many scientists theorize Venus once had an abundant liquid water supply, so if so, that's three planets within a band of the Solar System, and thus is stands to reason that liquid water exists at least in similarly configured extra solar systems, right? Of course that doesn't answer the question of whether water is either endogenous or exogenous, or a little of both, but if asteroids and comets can carry water, enoug
  • by tompaulco ( 629533 ) on Saturday November 14, 2015 @10:49AM (#50929915) Homepage Journal
    " If true, the results suggest that water-rich planets may abound in the universe."
    Or if the other theory about asteroids is true, then water-rich planets may abound in the universe. So this changes nothing. Also, both theories explain why all the planets in our solar system are so rich and abundant in water. Except that they are not.
    Hydrogen is the most common element in the universe, making up about 3/4 of the mass of the universe. Oxygen is the third most common, but the 2nd most common, Helium, takes up almost 1/4 of the mass of the universe. Oxygen makes up about 1/70th as much mass in the universe as hydrogen. However, somewhat unique on Earth, Oxygen makes up about 64% of the mass of Earth. Atmospheric Oxygen in the other planets is negligible, and even Oxygen combined in other compounds on other planets is a much smaller percentage than on Earth. Oddly, Hydrogen is relatively uncommon on Earth when compared to its abundance in the rest of the universe.
    • by dryeo ( 100693 )

      I doubt very much that oxygen makes up 64% of the mass of the Earth. Do you have a citation?
      According to https://en.wikipedia.org/wiki/... [wikipedia.org], the main composition of the Earth by mass is iron at 32.1% though I am somewhat surprised that oxygen is number 2 at 30.1%, which is a lot of oxygen. There are estimates that put the percent of oxygen in the crust at 47% which is close enough to half but still not 64%. And of course the atmosphere is mostly nitrogen at aprox 78% and almost 21% oxygen, thanks to the hard

      • I tried to go back through my history and see what sites I visited and where I might have gotten the 64% number. However, I wasn't able to find the site where that number came from. I would have to guess that I was cruising wikipedia, based on the history. Perhaps somebody updated the article. It does show 47% or 49% (depending on which article you read) in the crust as you mention, and also 88% in the ocean and 23% (by mass, 21% by volume) in the atmosphere. The mantle also contains about 41% by mass of Ox
        • by dryeo ( 100693 )

          Probably just a typo somewhere or if you're like me, a mental hiccup. Shit, I thought that I said by mass for the total and yet I obviously didn't and of course the atmosphere numbers I quoted were by volume which I missed, having found the numbers I remembered.

  • by anwyn ( 266338 ) on Saturday November 14, 2015 @10:50AM (#50929921)
    Remember the current theory of the formation of the moon! The collision could eject a huge amount of H (in various forms), into space. This gives an opportunity for isotopic sorting, the lighter H having a greater chance of being blown away by the solar wind, the heavier having a greater chance of coming back.
  • by Cacadril ( 866218 ) on Saturday November 14, 2015 @03:15PM (#50931095)
    The article says that the comets are much richer in deuterium than Earth's water. The small inclusions of water in primordial rocks match the isotopic balance of sea water. That is why the comet theory is now relatively disfavored.

    But how did the isotopes get differentially distributed in the primordial cloud, so that the deuterium collected in the outer regions where the comets formed, rather than in the region between Jupiter's orbit and the Sun itself?
    • by Anonymous Coward

      Deuterium has enough of a mass difference with hydrogen that the chemical and physical properties differ a little. Processes that involve adsorption, freezing, and sublimation, like on a comet, can cause small changes in the isotropic ratio. The effect is quite small, but measurable. Even processes like the fixation of carbon by plants can cause measurable changes in isotropic ratio from subtle chemistry differences.

      • But then the isotopic composition of the water carried by the comets may have changed since the time when they supposedly brought water to the Earth? It's a long time since, and I would think that hydrogen diffuses more readily than the heavier deuterium.

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