Water: Did it come from outer space?

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[Ed. note: Alarmed to find that over two months have passed without a single blog entry, the editorial team of Waterblogged.info convened an emergency meeting.  After the usual orgy of finger-pointing and blame-gaming that characterizes any editorial meeting worthy of the name, the team appointed a Scapegoat Identification Task Force. At the next meeting, the SITF recommended censuring the editor and declaring him “one lazy-assed lagger.” Resorting to his usual self-pitying litany of “reasons”—depression, the demands of his “day gig,” etc. ad patheticum—the cornered serial procrastinator promised to begin posting regularly, although he demanded that the minutes reflect that he “really can’t see the point.”]

Apparently, the origin of water on earth—or more exactly, the relative ubiquity of water on its surface compared to other heavenly bodies—is unknown. One theory holds that volcanic eruptions spewed subterranean water up to the surface; another postulates that water was brought to us from space by water-laden meteors and comets.  According to one theory that we read in Water and Space, a paper written by water expert Elizabeth L. Chalecki, the earth is bombarded by tiny “cosmic snowballs”—comets of pure water—at a rate of 20 per minute. Over billions of years, this would have provided sufficient water to fill our oceans.

Judging from what we read here, that seems ridiculously oversimplified and unlikely. One major objection from another source is that if Earth were being pelted with mini-comets at such a high rate, the same would hold true for the Moon, and it would be evident. The comets would not melt and would pockmark our favorite natural satellite with new, observable craters every day.

It’s important to note, though, that there is plenty of water in space, as Chalecki notes: 

Surprisingly, we have found water, a simple unmistakable molecule, to be widely distributed in space. Earth’s moon, the driest of bodies, may have frozen water at the poles. Mars, with its red sands and carbon-dioxide atmosphere, has water frozen in ice caps and may have subterranean ice. Several moons of Jupiter, circling their forbidding gas giant, may have large quantities of ice. Europa, in particular, looks promising as a source of liquid water, kept from freezing by the heat of its volcanic core. We have even
found traces of water in far stellar clouds and galaxies.

This Wikipedia entry, The Origin of Water on Earth, lists five broad hypotheses, including the possibility that collisions with massive water-rich comets and asteroid filled the oceans eons ago.

  • The cooling of the primordial Earth to the point where the outgassed volatile components were held in an atmosphere of sufficient pressure for the stabilization and retention of liquid water.
  • Comets, trans-Neptunian objects or water-rich asteroids (protoplanets) from the outer reaches of the asteroid belt colliding with a pre-historic Earth may have brought water to the world’s oceans. Measurements of the ratio of the hydrogen isotopes deuterium and protium point to asteroids, since similar percentage impurities in carbon-rich chondrites were found to oceanic water, whereas previous measurement of the isotopes’ concentrations in comets and trans-Neptunian objects correspond only slightly to water on the earth.
  • Biochemically through mineralization and photosynthesis (guttation, transpiration).
  • Gradual leakage of water stored in hydrous minerals of the Earth’s rocks.
  • Photolysis: radiation can break down chemical bonds on the surface.
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