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Earth May Have Kept Its Own Water Rather Than Getting It From Asteroids (sciencemag.org)
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.
The important question isn't how, but why.
The flying spaghetti monster gave us water so that we could boil pasta.
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?
You can't see ANYTHING from a car, You've got to get out of the goddamned contraption and walk...Edward Abbey
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.
" 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.
If you are not allowed to question your government then the government has answered your question.
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.
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?
There is no substitute for common sense. Especially, no body of rules will do.