Where Earth got its water

By | June 8, 2023

Sean Raymond writes:

When Carl Sagan famously called Earth the “pale blue dot,” he was judging a book by its cover. Even though three quarters of our planet’s surface is covered by oceans, our planet is actually very dry. Water makes up about one part in a thousand of Earth’s mass (most of it is simply rock and iron). I say “about” because we don’t know exactly how much water is trapped in Earth’s interior. Estimates range from less than one “ocean”—defined to represent the sum of all of Earth’s surface water (oceans, lakes, glaciers, and so on)—to more than 10. Regardless of the exact water budget, Earth is about 10 times drier than crackers (which typically contain about 2 percent water).

A gripping new story about the origins of water is emerging from analyses of meteorites. The old story was that water-rich asteroids from afar “delivered” all of Earth’s water during our planet’s formation. New results show that, contrary to this narrative, most of Earth’s water was sourced locally, much closer to the sun.

Meteorites are rocky leftovers or remnants from the time when the planets were forming that managed to survive the fall to Earth. They might have broken off as fragments from any rocky body after a powerful impact. Most meteorites are fragments of asteroids, whose orbits were slowly perturbed over millions of years to cross Earth’s path and crash down. Sometimes, chunks of rock from the moon and Mars, launched into space after an impact, could land on Earth, pulled in by its gravity, as lunar or Martian meteorites. Like spilled coffee grounds and cream drops, these chunks are evidence of what went into the cup long after it had been brewed and drunk. [Continue reading…]

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