Scientists think they've found the source of Earth's water

And it could substantially reduce estimates for the age of the moon.
Brad Bergan

Scientists may have finally discovered the answer to a long-standing question about the origin of life on Earth.

For decades, the consensus was that Earth got its water from asteroids or perhaps from a collision that led to the moon's formation.

But by looking at moonrocks, scientists discovered something else.

According to a study published Monday in the journal Proceedings of the National Academy of Sciences, the abundance of water on Earth was either here in the first place or deposited by an extremely wet (nearly pure H2O) object in the past.

This is a gigantic step forward in the history of our planet's evolution and, in turn, the preconditions for life as we know it. But it also moves our estimates of the moon's age back substantially.

Investigating an ancient, apocalyptic impact

It's a scientific fact that the Earth-moon system formed from a single impact between two gargantuan bodies in the solar system's early days. This is why the moon's history is joined-at-the-hip to the Earth's. But, unlike Earth, the moon doesn't experience the joys of plate tectonics or weathering, which means billion-year-old clues to ancient events in the solar system haven't been erased and buried. Despite roughly 70 percent of the Earth's surface lying underwater, our world is pretty dry compared to other bodies in the solar system.

Of course, the moon is drier, which is why for a long time, the scientific consensus was that the lack of water on the Earth-moon system was the result of this originating impact that created the two bodies — flinging "volatile" elements like water into space.

An examination of moon rocks' isotropic makeup by a team of researchers from Lawrence Livermore National Laboratory has discovered that the bodies with roles in the ancient impact weren't rich in volatile elements before it happened.

They determined this lack of volatility by employing the relative quantity of 87Rubidium (87Rb), a volatile and radioactive isotope that's calculated via the isotope's "daughter isotope": 87Strontium (87Sr).

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This discovery enabled the researchers to work out the levels of Rb in the Earth-moon system when it first came into being. With that in new information, the team concluded that the levels of 87Sr — an indicator of how much water is on the moon — were so unconscionably low that the bodies involved in the moon-forming collision were already dry start with. Crucially, not much water could have been added to our planet afterward.

Earth was probably "born with" water, and the moon is much younger

"Earth was either born with the water we have, or we were hit by something that was pure H2O, with not much else in it," explains Greg Brennecka, a cosmochemist, and co-author of the new study, in a statement. "This work eliminates meteorites/asteroids as a possible source of water on Earth and points strongly toward the 'born with it' option."

And this does more than change the source of Earth's comparably abundant water we see today. The new work suggests that the two large bodies that collided and formed the moon both must have hailed from the inner solar system. This also means that apocalyptic impact couldn't have happened more than 4.45 billion years ago — substantially reducing the moon's age.

Paradigm shift — Our scientific grasp of the origin of life, the Earth, and its moon is still evolving, which serves to remind us that no matter how advanced we think our science has become, we still have a long, long way to go before we understand how we even got here. It boggles the mind.

Study Abstract:

The origin of volatile species such as water in the Earth–Moon system is a subject of intense debate but is obfuscated by the potential for volatile loss during the Giant Impact that resulted in the formation of these bodies. One way to address these topics and place constraints on the temporal evolution of volatile components in planetary bodies is by using the observed decay of 87Rb to 87Sr because Rb is a moderately volatile element, whereas Sr is much more refractory. Here, we show that lunar highland rocks that crystallized ∼4.35 billion years ago exhibit very limited ingrowth of 87Sr, indicating that prior to the Moon-forming impact, the impactor commonly referred to as “Theia” and the proto-Earth both must have already been strongly depleted in volatile elements relative to primitive meteorites. These results imply that 1) the volatile element depletion of the Moon did not arise from the Giant Impact, 2) volatile element distributions on the Moon and Earth were principally inherited from their precursors, 3) both Theia and the proto-Earth probably formed in the inner solar system, and 4) the Giant Impact occurred relatively late in solar system history.