Meteorites That Made Earth Brought Its Water, Too

A piece of the meteorite Sahara 97096 (about 10 centimeters, or roughly 4 inches, long), an enstatite chondrite (EC) meteorite that contains so much hydrogen that, if Earth formed entirely of this material, it would have had 23 times the total mass of water present in the Earth’s oceans. (Credit: Laurette Piani, Museum of National History in Paris.)

(CN) — Earth’s water may owe its existence to hydrogen-bearing meteorites that crashed into the planet in its infancy, researchers suggest in a new study on the origin of the blue marble’s blue.

Unlike every other planet in the solar system, Earth is extremely wet: its oceans, the atmosphere’s humidity and the ground’s moist geology stand in defiance against models of the solar system’s formation, which say the Earth should be as dry as the rest of its rocky planetary neighbors.

The answer to this mystery may lie in Earth’s component materials. Enstatite chondrite meteorites, or ECs, rocks formed near the center of the hot solar nebula that created our solar system, compose a large portion of the building-block rocks that accreted over time to compose the Earth.

“These meteorites are quite rare … and contain very special minerals, such as calcium- or magnesium-sulfides, nitrides, silicides, that are very uncommon on Earth,” Laurette Piani, a cosmochemistry researcher at France’s Centre de Recherches Pétrographiques et Géochimiques and lead author of the study published Thursday in Science, said via email. “I found them very beautiful to watch under a microscope and also fascinating because they are probably the best analogues we have of the Earth’s building blocks.”

Long considered some of the solar system’s driest objects due to their formation in that hot nebula, ECs’ hydrogen concentrations were very difficult to measure — until now.

“The water in ECs fits well (from the point of view of hydrogen isotopes) to the water believed to be retained in the mantle rocks and a bit less with the surface water. It is commonly thought that the Earth’s mantle could contain between 1 and more than 10 times the amount of water present in the oceans,” Piani said.  

Piani led five colleagues in measuring 13 EC specimens’ hydrogen content. They found that the meteorites contain much more hydrogen than scientists previously thought. They predict that ECs and similar materials came together during Earth’s early formation period, delivering enough hydrogen to the fledgling planet’s mantle to account for its water.

“We propose in the paper that the water present in the Earth’s mantle was directly inherited from EC-like material and present from the beginning of the Earth formation, while the surficial water (oceans) could be made out of about 95% of EC-like material and 5% of hydrated asteroids,” Piani said.

Additional evidence arrives in the form of the meteorites’ deuterium-to-hydrogen (D/H) ratio and their isotopic composition, which closely align with the composition of the Earth’s mantle. This, the group says, suggests that Earth’s water may come from the rocks that built the planet itself.

“These observations suggest that the [hydrogen] contents of ECs were established during the early solar system and have experienced minimal modifications on Earth,” the authors write in the paper.

Determining that EC meteorites were rich in hydrogen was no small feat.

“To do the measurements of hydrogen in ECs, we first had to select carefully meteorite pieces that were well preserved from the terrestrial water contamination. This is not that easy because ECs are quite rare and easily altered in the Earth’s surficial conditions,” Piani said.

As a result, Piani and her team had to set aside some meteorite samples that had become too contaminated.

“Then we used two analytical techniques, the conventional mass spectrometry to measure the bulk hydrogen content and D/H ratios of each EC and the secondary ion mass spectrometry (SIMS) to measure hydrogen in a specific mineral phase that we thought could contain hydrogen,” Piani said. “For the bulk hydrogen content, we had to take care of removing adsorbed atmospheric water at the surface of the meteorite prior to the analysis.”

The SIMS measurements verified that the chondrites’ chondrules — round mineral grains found within chondrites such as EC meteorites — were full of water.

“Using SIMS, we confirm that some phases in ECs contain a high amount of water: we found that a part of the hydrogen is present dissolved in the silicate glass of chondrules,” Piani said. “Chondrules are among the most abundant constituents of primitive meteorites and are among the first solids to form in the solar system.”

Measuring the ECs’ isotopic composition also helped the group weigh their theory against another hypothesis aiming to explain Earth’s water, according to which most of the planet’s water was delivered primarily by water-rich carbonaceous chondrites.

“The isotopic ratio is a type of signature akin to DNA for a chemical element. We found the hydrogen isotopic composition of enstatite chondrites to be similar to the one of the water stored in the terrestrial mantle,” Piani said. “It thus appears that the Earth might have been formed from a material that both contains enough water and possesses the right isotopic composition to explain almost all of the water present today.”

The researchers’ findings suggest that Earth’s water was developed at the same time as the planet’s buildup, rather than arriving solely from external deliverers.

“I think one of the greatest contributions of our paper is to show that ECs could also greatly contribute to the Earth’s water budget and that water-containing rocks were present in the inner solar system at the time of the rocky planet formation,” Piani said.

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