Where on Earth did water come from?

Data from the analysis of lunar samples suggest that, although the Earth and Moon formed as a result of a giant impact, they have largely retained the original abundance of volatile elements, including water. Credit: Image by Adam Connell / LLNL

The water supply on Earth is incredibly important for its ability to sustain life, but where does this water come from? Was it present at the formation of the Earth or was it later delivered by meteorites or comets from space?

The source of water on Earth has been running for a long time, and scientists from Livermore National Laboratory (LLNL) think they have the answer – and they found it by looking at rocks from the moon.

Because the Earth-Moon system formed together from the impact of two large bodies very early history of the solar system, their stories are very connected. And because the Moon lacks plate tectonics and weathering, processes that predispose to Earth or obscure evidence, the Moon is actually a great place to find clues to the history of Earth’s water.

Although almost 70 percent of the Earth’s surface is covered with water, the planet as a whole is a relatively dry place compared to many other objects in the solar system. And the moon is still dry. The usual wisdom was that the lack of volatile species (such as water) on Earth – and especially on the Moon – was due to this strong impact, which caused the depletion of volatile elements.

But looking at the isotopic composition of lunar rocks, the team found that the bodies involved in the impact that formed the Earth-Moon system had very low levels of volatile elements before the impact, not because of it. In particular, the team used the relative amount of volatile and radioactive isotope rubidium-87 (87Rb), which is calculated from its daughter isotope strontium-8787Sr) to determine the Rb budget in the Earth-Moon system when it was formed. The team found that because 87The senior, taking into account the moon’s long-term volatile budget, was so low that the collided bodies were initially dry, and little has been added since then.

“The earth was either born with the water we have or we were hit by something that was basically pure H2Oh, there is nothing else in it. This work excludes meteorites or asteroids as possible sources of water on Earth and strongly points to the option “born with it”, – said cosmochemist Greg Brenneka, co-author of the work.

In addition to the significant narrowing of the potential source of water on Earth, this work further shows that the large bodies that collided must have come from the inner solar system, and this event could not have occurred before 4.45 billion years ago, which greatly reduces window formation months.

According to Lars Borg, lead author of the study: “There were only a few types of materials that could unite the Earth and the Moon, and they were not exotic – they were probably just large bodies that formed in about the same area that collided. with each other a little over 100 million years after the formation of the solar system … but thankfully they did just that. ”

Reference: “The Origin of Volatile Elements in the Earth-Moon System”, Lars E. Borg, Gregory A. Brenneck and Thomas S. Kruer, February 14, 2022, Proceedings of the National Academy of Sciences.
DOI: 10.1073 / pnas.2115726119

The study is in Proceedings of the National Academy of Sciences. LLNL scientist Thomas Kruer also contributed to the study. The work was funded by c NASA and a laboratory research and development program. Where on Earth did water come from?

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