Where on earth did the water come from?

Evidence from analysis of lunar samples indicates that although the Earth and Moon formed from a giant impact, they mostly retained an abundance of primordial volatile elements, including water. Credit: Adam Connell/LLNL Photo

Earth’s supply of water is very important to its ability to sustain life, but where did that water come from? Did they exist when Earth formed or were they delivered later by meteorites or comets from outer space?

The source of Earth’s water has long been the subject of debate and scientists at Lawrence Livermore National Laboratory (LLNL) think they have the answer — and they found it by looking at moon rocks.

Since the Earth-Moon system was formed together from the influence of two large bodies very early on History of the Solar SystemTheir history is closely related. And since the Moon lacks plate tectonics and atmospheric conditions, processes that tend to erase or obscure evidence on Earth, the Moon is actually a great place to look for clues to Earth’s water history.

Although nearly 70 percent of the Earth’s surface is covered in water, the planet in general is a relatively dry place compared to many other bodies in the solar system. The moon is drier. The conventional wisdom was that the lack of volatile species (such as water) on Earth – especially the Moon – was due to such a violent effect that the volatile elements were depleted.

But by looking at the isotopic composition of the lunar rocks, the team found that the objects involved in the impact that shaped the Earth-Moon system had very low levels of volatile elements before, rather than because of, the collision. Specifically, the team used the relative amount of the volatile and radioactive isotope rubidium-87 (⁸⁷Rb), which is calculated from its daughter isotope strontium-87 (⁸⁷Sr), to determine the Rb budget in the Earth-Moon system when it was formed. The team found it because ⁸⁷Sr, a proxy for the moon’s fluctuating long-term budget, was very low, and the objects that collided must have been dry at first, and not much could have been added since.

“Earth was either born with the water we had, or we were struck by something that was essentially pure H₂Oh with not much in it. This work eliminates meteorites or asteroids as potential sources of water on Earth and points strongly toward the “born with it” option, said cosmic chemist Greg Brenica, co-author of the paper.

In addition to significantly narrowing the potential source of Earth’s water, this work additionally reveals that the large objects that collided must have come from the inner solar system, and the event could not have occurred 4.45 billion years ago, which greatly reduces the formation. Moon window.

According to Lars Burg, the study’s lead author: “There were only a few types of material that could combine to form Earth and the Moon, and it wasn’t strange – it’s likely that both were just large bodies that formed roughly in the same region that just happened to smash into each other after more than one year. A little 100 million years after the formation of the solar system…but we are fortunate that they did just that.”

Reference: “The Origin of the Volatile Elements in the Earth-Moon System” by Lars E. Burg, Gregory A. Bernica, and Thomas S. Kroger, Feb 14, 2022 Available here. Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2115726119

The search appears in Proceedings of the National Academy of Sciences. LLNL scientist Thomas Kruijer also contributed to the research. The work was funded by NASA and a laboratory-oriented research and development programme.