How Did Water Appear on Earth? Scientists Think It's Down to the Giant Crash That Formed the Moon

apollo 8, moon, earth, earthrise
Taken aboard Apollo 8 by Bill Anders, this iconic picture shows Earth peeking out from beyond the lunar surface as the first crewed spacecraft circumnavigated the moon. NASA

Earth's water originated from the event that is thought to have formed the moon early in the solar system's history, according to research published in the journal Nature Astronomy.

According to what's known as the "giant impact hypothesis," the moon formed from debris after a Mars-sized body called Theia—which was thought to have originated in the inner solar system—struck the proto-Earth around 4.5 billion years ago.

But in a new study, scientists from the University of Münster in Germany suggest that not only did Theia come from the outer solar system but it was also responsible for bringing water to our planet.

The source of water on Earth and other rocky planets like Mars has long been a topic of contention for scientists, particularly because our planet formed in the "dry" inner solar system where there was a lack of the substance.

"The formation of the moon and the origin of Earth's water are two of the fundamental open questions in cosmo-chemistry," Gerrit Budde, the study's lead author, told Newsweek. "Unraveling these processes is essential to understand the early evolution of the solar system, the formation of planets and the development of life."

Previous studies have shown that, at its very beginning, the solar system was essentially split in half for the first few million years, with the formation of Jupiter likely acting as a barrier against the exchange of materials between the inner and outer regions.

The outer solar system was populated by meteorites containing water-rich "carbonaceous" material. Meanwhile, the inner solar system was filled mostly with non-carbonaceous meteorites that lacked water.

Among the most popular explanations for the origin of Earth's water is one suggesting that "carbonaceous" bodies from the outer solar system delivered significant amounts of carbonaceous material to Earth. However, scientists are not sure about when these outer solar system bodies brought this material to our planet, and what the quantities involved were.

To try to address these issues, the scientists in the study analyzed data on variants of a silvery-white metal called molybdenum obtained from meteorite and terrestrial rock samples.

Isotopes—variants of a chemical element that differ in the number of neutrons—allowed the scientists to clearly distinguish between carbonaceous and non-carbonaceous materials, thus providing them with a "genetic fingerprint" of material from both the inner and outer solar systems.

"By comparing the molybdenum isotope composition of Earth and meteorites, for the first time we were able to constrain the timing and amount of the addition [of] outer solar system material to the Earth, as well as the origin of the moon-forming impactor and Earth's water," Budde said.

"In short, Earth's water was delivered by water-rich carbonaceous material that derived from the cold outer solar system, probably beyond the orbit of Jupiter," he said. "This happened relatively late at the end of Earth's main growth history, and this material was delivered most likely by the giant impactor—Theia—that also caused the formation of the moon. In turn, this means that the impactor itself originated from the outer solar system—it is generally assumed that Theia originated in the inner solar system near the Earth."

According to the researchers, the delivery of water to the Earth and the formation of the moon—which stabilizes the planet's axis—were both essential requirements for Earth's habitability and the development of life.

"Our approach is unique because, for the first time, it allows us to associate the origin of water on Earth with the formation of the moon," Thorsten Kleine, another author of the Münster study, said in a statement. "To put it simply, without the moon there probably would be no life on Earth."