The supply of water on Earth is very important because of its ability to sustain life, but have you ever wondered where that water comes from? Was water present when Earth formed, or was it sent later by meteorites or comets from outer space?
The source of Earth's water has long been a subject of debate, and scientists Lawrence Livermore National Laboratory (LLNL) think they have the answer. They found it by looking at rocks from the Moon.
Since the Earth-Moon system formed together from the collision of two large bodies early in the history of the Solar System, their histories are closely related. The moon is actually a good place to look for clues to Earth's water history. But because the Moon lacks plate tectonics and weathering, this tends to erase or obscure evidence on Earth.
As quoted from News Wise, although 70% of the Earth's surface is covered with water, overall, the planet is a relatively dry place compared to many other objects in the Solar System. Our natural satellite, the Moon, is even drier.
Conventional wisdom is that the lack of volatile species (such as water) on Earth, and specifically the Moon, is due to a violent impact that causes the depletion of volatile elements.
But by looking at the isotopic makeup of the Moon's rock, the team found that the objects involved in the collisions that formed the Earth-Moon system had very low levels of volatile elements before the collision, not because of that.
Specifically, the team used the relative amounts of the volatile and radioactive isotope rubidium-87 (87 Rb), as calculated from its daughter isotope strontium-87 (87 Sr), to determine the composition of Rb in the Earth-Moon system when it formed.
The team found that because 87 Sr, a proxy for the Moon's long-term volatility composition, is very low. The colliding bodies must have been dry from the start, and there wasn't much to add since then.
"Earth was born with the water we have, or we were hit by something that's basically pure H2O, without much else in it. This work rules out meteorites/asteroids as possible sources of water on Earth and points strongly toward the 'born with that'," said cosmochemist Greg Brennecka, a co-author of the paper.
In addition to narrowing down the potential sources of Earth's water, the study also revealed that the massive colliding objects must have originated in the inner Solar System, and that the event was unlikely to have occurred before 4.45 billion years ago.
According to Lars Borg, lead author of the study: "There are only a few types of material that could be combined to form the Earth and the Moon, and they weren't exotic, they were probably just large bodies that formed around the same area that happened to bump into each other more or less than that. 100 million years after the Solar System formed. Lucky for us, they did just that."
The research appears in the Proceedings of the National Academy of Sciences. LLNL scientist Thomas Kruijer also contributed to the research. This work was funded by NASA and the Laboratory Directed Research and Development program.