Earth’s Water Supply Survived Moon-Forming Impact

Earth’s Water Supply Survived Moon-Forming Impact
Image Credit: NASA / JPL-Caltech

The image above shows an artist’s impression of the impact between Earth and a Mars-sized object in an event that is widely accepted as the mechanism that formed Earth’s Moon only about 100 million years or so after the formation of the solar system.

A recent study by a team of researchers under the leadership of Richard Greenwood of the Open University suggests that contrary to popular belief, Earth was already supplied with an abundant water supply when the collision that had formed the Moon occurred. Prior to Greenwood’s research that was published in the scientific journal, Science Advances, many, if not most planetary scientists were of the opinion that at the time the collision occurred, Earth was too hot to hold water, and that the presence of water on Earth is the result of collisions with comets and other bodies subsequent to the collision that formed the Moon.

In short, the collision theory holds that the remains of the impactor and the material ejected from Earth mixed so completely that the resulting mixture of material was nearly completely homogenous. In fact, rock samples brought back from six Apollo missions turned out to be virtually indistinguishable from similar material that was known to have formed on Earth.

However, the lunar samples were not exactly the same as terrestrial material in terms of some of the isotopes that both contained, which led some investigators to believe that these slight differences were the result of subsequent impacts on the Moon that formed some of its surface features, such as major craters and impact basins. To resolve this question, Greenwood and his team of researchers compared the samples recovered during all six Apollo landings with a variety of different terrestrial volcanic rocks from Earth’s ocean floors.

The aim of the study was to compare concentrations of specifically oxygen isotopes in both the lunar and terrestrial material in order to determine the degree of mixing that took place during the collision that formed the Moon.

The results were rather surprising. While a difference of between 3 and 4 parts per million in the oxygen isotope content between the lunar and terrestrial basaltic rocks was found, the researchers could find no meaningful differences between the lunar samples and terrestrial olivine, which is a common constituent of sub-surface material on Earth.

This similarity is not only clear evidence of how efficient the mixing process was during the Moon-forming collision; the finding also places strict and clearly defined limits on both the types and relative volumes of material that could have been added to Earth (or the Moon) subsequent to the impact.

Greenwood explained the implications of his findings thus, “If most of the water on Earth had arrived after the collision, we would expect the lunar and terrestrial rocks to have distinctly different oxygen compositions. This suggests that liquid water on Earth must have existed at an earlier stage, prior to the Moon-forming impact”, and added, “Our research shows that water is also extremely resilient and can survive an event as catastrophic as two planets colliding.”

The implications of this finding are far reaching with respect to the search for life outside of the solar system, because according to Greenwood, if this can happen in the solar system, it is almost certain that it can, or could have happened in many other places throughout the Universe.

Therefore, based on this study, exoplanets with liquid water on their surfaces may be much more common that was hitherto thought, and after all, as Greenwood says,” where there’s [liquid] water, there could also be life”.

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