Isotopic Signal Reveals Echo of Ancient Earth

A group of scientists may have identified an echo of ancient Earth, before it collided with a celestial body and created the Moon, found in the form of an unexplained isotopic ratio.

About 4.5 million years ago the Moon was created after a Mars-sized body - dubbed "Theia" - collided with Earth, according to the long-accepted theory. Supposedly, the heat generated by the collision would have caused the whole planet to melt, before some of the debris cooled and spun off to create the Moon.

Now, however, a team from Harvard University believes that have found a signal that only part of Earth melted, and that an ancient part still exists within its mantle.

"The energy released by the impact between Earth and Theia would have been huge, certainly enough to melt the whole planet. But we believe that the impact energy was not evenly distributed throughout the ancient Earth," lead researcher Associate Professor Sujoy Mukhopadhyay explained in a press release. "This means that a major part of the impacted hemisphere would probably have been completely vaporized, but the opposite hemisphere would have been partly shielded, and would not have undergone complete melting."

Mukhopadhyay and his colleagues analyzed noble gas isotopes from deep within Earth's mantle, and compared these results to isotope ratios closer to the surface. It is known that material that surfaces from within the deep mantle (brought up via mantle plumes) has a lower ratio than that normally found nearer the surface.

But they found that 3He to 22Ne ratio from the shallow mantle is significantly higher than the equivalent ratio in the deep mantle.

This implies that "the last giant impact did not completely mix the mantle and there was not a whole mantle magma ocean," Mukhopadhyay said.

Additional evidence comes from analysis of the 129-Xenon to 130-Xenon ratio, which shows that the ancient part of Earth's mantle has a formation age to within the first 100 million years of Earth's history.

"The idea that a very disruptive collision of Earth with another planet-sized body, the biggest event in Earth's geological history, did not completely melt and homogenize Earth challenges some of our notions on planet formation and the energetics of giant impacts," Mukhopadhyay concluded. "If the theory is proven correct, then we may be seeing echoes of the ancient Earth, from a time before the collision."

The findings will be presented at the Goldschmidt conference in Sacramento, California.