Two independent studies presented within just a week of each other have added to the growing body of evidence that the Moon formed from a dramatic collision between primordial Earth and another Mars-sized astronomical body around 4.5 billion years ago.
Last week, a study was published in the journal Science which compared oxygen isotopes between lunar samples collected from Apollo missions and samples collected from Earth. If this dramatic event took place we would expect to see differences in the chemical compositions. Indeed, scientists found subtle differences between the isotope ratios which supported the collision theory. Furthermore, the data gathered also offered some insight into the geochemistry of the colliding object, which has been named Theia.
Now, a team of scientists from Harvard University believe that they have obtained further information from isotopic ratios which may represent signals, or echoes, from a pre-collision Earth. More specifically, they believe that their data suggests that the violent collision caused some, but not all, of the Earth to melt, and that an ancient remnant still exists deep within the Earth’s mantle. The results are being presented at the Goldschmidt conference in California.
According to popular theories about the collision between Theia and Earth, the event would have generated so much heat that Earth would have completely melted. Shortly afterward, some of the debris would have been flung out into space, destined to become the Moon. However, Harvard scientists believe their evidence contradicts this theory and suggest that only a portion of Earth melted.
“The energy released by the impact between Earth and Theia would have been huge, certainly enough to melt the whole planet,” said lead researcher Sujoy Mukhopadhyay in a news-release. “But we believe that the impact energy was not evenly distributed throughout the ancient Earth. 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.”
The team obtained their evidence by comparing the ratios of noble gas isotopes (variants of a particular element that differ in the number of neutrons) from deep within the Earth’s mantle with those nearer the surface of the mantle. They found that the ratios of Helium-3 and Neon-22 were significantly higher in samples obtained from the shallow mantle. According to Professor Mukhopadhyay, this suggests that the collision did not completely mix the mantle.
Furthermore, the scientists found evidence for the persistence of early-formed chemical signatures in the deep mantle by analyzing Xenon isotope ratios (129Xe/130Xe). 129Xe is produced by the slow decay of Iodine-129, and by probing 129Xe/130Xe ratios the team was able to date the creation of the ancient part of the mantle to within the first 100 million years of Earth’s timeline.
“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,” said Mukhopadhyay. “If the theory is proven correct, then we may be seeing echoes of the ancient Earth, from a time before the collision.”