Mars-sized body struck Earth, formed moon, study says

Chuck Bednar for – @BednarChuck

Geologists from the University of Maryland have uncovered new evidence supporting the belief that the moon formed after a roughly Mars-sized body struck and merged with Earth, causing the formation of a large debris cloud that ultimately coalesced and formed our satellite.

While that notion of lunar formation has been widely accepted over the past three decades, it did have some issues: namely that the isotopic compositions of the Earth and the moon (their genetic fingerprints, essentially) appeared to be too similar to one another to support the scenario.

Problems with the current theory of lunar formation

The size of the moon and the physics of its orbit around support this merger theory, but if a third-party was involved in the formation process, than the moon should carry the so-called “isotopic fingerprint” of that entity. Since the third-party, which has been dubbed Theia by scientists, came from elsewhere in the solar system, it should have a far different fingerprint than Earth.

To investigate, UMD geology professor and study co-author Richard Walker and his colleagues generated a new isotopic fingerprint of the moon that focused on an isotope of Tungsten that can be found in both Earth and the moon. Their research, which has been published Wednesday in the journal Nature, is the first to reconcile the unexpectedly similar isotopic fingerprints of Earth and the moon with the currently accepted model of lunar formations, the authors claim.

“The problem is that Earth and the moon are very similar with respect to their isotopic fingerprints, suggesting that they are both ultimately formed from the same material that gathered early in the solar system’s history,” Professor Walker explained. “This is surprising, because the Mars-sized body that created the moon is expected to have been very different. So the conundrum is that Earth and the moon shouldn’t be as similar as they are.”

Differences in a Tungsten isotope help resolve the issue

Other theories have surfaced over the years to help explain the similarities, including one which suggested that the debris cloud caused by the impact mixed with the Earth before condensing to form the moon, and another suggesting that an unusual collision caused the moon to form out of materials originating from Earth, not Theia itself.

Evidence indicates that both Earth and the moon gathered additional material after the primary impact took place, and that our planet then collected more of this debris and dust. This dust and debris contained high amounts of Tungsten, including relatively small amounts of a lighter isotope called Tungsten-182. In theory, the moon should have more Tungsten-182 than Earth, and that’s exactly what Walker and his colleagues found when comparing rocks.

“The small, but significant, difference in the Tungsten isotopic composition between Earth and the moon perfectly corresponds to the different amounts of material gathered by Earth and the moon post-impact,” the professor said. “This means that, right after the moon formed, it had exactly the same isotopic composition as Earth’s mantle.”

The discovery supports the notion that the mass of material that was created by the impact and which went on to form the moon mixed together before cooling, explaining the overall isotopic similarities between the moon and Earth, the researchers said. It also all but eliminates the idea that Theia had a similar composition to Earth, or that the material involved in lunar formation came primarily from our planet.


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