Earth and moon are not 'identical oxygen twins,' researchers say

IMAGE Research Assistant Erick Cano works in UNM's Center for Stable Isotopes.   Image Steve Carr

IMAGE Research Assistant Erick Cano works in UNM's Center for Stable Isotopes. Image Steve Carr

There are multiple theories which try to explain the formation of the Moon, but the giant-impact hypothesis is the most reliable on.

There has been no shortage of theories on how the moon formed over the years, including asteroid collisions or even collisions with ancient planets. Recently, through a new study published in the journal Nature, new evidence was presented to support this theory.

The odds of Theia having a similar composition to the Earth are extraordinarily small, so theorists and researchers were left scratching their heads; That is, until planetary scientist Erick Cano and his team performed a meticulous and methodical reanalysis of the lunar samples we've so far been able to bring back to Earth.

Models made by scientists predicted that around 70 to 90 per cent of the Moon should have been made up of mooshed and reformed Theia, which means that the oxygen isotopes of Moon would have been different from Earth's oxygen isotopes.

In addition, their findings help clarify one of the main issues of the giant-impact hypothesis.

'While the Earth rocks occupy a very narrow range of oxygen isotope compositions, the lunar samples demonstrated almost three times that variability, ' Erick Cano from the University of New Mexico told the New Scientist. That would suggest early Earth and Theia featured similar oxygen compositions, an unlikely coincidence.

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Another explanation is the fact that everything got mixed during the impact, but it seems that this is not so likely, according to simulations.

Although the Giant Impact Hypothesis can nicely explain numerous geochemical similarities between Earth and Moon, the extreme similarity in oxygen isotopes has been hard to rationalize with this scenario: either the two bodies were compositionally identical in oxygen isotopes to start with, which is unlikely, or their oxygen isotopes were fully mixed in the aftermath of the impact, which has been hard to model in simulations. These theories included the idea of Theia fusing with Earth, creation of a cloud of dust that went on to become Earth and Moon, Theia and Earth forming really close to each other, and more.

New research, by Erick Cano and his team at the University of New Mexico, looked at the rock samples from the Moon once more.

They examined samples of rocks gathered from a number of different lunar sites and with both high and low titanium basalts.

Their analysis showed the exact oxygen isotope composition depended on the makeup of the rock.

'Typically, all the lunar samples are lumped together as a whole and the average value of all the lunar samples measured in the study is taken as the value for the moon, ' he said. "Based on the results from our isotopic analysis, Theia would have an origin farther out from the Sun relative to Earth and shows that Theia's distinct oxygen isotope composition was not completely lost through homogenization during the giant impact".

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