About 4.5 billion years ago, something the size of Mars collided with the newly formed Earth, causing a colossal explosion. It is believed that this object not only merged with the Earth, but also threw out some of the matter that became the Moon.
This story is known as the giant collision hypothesis; an object the size of Mars is called Thea; and now scientists believe they have found traces of Thea on the moon.
The giant collision hypothesis has been the preferred model for explaining the formation of the moon for many years.
“This model was able to account for recent observations from samples returned by the Apollo missions, which included low iron content on the Moon relative to Earth,” researchers at the University of New Mexico wrote in their paper.
Models predicted that about 70-90 percent of the moon would be reformed Thea. However, the oxygen isotopes in the lunar samples collected by the Apollo astronauts were very similar to terrestrial oxygen isotopes – and very different from oxygen isotopes in other objects in the solar system.
One possible explanation is that the Earth and Thea had similar compositions. Secondly, during the impact, everything was mixed, which, according to the simulation data, is unlikely.
Moreover, Thea's chances of having a composition similar to Earth are actually extremely small. This means that if the Moon is primarily Thea, its oxygen isotopes must be different from Earth's oxygen isotopes.
This close similarity was a major problem for the giant collision hypothesis. Over the years, researchers have published several articles trying to explain this.
This is where the idea came from that Thea had merged with the Earth. Another study suggested that the collision produced a cloud of dust, which then turned into the Earth and the Moon. There have been suggestions that, perhaps, Thea and Earth formed very close to each other.
Scientist Eric Cano and his colleagues took a different route: careful reanalysis of lunar samples.
They obtained a number of samples from different types of rocks collected on the Moon – both high and low titanium from the lunar rock; anorthosites from the highlands and norites from the depths, uplifted during a process called the flip of the lunar mantle; and volcanic glass.
For the new analysis, the research team modified the standard isotope analysis technique to obtain highly accurate measurements of oxygen isotopes. And they did find something new: this oxygen isotopic composition varied depending on the type of rock being studied.
“We are showing,” they wrote in their paper, “that the method of averaging the isotopic data of the Moon, while ignoring the lithological differences, does not give an accurate picture of the differences between the Earth and the Moon.”
The researchers found that the deeper the origin of the rock sample, the heavier the oxygen isotopes compared to those on Earth.
This difference could be explained if, during the impact, only the outer surface of the Moon was crushed and mixed, resulting in a resemblance to Earth. But deep inside the Moon, a portion of Thea remains relatively intact, and its oxygen isotopes have remained closer to their original state.
The study argues that this is fairly accurate evidence that Thea may have formed further in the solar system and moved inward ahead of the large collision that created the moon.
“Obviously, Thea's distinct oxygen isotopic composition was not completely lost as a result of homogenization during the giant collision,” the researchers concluded.
'Thus, this result removes the need for giant impact models to include a mechanism for complete homogenization of oxygen isotopes between two bodies and provides a basis for future modeling of the formation of the moon.'
The study was published in the journal Nature Geoscience.
Sources: Photo: (Mark Garlick / Science Photo Library / Getty Images)
