Why doesn't the moon earth collide?

Earth and moon are not "twins"

According to popular theory, the moon is the result of a catastrophic collision between the early Earth and a Mars-sized protoplanet. So far, however, this scenario does not seem to fit with findings that the moon and earth are almost geochemically identical - there is almost no trace of the protoplanet. Now researchers may have solved at least part of that puzzle. Because their new analysis of Apollo moon samples from different terrain shows striking differences in the oxygen isotopes. According to this, old lunar crustal rocks and some basalts actually have earth-like proportions, but this does not apply to rocks from deeper layers of the moon. This sheds new light on the collision and the whereabouts of the protoplanet.

Our moon owes its existence to a cosmic catastrophe: around 4.5 billion years ago the earth collided with a Mars-sized protoplanet. With this hit the impactor "Theia" was completely destroyed, its debris collected together with smaller portions of earthly crustal and mantle rock in an orbit around the earth. From these the moon then formed, around three-quarters of which would have to consist of rock material from the former protoplanet. So much for the common scenario. But there is a catch: the moon and earth are too similar. If the moon had actually emerged from the debris of a protoplanet, the isotope signatures of the rocks of the earth and moon would have to be different. Because every celestial body in the solar system has its own typical isotope signature, even asteroids.

Isotope value puzzle

But this does not seem to be the case with the moon and its predecessor Theia: The isotope compositions of some elements, including silicon, chromium, tungsten and titanium, are almost identical in the earth and moon, and the isotope patterns of water molecules of terrestrial and lunar origin are similar . So far, the same has applied to the proportion of the oxygen isotope 17-O: "Analyzes of lunar basalt samples have resulted in average values ​​that are practically indistinguishable from those on Earth," report Erick Cano from the University of New Mexico and his colleagues. Some planetary researchers therefore suspect that Theia may have been a chemical twin of Earth - the protoplanet must therefore have formed in roughly the same orbit and solar distance as the early Earth. Alternatively - there is also this hypothesis - earth and Theia would have to be almost completely evaporated during the collision so that their debris could then mix almost homogeneously. Both scenarios, however, can only be reproduced in models to a limited extent.

That is why Cano and his team have now started looking for a simpler explanation: The isotope values ​​may depend more on the type of moon sample than previously assumed. To check this, they subjected rock samples from as different as possible lunar terrain to a new analysis with regard to the proportion of the oxygen isotope 17-O. The samples included various rocks from the Mare areas and the lunar highlands, as well as volcanic glass. For comparison, the researchers also determined the 17 O values ​​of various rocks from the Earth's mantle.

Did Theia come from further outside?

The analyzes revealed that if you take the average of all the lunar rock samples examined, the oxygen isotope values ​​actually correspond almost exactly to those of the earth. "But it is far more striking that the lunar samples have almost three times as high a variability in the 17-O values ​​than the terrestrial ones," report Cano and his team. Accordingly, the previous assumption that lunar rocks have the same oxygen isotope values ​​is not correct - there are clear deviations in the details. Specifically, the researchers found that the titanium-rich Mars basalts and highland rocks have significantly lower 17-O proportions than the greenish volcanic glass. As they explain, this volcanic glass comes from sources more than 400 kilometers deep in the magma ocean of the young moon. "We therefore assume that the high 17-O content of this glass is representative of the rock melt that comes deep from the lunar mantle," said Cano and his colleagues.

But what do these results mean for the collision scenario and the nature of the protoplanet Theia? According to the scientists, this indicates that the protoplanet had a slightly different composition than the earth - and that relics of this celestial body have been preserved in the interior of the moon. “If the oxygen isotopes of terrestrial celestial bodies in the inner solar system tend to have higher 17-O values ​​with increasing distance from the sun, then Theia could have originated further out than the earth,” the researchers say. After the collision, the debris of this protoplanet mainly gathered inside the newly formed moon. Therefore, rocks from the deep moon mantle contain higher proportions of the heavier oxygen isotope 17-O. "Theia's isotopic composition was not completely homogenized during the collision," said Cano and his colleagues. The crustal rock of the moon, on the other hand, mixed with the remains of the slowly condensing silicate vapor. This remaining vapor contained lower 17-O values ​​and was only absorbed into the outer layers of the already solidifying lunar magma ocean. According to the researchers, this explains why the lunar crustal rocks contain less 17-O than the lunar mantle.

Source: Erick Cano (University of New Mexico, Albuquerque) et al., Nature Geoscience, doi: 10.1038 / s41561-020-0550-0

March 9, 2020

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