According to current theory, the moon is the result of a catastrophic collision between the early Earth and an Mars-sized protoplanet. So far, however, this scenario does not seem to match findings that the moon and earth are almost identical geochemically – almost no trace of the protoplanet is missing. Now researchers could have solved at least part of this puzzle. Because their re-analysis of Apollo moon samples from different terrain shows striking differences in the oxygen isotopes. Accordingly, old lunar crustal rocks and some basalts actually have earth-like proportions, but this does not apply to rocks from the deeper layers of the moon. This sheds new light on the collision and whereabouts of the protoplanet.
Our moon owes its existence to a cosmic catastrophe: around 4.5 billion years ago, the earth collided with an Mars-sized protoplanet. With this hit, the impactor “Theia” was completely destroyed, its debris collected together with smaller portions of earth’s crust and mantle rock in an orbit around the earth. From these the moon was formed, which would have to consist of around three quarters of rock material of 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 would have to be different from earth and moon. Because every celestial body in the solar system has its own typical isotope signature, even asteroids.
Riddle of the isotope values
However, 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 for earth and moon, and the isotope patterns of water molecules of earthly and lunar origin are similar . Something similar has previously been true for the proportion of the oxygen isotope 17-O: “Analyzes of lunar basalt samples have shown 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 the earth – the protoplanet must therefore have originated at approximately the same orbit and distance from the sun as the early Earth. Alternatively – and this hypothesis also exists – earth and Theia should have evaporated almost completely during the collision so that their rubble could then mix almost homogeneously. However, both scenarios can only be reproduced to a limited extent in models.
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 the respective moon samples than previously assumed. To check this, they subjected rock samples from the most diverse lunar terrain to a new analysis with regard to the proportion of the oxygen isotope 17-O. The samples included various types of rock 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 outside?
The analyzes revealed: If you take the average of all examined moon rock samples, the oxygen isotope values actually correspond almost exactly to that of the earth. “But it is far more striking that the lunar samples have almost three times as much variability in the 17-O values as the earthly ones,” report Cano and his team. Accordingly, the previous assumption that lunar rocks have the same oxygen isotope values is not correct – there are significant deviations in the details. Specifically, the researchers found that the titanium-rich Mars basalts and highland rocks have significantly lower 17-O fractions than the greenish volcanic glass. As they explain, this volcanic glass comes from sources that were 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 melts that come from the lunar mantle,” says 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 are preserved inside the moon. “If the oxygen isotopes of terrestrial celestial bodies in the inner solar system tend to be higher 17-O values as the distance from the sun increases, then Theia could have originated further away than the earth,” said the researchers. After the collision, the debris of this protoplanet primarily collected inside the newly formed moon. This is why rocks from the deep moon’s mantle contain higher proportions of the heavier oxygen isotope 17-O. “Theia’s isotope composition was not completely homogenized during the collision,” says Cano and his colleagues. The moon’s crustal rock, 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 taken up in the outer layers of the already solidifying lunar magma ocean. According to the researchers, this explains why the lunar crust 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