About 4.5 billion years ago, a momentous catastrophe occurred: a protoplanet called Theia collided with the young Earth and the moon was formed from the rubble. But where did Theia come from? And what was it made of? Planetary scientists have reopened these questions. To do this, they carried out new, high-resolution analyzes of iron isotopes from samples of lunar rock, meteorites and terrestrial rock. In addition, they also included isotope ratios of other elements. This confirmed that the Moon and Earth’s mantle are almost indistinguishable for most isotopes. Theia and the young Earth must therefore have originated in approximately the same area of the primordial cloud. However, some deviations in molybdenum isotopes suggest that the protoplanet Theia was a little closer to Sone than our home planet at that time.
Our moon owes its existence to a serious collision: around 4.5 billion years ago, the Earth collided with the Mars-sized protoplanet Theia. This was completely destroyed, and part of the earth’s rock shell may have temporarily melted and evaporated. The moon was formed from the debris of the collision and, according to current theory, consists largely of the remains of Theia. Because every celestial body in the solar system has its own, typical isotope signature, the lunar rock would therefore have to be different from that of the Earth. But that’s exactly not the case: the Moon and Earth have almost identical isotope values for many elements – both titanium, silicon, chromium and tungsten as well as hydrogen and oxygen. Some planetary researchers therefore suspect that Theia was a chemical twin of Earth and was formed in the same zone of the primordial solar cloud.
However, it would also be conceivable that the debris from Theia and Earth mixed more strongly after the collision than current models suggest – or that the Moon was formed largely from evaporated Earth material.
Searching for traces of iron isotopes
It is still unclear which of these scenarios applies, because two factors in particular make the reconstruction difficult: So far, researchers have not been able to identify any intact parts of Theia’s rubble. Therefore, we do not know what the protoplanet was composed of before the collision. On the other hand, since their formation, the Moon and Earth have undergone a wide variety of geological processes that have changed the distribution and concentrations of the various elements and isotopes on their surface and in their interior. These must always be taken into account when comparing isotopes. Nevertheless, some conclusions can be drawn about the place of origin of a celestial body from such analyses.
“The composition of a body archives its entire formation history, including its place of origin,” says senior author Thorsten Kleine from the Max Planck Institute for Solar System Research (MPS) in Göttingen.
To find out more about Theia, Kleine, first author Timo Hopp from MPS and their colleagues once again looked for telltale clues in the isotope values of the moon, earth and meteorites. The metorites – chondrites with different carbon contents – served as comparison objects. Previous analyzes had already shown that the carbon-containing chondrites formed in the outer areas of the solar system have different isotope ratios for certain elements than the non-carbon-containing chondrites formed closer to the sun. This reflects the isotope distribution in the gas and dust cloud of the early solar system – and should therefore also provide clues about where planets were formed.
Hopp and his team’s analysis initially focused on the isotopes of the element iron. To do this, they examined 15 terrestrial rocks, six lunar samples from the Apollo missions and 20 different meteorites for their iron isotope distribution. As with other elements, these comparisons showed little difference between terrestrial and lunar rocks. The material of the silicate-rich rock layers of both celestial bodies is indistinguishable in terms of its iron, the team reports. Compared to meteorites, lunar and Earth samples showed clear deviations from the carbonaceous meteorites formed far outward, but good matches with the non-carbonaceous chondrites closer to the sun.
Theia orbited closer to the sun than Earth
These results alone are therefore not enough to solve the mystery of Theia, the Moon and the Earth. That’s why Hopp and his colleagues used a model to help them. The aim was to find out which compositions and sizes of Theia and the early Earth most likely lead to the isotope values that can be measured today. In this “reverse engineering”, the researchers included not only their iron results but also the results of isotope comparisons of other elements such as chromium, molybdenum and zirconium. It turned out that both Theia and the Earth were formed in the inner solar system and in a similar area of the primordial solar cloud. “Erde and Theia were probably neighbors,” says Hopp. This could explain why today’s earth and moon rocks are so similar: the two celestial bodies from which they come were chemically and isotopically similar.
However, there is a small difference in certain molybdenum isotopes, which are typically more present near the sun. “If this is confirmed, then we consider a scenario to be likely in which Theia formed closer to the sun than the planetary building blocks of the young Earth,” explain the researchers. The protoplanet Theia could have orbited the sun just within the Earth’s orbit before it collided with the primordial Earth and was destroyed.
Source: Timo Hopp (Max Planck Institute for Solar System Research, Göttingen) et al., Science, doi: 10.1126/science.ado0623