The Moon is thought to have formed after a planetary embryo, known as Theia, collided with the proto-Earth 4.5 billion years ago. This so-called Giant Impact was the last major event during Earth’s accretion, and its effects on the composition of the Earth and the newly forming Moon would be measureable today. Recent work on lunar samples has revealed that the Moon’s water was not lost as a result of this giant impact. Instead, the Moon appears to contain multiple hydrogen reservoirs with diverse deuterium-to-hydrogen (D/H) ratios. For the first time, we incorporate hydrogen isotopic measurements of lunar samples to help constrain the composition of Theia. We show that the Moon incorporated very low-D/H (δD ≈ -750‰) materials that only could have derived from solar nebula H₂ ingassed into the magma ocean of a large (∼0.4 M_E) planetary embryo that was largely devoid of chondritic water. We infer Theia was a very large body comparable in size to the proto-Earth, and was composed almost entirely of enstatite chondrite-like material. These conclusions limit the type of impact to a “merger” model of similarly-sized bodies, or possibly a “hit-and-run” model, and they rule out models that mix isotopes too effectively.

人们认为月球是在45亿年前的行星地球（称为Theia）与原始地球相撞之后形成的。所谓的“巨人撞击”是地球增生期间的最后一个重大事件，它对地球和新形成的月球组成的影响今天可以测量。最近对月球样品的研究表明，由于这种巨大的撞击，月球的水并未损失。取而代之的是，月球似乎包含多个具有不同氘氢比（D / H）的氢储层。我们首次结合了月球样品的氢同位素测量，以帮助限制Theia的成分。我们表明，月球掺入非常低d / H（δ d≈-750‰），只有可能从太阳星云ħ衍生的材料₂吞入了一个大型（〜0.4 M _E）行星胚胎的岩浆海洋，该行星基本上没有软骨组织水。我们推断，Theia是一个非常庞大的尸体，大小与原始地球相当，并且几乎完全由顽辉石陨石样材料组成。这些结论将影响的类型限制为类似大小的物体的“合并”模型，或者可能是“即撞即走”模型，并且它们排除了过于有效地混合同位素的模型。