Moon-forming Impactor Theia originated in inner Solar System

4.5 billion years ago, the planets were being assembled in the debris disk surrounding a newborn Sun. Some planets were growing and accumulating material, while others were being battered to bits and stripped to the cores, with the gas giants mediating the process. In this chaotic infancy of the Solar System, a protoplanet called Theia smashed into the Earth, changing its composition and orbit, and forming the Moon. The Moon may be material made up of mostly of a primordial Earth, a fragment of Theia, or a combination of the two. There are no significant differences in the chemical fingerprints of the rocks from the Moon and Earth, causing scientists to turn to meteorites as a benchmark. 

Meteorites are fragments of asteroids that reach the surface of Earth. These asteroids preserve the conditions in which they were formed. The asteroids that formed in the outer Solar System had different isotope ratios than those that formed in the inner Solar System, because of the complex interplay of dust, gas and ices with solar radiation. Now while fragments of Theia with its unique isotope ratios can be isolated in theory, the material is buried deep within the interior of Earth. The scientists used innovative reverse engineering techniques to compute what kind of material Theia would have to be made up of, to result in the material that makes up the Earth and Moon today. 

The research using meteorites as benchmarks indicates that the isotope ratio of Theia more closely matches the asteroids that formed in the inner Solar System, in orbits closer to the Sun than the Earth is. The mathematical calculations reveal a wide range of formation scenarios and possible chemical makeups of the Earth and Theia, but some of these are incompatible with the known ongoing processes in the early Solar System. A paper describing the findings has been published in Science. The research also indicates that the isotope ratios of Theia are vastly different from that of the Earth. When the Earth was molten, most of the Iron sunk towards the core with the heavier elements. It is possible that most of the iron on the surface of the planet was delivered by Theia.