Exploration Mysteries: Theia Impact

Almost since the beginning of the solar system, Earth has taken beating after beating. But one of its most cataclysmic blows led to the foundations of life here.

Astronomers hypothesize that 4.5 billion years ago, one of the solar system’s earliest planets named Theia smashed into the Earth. It caused major changes to the Earth’s makeup. Most notably, it created our Moon. A growing amount of evidence supports this theory. 

Baptism by fire

The solar system began with a gravitational collapse. After that came a fast spinning disk of particles, dust, and gas. Soon after, planet embryos (or planetesimals) started to form through accretion — the accumulation of particles to form a larger object.

The planetesimals had approximately 10% of the Earth’s mass. Supposedly, the early solar system hosted around 20 small planets. However, these newly formed bodies had to undergo yet another tumultuous stage: collision. 

Planetary collisions among planetesimals was completely normal and resulted in larger terrestrial bodies. One of these protoplanets was Theia, named after a Greek Titan and, suitably enough, the mother of the moon goddess. Proponents of the Theia hypothesis, which popped up in the 1970s, claim that it was the size of Mars before its great demise. Possibly, gravitational pushes and pulls from other planets nearby put Theia on a collision course with Earth. 

When worlds collide

Initially, astronomers believed the Moon had formed by some other means. Some thought a single planetary body had split into the Earth and Moon. Others suggested that the Moon was a solitary body that wandered nearby and got caught in Earth’s orbit. Some even posited that the Earth spun so fast that a piece of it broke off and began to orbit. 

moon and earth

Moon with a view of Earth. Photo: Shutterstock


If Theia struck Earth, debris from both planets would have flown out. It would have formed a brief asteroid belt in the shape of a disk. Shortly after, pieces of Theia and Earth probably merged to form the Moon, which then started to orbit the Earth.

Theia’s mantle and core could have merged with Earth’s. This Theia Mantle Material (TMM) would have been 0.017 to 0.026 of Earth’s mass, according to geodynamcist Qian Yuan. The impact could have created our magnetic field, increased our planet’s speed, and even shaped our atmosphere. Most notably, Theia could have provided the Earth with water.


Any evidence for the Theia impact theory lies thousands of kilometers down, in the Earth’s mantle. Based on seismic wave readings, researchers in the 1980s pinpointed some anomalies beneath the African continent and Pacific Ocean. These are called LLVPs (large low-shear velocity provinces) and occur in the lowest part of the mantle. Irregular shaped blobs 1,000km thick, they differ in density from the rest of the mantle.

Seismic waves slow down considerably moving through them compared to the surrounding mantle. Remnants of LLVPs could occur on some islands around Africa and in the Pacific Ocean, where extremely ancient basalt is present. 

At first, researchers thought the LLVPs were the result of subducted oceanic crust or chemical differentiation. However, the structures are too complex to come from simple plate tectonic movement. 

solar system

Illustration of the early solar system. Credit: NASA


Along with this evidence, we have exciting possibilities from the Moon itself. The Apollo 11 mission in 1969 brought back about a tonne of lunar rock samples. These also point to a possible connection with Theia. The rocks date back 4.5 billion years, when this supposed collision took place. Their composition is almost the same as the Earth’s. This strongly suggests that the Moon came from a piece of the Earth. The moon might have been molten at first, pointing to a possible high energy impact origin. 

Universities have also used supercomputers to generate simulations of the scenario. Scientists debate whether Theia struck the Earth head on or at an angle. They tested Theia’s spin, angle, speed, etc. They found that for the Moon to form at its current size, Theia would have had to hit Earth at a 45° angle and at a speed of 4kps.

If it was a head-on collision at a terribly fast speed, Earth would have vaporized beyond repair, and the Moon would just be a random hunk of rock in the ether of space.


The Theia impact theory is plausible. The beginning of our solar system was certainly tumultuous, and the proto-planets that existed before our original eight likely collided with each other to give us exactly this group we know today.

The best strategy would be to return to the Moon, as NASA plans to do, to collect more samples and study the landscape thoroughly. Perhaps the answer lies there. We will see what happens in the next few years when we return to that still-mysterious object, which continues to hold so many secrets.

Kristine De Abreu

Kristine De Abreu is a writer at ExplorersWeb.

Kristine has been writing about Science, Mysteries and History for 4+ years. Prior to that, Kristine studied at the University of Leicester in the UK.

Based in Port-of-Spain, Kristine is also a literature teacher, avid reader, hiker, occasional photographer, an animal lover and shameless ramen addict.