The formation of Earth’s Moon remains a compelling mystery, with the leading scientific explanation centering on a catastrophic event involving a hypothetical ancient planet named Theia. This theoretical collision, occurring billions of years ago, profoundly shaped both the Moon and Earth. Understanding Theia’s impact provides insight into the dynamic early history of our solar system.
Theia: A Hypothetical Protoplanet
Theia is envisioned as a protoplanet that existed in the early solar system. Scientists estimate its size was roughly comparable to that of Mars, with a diameter of approximately 6,102 kilometers (3,792 miles). Theia is believed to have formed in an orbit similar to Earth’s, possibly in one of the stable Lagrange points within the Earth-Sun system, where gravitational forces allowed it to accumulate material over time. Its existence is not directly observed but inferred through detailed scientific models and the evidence left behind by its proposed collision. While its exact composition is debated, it likely shared similarities with the early Earth, as both formed in a similar region of the solar system. The name “Theia” itself is derived from Greek mythology, where Theia was the Titan mother of Selene, the goddess of the Moon, a fitting parallel to its hypothesized role.
The Giant Impact Hypothesis
The Giant Impact Hypothesis posits that around 4.5 billion years ago, Theia collided with the early Earth, often referred to as “proto-Earth.” This was a high-energy, oblique impact, meaning Theia struck Earth at an angle rather than head-on. Computer simulations suggest the impactor could have been traveling at speeds between 8 to 20 kilometers per second (5.0–12.4 miles per second). The immense energy released during this collision would have vaporized and liquefied vast amounts of material from both Theia and proto-Earth’s outer layers, primarily their mantles and crusts. This catastrophic event ejected a significant plume of molten rock and vapor into orbit around Earth. The metallic core of Theia is thought to have merged with Earth’s core, while the ejected silicate material formed a debris disk around Earth.
Scientific Evidence for Theia’s Collision
Multiple lines of scientific evidence support the Giant Impact Hypothesis. One of the strongest pieces of evidence comes from the striking similarity in the stable isotopic compositions of lunar and terrestrial rocks, particularly oxygen isotopes. Samples from Apollo missions show that the ratios of oxygen isotopes (like oxygen-16, oxygen-17, and oxygen-18) on the Moon are nearly identical to those found in Earth’s mantle. This suggests that the material forming both bodies underwent thorough mixing during the collision, implying a common origin for much of their composition. Another piece of evidence is the Moon’s relatively small iron core compared to Earth’s. Earth has a dense, large iron core, and if the Moon had formed independently, it would likely have a proportionally larger core. The Giant Impact Hypothesis explains this by proposing that Theia’s iron core largely merged with Earth’s, leaving the Moon to form primarily from the iron-poor, silicate-rich mantle material ejected during the impact. Furthermore, the Moon’s depletion in volatile elements, such as potassium, sodium, and zinc, provides additional support. These elements have lower boiling points and would have easily vaporized under the extreme temperatures generated by a giant impact, resulting in the Moon’s lower abundance of these elements. The Earth-Moon system’s anomalously high angular momentum also aligns with the hypothesis, as the impact would have imparted a significant amount of rotational energy to the proto-Earth and the resulting debris disk.
The Legacy of Theia: Formation of the Moon and Earth’s Characteristics
The most direct consequence of Theia’s impact was the formation of the Moon. The ejected material, consisting of vaporized and molten rock, coalesced in orbit around Earth. Over time, gravitational forces caused this debris to aggregate, forming the spherical body we know as the Moon within a few million years after the collision. Beyond the Moon’s creation, the impact also significantly influenced Earth’s characteristics. The collision is thought to have contributed to Earth’s current axial tilt of approximately 23.5 degrees, which is responsible for the planet’s seasons. The impact also likely caused Earth to rotate much faster initially, with some models suggesting a day as short as 2 to 5 hours immediately after the event. Tidal interactions between the Earth and the newly formed Moon have since gradually slowed Earth’s rotation to its present 24-hour day. The immense energy from the impact also generated enough heat to melt large portions of Earth, contributing to the differentiation of its internal layers, including the core, mantle, and crust.