Theia (moon)

Theia is the name given to a hypothetical protoplanet that is believed to have collided with the early Earth, leading to the formation of the Moon. According to the giant impact hypothesis, this immense collision occurred approximately 4.4 to 4.5 billion years ago and was a decisive event in shaping both the Earth and its satellite. The name Theia originates from Greek mythology, referring to the Titaness who was the mother of Selene, the goddess of the Moon.

Background

During the formative years of the Solar System, planetary embryos known as protoplanets frequently collided and merged to form the larger planets we observe today. In this chaotic period, Theia is believed to have been a Mars-sized celestial body orbiting the Sun in a region close to the early Earth. Gravitational interactions likely destabilised its orbit, causing it to eventually collide with the proto-Earth.
The collision was immensely energetic, releasing enough heat to vaporise and melt significant portions of both planets. The impact ejected a vast quantity of material from the Earth’s outer layers and from Theia itself into orbit around the Earth. This debris gradually coalesced under gravity, eventually forming the Moon.

Characteristics and Composition

Theia is estimated to have been roughly 10 % of Earth’s mass, possibly similar in size to Mars. Its composition is presumed to have been silicate-rich, resembling that of Earth’s mantle. When the impact occurred, Theia’s metallic core may have merged with that of the Earth, contributing to the present size and density of Earth’s core. The remaining lighter material, composed primarily of silicates, formed the debris disk from which the Moon accreted.
One of the most significant pieces of evidence supporting the Theia hypothesis is the remarkable similarity between the isotopic compositions of lunar and terrestrial rocks. Samples of lunar soil brought back by the Apollo missions show nearly identical oxygen isotope ratios to those of Earth’s mantle, suggesting that the Moon formed largely from material originating from Earth and not from a distinctly foreign body.

The Giant Impact Hypothesis

The giant impact hypothesis explains several otherwise puzzling features of the Earth–Moon system:

• Relative Size of the Moon – The Earth has an unusually large moon compared to its size. A massive collision would account for the formation of such a large satellite.
• Low Iron Content of the Moon – The Moon’s relatively small iron core suggests that the metallic components of Theia merged with Earth, leaving the Moon deficient in heavy elements.
• High Angular Momentum – The impact would have imparted the necessary rotational energy to produce the current angular momentum of the Earth–Moon system.

Modern simulations indicate that the Moon could have formed rapidly—perhaps within hours or days after the collision—through the accretion of molten and vapourised rock from the debris disk.

Geological Evidence and Remnants

Recent geophysical studies propose that remnants of Theia may still exist deep within Earth’s interior. Seismic studies have revealed large, dense regions at the base of the mantle, known as large low-velocity provinces (LLVPs). Some scientists hypothesise that these could be ancient fragments of Theia’s mantle that were absorbed into Earth after the collision.
Moreover, isotopic analyses of rocks from both Earth and the Moon continue to provide clues about the degree of mixing and the precise nature of the impact. The presence of identical tungsten and oxygen isotope signatures suggests that Theia’s material was extensively blended with that of Earth during the collision.

Alternative Theories and Criticisms

While the Theia hypothesis remains the most widely accepted explanation for the Moon’s origin, several aspects continue to be debated:

• Isotopic Uniformity – If Theia originated from a different region of the solar nebula, it should have had a distinct isotopic signature. The nearly identical isotopic ratios between the Earth and Moon require either complete mixing during the impact or an improbable similarity between both bodies before the collision.
• Impact Conditions – The exact angle, speed, and size of the impactor are uncertain. Various models suggest glancing or head-on impacts, each producing different results for Earth’s tilt, rotation, and Moon’s composition.
• Alternative Models – Some newer models propose multiple smaller impacts, or a transient structure known as a synestia, in which the Earth temporarily became a vast rotating cloud of molten rock and vapour before the Moon condensed out of it.

Despite these debates, the Theia hypothesis remains the most comprehensive model consistent with the available physical and chemical evidence.

Broader Implications

The idea of Theia has far-reaching implications for understanding planetary formation and evolution:

• It illustrates the violent and dynamic processes that shaped the early Solar System, where planetary collisions were common.
• It provides insight into why Earth possesses its unique combination of size, core composition, and axial tilt.
• It offers an explanation for the Moon’s crucial role in stabilising Earth’s rotation and climate over geological time.
• It guides research into exoplanetary systems, where similar moon-forming collisions might occur.