Giant impacts are cosmic collisions between celestial bodies on a massive scale, capable of profoundly altering one or both objects. Such impacts involve immense energy release, leading to significant changes in a planet’s structure, composition, and dynamics.
The Moon’s Origin
The leading scientific explanation for the Moon’s formation is the Giant Impact Hypothesis. This theory suggests that approximately 4.5 billion years ago, a Mars-sized protoplanet named Theia collided with the early Earth.
This colossal collision would have released an enormous amount of energy, causing both Theia and the early Earth to partially melt and eject a vast amount of material into orbit around Earth. This ejected debris then coalesced to form the Moon. Scientists believe the Moon formed relatively quickly after the impact, possibly within a few hours.
Evidence supporting the Giant Impact Hypothesis includes the Moon’s similar orientation in orbit to Earth’s rotation. The stable isotope ratios of lunar and terrestrial rocks are also nearly identical, suggesting a common origin for their materials. This similarity is particularly significant because Earth is isotopically distinct from other bodies in the solar system, yet nearly identical to the Moon.
The Earth-Moon system possesses an unusually high angular momentum. Lunar samples indicate that the Moon was once molten to a substantial depth. Recent research has also identified two large low-shear-velocity provinces (LLSVPs) deep within Earth’s mantle, which are suspected to be sunken remnants of Theia’s mantle.
Shaping the Solar System
Giant impacts extend beyond the Earth-Moon system, shaping the planets we observe today. Mercury, for instance, has an unusually large iron core, making up an estimated 60% of its mass, compared to about 30% for Earth, Venus, and Mars. One theory suggests this disproportionate core size resulted from a massive impact that stripped away much of its original rocky mantle.
Mars also bears the scar of a likely giant impact in its northern hemisphere, known as the Borealis basin. This immense, flat area covers a vast area and is believed to have formed from a collision with a large object early in Mars’ history. The resulting elliptical shape of this basin was identified through complex analysis of Martian topography and gravity data.
Uranus presents another compelling case with its extreme axial tilt of nearly 98 degrees, causing it to orbit the Sun on its side. The prevailing hypothesis attributes this unusual orientation to a massive collision with a protoplanetary body in the early Solar System, potentially an object twice the mass of Earth or larger. Some studies also suggest a series of at least two smaller impacts could have caused this tilt.
The formation of the Pluto-Charon binary system also points to a giant collision in the outer solar system. While earlier models suggested a destructive impact that melted and mixed the bodies like a lava lamp, newer simulations propose a gentler “kiss-and-capture” mechanism. In this scenario, two icy worlds collided and temporarily stuck together, rotating as a snowman-shaped object before separating into their current orbiting configuration, largely preserving their original compositions.
Detecting Cosmic Collisions
Scientists employ various methods to uncover evidence and confirm these ancient, massive collisions. Analyzing lunar samples brought back by missions like Apollo provides chemical and isotopic clues. The composition of Moon rocks shares similarities with Earth’s crust, supporting a shared origin.
Computer simulations play a role in recreating impact scenarios and predicting their outcomes. Researchers run simulations, varying factors such as size, speed, composition, and impact angles of colliding objects, to understand how atmospheres are affected or how planetary bodies form. These models can also help determine if a collision would lead to atmospheric loss, gain, or even the complete obliteration of a planet.
Interpreting large-scale surface features on planets and moons also provides strong geological and topographical evidence of past impacts. Features like vast basins, unusual axial tilts, and differences in hemispheric terrains can be remnants of ancient collisions.
Studying cratering records on celestial bodies helps scientists understand the history of bombardment across the solar system. By analyzing the size, number, and erosion levels of impact craters, researchers can estimate the ages of planetary surfaces and infer periods of intense bombardment. The Moon’s surface, largely untouched by erosion, serves as a time capsule, preserving a detailed record of impacts that helps to understand Earth’s own bombardment history.