Building Earth Through Cosmic Collisions: How Giant Impacts Shaped Rocky Planets
Haruya Maeda and Takanori Sasaki of Kyoto University investigate how Earth-like planets form through violent collisions and chemical reactions. Their study combines computer simulations of planetary crashes with models of how molten rock, atmospheres, and planetary cores exchange elements. The central question: how did repeated giant impacts, happening while the early Solar System’s gas disk was fading away, set the stage for Earth’s current composition?
The Challenge of Making Earth
In the early Solar System, small rocky bodies called protoplanets smashed together to build larger worlds. Each collision released enough energy to melt the surface, creating a global “magma ocean.” At the same time, protoplanets could capture hydrogen-rich atmospheres from the surrounding disk of gas. Past studies showed that these molten layers and captured gases enabled important chemical reactions, such as forming water or transporting hydrogen into the iron core. But a puzzle remains: how do multiple giant impacts, occurring as the disk gas gradually disappears, affect the balance of elements that define Earth’s structure?
Modeling Collisions and Chemistry
Maeda and Sasaki built two models. First, they used “N-body” simulations, which track the gravitational interactions of many protoplanets over millions of years. These simulations showed how 21 small protoplanets around 1 AU (Earth’s distance from the Sun) gradually merged into a handful of rocky planets. Second, they added chemical equilibrium calculations, which simulate how hydrogen, oxygen, and iron exchange between the atmosphere, magma ocean, and core after each giant impact. Together, these models followed not just the physical growth of planets, but also their chemical evolution through time.
Too Much Hydrogen, Too Soon
The results showed that the timing of impacts is crucial. Early collisions, when the gas disk was still dense, allowed protoplanets to acquire thick hydrogen-rich atmospheres. Much of this hydrogen was pulled into their iron cores, leaving them with “density deficits” larger than Earth’s. But if planets formed solely this way, they would end up too hydrogen-rich compared to our Earth. On the other hand, if impacts occurred too late, after the disk gas had nearly vanished, there wasn’t enough hydrogen left to match Earth’s current composition.
A Balancing Act Through Late Impacts
The key, the authors argue, lies in late giant impacts. These later collisions, occurring after the gas disk had mostly dispersed, could strip away excess hydrogen and mix hydrogen-rich protoplanets with hydrogen-poor ones. This process reduced the overall hydrogen in the core, bringing the final planet’s interior closer to Earth’s observed density. In some simulations, collisions produced planets at around 1 AU with masses and chemical compositions remarkably similar to Earth, suggesting this sequence of early “overloading” and later “balancing” may explain our planet’s properties.
Why This Matters
The study emphasizes that forming an Earth-like planet is not just about smashing rocks together, it is about when those collisions happen relative to the fading of the Solar System’s gas. Hydrogen, oxygen, and other elements exchanged between molten layers and atmospheres played a decisive role in shaping Earth’s water supply and the structure of its core. Maeda and Sasaki’s integrated approach shows that Earth may be the product of both chaotic impacts and delicate chemical timing.
Source: Maeda
