Artist’s impression of the early Earth. Credit: SwRI/Simone Marchi, Dan Durda.

The evolution of early Earth was significantly influenced by collisions of planetary materials with Earth’s surface. It is generally accepted that these events modified the composition of Earth’s atmosphere and its solid components. Thanks to lunar data, the frequency of collisions is well constrained for Earth’s history 4.0 to 3.5 billion years ago, but the frequency of impacts 3.5 to 2.5 billion years ago is less certain. Consequently, the relationship between impactors and their effects on the evolution of Earth’s atmosphere during this era has not been explored in detail.

Simone Marchi at the Southwest Research Institute and colleagues sought to better constrain the amount of material delivered to Earth during the Archean Eon 4.0 to 2.5 billion years ago by constructing a bombardment model. The model considered projectiles coming from the main asteroid belt as well as leftover planetesimals in the terrestrial-planet region. The model was calibrated using terrestrial impact spherule layers that record about 20 large impacts 3.5 to 2.5 billion years ago. They concluded that during the Late Archean, the impactor flux was significantly higher than previously estimated.

The Late Archean preceded a major shift in Earth’s atmospheric oxygen content dubbed the “Great Oxygenation Event.” At this time, microscopic life emerged, causing Earth’s atmosphere to transform from oxygen-poor to oxygen-rich. This led the team to explore how the early impactor flux might have affected the oxygenation of the atmosphere. They determined that objects impacting Earth might have delayed the oxygenation of Earth’s atmosphere because the impactors likely consisted of materials that reacted with oxygen and removed it from the atmosphere. Additionally, they concluded that atmospheric oxygen levels could have fluctuated dramatically as these impact-induced reactions competed with microscopic life-generating oxygen. These results demonstrate how early planetary processes could have affected the long-term evolution of Earth’s atmosphere. READ MORE