Shaping of rocky planets traced to final stages of formation

Rocky planets form from a disk of dust and gas surrounding young stars. During this process, small rocky bodies merge into planetesimals and then into full-sized planets. The Earth, the researchers note, likely reached 99% of its mass within 60 to 100 million years of solar system formation, but the remaining 1%-the so-called "late accretion" phase-played a disproportionately large role in shaping planetary characteristics.

"We examined the disproportionate role late accretion - the final 1% of planetary growth - plays in controlling the long-term evolution of the Earth and other terrestrial planets," said lead author Dr. Simone Marchi of SwRI. "Differences in planets' late accretions may provide a rationale for interpreting their distinct properties. We made advances constraining the history of late accretions, using large-scale impact simulations and understanding the consequences of interior, crustal and atmospheric evolution."

The influx of geochemical data from both meteorites and Earth rocks has sharpened scientists' ability to trace planetary development. Planetary impacts, researchers found, influenced tectonic systems, volatile inventories, and atmospheric makeup. Venus and Earth's water and atmospheres, Mars' surface variability, and Mercury's dense core all reflect late-stage collisions.

"Impact histories should play a critical role in the search for habitable exoplanets like Earth," Marchi said. "The habitability of a rocky planet depends on the nature of its atmosphere, which is tied to plate tectonics and mantle outgassing. The search for Earth's twin might focus on rocky planets with similar bulk properties - mass, radius and habitable zone location - as well as a comparable collision history."

Geologic processes can obscure the impact record, but researchers use lunar data and dynamic modeling to reconstruct collision histories. Marchi noted that the behavior of impactor material offers key insights into a planet's crust and mantle evolution. By studying how certain metals partition during impacts, scientists can deduce when planetary layers formed.

Planetary atmospheres are also significantly influenced by impacts. Some collisions strip atmospheres, while others deliver volatiles like carbon and water. The resulting atmospheric makeup is critical to understanding a planet's potential to host life.

"These processes almost certainly played a role in the prebiotic chemistry of early Earth, but their implications in the origin of life remain a mystery," Marchi said.

Research Report:The shaping of terrestrial planets by late accretions