Tim Johnson, a Professor in Curtin University’s School of Earth and Planetary Sciences and its Curtin Frontier Institute for Geoscience Solutions, led a 2026 study in *Science* revealing that asteroid impacts melted Earth’s early crust, explaining the scarcity of Hadean-era rocks and the delayed formation of continents. His team’s models showed that impact heating dominated over internal planetary heat, creating a molten, silica-rich crust that recycled into the mantle, with material sinking at least 600 kilometers deep. This process, they argue, erased most of the Hadean crust, while hydrothermal systems from impacts may have fostered life’s emergence.
The Crust That Never Settled
Johnson’s research, published in *Science* and titled “Impact heating and the hidden Hadean” (DOI: 10.1126/science.aeb5402), challenges long-held assumptions about Earth’s early history. By simulating impact-driven melting, his team found that the Hadean crust—formed after Earth’s initial cooling—was repeatedly recycled into the mantle, preventing the preservation of ancient rocks. “We need to start taking seriously the outputs of these models rather than just say, well, we can’t find any rocks, so let’s give up,” Johnson said. The study attributes the near-total absence of shock-deformed Hadean zircons to this intense melting, which absorbed and scattered shock waves before they left lasting deformation in surviving crystals. Source 1
This recycling explains why so little Hadean crust survived to the present. Johnson noted that “the Earth is extremely good at covering the tracks of its history,” but he remains optimistic about future discoveries. “I also know another group has found a rock which is possibly even older. Hopefully you will be able to read about it in the next couple of months,” he added. Source 1
Hydrothermal Havens and the Seeds of Life
While the Hadean crust melted, asteroid impacts also created hydrothermal systems that may have nurtured life. A 2026 study in *AGU Advances*, led by Amanda Alexander of the Southwest Research Institute, modeled how impacts fractured Earth’s crust, generating porous networks for water circulation. “A single large impact during the early Earth era could generate hydrothermal activity 100 times more powerful than the entire Yellowstone Park today,” Alexander noted. These systems, she argued, provided ideal conditions for prebiotic chemistry, with heat and minerals driving organic molecule formation. Source 3
The simulations revealed that Earth’s upper 8 kilometers remained permeable from 4.3 to 3.5 billion years ago, aligning with the emergence of life’s earliest traces. “The bombardment was a catastrophe from the perspective of dinosaurs, but it likely created the environment for prebiotic chemistry,” Alexander said. This contradicts the traditional view of impacts as purely destructive, instead framing them as catalysts for habitability. Source 4
Life’s Resilience in the Fire
Contrary to fears that impacts sterilized Earth, a study by Oleg Abramov of the University of Colorado Boulder suggests life could have thrived underground. Using computer models, Abramov’s team found that less than 25% of the crust melted during the Late Heavy Bombardment, leaving hydrothermal vents as refuges for hyperthermophilic bacteria. “Even under the most extreme conditions we imposed, Earth would not have been completely sterilized by the bombardment,” Abramov said. Source 6
These findings challenge the notion that the Hadean eon was a “hellish” period devoid of life. Instead, they suggest that life may have originated as early as 4.4 billion years ago, coinciding with the formation of Earth’s first oceans. “It opens up the possibility that life emerged as far back as 4.4 billion years ago,” Abramov added. This aligns with the idea that impacts, while violent, created the conditions necessary for life’s survival and evolution. Source 6
The Legacy of Cosmic Collisions
The interplay between impact-driven melting and hydrothermal activity reshaped Earth’s geological and biological trajectory. As impact heating waned around 3.9 billion years ago, the crust thickened, enabling plate tectonics and the formation of stable continents. “As soon as you can create thick crust and you can create a mantle lithosphere underneath, you can start building continents,” Johnson explained. This transition, he argued, “is likely not a coincidence” with the emergence of the Acasta Gneiss. Source 2
Looking ahead, the search for older rocks could further refine these models. Meanwhile, the role of impacts in life’s origin remains a focal point. “Because life could have originated or evolved in hydrothermal environments, it is important to understand and quantify the generation of these systems by impacts on the early Earth,” Alexander said. Source 5
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