New research shows that more than four billion years ago, giant asteroid impacts heavily reprocessed — or mixed, buried, and melted — Earth’s surface. A new terrestrial bombardment model based on existing lunar and terrestrial data sheds light on the role asteroid bombardments played in the geological evolution of the uppermost layers of Earth during the Hadean period (approximately 4 to 4.5 billion years ago).
“When we look at the present day, we have a very high fidelity timeline over the last about 500 million years of what’s happened on Earth, and we have a pretty good understanding that plate tectonics and volcanism and all these kinds of processes have happened more or less the same way over the last couple of billion years,” said Lindy Elkins-Tanton, director of the School of Earth and Space Exploration at Arizona State University in Tempe.
But, in the very beginning of Earth’s formation, the first 500 million years, there’s a less well-known period that has typically been called the Hadean (meaning “hell-like”) because it was assumed that it was wildly hot and volcanic, and magma covered everything was — completely unlike the present day.
Terrestrial planet formation models indicate Earth went through a sequence of major growth phases. These included accretion of planetesimals and planetary embryos over many tens of millions of years; a giant impact that led to the formation of our Moon; and then the late bombardment, when giant asteroids, dwarfing the one that presumably killed the dinosaurs much later, periodically hit ancient Earth.
While researchers estimate accretion during late bombardment contributed less than 1 percent of Earth’s present-day mass to the planet, giant asteroid impacts still had a profound effect on the geological evolution of early Earth. Prior to four billion years ago, voluminous impact-generated melt resurfaced our planet. Furthermore, large collisions as late as about four billion years ago may have repeatedly boiled away existing oceans into steamy atmospheres. Despite heavy bombardment, the findings are compatible with the claim based on geochemical data that liquid water on Earth’s surface existed as early as about 4.3 billion years ago.
A key part of Earth’s mysterious infancy period that has not been well quantified in the past is the kind of impacts Earth was experiencing at the end of accretion. How big and how frequent were those incoming bombardments, and what were their effects on Earth’s surface? How much did they affect the ability of the now cooling crust to actually form plates and start to subduct and make plate tectonics? What kind of volcanism did Earth produce that was different from volcanoes today?”
“We are increasingly understanding both the similarities and the differences to present day Earth conditions and plate tectonics,” said Elkins-Tanton. “And this study is a major step in that direction, trying to bridge that time from the last giant accretionary impact that largely completed Earth and produced the moon, to the point where we have something like today’s plate tectonics and habitable surface.”
The new research reveals that asteroidal collisions not only severely altered the geology of Earth in the Hadean period, but likely played a major role in the subsequent evolution of life on the planet as well.
“Prior to approximately four billion years ago, no large region of Earth’s surface could have survived untouched by impacts and their effects,” said Simone Marchi of NASA’s Solar System Exploration Research Virtual Institute at the Southwest Research Institute in Boulder, Colorado. “The new picture of the Hadean Earth emerging from this work has important implications for its habitability.”
Large impacts had particularly severe effects on existing ecosystems. Researchers found that on average, Hadean Earth could have been hit by one to four impactors that were more than 600 miles (1,000 kilometers) wide and capable of global sterilization, and by three to seven impactors more than 300 miles (500km) wide and capable of global ocean vaporization.
“During this time, the lag between major collisions was long enough to allow intervals of more clement conditions, at least on a local scale,” said Marchi. “Any life emerging during the Hadean eon was likely resistant to high temperatures, and could have survived such a violent period in Earth’s history by thriving in niches deep underground or in the ocean’s crust.”
