Asteroid impacts may have buried Earth's earliest rocks
August 04, 2014
New research indicates that more than four billion years ago giant asteroid impacts heavily “reprocessed” or melted, mixed, and buried Earth's surface.
The study focuses on changes in the uppermost layers of the early Earth during the geologic eon called the “Hadean,” about 4 to 4.5 billion years ago.
Researchers from institutions including NASA's Solar System Exploration Research Virtual Institute at the agency's Ames Research Center in Moffett Field, Calif. published their findings in a paper in the July 31 issue of the journal Nature.
“A large asteroid impact could have buried a substantial amount of Earth's crust with impact-generated melt,” said Yvonne Pendleton, director of the institute. “This new model helps explain how repeated asteroid impacts may have buried Earth's earliest and oldest rocks.”
Planet formation models indicate Earth went through a sequence of major growth phases: initially accretion of planetesimals planetary embryos over many tens of millions of years, then a giant impact that led to the formation of our moon. Following this the models point to a “late bombardment” when giant asteroids several tens to hundreds of miles in size periodically hit ancient Earth, dwarfing the one that much later killed the dinosaurs, an object estimated to be six miles wide.
Researchers estimate this bombardment contributed less than one percent of Earth's present-day material, but still had a profound effect. Prior to four billion years ago Earth would have been “resurfaced” repeatedly by impact-caused melting. Furthermore, large collisions may have repeatedly boiled away existing oceans into steamy atmospheres. Despite this, the findings are said to be compatible with the claim of liquid water on Earth's surface as early as about 4.3 billion years ago based on geochemical data.
The new research reveals that asteroidal collisions not only severely altered the geology of the Hadean eon Earth, but likely also played a major role in the subsequent evolution of life.
“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, senior researcher at the Southwest Research Institute in Boulder, Colorado, and the paper's lead author. “The new picture of the Hadean Earth emerging from this work has important implications for its habitability.”
The researchers found that on average, Hadean Earth could have been hit by one to four impactors that were more than 600 miles wide and capable of global sterilization, and by three to seven impactors more than 300 miles wide and capable of global ocean vaporization.
“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 likely needed to be resistant to high temperatures, and could have survived such a violent period... by thriving in niches deep underground or in the ocean's crust.”
Source : www.world-science.net
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