artistic conception of the early Earth
August 1, 2014

Asteroid Collisions Had Major Impact On Surface Of Hadean Earth

redOrbit Staff & Wire Reports - Your Universe Online

Over four billion years ago, giant asteroid impacts collided with the Earth’s surface and gave the planet a facelift of sorts – mixing, burying and melting the landscape, according to a new terrestrial bombardment model devised by an international team of researchers from various academic and government institutions.

The new model was based on existing lunar and terrestrial data, and according to the authors of a new paper published this week in the journal Nature, it sheds new light on the role that asteroid bombardments played in the geological evolution of the uppermost layers of the Hadean Earth, approximately four to 4 1/2 billion years ago.

While the Earth is approximately 4.5 billion years old, Becky Oskin of LiveScience explained it is unusual to find rocks more than 3.8 billion years old. The new model suggests that older rocks might have been destroyed when the surface was hammered by asteroids and comets during an event known as the Late Heavy Bombardment.

Lead author Simone Marchi, a planetary scientist with the Southwest Research Institute (SwRI) Planetary Science Directorate in Boulder, Colorado, told Oskin that the Earth’s surface “was heavily affected by all these collisions,” and that there was “no doubt the crust was excavated, mixed and buried as a result of this bombardment.”

“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,” added Lindy Elkins-Tanton, director of the School of Earth and Space Exploration at Arizona State University, in a university statement.

The Hadean period marks the first 500 million years of the Earth’s formation, the researchers said. The name, which means “hell-like,” was given to this era because it is believed that it was extremely hot and the land was covered with magma due to volcanic activity.

According to terrestrial planet formation models, the Earth underwent a series of major growth phases: accretion of planetesimals and planetary embryos over tens of millions of years, then a massive impact that resulted in the formation of our moon, and then the Late Heavy Bombardment.

While the asteroid collisions during this event are believed to have contributed less than one percent of the modern-day mass of the Earth, those impacts nonetheless had a significant effect on the geological evolution of the planet early on in its life span, the researchers said. More than four billion years ago, it was resurfaced multiple times by impact-generated melt, and the collisions could have repeatedly caused oceans to boil away into the atmosphere.

“Despite the heavy bombardment, the findings are compatible with the claim of liquid water on Earth's surface as early as about 4.3 billion years ago based on geochemical data,” researchers from NASA’s Solar System Exploration Research Virtual Institute (SSERVI) at the Ames Research Center in California, said in a statement.

“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 on Earth as well,” the US space agency added. “Large impacts had particularly severe effects on existing ecosystems.”

In fact, the study authors reported that, on average, Hadean Earth could have been struck by as many as four impactors more than 600 miles wide and capable of global sterilization, as well as three to seven others at least 300 miles wide and capable of vaporizing the planet’s oceans.

“During that time, the lag between major collisions was long enough to allow intervals of more clement conditions, at least on a local scale,” Marchi explained. “Any life emerging during the Hadean eon likely needed to be 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.”

Image 2 (below): Spatial distribution and sizes of craters formed on the early Earth. Each circle indicates the final estimated crater size. Color-coding indicates the time of impact. Credit: Simone Marchi et al. 2014