History of Extra-terrestrial Impacts Revealed in Ancient Sediments

Research by NASA and international scientists concludes giant asteroids, similar or larger than the one believed to have killed the dinosaurs, hit Earth billions of years ago with more frequency than previously thought.

Extra-terrestrial Impacts

To cause the dinosaur extinction, the killer asteroid that impacted Earth 66 million years ago would have been almost 6 miles (10 kilometres) in diameter.  By studying ancient rocks in Australia and using computer models, researchers estimate that approximately seventy asteroids the same size or larger impacted Earth 1.8 to 3.8 billion years ago.  During the same period, approximately four similarly-sized objects hit the moon.

The Archean Eon represents a period in the Earth’s history from approximately four billion to 2.5 billion years ago.  The Archean, in relation to other Eons assigned to the modern geological time-scale is the largest Eon in terms of the period of time covered.  It is also the most difficult to define, essentially the beginning of the Eon being set by geologists to mark the end of the so-called “Great Extra-terrestrial Bombardment, otherwise known as the Late Heavy Bombardment” and the end to coincide with the change in the fundamental nature of the planet’s atmosphere with the increase in oxygen levels.

Late Heavy Bombardment

NASA scientists collaborating with a number of other international research bodies have concluded that the “Great Extra-terrestrial Bombardment” may have been more intense than previously thought.

Commenting on the study, the paper having been published in the scientific journal “Nature”, Yvonne Pendleton, Director of NASA’s Lunar Science Institute based in California stated:

“This work demonstrates the power of combining sophisticated computer models with physical evidence from the past, further opening an important window to Earth’s history.”

Evidence for these impacts on Earth comes from thin rock layers that contain debris of nearly spherical, sand-sized droplets called spherules.  These millimeter-scale clues were formerly molten droplets ejected into space within the huge plumes created by mega-impacts on Earth.  The hardened droplets then fell back to Earth, creating thin but widespread sedimentary layers known as spherule beds.

William Bottke of the Southwest Research Institute, in the aptly named Boulder (Colorado), said:

“The beds speak of an intense period of bombardment of Earth.  Their source long has been a mystery.”

The team’s findings support the theory Jupiter, Saturn, Uranus and Neptune formed in different orbits nearly 4.5 billion years ago, migrating to their current orbits about 4 billion years ago from the interplay of gravitational forces in the young solar system.  This event triggered a solar system-wide bombardment of comets and asteroids, the young planet Earth and its recently formed satellite we now call the moon were not spared and suffered from this barrage as space rocks were sent hurtling towards the inner solar system.

In the paper, the team created a model of the ancient main asteroid belt and tracked what would have happened when the orbits of the huge, gas-giant planets changed.  They discovered the innermost portion of the belt became destabilised and could have delivered numerous big impacts to Earth and the moon over long time periods.

Twelve Mega-impacts

The team has concluded that at least twelve mega-impacts produced spherule beds during the so-called Archean period 2.5 to 3.7 billion years ago, a formative time for life on Earth.  Ancient spherule beds are rare finds, rarer than rocks of any other age.  Most of the beds have been preserved amid mud deposited on the sea floor below the reach of waves.

The impact believed to have killed the dinosaurs was the only known collision over the past half-billion years that made a spherule layer as deep as those of the Archean period.  The relative abundance of the beds supports the hypothesis for many giant asteroid impacts during Earth’s early history.

The frequency of the impacts indicated in the computer models matches the number of spherule beds found in terrains with ages that are well understood.  The data also hint at the possibility that the last impacts of the Late Heavy Bombardment on Earth made South Africa’s Vredefort crater and Canada’s Sudbury crater, both of which formed about two billion years ago.

Bruce Simonson, a geologist from Oberlin College (Ohio) stated:

“The Archean beds contain enough extraterrestrial material to rule out alternative sources for the spherules, such as volcanoes.”

The impact study team also included scientists from Purdue University in West Lafayette, Indiana.; Charles University in Prague, (Czech Republic); Observatorie de la Cote d’Azur in Nice, France; and Academia Sinica in Taipei, Taiwan.

Sourced from a Press Release – NASA

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