Artistic rendering illustrates large asteroids penetrating Earth’s oxygen-poor atmosphere.
Credit: SwRI/Dan Durda, Simone Marchi


It is a messy business – asteroid bombardment of the Earth.

The Earth was a “full stop” planet to a substantial number of large impacts throughout the late Archean era. Around 2.4 billion years ago, during the tail end of this bombardment, the Earth went through a major shift in surface chemistry triggered by the rise of atmospheric oxygen, dubbed the Great Oxidation Event (GOE).

A team led by Southwest Research Institute (SwRI) has updated its asteroid bombardment model of the Earth with the latest geologic evidence of ancient, large collisions.

These new findings correspond to the geological record, which shows that oxygen levels in the atmosphere varied but stayed relatively low in the early Archean eon.

SwRI-led study updated bombardment models based on small glassy particles, known as impact spherules, that populate multiple thin, discrete layers in the Earth’s crust, ranging in age from about 2.4 to 3.5 billion years old. Spherule layers — such as the one shown in this 5-centimeter, 2.6-billion-year-old sample from Australia — are markers of ancient collisions.
Credit: UCLA/Scott Hassler and Oberlin/Bruce Simonson

Oxygen scarcity

Impacts by bodies larger than six miles (10 kilometers) in diameter may have contributed to its scarcity, as limited oxygen present in the atmosphere of early Earth would have been chemically consumed by impact vapors, further reducing its abundance in the atmosphere, according to a SwRI statement.

“Impact vapors caused episodic low oxygen levels for large spans of time preceding the GOE,” said SwRI’s Simone Marchi, lead author of a paper about this research in Nature Geoscience. “As time went on, collisions become progressively less frequent and too small to be able to significantly alter post-GOE oxygen levels. The Earth was on its course to become the current planet.”

Droplets of molten rocks

When large asteroids or comets struck early Earth, the energy released melted and vaporized rocky materials in the Earth’s crust.

Small droplets of molten rock in the impact plume would condense, solidify and fall back to Earth, creating round, globally distributed sand-size particles.

Known as impact spherules, these glassy particles populated multiple thin, discrete layers in the Earth’s crust, ranging in age from about 2.4 to 3.5 billion years old. These Archean spherule layers are markers of ancient collisions.

Credit: NASA


Free oxygen

Nadja Drabon, a Harvard assistant professor of Earth and planetary sciences, was part of a team that analyzed remnants of ancient asteroids and modeled the effects of their collisions to show that the strikes took place more often than previously thought and may have delayed when oxygen started accumulating on the planet.

The new models can help scientists understand more precisely when the planet started its path toward becoming the Earth we know today.

“Free oxygen in the atmosphere is critical for any living being that uses respiration to produce energy,” Drabon said in a Harvard statement. “Without the accumulation of oxygen in the atmosphere we would probably not exist.”

To access the new paper – “Delayed and variable late Archaean atmospheric oxidation due to high collision rates on Earth,” go to:

Also, go to this SwRI press release —

“SwRI-led team produces a new Earth bombardment model – New model applied to understand how oxygen levels in Earth’s atmosphere evolved,” at:

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