How asteroid, comet strikes may have delayed evolution of the atmosphere

The Southwest Research Institute team, led by the Southwest Research Institute, updated planetary bombardment model with the most recent geologic information. They then applied the models to determine how impacts might have affected oxygen levels in Earth's atmosphere in Archean eon 2.5 to 4 billion year ago. This illustration shows how large asteroids can penetrate Earth's oxygen-poor atmosphere. Credit: SwRI/Dan Durda; Simone Marchi
Earth's weather was cloudy and prone to asteroid impact between 2.5 billion and 4 billion years ago. This time is known as the Archean Eon.

It was not unusual for comets or asteroids to strike Earth back then. The earliest planet's atmosphere was altered by the impact of comets that were larger than six miles in diameter. Geologists have accepted this information. However, it is not clear how many of these large asteroids would impact the planet and how the oxygen levels will be affected by their impacts. Researchers now believe they have the answers.

Nadja Drabon is a Harvard assistant professor in Earth and Planetary Sciences. She was part of a team which analysed the remains of ancient asteroids. They then modeled the effects of collisions. This revealed that strikes occurred more frequently than previously thought. It may also have delayed the time when oxygen began to accumulate on the planet. These models will help scientists better understand the exact moment when the planet began its journey to becoming the Earth we know today.

Drabon stated that oxygen is essential for all living things that use respiration to generate energy. "We wouldn't exist if there wasn't enough oxygen in the atmosphere."

Simone Marchi, scientist at Boulder's Southwest Research Institute, led the work.

The Southwest Research Institute's team has updated the planetary bombardment models to better understand how large impacts such as this one may have affected oxygen levels on Earth's surface in the Archean Eon, 2.5 to four billion years ago. Credit: SwRI/Simone Marchi

Researchers found that existing planetary bombardment models underestimate the frequency of asteroids or comets hitting Earth. New collision rates suggest that impactors could hit Earth roughly every 15,000,000 years. This is about 10 times more than the current models.

After examining records of seemingly ordinary pieces of rock, scientists discovered this. These ancient pieces of rock are impact spherules. They were formed during fiery collisions when large asteroids and comets hit the planet. The impact energy melted and vaporized the Earth's crust rocky materials, causing them to explode in a huge plume. The small droplets of molten rocks in the cloud would then condense, solidify and fall back to Earth as sand-sized pieces that would settle onto the Earth's crust. These layers are often only a few inches thick and are difficult to locate.

Drabon stated that you don't need to do any long hikes. You just look at the rocks and try to find impact particles. They can be easily overlooked.

A SwRI-led study has updated bombardment models using small glassy particles known as impact spherules. These tiny particles populate many thin, discrete layers of the Earth's crust and range in age from 2.4 to 3 billion years. Ancient collisions are indicated by spherule layers like the one in this 5-centimeter, 2.5-billion-year old sample from Australia. Credit: UCLA/Scott Hassler, Oberlin/Bruce Simonson

Drabon is one of many scientists who have benefited from this. She said that evidence has been discovered for additional impacts over the past few years that were not previously recognized.

These new spherule layer increased the number of impact events that were recorded during the early Earth. The Southwest Research Institute team was able to update their bombardment models and discovered that the collision rate was underestimated.

The researchers then created models of how all these impacts might have affected the atmosphere. The accumulated effects of meteorite impact by objects greater than six miles likely created an oxygen sink, which sucked most oxygen from the atmosphere.

These findings are consistent with the geological record which shows that oxygen levels varied in the atmosphere but remained relatively low during the Archean eon. This was true until the end of the bombardment, which occurred around 2.4 billion year ago. A major shift in the Earth's surface chemistry was then triggered by an increase in oxygen levels, known as the Great Oxidation Event.

Marchi stated in a statement that "as time passed, collisions became progressively less frequent and smaller to be able alter post-GOE oxygen levels." "The Earth was on its way to becoming the current planet."

Drabon stated that the next steps in this project are to put their modeling skills to the test and see what they can model in rocks.

"Can we trace the oxygen sucked from the atmosphere in rock records?" Drabon wondered.

Explore more Early Earth was bombarded with a series of asteroids the size of a city.

More information: Simone Marchi, Delayed, and variable late Archaean atmospheric oxygenation due to high collisions rates on Earth, Nature Geoscience (2021). Journal information: Nature Geoscience Simone Marchi, Delayed and variable late Archaean atmospheric oxidation due to high collision rates on Earth,(2021). DOI: 10.1038/s41561-021-00835-9