Our planet was an asteroids punching bag during a time in Earth's history that was between 2.5 and 4 billion years ago.
Earth was pelted with large-sized space rocks during this period, which is quite a contrast to our current relative peace. Although this activity could have caused significant changes in the chemistry of Earth's atmosphere, the extent and form of these alterations, particularly the impact on oxygen levels, is difficult to quantify.
A study of tiny, once-molten particle in Earth's crust revealed that there were more asteroid impacts than we thought. This may have led to a delay in the oxygenation of Earth’s atmosphere.
These particles are known as impact spherules. They're formed when an asteroid hits Earth. The intense heat causes the crust to melt and then spray into the atmosphere. The material forms a layer in the crust of the planet's surface when it cools and settles.
These spherules were discovered in more drill cores and excavations in recent years. This suggests that the asteroid collision rates may be 10x higher than previously thought. This would have had a greater impact on Earth's oxygen levels that previous models.
Simone Marchi, planetary geologist at the Southwest Research Institute, says that current bombardment models underestimate the number late Archean-era spherule layer count. "[This suggests] that the impactor flux was at that time up to 10x higher than previously believed."
All this extra rock from outer space creates chemistry which results in more oxygen being retained from the atmosphere.
Understanding planet habitability is fundamentally dependent on understanding how, when and why Earth's atmosphere was rich in oxygen. The oxygen-rich atmosphere is essential for multicellular organisms. Without it, most of them wouldn't survive.
However, oxygen levels didn’t begin to rise in the Great Oxidation Event (or as we refer it) until the appearance of photosynthesizing Cyanobacteria 2.4 billion years ago.
The team's new analysis suggests that steroid bombardment could have been one mechanism to prevent oxygen levels rising. Space rocks repeatedly collided into Earth's surface, causing their impact vapors to remove the small amount of oxygen in the early atmosphere.
Laura Schaefer, an astronomer and geologist at Stanford University said that "Late Archean bombardment of objects larger than 6 miles in diameter would produce enough reactive gases to completely absorb low levels of atmospheric oxygen."
This pattern is consistent with evidence of so-called "whiffs" of oxygen, relative steep but temporary increases in atmospheric oxygen around 2.5 billion years ago.
"We believe that the whiffs were destroyed by impacts that removed oxygen from the atmosphere. This is consistent in the large impacts that were recorded by spherule layer in Australia's Bee Gorge or Dales Gorge.
The new analysis of spherule layer by the team challenges existing impact models. It also scales up intensity of collisions. This means that an asteroid larger then 10 km (6 miles) would have struck Earth once every 15,000,000 years.
Although it may seem rare, this is a significant number of large asteroids, and 10 times more common than we thought.
The cumulative oxygen sink effect from these impacts was then shown by modeling. After bombardment had slowed down, oxygen levels started to rise. This changed the Earth's surface chemistry, and transformed it into a habitable planet. Researchers now believe this is not an accident.
Marchi stated that "Impact Vapours Caused Severe Low Oxygen Levels for Large Spans of Time Prior to the Great Oxidation Event."
"As the years passed, collisions became less frequent and smaller to have an impact on post-Great Oxidation Event oxygen levels. The Earth was well on its way to becoming the planet it is today.
Nature Geoscience published the research.
