Around 800 million years ago, Earth was bombarded by meteorite fragments belonging to a huge, disintegrated 100-km-wide asteroid. The collective force of impact is believed to have been 30 to 60 times more than the Chicxulub impact — the asteroid that hit our planet off the coast of Mexico and wiped out the dinosaurs in one swift blow at the end of the Cretaceous.
However, you wouldn’t have any way of knowing this, as any crater older than 600 million years has been erased by the relentless grinding and sweeping of erosive forces. But the moon, which has such a minimal atmosphere it virtually doesn’t exist, still bears the pockmarks of this ancient onslaught that affected both Earth and its satellite.
In a new study published in Nature Communications, Japanese researchers at Osaka University describe the age and formation of 59 lunar craters with a diameter greater than 20 kilometers.
The humongous craters were imaged by the Terrain Camera (TC) onboard the lunar orbiter spacecraft Kaguya, built by the Japanese Space Agency (JAXA). But in order to determine their age, the researchers led by Kentaro Terada, professor at the Department of Earth and Space Science at Osaka University, had to examine the density of relatively small craters in the ejecta of the larger craters.
I said “relatively small” because some of the craters could be as large as 1 km in diameter. But that pales in comparison to the Copernicus crater, which is 93 km in diameter. For the Copernicus crater alone, the researchers were able to identify 860 smaller craters with a diameter of 0.1-1 km.
“In this study, we used the Moon as “a witness to the history of the solar system”, because the moon surface has no erosion and well preserves the impact history of Earth-Moon system. This is a “quite new idea”, shedding light on the veiled impact history of the Earth before 600 million years,” Terada told ZME Science.
Overall, 8 of 59 craters that the researchers examined were formed simultaneously. Based on crater scaling laws and collision probabilities, Terada and colleagues estimate that up to 5 trillion tones worth of meteoroids impacted the Earth-Moon system immediately before the Cryogenian (720-635 million years ago).
“Since I have looked at the data that indicated asteroid shower on the Earth-Moon system, I have been excited, and I wanted to publish this exciting new insight to all over the world as soon as possible. But it took two years since my first submit until publish. My biggest challenge was to persuade the reviewers,” Terada said as he remembered the struggles of publishing such an innovative technique.
According to Terada, the meteoroid shower was generated by the collision and disruption of two big asteroids with a diameter greater than 100 kilometers. For comparison, the devastating Chicxulub impact that led to the extinction of 75% of all animal and plant species more than 65 million years ago was generated by an asteroid 10 to 15 km in diameter.
Some of these fragments fell on Earth and the Moon, but also other terrestrial planets in the solar system, as well as the sun itself. Other fragments strayed into the asteroid belt, where they still roam today as part of the Eulalia family. Other remnants entered an orbital evolution as near-Earth asteroids, like Ryugu and Benny, which show rubble-pile structures. In 2018, JAXA landed two small rovers and a probe on the surface of Ryugu — marking an unprecedented feat.
Snowball Earth and life thereafter
After this massive meteorite shower bombed the Earth-Moon system, around 700 million years ago, the planet turned into a snowball. Virtually all of Earth’s surface became engulfed in polar ice sheets and the oceans turned to slush. Massive volcanic eruptions then spewed so much greenhouse gases into the atmosphere that Earth became so hot oceans were near their boiling point. Then, the ash-darkened skies triggered global cooling again, and Earth returned to its snowball form yet again.
It’s rather difficult to imagine a more inappropriate time for the tiny single-celled and multicellular organisms that came into existence. But oddly enough, this difficult geological period, known as the Cryogenian, coincides with the moment complex animal life evolved. Out of the alternating ice and inferno sprang the first complex creatures that would eventually evolve into jellyfish and corals, snails, fish, dinosaurs, birds, and, at some point, humans.
Where do these 800-million-year-old impacts fit into all of this? The study suggests that the meteorites deposited 100 billion tonnes of phosphorus on Earth, “which is one order of magnitude higher than the total phosphorus amount of the modern sea (assuming that the volume of modern seas is 13.5×10^8 km3 and the concentration of P is approximately 3 μg/litre),” Terada told ZME Science.
“Interestingly, Reinhard et al. (Nature 2017) found that the average P content of late Tonian samples is more than four times greater than that of pre-Cryogenian samples and noted that a fundamental shift in the phosphorus cycle may have occurred during the late Proterozoic Eon after 800 Ma (until 635 Ma),” the Japanese researcher added.
While there’s no way of telling at this point how the course of life was impacted by this ancient meteorite shower, the extraterrestrial phosphorus may have seeded enough nutrients to kick start the evolution of complex life.
“In general, large-scale changes in marine biogeochemical cycles are undoubtedly forced by tectonic and magmatic processes and chemical weathering of the continental crust, but our new finding suggests that the flux of extra-terrestrial bioavailable elements might also have influenced marine biogeochemical cycles,” Terada said.
“I am glad that this research will lead to the advancement of earth science from the point of view that environmental changes 800 million years ago may have been caused by extraterrestrial factors,” he concluded.