In a new paper published in Nature Communications, Kentaro Terada from Osaka University in Japan and colleagues report on their analysis of impact craters on the Moon as revealed by the Kaguya lunar orbiter. They found that many of the impacts occurred around the same time, about 800 million years ago. After first calculating the total asteroid mass that hit the Moon, they extrapolated the amount that also would have hit Earth, with its much stronger gravity. Altogether, they estimate that some 40 to 50 trillion tons of asteroid material must have impacted our planet. To put this into perspective, it’s 30 to 60 times greater than the mass of the Chicxulub impactor that wiped out the non-avian dinosaurs 65 million years ago.
The timing of the impacts suggests that they caused a period of extreme cold known as the Cryogenian. This era is known for several “Snowball Earth” events when all, or nearly all, of our planet was covered by snow and ice, perhaps with a thickness of up to two kilometers. The so-called Sturtian glaciation was the first of these events, and it kept Earth in deep freeze between 717 and 643 million years ago. The Sturtian was also the longest and most intense of the Snowball Earth events, and it matches most closely the date Terada et al. have estimated for the lunar impacts.
Even though the timing may seem a bit off, all these dates have large error margins, and from later asteroid impacts we know that impacts kick up huge amounts of dust that trigger subsequent icehouse conditions, as happened during the Ordovician period several hundred million years later.
We have little idea what caused the asteroid impacts 800 million years ago. Craters from these events are no longer visible on Earth due to the constant reworking of our planet’s surface by processes ranging from tectonic shifts to erosion. We can say, however, that this kind of asteroid pummeling must have been catastrophic to the ecology of early Earth and its biosphere, which was mostly microbial at the time, but already included the first animals, such as sponges.
Intriguingly, these impacts might not have been all bad for life on Earth. The findings by Terada’s science team suggest that about 100 billion tons of phosphorus were brought to Earth by those asteroids 800 million years ago, and we know how important that element is for life. In fact, what seems like a catastrophe may very well have been critical for creating the rich and diverse biosphere that developed later on, after the shock of the impacts had long subsided.