Around 800 million years ago, a shower of small asteroids smacked into the Earth and the Moon. Given that they fell more or less at the same time, plausibly the cluster of asteroids was created by the disintegration of one or more bigger asteroids.
The scars of that celestial trauma have long disappeared on the geologically turbulent surface of Earth but remain starkly clear on the Moon, Kentaro Terada of Osaka University, Tomokatsu Morota of the University of Tokyo and Mami Kato of Nagoya University report in Nature Communications on Tuesday.
Each small asteroid, called a meteoroid, may have been puny, but their collective impact was anything but. Based on collision probabilities with the Earth and Moon and other calculations, the team estimates that Earth alone was hit by a total asteroid mass of 40 to 50 million billion kilograms.
That is between 30 to 60 times more than the mass of the asteroid that caused the Chicxulub crater and decimated the dinosaurs.
And all this happened just before the Cryogenian, aka the transition from a greenhouse planet to Snowball Earth – which might not be coincidence, Terada, professor of Isotope Cosmochemistry and Geochemistry at Osaka University, tells Haaretz.
As for the Moon, the team estimates that the total mass of the asteroid shower on that body extrapolates to an impactor 10 to 13 kilometers (6 to 8 miles) in diameter. If Tel Aviv was a circle, this would have been bigger.
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Studying geological phenomena on Earth has to start with spotting them. Over the eons, tectonic movements and erosion reshape the landscape. Even if it survives, an impact crater can be so big that it escapes notice. Chicxulub, for instance, is about 150 kilometers in diameter, and half of it lies under the sea. It took decades from the discovery of anomalies by geophysicists looking for oil in the late 1970s for the crater’s existence to be accepted.
Scars on the Moon are hardier, even though the Moon also experiences “weathering,” erosion and seismic spasms, it turns out. Seismometers placed by American astronauts on the lunar surface revealed moonquakes up to 5 on the Richter scale in the mantle and crust. They may be, it has been postulated, echoes from a particularly violent impact in the Moon’s early years, 4.3 billion years ago. And at least some moonquakes may stem from the Moon continuing to cool and contract.
But all in all, the lunar surface retains impact scars for eons, and now the Japanese team reports studying the formation ages of 59 large lunar craters with a diameter greater than 20 kilometers.
The study is based on images captured by the lunar orbiter spacecraft Kaguya (“Moon Princess”), launched by the Japan Aerospace Exploration Agency in 2007. The team also factored in data from the glass spherules that formed when meteorites slammed into silicate soil and were brought earthward by some of the Apollo missions.
Eight of the 59 lunar craters were created at about the same time, the team deduced. They postulate that a sporadic bombardment by meteoroids occurred across the whole Moon and dated the sequence of impacts to about 800 million years ago.
Truth be told, there is some discrepancy in the chronology. But the main point, the scientists explain, is this: “The most important of the new findings is that eight craters, including Copernicus, show identical relative ages based on a constant flux model.”
It seems the Earth, for one, has experienced this sort of thing before: almost half a billion years ago, apparently, an asteroid suffered an impact in outer space and broke up. Its debris caused a shower of meteors on Earth that lasted millions of years, and may have resulted in the Ordovician “icehouse conditions” and extinction event, which was followed by the great Ordovician biodiversity event.
And maybe it wasn’t the first time that happened. About 800 million years ago, the team estimates, a total asteroid mass of 40 to 50 million billion kilograms collided with the Earth, which likely would have had a dramatic result.
Colder, colder, freezing point
To estimate the date of the large impacts on the Moon, the team calculated the density of later impact marks between 100 meters to 1 kilometer “atop” the 59 large craters (i.e., in the ejecta).
One of the 59 is the famous Copernicus crater, around which the team detected and examined no less than 860 later craters, each 100 to 1,000 meters in diameter.
They concluded that eight of the 59 craters had formed at the same time, which means the Moon had been hit by an asteroid shower. If it had, the Earth had too.
The timing of this meteoroid shower was shortly before the onset of the Cryogenian era, which began about 720 million years ago.
The Cryogenian was apparently the most extreme ice age the planet has known, though scientists continue to argue over whether the early life-forms on the benighted planet were living on Snowball Earth – completely frozen over – or Slushball Earth, which retained some open ocean.
Either way, despite the inhospitable conditions, some unicellular life in the seas at least survived and the first known sponges, which are animals, appeared. Despite suspicions that the icebound oceans were woefully short of oxygen, fossils of early sponges appear between fossil bacterial mats from the period.
Until now, the thinking has been that the Cryogenian was caused by intense volcanism as the supercontinent Rodinia broke up. Now, based on the exquisite study by the Japanese team, a different theory may come to mind.
Could the meteoroid shower peppering Earth around 800 million years ago have kicked up enough dust or caused volcanism – or in some other form, caused the equivalent of an extremely protracted volcanic winter?
Maybe, maybe not, Terada answers. But it could be. “It is considered that Chicxulub impact formed the dusty atmosphere over years, causing the darkness and cooling 66 million years ago,” he tells Haaretz.
Also, he adds: “A meteoroid shower occurred on the Earth 470 million years ago. Recently, Schmitz et al. (2019) suggested that after the meteoroid shower, the extraordinary amounts of dust during 2 million years cooled the Earth and triggered the Ordovician icehouse conditions, sea-level fall, and major faunal turnovers related to the Great Ordovician Biodiversification Event.”
The mass the team calculated for the asteroids that hit Earth 800 million years ago is 10 to 100 times greater than the Chicxulub impact, and also compared with the meteoroid shower 470 million years ago. So, Terada sums up, it is plausible that this celestial shower they now report might have triggered the Snowball or Slushball. You can decide for yourself which theory you prefer.
All we can say at this point, as Terada implies: The chronology of the asteroid showers detected on the moon, and therefore inferred for Earth, looks awfully like a smoking gun behind the planetwide glaciation – a condition that persisted for some 100 million years. Ironically, for the geologically emotional among us, the very advent of worldwide glaciers crushing the land below would have erased the scars of the asteroids that caused them.