This is an artist’s depiction of a 10-kilometer (6-mile) diameter asteroid striking the Earth. New evidence in Australia suggests an asteroid 2 to 3 times larger than this struck Earth early in its life. Credit: Don Davis/Southwest Research Institute.

Ancient Asteroids Kept Pelting Earth in a ‘Late-Late’ Heavy Bombardment

25 Apr , 2012 by


Even though the Late Heavy Bombardment is somewhat of a controversial idea, new research has revealed this period of impacts to the Earth-Moon system may have lasted much longer than originally estimated and well into the time when early life was forming on Earth. Additionally, this “late-late” period of impacts — 3.8 billion to 2.5 billion years ago — was not for the faint of heart. Various blasts may have rivaled those that produced some of the largest craters on the Moon, and could have been larger than the dinosaur-killing impact that created the Chicxulub crater 65 million years ago.

“Our work provides a rationale that the last big impacts hit over an extended time,” said William Bottke principal investigator of the impact study team at the NASA Lunar Science Institute’s Center of Lunar Origin and Evolution (CLOE), based at the Southwest Research Institute (SwRI) in Boulder, Colorado.

The evidence for these prodigious impacts comes from bead-like impact ‘spherules’ found in millimeter- to centimeter-thick rock layers on Earth and date from the Archean period of Earth’s history, more recent than the estimated LHB period of 4.1 to 3.8 billion years ago.

“The beds speak to an intense period of bombardment of Earth,” Bottke said. “Their source long has been a mystery.”

The millimeter-scale circles and more irregular gray particles are formerly molten droplets ejected into space when an asteroid hit the early Earth. The image at left is from the Monteville layer in South Africa. Courtesy Bruce Simonson, Oberlin College and Conservatory

The circles seen in the image above are all formerly molten droplets ejected into space when an asteroid struck the Earth about 2.56 billion years ago. The droplets returned to Earth and were concentrated at the base of the Reivilo layer in South Africa.

The spherules still contain substantial extraterrestrial material, such as iridium (176 parts per million), which rules out alternative sources for the spherules, such as volcanoes, according to Bruce Simonson, a geologist from the Oberlin College and Conservatory who has studied these ancient layers for decades.

The timing of these impacts also coincides with a record of large lunar craters being created more recently than 3.8-billion years ago.

At least 12 spherule beds deposited between 3.47 and 1.7 billion years ago have been found in protected areas on Earth, such as in shales deposited on the seafloor below the reach of waves.

From these beds, the team found evidence of approximately 70 impacts on Earth during this time period that were likely larger than the Chicxulub impact.

In their paper, which was published in Nature, the team created a computer model of the ancient main asteroid belt and tracked what would have happened when the orbits of the giant planets changed. They extended the work of the Nice Model, which supports the theory that Jupiter, Saturn, Uranus and Neptune formed in different orbits nearly 4.5 billion years ago and migrated to their current orbits about 4 billion years ago, triggering a solar system-wide bombardment of comets and asteroids called known as the LHB.

This image shows a representation of how the giant planets have migrated to the current orbits, destabilizing the extension of the primordial asteroid belt closest to Mars. This drove numerous big impactors onto orbits where they could hit the terrestrial planets, though over a long enough time span that this drawn-out barrage may have lasted more than a billion years. The frequency of these impacts on Earth was enough to reproduce the known impact spherule beds. Image Courtesy David Kring, Center for Lunar Science and Exploration, and the Lunar and Planetary Institute

The new computer model shows that the innermost portion of the asteroid belt could have become destabilized, delivering numerous big impacts to Earth and Moon over longer time periods.

Have there been any previous indications about this period of impacts?

“The problem is that we have almost no Archean rocks,” Bottke told Universe Today. “The oldest terrestrial craters, Sudbury and Vredefort, are 1.85 and 2.02 billion years old. The spherule beds are our only window into impacts prior to this time.”

Also, Bottke said, the number of people who look for impact spherules is almost equally scarce. “People such as Bruce Simonson, Don Lowe, Gary Byerly, and Frank Kyte, have been carrying on a long, lonely quest to try to get people to consider the implications of their work, which are deeply profound, in my opinion,” Bottke said.

As for finding evidence of this later period of impacts on the Moon, Bottke said the problem there is the lack of solid ages for most impact events.

“This means it is difficult say anything definitive about the timing of major impacts,” Bottke said. “We are working this problem now with Michelle Kirchoff, who is counting craters on top of large lunar craters. This can be done now that we have LRO data.” (Listen to a podcast interview of Kirchoff on the 365 Days of Astronomy.)

Still, Bottke said, without using “fancy dynamics,” they can address some issues.

“Studies in the post-Apollo era suggested that the Moon has four 160-300 km craters that formed after Orientale, whose age is 3.7-3.8 billion years ago and (i.e., K/T-sized events or larger),” he said. “Crater counts from the Galileo mission and Apollo-era geologic analyses suggest at least one of these events took place near 3.2-3.5 billion years ago. If we account for the gravitational cross section of the planets, we know that for every lunar event, we should get about 20 on the Earth. So, from this argument alone, one should get a lot of big impacts on the Earth after the formation of Orientale.”

The new study fits with the available constraints about impacts on the Moon as well as finding the right distribution of spherule beds on Earth.

The best way to confirm everything, however, Bottke said, would be if more lunar rocks from various locations were available for study.

Read the team’s paper in Nature.

Further reading:
Press release from SwRI.
NLSI press release

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April 26, 2012 11:36 AM
“If we account for the gravitational cross section of the planets, we know that for every lunar event, we should get about 20 on the Earth.” – One thing is as dramatically evident in light of that fact as the Moon in our night sky – at one time our Planet was subjected to a violent pummeling from space. “Dawn Reveals More of Vesta’s Secrets” by N. Atkinson: “Many of the materials seen by Dawn are composed of iron and magnesium-rich minerals, which often are found in Earth’s volcanic rocks.” – Would I be incorrect to conclude that means they were formed interior to a large object (like a Moon/Planet)? Do such compositions at least suggest, from this… Read more »
Torbjorn Larsson OM
Torbjorn Larsson OM
April 26, 2012 9:10 PM
I don’t think you can say that there was a single responsible process for the asteroid belt. – The asteroids formed by accretion, see the Vesta and similar results. – The asteroids were gravitationally scattered, see Bottke et al theory and similar results. – And the asteroids have been high velocity bombarded and in cases shattered, see the recent Hayabusa publications and similar results: “But what can explain these high-velocity impacts on the nano-scale? Over millions to billions of years, these space rocks collide with one another. Although the rate of asteroid collision is low, it happens regularly enough over the evolution of the solar system, produce huge quantities of debris. The debris, by its nature, will be… Read more »
April 27, 2012 11:45 AM
The only issue I would take from your reply is “The asteroids formed by accretion, see the Vesta and similar results.” As for gravitational scattering, in-house bombardment and shattering through time (creating “families”), “high-velocity impacts” within, and without the “Belt”, “resonance” groups(?) and clusters over spans of age, do not spin-beyond what I had in mind. In fact these things make perfect sense in its broad frame (to a layman, anyway). And yes, I believe an Event explains the rubble belt – and oddly tilted Giants, surface-stripped worlds, missing moons(?), and hell-imprisoned planets (if not all by direct results, then from the dust-settling aftermath), etc. One titanic Event engulfing, sweeping through the entire Solar System (Earth-Moon not having… Read more »
Torbjorn Larsson OM
Torbjorn Larsson OM
April 26, 2012 9:02 PM
Nice! A lot of more evidence for Bottke (et al) theory, and a prominent publishing. I was going to say that we have discussed this before on UT, but googling turns up that in fact UT has reported on this research as well. Unfortunately astronomers haven’t learned to publish to arxiv, so the best way to understand what Bottke et al is about is his SETI talk where he explains all the evidence for it. From looking at the supplementary material, they have now summed up and added LHB predictions that strengthen the late, late tail theory. Something that is detracting but circumstantial is that both models of core formation and observations of mantle inhomogeneities makes the hypothesis… Read more »