Digging into the Late Heavy Bombardment

by Paul Gilster on April 11, 2014

The Barberton greenstone belt is considered one of the oldest pieces of continental crust on the planet. About 100 kilometers long and 60 kilometers wide, the belt is in South Africa east of Johannesburg and not far from the border of Swaziland, a region where gold was first discovered in South Africa. Greenstone belts, however, are numerous, widely distributed geographically and throughout geological history, all of them marked by the characteristic green hue imparted by the metamorphic minerals within their rocks. The Barberton greenstone belt is now yielding evidence of a massive ancient impact well over three billion years old.

The paper on this work is slated to appear in the journal Geochemistry, Geophysics, Geosystems, where scientists will make the case that the impact they are tracking occurred 3.26 billion years ago at the end of the Late Heavy Bombardment, a period between three and four billion years ago when numerous large asteroids are thought to have struck the planet. The impact may have caused a major shift in plate tectonics, and characterizing it will help us better understand the conditions life struggled against in its earliest evolution.

It is possible that changes to the environment caused by impacts like this one may have wiped out existing microscopic organisms, only to allow other organisms to evolve. What’s mind-boggling here is the sheer size of the event. Have a look at the graphic below, and note in particular the comparison of the impact crater to the island of Hawaii.

014_2091_Asteroid and crater combo Graphic for Pi Dept

Image: A graphical representation of the size of the asteroid thought to have killed the dinosaurs, and the crater it created, compared to an asteroid thought to have hit the Earth 3.26 billion years ago and the size of the crater it may have generated. A new study reveals the power and scale of the event some 3.26 billion years ago which scientists think created geological features found in a South African region known as the Barberton greenstone belt. Credit: American Geophysical Union.

The asteroid, according to this American Geophysical Union news release, would have been three to five times larger than the Chicxulub impactor considered to have played a huge role in the extinction of the dinosaurs. Striking the Earth at 20 kilometers per second, the object would have created a crater nearly 500 kilometers across, a larger jolt than a 10.8 magnitude earthquake, creating tsunamis thousands of meters deep. The researchers believe the sky would have become red hot and the tops of the oceans would have boiled.

“We are trying to understand the forces that shaped our planet early in its evolution and the environments in which life evolved,” said Donald Lowe, a geologist at Stanford University and a co-author of the study. As to the asteroid itself, Lowe added, ““We knew it was big, but we didn’t know how big.” The team’s model shows that while the Chicxulub event is estimated to have released a billion times more energy than the Hiroshima and Nagasaki bombs, this more ancient impact would have been far more powerful, and it was just one of many in the Late Heavy Bombardment.

The actual site of most of the impacts during the LHB is unknown, the victim of erosion, crustal movement and evolving geology, and the researchers believe the asteroid they are studying impacted thousands of kilometers away from the Barberton greenstone belt, though its seismic waves would have been responsible for the geological formations found in the region. Clearly the early Solar System was a chaotic and dangerous place, one in which the great experiment of life was under continuous threat. To map an impact that occurred more than three billion years ago is to chart the dimensions of ancient catastrophe, a time when vaporized rock fell as rain.

The paper is Sleep et al., “Physics of crustal fracturing and chert dike formation triggered by asteroid impact, ~3.26 Ga, Barberton greenstone belt, South Africa,” to be published in Geochemistry, Geophysics, Geosystems (abstract).

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{ 22 comments… read them below or add one }

RobFlores April 11, 2014 at 13:21

Do we know for a fact that some micro fauna survived this impact.
I know it’s hard to kill all life on a planet, but I don’t see many ecological
niches surviving such a large impact. Boiled Oceans, Cauldron of atmospheric heat. The only organism that could survive that I can see it
deep crust dweller, and only if the crust remained solid in some spots.
It interesting to speculate what might happened if we had foreknowledge of a impending collision of 2 sizeable bodies and saw that the inner solar system was going to be a shooting gallery for the next millions of years.
While the outer solar system might be safer, with hundreds of objects
some are bound interact with the inner planets and get sent in long period
elliptical orbits deep to the gas giants. So no place but probably Neptune
or Pluto pr another KBO is safe. Lousy choices.
Would this be a catalyst for migration to the outer solar system where it is relatively safer. Or would some segments of humanity seek another home planet in a nice quiet HZ of some main sequence star.

ljk April 11, 2014 at 14:09

Did you ever think maybe that massive impact *stimulated* more life, or life that could better handle the rough environment and thus really survive on Earth once things calmed down?

Alex Tolley April 11, 2014 at 15:07

It is possible that changes to the environment caused by impacts like this one may have wiped out existing microscopic organisms, only to allow other organisms to evolve.

If true, then genesis is not so hard to restart and we might expect life to be ubiquitous in the universe.

It would also suggest that the earliest evidence of life on Earth predates this event and therefore that it should be different from current life.

Unfortunately, as we see no evidence for different evolutionary trees, we cannot determine whether life survived this event, completely restarted, or the original forms reseeded via panspermia, perhaps from Mars.

andy April 11, 2014 at 17:48

If you look at the outer solar system, there are some rather large objects on unstable (on the timescale of several millions of years) orbits — some of the centaurs are fairly large, e.g. (2060) Chiron and (10199) Chariklo are in the ~200 km diameter range, and it seems that this population is being replenished from the Edgeworth-Kuiper belt and scattered disc.

There is therefore a non-zero probability that one of the major planets can experience an extremely large collision before the Sun becomes a red giant, even without considering the possibility that the inner solar system may become destabilised by a Mercury-Jupiter secular resonance.

Observations of the BD+20°307 system indicate this kind of thing does indeed happen in relatively “mature” planetary systems.

Eniac April 11, 2014 at 19:47

The best explanation is that life first emerged after the LHB, possibly much later. Everything else is pure, irrelevant speculation in need of being cut down by Occam’s Razor.

Eniac April 11, 2014 at 19:49

Oh, and cetero censeo, the dinosaurs are not extinct. They evolved into birds. Whether a meteor is to be blamed/credited for that is similarly blatant speculation.

Rob Henry April 11, 2014 at 19:58

The frightening part to me is that, despite the spin, 3.26 billion ya is NOT part of the Late Heavy Bombardment. That objects that size were still hitting the Earth without one off cause (such as was the LHB) could imply that we seriously underrate the modern threat.

Unlike the LHB proper, where half a dozen impacts could have evaporated all oceans and had every inch of the Earth’s surface radiating at thousands of degrees, this smaller impact would not have touched the planets ‘deep hot biosphere’. It would explain why all the deepest rooted extant organisms (at least as measured from rRNA) are extreme thermophyles without need of postulating that that was the original form before the divergence of life’s trichotomy.

Anyhow this just emphasizes our need to return to the moon, where every square kilometer is estimated to hold about 100kg of Earth-rock from the LHB alone. Only their can we study this time in the detail that we really need to.

Glaas April 12, 2014 at 9:53

@ RobFlores,
An alternative would be to live in space stations and dodge the rubble, as the ISS does today. It would actually offer loads of easily accessible heavy elements as debris from the destroyed planets! We’ll go interstellar before we mine the center of Earth, it’s really an inaccessable part of the universe 6300 km under our feet.

Denver April 12, 2014 at 10:21

A look at the literature finds three large impact event horizons (spheroids, tektites, iridium, impact-diamonds) spread across 250 million years recorded in greenstones exposures in South Africa and Oz. The one dated 3.26Gy is the latest of three. The locations of the impacts are unknown (though there is a suspected ancient 500km crater in the Vredefort (Reitz Ring) area of SA. These impacts are not related to the Terminal Lunar Cataclysm (3.9Gy-3.8Gy) as they happened hundreds of millions of years after 3.8Gy.

Considering Mars has impact basins in excess of 4000 km diameter (Utopia), and the Moon has impact basins 2000-3000km (SP-Aiken, Oceanus Procellarum) in diameter, Earth “took” at least five 300-500 km diameter bolide impacts, creating basins 4000-6000 km in diameter, during LHB/TLC, so the 500 km crater, mentioned above, no longer looks so daunting.

References: Hargraves R. B. and Fuller A. O. (1981) The Reitz ring: a possible circular structure, 350-500 km in diameter, in South Africa. Precambrian Research 14(2):99-106. http://www.sciencedirect.com/science/article/B6VBP-48BDX3S-2K/2/c533f2bfb9a3108ff106a8d893dfc26e

Tangentially, it is theorized that greenstones (heavily metamorphosized igneous rock) are a Terran analog of the lunar mare and were created during the LHB/TLC.

Tony Mach April 12, 2014 at 10:33

There is this very excellent talk by Don Lowe on the LHB and spherules in the BGB, which I can recommend in that context:
http://www.youtube.com/watch?v=JAMm_0t5kcM&feature=share&list=PLzJ2a4zDu_YkoKEFjE8LprVCDYjT32ReY&index=3

It was made as part of the SETI Talks series.

Jim Early April 12, 2014 at 11:46

Rob, life around deep sea vents should have survived, particularly if only the top of the oceans boiled.
I thought most of the LHB events took place around 4.0-3.8 MYA. This would be quite late for such a massive event.

Alex Tolley April 12, 2014 at 12:07

…the dinosaurs are not extinct. They evolved into birds. Whether a meteor is to be blamed/credited for that is similarly blatant speculation.

The timing is way out. For reference:
http://evolution.berkeley.edu/evolibrary/article/evograms_06

Alex Tolley April 12, 2014 at 12:11

this smaller impact would not have touched the planets ‘deep hot biosphere’. It would explain why all the deepest rooted extant organisms (at least as measured from rRNA) are extreme thermophiles without need of postulating that that was the original form before the divergence of life’s trichotomy.

I agree that this is the most parsimonious explanation.

Michael April 12, 2014 at 14:20

@Glaas April 12, 2014 at 9:53

‘An alternative would be to live in space stations and dodge the rubble, as the ISS does today…’

If these massive objects hit a planet there is enough energy to eject a significant amount ~10% or more of the impactor/planet (velocity/mass dependant) back out into space and even escape the planet completely.

Michael April 13, 2014 at 2:13

Just a thought, if life had got started and was ejected into space by the impact it could have returned thousands of years later as some of the material from the impact returned back to earth when the events effects had subsided.

Tony Mach April 13, 2014 at 5:04

One more thing from the Don Lowe talk I linked above (and a talk from Bill Bottke, following in that playlist): Probably our reconstruction of the LHB is a bit skewed, as the Apollo sites are all biased towards one big impact basin – this results in reconstructions of the LHB as we have grown up with, and as we all know we thing/thought that the LHB was one very short but very intense burst. The data from BGB (together with Nice model) instead are more consistent with the LHB lasting longer, and slowly “fading away”.

We need more samples from the Moon, from a wider selection of landing sites: further “towards the limb”, away from the basins, and additionally we need to go to the far side of the Moon…

Rob Henry April 13, 2014 at 17:06

Tony March, yes, the evidence in favour of the LHB is not as unequivocal as once seemed, but, from what I’ve seen, more moon rock gathered from a wider area could well see it firmly re-established in its strongest form (a narrow window with impact rates a hundred that of just before it or just after it). Yet another reason to return to the moon.

A third reason might be to study Martian rock from its earliest epic. Several scientists have found cause to speculate that early Mars was a better place for the origin of Earth’s life than early Earth was. The moon’s surface would also be loaded with rock from those proposed times of lithopanspermia.

Michael Simmons April 13, 2014 at 20:59

Could someone explain “16 Psyche”?
If its the iron core of a planet that broke apart, how big would that planet have been? 1000km? diameter
Could it be the source the large post LHB impactors we are taking about?

ljk April 14, 2014 at 9:07

I will probably regret asking this, but how is the theory of a celestial impact bringing about the demise of the dinosaurs (or at least the big lizardy ones) 65 million years ago only speculation? I thought there was some serious evidence for it going back to the 1980s?

Try not to make me feel like a complete fool for asking, okay? I’m very sensitive when my decades old ideas are trampled upon in a few brief paragraphs on this blog.

torque_xtr April 14, 2014 at 13:21

The idea about Earth ejecta somewhere else in the Solar system is fascinating. Based on some recent works on lithopanspermia http://online.liebertpub.com/doi/abs/10.1089/ast.2013.1028, there could be billions and jillions of tonnes of ejecta from all epochs of all inner planets on the stable orbits somewhere in the asteroid belt. Archaean and maybe even Hadean Earth – the only way to find out if the life originated before LHB – noachian Mars, possibly even early ocean Venus – and all perfectly conserved. The asteroid belt may turn out a much better place for archaeology than the planets themselves some 50 years into the future…

Alex Tolley April 14, 2014 at 17:09

@ljk
The Chicxulub impact was most definitely coincident with the end of the dinosaurs. AFAIK, no definite post KT event fossils have been proven to exist, although there have been claims.

Richard Muller has claimed that the meteor impact was coincidental and that the extinction was part of a cyclical 62.5 my cycle. He bases this on the numbers of species vs time. This hypothesis got some traction when it was published, although I personally don’t buy it.

Other theories have also been posited, but none seem as good at the meteor impact which is highly coincident and seems like a smoking gun since we know the timing and impact point.

Why the complete dinosaur lineage went extinct, leaving just their earlier bird lineages is fully understood. It seems to me that small, warm bloodied dinosaurs should have survived, just like the birds and mammals. There is a nice review on the KT extinction in Wikipedia with plenty of references.

ljk April 14, 2014 at 22:23

Some pieces of ancient Earth may be on the lunar surface.

Yet another reason to explore the Moon: Well preserved pieces of our planet’s early history.

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