Late Heavy Bombardment Myth Is Dying
A favorite tale from the Apollo era, the so-called Late Heavy Bombardment, is being bombarded itself.
It was a nice story while it lasted. It explained craters on the moon and the timing of life on the earth. It gave TV animators something to do. But it never happened. At Nature, Adam Mann writes that “New analyses undermine a popular theory about an intense asteroid storm 4 billion years ago.” First, he recounts the old myth:
Early in Earth’s history, roughly half a billion years after the planet formed, all hell broke loose in the inner Solar System. A barrage of asteroids — some the size of Hong Kong — pummelled the globe intensely enough to melt large parts of its surface. This incendiary spree around 4 billion years ago vaporized most of Earth’s water and perhaps even sterilized its exterior, killing off any life that might have started to emerge. Only after this storm of impacts passed did the planet become safe enough for hardy organisms to take firm root and eventually give rise to all later life.
What happened to this story that has been repeated since the 1970s? Increasing numbers of geophysicists are calling the LHB fake science.
That horrific episode, known as the Late Heavy Bombardment (LHB), has been an integral part of Earth’s origin story for decades, ever since geologists did a systematic study of samples brought back from the Moon by NASA Apollo missions. But now, the once-popular theory has come under attack, and mounting evidence is causing many researchers to abandon it.
The reason that moon rocks were used to fortify the LHB is that earth rocks have been recycled by plate tectonics, he says. The moon, though, was supposed to preserve a pristine record of early solar system history.
“… a key chapter of Solar System history might be vanishing before their eyes.”
The fallout from this change of script is enormous, Mann writes.
Settling the debate could have major ramifications for some of the biggest questions in geoscience: when did life emerge and what were conditions like on early Earth? But some researchers think that fresh samples will be needed to finally put this conundrum to rest. They are looking with hope at the United States’ recent pledge to send astronauts back to the Moon — although no timeline has yet been set. In the meantime, the community is grappling with the fact that a key chapter of Solar System history might be vanishing before their eyes.
“The Late Heavy Bombardment was seen as one of the great triumphs of the Apollo era,” says geochemist Mark Harrison of the University of California, Los Angeles. “There’s no question that something has happened in the past few years that has profoundly upset the apple cart.”
What happened? The LHB myth began when Caltech scientists measured the same radiometric date, 3.96 billion years, in moon rocks from four widely separated sites. Using scientific divination, they envisioned a flurry of impacts all around the same time, giving it the name Late Heavy Bombardment. But not everyone liked the story. “The idea was immediately divisive,” Mann says, “in large part because of ambiguity in the rock dating.”
But making sense of the argon and potassium concentrations can be difficult because the same ratio could have been caused by a concentrated barrage that heated the rocks and released 40Ar some 3.95 billion years ago, or by a long, dwindling asteroid torrent that released it in fits and starts before fizzling out at about the same time.
The LHB story was supported in 2000 by another study of moon rocks that had fallen to earth. These, presumably, provided a more random sample. The ages of those rocks, however, did not indicate melting before the supposed LHB, and so were thought to show a later event.
Not everyone liked the LHB, though, because it seemed like an ad hoc event. What would cause a flurry of asteroids to strike the inner solar system after things should have settled down? In 2005, another idea, the “Nice model” (named after the city of Nice in France where it was conceived), proposed that interactions of Jupiter with the Kuiper Belt might have sent objects in to create the LHB. Most planetary scientists accepted this as the source of the unusual bombardment. This was “the real turning point” that cemented the LHB myth. By then, reporters, trusting the scientists treated the LHB as accepted truth.
The story began to unravel in 2009, when planetologists began to suspect that all the Apollo samples with the same date had come from Mare Imbrium. Crater count dating and re-analysis of radiometric dates began to spread the ages, not indicating a spike at 3.96 billion years. Mark Harrison may have said too much:
Others are still scrutinizing the original Apollo evidence. To determine the samples’ ages, researchers heated the rocks to release argon, slowly ramping up the temperature. But as far back as 1991, Harrison had pointed out that the process won’t work well for rocks containing multiple minerals. Different minerals will release their argon at different temperatures. A sample heated to 400 °C might provide an age of 2 billion years; to 500 °C, an age of 2.5 billion. Researchers have tried to extrapolate from this behaviour, but Harrison says the complex patterns often lead them to pick essentially arbitrary ages. “This is quackery,” he says. “There’s no physical basis for it.”
Others disagree, so it has taken awhile for the myth of the LHB to evaporate.
Such back and forth underscores how difficult it can be to tease small clues out of extremely ancient rocks. “Sherlock Holmes was good at resolving mysteries that happened last year,” says David Nesvorný, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. “This all happened 4 billion years ago.”
Meanwhile, the Nice Model, that seemed to clinch the LHB, has come under attack as well. Modelers are realizing that getting a late bombardment to work is like trying to balance a pencil on its tip, Mann writes. One of the architects of the Nice Model now “admits that the first versions took fine-tuning to get the reshuffling to occur so late.” He now predicts that the LHB idea will be abandoned, after the last holdouts change their minds.
How could astronomers be so wrong? We find multiple sources of self-deception: cherry-picking data, seeing what they wanted to see (like in a Rorschach Test), and assuming that other scientists knew what they were talking about.
Astronomer William Hartmann, a visiting scientist at the International Space Science Institute in Bern, thinks the current situation proves that the idea of a cataclysm was never particularly robust. Various research communities “kind of had the impression that the other community had really solved this”, he says. “A paradigm structure was built up from supporting evidence, none of which was actually conclusive in itself.”
…. So far, efforts to clinch the LHB debate with evidence from other likely victims — Mercury, Venus, Mars and objects from the asteroid belt — have proved inconclusive. Each camp accuses the other of cherry-picking favourable data and not looking at the total picture. “It’s a Rorschach test,” says Norman. “People see what they want to see and disregard the rest.”
Mann thinks that this will help explain the origin of life on earth if organisms didn’t have to survive an LHB. But can we trust scientists now?
However things eventually shake out, the new evidence will shift careers and rewrite textbooks. Yet, perhaps because of the long-lived nature of this debate, those trying to make sense of the LHB remain flexible, sceptical and surprisingly lighthearted.
“We are close friends and therefore we disagree all the time and then go drink a beer together,” says Bottke. “One should carry models lightly and be prepared to drop them if something better comes along, because it happens all the time.”
Maybe they were drinking too much before, not after, they dreamed up the LHB.
This is an important article for a number of reasons. One issue concerns radiometric dating. The public is taught that it is infallible, but we see the scientists themselves disputing the results and pointing out sources of error in interpretation. Another issue is the sociology of science: these guys commit intellectual inbreeding inside their cliques. Sure, they may disagree about details, but they drink beer together and all know each other. How likely are they to think totally outside the box? The agreed-on dogmas of their beliefs—billions of years and naturalism—are set in stone. They’ll never listen to a creationist pointing out flaws in the Moyboy Philosophy. All they’re doing is reshuffling the pieces of their naturalistic puzzle when inconvenient evidence shows up.
Most disturbing is the cavalier attitude they have about misleading the public. For decades this has gone on! The public is fed fake science, reinforced by animators and reporters who treat ‘scientists’ like shamans, the dispensers of supreme wisdom about the past. Now, we learn that there never was an LHB, and these guys go out and laugh and drink beer over it. It’s time for some accountability in science. In other human endeavors, the ones who sold the public a bill of goods would be fired and shamed.