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CATACLYSM’S END A popular theory about the early Solar System comes under fire.

BY ADAM MANN

arly in Earth’s history, roughly half a billion ever since geologists did a systematic study of They are looking An artist’s impression years after the planet formed, all hell broke samples brought back from the Moon by NASA with hope at the of the early Earth, loose in the inner Solar System. A barrage Apollo missions. But now, the once-popular United States’ recent bombarded by Solar of asteroids — some the size of Hong theory has come under attack, and mounting pledge to send astro- System debris. Kong — pummelled the globe intensely evidence is causing many researchers to aban- nauts back to the Eenough to melt large parts of its surface. This don it. A growing community of planetary Moon — although no timeline has yet been set. incendiary spree around 4 billion years ago scientists thinks that things quietened down In the meantime, the community is grappling vaporized most of Earth’s water and perhaps relatively quickly, with a steadily decreasing rain with the fact that a key chapter of Solar System even sterilized its exterior, killing off any life that of asteroids that ended a few hundred million history might be vanishing before their eyes. might have started to emerge. Only after this years after Earth and the Moon formed. “The Late Heavy Bombardment was seen as storm of impacts passed did the planet become Settling the debate could have major rami- one of the great triumphs of the Apollo era,” says safe enough for hardy organisms to take firm fications for some of the biggest questions in geochemist Mark Harrison of the University of LAB IMAGE NASA/GODDARD root and eventually give rise to all later life. geoscience: when did life emerge and what California, Los Angeles. “There’s no question That horrific episode, known as the Late were conditions like on early Earth? But some that something has happened in the past few Heavy Bombardment (LHB), has been an researchers think that fresh samples will be years that has profoundly upset the apple cart.” integral part of Earth’s origin story for decades, needed to finally put this conundrum to rest. The Solar System formed some 4.6 billion

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NEWS FEATURE years ago, after the centre of a this whole idea was wrong,” says massive cloud of gas and dust Swindle. Instead, they found no collapsed into a dense sphere SAMPLING THE MOON evidence of impacts before the In the 1970s, dating of some lunar material suggested a 3 that became our Sun. Pebbles spike in asteroid impacts long after the Solar System hypothesized time of the LHB .

in a dusty disk orbiting the formed — a Late Heavy Bombardment. However, this idea “That kind of pushed me to a dif- NASA MOON IMAGE:

is now being questioned, in part because some evidence Billion years ago star continuously collided and Present ferent side of the fence,” he says. suggests that samples from multiple missions might have day sometimes stuck together. After been ejected from one impact area: Mare Imbrium. But researchers still wondered tens of millions of years, these how a bombardment could come agglomerations had built up so long after the Solar System into planetesimals — the begin- formed. By the half-billion-year nings of the planets. Other rocky mark, most of the leftover debris fragments remained, crashing should either have been cast into their larger kin and leav- Mare out or have settled into stable ing deep craters. Over time, the Imbrium  zones such as the main asteroid Solar System thinned out, leaving Apollo 15 belt, which sits between Mars something like the configuration (Jul 1971) Mare and Jupiter, or the Kuiper belt we see today. Serenitatis beyond Neptune. Nobody could Most of the evidence of Apollo 17 come up with a physical rea- (Dec 1972) Luna 24 this violent history has been Apollo 12 Apollo 14 son for the unexpected drama Mare (Aug 1976) (Nov 1969) (Feb 1971) erased on Earth by the churn- Tranquilitatis at such a late date. “Where did ing of tectonic plates. But the Luna 20 you have the bodies in the Solar Apollo 11 (Feb 1972) 2 scarred surface of the Moon, Mare (Jul 1969) System that could hang around Apollo 16 long inert, retains a lengthy Nubium Luna 16 for 600 million years and then (Apr 1972) record of impacts. Some of that Mare (Sep 1970) come screaming in and hit the Nectaris record — roughly 382 kilograms Moon?” asks Cohen, who is now of lunar rock and soil — was col- at NASA’s Goddard Space Flight lected by Apollo astronauts and NASA Center in Greenbelt, Maryland. carried back to scientists eager landings A potential answer arrived in to see what the samples might Soviet Union 3 2005, with the emergence of what reveal about the Moon’s history. sample-return came to be known as the Nice missions In 1973, the year after the last model, after the French city where Apollo landing, a group at Shef- it was conceived. Originally pro- field University, UK, reported Early fossil evidence of life on Earth. posed to explain odd orbital a curious pattern in samples 3.5 behaviour by distant icy objects from four separate Apollo mis- Time rst proposed for in the Kuiper belt, the conjecture sions as well as a Soviet Luna Late Heavy Bombardment. 3.95 asserted that the Solar System’s mission. Radiometric dating of Possible upsurge in asteroid impacts. 4 outer planets had formed much each one returned the same age: 4.–4.0 closer to one another than they 1 3.95 billion years . A team at the Mars-sized body hits proto-Earth, are now. Computer simulations 4 California Institute of Technol- ejecting material to form the Moon. 4.5 showed how the massive gravi- ogy (Caltech) in Pasadena cor- Earth forms. tational pull of Jupiter and Saturn roborated the findings the same ~4.54 could have created an instability Solar System begins to coalesce. year2. 4.6 that ultimately bumped Uranus and Neptune into more distant CURIOUS CHRONOLOGY orbits, knocked comets out of The confluence of ages suggested that a flurry concentrations can be difficult because the same remote reservoirs and kicked asteroids out of of objects struck the Moon in a narrow 50-mil- ratio could have been caused by a concentrated the main belt. lion-year window, leaving behind countless barrage that heated the rocks and released 40Ar The Nice model offered huge support for the impact craters — including as many as a dozen some 3.95 billion years ago, or by a long, dwin- LHB. “I think this helped cement this idea,” says of the Texas-sized basins that scar the surface. dling asteroid torrent that released it in fits and physicist Nicolle Zellner of Albion College in Because it seemed to represent a final surge of starts before fizzling out at about the same time. Michigan. Geologist Marc Norman of the Aus- pandemonium after the Solar System’s chaotic The first really new data arrived in 2000. tralian National University in Canberra agrees. genesis, the Caltech team named the event the Planetary scientist David Kring, cosmo­chemist “That was the next real turning point,” he says. terminal lunar cataclysm, although it later Timothy Swindle and planetary scientist became more popularly known as the LHB. Barbara Cohen, all then at the University of CATACLYSMIC CONFUSION The idea was immediately divisive, in large Arizona in Tucson, collected lunar meteorites Yet just when the idea of the LHB finally part because of ambiguity in the rock dating. that had fallen to Earth after being blasted seemed unimpeachable, holes began to appear. This was done primarily by measuring the from the Moon’s surface by asteroid strikes. Apollo data and ‘crater counting’, which esti- rocks’ ratio of argon-40 atoms to radio­active They hoped such rocks would provide a more mates the order in which craters were laid potassium-40. 40K decays into 40Ar with a half- random sample of the Moon’s crust than those down on the basis of how they overlap, had life of 1.25 billion years. At high temperatures, from Apollo, which represent at most 4% of the indicated that three of the largest crater that 40Ar can leak out of minerals. That makes lunar surface. But when the results came back, basins on the Moon’s near side — Imbrium, the ratio of these two isotopes a kind of clock: they showed a curious, and familiar, pattern. Nectaris and Serenitatis — might all be the more time that has elapsed since a rock “Frankly, I thought we’d measure a bunch of about 3.95 billion years old (see ‘Sampling was hot, the more 40Ar should be present. But these and have ages running back to 4.3 and 4.4 the Moon’). But high-resolution maps from making sense of the argon and potassium [billion years] and prove once and for all that NASA’s Lunar Reconnaissance Orbiter, which

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FEATURE NEWS

started circling the Moon in 2009, spotted Meanwhile, the Nice model has proved suggests that the microbes at the base of the rays of debris extending from Imbrium5. This less helpful to the idea of an LHB than it once tree of life were hyper­thermo­philes — that is, suggested that the impact that formed the seemed. More-advanced simulations of the organisms that thrived in extreme heat. The crater might have knocked rocks into nearby early Solar System’s gravitational interactions intense conditions created by a rain of aster- Serenitatis, contaminating the Apollo samples indicate that the planetary reshuffling probably oids could have resulted in a number of pockets picked up there. In 2010, a reanalysis of rocks happened shortly after formation, not with a where life might have emerged. thought to have been ejected from Nectaris delay of hundreds of millions of years10. Nes- So far, efforts to clinch the LHB debate with indicated that they were also chemically and vorný likens delaying the reshuffling — and so evidence from other likely victims — Mercury, geologically similar to Imbrium material6. “We keeping the Solar System hovering on the edge Venus, Mars and objects from the asteroid started realizing that maybe we were sampling of instability — to trying to balance a pencil on belt — have proved inconclusive. Each camp Imbrium over and over,” says Zellner. its tip. “It’s really hard to put the pencil there accuses the other of cherry-picking favourable The data from lunar meteorites didn’t data and not looking at the total picture. “It’s necessarily help. Although none of the samples a Rorschach test,” says Norman. “People see seemed to be older than 4 billion years, some what they want to see and disregard the rest.” were billions of years younger than that3, with “PEOPLE SEE WHAT The only thing that researchers say will sub- no obvious spike around 3.95 billion years. And stantially move the needle is new samples from the Apollo samples held other surprises. Since THEY WANT TO SEE the Moon. Kring, now at the Lunar and Plan- 2012, detailed study7 of microscopic regions etary Institute in Houston, Texas, has developed in the rocks has turned up ages of as much as some concepts for sample-return missions, 4.2 billion years, much older than any seen AND DISREGARD including one that would see astronauts collect- before, suggesting that there had been signifi- ing rocks from the South Pole–Aitken basin, the cant impacts earlier than the proposed spike. largest and oldest impact crater on the Moon. Prodded in part by these revelations, some THE REST.” However, the next human mission to the Moon researchers proposed8 a longer-lasting LHB that is still a long way off. The first new lunar rocks to began around 4.1 billion or 4.2 billion years ago. in such a way that it falls in an hour,” he says. be carried back to Earth may come from China’s But that idea had one major strike against it: One of the original architects of the Nice Chang’e-5, a robotic mission currently planned some of the most ancient crystals on Earth, from model, astronomer Alessandro Morbidelli of for 2019. It aims to collect samples from the vol- the Jack Hills range in Australia, suggest9 that the Cote d’Azur Observatory in Nice, admits canic Mons Rümker formation, an area younger the planet was a fairly clement place then, with that the first versions took fine-tuning to get than those explored by Apollo astronauts. relatively low temperatures and ample water. the reshuffling to occur so late. He no longer Although no single exploration effort is believes in the LHB, and sees many others in likely to end the dispute, researchers’ improved HOT TOPIC the field trading in the idea of a sudden aster- understanding of the Moon and how to Others are still scrutinizing the original Apollo oid deluge for that of a long, declining tail of determine the ages of samples should provide evidence. To determine the samples’ ages, bombardment. “My prediction is people will greater confidence in the results. researchers heated the rocks to release argon, abandon the cataclysm,” he says. However things eventually shake out, the slowly ramping up the temperature. But as far Even those who remain tied to the LHB have new evidence will shift careers and rewrite back as 1991, Harrison had pointed out that had to modify their ideas. Planetary scientist textbooks. Yet, perhaps because of the long- the process won’t work well for rocks contain- William Bottke of the Southwest Research Insti- lived nature of this debate, those trying to make ing multiple minerals. Different minerals will tute agrees that there is no longer much support sense of the LHB remain flexible, sceptical and release their argon at different temperatures. A for a single, short spike. He says the best reading surprisingly lighthearted. sample heated to 400 °C might provide an age of the evidence, including samples from ancient “We are close friends and therefore we of 2 billion years; to 500 °C, an age of 2.5 billion. Earth and radiometric dates in meteorite rocks, disagree all the time and then go drink a beer Researchers have tried to extrapolate from is a more drawn-out surge of bombardment that together,” says Bottke. “One should carry this behaviour, but Harrison says the complex began around 4.1 billion or 4 billion years ago, models lightly and be prepared to drop them patterns often lead them to pick essentially with a relative lull before that, consistent with if something better comes along, because it arbitrary ages. “This is quackery,” he says. the existence of surface water in that period. happens all the time.” ■ “There’s no physical basis for it.” Astronomer William Hartmann, a visiting Swindle says the argon heating situation is scientist at the International Space Science Adam Mann is a freelance journalist based in not necessarily as bad as Harrison makes it out Institute in Bern, thinks the current situation Oakland, California. to be; Apollo samples can be found whose ages proves that the idea of a cataclysm was never 1. Turner, G., Cadogan, P. H. & Yonge, C. J. Proc. Lunar don’t change significantly with temperature, particularly robust. Various research com- Sci. Conf. 4, 1889–1914 (1973). and their dates — whether they refer to one or munities “kind of had the impression that the 2. Tera, F., Papanastassiou, D. A. & Wasserburg, G. J. Abstr. Lunar Planet. Sci. Conf. 4, 723–725 (1973). multiple impacts — still cluster around 3.95 bil- other community had really solved this”, he 3. Cohen, B. A., Swindle, T. D. & Kring, D. A. Science lion years. Cohen says that other chronometers, says. “A paradigm structure was built up from 290, 1754–1756 (2000). such as those using radioactive isotopes of supporting evidence, none of which was actu- 4. Gomes, R., Levison, H. F., Tsiganis, K. & Morbidelli, A. Nature 435, 466–469 (2005). rubidium and uranium, corroborate the argon ally conclusive in itself.” 5. Spudis, P. et al. J. Geophys. Res. Planets 116, ages (although Harrison counters that the dates If an LHB did not happen, that could make it E00H03 (2011). can differ by as much as 600 million years). easier to explain how life emerged. Evidence of 6. Norman, M. D., Duncan, R. A. & Huard, J. J. Geochim. Cosmochim. Acta 74, 763–783 (2010). Such back and forth underscores how microbial life has been found in rocks that are 7. Norman, M. D. & Nemchin, A. A. Early Solar System difficult it can be to tease small clues out of around 3.5 billion years old. But those fossils Impact Bombardment II Conf., Lunar and Planetary extremely ancient rocks. “Sherlock Holmes seem quite complex, suggesting that they had Institute (Houston), Abstr. 4013 (2012); available at go.nature.com/2mbdwvc was good at resolving mysteries that happened been evolving from earlier forms for at least a 8. Morbidelli, A., Marchi, S., Bottke, W. F. & Kring, D. A. last year,” says David Nesvorný, a planetary few hundred million years, during the origi- Earth Planet. Sci. Lett. 355–356, 144–151 (2012). scientist at the Southwest Research Institute nally hypothesized time of the LHB. Without 9. Watson, E. B. & Harrison, T. M. Science 308, 841–844 (2005). in Boulder, Colorado. “This all happened the cataclysm, such an ancient genesis might 10. Deienno, R., Morbidelli, A., Gomes, R. S. & 4 billion years ago.” make more sense. Then again, some evidence Nesvorný, D. Astron. J. 153, 153 (2017).

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