20 April 2016 [In social media today]
Two specks of rock that formed when Earth was young suggest we’ve got to rethink everything from the story of the solar system to the origins of life.
OF THE 200,000 shards of rock that Mark Harrison has retrieved from Australia since the mid-1980s, only one contained what he was looking for. Two flecks of graphite, each barely the size of a red blood cell. Small, perhaps, but capable of overturning everything we know about life on Earth. Harrison, a geologist at the University of California, Los Angeles, remembers thinking to himself: “By golly, they’re a dead ringer for a biogenic origin.” Biogenic means made by life – but how? These graphite flecks were found in a zircon crystal that had lain trapped deep in the Jack Hills of Western Australia for 4.1 billion years. So they seem to imply our planet was inhabited at least 300 million years earlier than anyone had previously imagined.
What’s more, these first living organisms would date from a time before our planet was thought capable of harbouring any life at all. In these early years, Earth was supposedly a molten hellhole racked by volcanism and bombarded by space debris, zinging around a solar system yet to find inner peace. If Harrison’s fossils are all they seem, they wouldn’t only rewrite the history of life and Earth – but the entire solar system’s as well.
When it came to explaining how these things all got started, we thought we had it more or less worked out. Some 4.6 billion years ago, a vast cloud of dust and gas in some corner of an unremarkable galaxy began to collapse into a dense ball of matter. As more and more surrounding material was pulled towards it, the temperature and pressure at its core increased, to the point where nuclear fusion kicked in. This released vast quantities of energy and marked the moment our sun became a star.
As the newborn star slowly began to spin, smaller bodies started to coalesce in orbit around it. Close in, vast quantities of water ice were boiled away, leaving only metallic compounds behind to form the smaller rocky planets. Further out, cooler temperatures allowed giant worlds of ice and gas to form. All in a single plane along smooth, near-circular tracks.
Rewriters of the solar system: tiny shards of zircon. Elizabeth Bell, et al./UCLA
It was a nice story, but as further details emerged, it became apparent that this picture was incomplete. For one thing, it struggled to explain the quantity and distribution of the so-called Trojan asteroids, thousands of tiny bodies that chase after Jupiter in its orbit. The Kuiper belt, the icy band beyond Neptune that Pluto belongs to, was equally difficult to justify: many of its bodies orbit at far greater angles to the planetary plane than the conventional picture would allow. Perhaps most perplexing of all, however, was the evidence our cosmic neighbourhood had once been under heavy bombardment. Rocks returned to Earth by the Apollo astronauts suggested the widespread cratering on our own moon was the result of a protracted assault which took place 3.9 billion years ago – a ruction the conventional model found hard to explain.
The solution, named after the city in France where it was devised in 2005, was the Nice model. In this refinement of the traditional story, our solar system’s four giant planets started out much closer together than they are today. This configuration was unstable, leading to hundreds of millions of years of gravitational tussling, during which the giant planets migrated into their current positions, disturbing the millions of tiny bodies littering the ancient solar system. Many fell under Jupiter’s gravitational influence, becoming its Trojan followers, while others settled in the solar system’s outer regions as highly angled denizens of the Kuiper belt.
Meanwhile, asteroids in the band between Mars and Jupiter were dislodged from orbit, many going on to collide with the innermost planets. This period of intense activity, known as the Late Heavy Bombardment (LHB), would have left deep craters on the moon and given our fledgling planet a serious knock during the turbulent early stages of its development.
The small number of surviving solid rocks from this period have led us to picture early Earth as a fiery world covered in volcanoes bursting through a molten crust. The LHB’s few hundred million years of constant collisions contributed to a nightmarish landscape so extreme that the geological period is known as the Hadean, after the Greek god of the underworld. The existence of life in such a hellscape was considered preposterous. Instead, the first traces of biogenic carbon, dated at 3.8 billion years old, neatly coincide with the time Earth was finally at peace and the bombardment from outer space had slowed.
Hence the excitement if Harrison’s fleck of graphite really is what it appears to be: evidence not only of our planet’s oldest known life form, but one that emerged at an impossible time. His smoking gun was the ratio of isotopes carbon-13 and carbon-12, within the sample. “If you were looking at this carbon ratio today, you would say it was biogenic,” he says.
Astonishing as it is, Chris Ballentine from the University of Oxford cautions against getting carried away. “It is one inclusion in one zircon,” he says. “But this sets the bar for people to find more and really show there was life around back then.”
Hellmouth: Australia’s Jack Hills hark back to Earth’s violent youth. Birger Rasmussen/Curtin University
Life or no life, it’s just the latest piece of evidence from the Jack Hills suggesting Earth’s hellish youth was more short-lived than astronomers thought possible. As far back as 1999, geologists uncovered other zircons in this astonishing terrain that indicated part of Earth’s surface had cooled and solidified 4.4 billion years ago. What’s more, measurements of how much oxygen the rocks contained suggested that Earth had been mild enough to support liquid water.
Further evidence that not all was right in the established picture of Earth and the solar system came in 2013, when Judith Coggon, then at the University of Bonn, Germany, was analysing another contender for the planet’s oldest rock – on the other side of the world in Greenland. There she found evidence that Earth contained significant quantities of gold and platinum as far back as 4.1 billion years ago – even though these metals were thought to have been delivered only later by the Late Heavy Bombardment.
Yet more contention came last year, when Nathan Kaib from the University of Oklahoma, along with John Chambers from the Carnegie Institution in Washington DC, published the results of their latest simulations of solar system formation. What they found seemed to sound the death knell for the Nice model. In 85 per cent of cases, the inner solar system ended up with fewer than the four rocky worlds it has today. “More often than not you lose Mercury,” says Kaib. Only 1 per cent of the time could they create a solar system that looked like the one we recognise. It would not be the first time the Nice model has been modified to take account of problems (see “Mystery of the missing planet“), but this was a problem of a different magnitude. “It seems very unlikely that you can get the outer solar system architecture and protect the inner planets,” he says.
Kaib has a surprisingly simple solution. The giant planets still migrated, producing the Jovian Trojans and the Kuiper belt, but they did so much earlier – while the innermost planets were still forming. By turning up to the party fashionably late, Earth dodged a bullet. The early migration of the giant planets would have scattered most of the larger impactors by the time Earth’s formation was complete. That works well, says Zoë Leinhardt, from the University of Bristol, UK. “The latter part of Earth’s formation would have been calmer, as opposed to having formed and then being smacked upside the head.”
It’s an appealing theory, explaining not only why the solar system looks the way it does, but how Earth became friendly to life so early. But one final mystery remains. If the giant planet migration happened before Earth and the moon had formed, then something else must have been responsible for the craters on the lunar surface. But what?
Up for grabs: the origins of the moon’s impact craters. NASA
David Minton from Purdue University thinks the answer lies closer to home. “In the Nice model, most of the LHB impactors come from the asteroid belt,” he says. “But the distribution of crater sizes on the moon and the distribution of asteroids don’t match.” Matija Cuk of the SETI Institute agrees. “If the LHB really was just asteroids being thrown at the moon en masse, there should be a lot more big lunar basins, and there aren’t,” he says. Minton believes he might have found an alternative source for the LHB: Mars.
He’s still working on the finer details, but he presented the concept to the American Astronomical Society’s Division on Dynamical Astronomy at their meeting in May 2015. One fact working in its favour is that the Red Planet’s northern hemisphere is low-lying and considerably flatter than the highlands in the south. “Many have suggested that’s because the northern area is a giant basin formed by a 2000-kilometre impactor,” says Minton. Debris thrown up by the formation of this so-called Borealis basin could have bombarded the moon, and Earth, 3.9 billion years ago.
Cuk has an even more radical explanation. “To me it is not clear at all that there was a spike in lunar bombardment 3.9 billion years ago,” he says. The Apollo samples that led to the assumption were returned from several different sites on the moon, with many showing evidence of impacts clustered around that time. But Cuk believes the Apollo samples all came from the impact or impacts that formed the Imbrium basin – one of the large, dark patches that makes up the “Man in the Moon”. Rocky shrapnel from this event could have contaminated disparate parts of the lunar surface, meaning that what at first looked like a host of simultaneous impacts might have only been a handful. “The idea of the carpet-bombing of the moon 3.9 billion years ago has gone away,” he says. If you could prove the impacts that caused the cratering on the moon were less of a spike and more of a steady drip, then the Nice model could be saved after all. Just as crucially, it would have profound implications for conditions on our infant planet. “If the impacts were more smeared out, early Earth wouldn’t have been total hell,” says Cuk.
Either way, with relative calmness kicking in sooner in Earth’s history, life could have emerged more quickly to leave its mark in the Jack Hills zircon. “Pushing giant planet migration back earlier would be consistent with what we found,” says Harrison. Future work will look at cementing this idea. Harrison has already identified another graphite inclusion in a separate Jack Hills zircon and will be analysing the ratio of its carbon isotopes within the next few months.
If Harrison’s hunch is right, then the life forms we had previously thought of as our earliest ancestors, dating from 3.8 billion years ago, weren’t the beginning of the evolutionary tree at all. Instead life on Earth began hundreds of millions of years earlier, almost as soon as the planet was ready for it. Such a scenario would raise hopes for the speed and ease with which biology can take hold, and of its aptitude for sticking around in an unfriendly cosmos. According to Harrison, “it makes the notion of life elsewhere in the universe that much more likely.” Our revised history could point to a more interesting future.
Mystery of the missing planet
Ever sensed something was missing? Researchers modelling our solar system have. Their best stab at explaining how our cosmic neighbourhood came to be suggests there shouldn’t be four giant planets in the outer solar system – there should be five.
In 2011, simulations suggested that without this mysterious fifth planet, intense gravitational interactions in the early solar system would have had disastrous consequences. As the four giants slowly creaked into their current positions, they would have disrupted their smaller neighbours, making the modern solar system all but impossible.
But with five giant planets jostling for supremacy, the migration would have taken place quickly enough to leave the innermost rocky planets virtually unharmed. What’s more, one of the quintet would have been slingshotted into the furthest reaches of the solar system, leaving us with the four outermost planets as we see them today.
Where exactly did this guardian angel end up? Speculation surrounding the existence of “Planet Nine” has long bubbled under the surface, but earlier this year the excitement finally burst when two astronomers from the California Institute of Technology announced they might have found it. With an orbital radius 600 times greater than Earth’s, this candidate Planet Nine would take at least 10,000 years to complete a single lap of the sun, making it one of our solar system’s most distant objects. Its existence has been inferred from the unusual clustering of half a dozen small objects beyond Pluto, which would be difficult to explain without a distant planet’s gravitational pull. The logic holds up, but direct observation has eluded us thus far.
It would be a remarkable find. Its discovery, while a pain for textbook publishers and quiz show contestants, would support the five-giant scenario for the solar system’s formation. “If Planet Nine exists, that’s where it must have come from,” says Matija Cuk of the SETI Institute in California.
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