Video: Why the moon is mainly made from the Earth
HUMANITY has witnessed some pretty loud bangs during our short sojourn on Earth. Take Krakatoa. When the Indonesian volcano exploded in 1883, the din was audible 3000 kilometres away, and the ash thrown into the atmosphere cooled the world for decades. Then there are the explosions of our own making. The most powerful nuclear weapon ever detonated, the , created a 10-kilometre wide fireball in the atmosphere.
But if , a planetary scientist at the VU University in Amsterdam, the Netherlands, is right, these cataclysms are nothing compared with an experience Earth went through 4.5 billion years ago. With the paint barely dry on the new planet, a giant nuclear reactor deep in its interior went super-critical. The result was an atomic bomb that dwarfs our puny efforts. Detonating with the force of 11,000 billion Tsars, the explosion was enough to rip our infant world open.
It is a controversial idea, but there is circumstantial evidence if you want to find it, from traces of smaller “fossil reactors” deep underground in equatorial Africa to the conspicuous imbalance between the heat Earth gives out and the amount it receives from the sun. But van Westrenen makes a more audacious claim. The , he says, is the serene body that watches over us most nights: the moon.
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Accounting for the moon’s origin has always been a problem. It is just too big. No other planet in our solar system has a satellite that is proportionally so large: it is over one quarter of Earth’s diameter. Such a body could not have been captured in passing, as other planets are thought to have snared their smallest satellites. In 1879, George Darwin, the astronomer son of Charles, proposed a different idea. He suggested that the early Earth spun so quickly it fell apart, spitting a bit of itself into space.
“George Darwin, son of Charles, reckoned that Earth spun so quickly it fell apart to make the moon”
That idea was popular for a time, but fell foul of planetary dynamicists in the early 20th century, who found that the numbers just did not add up. They looked at the Earth and moon’s angular momentum, a measure of the rotational energy stored in a body. The total amount in a given system – such as Earth and the moon – always stays the same, unless there is an interaction with an outside body. If the moon started off as part of Earth, then the angular momentum of today’s Earth and moon is the amount Earth had to play with on its own in the past.
If that was the case then Earth pre-break-up must have been rotating faster in the past. Indeed, the extra angular momentum would have shortened its day to just 4 hours. The problem with Darwin’s hypothesis was that for Earth’s outwardly directed centrifugal force to overwhelm the inwardly acting gravitational force and break the planet apart, it would have had to be rotating even faster, spinning about once every 2 hours.
As Darwin’s idea fell out of favour, another has taken its place. Known as the giant impact hypothesis or “big splat“, the idea is that a game of interplanetary billiards sent a Mars-sized object careering towards the infant Earth. Striking our planet a glancing blow, this foreign body shattered on impact, sending up a giant plume of debris that eventually coalesced to become the moon.
At first, there was nothing much to favour the big splat over any other explanation for the moon. “It was proposed because nothing else worked,” says , a planetary scientist at Harvard University. But that has changed as we have refined our picture of what the early solar system was like. Evidence suggests planets formed when asteroid-like rocks smashed into one another, coalescing to build bigger and bigger bodies. It is perfectly reasonable to expect huge impacts in the latter stages of this process. “We know that impacts are important to planet formation,” says Cuk.
Be that as it may, we may be forced to think again. The big splat itself could be quashed by new analyses of moon rocks brought back by the Apollo astronauts. According to the giant impact hypothesis, these did not all come from Earth, so you would expect them to show some differences in composition compared with terrestrial rocks and, in particular, contain different amounts of isotopes of the same element.
And that is the problem. When cosmochemist Junjun Zhang from the University of Chicago and colleagues completed an analysis in unprecedented detail of moon rocks last year, they found that the . Then in February this year, Hejiu Hui, a geologist from the University of Notre Dame in Indiana, and his colleagues discovered that several samples thought to be fragments of the first crust formed on the moon, including the famous Genesis rock brought back by Apollo 15 astronaut David Scott, . In the hellish aftermath of a giant impact, the heat generated should have melted the rocks and driven off the water.
Blast in the past
Hui is in no doubt of the significance of the findings. “This does challenge the giant impact scenario,” he says. Van Westrenen is more forthright: “The chemical composition of the moon deals the original giant impact scenario a fatal blow. It cannot be right.”
Taken at face value, the findings strongly suggest that the moon was once a part of Earth that was somehow blasted into space without being contaminated by rocks from a colliding planet. To avoid the angular momentum problem that plagued Darwin’s solution, however, a massive energy kick has to be delivered quickly and cleanly. Van Westrenen’s calculations show it must be the equivalent of 40 million billion atomic bombs of the size dropped on Hiroshima.
It was nuclear geophysicist Rob de Meijer at the University of the Western Cape in Bellville, South Africa, who first drew van Westrenen’s attention to a possible source. The idea that self-sustaining nuclear reactors might be buried in Earth has been around for 60 years. It seems almost certain that small ones were once active. In 1972, the French Alternative Energies and Atomic Energy Commission (CEA) was mining the Oklo region of Gabon in West Africa for uranium when it discovered a significant depletion of the uranium isotope U-235 that suggested it had been processed as if by a nuclear reactor.
Further exploration led to the uncovering of 16 natural fossil reactors between 1.5 and 10 metres across. Each was active around 2 billion years ago and probably continued on and off for a few hundred thousand years, kicking out around 100 kilowatts of power until they exhausted their supply of uranium.
Bigger reactors have also been proposed – indeed, it has been suggested that Earth’s core harbours a massive nuclear reactor. Van Westrenen was quickly convinced that something similar could explain the origin of the moon. “A nuclear blast is the only thing we could come up with that could produce the necessary energy quickly enough,” he says.
It would need something a lot larger than the Oklo reactors, though, and energy would have to be generated in a subtly different way – more akin to our fast breeder reactors. The basic idea is that heavy elements such as uranium, thorium and plutonium were concentrated in dense rocks that sank deep into Earth shortly after its formation. They accumulated at the boundary of the outer core and the mantle, where the restless geological forces brought them closer together to form large reservoirs.
Decaying radioactive nuclei within these rocks spit out fast-moving neutrons that can set off reactions of their own. But if the neutrons strike the right type of nucleus, such as uranium-238, they can be absorbed. The result is plutonium-239, which is itself a fissile material. If this absorption goes on unchecked, the fissile material builds up until enough fuel is present to go supercritical and explode.
An internal nuclear reactor could explain why Earth, like many of the planets in the solar system, gives out conspicuously more energy than it receives from the sun. This surplus energy powers the Earth’s magnetic field, volcanoes and earthquakes, and much of it is thought to come from radioactive processes within the planet. That seems to be confirmed by a steady stream of ghostly neutrinos, caught by the and neutrino detectors, based in Japan and Italy, respectively. The energy of their quarry shows all the hallmarks of by-products of nuclear reactions, coming up from Earth’s interior. What is not clear is whether these neutrinos are coming from the natural radioactive decay of elements within Earth, or whether natural reactors are enhancing their release in certain regions. A definitive answer would require a global network of neutrino detectors capable of building up a map of radioactive deposits within our planet.
“An internal nuclear reactor could explain why Earth gives out more energy than it receives from the sun”
Even if evidence for global “georeactors” was found, most people would still need a lot of convincing that they were capable of forming the moon. Some form of the standard scenario still has Cuk’s vote. “I don’t think you can separate the moon’s formation from a giant impact,” he says.
Having said that, he admits . Ironically, his idea starts with the conservation of angular momentum – the cast-iron concept that put paid to Darwin’s earlier hypothesis of the moon budding off from Earth.
Giant impacts have the problem that they impart a lot of energy to the Earth – so much so that the planet starts spinning faster than the 4-hour rotation that conservation of angular momentum says was possible at the point the moon was formed. But that’s if the Earth and the moon form a closed system. Together with his Harvard colleague Sarah Stewart, Cuk devised a cunning way to siphon off excess angular momentum using a third body: the sun.
The idea is that a peculiar alignment of the sun, Earth and moon created a situation known as an evection resonance. This trapped the moon in orbit, preventing the steady drift away from Earth that it has been embarked on ever since it formed. Such a situation could have persisted for 100,000 years or so, says Cuk. In that time, Earth, sun and moon were locked together in a gravitational threesome during which the early Earth’s excess angular momentum could be transferred through the moon to the sun. Eventually the moon broke out and started to recede again from Earth – as it still is, by a few centimetres every year.
The real pay-off came when Cuk and Stewart worked out what this meant for a giant impact. This mechanism allows Earth to spin faster in the past, so it would need less of a smacking to catapult the moon into orbit. Instead of a Mars-sized impactor, one with just half the mass could have hit Earth at a steeper angle, burying itself deep inside our world. Cuk and Stewart’s computer simulations show that would provide just enough energy to – providing a moon isotopically indistinguishable from Earth.
A different sort of “giant impact lite” has been proposed by planetary scientist of the Southwest Research Institute in Boulder, Colorado. She envisages . In the ensuing coalescence that gave birth to our planet, the moon was formed from the leftovers, ensuring both bodies were made from the same ingredients.
These two models are very different, but they both have the advantage of saving something of the giant impact model without having to propose anything as wacky as a vast, explosive nuclear reactor deep inside Earth. Van Westrenen is unruffled, pointing out that a faster spinning Earth as envisaged in Cuk’s model makes the energy required to form the moon during a nuclear detonation lower, too.
He has a proposal to test his idea. With their ability to change one element into another, deep-Earth reactors would increase the level of the isotope xenon-136 in the ejected material that formed the moon. This isotope is only formed in violent astrophysical processes such as supernovae, or through the radioactive decay of elements such as uranium and plutonium. Any excess in the moon’s xenon level compared with that found in meteorites, which represent chemically unchanged material from the solar system’s dawn, would indicate that nuclear processes were in play during our satellite’s birth. In principle, xenon levels could be measured by future lunar drilling experiments.
Such excavations are most probably decades away, however. In the meantime, the competing explanations for the moon’s origin will continue to slug it out. “There is a lot more work to be done,” says Cuk. The good news is that, whatever the outcome, there’s no ticking time bomb under our feet: the relatively short-lived isotopes that would have helped to power van Westrenen’s explosion have mostly decayed away by now. Whether or not Earth truly did explode one day 4.5 billion years ago, we are unlikely to experience its like again.
This article appeared in print under the headline “The day the Earth exploded”
