WHATEVER FLOATS YOUR CRUST: A new study of the moon provides further evidence for the dominant theory of its geologic evolution.
Image: NASA/JPL/USGS
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The leading hypothesis for the moon's formation contends that a massive impact billions of years ago knocked a wealth of planetary material off of Earth, which coalesced into our lunar companion.
But that coalescence was not a serene process—the heat of formation left the nascent moon coated with an ocean of magma, researchers studying samples returned by Apollo 11 theorized in 1970. As that ocean cooled, its most buoyant components floated to the top, forming an outer shell over denser layers of rock.
Now, a geological survey of the moon conducted using data collected by the recently retired Kaguya spacecraft supports this magma-ocean hypothesis, finding that the upper layer of the moon's crust is indeed rich in low-density rock of exceptional purity. The results from the Japan Aerospace Exploration Agency's Kaguya, formerly known as the Selenological and Engineering Explorer (SELENE), are published in this week's Nature. (Scientific American is part of the Nature Publishing Group.)
The Kaguya orbiter looked for anorthosite, a type of rock enriched in the relatively low-density mineral plagioclase. The spacecraft's survey found that in fresh craters and basins across the lunar highlands, the terrain that constitutes the majority of the moon's crust, anorthosite was not only prevalent but was almost entirely pure—that is, the rock was nearly 100 percent plagioclase. The study's authors speculate that exceptionally pure anorthosite may make up a global layer of the moon's crust from roughly three kilometers to 30 kilometers below the surface.
The ubiquity of the buoyant crustal rock points to a formation process that worked the same way across the moon, such as a magma ocean. The Kaguya results show that anorthosite formation "is really a global process," says John Longhi, a petrologist at the Lamont-Doherty Earth Observatory of Columbia University and currently a visiting scholar at Duke University, who did not contribute to the study. "Forming it by any other means than a magma ocean seems very doubtful, even though I've proposed one of those models. This research seems to suggest that there's really no other way."
Longhi's comments are significant, given that Paul Warren, a geochemist at the University of California, Los Angeles, calls Longhi "the author of, I think, the leading contender versus the magma ocean." In that model, plagioclase-rich rock rose to the upper crust as a result of reheating after an ocean of magma had already crystallized. Some other theories dispense with the need for a magma ocean entirely.
Warren, who did not participate in the Kaguya study, says that the new anorthosite research "implies that you had an awfully efficient process for purifying that mineral." The rock's prevalence and purity "is hard to reconcile with any kind of serial model where you have things happening in a piecemeal way," Warren says. "All that tends to weigh for a globally consistent process that is most readily explained with a magma ocean model."
Both Longhi and Warren harbor some skepticism about the extraordinary level of plagioclase purity in the anorthosites detected by Kaguya but maintain that the general implications remain relevant. Extremely homogenous composition is nearly impossible to find via remote sensing of the moon, Warren notes, because lunar soil is such a hodgepodge, and it is bound to contaminate any given surface to some extent. "Whatever extreme compositions might occur in the underlying crust, they are bound to be dampened" by deposited soil, he says.
"I find it a little hard to believe in detail," Warren says of the Kaguya paper. "But nonetheless, even if the details might be a little off, the import of what they've got is very great."
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Add CommentExcellent research of Japan Aerospace Exploration Agency's Kaguya spacecraft. Yes, indeed, a massive cataclysmic impact of another planet with Earth 4.5 billion years ago vaporized or melted down a large chunk of Earth and the other incoming planet that eventually coalesced into the moon. This was before life emerged on Earth 3.9 Ga. Kaguya findings of nearly 100 percent plagioclase provide convincing evidence of the magma ocean model of formation.
Reply | Report Abuse | Link to thisExcellent research of Japan Aerospace Exploration Agency's Kaguya spacecraft. Yes, indeed, a massive cataclysmic impact of another planet with Earth 4.5 billion years ago vaporized or melted down a large chunk of Earth and the other incoming planet that eventually coalesced into the moon. This was before life emerged on Earth 3.9 Ga. Kaguya findings of nearly 100 percent plagioclase provide convincing evidence of the magma ocean model of formation.
Reply | Report Abuse | Link to thisExciting results from Japan Aerospace Exploration Agency's Kaguya spacecraft. Yes, a massive cataclysmic impact of another planet with Earth 4.5 billion years ago vaporized or melted down a large chunk of Earth and the other incoming planet that eventually coalesced into the moon. This was before life emerged on Earth 3.9 Ga. Kaguya findings of nearly 100 percent plagioclase provide convincing evidence of the magma ocean model of formation.
Reply | Report Abuse | Link to thisthe models i've heard described suggest the impact debris was thrown into a low circular orbit around earth, akin to a halo effect, then subsequently coalesced into present luna and started drifting out into high orbit. if that was the process, how did the magma remain in liquid form over what must have been (literally) ages? or does it suggest a chunk of magma was blown off in a single mass, forming an instant binary system with a distinct lunar orb from the outset?
Reply | Report Abuse | Link to thisThe magma would have been generated as the orbiting debris rained down upon the lunar surface, its kinetic energy being transformed into thermal energy upon impact. This research suggests that the layer of anorthosite could be up to 30km deep, and the denser material that crystallized out of this magma must be even deeper still! An ocean of magma this deep, and that spans the entire lunar surface would be able to store huge amounts of thermal energy for a very long time, as it could only have lost energy by radiating it into space at its surface, which, though large, is nothing compared to the reservoir beneath it. Therefore, this ocean could have remained liquid for eons.
Reply | Report Abuse | Link to thisWow, I'd love to see a simulation of this process.... from something striking the earth through the formation of the moon.... I hope someone animates a realistic possible view of what might have happened, and posts it for us to see.
Reply | Report Abuse | Link to thisAccording to the article, a widely acceped hypothesis of the formation of the moon is that a wealth of planetary material leaves the earth, and finally forms the moon, and during the process, the most buoyant components floated to the top which give rise to the magma-ocean.Further discovery by the retired Japan spacecraft show that the so called magma ocean is of a high degree of purity, named about 100 percent plagioclase. I wonder that what a kind of powerful force will knock such a enormous part of the earth to form the moon, and how can the magma ocean would maintain so higher degree of purity in the space which consists so many contaminates?
Reply | Report Abuse | Link to thisThe moon coalesced relatively quickly after impact - there are some nice hydrodynamic simulations on the web performed by Robin Canup. These all suggest that formation was rapid - a matter of days after impact - with the energy generated by the impact providing substantial heat (how hot, its not easy to calculate, but enough to provide a silicate atmosphere which would allow isotopic equilibration of the oxygen between impactor/moon and the early Earth).
Reply | Report Abuse | Link to thisIn answer to Davids question above, the Earth had a core before the moon forming event - we know this from various isotopic systems, notably tungsten/hafnium. The core would have most likely formed at the same time as accretion progressed. We have direct evidence of similar processes occurring very early on in meteorite materials. More detailed knowledge of the conditions and processes of core formation is obviously hard to come by. But, by performing detailed experiments at high pressures and temperatures we can look at how elements behave under planetary forming processes. Of late, we are also able to look at the subtle differences in isotopic composition between bodies and explore the the processes that may influence these small differences. For instances, the small differences in silicon isotopes between Earth moon and asteroids *may* be due to conditions of core formation. regards.
Personally, I go with the binary theory of formation, whereby the early earth is split in two. One thing no one seems to have considered, however, is this: if the aforesaid event had not happened, how would women regulate their moon cycle?
Reply | Report Abuse | Link to thisA new thory of the evolution of earth and the moon, they were created at the same time expulsed by the sun, in a sort of spin,arround one another, keeping face to face, their in between filed with sunplasma. After some time the thing cooled of and organic life became possible and indeed life literally exploded growing to overprorposional expressions animal or vegetal. A new planet suddenly arrived hit the earth, exploded(leftover the asteroid-belt),made the earth spin faster and desengage the moon and have its own gravity and on the same time attact all of the water wich was suspended between the aerth and the moon,so in the explosion of the astplanet made melt the surface of the moon and the earth, on earth the ocean,since then made sand of what happend then, I can here add that fossile rest are found everywhere on earth and the northpole was facing the moon!
Reply | Report Abuse | Link to thisI finally read all the comments, with interest and appreciation for the information received. What I get from it, though, is that it doesn't bode well for advanced life on any great multitude of planets---at least, not as we know it---if a collision with another suitably sized planetoid is required to blow away the required elements that would otherwise make for another Venus (even those planets that sit in the hot spot for life like Venus, Earth, and Mars). It seems likely, then, that all we're likely to find out there are bacteria, viruses, and prions---along with the occasional hint of civilizations that abandoned their home planet hundreds of thousands of years, ago.
Reply | Report Abuse | Link to thisjoe poppa, I don't see how life could not form on a multitude of planets under entirely different circumstances. We are limited to only our example of life on this planet with its specific chemistry and geological history. Life here was a product of happenstance and has restarted at least five times in our planets history. There may be millions or billions of ways life can assemble given the available resourses of the host planet. And as you suggest, some life may have left their planets thousands of years ago suggesting the path that life took on our planet may not have been the most efficient. Could there be conditions that would spawn life and then accelerate the process of intelligence by millions of years? I think given the billions of star systems it is entirely feasible. But if those life forms left their home planets, where did they go? Where are they now? I feel that we are living in an age, or at least close to it, that may begin to get the answer to many of the questions. Given the advances in genetic engineering, it is possible we may still live long enough to see some of these answers. How exciting it will be when we finally find life on another planet and even perhaps have the opportunity to study it to compare how it evolved compared to ours. What new ways of thinking may come of it?
Reply | Report Abuse | Link to thisjoe poppa, I don't see how life could not form on a multitude of planets under entirely different circumstances. We are limited to only our example of life on this planet with its specific chemistry and geological history. Life here was a product of happenstance and has restarted at least five times in our planets history. There may be millions or billions of ways life can assemble given the available resourses of the host planet. And as you suggest, some life may have left their planets thousands of years ago suggesting the path that life took on our planet may not have been the most efficient. Could there be conditions that would spawn life and then accelerate the process of intelligence by millions of years? I think given the billions of star systems it is entirely feasible. But if those life forms left their home planets, where did they go? Where are they now? I feel that we are living in an age, or at least close to it, that may begin to get the answer to many of the questions. Given the advances in genetic engineering, it is possible we may still live long enough to see some of these answers. How exciting it will be when we finally find life on another planet and even perhaps have the opportunity to study it to compare how it evolved compared to ours. What new ways of thinking may come of it?
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