MOON STRUCK: A simulation of Earth's moon absorbing an impact by a smaller companion moon billions of years ago offers an explanation of puzzling features on the lunar surface.
Image: Martin Jutzi and Erik Asphaug
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For tens of millions of years—a mere sliver of astronomical time—the night sky above Earth may have been a bit more populous than it is today. For that brief period, our planet may have had not one but two moons, which soon collided and merged into our familiar lunar companion. No one would have been around to see the second moon—the lunar merger would have occurred nearly 4.5 billion years ago, shortly after Earth had formed.
The two-moon hypothesis, put forth in a study in the August 4 issue of Nature, would help explain why the moon's two hemispheres are so different today. (Scientific American is part of Nature Publishing Group.) The familiar hemisphere facing Earth is covered by low, lava-filled plains (seen as the darker gray areas on the moon's "face"), whereas the far side, which is never visible from Earth, is a collection of rugged, mountainous highlands. Those highlands, according to the new hypothesis, would be the remains of the smaller, short-lived satellite following its collision with the moon that now hangs overhead. The key is that the moonlet's impact would be slow enough to pancake its material across one face of the moon rather than excavating a large crater.
"Usually when you think of two objects colliding, one of them leaves a big hole," says Erik Asphaug, a planetary scientist at the University of California, Santa Cruz, who co-authored the new study with Martin Jutzi of the University of Bern in Switzerland. "At a low enough impact speed, you actually deposit material." The effect of two moons coalescing at subsonic speeds is an almost fluid merger, "like you literally threw a cow pie on the ground and there it is," Asphaug says. "The physics are basically the same."
The leading hypothesis for the moon's creation itself involves an impact, this one a higher-speed crash of a Mars-size body into the nascent Earth. That collision, as the story goes, packed enough punch to kick up a ring of debris around Earth that coalesced into the moon. If an accompanying moonlet formed in the aftermath of that collision, simulations have shown, the system would be unstable, pushing the moonlet into a sudden demise in a collision with the dominant moon or with Earth. But certain orbital safe havens known as Trojan points, leading or trailing the moon in its orbit around Earth, would allow a moonlet to hang around for tens of millions of years before meeting its end.
By that time, the two objects would be at very different stages of evolution: a moonlet roughly one third the diameter of the moon would have cooled and solidified, whereas an ocean of magma would persist on the larger moon. In Jutzi and Asphaug's computer simulations, the pancaking of a solid moonlet against a partly molten moon would provide enough material to create the elevated highlands on one hemisphere and would displace huge amounts of magma to the opposite hemisphere.
One attractive feature of the new hypothesis is that it tidily explains why the near and far sides of the moon are not only topographically but compositionally different. Several sites on the near side sampled by Apollo astronauts had rocks enriched with KREEP—for potassium (K), rare earth elements (REE) and phosphorus (P)—which resists crystallization from magma and hence remains in a molten state until the entire magma ocean has solidified. But KREEP is scarce on the lunar far side. The hypothesized moonlet pushing a cooling magma ocean and its KREEP to the lunar near side would explain the dichotomy. "The momentum of that impact squashes the KREEP onto the other hemisphere," Asphaug says. "There's this compositional puzzle that we did not set out to explain but our model does help to explain."
The planetary scientist who, along with Asphaug, helped vault the giant-impact mechanism for the moon's formation into wide acceptance, sees value in the new hypothesis. "It's an old problem of trying to understand why there's this elevation dichotomy on the far side of the moon," says Robin Canup of the Southwest Research Institute in Boulder, Colo., whose 2001 study with Asphaug identified a Mars-size impactor as the likely moon-yielding culprit. "A nice new explanation that seems plausible for an old problem in planetary science is always a great thing."
A NASA lunar mission scheduled to launch in September, the Gravity Recovery and Interior Laboratory (GRAIL), may help settle the question of whether the moon once had a smaller companion. GRAIL, which will map the moon's gravitational field to expose variations in its near-surface density, may be able to detect the residual effects of a long-lost moonlet pancaked across the lunar far side. "I think it may be testable with time," Canup says. "The type of alteration that they're predicting, you might be able to see some evidence for or against that in something like future GRAIL data."
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25 Comments
Add CommentSo would discoveries such as these better help us explain/understand how water ended up on Earth? What if this moon happen to be of liquid composition?
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But cdeachus, Wouldn't that leave water on the moon? Or at least evidence of having it? Moons colliding might leave discernible layers of mineral evidence.
Reply | Report Abuse | Link to thisWe have a lot of water, musta been a lotta moon.
Zero, and multi, mooned planets seem to have extraordinary weather patterns.
I should think a forming solar system would leave a few dinks and dents.
Strange how things worked out so perfectly for Earth's inhabitants.
I thought the accepted theory was that since the moon is tidaly locked with one face towards the Earth it was shielded from further bombardment and the maria were allowed to cool unhindered.
Reply | Report Abuse | Link to thisHi Scilo,
Reply | Report Abuse | Link to thisVery true. I was leaning more towards the idea of a moon (with liquid water composition) slamming into the earth during planetary develoment. Possibly leaving an abundance of water to earth and relatively small traces on the newly formed moon. It just frustrating that we as humans are still struggling to explain our planet, solar system, galaxy, and so on; just theories. But on the other hand, it is always exciting to whiteness new discoveries. I feel we live in a exciting time and that we’re on the brink of a huge discovery.
AZComicGeek,
Yes. That part of the theory was making me scratch my head as well.
That sounds like a simpler explanation. Occam's Razor...
Reply | Report Abuse | Link to thisOr perhaps tidal forces kept the single moon's Earth locked side molten for a longer period than the cooler outer side, allowing the slowly cooling mountains to form and the iron deposits to move to the Earth side. Keep in mind the 'new' moon was much nearer to the Earth than it is now...
Reply | Report Abuse | Link to thisI thought that the moon slowed its rotation over time due to the gravitational interaction with the earth and that the moon was not in lock step with earth until relatively recently. I also understood that earth's rotation was actually faster back then as well and slowed due to the moon's influence. So the hypothesis that earth facing side of the moon was protected would only be true for "recent" history.
Reply | Report Abuse | Link to thisI may be wrong, but I think I recall that the recent Japanese orbiters determined that the moon does not have an iron core per se, but that several large iron nodules are located within the moon's near side, keeping it facing the Earth. It's difficult for me to imagine that this condition could have changed over time, although its rotation rate certainly would decrease in lock step with the Earth's...
Reply | Report Abuse | Link to thisHowever, wikipedia [http://en.wikipedia.org/wiki/Moon] does state:
"The Moon is in synchronous rotation: it rotates about its axis in about the same time it takes to orbit the Earth. This results in it nearly always keeping the same face turned towards the Earth. The Moon used to rotate at a faster rate, but early in its history, its rotation slowed and became tidally locked in this orientation as a result of frictional effects associated with tidal deformations caused by the Earth."
It refers to a 1973 paper:
Alexander, (1973), "The Weak Friction Approximation and Tidal Evolution in Close Binary Systems",
http://adsabs.harvard.edu/abs/1973Ap&SS..23..459A
I don't know which view is correct, but I suspect the 1973 findings have been established and likely taught for many decades now.
It has also been mentioned that a slow speed collision of two moons in high velocity, close proximity orbits of the Earth would be extremely unlikely...
Reply | Report Abuse | Link to thisNo cow I ever saw on the farm ever "threw" her pie onto the ground. LOL...
Reply | Report Abuse | Link to this"But KREEP is scarce on the lunar far side." Oh really? How do we know since we never sent any astronauts there? But from what I've been told UFOnauts do have a base there so their comings and goings can not be seen from Earth. Maybe we should ask them about KREEP.
If a collision with earth created the moon and moonlet how might they differ in composition as the theory suggests?
Reply | Report Abuse | Link to thisAtlanta Terry,
Reply | Report Abuse | Link to thisI, too, wondered how cosmologists seem to know so much about the composition of the Dark Side of the Moon. There has been no physical remnants of that hemisphere brought back to Earth for study. I realize that instrumentation exists that can help form rational hypotheses for much of what we don't know about the cosmos, but as of now that's all its capable of doing - forming hypotheses. Making blanket (and unproven) assertions about what is or isn't on the Dark Side subtracts from the theory as a whole. At any rate, we'll find out all the secrets of the universe, near and far, when God decides to let us in on them. We just need to keep plugging away with our research.
Occam's razor only applies, when two theories of differing complexity explain the observations equally well.
Reply | Report Abuse | Link to thisWater on earth is easily explained by the collision with the earth, both during the Hadean and post Hadean periods, of many comets. These also collided with Mars and Venus. The comets would have produced seas and oceans of fresh water. Evaporation and runoff of the land would have produced a salt ocean after a few million years.
Reply | Report Abuse | Link to thisThe Pacific Ocean missing part might be a part of the moon. A small Moon or a meteorite with a great long focal distance around the sun slowly touch the earth enough hot and soft to enable a collision removing the upper part of Pacific Ocean. As leaving the earth it strike the other small moon by a complete hazard. It is possible
Reply | Report Abuse | Link to thisYou are correct sir!
Reply | Report Abuse | Link to thisI find new explanations for previously discovered puzzles intyeresting, but the fact that these explanations may be testable even more so. Waiting for the data from the (holy) Grail...
Occams Razor? not a law (either scientific or philosophic), just a useful rule of thumb. A gamblers tool, suggesting we stick with the explanations of the highest probability. in the universe, sometimes, longshots come thru.
Reply | Report Abuse | Link to thisHow do we know the composition of the "dark side of the moon?" a stupid question. we have seen it. we have examined it in detail, spectroscopicaly. (with orbiting telescopes, spacecraft and other assorted satelites)
The two-moons hypothesis has several weaknesses: 1) as pointed out above, if the two moons had originated from a collision with Earth, why would their compositions be different? 2) the theory of a Giant Impact forming the Earth-Moon system has been seriously undermined by new evidence of lunar melt inclusions containing volatiles (including water) that could not have survived a very high energy event; 3) the velocities of the two moons would need to be very similar--highly unlikely; and 4) the maria were formed long after this hypothesized event, yet the hypothesis provides no explanation of why they cover 31.2% of the near side and only 2.5% of the far side--a better hypothesis would include an explanation of this disproportion. All in all, while intriguing, the two-moon hypothesis has too many weaknesses to merit acceptance; and I have probably missed some weaknesses. I think that there is a much better explanation for the skewed topography and elemental composition of the Moon. It requires us to revisit the capture theory of the origin of the Earth-Moon system, using the assumption that the Moon showed up in the vicinity of the Earth in a comet-like form that, Slinky-like, bent itself around the Earth. Search "The Outer Solar System Origin of the Terrestrial Planets".
Reply | Report Abuse | Link to thisTwo questions:
Reply | Report Abuse | Link to thisWhy would two moons formed from the same ring of debris have such different compositions?
Why does the smooth side of the moon face the earth? The probabilty of this happening by chance is extremely remote.
Hello AZComicGeek! If you consider how small the Earth appears in the lunar sky, it becomes obvious that the Earth is not much of a shield; there are relatively few trajectories for collisions with the near face that are blocked by the Earth. On the other hand, the asymmetry could explain why the Moon finished up with this face locked to the Earth.
Reply | Report Abuse | Link to thisThe KREEP, Iron, Siderofiles, and Thorium found in the near side of the moon are near 5 big mass concentrations...
Reply | Report Abuse | Link to thisThat kind of stuff is expected in the core of a moon not its surface...
May be there was not just one big impact in the far side, but at least five small ones in the near side?
The residues of the cores are near the moon surface, and released some amount of that stuff.
You can look at the NASA's Lunar Prospector maps to see the relation:
Reply | Report Abuse | Link to thishttp://lunar.arc.nasa.gov/dataviz/datamaps/
The moon is bound to lock up along the axis of asymmetry, since the tides act as a radial stretching force that the axis will natually line up with. (See 'Neutron Star' by Larry Niven for a brief exposition.) I suppose the moon could have lined up the other way, maria facing out. Imagine the head scratching Luna-3 would have provoked in that case!
Reply | Report Abuse | Link to thisTidal locking occurred very early, when the moon was close. Tidal forces were then very strong, being proportional to the inverse cube of the separation. Ever since then, the same tides have been increasing the earth - moon separation, but they are still strong enough to keep the moon's rotation locked. In the distant future, the sun's tide will act to draw earth and moon closer, while increasing the separation of the sun and the eart-moon system.
Reply | Report Abuse | Link to thisKREEP is rich in thorium, which has a characteristic gamma spectrum that is easily detected at a distance by an orbiting satellite. There have been plenty of these. Remember Clementine? India has sent one too.
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