THEORY OF RELATIVES: Earth and the moon, as seen from the Mars Reconnaissance Orbiter, seem to share a common origin.
Image: NASA/JPL-Caltech/University of Arizona
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The moon may be a chip off the old block after all.
The most commonly invoked explanation for lunar formation holds that a giant protoplanet, sometimes called Theia, struck the newly formed Earth 4.5 billion years ago and created a cloud of debris that quickly coalesced into the moon. But that hypothesis has suffered from a nagging flaw. Simulations of moon-forming collisions have shown Theia would have been the primary donor of lunar material. But analyses of Apollo moon rocks have shown that the moon seems in many ways a chemical clone of Earth, not Theia.
“The giant impact theory explains many traits of the system—that’s why it’s favored—but this [discrepancy] is a little tricky,” says planetary scientist Robin Canup of the Southwest Research Institute in Boulder, Colo., who played a key role in developing the Theia idea. “This has been a thorn in the side of the impact theory for some time.”
That thorn may be on its way out. A pair of papers published online October 17 in Science, one by Canup and one by planetary scientists at the SETI Institute in Mountain View, Calif., and Harvard University, demonstrate two different ways that a giant impact could produce a moon with the observed chemical similarities to Earth.
In Canup’s model, the impactor is substantially heftier than the canonical Theia—instead of a Mars-size object colliding with the much larger proto-Earth, her new study proposes a smashup of two comparably sized objects. “The set of impacts that I identify that can do this involve a much larger impactor than had been considered before,” Canup says. “The type of impact that I’m advocating here is the collision of two half-Earth-mass objects. They merge to form the Earth.” The moon would then form from the leftover debris, naturally explaining its similarities to Earth.
A different conception, from Matija Ćuk of the SETI Institute and Sarah Stewart of Harvard, invokes a small, high-velocity projectile smacking into a fast-spinning proto-Earth. Like an interplanetary mortar, that high-energy impact would fling out a cloud of debris composed primarily of material from Earth. “The crucial difference is that Earth is spinning faster,” Ćuk says. “If you hit it hard it’s easier for the pieces to fly into space.”
Both studies build on the recent finding by Ćuk and Stewart that gravitational interactions with the sun can quickly sap angular momentum from the newborn Earth-moon system. As a result, Earth may have been spinning much faster after lunar formation than had previously been thought plausible—a day on Earth may have lasted only two or three hours immediately after the impact. And the possibility of a fast-spinning Earth opens the door to types of collisions that had not been considered viable before.
Indeed, the true impact of the new studies is not in the specifics of the revised lunar-forming models but in the fact that such revisions now appear plausible, says Erik Asphaug, a planetary scientist at the University of California, Santa Cruz. “It’s not so much that they’ve come up with a model that works; it’s that they have taken away a constraint that we thought was sacrosanct for the last 20 years,” he says.
Ćuk, too, foresees the opening of a new chapter in unraveling the story of the moon’s birth. “This is going to be, I hope, the first of a new batch of papers, rather than the final word,” he says. “The thing that really surprised me and Sarah is, we didn’t try very hard—this kind of came out pretty much by itself. So that’s promising. We didn’t have to look far and wide for something that worked.”
The only catch is that Theia’s size and the magnitude of its impact, which once seemed to be fairly well understood, are now open to debate. And many more plausible scenarios that can explain the Earth-moon system may now come to the fore. “That is my worry—I wonder if moon formation may have become an unsolvable problem,” Asphaug says. “If you can have an Earth that is spinning with pretty much any spin rate, suddenly all bets are off.”
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30 Comments
Add CommentMy interpretation of "Beast of 616" is an asteroid 1 mile in diameter hitting the earth and raising Atlantis. May 21 2016.
Reply | Report Abuse | Link to thisWhat about the evidence for a pre-Earth crust? For pre-Earth life and a pre-Earth ocean? Whatever happened to the planetary magma ocean that followed when the Earth re-solidified?
Reply | Report Abuse | Link to thisWas there an impact because there was an impact, or because destructive impacts bring joy to the hearts of 12 year old boys deep in the souls of grown men everywhere?
Alan6302; please never read Scientific American or comment on anything ever again. You obviously don't understand what the word science means, and if you actually put faith into things like you said, you never will.
Reply | Report Abuse | Link to thisFrom the story on Phys.org, the main piece of evidence is that the Moon only contains heavy zinc isotopes; the lighter isotopes failed to condense out of gaseuos form before being blown out of the gravity well.
Reply | Report Abuse | Link to thisI'm still hedging my bets on this one, but it's looking more likely by the day.
@ alanwhatsit: Atlantis? Really? Have you checked your pills yet today?
You better reconsider your "12-year-old-boy-who-loves collisions" bias because Robin Canup is a (censored)-year-old female astrophysicist who,incidentally, is about to have a baby! Evidently females love collisions, too. Especially when they best explain the observed facts. Good luck, Robin, on your collision simulation and also on you real-world event.
Reply | Report Abuse | Link to thisI would imagine that such an impact would give rise to a somewhat eccentric orbit about the sun. And yet Earth has one of the least eccentric orbits in the solar system. How does the Theia hypothesis account for this?
Reply | Report Abuse | Link to this@Chris: I don't think the impact would automatically mean orbital eccentricity at all. What are you basing your theory on?
Reply | Report Abuse | Link to thisI would actually imagine that the shifting of Jupiter and Saturn orbits that lead to their 2:1 orbit ratio, which had pushed Neptune and Uranus to where they are now, would have had much greater effect on the orbits of the inner solar system planets as well.
Re: "A pair of papers published online October 17 in Science...demonstrate two different ways that a giant impact could produce a moon..."
Reply | Report Abuse | Link to thisIf the information available to me is correct, impacts (i.e., collisions) do not produce anything but rubble. What, exactly, was the demonstration that produced the moon?
That's the main issue with the hypothesis. I bet, though, that pre-Earth's orbit was highly eccentric.
Reply | Report Abuse | Link to thisBill, have you ever heard of gravity? With sufficiently large masses involved, it can actually fuse rocks and dust. Why do you think that we have an atmosphere? The Earth's gravitational field picked up a bunch of gas from the early solar nebuls.
Reply | Report Abuse | Link to thisWe agree with Mr. Bill_Crofut that impacts (i.e., collisions) do not produce anything but rubble. If we look at the chemical composition of the Sun’s surface, we find the dominant element is hydrogen – 71 percent. In the case of Earth’s crust, it is oxygen – 46.6 percent. But what about the Moon? Yes; it is covered with silicate like Earth. But what about hydrogen, helium and oxygen? It is totally different. The Earth’s core is composed of mostly molten iron. The Moon a small iron core perhaps 300 km in radius, with a temperature of about 1500 K, too cool to melt the rock, surrounded by a soft asthenosphere that do not permit S-waves, implying it must be plastic. The Mare covers of the moon are less reflective than the “highlands” as a result of their iron-rich compositions, and hence appear dark to the naked eye. The maria cover about 16 percent of the lunar surface, mostly on the near-side visible from Earth. The few maria on the far-side are much smaller, residing mostly in very large craters. The Near side has more mare: 32% of its surface is mare covered compared with 2% of the far side (globally mare cover 17%).
Reply | Report Abuse | Link to thisThere is interesting similarities between the Moon and the Mercury. If the crust of Moon is heated enough, it will resemble the crust of Mercury. There are many other similarities between the two like only one side of their surface is always faced towards their principal body, strength of magnetosphere, etc. Thus, the theories of the origin of Moon are all wrong. The origin of Mercury is linked to the origin of Moon. Both were formed neither due to impact of two bodies (it will impossible for Mercury), not due to collision or split. Their formation should be linked to the conservation laws. While the cosmic background temperature is very low, it is very high at the stars and galaxies. Thus, the accumulation of high temperature bodies are linked to a corresponding process of evolution of low temperature bodies following the conservation laws. Both have the same source of origin – cosmic nebulae.
mbasudeba@gmail.com
And you don't know what you're talking about, as evidenced by your first sentence.
Reply | Report Abuse | Link to thisИзменение формы Земли изменило скорость вращения Земли,угловой наклон это нарушило равновесие в ситеме Земля-Луна.Луна приблизилась к Земле на такое растояние,что попала в сферу тяготения Земли (не путать с гравитацией).Не дойдя до точки Лагранжа,Луна (во время лунного затмения)обмениваясь с Землёй апокалептическими выбросами меняя географические полюса(смена полюсов Луны доказана сотрудниками парижского Института Физики Земли) одновременно с Землёй,стала удаляться от Земли.Земная атмосфера вместе с земным грунтом(лунный лёд) попала на Луну также как брекчий на Землю.
Reply | Report Abuse | Link to thisRespect Sir,
Reply | Report Abuse | Link to thisWill you please clarify your statement? We may not know anything, but you can be a good teacher.
Regards,
basudeba
Try smashing together two really big lumps of molten rock and iron at high speed in a vacuum. You will get a big fused mass of rapidly stratifying rock and metal, and a bunch of ejecta.
Reply | Report Abuse | Link to thisI should note that I am hedging my bets on the Theia hypothesis until further information is forthcoming. It does sound cool, though.
Basudeba (comment 11),
Reply | Report Abuse | Link to thisThank you for your support.
Bird/tree/dinosaur/etc. geek (comments 10),
Reply | Report Abuse | Link to thisThe effects of gravity are observable (even if gravity itself is not). Where is the observable evidence that gravity, regardless of the masses involved, fuses rocks and dust into moons?
comment 15,
Are you reporting on the results of an experiment someone has conducted?
Since I apparently got deleted:
Reply | Report Abuse | Link to thisI cannot argue with you, as you seem to be incapable of rational debate. I will reference you to this page:
http://en.wikipedia.org/wiki/Modern_geocentrism#Religious_and_contemporary_adherence_to_geocentrism
I must admit that you have taught me something: Before I "met" you, I thought that such people no longer existed. Obviously, I underestimated the utter irrationality of humanity.
I suspect 6302 was trying to be humorous. Perhaps he was trying to imply these approaches to solving the moon/earth origins were as plausible as the book of revelation in describing the end of the world.
Reply | Report Abuse | Link to thisWould you please stop referring to yourself as "we"? It sounds disturbingly like something out of a distopian novel.
Reply | Report Abuse | Link to thisYour point is unclear and your views unscientific. Please go learn basic physics.
Furthermore, I already stated that I am hedging my bets on the Theia hypothesis for now. If you and Bill_Crofut want to say that it is impossible, when you both appear to hold baseless fringe views that have literally thousands of papers that disprove them, prepare to be ignored.
It's hard to tell. And if he means it, he needs to double-check his meds.
Reply | Report Abuse | Link to thisnow, Ladies and Gentlemen, why not consider the more plausible common model not only of the moon but also of the star and galaxy formation to boot in this fractal universe of ours.
Reply | Report Abuse | Link to thisAs Dr.Krumholz of Princeton U explains, "Instead, stars form by fragmentation, and the fragmentation process determines their masses." For more, please see: http://www.sittampalam.net/StarFormation.htm.
(I cannot help thinking if the good doctor and his team of researchers had got this fragmentation idea from my book and web page illustrations published many years before the team's article here in Physics Web!)
The moon, too, could well have fragmented from a molten early earth during one of the latter's violent, nova-like eruptions, when a massive chunk of the lava-like outer crust of the earth would have (Houston, we have) lifted off! Please see the illustrations in: http://www.sittampalam.net/TheSun.htm
Thank you all. Cheers!
www.toe.tv
Must have been one hell of an eruption. Either that, or you're out of your depth.
Reply | Report Abuse | Link to thisSadly, I am pretty sure that 6302 is being serious, based on many past comments of his. Some people are fun to argue/discuss things with (like Bird), but some are best left alone in their darkness.
Reply | Report Abuse | Link to thisBy the way, I agree with Bird on this one, plausible, but the jury is still out.
Thank you! It's good to have a friendly rival.
Reply | Report Abuse | Link to thisBird/tree/dinosaur/etc. geek comment 18 (comment 18),
Reply | Report Abuse | Link to thisIf you are truly concerned about rational debate, please rationally debate me on the following:
A geocentrist and a heliocentrist can stand side by side in an open field facing east at dawn and observe the sun ascend above the horizon (http://www.youtube.com/watch?v=Cw_JfsU0OWY). They can return to that same open field facing west at dusk and observe the sun descend below the horizon (http://www.youtube.com/watch?v=8tyNaJxZ76A). The geocentrist will accept what been observed as reality. The heliocentrist, by reason of his position, must not only reject what has been observed as reality, but must believe (for it is a faith commitment) that precisely the opposite of what has been observed is reality.
Re: http://en.wikipedia.org/wiki/Modern_geocentrism#Religious_and_contemporary_adherence_to_geocentrism
“Geocentrism as a separate religious belief, however, never completely died out.”
Please explain to me the religious content of the two videos above which will be the basis of our debate.
Where did Theia come from? Could it have been in an orbit similar to Earth's, in which case the 'When worlds collide' animation showing a high speed collision may be fanciful, and an orbital dance followed by a merger more realistic? Would not their gravity produce mutual disruption before the planets even touched? If Theia was similar in size to the present Earth, what happened to the rest of it?
Reply | Report Abuse | Link to thisRubble, in the early solar system, could coalesce into a coherent orbiting body which in English is called the Moon.
Reply | Report Abuse | Link to thisThe early solar system had so many collisions and movements that we do need a computer to help make sense of it all. Gas was ubiquitous in that early solar system, and gravitationally picking up and producing nitrogen, carbon dioxide, argon, and others was the normal occurrence. The comets now are in neat orbits, except for a very rear comet collision which produces non orbiting comets. But in the early solar system, many comets were out of orbit, and passing through the planets, and colliding with them. The multiple comet collisions explain our water, which has accumulated into oceans. But before this, when our earth was a bit smaller, a similar sized body could have collided, producing rubble which became the moon, and which probably moltenized much of the earth, allowing iron, uranium, and other heavy metals to settle at our core. This also would have produced a storm of volcanism. after all this settled, the comets came with their water. Oxygen was a biological product. Then came the dinosaurs, and finally we have McDonalds.
Reply | Report Abuse | Link to thisJacobSilver,
Reply | Report Abuse | Link to thisAre you aware of any experiment which would verify that rubble does coalesce in such a manner as to form a moon?