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    Thursday, February 15, 2007


    The Universe Creates Our Home

    Posted on: Wednesday, 17 November 2004, 06:00 CST

    When Worlds Collide: How the Moon Was Formed

    Click to enlarge

    In astronomical circles, it is pretty much official. The Moon was created when a body about the size of Mars slammed into the newborn Earth. In the cataclysm, the molten iron core of the impacting body sank to the Earth's core while its molten mantle splashed out into space to form a ring of debris. This congealed into the Moon.

    The Moon, originally about 20 times closer to the Earth, gradually moved out to its current location. This "Big Splash" picture, proposed by William Hartmann, Al Cameron and their colleagues in 1975, is very well-received. For instance, it explains why the Moon contains essentially no iron.

    Unfortunately, it has a big problem. It concerns the body that collided with the Earth. "Where did it come from?" says Richard Gott of Princeton University in New Jersey. "The clues suggest a seemingly impossible location."

    One such clue comes from comparing the composition of the Earth and Moon. Cosmologists are pretty sure that the disc of swirling debris from which the planets congealed had a different composition at different distances from the newborn Sun.

    The Mars-mass body would, therefore, not have had the same make- up as the Earth. In the impact, the Earth and Moon would have been contaminated by different amounts of this material, which means, when we examine terrestrial and lunar rocks, we should see marked differences in composition. "The bizarre thing is, we don't," says Gott.

    Take oxygen. It comes in three types - oxygen-16 and two heavier and rarer types, oxygen-17 and oxygen-18. The relative proportions of these are like a chemical "fingerprint". The prediction of the Big Splash scenario is that the Earth's oxygen fingerprint will be quite different from the Moon's. But it isn't. It's pretty much identical.

    The oxygen evidence forces the conclusion that the body that hit the Earth and created the Moon formed at exactly the same distance from the Sun as the Earth. This is also indicated by computer simulations of the birth of the Moon, which show that the impactor came in at relatively low speed, characteristic of bodies in the Earth's vicinity. "But if the impactor formed at the same distance from the Sun as the Earth, there is a big problem understanding how it ever managed to grow as big as Mars," says Gott.

    The accepted theory of the birth of the planets is that they gradually "accreted" from debris pulled in by their gravity. The bigger they got, the stronger was their gravity and the more matter they pulled in. Since it is a process in which the rich get richer and the poor poorer, the impactor should have been gobbled up by the proto-Earth long before it reached the mass of Mars. So, why wasn't it?24 Hours of Chaos: The Day The Moon Was Made By Robert Roy BrittSenior Science Writerposted: 02:00 pm ET15 August 2001For 25 years, scientists have pondered a theory that the Moon was created when an object the size of Mars crashed into Earth less than 100 million years after the Sun was born, some 4.6 billion years ago. The general idea has been run through the paces and massaged into shape and is now the favored explanation.

    Gott set out to solve the puzzle with Princeton colleague Edward Belbruno. They began by asking: is there some special location at the Earth's distance from the Sun where a body could grow to the mass of Mars? Immediately, they realised there is.

    In fact, there are two places. These are the "Lagrange- 4" and "Lagrange-5" points, whose existence was first suggested by the French mathematician Joseph Louis Lagrange in 1772. One lags 60 degrees behind the Earth as it orbits the Sun and the other precedes the Earth in its orbit by the same amount.

    At the Lagrange points, all the forces in the Sun-Earth system miraculously balance each other. What's more, any slow-moving debris that happens to find its way there becomes hopelessly trapped in a kind of interplanetary Sargasso Sea.

    Gott and Belbruno say the Lagrange points are places where matter would naturally have accumulated and where a body could have grown in peace without being affected by the fast-growing Earth. Eventually, when it had reached the mass of Mars, the gravity of other embryonic planets in the Solar System, such as Jupiter, would have tugged it repeatedly, perhaps over millions of years, until it was ejected from the Lagrange point.

    In computer simulations, Gott and Belbruno have followed the subsequent course of events. They find nothing can prevent the inevitable - a titanic collision with the Earth. Everything appears to fit. The impactor comes in on a low-velocity orbit, delivering a glancing blow on the Earth. Gott and Belbruno's simulations show that, in a quarter of encounters, the end result is a body exactly like the Moon.

    If Gott and Belbruno are right, the Earth had once had a planetary which shared its orbit round the Sun. "It's a clever idea which would solve some obvious problems," says Carl Murray of Queen Mary University in London. But he thinks work still needs to be done to prove it.

    The most interesting consequence of Gott and Belbruno's scenario is its implications for our prospects of finding extraterrestrial life. The Earth has the biggest moon compared to its size of any planet in the Solar System (Pluto also has a big moon but is rarely considered a full-blown planet nowadays). And a giant moon has been important for the evolution of life.

    The Earth, for instance, spins around its axis like a top. And, in common with all tops, it has a tendency to wobble wildly. Such wobbles would cause severe changes in the Earth's climate, with grave consequences for life. But every time the Earth tips too far over on its axis, the Moon's gravity rights it. The Moon has, therefore, ensured a relatively stable climate for the evolution of life over billions of years.

    And this is not the only way that the Moon has been important in the evolution of life. The tides created by the Moon, which are three times bigger than those created by the Sun, leave large areas of the ocean margins high and dry twice a day.

    Hundreds of millions of years ago, this enabled marine creatures to gradually adapt to arid conditions - the first step in the conquest of the land. But the Moon's key importance in the evolution of life has a depressing consequence for our prospects of finding ET life. The reason is that the kind of collision needed to create a big moon has always seemed an extremely unlikely event.

    Gott and Belbruno don't see it like that. They say that the formation of a large Mars-mass body at one of the Lagrange points of other planetary systems may not be that uncommon at all. And, since their simulations show a big moon created in a quarter of cases, the formation of a big moon may be more likely than anyone expected. They even speculate that there may exist planetary systems in the Galaxy, where two or more terrestrial planets have big moons.

    Is there any way of proving Gott and Belbruno's scenario? At first sight, it would appear to be difficult. After all, the Moon was formed in a tremendously violent manner and the impactor was utterly destroyed. It would be highly unlikely that any unprocessed material from that time could have survived to the present day. "But perhaps not impossible," says Gott.

    Gott and Belbruno point to an asteroid, or chunk of interplanetary rubble, discovered in 2002. "2002 AA29" is barely the size of a football pitch and is currently in a orbit which periodically brings it within a mere 5.8 million kilometres of the Earth. The peculiar orbit is very similar to the one the impactor that created the Moon would have been in 4.55 billion years ago. "You have to ask yourself, how did 2002 AA29 get in that orbit?" says Belbruno.

    An intriguing possibility is that it might have been associated with Lagrange-4 or Lagrange-5 in the distant past and at some point was kicked out. If so, 2002 AA29 may carry the imprint of the material from which the impactor and the Earth were formed. Bizarrely, 2002 AA29 has been picked out by planetary physicists as an asteroid that would be relatively easy for a space probe to visit.

    Gott and Belbruno suggest that a mission to return a sample would be most interesting. If it found iron and material with the same oxygen fingerprint as the Earth and Moon, it would support the Lagrange point scenario. If it contained no iron, it could be a bit of the splashed out material from the impact that formed the Moon. "Either way, we think 2002 AA29 could tell us about the origin of the Earth and Moon," says Gott. "It may be the most valuable chunk of rock in the Solar System."

    Marcus Chown is the author of `The Universe Next Door: Twelve Mind-Blowing Ideas from the Cutting Edge of Science', published by Headline, pounds 7.99

    Source: Independent, The; London (UK)

    24 Hours of Chaos: Click to see how the Moon was made.


    24 Hours of Chaos: The Day The Moon Was Made By Robert Roy BrittSenior Science Writerposted: 02:00 pm ET15 August 2001
    For 25 years, scientists have pondered a theory that the Moon was created when an object the size of Mars crashed into Earth less than 100 million years after the Sun was born, some 4.6 billion years ago. The general idea has been run through the paces and massaged into shape and is now the favored explanation
    But attempts to model cousins of that theory on computers generate inexplicable side effects.
    In one of two leading computer models, Earth was creamed while it was still gathering mass, during a brief time when it was only half its current size. All the rocky inner planets are thought to have formed this way, a method rapid accumulation of matter called runaway growth.
    But if the Moon was carved out during Earth's growth phase, then it would have been around when Earth continued bulking up by swallowing tremendous numbers of large asteroids. Some of these iron-rich rocks would have hit the Moon, too. Yet the iron is not there.
    In the other model, the aggressor was three times as massive as Mars and created an excess of rotation in the Earth-Moon system that simply doesn't exist today.
    Now researchers have harnessed the latest in computing power to provide the most detailed model ever made of the cosmic scene that supposedly created the Moon. The result, a 3-D animation of the blast and subsequent chaos, is comforting. It shows that the Moon could have formed when a Mars-sized object hit a fully formed Earth.
    The collision would have given Earth its spin, defined what we now call an equator, and put enough material into orbit at the right distance from Earth to allow the formation of a satellite that generations would later swoon over.
    24 hours of chaos
    Robin Canup of the Southwest Research Institute has been modeling the Moon's formation for eight years. On previous studies, she has worked with William Ward and Alastair Cameron, who represent one of two separate research groups that developed the original impact theory back in the mid-70s. (William K. Hartmann and Donald R. Davis were the other team.)
    As Canup knows, all ideas about how the Moon formed must contend with one important fact: The Moon contains very little iron. Earth, on the other hand, is loaded with iron, the bulk of it tied up in the planet's core.
    So the Moon is thought to have been pieced together by the bits that got blown off the upper layers of Earth, as well as the outer portions of the object that hit Earth.
    Canup's latest effort, produced with the help of Erik Asphaug of the University of California, Santa Cruz, is like a small scene in a blockbuster disaster movie -- the first 24 hours of time in the epic calamity that made the Moon. It is detailed in the Aug. 16 issue of the journal Nature.
    The model treats the debris created by the collision as more than 20,000 computational lumps, or particles, all of which are given their own gravity to play with as the cataclysm unfolds.
    In a telephone interview, Canup described the day the Moon was made:
    A dark, lifeless object less than half as massive as Earth careens around a newborn Sun. It is one of many planet-sized bodies hoping for a long career. But its orbit is shaky. It's future grim. It is a character actor on the grand stage of the solar system, a player of great ultimate consequence but one destined to never see its name in lights.
    This doomed "protoplanet" travels a path that crosses the orbits of similar objects and, ultimately, cannot last. Eventually, the nameless protoplanet meets up with a fledgling Earth.
    It is not a head-on collision, but rather a glancing blow. The impact imparts what astronomers call angular momentum into the system. It sets Earth to spinning on its axis and creates a Moon that would go round and round the host planet for billions of years.
    The shock of the impact strips material from the outer layers of Earth and the impacting object. The mostly iron cores of both bodies meld into Earth's core. It is like a compact car merging onto the highway and colliding with an S.U.V. -- glass, trim and hubcaps fly, but the two chassis remain hopelessly tangled.
    All told, about 2 percent of the combined mass of the objects -- mostly rocky stuff that's largely bereft of iron -- begins to orbit the Earth. About half of this eventually becomes the Moon.
    Some of the stripped material is heated so fantastically that it vaporizes and expands into the surrounding vacuum of space.
    "The material that was vaporized expands into a cloud that envelops the whole planet," Canup explained.
    Meanwhile, a long arm of solid matter is winging its way around Earth. Some of it develops into a clump that slams back into the planet. The rest is flung into orbit, all pretty much along a plane that mimics the path of the incoming object. This plane slices through what is now Earth's equator, and it is roughly the same plane along which the Moon orbits.
    "The object came in and hit, and that's what set the Earth's rotation and what its equator would be," Canup said.
    The model assumes Earth was not spinning before the impact, though it might have been. If it were already spinning, Canup said the model could be tweaked to account for that fact and would still work.
    "For the first time, we demonstrated with simulations that a single impact can give you an iron-depleted Moon of the right mass, and the current mass of the Earth, and the current angular momentum of the Earth-Moon system," Canup said.
    Though the model covers only a day's time, Canup said shortly thereafter the material in outer regions began to cool. Gradually, small clumps would have formed, collided with one another, and grown. Based on other models, she said it would have taken between 1 and 100 years to make a Moon after the impact.

    Case is not airtight
    The new model is a significant improvement over previous efforts, which treated gravity as an overall issue or worked with no more than 3,000 computational lumps. But it is just one step toward a fuller understanding of what really happened.
    Jay Melosh, a University of Arizona researcher who is known for his work in modeling asteroid impacts, told the new model is an incremental step rather than a trailblazing one. And there are outstanding questions about some assumptions made.
    "Their case is not airtight," Melosh said.
    In a review of the work that also appears in Nature, Melosh argues that the real promise is in how computers are becoming powerful enough to handle the complicated scenario of a such a colossal impact.
    "Not only does such a collision involve all the details of shock physics, melting and vaporization, but the mutual interactions of all those hot fluids squirting around in space have to be taken into account," Melosh writes.
    He says that as with any attempt to model the Moon formation, the results hinge on an incomplete understanding of how the energy, density and pressure would affect the material of which the Earth and Moon are composed.
    And Canup acknowledges that there is not, and never will be, direct physical remains of the Moon-forming impacter. The ensuing drama was so hot, and the characters so well-mixed, that there are no ancient layered deposits to provide clues, as are found by people like Melosh who study smaller and more recent asteroid impacts.
    But Melosh said the prospects for better models are promising. In fact, he is working with Canup and Asphaug on ways to refine the new model to better account for the shock and fluid dynamics. And he figures others will soon use the improved computing power and more capable software packages to produce their own scenarios.
    "More studies of this kind will be published in the not-too-distant future," Melosh said.
    Other ways the Moon might form
    The new study strengthens just one theory of how the Moon might have formed. Other scientists have suggested that the Moon developed elsewhere in the solar system and was captured by Earth. An even more remote possibility is that the Earth and Moon condensed together out of the material that formed the solar system.
    Another idea is that gravitational interactions between the Earth, the Sun, and other developing planets simply tore Earth apart and the Moon formed from this debris.
    But the majority of researchers prefer the impact theory. And though a similar impact would be extremely unlikely today, it was a fairly common occurrence back when the solar system was forming.
    "The last stages of planetary accumulation were very violent," Melosh said. "An event of this type within 100 million years of the birth of the solar system is not rare at all."
    The Moon is not the only result of this chaos. In fact, the present spacing between planets "evolved by a sort of natural selection involving the demise of intervening objects whose orbits were not so stable," Melosh said.
    Click here for more news and information about the Moon.
    Tilted View of Our Moon's Formation
    More Moons Around Earth? Its Not So Loony
    Scientists Offer Answer to Baffling Lunar Illusion
    Sudden Increase In Asteroid Impacts May Have Fueled Life


    Moons like Earth's are few and far between

    Movie Camera
    • 15:10 21 November 2007
    • news service
    • David Shiga

    Moons created from massive collisions the way Earth's may have been, are a rarity in the universe, suggests a new study.

    Our Moon probably formed when a Mars-sized object slammed into the newly-formed Earth, spraying debris into space. Some of this then coalesced to form our Moon.

    Earth appears to be the only planet in our solar system with a moon that formed in this way. Other moons are thought to be objects that were captured by their planet's gravity or that formed simultaneously with their planet.

    However, a smaller-scale collision may have spawned dwarf planet Pluto's moon, Charon. (Watch an animation showing an artist's impression of a collision between large rocky space objects.)

    There is some evidence that collisions between rocky bodies happen in other solar systems.

    Collision debris

    On 14 November, Joseph Rhee of the University of California in Los Angeles, US, and his team of astronomers said they had found large quantities of warm dust around a star in the Pleiades cluster, similar conditions existed in our solar system when the Moon formed and so seem a pre-requisite for the collisions needed for moon-making. This dust could have arisen from just such a collision. Until now, however, it has been unclear how common such collisions are.

    Now, a survey of 400 stars suggests collisions big enough to produce something like Earth's Moon happen in only one out of every 10 to 20 solar systems.

    Nadya Gorlova of the University of Florida in Gainesville, US, led a study that used the infrared Spitzer Space Telescope to look for signs of dust around stars in a young cluster called NGC 2547 about 1400 light years from Earth.

    The cluster is ideal for addressing the question, because its member stars are about 30 million years old. The era when big collisions are thought to be common in solar systems lasts from about 10 to 50 million years after a star is born.

    'Teenage' solar system

    The team searched for signs of dust in the infrared light spectrum of the stars in the cluster. They found just one star with enough surrounding dust to suggest a moon-forming collision may have occurred there.

    Taking into account the fact that the dust from such collisions is only briefly observable because it is quickly blown out of the system, the astronomers calculate that these collisions must occur in about 5 to 10% of solar systems.

    Moons like Earth's may be even more rare than this suggests, because not every collision between objects of the appropriate size will necessarily produce a moon, says team member George Rieke of the University of Arizona in Tucson, US.

    The survey provides an insight into what our own solar system may have been like in its early stages, says Marc Kuchner of NASA's Goddard Space Flight Center in Greenbelt, Maryland, US.

    "This kind of survey tells us what the solar system might have been like as a 'teenager' – almost grown up but still a little bit wild," he told New Scientist.

    Journal reference: Astrophysical Journal, (DOI: 10.1086/521671)


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