The primordial planet believed to have smashed into baby Earth, creating a cloud of debris that eventually formed into the moon, was chemically a near-match to Earth, a new study shows.
The finding, reported in this week's Nature, helps resolve a long-standing puzzle about why Earth and the moon are nearly twins in terms of composition. Computer models show that most of the material that formed the moon would have come from the shattered impactor, a planetary body referred to as Theia, which should have a slightly different isotopic makeup than Earth.
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"For some 30 years this contradiction was a major challenge to physicists grappling with the formation of the moon. The hope was that better simulations might resolve this issues, but it turned out that the progress with simulations gave essentially the same results, giving rise to the ‘isotope crisis,' as this problem came to be called," astronomer Alessandra Mastrobuono-Battisti, with the Israel Institute of Technology in Haifa, wrote in email to Discovery News.
Using advanced computer modeling, Mastrobuono-Battisti and colleagues ran dozens of simulations of later-stage planet formation, each time starting with 85- to 90 planetary embryos and 1,000 to 2,000 planetesimals extending from about halfway between the orbits of Mercury and Venus to within 50 million miles or so of Jupiter's orbit.
Within 100 million to 200 million years, each simulation typically produced three to four rocky planets as a result of colliding embryos and planetesimals, the scientists found. Looking particularly at the last moon-forming impact scenarios, the scientists assessed the likelihood that Theia and Earth had the same chemical composition.
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"It turned out it is not a rare event ... On average, impactors are more similar to the planets they impact compared with different planets in the same system," Mastrobuono-Battisti said.
"Our study was the first to reconsider this issue, now exploring it with large data and ... wide range of models. One should always be careful when basing the assumptions on limited data," she added.
Related papers, also published in Nature, home in on slight variations in an isotope of tungsten found on Earth and on the moon, which continue to raise questions about the moon's formation.
"It is very unlikely -- but not impossible -- that two very different sized bodies developed the exact same tungsten isotopic composition," University of Maryland astronomer Richard Walker told Discovery News.
"I think all three papers work to explain the formation of the moon within the framework of a giant impact. I don't think we have a better alternative at this time," he added.
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Additional studies may depend on sacrificing Apollo moon rocks in an attempt to more precisely measure lunar tungsten.
"It may be worth it. The committee that considers such requests will have to be convinced of the merit of continued work," Walker said.
A second tact is to get a sample from another inner solar system planet.
"Venus would be really difficult, but a sample from it would tell us whether or not Mars (which is considerably different from Earth and the moon) is the odd man out," Walker said.
A sample of Mercury already may be on Earth, in the form of a rare, and still unidentified, meteorite.