Collision with Earth's 'Little Sister' Created the Moon

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.

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.

Artist impression of a smaller planetary body colliding with early Earth -- the debris of which went on to form the moon.

On Saturday morning, the moon dimmed and, in some cases, turned a gentle reddish hue, as the first of two total lunar eclipses of 2015 wowed the planet. Here are some of the most dazzling photos of this so-called "blood moon" from around the world.

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Occurring shortly after the March 20 total solar eclipse that put on a show over Europe and Scandinavia, Saturday's lunar eclipse is no coincidence. Solar eclipses occur when the moon is at a node in its orbit -- the location at which the moon will pass directly between the sun and Earth. Half a lunar orbit later, it seems logical that the moon should slip behind the Earth, passing into Earth's shadow.

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The eclipse consists of two parts: the penumbra and the umbra. The penumbra occurs when the moon is partially in the Earth's shadow; the umbra is when the moon passes through totality. Although totality couldn't be seen by all skywatchers, anyone who could see the moon on Saturday morning would have at least viewed a partial eclipse.

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Lunar eclipses are always a wonderful astronomical event to watch. In this photo, an astronomer shoots a laser pointer at the moon during the eclipse over Indonesia.

This composite image shows the total lunar eclipse in the sky over the Kingkey 100 (KK100) Tower in Shenzhen city, south China's Guangdong province.

Steeped in myth and legend, the "blood moon" was often seen as a bad omen or even a sign of the apocalypse. But the name refers solely to the color of the eclipse during totality -- light from the sun refracts through the Earth's atmosphere, tainting the full moon. Depending on viewing conditions and atmospheric effects, the moon may just dim, producing a faint ruddy hue, or it could produce a very obvious red glow.

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Interestingly, Saturday's lunar eclipse experienced the shortest totality of the 21st century, lasting for only 4 minutes and 43 seconds.

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The red hue is evident in this stunning photo of the total lunar eclipse over Shenzhen city, south China's Guangdong province.

If you missed Saturday's lunar eclipse, fear not. You have enough time to start planning our observing time for the Sept. 24, 2015, eclipse that will be visible over Europe, the Middle East, Africa, and the Americas.