Scientists have pinned down the birth date of the moon to within 100 million years of the birth of the solar system — the best timeline yet for the evolution of our planet's natural satellite.

This new discovery about the origin of the moon may help solve a mystery about why the moon and the Earth appear virtually identical in makeup, investigators added.

Scientists have suggested the moon was formed 4.5 billion years ago by a gigantic collision between a Mars-size object named Theiaand Earth, a crash that would have largely melted the Earth. This model suggested that more than 40 percent of the moon was made up of debris from this impacting body. (Current theory suggests that Earth experienced several giant impacts during its formation, with the moon-forming impact being the last.)

However, researchers suspected Theia was chemically different from Earth. In contrast, recent studies revealed that the moon and Earth appear very similar when it comes to versions of elements called isotopes — more so than might be suggested by the current impact model. (Isotopes of an element have differing numbers of neutrons from one another.)

"This means that at the atomic level, the Earth and the moon are identical,"study lead author Seth Jacobson, a planetary scientist at the Côte d'Azur Observatory in Nice, France, told Space.com. "This new information challenged the giant impact theory for lunar formation."

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A new study has determined with 99.9% accuracy that the Moon was formed 95 million years after the birth of the Solar System. This pegs the Moon as being a lot younger than we previously thought — and, perhaps more importantly, that our fair Earth’s development was rather tardy, taking took a lot longer to form than we previously thought. The study also confirms that the Moon was created when a Mars-sized planet (often called Theia) collided with Earth some 4.5 billion years ago, ejecting a massive amount of debris that quickly coalesced into our planet’s only natural satellite. As always with new data like this, it actually poses more questions than it answers — but that’s why we love science so much, right?

Historically, all attempts at aging the Moon have revolved around radiometric dating — i.e. testing the decay of radioactive elements found in lunar samples. Radiometric dating is pretty good, but it has fairly limited accuracy over a time span of billions of years (previous attempts at dating the age of the Moon have ranged from 30 to 100 million years after the Solar System was born). This new method, devised by a worldwide group of researchers, takes a very different approach. It’s fairly complex, but I’ll try to boil it down for you.

As you know, Earth has an iron core. What you probably didn’t know is that there is a class of elements called siderophiles (literally “iron-loving” in Greek), including platinum, gold, and iridium, that readily dissolve in iron (either as a solution or molten). These siderophiles naturally tend to sink towards the core of the Earth, where there’s lots of iron — which explains why these elements so rarely occur in the Earth’s crust (where we do all of our mining). Anyway, the theory (which is very well supported by this point) is that Theia’s collision drove almost all of these siderophiles into the Earth’s core. The theory postulates that the crust was so stripped of these rare elements that almost all of the siderophiles that we find today must’ve come from later, smaller impacts (asteroids).

By using a series of 259 computer simulations, the researchers — led by Seth Jacobson, a planetary scientist at the Côte d’Azur Observatory in France — found they could work out how heavy the Earth was before the Theia impact, how heavy it was after, and then how heavy we are now, after 4.5 billion years of further asteroid impacts. Along the way, they worked out that the Earth-Theia impact must’ve occurred around 4.47 billion years ago, or about 95 million years after the Solar System came into existence. [Research paper: doi: 10.1038/nature13172 - "Highly siderophile elements in Earth’s mantle as a clock for the Moon-forming impact"]

What’s the significance of all this? Well, it’s always interesting to know more about how the Solar System and its planets formed, as it can inform us about the science and geology of other local planets, and also about other planets elsewhere in the universe. In this case, the study highlights that the Earth did not finish forming until around 100 million years after the birth of the Sun — but Mars appears to have taken only a few million years to form completely. Why is there such a huge disparity, and is such a disparity common, or is Earth (or Mars) special? There is now a lot of attention on Venus, which is very similar in size and composition to Earth — but because no asteroids from Venus have ever landed on Earth, and because we haven’t sent a lander there, we don’t know how long it took to form. We also believe that huge impacts, like the Earth-Theia event, were pretty common back then — but if so, why doesn’t Venus have a moon?

Also, just in case you were wondering, there are other theories of how the Earth and Moon came to be. Most of them are a little silly, such as the Earth spinning fast enough that its molten surface was flung into space by centrifugal force, where it cooled into the Moon. A better theory, which I quite like, is that the Earth and Moon were both created at the same time, by a giant collision of two planets, which ejected some debris that became the Moon, and then re-collided (more gently this time) to coalesce into Earth. This quite neatly explains why the Earth and Moon are “atomically identical” when it comes to the presence of various elemental isotopes.

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An international team of planetary scientists determined that the Moon formed nearly 100 million years after the start of the solar system, according to a paper to be published April 3 in Nature. This conclusion is based on measurements from the interior of Earth combined with computer simulations of the protoplanetary disk from which Earth and other terrestrial planets formed.

The team of researchers from France, Germany and the United States simulated the growth of the terrestrial planets (Mercury, Venus, Earth and Mars) from a disk of thousands of planetary building blocks orbiting the Sun. By analyzing the growth history of Earth-like planets from 259 simulations, the scientists discovered a relationship between the time Earth was impacted by a Mars-sized object to create the Moon and the amount of material added to Earth after that impact.

Augmenting the computer simulation with details on the mass of material added to Earth by accretion after the formation of the Moon revealed a relationship that works much like a clock to date the Moon-forming event. This is the first "geologic clock" in early solar system history that does not rely on measurements and interpretations of the radioactive decay of atomic nuclei to determine age.

"We were excited to find a 'clock' for the formation time of the Moon that didn't rely on radiometric dating methods. This correlation just jumped out of the simulations and held in each set of old simulations we looked at," says lead author of the Nature article Seth Jacobson of the Observatory de la Cote d'Azur in Nice, France.

Published literature provided the estimate for the mass accreted by Earth after the Moon-forming impact. Other scientists previously demonstrated that the abundance in Earth's mantle of highly siderophile elements, which are atomic elements that prefer to be chemically associated with iron, is directly proportional to the mass accreted by Earth after the Moon-forming impact.

From these geochemical measurements, the newly established clock dates the Moon to 95 ±32 million years after the beginning of the solar system. This estimate for the Moon-formation agrees with some interpretations of radioactive dating measurements, but not others. Because the new dating method is an independent and direct measurement of the age of the Moon, it helps to guide which radioactive dating measurements are the most useful for this longstanding problem.

"This result is exciting because in the same simulations that can successfully form Mars in only 2 to 5 million years, we can also form the Moon at 100 million years. These vastly different timescales have been very hard to capture in simulations," says author Dr. Kevin Walsh from the Southwest Research Institute (SwRI) Space Science and Engineering Division.

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