Updated June 6: Another similar study, published today in Science, appears to confirm that the Moon was created when the planet Theia struck Earth 4.5 billion years ago. Previous studies had failed to find evidence of Theia in lunar rock samples — but now a more thorough analysis has discovered material of alien origin that lines up with the Theia-Earth giant impact hypothesis. [DOI: 10.1126/science.1251117 – “Identification of the giant impactor Theia in lunar rocks”]. The original story remains below.

Original story

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.

The impact of Earth and Theia, as depicted by a NASA artist

The impact of Earth and Theia, as depicted by a NASA artist. (The photo at the top of the story shows the same thing, but visualized by a National Geographic artist instead.)

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”]

Buzz Aldrin on the Moon, Apollo 11

Buzz Aldrin on the Moon, Apollo 11

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.

[Featured image credit: National Geographic]