Cosmic Archaeology

the_earth_seen_from_apollo_17

The Earth is a very beautiful planet. If we hadn’t realised it before, this was made particularly clear in the images taken by Apollo astronauts. A blue and white jewel suspended in the immensity and darkness of space. But that beauty hides a violent past. To form a planet the size of Earth you need to catastrophically merge literally millions of tiny worlds.  Such mini-planets were formed shortly after the birth of the Solar System and in a very real sense are Earth’s true ancestors.

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The Solar System was originally populated by small bodies known as planetesimals. These merged to form the terrestrial planets, including Earth. (Painting copyright William K. Hartmann; image: PSI).

But what were they like, these ancestral asteroids? Well of course most of them were lost, as they became incorporated into the growing terrestrial planets: Mercury, Venus, Earth and Mars. However, some debris remained, locked into the asteroid belt between the orbits of Jupiter and Mars. Chunks from these objects are delivered to Earth as meteorites. But how can we make some sense of this debris and so construct a coherent picture of the very earliest stages of Solar System evolution?

In a review paper that has just come online in the journal Chemie der Erde, we have looked in detail at the results from one important technique that is particularly powerful tool in this forensic cosmic archaeology. Oxygen isotope analysis allows us to make connections between different meteorite samples and place them into coherent groups. It also helps us to make connections between different meteorite groups and so identify which are from distinct asteroids and which are just chips from the same body.

It turns out that we seem to have samples from about 110 distinct asteroids in our global meteorite collection. Sounds a lot? Well not really! A simple calculation shows that you would need at least 400,000 asteroids the size of 4 Vesta to form the planets of the present inner Solar System. And by asteroid standards Vesta is already massive, being the second largest body in the main belt after Ceres.

But it’s not all doom and gloom. Armed with only a tiny fraction of the original ancient asteroid population we can in fact say a lot about the conditions that existed during the earliest stages of Solar System evolution. And it was certainly a violent and chaotic place. In fact, it turns out to be two places! The inner Solar System, where the Earth formed, was a particularly hot and energetic environment; while the outer Solar System was cooler and more benign. This dichotomy is reflected in the meteorites that arrive on Earth today. Samples from the very earliest-formed asteroids, termed achondrites, show the signs of extensive melting and segregation into a metal core and rocky outer layers, just like Earth. In comparison, material that may be samples from outer Solar System bodies, known as carbonaceous chondrites, never experienced such extensive processing and so preserve evidence of the Solar System’s even earlier past, as it formed from a collapsing cloud of dust and gas.

So it seems that great beauty can and does emerge from chaos and catastrophe!

To find out more about how oxygen isotope analysis has influenced our understanding of early Solar System processes why not download our article from the Chemie der Erde website. The article is Open Access.

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Oxygen isotope fluorination line and MAT 253 mass spectrometer at the Open University. Oxygen isotope analysis is a powerful technique for deciphering the relationships between different meteorite groups.

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