Venus (left) and Mercury (right) (images courtesy of NASA)

A little while back, we had a fascinating talk from Professor Albert Jambon (Université Pierre et Marie Curie, Paris) about a strange group of meteorites known as angrites. Now, you might wonder how one group of meteorites can be any more exotic than another. But angrites are really, really strange. They are all igneous in origin, meaning they formed from hot, molten material, and so resemble volcanic rocks found on Earth. But their chemical composition is unique (very low sodium and potassium, high calcium compared to aluminium). As a result, angrites have mineral compositions like nothing else in the Solar System. They are also really old and give dates very close to the age of the Solar System itself . So, where do they come from?

Back in 2003, Professor James Papike (University of New Mexico) and his colleagues suggested that angrites might be samples from Mercury. Further support for this view came from Professor Anthony Irving (University of Washington) and his Team, who studied a number of newly found angrites. It was once thought highly improbable that a sample ejected from Mercury could make it to Earth. But recent calculations suggest that this may not be as difficult as once thought. An alternative source for angrites was suggested by Professor Jambon a number of years ago. Based on an interpretation of oxygen isotope data (see “isotope” in “Jargon Buster” on toolbar) collected at the Open University, he speculated that they could be samples from Venus.

So, are angrites from Mercury or Venus? The ancient ages given by angrites suggest that they must have come either from a body that was too small to sustain volcanic activity for long, or alternatively from a large object that was destroyed early in the history of the Solar System. In that case, neither Mercury nor Venus fit the bill. A bit of a let down? Well, maybe not.

Intriguingly, there is some evidence from mineral textures and magnetic properties, which points to a relatively large body as the original source of the Angrites. This would fit in with the results of numerical simulations, showing that shortly after its formation, the Solar System was populated by large numbers of mini-planets, the so-called Moon-to-Mars sized protoplanets. These in turn collided to form the larger planets we see and live on (the Earth) today. In his talk, Professor Jambon suggested that the angrites may originally have formed on one such protoplanet. He further speculated that this may have become disrupted during the formation of Venus, but with some material surviving as a small asteroid. It would be this unidentified asteroid which supplies the angrite meteorites that impact Earth today.

So, does this model solve the so-called angrite mystery? Well, perhaps, but then again perhaps not. With increasing numbers of angrite samples being found in hot and cold desert regions, our knowledge of this fascinating group continues to grow. There are certainly a lot more surprises in the pipeline… But one thing is sure: the angrites still have a lot to tell us about the early history of our Solar System!