The Hoba iron meteorite was found in 1920 in Namibia. The single slab-like mass measures 2.7m x 2.7m x 0.9m and weighs in excess of 60 tons. Due to its large mass it has never been moved from its original site. It is the largest single intact meteorite in the world. (image: wikipedia)
Experts can really drive you up the wall! Just when it seems like everything has been nicely sorted, they go and change their minds and then it’s back to the drawing board. It’s no different in the world of meteorites: let me explain…
Experts always like to divide things up. Meteorites come in three main flavours: irons, stones and stony-irons. Everyone knows what an iron looks like. They are those huge lumps of metal that form the centrepiece of museum meteorite displays. When sliced, polished and etched, many irons show a beautiful criss-cross structure, known as Widmanstätten pattern. No doubt about it, irons are always the real stars of the show. Stones generally look a lot less spectacular and are made up mainly of rocky minerals. At first glance, they look much like rocks found on Earth. Finally, stony-irons are, you guessed it, more or less a mixture of the other two. Nice and straightforward then!
But experts just love to catch you out. They will ask questions like: “Which of the three sorts of meteorites are the most important?” That’s an easy one, you think… No contest, it must be irons. Now comes the interesting bit. A few years ago, the experts would have been happy to give you a knowing smile and reply: “Well, irons are lovely, but not really that crucial. They make up only a very tiny proportion of meteorite falls”. They would have also been happy to tell you that irons are just bits from the cores of melted asteroids and probably formed relatively late in Solar System history. In comparison, stones are much more significant (85% of all falls), and the largest sub-grouping of stones, known as chondrites, are the oldest meteorites of all.
Of course, you do have to trust the experts, don’t you? A bit strange, though… They show you these wonderful specimens of irons and, at the same time, claim that they are not that important. Perhaps you even remember from a previous museum visit that the world’s largest meteorite is an iron. The Hoba meteorite, as it is called, is so huge it had to be left in the ground in Namibia, where it was discovered. Based on all this, you would certainly have some lingering doubts about the experts’ views. A good thing too, because it turns out that the experts were in fact wrong and you were right all along…!
A quiet revolution has been taking place in the study of meteorites and you will be pleased to know that irons are at the centre of it. As the experts said, irons are thought to have formed the cores of asteroids that were once totally molten. Like some giant blast furnace, the dense molten metal would have drained to the centre of the molten body under the influence of gravity, leaving almost pure rocky material forming its outer layers. But what caused the asteroid to heat up in the first place? The most likely answer is the heat given out by the decay of a short-lived isotope 26Al, which would have almost totally decayed to its daughter isotope 26Mg within a few million years of Solar System formation. So those irons that the experts believed formed relatively late must, in fact, be much older than originally thought. Putting an age on iron meteorites has been the subject of very intensive study in recent years.
New results presented at the Meteoritical Society Meeting in New York earlier this summer by Thomas Kruijer of EHT Zurich indicate that iron meteorites are samples of asteroids that formed and subsequently melted within the first million years of Solar System history. It used to be thought that unmelted asteroids with a more “primitive” composition, known as chondrites, were the first to have formed. However, these new results, in agreement with other recent studies, demonstrate that the earliest asteroids rapidly melted and separated into a distinct metal core and silicate-rich outer layers. Since iron meteorites are thought to be samples from at least 50 different asteroids, it is clear that the early Solar System was a hugely dynamic environment.
What is starting to emerge from these recent studies of iron meteorites is a new picture of the earliest stages of Solar System history. The very rapid formation of molten asteroids is an unexpected finding. It has major implications, not just in the field of meteorite studies, but also in terms of how the larger planets like Earth were formed.
For the moment though, it is rather nice to report that those huge lumps of iron that sit at the centre of museum displays are finally being given the full attention that they truly deserve. Thank goodness the experts have finally caught up with your instincts!