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Meteorites demystified: A beginner’s guide (Part 2)

Helen Gould (UK) Chemistry is the key to identifying the source of a meteorite. The commonest rock in the Solar System – and on Earth – is basalt. Erupted at mid-ocean ridges and many hotspot volcanoes, it also floors the oceans. However, each of these situations can be identified as geochemically different from one another. Some meteorites have geochemical signatures associated with individual asteroids, being either enriched or poor in specific minerals. The ratios of their minerals are plotted against one another, then the shape and co-ordinates of the plots are cross-referenced to a database. This process has allowed distinct groups of meteorites with similar geochemistry to be identified, suggesting that the meteorites in each cluster plotted came from the same source. There are five sub-groups of achondrites of various chemical composition, including eucrites, diogenites, SNC, lunar achondrites and ureilites. The name means they don’t contain chondrules. Most are of igneous origin, but lunar achondrites resemble fragmental sedimentary rocks. The only “weathering” on the Moon comes from impacting meteorites, but this breaks up rocks and reforms them into breccias – jumbles of jagged fragments fused together. Eucrites Eucrites are basaltic meteorites containing low-calcium proxenite and plagioclase feldspar with metallic iron, troilite (iron sulphide) and silicates. They probably all crystallised at or just below the surface of their source bodies. Fig. x. Eucrite. Diogenites Diogenites consist of calciumpoor pyroxenite, which is an igneous rock resembling the ocean crust. Fig. x.. Diogenite. SNC SNC meteorites have been identified as coming from Mars. … Read More

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Meteorites demystified: A beginner’s guide (Part 1)

Helen Gould UK) What are meteorites? Lumps of rock left over from the formation of the solar system or “chipped off” planets during major impacts can become trapped in the Earth’s gravitational field and fall as meteorites. The three main types are iron, stony and stony-iron. All of these are discussed in this article. In particular, I consider two important questions: Why are they so important? Because they represent the growth (accretion) of planets, they carry clues to our Solar System’s formation.How do we know we are dealing with a meteorite? Like other rocks, meteorites record events. Most of their minerals are familiar but some have higher or lower concentrations than rocks found on Earth, suggesting an extra-terrestrial origin.Irons Fig. 1. Iron meteorite. Most contain 7-15 wt % of Nickel (Ni) metal, with traces of other minerals. At room temperature, instead of a single mineral, this forms a Widmanstätten structure, whose intergrowth lamellae show two different minerals, one with about 40% Ni, the other with only about 5% Ni, and indicate slow cooling from greater than 700°C. Iron (Fe) meteorites have usually been completely melted, proving they formed in asteroid cores. So even asteroids are differentiated – like the major planets – with a core and mantle which solidified slowly. Widmanstätten patternsAlso known as Thomson structures, these are figures of long nickel–iron crystals, found in the octahedrite iron meteorites and some pallasites. They consist of a fine interleaving of kamacite and taenite bands or ribbons called lamellae.Stony-irons Stony-iron meteorites probably … Read More

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Plate tectonics (Part 2): A closer look

Helen Gould (UK) As we saw last time (Plate tectonics (Part 1): What are they?), the Earth is a pretty dynamic place, with tectonic plates moving about on the surface, driven by convection cells in the upper mantle. But producing a workable theory, which combined most of the observations of geological evidence, took years. It was known that the centres of continents were extremely old, and that some areas around the continental “cratons” didn’t seem to belong because they contained completely different types of rocks. Combining continental drift with seafloor spreading and mantle convection currents produced the idea of plate tectonics, and provided an explanation for the odd rocks on areas fringing some cratons. These “microplates” had come from other areas of the Earth, where different geological processes had produced different rock types. The role of density in recycling: oceanic and continental crust The physical features of the ocean basins and continental mountain ranges are known as the “crustal dichotomy” (splitting of the crust into two equal parts), and because these types of feature are essentially dissimilar, they have their own rock types. Basalt is the commonest rock both in the Solar System and on Earth, where it forms the ocean floor, along with various sedimentary rocks deposited underwater which make up another 5% of the total oceanic crust. Continents typically consist of coarse-grained rocks related to granites, which solidify below ground. Comparing similar-sized pieces of basalt and granite in the hand will establish obvious physical differences between them. Basalt’s … Read More

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Book review: Collecting spacerocks: a guide to Meteorites, Tektites and Impactites, by David Bryant

Jon Trevelyan (UK) This is a nice little guide for the non-specialist collector of all things that go bump from above (and the effects they have on the rocks they impact). As is clear from the title, the book covers three wide categories: meteorites, tektites and impactites. Broadly speaking, a meteorite is any piece of solid material that has arrived on Earth from space (and, of course, is not from the Earth in the first place, like space junk). Tektites are small, black, glassy objects found in great numbers in a roughly equatorial belt, which are thought to have been formed from molten debris by the impact of massive asteroids and/or comets. And impactites are rocks created or modified by the impact of a meteorite on the surface of the Earth. With that in mind, the book covers: what meteorites are; their origins and classification; tektites and impactites; what makes the big holes that you find in them; meteorites in human history; meteorites and meteorwrongs(!); obtaining, preparing and displaying meteorites; and where you can see them. That is, everything an amateur needs to begin to understand and enjoy the subject as a hobby. In fact, it is a useful and accessible guide to all those who appreciate that, unlike most of the (passive) subjects of astronomy (distant stars, black holes, spiral galaxies and so on), meteorites and their associates are quite literally tangible evidence of the moon, mars, and far distant objects and asteroids. For a self-published book, the style … Read More

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Pallasites: The meteorite jewels in the crown

David Bryant (UK) Perhaps unsurprisingly (as a professional dealer in space rocks), I find all meteorites equally fascinating and, in their own way, aesthetically appealing. However, I have to admit, the meteorites known as the Pallasites, with their beautiful structure of olivine fragments suspended in a nickel-iron matrix, are probably the most visually exciting, particularly to the non-specialist. In addition to their undoubted beauty and rarity, Pallasites offer us an intriguing glimpse into the interior of a planet that make them among the most scientifically important of all meteorite types. The name Pallasite is derived from that of the German naturalist, Simon Peter Pallas. Pallas was one of those amazingly observant and gifted polymaths, who seem to have been a lot more abundant during the eighteenth century, as well as lending his name to a whole class of meteorite, an eagle, a warbler, two species of bat, a wild cat and dozens of other plants and animals. In 1772, Pallas obtained a 680kg lump of metal that had been found near Kransnojarsk in Siberia. When it was examined in St Petersburg, it was identified as a new type of stony meteorite. In keeping with tradition, it was named after the location where it was found, but, uniquely, the whole class of meteorites was named for Pallas. There is still some debate about the actual origin of Pallasites. Although some meteorologists contend that the stony-iron structure resulted from a collision between a nickel-iron asteroidal core and a chunk of mantle material … Read More

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Urban micrometeorites: A myth?

Jon Larsen (Norway) Is it possible to find micrometeorites in populated areas? The question has been raised for nearly a century and, despite numerous attempts to find them, the answer up to this day has been a very short “no”. Meanwhile, our knowledge about these amazing stones has gradually increased. There is a continuous evolutionary line in the research on micrometeorites, from the early pioneers, John Murray and Adolf Erik Nordenskiöld in the nineteenth century, to Lucien Rudaux and Harvey H Nininger. With Donald E Brownlee and Michel Maurette in the 1960s, micrometeoritics became real science. During the past two decades, this research has accelerated thanks to, among others, Susan Taylor, who extracted micrometeorites from the water well at the South Pole, Matthew Genge, who figured out the classification, and other splendid researchers, in addition to the space probes that have returned to Earth with dust samples from comets and asteroids. Today, there is a growing literature about micrometeorites, but still the answer to the initial question is “no” and urban micrometeorites have been considered an urban myth. Micrometeorites have been found in the Antarctic, but also, to some extent, in prehistoric sediments, remote deserts and in glaciers – places that are clear of the confusing anthropogenic influence. The wall of contamination has been considered insurmountable. It is therefore with pride and joy that I can report here about a project involving the systematic examination of all sorts of anthropogenic and naturally occurring spherules in an empirical search for micrometeorites … Read More

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Exploding star missing from formation of solar system

Chelsea Leu (USA) A new study, published by University of Chicago researchers challenges, the notion that the force of an exploding star forced the formation of the solar system. In this study, published online in Earth and Planetary Science Letters in November 2012, authors Haolan Tang and Nicolas Dauphas found the radioactive isotope iron 60 – the telltale sign of an exploding star – low in abundance and well mixed in solar system material. As cosmochemists, they look for remnants of stellar explosions in meteorites to help determine the conditions under which the solar system formed. Some remnants are radioactive isotopes, that is, unstable, energetic atoms that decay over time. Scientists in the past decade have found high amounts of the radioactive isotope iron 60 in early solar system materials. “If you have iron 60 in high abundance in the solar system, that’s a ‘smoking gun’ – evidence for the presence of a supernova,” Dauphas, professor in geophysical sciences, told me during a meeting in his office in October 2012. Iron 60 can only originate from a supernova, so scientists have tried to explain this apparent abundance by suggesting that a supernova occurred nearby, spreading the isotope throughout the explosion. However, Tang and Dauphas’ results were different from previous work. They discovered that levels of iron 60 were uniform and low in early solar system material. They arrived at these conclusions by testing meteorite samples. To measure iron 60’s abundance, they looked at the same materials that previous researchers had … Read More

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Lavas from Hawaiian volcano contain fingerprint of planetary formation

Dr Steve Koppes (USA) Hikers visiting the Kilauea Iki crater in Hawaii today walk along a mostly flat surface of sparsely vegetated basalt. It looks like parking lot asphalt, but, in November and December 1959, it emitted the orange glow of newly erupted lava. Now, a precision analysis of lava samples taken from the crater is giving scientists a new tool for reconstructing planetary origins. The results of the analysis, by the University of Chicago’s Nicolas Dauphas and his associates, were published in the 20 June 2008 issue of the journal Science. Fig. 1. Eruption Hill in Kilauea Iki crater on the Big Island of Hawaii. In December 1959, lava spurted 580m feet high from this location. Working with lava samples from the crater, scientists at the University of Chicago and elsewhere have devised a new tool for reconstructing planetary origins. (Photo: Steve Koppes.) The researchers selected Kilauea Iki for their study because scientists have drilled it for samples many times over the years as it cooled. This sequence of samples makes the lava lake a perfect site for studying differentiation – the separation of minerals and elements as magma cools and hardens. In particular, a close examination of iron isotopes – the slight variations the element displays at the subatomic level – can tell planetary scientists more about the formation of crust than they previously thought, according to Dauphas and co-authors, Fang-Zhen Teng of the University of Arkansas and Rosalind T Helz of the US Geological Survey. Dauphas informed … Read More

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Book review: The Tunguska Mystery, by Vladimir Rubtsov and Edward Ashpol

It appears that I was naive to assume the Tunguska explosion of 1908 had been adequately explained. It was a meteorite or, more probably, a comet that exploded above a remote area of Siberia. Wrong! This fascinating book shows that we still await an adequate scientific explanation and the jury is still out on what precisely the object was.