Meteorites are samples of extraterrestrial material that we find on Earth, and that have come from other bodies in the Solar System, particularly the asteroids but also Mars and the Moon. Well over 30000 have been collected. They are of enormous importance in establishing the chronology of events in the Solar System, the nature of those events, and the composition of the Sun plus its family, as you will see.

3.3.1 Meteors, Meteorites, and Micrometeorites

You have probably seen a 'shooting star', a bright streak of light that flashed across the sky for a second or so before disappearing. You might even have been lucky enough to see a spectacularly bright example, called a fireball, or a bolide if it explodes. These phenomena are caused by meteors, small bodies that have entered the Earth's atmosphere at great speed, mostly in the range 10-70 km s-1. Sometimes the sonic boom produced by the supersonic speed of the body can be heard. They ionise the atmosphere as they travel, and their surfaces become very hot. The streak of light is the glow from the ionisation. In space, the parent body of a meteor is typically less than a few millimetres across. □ What are such bodies called?

Such bodies are called micrometeoroids, or dust if smaller than about 0.01 mm (Section 3.1).

Most meteors vaporise completely at altitudes above 60 km. The larger ones, greater than a few tenths of a metre across, usually reach the ground, often fragmenting in the atmosphere or on impact. As you have seen, a fragment, or the whole object from which a set of fragments came, is called a meteorite. Meteorites that are seen to fall are, unsurprisingly, called falls, and there can be no doubt that a fall came from the sky. For a handful of falls there are sufficient observations of the path through the atmosphere for accurate orbits to have been obtained. These orbits resemble those of the NEAs, suggesting an ultimate origin in the asteroid belt.

Only about 1 in 20 of the collected meteorites have been seen to fall. The rest have been found on the Earth's surface some time later. Naturally, these are called finds. You might wonder why a rock on the ground should be thought to have fallen there from the sky. One indicator is a fusion crust on its surface (Plate 25(a)). This is evidence of high-speed travel through the atmosphere. Some of the meteorite burns off in a process called ablation, and the fusion crust is the millimetre or so layer of heat-modified material overlying almost pristine material underneath. However, though this indicates that a rock has arrived at a location via a rapid passage through the atmosphere, it does not establish that it came from interplanetary space. This can be learned from detailed study of its structure and composition, a topic for Section 3.3.2.

Deserts and the Antarctic ice sheets are particularly good places to find meteorites, because small rocky bodies on the surface stand out. Also, in the Antarctic, ice flows concentrate meteorites into glaciers, where subsequent sublimation of ice exposes long-buried meteorites.

A typical unfragmented meteorite is of the order of 10 centimetres across, and has a mass of a few kilograms. Bigger parent bodies tend to fragment, unless they are predominantly iron (Section 3.3.2). For example, the known fragments of the Murchison meteorite seen to fall near the town of Murchison in Australia in 1969 amount to about 500 kg. A particularly massive meteorite was observed to fall near the town of Allende in Mexico, also in 1969. Fragments amounting to over 2000 kg have been recovered. More recently, in 2003, the Park Forest meteorite was observed to break up over the area of this name near Chicago, USA. Many fragments, each a few kilograms, have been recovered. It is estimated that the parent body had a mass of 10 000-25 000 kg. This was the eighth meteorite to have had its orbit accurately determined. The larger the meteorites, the rarer they are. A meteorite of the mass of Murchison, or larger, will arrive at the Earth's surface roughly once a month, but most of these land in the oceans, or in remote areas where they go undiscovered.

Smaller meteorites are more common. The really small ones, a few millimetres or less across, are placed in a separate category called micrometeorites. One type is found in abundance in ocean sediments, where their nature is recognised through their spherical form. They are resolidified small bodies that melted in the atmosphere, or resolidified droplets from larger bodies. At sizes below about 0.01 mm, the most common type is a fluffy aggregate of tiny particles, also found in sediments, but also collected by high-flying aircraft. These have traversed the Earth's atmosphere without melting because they are slowed down before they reach their melting temperatures. In many cases those collected might be fragments of larger fluffy aggregates. These dust particles float gently to Earth, and are so common that if you spend a few hours out of doors, even as small a target as you is likely to collect one. Alas! You do not recognise this extraterrestrial mote among all the dust of terrestrial origin that you collect.

Overall, extraterrestrial material is currently entering the Earth's atmosphere at a rate of about 108kg per year, mainly in the form of meteors that completely vaporise. □ What is this as a fraction of the Earth's mass? This is only just over 1 part in 1017 of the Earth's mass.

More evidence that meteorites of all classes are of non-terrestrial origin comes from the isotope ratios of certain elements, such as oxygen. The isotope ratios are strikingly different from those found in the Earth's crust, oceans, atmosphere, and Antarctic ice. In most cases the non-terrestrial ratios are consistent with general Solar System values. However, in many meteorites there are tiny refractory grains with very different ratios, indicating that these grains have survived from before the birth of the Solar System. The range of isotope ratios suggests several sources, including condensation in the winds from red giant stars and from the material ejected in supernova explosions. Prominent in these grains are nanometre-sized diamonds, but silicon carbide (SiC), graphite, and corundum (Al2O3) are also found.

Question 3.10

Why are most meteorites never found? (Four short reasons will suffice.) 3.3.2 The Structure and Composition of Meteorites

Three main classes of meteorite are defined: stones, stony-irons, and irons. Figure 3.14 shows these classes in the relative numbers in which they occur in falls. Finds are excluded because of a strong observational bias that favours irons. As their name suggests, irons are composed almost entirely of iron, and resemble more or less rusty lumps of metal. Stones, as their name suggests, look superficially much like any other stone. Irons thus look much odder than stones, with the result that a much larger fraction of irons are found than of stones. Furthermore, some stones suffer more rapid degradation than irons.

Iron meteorites, as has just been mentioned, consist almost entirely of iron. This is alloyed with a few per cent by mass of the metal nickel, and small quantities of other materials. Naturally occurring terrestrial iron is almost always combined in compounds with non-metals, and so an extraterrestrial origin for irons is at once suspected, particularly given the variety of geological environments in which irons are found. This suspicion can be reinforced by cutting an iron, polishing the fresh surface, and etching it with a mild acid. A pattern emerges like that in Plate 25(b). This is called a Widmanstatten pattern, after the Austrian director of the Imperial Porcelain Works in Vienna, Alois von Widmanstatten (1754-1849), who discovered the pattern in 1808. The pattern arises from adjacent large crystals that differ slightly in nickel content. The large size of the crystals is the result of very slow cooling, 0.5-500 K per Ma, indicating that solidification took place deep inside an asteroid at least a few tens of kilometres across. Such slow cooling in the rare bodies of metallic iron in the Earth's crust is extremely unusual.

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