Meteorites and tektites

David Bryant (UK)

The Solar System formed around five billion years ago from a cloud of dust and debris orbiting the Sun. By a process of accretion and remelting by electrical discharges within the dust cloud, this material condensed into spherical particles called chondrules. By collision, the chondrules fused together to form larger and larger planetesimals and these aggregated to form asteroids and the planets themselves. All the rocky planets (that is, Mercury, Venus, Earth and Mars), together with many of their satellites, show evidence of the collisions that formed them.

Debris from the original formation of the Solar System is still abundant. Many tonnes fall onto the Earth every year as meteorites – perhaps as much as 300 tonnes each day.

These can be broadly classified into the three types discussed below.

(1) Stony meteorites

(a) Chondrites are debris from the original condensation of the Solar System and are undifferentiated. That is, the various elements of the original solar cloud are all present. For this reason, they are attracted to a magnet because of the nickel-iron they contain (within more massive bodies, like asteroids and planets, the heavy elements migrated inwards to form a core).

Chondrites are classified using an alphanumeric system that refers to the abundance and size of their chondrules (on a scale of 3 to 6, with 3 meaning there are abundant chondrules present and 6 meaning there are indistinct or sparse chondrules) and their iron content (referred to as either ‘L’ for low or ‘H’ for high). For example, a chondrite might be of a type L4-6.

(b) Achondrites are extremely rare. They contain no iron or chondrules and almost
certainly represent debris from impacts on asteroids, the Moon, Mars or Mercury. The main groupings are: eucrites, diogenites and howardites, derived from asteroids such as Vesta; lunaites from the Moon; and the SNC group (shergottites, nakhlites and chassignites) from Mars.

(c) Carbonaceous chondrites represent less than 6% of all meteorites. They contain dark organic matter (including amino acids!) and very little, if any, free iron. They may give clues about the origins of life in the Universe.

(2) Stony irons (for example, pallasites and mesosiderites)

These are very rare meteorites and usually consist of a nickel-iron matrix with crystals of olivine. It is generally believed that pallasites derive from the core-mantle boundary of large ‘A’ type asteroids. However, it is possible that some were formed during collisions between stony and metallic bodies. This is certainly the origin of the rare mesosiderites that consist of a eucritic matrix in which is suspended large amounts of nickel-iron.

(3) Meteoric Irons

Most iron meteorites were formed in the cores of small, differentiated asteroids that were disrupted by devastating impacts shortly after their formation. They are true remnants of other worlds that once existed in the early Solar System and are classified according to their structure and composition. When polished, many show the classic Widmanstätten pattern, which is believed to be the result of very slow cooling in a low gravity environment.

There are three main structural groups of iron meteorites:

• Hexahedrites have bandwidths of greater than 50mm.

• Octahedrites have bandwidths from 0.2mm to 50mm.
• Ataxites (silicated irons with high nickel content) show no banding.

Tektites

Tektites are glassy objects found in a number of locations around the World. Most are blackish in colour and show signs of exposure to intense heat. The exceptions to this include the beautiful (and rare!) green moldavites from the Czech Republic, yellow, green or white Egyptian desert glass and the green-grey darwinites from Australia.

Locations where tektites can be found:

1) Georgiaites	5) Ivory Coast tektites
2) Bediasites	6) Indo-Chinese tektites
3) Moldavites	7) Australian tektites
4) Egyptian Desert Glass	8) Darwinites

There has been much controversy about how tektites formed. The shapes and apparent ablation of some types and their distribution in strewn fields suggests a meteoric origin, but their composition is similar to some terrestrial minerals. Until recently, geologists generally believed that all tektites were created when meteoric impacts on the Earth hurled molten surface rocks high into the atmosphere. However, an increasing number of experts feel that the following facts point to an extraterrestrial origin:

• Tektites have not been shown to be associated with craters.
• Apollo 12 sample 12,013 contains material that is virtually identical in chemistry to Southeast Asian tektites.
• There is too little water associated with tektites for them to be terrestrial.
• Many tektites show evidence of ablation and orientation consistent with a high-speed passage through the atmosphere.

Hal Povenmire and John O’Keefe, two highly respected geologists who have both worked with NASA, have constructed a very plausible theory that many tektites were produced during recent crater-forming events on the Moon. The ejecta from these impacts was captured by the Earth’s gravity to form a ring like those around the four gas giant planets. This material gradually fell to Earth to form the bands of strewn fields either side of the equator.

Finding a meteorite

Despite the huge amount of meteoric material that arrives on Earth every day, the chances of finding one in the UK are slim – less than 50 people have had the good fortune to have done so! Unfortunately, since most meteorites resemble dark chunks of gravel, most must simply go unnoticed until the British climate rusts them away to dust!

Those that have been found were generally seen falling, making their location a much simpler matter. Therefore, the majority of ‘finds’ are made in desert regions (including Antarctica) where they are both more noticeable and less like to be degraded by the weather. Probably the richest source of supply is Northwest Africa – countries such as Morocco, Mauritania and Tunisia have produced literally thousands of meteorites over the past few years. In fact, so many have been removed that prices have tumbled and unclassified common chondrites can now be purchased for only a few pounds.