Meteorites: ‘rocks’ from space

Dr Vic Pearson (UK)

Every year, thousands of tonnes of dust and rock penetrate the Earth’s atmosphere. The fiery passage of these objects produces the familiar ‘shooting star’ phenomenon, known as meteors. Much is destroyed during this descent, but some material is delivered to the Earth’s surface, either as meteorites or micrometeorites (the latter being typically less than 1mm). However, only 1% of the surviving material is large enough for identification and recovery, making meteorites and micrometeorites much sought after, both scientifically and commercially.

Naturally, bombardment by extraterrestrial materials has been ongoing throughout Earth’s history. The early Solar System would have been a turbulent time and the young Earth was subject to much greater amounts of extraterrestrial infall than today. Our geological record contains many impact craters, and fossilised meteorites have been found in Ordovician sediments in Sweden (Schmitz et al., 2001), thought to be the result of the catastrophic break-up of an asteroid 470 million years ago.

Today, it is accepted that the majority of meteorites are fragments of asteroids broken off during collisions with other extraterrestrial objects, perturbed from their orbits by the gravitational effects of Jupiter. The origins of micrometeorites are less well constrained and evidence abounds for both cometary and asteroidal origins.

Types of meteorites

Meteorites are generally classified as either falls or finds. Falls are those that are seen to fall, generally producing a fireball and vapour trail, and are collected relatively soon afterwards. Finds are those that are not seen to fall and are collected tens, hundreds or even thousands of years after their arrival on Earth.

Based on their chemical make-up, meteorites are more specifically divided into three principal types: irons, stony-irons and stones.

Iron meteorites are principally composed of iron-nickel metal alloys (kamacite and taenite) and constitute over 40% of all meteorites finds as a result of their ability to survive atmospheric entry. However, they represent only 5% of all observed falls. Iron meteorites are believed to represent the cores of larger asteroids, analogous to the core of our own planet Earth.

Through catastrophic collision with other bodies after the formation of the core, this core was exposed and liberated from the asteroid. Iron meteorites are characterised by crystalline features known as Widmanstätten patterns formed by the differential cooling rates of iron and nickel when they were once a cooling, molten metallic mass. This structural pattern was traditionally used to classify iron meteorites, with the angle of the criss-crossing pattern indicative of the ratio of iron to nickel. However, this technique has since been replaced by chemical classification methods based on the presence of trace elements, and has resulted in 13 different groups and some ungrouped iron meteorites.

One of the most famous iron meteorites is the Hoba meteorite found near Grootfontein in Namibia. This is the largest known single meteorite, being over 60 tonnes in weight and around three metres in diameter. It is estimated to have fallen about 80,000 years ago and, unsurprisingly, has not been moved since its fall. It is now a national monument and tourist attraction. The Canyon Diablo iron meteorite has similar fame due to its role in the formation of the 1.2km Barringer (Meteor) Crater in Arizona, approximately 50,000 years ago.

The total mass of this meteorite has been estimated at several hundred thousand tonnes, however the majority of it was vaporised or melted on impact. Many fragments have been found around the crater rim and the area has been designated a national landmark, with an exhibition and information centre.

Stony-iron meteorites contain approximately equal proportions of iron and nickel metal (akin to iron meteorites) and silicate and oxide minerals (akin to stony meteorites). They are much rarer than iron or stony types and account for less than 2% of all meteorites.

There are two types of stony-iron meteorite, pallasites and mesosiderites, each of which is thought to have distinct extraterrestrial origins. Pallasites are composed of well-formed and often gem-quality olivine crystals set within a metallic matrix. These are believed to be sourced from the transition region between an asteroid’s core and mantle. Mesosiderites, however, contain abundant brecciated pyroxene and feldspars (similar to those within some stony achondrite meteorites) set within a metallic matrix and are believed to result from the collision of two differentiated asteroidal bodies.

Stony meteorites contain mostly silicate and oxide minerals and are divided into two further categories: achondrites and chondrites. The distinguishing feature used to classify a meteorite as being either chondritic or achondritic is the presence of chondrules. These are typically 0.1mm to 1mm glassy spherules, visible under a petrographic microscope. No single formation process has been identified for their origin, with current theories ranging from the heating of dust within the Solar Nebula before our planets formed, to the ejection of material during planetary volcanism.

Chondrites are, in essence, sedimentary in nature, being composed of several different components that have accumulated through a complex series of events during and following the accretion of their parent asteroid. Their chemistry is similar to the average chemistry of the Solar System and the presence of interstellar grains (nanodiamonds, graphite and silicon carbide) that pre-date the Solar System suggest that chondrites are the most primitive (that is, the most pristine) of all Solar System objects, lacking evidence of major planetary processing.

Antoine Lavoisier carried out the first crude chemical analysis of a chondrite in 1772 and, today, chondrites are the most thoroughly analysed extraterrestrial materials. They are sub-divided into Ordinary (types H, L and LL), Carbonaceous (C), Enstatite (E) and Rumaruti (R) chondrites.

Achondrites are a diverse group of meteorites that lack chondrules since they were produced by the crystallisation and partial melting of magma. Although they constitute only about 8% of all known meteorites, they are important because they are efficient indicators of planetary evolution and processing, similar to that which has differentiated the Earth. The Howardites, Eucrites and Diogenites, possibly from the asteroid 4-Vesta, are examples of achondritic meteorites. Those meteorites from the Moon and Mars are also achondrites.

Naming meteorites

Regardless of the circumstances of their retrieval, meteorites are always named after a locality or geographic feature near their site of recovery. In the case of meteorites found in areas where distinct geographical locations are few (for example, hot or cold desert environments), they are named using a prefix of their location (for instance, ALH for Allan Hills of Antarctica or SAH for the Sahara) followed by a number, characteristic of the year, and order in which they were collected.

Public interest in meteorites

Falls are the meteorites that attract the most media attention and public interest because of their spectacular fireballs or the circumstances of their impact on Earth. On 9 October 1992 at 23.48 Universal Time (UT), a greenish fireball was witnessed from Kentucky to New York. Over 16 witnesses caught the 40-second event on video camera, documenting an object fragmenting and eventually falling in the suburban Peekskill area of New York. Indeed, a single 12kg rocky fragment was recovered from the driveway of a home after it had impacted and penetrated the boot of a 1980 Chevy Malibu. Later classified as an H6 chondrite, the Peekskill car and meteorite have become world famous owing to the astonishing fall and the recognisable fireball videos that are widely available on the Internet.

The Tagish Lake meteorite fell on 18 January 2000 at 16.43 UT. A brilliant fireball was seen over the Yukon Territory, northern British Colombia and some parts of Alaska. Over 70 eyewitnesses witnessed an explosion with an accompanying dust cloud and vapour trail. US Department of Defence satellites also detected the fireball with infrared and optical sensors.

The explosion occurred at an altitude of 25 km and eyewitnesses took several photographs between one and two minutes after detonation. With a velocity of about 16 km a second, this 5 m diameter meteorite plunged into the ice-covered Tagish Lake. A week later, local man, Jim Brookes, found the first fragments of the meteorite in the Taku Arm of the Tagish Lake. Easily seen against the ice, he collected several fragments (up to 1kg) in clean plastic bags and kept them frozen before handing them over to scientists. Further search parties collected another 200 fragments (5kg to 10kg) and it was estimated that the strewn field, the region covered by fragments of the meteorite, was at least 3km by 16km.

The most recent fall to make headlines was that of a meteorite in Peru on 18 September 2007. The fall area was near to the rural village of Carancas, near the Bolivian border, and locals and collectors recovered over 340kg of meteoritic fragments. Although the fireball was witnessed and the meteorite created a crater approximately 14m in diameter, the associated furore was not due to the impact but to an apparent medical side effect. Initial reports from the area suggested that over 200 local people had been struck down with a sickness immediately after the meteorite’s fall, with the sickness resulting from the inhalation of noxious gases released by the meteorite. Police were even supplied with oxygen masks to wear during their investigative duties.

Although this mystery has not yet been solved, suggestions for terrestrial explanations have included changes to the local groundwater table invoking mild arsenocosis from mud-rich sediments, to meteorite-water interactions causing the release of sulphurous fumes. It is highly unlikely that an extraterrestrial ‘vomiting bug’ was delivered to Earth! The crater area has since been designated as protected and recent reports suggest that a Japanese team of investors may construct a space museum to protect the crater from the Peruvian rainy season and to enable further excavations to be made.

Meteorites in history and mythology

The suspicion aroused by the Carancas fall is typical of the public reaction to these events over history and meteorite falls have been well documented due to the fear and awe they invoke. Many religions considered them sacred and holy in origin, and they are embedded in the culture of many societies. A dagger made of meteoritic iron was found in Tutankhamen’s burial chamber.

The most famous example is the Hadschar al Aswad of the Islamic world, a ‘black stone’ that is the centre of any pilgrimage to Mecca’s Kaaba and is believed to be a meteorite given to Abraham by the angel Gabriel. It is said to have played a pivotal role in the Prophet Mohammed’s life, however its meteoritic origin is hotly disputed. In Greek mythology, Kronos, father of Zeus, is believed to have thrown down a mighty rock – the omphalos stone – signifying the centre of the universe. This rock, generally now believed to have been a meteorite, became the centre-point of the Sanctuary of Apollo at Delphi. There is a rock on display in the Delphi museum that is unlikely to be the original omphalos stone, so its extraterrestrial source cannot be ascertained and, therefore, it remains a Greek legend.

The antiquities are not the only source of meteorite legends. On 7 November 1492, a large stone fell into a wheat field outside the then German (now French) town of Ensisheim in Alsace, which was then still part of the Holy Roman Empire. Believing this to be a sign of good fortune, locals began chipping fragments from this ‘thunderstone’ for personal talismans. This attracted the attention of Emperor Maximillian, who ventured to Ensisheim to hold court over the meteorite and to investigate the meaning of its delivery. He ordered that it be preserved in the local church as it represented future luck in the wars between the Holy Roman Empire and the Turks. Today, it is preserved in the Regency Palace of Ensisheim as part of a larger collection of meteorites.

Finding meteorites

Finds are less reliable for scientific analysis as they have undergone terrestrial processes such as weathering and contamination after their fall that can affect their chemical make-up. In 1969, a team of glacial geologists discovered meteorite fragments near the Yamato Mountains of Antarctica. Since then, over 20,000 meteorites have been found in Antarctica by several dedicated international expeditions (Koeberl and Cassidy, 1991). Recovery of meteorite fragments in Antarctica is highly efficient as they can be easily identified on the blue ice sheets (Huss, 1990) and the Antarctic population a large number of meteorites that are rare or unknown in non-Antarctic locations.

Although prone to contamination, the cold Antarctic environment preserves meteorites for longer, resulting in the long residence times of some meteorites.  The low humidity of hot desert regions such as the Sahara and the Nullarbor Plain of Australia also enables meteorites to be preserved for long periods of time. In addition, the contrast of meteorites with the desert floor and minimal vegetative camouflage make hot deserts excellent sites for meteorite recovery. However, many are heavily weathered and rusty in appearance, inhibiting effective scientific analysis.

The origins of meteorites

Since the 1950s, only seven meteorite-producing fireballs have been photographed, allowing their orbits to be reconstructed and traced back to the asteroid belt. The possible parental asteroid of different meteorite types may be investigated by astronomers, using reflectance spectroscopy – a technique which uses the reflected light from an asteroid’s surface to determine its composition.  Several meteorites and asteroids have been found to have similar compositions (Gaffey, 1993; Lupishko and di Martino, 1998). For example, some C-type asteroids exhibit weak absorptions that can be attributed to phyllosilicate minerals, the products of secondary aqueous alteration in some carbonaceous meteorites, and so the parent bodies of carbonaceous meteorites may be C-type asteroids (Bell, 1989; McFadden, 1994). Similarly, the Tagish Lake meteorite may be spectrally similar to D-type asteroids (Hiroi et al., 2001).

The most successful spectral pairing of meteorites with asteroids is of the group of meteorites called Howardites, Eucrites and Diogenites (HEDs) with the asteroid 4-Vesta. 4-Vesta is a 530 km diameter differentiated asteroid, the second largest in the asteroid belt, and is characterised by its unusual shape and massive impact crater in its southern hemisphere. It is believed that this impact resulted in the ejection of fragments, some of which are retained in the asteroid belt as ‘Vestoids’ and some of which have made their way to Earth as the HEDs.

Yet, asteroids are not the only source of meteorites. Forty meteorites have been identified that, based on their chemistry and mineralogy, are believed to be impact ejecta from the lunar surface (Mason et al., 1970; Mayeda et al., 1983). The first lunar meteorite was discovered in 1979 by a Japanese collection team in Antarctica, although they did not recognise it as being from the moon. It was not until an ANSMET (US Antarctic Search for Meteorites) team also discovered a similar meteorite in Antarctica and compared it to the Apollo samples that it was ascertained that these two samples represented meteorites from the lunar surface.

Since then, over 112 individual stones of lunar meteorites have been found in hot and cold deserts, representing about 50 individual falls that have fragmented during atmospheric entry. As well as providing information about the areas of the Moon un-sampled by the Apollo astronauts, lunar meteorites can also give significant insights into the formation of the Moon and the effects of space weathering.

Perhaps the most famous group of meteorites are those which originate from the planet Mars. A martian origin for several achondrites was first suggested in the early 1980s after studies of their intricate mineralogy and chemistry suggested a similarity to the martian surface. Indeed, the noble gases (Ar, Kr, Xe) trapped within the igneous matrix were consistent with measurements made of the martian atmosphere by the Viking landers in the 1970s (Bogard and Johnson, 1983).

Martian meteorites are often known as SNCs after the type specimens Shergotty, Nahkla and Chassigny – three distinct groups of martian meteorites, all of which are made up of igneous rocks. Legend has it that the Nakhla meteorite is responsible for the only recorded fatality due to a direct meteorite strike, allegedly killing a dog on impact near Alexandria, Egypt in 1911, although this has not been corroborated with any evidence. To date, over 70 martian stones have been collected, representing over 30 individual falls. Martian meteorites have been crucial to the development of environmental models of Mars, showing mineralogical and chemical signatures of past, wet martian regions.

In the search for life on mars, martian meteorites have proved invaluable and gained infamy. In December 1984, ANSMET found an unusual 2kg stone in the Allan Hills region of Antarctica resembling other meteorites classified as martian. However, dated at 4.5 billion years, ALH84001 is the oldest martian meteorite; all others have crystallisation ages of between 160 million and 1.3 billion years. ALH84001 is not infamous because of its relatively ancient age, but because suggestions have been made that this sample contains evidence of fossilised life from the martian surface. In 1996, a group of NASA researchers (McKay et al., 1996) published a report inferring that carbonates found within the meteorite were of biological origin and high-resolution images showed nanometre-sized, rod-like structures that could represent primitive fossilised bacteria.

Although martian meteorites do contain very small amounts of carbon, and some organic molecules have been tentatively identified, these may be sourced from processes unrelated to life in the same way as the organic molecules that have been identified in star-forming regions and in the carbonaceous asteroids. The ALH84001 claim has proved to be controversial and ultimately, it was not corroborated. However, the hunt continues for any chemical evidence of biogenic activity in the martian surface.

Although at the centre of many controversies, both scientific and cultural, meteorites remain pivotal tools for deciphering the history of our Solar System and its origin and processes. They may tell us about the space environment outside our own star system, how our own planet Earth evolved, or may even provide clues to the origins of life on Earth (or elsewhere).

References

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