An expert guide to meteorites

Meteorites are small rocks from space that impact the Earth. They are exceedingly rare: the collective weight of every meteorite known to exist is less than the world’s annual output of gold.

The vast majority originate from the asteroid belt and hail from 100 to 150 different asteroids. Asteroid meteorites are the oldest rocks around — a few hundred million years older than the oldest existing Earth rocks, and approximately 60 million years older than the Moon itself.

A few hundred meteorites come from the Moon and Mars; they were ejected by collisions into interplanetary space where they eventually assumed Earth-crossing orbits.

Although there are more than 60,000 meteorites in the world’s collections, about two-thirds will never be available to the general public. And the resource is barely growing: each year, hunters recover only about eight to 10 fresh falls and a few hundred weathered non-Antarctic ‘finds’ (most of which weigh less than 200 grams).

For scientists to classify a meteorite into its proper category, the rock must be broken or cut; only then can well-characterised samples be offered for sale. Meteorite hunters, researchers and dealers work together in a worldwide enterprise to discover new specimens, uncover details about the origin of the solar system, and make samples available to the discerning collector.

The three main varieties of meteorite

Stones (95 per cent of meteorite falls): these are silicate rocks (some resembling terrestrial volcanic rocks) derived from melted and unmelted asteroids, the Moon and Mars. Some were melted in their parent bodies; in others one can discern the raw ingredients of the planets. Some stones may have brought water and organic compounds to the Earth, facilitating the origin of life.

Irons (4 per cent of falls): these are metallic iron-nickel masses, predominantly from the cores of melted asteroids.

Stony irons (1 per cent of falls): these half-stone, half-metal samples are formed on or within melted asteroids by the mixing of metal core material with silicate rocks. One class of these meteorites, the pallasites, are widely considered the most beautiful extraterrestrial substance known.

Meteorites and the origins of life

Meteorites have been pelting the Earth throughout geological history. Sixty-six million years ago, an asteroid (essentially a giant meteorite) slammed into the Yucatán Peninsula in Mexico, causing a mass extinction that wiped out the dinosaurs. Had that meteorite missed, there would be no cows, cats, pigs or people on this planet right now.

Some stone meteorites contain hundreds of organic compounds, including amino acids, the building blocks of proteins. Many scientists propose that the Earth provided a suitable environment for life to commence after meteorites and comets brought water and organic material to the planet’s surface. Had it not been for these extraterrestrial wanderers, there may have been no life on Earth at all.

Lunar meteorites

Specimens of the Moon are among the rarest substances on Earth. Less than 650 kg of lunar meteorites are known to exist — all would fit within five footlockers — and a significant portion of these precious samples are controlled by governmental institutions. Only about 0.6 per cent of known meteorites are lunar; in fact, they are so scarce that, to date, no examples have been found in Europe, Asia, North America or South America.

Scientists identify Moon rocks by their specific textural, mineralogical, chemical and isotopic signatures.

Lunar meteorites are ‘breccias’: stones made of rock fragments, glass shards and glass spherules cemented together by interstitial impact melt. They are derived from the near-surface regions of the Moon; this is the lunar regolith, the layer of impact-fragmented and pulverised rock and mineral grains that overlies lunar bedrock.

Some lunar meteorites are identical to Moon rocks collected by the Apollo astronauts. The light-coloured, heavily cratered regions of the Moon are called the ‘lunar highlands’; they are made up mainly of anorthosite — a light grey rock rich in calcium-aluminum silicate. The dark regions of the Moon are the ‘maria’ — impact basins that were flooded with basalt, a dark grey volcanic rock.

Martian meteorites

Mars’s heavily cratered southern hemisphere attests to its bombardment by asteroids; and the relatively low surface gravity of the red planet (only 38 per cent as strong as the Earth’s) suggests that it would be feasible to launch rocks during a giant impact.

But there was no proof that Martian meteorites had actually landed on Earth until 1983, when NASA scientists analysed the gas bubbles trapped inside impact-melted glass within a basaltic meteorite found in Antarctica. The chemical and isotopic composition of those bubbles precisely matched that of the atmosphere measured on the surface of Mars by the Viking spacecraft lander in 1976.

The putative Martian basalts have relatively young crystallisation ages (180 million to about 2 billion years before present); this shows that these rocks cannot be from asteroids, because those small bodies had cooled completely more than four billion years ago.

The case for the Martian origin of these rocks is essentially closed, since even the most sceptical meteorite scientists admit that there is at least a 95 per cent probability that these samples are from Mars.

Although there are close to 200 Martian meteorites in collections worldwide, many of the specimens are actually different pieces of the same rock. A good estimate of the number of separate Martian meteorites is 135, giving scientists unparalleled access to evidence bearing on the geological history of the Martian crust. These rocks are scientifically important, commercially valuable, and a key component of fine collections.

Dr Alan Rubin is a retired and recalled adjunct professor and curator of the UCLA Meteorite Collection