As the scientific community puzzles over the nature of ’Oumuamua, the cigar-shaped object from interstellar space discovered on 19 October, Dr. Alan Rubin discusses the origins of extraterrestrial rocks that we know rather more about
Meteorites are our principal source of extraterrestrial material. They are sometimes called the ‘poor man’s space probe’ because they land on Earth for free. These rocks hail from approximately 100–150 different asteroids as well as from the Moon and Mars, and they provide key information about our origins.
Asteroidal meteorites are also 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.
There are three main varieties of meteorites
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.
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.
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.
Much older ‘fossil’ meteorites have been preserved in 470-million-year-old limestone in Sweden; these rocks are impact debris from a major collision in the asteroid belt. Each year, samples from that disrupted body still land on Earth.
Although there are more than 60,000 meteorites in the world’s collections, two-thirds of these are from Antarctica and will never be available to the general public. And the resource is barely growing; each year there are only five or six fresh falls and 200 or so ‘finds’ (most of which weigh less than 200 grams and are appreciably weathered).
For scientists to classify a meteorite into the proper category, the rock must be broken or cut; only then can well-characterised samples be offered for sale. Meteorite hunters, meteorite researchers, and meteorite 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.
Mars’ heavily cratered southern hemisphere attests to its bombardment by asteroids; the relatively low surface gravity of the Red Planet (only 38 per cent as strong as the Earth’s) suggests 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 analyzed 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 crystallization 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 would probably 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, which provide 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.
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-aluminium silicate. The dark regions of the Moon are the ‘maria’ — impact basins that have been flooded with dark grey basalt. The basalt is similar in composition and texture to the dark igneous rocks found in Hawaii, Iceland, and on the floors of the Earth’s ocean basins.
Together the highlands and maria make up the features of the ‘Man in the Moon’, the ‘face’ that stares down at Northern Hemisphere observers squinting up at the heavens. But these two rock types (along with impact melt rocks) are also found in lunar meteorites — samples from major impacts that have blasted off the surface of the Moon.
Lunar meteorites are ‘breccias’ — stones made up of broken rock fragments, glass shards and glass spherules cemented together by interstitial impact melt. They are derived from the near-surface regions of the Moon known as the lunar regolith, which is the layer of impact-fragmented and pulverized rock and mineral grains that overlie lunar bedrock. The regolith has also been subjected to intense solar radiation and bombardment by cosmic rays and micrometeorites.
Although close to 300 lunar meteorites are currently known, many specimens are actually different pieces of the same meteorite; the number of separate lunar meteorites is probably closer to 155. From the existing samples on Earth, only about 0.4 per cent of known meteorites are lunar; they are so scarce that, to date, none have been found in Europe, Asia, North America or South America. In fact, about 12 per cent of recovered lunar meteorites are from Antarctica, and will most likely never be available to the general public.
Dr. Alan Rubin is a Research Geochemist in the Department of Earth and Space, UCLA