Meteorite

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STONES FROM THE SKY

As soon as the curious gaze of our distant ancestors turned skywards, they began to notice stars streaking across the Heavens. On any given cloudless night in any part of the world, provided sufficient patience is exercised, the celestial light show continues. These lights, called meteors (from the Greek μετέωρος (meteoros) which means ‘raised from the ground’), have been a part of the human experience of life on Earth for as long as our species has existed. We have never known skies without them. They continue to capture imaginations the world over, and as they blaze across the sky as if they are ‘shooting stars’, we continue to cast our wishes upon them.

But meteors, and their bigger and more brilliant equivalents, fireballs, are not stars at all. They are the products of something so unexpected, so out of the ordinary, that it was not until around two centuries ago that their origin and significance was realised by the minds of modern science. Meteors and fireballs have their origins not in astronomy, but in geology. They are the products of rocks falling from outer space.

Racing between twenty and seventy kilometres per second, a typical meteor would travel the length of the United Kingdom in just shy of half a minute. Like the prow of a ship parting the water, falling stones part the air in the Earth’s atmosphere as they press onwards across the sky, compressing the gas before their bow. The air caught in their path is pressed so strongly and so rapidly that its temperature instantly soars to thousands of degrees Celsius. It glows incandescent as a result.

The forward-facing surface of a falling stone is superheated to temperatures far exceeding that of freshly erupting lava from a volcano; the outer layers of the stone are slowly vaporised and unmade entirely. Most stones are utterly consumed as they fall, but a handful survive their fiery passage through the atmosphere, and make landfall on the Earth’s surface. We call these survivors ‘meteorites’.

The Field of the Sky

At twilight some 4,000 years ago in the Chaco region of what is today known as Argentina, people were awaiting the warm return of the Sun God from beneath the horizon when, the story goes, He came forth from the sky and fell to Earth. A brilliant light suddenly filled the half-lit sky and a clamorous noise spread through the air. He appeared before them as a large mass of iron, jet-black on the surface and silvery on the inside. Fires sprang all around Him. His coming to the Earth persisted in the collective memory and mythology of the native Chaco people for some 3,500 years until the arrival of Spanish conquistadores. Driven by the lust for gold, silver, and power, European colonisers claimed vast areas of South America for themselves in the sixteenth century. They discovered, no doubt much to their confusion, that the native people of the Chaco region wielded tools and weapons made from iron of an unusually high grade. There were local legends of a Sun God falling to the Earth as a large piece of iron in the distant past. In their arrogance, the colonisers dismissed these stories as mere rumour in the hope that the so-called piece of iron was in fact a large deposit of silver.

In 1576, the Spanish had the natives take them to the mass of metal. They were led along a network of well-trodden footpaths over a flat-lying plain void of waterways and rocks, but occasionally punctuated by hollows a few metres across. The area was named Piguem Nonraltá, which the Spanish translated as Campo del Cielo (Field of the Sky). Then they came upon it: a large, smooth-skinned piece of metal protruding out of the soft soil. It was almost two metres across, and weighed an estimated fourteen tonnes. Ignoring the fact that this was a spiritual place of worship for the native people, the Spanish broke pieces off the mass for investigation.

A blacksmith determined that the stone was made not of silver, but of high-quality iron. The colonisers mistakenly thought they had discovered an iron mine, and that this large piece, which became known as Meson de Fierro (Large Table of Iron), was just the tip. Many more pieces were found scattered over the area, but written reports of their discovery went largely unacknowledged for the next 300 years. The Meson de Fierro was visited for the last time in 1783 by another Spanish expedition, and its whereabouts have since been lost to history. It is unlikely the expedition of 1783 would have had the means to move such an enormous mass of iron: it was probably rolled over into a nearby hollow, and has since been buried by silt-laden floodwaters and shrouded from view.

The Campo del Cielo irons are not from an iron mine: they are meteorites, and we know with reasonable precision the date on which these enormous pieces of cosmic iron fell from the skies thanks to the bushfires they ignited. Vegetation and shrubbery were incinerated by the enormous fallen mass, and a vast area of the Field of the Sky was reduced to charcoal. By carefully measuring the isotopic fingerprint of the carbon in the charcoal, the date that the plants died–the date of the fire–can be measured: this in turn corresponds to the date of the meteorite fall, which turns out to be ~ 4,000 years ago, well within the lifetime of ancient stories and the ancestral memory of the native people.

Campo del Cielo is an unusually large meteorite. When it slammed into the Earth’s atmosphere it was probably one enormous body but, as it fell towards the ground, the heat melted and stripped most of it away. Immense forces and pressure also played a part in reducing the size of the rock, tearing it into many smaller (albeit still huge) fragments. Over a dozen large pieces of this meteorite fall have been unearthed in the Field of the Sky, and there are probably more that lie yet undiscovered beneath the soil.

A piece named ‘Optumpa’, which is one metre across and weighs over half a ton, can be seen today on public display in the Natural History Museum in London. I saw it when I was nineteen on a school outing, my first visit to the museum. I remember saying to my A-level geology teacher, Mr Currie, ‘I would love to work here one day.’ As it turned out, I ended up completing parts of my PhD research there. I have had a particular fondness for Optumpa ever since.

Sky metal

In the arid region of central Iran lies the ancient city of Tepe Sialk. Archaeological excavation began early in the last century and hundreds of artefacts have since been uncovered: ancient architecture, sophisticated pottery, and solemn tombs. Amongst the items are a trio of small spheres or beads made of iron that date back ~ 6,300 years. What purpose they served is unknown. They caught the eye of archaeologists, however, not because of their shape, but because of what they were made from. These iron spheres pre-date the Iron Age by about 3,000 years. The technology used to smelt or process iron did not exist when they were crafted.

Modern analysis of the beads reveals their celestial origin. They are pieces of ductile meteoritic iron that were hammered into spheres by skilled craftsmen. In the Age of Stone, this metal–dense, shiny, malleable and cool to the touch–must have seemed strange to the people of Tepe Sialk. It is not known whether these pieces of iron were seen falling from the sky or were simply happened upon in the desert surrounding the city, but either way, the small iron spheres are one of the most ancient examples of iron being fashioned into objects.

There is something stirring about humanity’s first contact with iron–a metal that so much of our modern civilisation is built upon–being iron of a celestial origin. It was the coming together of two stories that at first seem so separate but, as we will discover, are intimately entwined: outer space and humanity.

Fit for a king

Among the many treasures discovered in King Tutankhamun’s tomb when it was unearthed in 1922 was a handsome iron dagger. With a handle of gold and a sheath to match, it had been placed inside the linen wrappings of the deceased pharaoh’s mummified body before he was sealed inside his sarcophagus.

Lingering doubt about the spiritual significance placed on meteoritic iron was cast aside when the chemical character of this dagger was measured at the Egyptian Museum of Cairo. By irradiating the dagger with a focused beam of electrons, the object began to fluoresce in X-ray light, and by deciphering the ‘colours’ (wavelengths) of those X-rays, its chemical composition was revealed.

The dagger was found to be made of almost pure iron with a large serving of nickel: this iron, without a doubt, has a celestial origin.¹ Metal with this particular blend of elements does not form on Earth. Tutankhamun’s dagger is made from an iron meteorite. Only the most precious objects were fit for the tomb of a pharaoh, and so it is likely that the metal held great spiritual significance. As is the case with the spheres of Tepe Sialk, we may never know for sure if its creator saw the meteorite falling from the sky or simply happened upon it atop the sands of the north-east African desert, but there are hints that they had at least some idea of its celestial origin.

Around the same time that Tutankhamun ruled over Egypt, a new hieroglyphic phrase came into use.

It translates into ‘iron from the sky’. The phrase was used to describe all types of iron, whether celestial or strictly Earthbound, and so, while a little ambiguous, it does suggest some recognition of where this iron came from. Some ancient cultures made a connection between meteors, fireballs and the strange objects that sometimes accompany them. They probably knew that lumps of iron occasionally fell from the sky, and they held these celestial stones in reverence. They knew these objects were important. This realisation, along with many others, was severed by the Dark Ages, however, and lay forgotten for over 2,000 years. However, the idea that stones could fall from the sky was rediscovered by modern science and Europe during the Enlightenment of the eighteenth century, in a series of chance events and serendipity.

The tranquillity of a pleasant evening in the summer of 1751 in north Croatia was disturbed by a rare cosmic event. A brilliant flash of light, easily visible above the soft evening skyglow, illuminated the sky over the village of Hraš´cina. Almost immediately after the bright flash, a booming sound rippled across the surrounding farmland and was heard over an area of almost 2,000 square kilometres; the roaring echoes of the explosion sounded like the deep rumbling of many horse-drawn carriages. Seven eyewitnesses out on evening strolls recalled seeing two balls of fire, linked by a glowing fiery chain, fall from the sky. Some even reported two large pieces of falling rock plunging into a freshly ploughed field, and large crevasses opening where they had struck the earth. The fallen pieces were later recovered from the soft soil, one of them from a depth of almost one and a half metres. A strange black surface crusted them as if they had been charred by intense fire, and concealed the metallic nature of the objects. The long trails of smoke left behind by the falling fires were left hanging in the evening air for several hours before dissipating into the night. The event was chronicled by a local priest: ‘In their ignorance, the common folk thought that the heavens had opened.’

And it is no wonder. Explosions in the sky were not an everyday occurrence in the mid-eighteenth century. The prevailing wisdom held that solid objects do not fall out of nowhere. That would be ridiculous: the heavens are perfect and without flaw, after all. Isaac Newton, one of the greatest and most influential scientists of the previous millennium, posited in his 1704 publication Opticks that outer space must necessarily be void of all small objects–including stones and pieces of metal–if his laws of gravity were to work. The common view held that meteors were a purely atmospheric phenomenon and had nothing whatsoever to do with the Heavens.

If there are no rocks in outer space other than the planets, the moons, and the occasional comet, then rocks could not possibly fall to the ground. Even so, seven eyewitnesses in northern Croatia gave sworn testimonies as to what they had seen: rocks falling from the sky, seemingly from nowhere.

With mind and eyes wide open

Ernst Florens Friedrich Chladni was born in 1756 in eastern Germany, and had been fascinated by physics and the natural sciences since boyhood. Because his father disapproved, Chladni wound up studying law and philosophy; he achieved his PhD in law at the age of twenty-six. Once his father died, Chladni quickly returned to his early passions. He went on to publish a seminal book on the physics of sound, Entdeckungen über die Theorie des Klanges (Discoveries in the Theory of Sound), in 1787: history now remembers him as the Father of Acoustics. Chladni is less well-known for his significant contribution towards another brand-new field of science: cosmochemistry.

Chladni was inspired by a conversation with Georg Christoph Lichtenberg, a prominent natural philosopher who had witnessed a spectacular fireball over Göttingen (north Germany) in 1791. Chladni asked what he made of the growing list of reported fireballs and the occasional reports of strange stones and metals falling from the sky. Lichtenberg responded that in his view, fireballs were not atmospheric but cosmic phenomena, originating in outer space. He speculated that the testimonies of stones and pieces of iron falling from the sky might really be genuine, though he didn’t truly believe this to be the case himself.

This conversation set Chladni’s imagination alight. He spent the following few weeks in Göttingen compiling a list of twenty-four well-documented fireballs, each observed between 1676 and 1783. Eighteen were accompanied by pieces of rock that allegedly fell from the sky, though few scholars trusted the authenticity of these claims. The rocks seemed to differ in their natures: some were stony, some were metallic, and some were a mixture. Chladni compiled their speeds, apparent sizes, flight paths, and other small details such as the number and violence of visible explosions, and the thunderous sounds made as they fell. The descriptions were astonishingly similar. Even though these accounts spanned more than a century and occurred across multiple continents, they bore a striking resemblance to one another. Adept at teasing out truths from witness accounts thanks to his legal training, Chladni deemed the accounts genuine. They were too similar not to be. What reason did the witnesses have to lie? If they were not telling the truth, how could their accounts be so similar?

Chladni shared his thoughts with the world in his 1794 publication Über den Ursprung der von Pallas gefundenen und anderer ihr änlicher Eisenmassen und über einige damit in Verbindung stehende Naturerscheinungen, which in English is shortened to Ironmasses. He argued that stones–both rocky and metallic–really do fall from the sky and are every bit as real as the Earth on which we stand. He argued that all fireballs, and the smaller lights sighted streaking through the sky called meteors, are caused by solid objects falling through the atmosphere at extraordinarily high speeds. This was the first time that somebody had explicitly linked fireballs and meteors (‘shooting stars’) with solid objects. It went against all conventional wisdom of the time. But Chladni went further still, concluding that the astonishing speed at which fireballs and meteors streak across the sky rules out an atmospheric origin: the meteor stones, or meteorites, must originate from the cosmic sphere, far above the atmosphere, to be travelling at such blistering speeds. They are not of this planet. Chladni also reasoned that other strange rocks that bore a resemblance to witnessed meteorite falls but were not found in association with a fireball–those with charred and blackened surfaces–also had a cosmic origin.

This radical departure from the prevailing worldview was not warmly embraced by the scientific community. Even Lichtenberg had great trouble believing it at first. But then another stone was witnessed falling from the sky the following year, and it happened to fall in just the right place, at just the right time, on just the right person’s land.

An extraordinary stone

The green rolling farmlands that stretch across the Wolds of the East Riding of Yorkshire are punctuated only by the occasional picturesque village, but one day in December 1795, the quiet was shattered by explosions emanating from the sky. The noise was heard in coastal villages fifteen kilometres away. And with the thunder still echoing across the fields, three ploughmen watched open-mouthed as a large rock fell from above and struck the ground with a dull thud. John Shipley, one of the ploughmen, was a mere eight metres away from where the rock landed. Sods of muck were hurled high into the air as it struck the soil. The twenty-five-kilogram stone, the size of a loaf, was travelling so fast that it sliced downwards through half a metre of earth and embedded itself into the solid rock below.

The owner of nearby Wold Cottage was a man by the name of Edward Topham, who was away on business at the time the meteorite fell. A playwright and founder of a scandalous newspaper called The World, Topham cut a flamboyant figure in late Georgian London, with his mutton-chop whiskers, unconventional sense of fashion, and charisma earning him a reputation as an eccentric (he was frequently the subject of caricaturists). But he was widely regarded as a fair and just man. He had retired with his three daughters (deemed ‘the best horsewomen in Yorkshire’) to Wold Cottage some years before. It was rumoured that he intended to spend the rest of his days farming, breeding greyhounds, and writing the history of his own life. His kennels were already considered the best in England, and his greyhound Snowball was praised as ‘one of the best and fleetest greyhounds that ever ran’. But the meteorite interrupted all his plans and his memoir was never to be completed.

Topham returned home to find that the fallen rock which the ploughmen had carried back to his cottage had caused something of an ongoing commotion: thirty to forty people passed by each day for almost three weeks to see it, and he amassed a pile of letters requesting more details. He recorded statements from each of the ploughmen and published them, along with his own thoughts on the strange event, in the Gentleman’s Magazine.² Crucially, Topham believed the ploughmen, and trusting Topham’s judgement, other people believed the ploughmen, too. But the mystery of exactly how the rock had come to fall out of the sky remained; Chladni’s outlandish idea that such objects originated in outer space had not yet caught on.

From Yorkshire to London

The Wold Cottage meteorite went on to become a national curiosity. In the Yorkshire volume of The Beauties of England and Wales, published between 1801 and 1815, the Wold Cottage event featured prominently. Being well connected in the capital, Topham had the meteorite sent to London to be placed on public display in the heart of the city. The exhibit was widely advertised in newspapers, including The Times.

For the modest price of one shilling (nearly £4 in today’s money), visitors could see the strange object for themselves. They also received a copy of the testimonies of the three ploughmen and a small illustration of the meteorite. When the President of the Royal Society, Sir Joseph Banks, paid his shilling to see the famous stone for himself, he noticed that it looked uncannily similar to a stone that apparently fell from the sky during a fireball event in Italy over a year previously. The stones were almost identical, yet they fell in different countries more than eighteen months apart.

Banks, however, was firmly of the mind that these stones had been generated in the atmosphere by meteors, rather than being the cause of meteors. His interest piqued, he tasked a talented young British chemist named Edward Howard with analysing the stones.

As a Fellow of the Royal Society, Howard was renowned for his dedicated research into synthesising new explosives for use in firearms (and suffered many injuries during the course of his experiments). Howard was loaned pieces of a further six meteorites, taking his total number of samples to eight. The stones differed in their geological characters: four were meteorites of stone, two were of pure metal, and two were a mixture of both stone and metal.

One of the iron meteorites was loaned by the Natural History Museum in London: it came from Campo del Cielo. The rock seen falling out of the sky ~ 4,000 years ago in South America was now being used for research purposes in a chemistry laboratory in Victorian London, marking a point in time where the spiritual and scientific significance of meteorites became irreversibly entwined.

Howard published his findings in 1802, and his paper remains one of the most important in the history of meteoritics.³ It was the first systematic documentation of the chemical and geological make-up of meteorites. A few chemists had attempted to discern the chemical nature of these stones previously, but Howard’s efforts were far more sophisticated. He took particular care with the stony meteorites, realising they were composed of innumerable individual grains, which he painstakingly divided into four separate components: strange rounded globules, yellow pyrites, small metallic blebs, and all sandwiched and held together by a fine ‘earthy’ substance made of crumbly rock. This must have been tedious work: the grains in stony meteorites are tiny. It would be like manually separating all of the poppy seeds from a bag of mixed bird food.

Howard discovered that the small metallic blebs in stony meteorites contained nickel. Nickel had been found in abundance in iron meteorites by French chemists; Howard replicated and confirmed these findings with his own set of irons and stony irons, too. It is unusual to find high levels of nickel in rocks of an Earthly origin, and so, for the first time, Howard chemically linked the stony meteorites with the iron and stony-iron meteorites. These rocks were unlike any described before them.

Their unworldly nickel-rich chemical compositions, and the distances and lengths of time between the apparent falls of similarly strange stones, pointed towards the previously laughable: a celestial origin. Howard had not only provided the first physical evidence in favour of Chladni’s hypothesis, but also fashioned the field of cosmochemistry, the chemical study of material from outer space.

It still seemed unbelievable, but as Howard himself beautifully put it: ‘To disbelieve on the mere grounds of incomprehensibility, would be to dispute most of the works of nature.’

Slowly, and with initial reluctance, the scientific world came round to the idea that meteors and fireballs are indeed caused by rocks falling to the Earth from space, and sometimes these rocks make it to the surface. It was fortuitous that the Wold Cottage stone happened to fall on Topham’s land when it did. Had it fallen on somebody else’s land–perhaps somebody with less of a gift for publicity–it may have wound up being used as a doorstop. (This really did happen to a meteorite named Lake House: it unceremoniously sat on the doorstep of an Elizabethan country house, from which the meteorite took its name, in Wiltshire (south-west England) for almost a century before its celestial origin was confirmed by scientists at the Natural History Museum in London.)

Today, the exact spot where the Wold Cottage meteorite made landfall is marked by a tall monument, commissioned by Topham himself and built from reddish-brown brick. On it sits an ornamental stone tablet, into which the following inscription is chiselled:

By the middle of the nineteenth century, all but the stubbornest intellectuals had accepted Chladni’s hypothesis. But there remained a huge problem: where exactly in outer space do meteorites come from in the first place?

Many other places

Chladni hypothesised that meteorites originated not merely outside the Earth’s atmosphere, but outside our Solar System entirely. He ascribed an interstellar (‘between the stars’) origin based on the lightning speed at which they enter our atmosphere. Chladni’s other theory was that meteorites may originate from a destroyed planet, but no evidence of large planetary fragments had been seen by telescopic observations of the night sky. It wasn’t long, however, before another hypothesis on the origin of meteorites was proposed.

In 1802, the same year that Howard published his work on the chemical nature of meteorites, Pierre-Simon Laplace, a French mathematician and astronomer, popularised the hypothesis that these meteorites originated closer to home. He suggested that they came from the Moon. Eruptions of Lunar volcanoes had been ‘observed’ by the German-British astronomer William Herschel in 1787 (this apparent observation, in time, turned out to be an error). Laplace hypothesised that if the powerful volcanic forces seen on the Earth are also active on the Moon, then ejecta thrown out by these volcanoes could be projected into space and onto the Earth. It seemed watertight. The hypothesis was so popular that in the Yorkshire volume of The Beauties of England and Wales guidebook, the Wold Cottage meteorite was described as a piece of the Moon.