Fire in the sky — a history of meteoritics
- February 3, 2022
- Maria Golia
The study of meteorites challenged the boundaries of the known universe, obliging nineteenth-century thinkers to revise their view of the solar system, and twentieth-century science to embrace them as a geological and evolutionary force.
The history of meteoritics may be understood as an epic journey into the anomalous. The learned men who first embarked ascribed to the Enlightenment ideal of objectively pursuing knowledge, but their conventional wisdoms had to be wrenched from their confident grasp. The politics of ideas, religious bias and the difficulty of having to theorise on the basis of second-hand observations, conspired to make the reality of meteorites, not to mention their cosmic origin, barely conceivable to the new men of science. Galileo noted that ‘the authority of thousands counts for nothing before the single voice speaking the truth,’ but that depends on whose voice it happens to be. Laymen’s reports of meteorite falls were discredited and scientists willing to believe them, obtain samples of the stones or entertain a probable cause for the phenomena were constrained in their thinking by the power of prevailing opinions.
At the turn of the eighteenth century the very notion of stones falling from the sky was infra dig and eye-witness accounts, however consistent in their details, were typically dismissed as the maunderings of credulous peasants. When a fireball exploded over the south of France in 1790 producing a shower of stones, teacher and naturalist Jean F.B. Saint-Amans asked for official testimonies, expecting none to be forthcoming. Yet he received a signed affidavit from a mayor of one of the affected towns including the depositions of 300 witnesses, and shared it with his friend, Pierre Bertholon, editor of the Journal des Sciences utile in Montpellier. Bertholon published the affidavit not as a testimony to the event but a lament that so many of his countrymen had yet to awaken to the Age of Reason:
‘How sad, is it not, to see a whole municipality attempt to certify the truth of folk tales… the philosophical reader will draw his own conclusions regarding this document, which attests to an apparently false fact, a physically impossible phenomenon.’
‘The noise that these meteors make in bursting, the dazzling light that they spread, the surprising shock they cause, stuns the majority of those who see them: they do not doubt that the burst had fallen all around them; they run, they look, and if they find, by chance, some little bit of black stone, surely this stone just fell. As the fable spreads, people all over the countryside search for stones and find thousands of them.’
For the study of meteorites to begin, their existence had to be acknowledged, or rather re-acknowledged, since falling stones had been documented throughout antiquity, and their possible cosmic origin proposed in the fifth century BC. Science is marked by reversals in thinking and discoveries that defy previous definitions of the natural world. But rather than an advanced theory or mathematical proposition, the birth of meteoritics demanded a return, along a zigzag path, to common sense.
The Royal Society of London, established in 1660, chose for its motto nullius in verba (‘take nobody’s word for it’), signalling its founders’ intention to establish facts via observation and experiment. Still, you had to take somebody’s word, and who better than Sir Isaac Newton, the Royal Society’s president from 1703 until his death in 1727 and whose Principia (1687) expounded the laws of motion and gravity. Newton’s orderly universe was the home of ‘…great bodies, Fixed Stars, planets and Comets’ but there were no odd bits flying about. Like other scientists of his time, Newton saw a higher power in nature’s workings:
‘This most beautiful system of the sun, planets, and comets, could only proceed from the counsel and dominion of an intelligent and powerful Being…’
Also like his contemporaries, Newton held ancient thinkers, especially Aristotle (384BC-322), in deep regard. Aristotle’s Meteorologica, a treatise on the Earth and its surroundings, maintained that solid bodies other than the Sun, Moon and planets could not exist in space, a theory congenial to Christianity’s belief in God’s perfect creation. Commenting upon the stone that had fallen at Aegospotami in Thrace c. 469-467BC (today’s Gallipoli Peninsula) Aristotle claimed it was a terrestrial rock, lifted by strong winds then dropped back to earth.
The Aristotelian tradition of comprehending the world through reasoning and searching for ‘natural’ circumstances would eventually ripen into empirical methodologies. But the belief meanwhile held that rare events contradicting existing theoretical models were aberrations, revealing nothing of nature as it ‘naturally’ functions and therefore was unworthy of inquiry. As Francis Bacon remarked in Novum Organum (1620) ‘the human understanding is, of its own nature, prone to suppose the existence of more order and regularity in the world than it finds.’ In the late eighteenth century reports of unruly stones were growing too frequent to be dismissed, but few minds were prepared to take on the degree of disorder required to explain them.
Unhindered by Aristotelian theories or Christian cosmologies, the Chinese were more conversant with astral phenomena. While European astronomers adopted the Greek system of constellations, eventually mapping 88, the Chinese divided the sky into 283 constellations. These smaller divisions allowed for greater accuracy in identifying the locations of cosmic events, only appreciated in the West in the nineteenth century, when the long-term motion of Halley’s Comet was determined using Chinese observations of its returns beginning in 230BC. By 28BC Chinese astronomers were regularly recording sunspot observations made with the naked eye. Astronomical events were documented in the official histories of Chinese dynasties. Provinces and districts produced regional histories, assembled by state officials and local scholars. The earliest reference to a meteorite is 645BC: ‘Five stones fell in Sung.’
Over time, people witnessing the appearance of large fireballs often noted the strange sounds that accompany them. Reports from different times and places mention ‘hissing,’ ‘swishing,’ ‘rushing,’ ‘popping,’ ‘buzzing’, and a ‘crackling’ or ‘vibrating’ in the air. Cultural factors clearly influence the descriptions. Gregory of Tours (580AD) recorded ‘a sound of as many trees crashing to the ground’; Chinese annals (817AD) note ‘a noise like a flock of cranes in flight’, whereas witnesses of the 1992 Peekskill New York fireball heard something ‘like a sparkler’. Aside from implying sensations beyond the auditory, these sounds differ from the explosions typically heard seconds to minutes after the fireball is extinguished in that they occur while it is still speeding across the sky.
Well into the twentieth century, the sounds were dismissed as an observational illusion owing to the disorientation of people witnessing the sudden appearance of a large, exceedingly bright, fast-flying object. ‘The explanation is without a doubt psychological,’ wrote a professor of mathematics and astronomy in 1939. One of the few to object was H.H. Ninninger (1887-1986) a self-taught American meteoriticist who thought the sounds should be regarded ‘as a problem in physics rather than psychology.’ The sounds, now called electrophonic, are described as resulting from the direct conversion of electromagnetic radiation into audible sound, but they are not yet fully understood.
Eighteenth-century scientists knew that sound ordinarily took time to travel, as did anyone who had observed the delay between a lightning bolt and the thunder clap, or the flash of a distant cannon and its boom. Having learned of a 1719 fireball over England and reports of it hissing ‘as if it had been very near at hand,’ Edmund Halley (a friend of Newton who determined the orbit of the comet bearing his name) calculated its probable distance from where the sounds were heard (‘60 English miles’) and dismissed them as the ‘effect of pure fantasy.’ Electrostatics and radio waves were unknown in Halley’s time. But the enduring view that these anomalous sounds were imaginary demonstrates the difficulty of objectively assessing someone else’s experience, and the (subjective) urge to dismiss the unknown as impossible. This hyper-wariness, combined with the relative rarity of fireballs and witnessed falls, helps explain why the science of meteoritics had such a hard time getting off the ground.
William Hershel’s 1781 discovery of Uranus had, however, proved that space held some surprises, Newton notwithstanding. Some of those willing to entertain the notion that stones did fall, maintained, like Aristotle, they had been somehow thrust into the air, possibly by volcanic activity, yet no one ever saw them going up, only coming down. The matter became more pressing with the first discoveries of asteroids in 1801 and 1802, small bodies that were neither comets, stars, nor planets and therefore should not have existed. Then on April 26, 1803, out of a clear blue sky, thousands of stones pelted L’Aigle (142 kilometres from Paris). France’s minister of the interior dispatched Jean-Baptiste Biot, a young and recently elected member of l’Institut National to investigate.
The first scientist to venture out of the lab or library to assess the phenomenon, Biot was thorough: interviewing eyewitnesses, collecting samples and carefully mapping the stones’ ‘strewn field,’ the elliptical area over which they fell. His report of ‘without a doubt the most astonishing phenomenon ever observed by man’ swayed influential minds and opinions.
Although a growing body of evidence, including chemical analyses, persuaded some members of Europe’s scientific community that meteorites may have come from space, theories that they were formed in Earth’s atmosphere or produced by lunar volcanoes persisted. But meteorites had finally passed from fable to fact. The 1824 edition of the French Dictionary of Natural History boasted a 46-page entry for pierres météoritiques marvelling how ‘…there was [recently] even some sort of stubbornness from savants to support the refutation (of meteoritic stones) and to ridicule those who were defending their existence.’
The same stubbornness on behalf of the scientific community delayed acceptance of meteorite impacts’ cosmic origin and the effects of the terrific craters they caused until the 1990s. At the end of the nineteenth century, geologists and astronomers were looking beyond Earth (where craters were virtually unexplored) to the moon, whose pock-marked surface was attributed to volcanic activity. Having studied some of the first photographs of the lunar landscape, Grove Karl Gilbert (1843-1918), chief geologist of the US Geological Survey, decided they were instead formed by impacts.
Gilbert attended the 1891 gathering of the American Association for the Advancement of Science when mineral merchant Arthur Foote described the unusual meteorites he found in the vicinity of Coon Butte, Arizona, containing microscopic diamonds. Foote barely mentioned that Coon Butte was the rim of a gigantic crater, now known as Meteor Crater, and soon to become the first of its kind ever studied on Earth. An estimated 200 million tonnes of rocks were displaced to form its perfect bowl, measuring nearly two kilometres across and 180 metres deep. In keeping with prevailing views, Foote thought the crater was volcanic and dismissed the small meteorites lying in its proximity as coincidence.
Gilbert instead proposed that the crater, like those on the moon, was made by an impact and the large mass that caused it might still be buried there. He soon set out ‘to hunt a star’ spending weeks examining the crater and hurling mud balls into mud surfaces to observe the holes they made, noting that an impact object, which would arrive obliquely, could still leave a round, not elliptical, crater. His estimates of the probable size of the mass (based on the amount of material thrown up to form the crater’s rim) were less satisfying. Gilbert’s calculations failed to take the crater’s 50,000 years of erosion into account; he arrived at a mass the same size as the hole, which made no sense. Nor did he find the magnetic anomalies that a large iron-rich body would have produced. Dismayed but faithful to accepted science, Gilbert concluded that the crater was volcanic and that the meteorites around it were coincidental.
To Daniel Moreau Barringer (1860-1929), a quintessential American entrepreneur, this was plain nonsense. Born in Raleigh, North Carolina, his father was a US congressman who shared an office with Abraham Lincoln. At military school his disrespect for authority won him an early dismissal. At fifteen he enrolled at the College of New Jersey (later Princeton University) where he befriended classmates like future US president Woodrow Wilson. Although he graduated in law at the University of Pennsylvania, Moreau, as he now called himself, opted out of a conventional career, partnering with a geologist and prospecting for minerals in Spain, South American, Mexico, California and the southwest United States. In 1897, he hit the jackpot, acquiring the mining rights for the Commonwealth Silver Mine in Pearce, Arizona.
Moreau heard of Arizona’s Coon Butte crater in 1902 in a chance encounter at the Tucson Opera with someone who knew the territory, and said that according to local legend, the bowl was formed by a meteorite. Intrigued but incredulous, Moreau procured samples of small meteorites from around the crater; analysis revealed 92 per cent iron, five per cent nickel and trace elements including iridium, platinum and microdiamonds. Metals like these sold for $125 USD per tonne at the time, and Moreau, riding a long lucky streak, figured there were at least ten million of them. Convinced the meteorite that formed the crater still existed, Moreau was ready to dig to the antipodes to find it. Apart from the monetary benefits, he wanted to prove academia wrong about the crater’s origin, calling their volcanic, and later ‘steam explosion’ theories ‘blind’ and ‘demented.’
Moving in the circles of Theodore Roosevelt, the trustees of Princeton and MIT, Moreau found deep-pocket investors, but when results failed to materialise, they abandoned him. Moreau, who took his fortune from the earth, put it back where he found it, drilling unsuccessfully for 26 years to find the meteoritic mother lode. His stubborn prospecting ruined him. Moreau never imagined that the mass (later estimated at 300,000 tons) had vaporised in the bombastic energies of impact (equivalent to approximately nine megatons of TNT) and the small scattered meteorites were all that remained. A nine-page spread in National Geographic Magazine ‘Tomb of a Giant Meteorite’ (1928) outlined mining operations at the site without mentioning Moreau or his quest. The article instead highlighted Gilbert’s investigations, perhaps in a nod to the esteemed US Geological Society, even though Gilbert had held firm to the crater’s mistaken volcanic origin until death. Moreau died in 1929, before the substantial evidence he had gathered would help prove the mandarins wrong.
In the 1950s, geologist Eugene Shoemaker (1928-1997) brought a unique perspective to Meteor Crater, as he’d recently studied nuclear blast zones at a testing field in Nevada and saw parallel signs of destruction. With a colleague he identified coesite, a quartz formed only under intense shock at high temperatures, in samples taken from both the blast zones and Meteor Crater proving its impact origin conclusively in 1963. Shoemaker’s next move was not to find other craters, but to look to space for what caused them. In the 1970s with his wife Carolyn and other colleagues, Shoemaker conducted a systematic survey of the inner solar system, tracking asteroids. Only twelve had so far been discovered, not because science lacked the means, but the interest. ‘Astronomers in the twentieth century essentially abandoned the solar system’ Shoemaker explained in an interview, ‘their attention was turned to the stars, the galaxies.’ When Shoemaker’s team focused San Diego’s Palomar Observatory telescope closer to home, the findings were startling. Asteroids and their detritus cross Earth’s path regularly.
In his Nature of the Stratigraphical Record (1973) British palaeontologist and geologist Derek Ager pointed out that what humans consider rare catastrophes are a routine aspect of Earth’s geological process. ‘The hurricane, flood or the tsunami may do more in an hour or a day than the ordinary processes of nature have achieved in a thousand years.’ While Ager refrained from listing meteorite impacts as geological game-changers, he cited respected paleontologist Digby McClaren who had linked them to mass extinctions in 1970. At the time, impact-related topics were treated by the scientific community like private indiscretions, acknowledged but rarely discussed. Geologists and palaeontologists, among others, followed the maxim ‘nature does not make jumps.’ Nothing significant occurs owing to some anomalous paroxysm, only well-observed, familiar processes. Since the simplest solution is always the best, the notion that extra-terrestrial forces directed earth-bound processes ‘violated [scientists’] parsimonious instincts.’
Yet the idea had been cropping up throughout the history of science. In the late 1600s Halley remarked to the Royal Society that the Bible’s great flood was possibly caused by a comet. A century later Pierre Simon Laplace (1749-1827), whose modified version of Kant’s nebular hypothesis remains current, thought that a large meteorite could kill off whole species. Over time, other scientists aired the possibility of mass extinction, and H.G. Wells (1866-1946), who studied biology and advocated Darwinian theory, essayed it for a wider audience in his two-volume non-fiction, Outline of History (1920), an international best-seller:
‘We do not know what jars and jolts the solar system may have suffered in the past… Some huge dark projectile from outer space may have come hurtling through the planets and deflected or even struck our world and turned the whole course of evolution into a new direction.’
Ager effectively ‘rehabilitated’ catastrophes by presenting them as relatively regular catalysts of the geological process, comparing Earth’s 4.5 billion year-old life to that of a soldier: ‘long periods of boredom and short periods of terror.’ But to scientists from a variety of disciplines, mass extinction was still not worthy of speculation.
That impacts and their consequences were so reluctantly explored reflects the same adherence to accepted theories that kept meteorites from being readily recognised as space rocks. To assess the effects of major impacts it was necessary to overcome attachments to familiar concepts but also the territoriality separating members of different disciplines and their suspicion of findings not originating in their own fields. The theory that the ‘age of the dinosaurs’ ended as a result of cataclysmic impact 65 million years ago nonetheless passed from heresy to orthodoxy over the next twenty years. Persuasive proof was found in a thin geological stratum encircling the earth between the layers corresponding to the Cretaceous (145 million years ago) and subsequent Tertiary periods, the so-called ‘K/T Boundary.’
In 1977, Geologist Walter Alvarez gathered samples of the peculiar stratum in Umbria and with his father Luis Alvarez, a nuclear physicist, enlisted a colleague at Berkeley to analyse it. Unheard-of quantities of iridium were found, an element abundant in meteorites and space dust but rare on Earth. Samples gathered from the K/T Boundary at worldwide locations produced the same results. The Alvarez’s paper, published in 1980, announcing their belief that the dinosaur extinction was triggered by a massive asteroidal or cometary impact was hotly contested, not least because its authors had trespassed paleontological territory. Habeas corpus was once more demanded – not the impact mass but its crater. Eugene Shoemaker proposed the Manson, Iowa crater, which was judged to have been formed nine million years too early. Then in 1990 a Houston Chronicle journalist pointed researchers towards Progreso, a Yucatan Peninsula port town near a ring-shaped depression which was 193 kilometres wide and 48 kilometres deep, discovered by an oil company in 1952.
At last, in 1991 Chicxulob, as the crater was named, was identified as the memento mori of a world-wrecking impact. The object that made it weighed over a trillion tonnes and was at least five kilometres wide. Nowadays even schoolchildren could tell you that an asteroid was behind the dinosaurs’ demise. But the controversy surrounding the K/T extinction is not over, with Princeton University researchers presenting evidence that extensive volcanic activity (in India’s Deccan Traps) had already done most of the work, killing off 70 per cent of Earth’s species, and that the Chicxulub impact only delivered the coup de grâce. Either way, it is now accepted that in its youth and at various points in its history, Earth experienced bombardments that altered planetary chemistries. Scientists are now convinced that Earth has experienced impacts ‘100 times more energetic [than Chicxulub]…with effects dwarfing that of the K/T,’ partially and perhaps fully vaporising oceans, presenting an evolutionary cul de sac for some types of life and favourable conditions for others, including us. Cosmochemists now speak of a ‘biology of revolutionary rather than evolutionary change.’
Our earliest ancestors were life forms capable of taking refuge in the murkiest mulch at the bottom of the sea, places that incidentally have yet to be as well-mapped as the surfaces of the moon and Mars. That homo sapiens has overcome the cosmic odds to not only exist but examine their place in the universe must give pause, especially considering that the process was randomly detoured by flying rocks. English astronomer Sir Fred Hoyle, a brilliant and eccentric cosmologist, compared the chances of natural selection producing results like us to a hurricane blowing through a junkyard and assembling a Boeing 747.
Humans are more likely to be killed by each other than a meteorite fall or asteroid strike. But since NASA began monitoring Near Earth Objects (NEOs) in 1984 the danger from space has been taken more seriously. Its ‘Spacewatch’ programme was expanded in the 1990s and renamed ‘Spaceguard’ following an event on Jupiter (July, 1994), the first of its kind ever predicted and observed from Earth. Comet Shoemaker-Levy, named for its discoverers (Eugene and Carolyn Shoemaker and David Levy) consisted of at least 20 fragments two-kilometres wide, including Nucleus G, which struck Jupiter with a force equal to six million megatonnes of nuclear bomb.
Earth has had several near misses. There was one in 1991, when an asteroid was spotted only after it had passed at a distance of 170,000 kilometres, ‘in cosmic terms the equivalent of a bullet passing through one’s sleeve without touching one’s arm.’ On February 15, 2013, a NEO passed, as had been predicted, an order of magnitude closer to the planet’s skin (17, 200 km), darting gracefully through a flock of orbiting telecommunications satellites without grazing so much as an antenna. But that same day an asteroid fragment struck Chelyabinsk, Russia, an unrelated, unpredicted event that prompted the US House Science Committee to convene a meeting to discuss threats from space. The search for killer asteroids and ways to deflect them from Earth’s orbit has lately become a matter of heightened public interest.
In what ranks as one of the history of science’s greatest reversals, meteoritics has grown from a narrow specialty to a field of astronomy and a cornerstone of planetary studies. Analyses of meteorites’ chemical and mineral components continue to illuminate earth’s interactions with space, the age and composition of our planet and the solar system and the evolution of stars. Meteorites have provided us with samples, free of charge, of the Moon and Mars. Some carry star dust, trapped before the birth of our solar system. Meteorites are not only a virtual goldmine of data – recent studies seem to indicate that Earth’s reserves of gold are themselves the result of massive meteorite bombardments. While sculpting Earth’s features, ongoing research suggests that meteorites may have delivered signature ingredients to the primal soup. Some carry traces of the amino acids that build proteins featuring both in organic structures and in enzymes, the catalysts that accelerate or regulate chemical reactions.
Intimately implicated in the nature of life on Earth, meteorites have served as signposts on the winding path of self-knowledge to humanity’s greater understanding of the universe, revealing both marvellous and terrifying truths. Just as they’ve helped author our planet’s epic from the start, so too may meteorites write the story’s end, as agents of annihilation. What space gives, space also takes away.