How do the versions of the Anthropocene defined by these dates differ in what they say about the relationship between the two histories that have become one? An Anthropocene which starts in the 1950s is a purportedly value-neutral just-the-facts-ma’am Anthropocene: its beginning marks the point at which, looking back, the strains on the Earth system first became apparent. It doesn’t worry about why humans are having their impact, just notes that this is the well-marked point at which the scale of that impact started to increase very rapidly.
The steam-engine starting point says that what matters is the technology behind that impact: new ways of exploiting fossil fuels that brought with them the power to level mountains, create new chemicals, wage global war and support populations of previously impossible size. An earlier start makes things more natural again, if you are willing to extend your notion of nature to human nature. When apes get smart, control fire and learn to farm, they change the planet before you know it. The Anthropocene thus becomes a seemingly unavoidable consequence of the evolution of modern humans.
The 17th-century threshold says something more challenging: that the Anthropocene began not with a technology nor as a consequence of human nature but as embedded in history and politics—specifically in the appropriation of American natures and the dispossession of native American people, and in the creation of a global economy built on the accumulation of capital and an expectation of exponential growth.
The different dates all identify things that matter in different ways. And that is why I am moved to add to the mix a suggestion made by David Grinspoon, an astrobiologist, in his book “Earth in Human Hands” (2016): the base of the Anthropocene is to be found at Tranquility Base.
To take Armstrong’s first footprint as the start of a new geological epoch would be to say very clearly that this epoch is of a very unusual kind. It is also to reinforce that the Anthropocene is a way of seeing as much as it is anything else—a way of seeing closely connected to the view of the Earth as a single and perturbable system that was given its iconic essence by the Apollo missions.
In making his case, Grinspoon points out that the Eagle’s landing site handsomely satisfies the geologists’ predilection for permanent markers distinguishing before and after: “The alien artifacts we left there will surely be detectible for as long as there is an Earth and a Moon.” It is undeniably a sign of the human fixed in time and space. It also stems from the same conflict as the bomb-test sediments favoured as a marker by others and matching, Mr Grinspoon says, their “symbolic potency”. Like them, it could only have been created by an entity that had “developed world-changing technology”. As Verne suggested in “From the Earth to the Moon”, the sort of technology that allows such travel is of its nature the sort of technology that is significant on a planetary scale.
Grinspoon’s suggestion has the further benefit, at least to my eyes, that if Tranquility Base marks the bottom of the Anthropocene, then the Anthropocene is a geological epoch that encompasses both Earth and Moon. That seems at once odd and reasonable. If indelible human influence means the Earth has entered a new geological age, surely it means the Moon has, too. To be sure, what has been done to the Moon is beneath trivial compared to what has been done to the Earth. But the background rate of change on the Moon is so slow that the human novelties might still be seen to matter.
Apollo brought the Moon substances and processes it has never seen before. Never before has moondust been bathed in the exhaust gases of rocket landings and takeoffs—gases which, for brief periods, made up a substantial part of the Moon’s ludicrously tenuous atmosphere. Never before has it had tire tracks traced across its surface or its boulders eroded by hammer blows. There is a sparse but real layer of human litter at six sites around the Moon, a strange sedimentation of abandoned experiments, blast-off detritus and sheer oddity—like the falcon’s feather, dropped in tandem with a hammer, to illustrate Galileo’s insight that, without air resistance, the two would fall at the same speed.
As yet these qualitatively unprecedented interventions do not quantitatively surpass even the Moon’s very meagre natural processes of change, as some of the human influences that define the Anthropocene do on Earth. The mass of human relics and rubbish on the Moon is less than 10% of the 1,800 tonnes that hit it every year in the form of interplanetary dust. But 1,800 tonnes is less than the takeoff weight of four large airliners. The practical and political prospects for moonbases and colonies will be dealt with in later chapters (spoiler: possible? Definitely, and on a smallish scale quite likely; large and/or enduring? Hard to say). But more than a few thousand tonnes moved on and off the Moon in a year is within the realms of feasibility. The traffic of supplies and personnel to and from America’s McMurdo base in Antarctica is many times larger.
Though the geologists may look askance, an interplanetary Anthropocene also has the benefit of honouring one of the great 20th-century developments in their own science. Gene Shoemaker and his astrogeological colleagues showed that the stratigraphic reasoning of their science, the approach that underlay centuries of argument about the boundaries of eons and ages and epochs, applied beyond the Earth. The relative age of surfaces could be defined in terms of which rocks sat on top of which other rocks on the Moon as well as it could in Montana. Indeed, the impacts that made up the Moon’s geological history lent themselves to stratigraphy from afar rather well. The ejecta from a large impact were often reasonably distinguishable, making clear a distinction between before—the surfaces they covered—and after—the ejecta layer and anything piled on top of it by subsequent impacts. Shoemaker’s first geological map of part of the Moon established the impact that created the crater Copernicus as one such epoch-defining event. Today the history of the Moon is divided into five impact-punctuated periods in this way: the pre-Nectarian, the Nectarian, the Imbrian, the Eratosthenian and the Copernican.
The astrogeologists went on to apply a similar stratigraphic understanding to every planetary surface they saw, not to mention sundry moons and asteroids. Mars, Mercury and Venus all have geologic periodisations of their own. In doing this they also revealed the role that some of those distant objects have had in the history of the Earth—the history of cosmic battery which the Earth’s face forgets but which the Moon’s remembers. If geology applies elsewhere, why should some of the boundaries of geological time not do so, too?
One answer is that, before Apollo, what happened on one planet did not matter to the next. But this is not entirely true. There has been at least one other crucial event that links the geological history of the Earth to the Moon. And, as it happens, years before Grinspoon voiced his proposal for the Anthropocene, a quartet of scientists argued that that earlier event, too, should be recognised as a boundary in the geological histories of both bodies.
AT THIS END OF THE GEOLOGICAL TIME SCALE, THINGS ARE WELL ordered. Until the Anthropocene is formally defined, if it ever is, humans live in the Holocene, a tiny sliver at the end of the 2.58m-year-old Quaternary Period, itself a subdivision of the 66m-year-long Cenozoic Era, the latest part of the Phanerozoic Eon.* The base of each is precisely defined—by a small but distinct climatic shift in the case of the Holocene, by the onset of the ice ages in the case of the Quaternary, by the thin level of iridium left behind by a dinosaur-killing asteroid in the case of the Cenozoic and, in the case of the Phanerozoic, by a 541m-year-old stratum in the cliffs of Fountain Head, on Newfoundland, just above which you can find the earliest fossilized burrows of a creature called Treptichnus pedum.†
At the other end of the time scale, unsurprisingly, things are considerably more rough and ready. The first of the Earth’s four eons—a bookend similar in duration to the Phanerozoic but at the other end of the shelf—is known as the Hadean. It has no defined beginning; most people just sort of assume it began when the Earth did, around 4,540m years ago. It is widely held to give way to the subsequent Archaean 4,000m years ago, but that is basically just because people have got into the habit of saying so�
��there is no particular rock boundary anyone can point to and say, “This is the top of the Hadean, and that is the bottom of the Archaean, and here’s why.” Nor is it clear what event or change that boundary might mark.
In 2010, in a fit of playful tidy-mindedness, four scientists—Colin Goldblatt, Euan Nisbet, Norm Sleep and Kevin Zahnle—wrote a short paper trying to put some of this right. As it happens, I know and like all four of them—and, like many of their colleagues, I consider at least three of them to be both brilliant and a bit batty. Their 2010 paper, “The Eons of Chaos and Hades”, reflects this. It is an attempt to stretch the geological time scale not just beyond the physical bounds of the Earth, as Grinspoon did, but back before its beginning. That it is undeniably fanciful. But it is not without sober sanction. No less an authority than the definitive “Geological Timescale” (1989) assembled by Brian Harland and others noted that “a pre-Hadean division to accommodate events prior to the earth’s formation may be considered: but not in this work.” And the events the proposed scheme seeks to fit into its framework are, as far as science can tell, things that must have happened in the sequence described, even if the actual dates for many of them are currently guesswork.
The story starts about 4.6bn years ago, at the point when a cloud of dust and gas which has begun to collapse in on itself due to its own gravity passes a threshold beyond which that collapse can but continue. It is the point at which the creation of a new star becomes a done deal. They make this moment of commitment the beginning of the Chaotian, an eon of swirling dust and gas.
They proceed to chop this first eon into two, the early Chaotian and the late. The boundary is “Let there be light”. At the dense heart of the disk into which the cloud is still collapsing, the core of the about-to-be Sun becomes hot and dense enough for nuclear fusion to take place. Gravity produces pressures so high that smaller atoms are squeezed together into bigger ones; a chain reaction takes off in which the energy from one such fusion triggers the next and the next. Very quickly the Sun becomes bright—far brighter, in the exuberance of its birth, than it is today. The solar wind of charged particles that has streamed away from it ever since begins as a gale.
In the late Chaotian the Sun dims back down. The now illuminated matter swirling around it—which has, in the process of collapse, sorted itself out chemically so that the elements and isotopes present differ in different zones of the disk—clumps together into bigger and bigger lumps. Fairly soon some lumps are big enough and hot enough to undergo their own inner transformations; their centres melt, and iron, which does not like to be bound up in dust-made stone, sinks down to the core. These bodies now have stone mantles and iron hearts—they are, as the cosmochemists say, differentiated.
We see fragments of both the earliest undifferentiated bodies and the later differentiated ones on Earth: they fall from the sky today as meteorites. Through the cunning study of the various isotopes they contain their ages are known with remarkable precision. Some of the differentiated bodies become the planetesimals that Grove Karl Gilbert theorized about. These planetesimals went on to hit each other, too, forming what are now known as “planetary embryos”. The bigger the objects, the more spectacular the collisions of accumulation.
In the outer solar system, where it is cool enough for volatile compounds such as water, methane and carbon monoxide to condense, these growing embryos wrap themselves in snow, ice and gas. The heavier they get, the more their increasing gravity can pull in; to those that have is more given. The biggest beneficiary of this positive feedback is Jupiter, which ends up weighing more than all the other planets of the solar system combined. Like a sun in miniature, it draws its own disk of gas and dust around it, a disk which produces four moons. A third of the age of the universe later, Galileo would trace their co-ordinated dance through his telescope, the first human eye ever to see them.
Towards the end of the Chaotian, the solar system is beginning to look quite familiar. Not all the planets have settled into quite the orbits they have today. But in the outer part Jupiter and the lesser gas balls are becoming recognisable, along with, farther out, billions of small icy bodies that never get swept up into anything bigger. In the inner part are almost-fully-assembled versions of Mercury, Venus and Mars, along with two other planets, Tellus and Theia. Theia is about the same size as Mars—which means about half the diameter of the Earth, and about a tenth of its mass. Tellus is about the size of Venus—almost as big as the Earth and about 90% as massive. It sits in an orbit very like that in which the Earth now sits.
And then, one day, something happens which, my friends say, brings the Chaotian to an end and gets the Hadean started. One day, here, is not a figure of speech; the boundary is as crisply defined as David Grinspoon’s July 20th 1969 base of the Anthropocene. It, too, is a moment of contact, a meeting of worlds just like the touchdown of a spacecraft or a footprint on a grey-dust plain. A distinction defined by a coming together.
It is, though, on a larger scale. In one of the most violent acts in the history of the solar system, Theia collides with Tellus. The resultant mess eventually resolves itself into a new arrangement of mass and motion—a planet a bit bigger than Tellus, spinning rapidly, with a satellite much smaller than Theia circling it in an orbit only hours long.
The Chaotian is over. Tellus and Theia are gone. The Earth and the Moon have been born in their place.
A WEEK BEFORE THAT FATEFUL DAY, AS WE NOW RECKON TIME, Theia was the same size in the Tellurian sky as the Moon is in the Earth’s sky today.* An hour before, it was as big as the dome of Saint Paul’s Cathedral seen from the north bank of the Thames, or that of the US Capitol seen from the pool that sits before it. The size of a Goodyear blimp, floating a few hundred metres above you.
Ten minutes before, it filled a third of the sky.
The land below may have been in day or in night. The incomer could have been crescent or gibbous, depending on the angle of the Sun. If crescent, swathes of its inverted night were bright-ashen-lit by the planet below. Mountain ranges hung down like damp-damaged paper from a ceiling, their shadows lengthening.
Or perhaps it came at midnight, totally eclipsed, an expanding absence of stars. Even then, it would have been something to see. The two planets’ magnetic fields would have met hours before the impact. In a solar wind much stronger than today’s, their mingling would have produced spectacular sheets and tangles of colour to light the land below and the land above.
Not that there was anyone there to see. Tellus and Theia may have had atmospheres and magnetic fields. Quite possibly they had oceans, too. They may even have had life. But if either of them did, it was simple, eyeless and probably deep below the surface. There were no birds to go silent in the sky, no animals to scurry and hide. No people to look up in terror as a ceiling spread over their world.
The airs and waters of Tellus would have felt the incomer’s presence, though, through a gravity in the sky. Its waters would have risen up in unaccustomed tides: a metre or so a week out, but 20 times that a day out; in the final hours a mountain range of water reaches up towards Theia. The atmosphere, too, is stretching spacewards; a computer model might tell you from how fast, and from how far, the compensating winds raced in to replace the air pulled up—whether they created some sort of hurricane, as air rushing to an eye of low pressure does above hotspots in today’s oceans. I cannot, myself, say. And minutes later, it would not have mattered. But I would like to know. Did the end which was a beginning come quietly or with the rushing of great winds?
Either way, it came. In the “standard model” of the Moon-forming impact—and what a story could be written about the catalogue of wonders that scientists litotically contrive to call “standard”—it came at 10km/s. That is fast: some ten times faster than a bullet, 30 times the speed of sound. But because planets are big, the collision itself is slow. At 10km/s, Theia merged with Tellus as slowly and surely as Armstrong would have sunk into the Sea of Tranquility had it been filled with Tommy Gold’
s quicksand dust.
More than ten minutes after the impact, there are parts of both planets which still do not know that it is going on. No shock wave, no blast of heat, no tidal wave or freakish jet stream has had time to reach them. It takes the shock waves almost 20 minutes to reach the mountain range of tidal water at the antipodes of Tellus; their encircling, narrowing, squeezing noose creates a waterfall jet up into space, like a child’s cupped hands clapping in a bath.
It takes most of an hour for the planetary merger to reach its climax.
Slow does not mean gentle. The kinetic energy of a planet moving at 10km/s is vast—about the same amount of energy as the Sun puts out in a day or that the Earth receives from the Sun in 6m years. Before the impact had been going on for a second it had released more energy than all the world’s nuclear weapons combined.
From that point of contact, the shock waves spread out as hemispheres, squeezing and heating the planets’ mantles intolerably. Behind the shock came low pressure, which liquefied the trillions of tonnes of overheated rock in a flash. A conical sheet of red-hot magma hundreds, then thousands, of kilometres long shot out from around the point of contact. Within that cone, the leading hemisphere of Theia was quickly melting.
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