Appearance
1.2 — The Birth of the Earth, and the Making of the Moon
Every atom of iron in your blood was made inside a star that exploded before the Sun existed. So was the calcium in your bones, the oxygen you are breathing, and the gold in a wedding ring. The Earth is assembled from the ash of dead stars, and the assembling took about fifty million years.
Here is how we know, and how it happened.
The cloud
About 4.6 billion years ago, in one arm of this galaxy, there was a cold cloud of gas and dust — mostly hydrogen and helium left over from the beginning of the universe, seeded with heavier elements cooked inside earlier generations of stars and scattered when those stars died. The cloud was enormous, thin, and doing nothing in particular.
Something disturbed it. The usual suspect is the shock wave from a nearby supernova, and there is real evidence for it: the oldest solids in meteorites contain the decay products of aluminium-26, an isotope with a half-life of only 717,000 years. Something must have made that aluminium and delivered it into the cloud within a very short time before the solids formed, and an exploding star is the only known source.
Once a patch of the cloud is compressed past a certain point, gravity takes over and does not stop. Denser regions pull in more material, which makes them denser, which makes them pull harder. The cloud collapses inward.
Two things then happen automatically, and both matter for the rest of this Part.
The first is heating. Falling material converts the energy of its fall into motion, and motion of particles is what heat is. The centre of the collapsing cloud gets hot, then very hot, and when it passes about 10 million kelvin at the core, hydrogen nuclei begin fusing into helium. That is the ignition of the Sun.
The second is flattening. Any real cloud has some slight overall rotation. As it shrinks, that rotation speeds up for exactly the reason a spinning skater speeds up when they pull their arms in — the same quantity of spin packed into a smaller radius means faster turning. Material that is falling straight down along the spin axis meets no resistance and arrives. Material falling in from the sides is moving sideways too fast to reach the centre and settles into orbit. The result, every time, is a disc.
The Sun took about 99.8 percent of all the material. The planets, every moon, every asteroid and every comet are built from the 0.2 percent left in the disc.
Why the inner planets are rock and the outer ones are gas
The young Sun was hot and the disc had a temperature gradient — scorching near the star, freezing far out. That gradient sorted the material by what could survive where.
Close in, only substances with very high melting points could exist as solids: metals such as iron and nickel, and silicate minerals, which are compounds built around silicon and oxygen. Water, ammonia and methane were vapour there and stayed vapour. Further out, past a boundary now called the frost line — around three times the Earth's distance from the Sun, out among what is now the asteroid belt — it was cold enough for water to freeze.
That boundary is why the Solar System has the shape it has. Water is made of extremely common elements, so once ice could exist as a solid, there was suddenly far more solid material available per unit of space. Bodies beyond the frost line grew large fast, got big enough for their gravity to capture hydrogen and helium gas directly from the disc, and became Jupiter and Saturn. Inside the frost line, the available solids were only metal and rock, there was much less of it, and the planets that formed there stayed small. Earth is a small planet made from the leftovers that could tolerate heat.
From dust to planet
The step-by-step is now well constrained by laboratory work, computer simulation and meteorites.
Dust sticks to dust. Grains a thousandth of a millimetre across collide gently and hold together through simple electrostatic attraction, the same effect that makes dust cling to a screen. This builds clumps up to roughly centimetre scale.
Then there is a genuine problem. Between about a centimetre and a metre, colliding bodies tend to bounce or shatter rather than stick, and worse, objects that size feel drag from the surrounding gas and spiral into the star within a few thousand years. This is called the metre-size barrier and it is an active research area. The leading answer is the streaming instability: pebbles orbiting through gas drag the gas along with them, which reduces the headwind on the pebbles behind, so pebbles clump into dense swarms, and once a swarm is dense enough its own gravity collapses it directly into a body several kilometres across. Simulations reproduce this, and the size distribution of asteroids it predicts matches what is observed.
Above a kilometre, gravity does the rest. These bodies — planetesimals — pull on each other, collide, and merge. Bigger ones grow faster because they have a bigger gravitational reach, so growth runs away and a few winners sweep up their neighbourhoods. Within a few million years the inner disc holds a few dozen bodies the size of the Moon or Mars, on crossing orbits.
And then they crash into each other, for another few tens of millions of years, until only four are left in the inner system: Mercury, Venus, Earth, Mars. The Earth is the sum of hundreds of such collisions, and the last really big one gave us the Moon.
Why the Earth is layered
While all of this was happening, the growing Earth was getting extremely hot. Three sources fed the heat: the kinetic energy of every impact, converted straight into heat on arrival; the compression of the interior under its own accumulating weight; and the decay of radioactive elements, which were far more abundant then than now because the short-lived ones had not yet run out.
The whole planet, or most of it, melted. And a molten planet does what any mixture of liquids of different density does when left alone — it separates.
Iron and nickel are dense. They sank. Silicate rock is lighter. It floated. The process is called differentiation, and it took perhaps 30 million years, releasing yet more heat as the falling metal converted its gravitational energy on the way down. The result is the layered planet of the next chapter: an iron core, a rocky mantle, and eventually a thin skin of the lightest rock of all.
This has a consequence that runs through the rest of this Part and through economics. Nearly all the iron, nickel, gold, platinum and other metals that came with the Earth's building blocks went down with the core and are permanently out of reach — the core holds enough gold to plate the entire surface knee-deep, and none of it will ever be mined. The metals we do mine are mostly either a late garnish, delivered by asteroids that hit after the core had already formed and sealed, or ores concentrated afterwards by water and heat inside the crust. Chapter 1.12 follows that thread to why some countries sit on wealth and others do not.
The collision that made the Moon
The Moon is strange, and the strangeness is the evidence.
It is very large relative to its planet — a quarter of Earth's diameter, where most moons are a tiny fraction of theirs. It has almost no iron core, so its overall density is low, while Earth's is high. It is bone dry and depleted in every element that boils easily. And the crucial clue: the oxygen isotope ratios of Moon rock are identical to Earth's, to the limit of measurement. That ratio varies measurably between Mars, the asteroid belt and Earth, so it works as a birth certificate for where in the Solar System something formed. The Moon's says: the same place as Earth.
No capture theory explains that. Neither does the Moon forming separately in orbit. The explanation that survives is the giant impact hypothesis.
About 4.5 billion years ago — roughly 60 to 100 million years after the Solar System began — a body about the size of Mars, given the name Theia, struck the young Earth. Not head-on, which would have shattered both, but a glancing blow at somewhere around 4 kilometres per second.
What happened next, in order. Theia's iron core, being dense, ploughed through and merged with Earth's core within hours. The outer rocky layers of both bodies were vaporised and thrown outward. A large fraction of that material was moving fast enough to leave the surface but not fast enough to escape, so it settled into a hot disc around what was left of the Earth. That disc cooled and gathered itself into the Moon within perhaps a hundred years — possibly less.
This explains every oddity. The Moon has almost no iron because the material it formed from came from the mantles of the two bodies, after both cores had already gone down. It is dry and depleted in easily boiled elements because it condensed out of rock vapour at thousands of degrees, and anything volatile simply never condensed. The isotope match follows if the debris disc was thoroughly mixed, which the extreme temperatures allow, or if Theia formed at a similar distance from the Sun in the first place. The Moon is unusually large because it was made from a serious fraction of two planets.
And there are three things the impact left behind that you personally live with.
The tilt. The Earth's axis is tipped 23.4 degrees from the vertical relative to its orbit. That tilt is why the Sun climbs higher in summer than in winter and why we get seasons at all. The impact is the most likely reason for the tilt, and the Moon is the reason it stays steady. Without a large moon, gravitational tugging from the other planets would let the tilt wander chaotically over tens of degrees on a timescale of millions of years, which is what happens to Mars, and the climate would swing with it.
The spin. The impact left the Earth turning far faster than now — a day of perhaps five or six hours. It has been slowing ever since, and that brings us to tides.
The tides. The Moon pulls harder on the near side of the Earth than on the far side, and that difference in pull stretches the planet slightly along the Earth–Moon line, raising a bulge in the ocean on both sides. The Earth then rotates underneath those bulges, which is why most coasts get two high tides a day rather than one. Friction as the water drags over the sea floor steals rotational energy from the Earth, and by an exchange required by physics, that energy goes into the Moon's orbit and pushes it further away. Laser reflectors left on the surface by Apollo astronauts let us measure it directly: the Moon is receding at 3.8 centimetres a year, and the day is getting longer by about 1.7 milliseconds per century. Growth bands in 400-million-year-old fossil corals show roughly 400 days in a year, exactly as that rate predicts running backwards.
Water, air, and the first solid ground
The young Earth had no ocean and no breathable air. Both arrived.
Where the water came from is still genuinely argued, and the honest answer is probably both of the two candidates. Some was there from the start, bound inside the minerals the planet was built from and released as the interior degassed through volcanoes. Some was delivered later by impacts from bodies that formed beyond the frost line. The test used is the ratio of deuterium — heavy hydrogen — to ordinary hydrogen, which differs between comets, asteroids and Earth's oceans. Earth's water matches a particular class of asteroid, the carbonaceous chondrites, far better than it matches most comets, which is why the current weight of evidence favours asteroid delivery plus native water, and disfavours the once-popular "comets brought the oceans" story.
The first atmosphere was volcanic: carbon dioxide, nitrogen, water vapour, sulphur compounds. No free oxygen at all, and this is not a minor detail — oxygen is chemically aggressive, and there was nothing to keep making it. Oxygen only appears once photosynthesis is invented, more than a billion years later, and the story of that transformation is in Volume V.
By 4.4 billion years ago there was already liquid water and solid crust. That is the message of the Jack Hills zircons from the last chapter. Those crystals contain a specific oxygen isotope signature that forms only when the rock they grew in had interacted with liquid water at low temperature. So within roughly 150 million years of the planet's formation — and within perhaps 50 million years of the Moon-forming impact — there were oceans and there was land. The Earth cooled from a magma world to a wet, rocky planet far faster than anyone expected before those crystals were measured.
Where this shows up in your life
The tide table at any Indian or British port is a direct consequence of a collision 4.5 billion years ago. So is the fact that your year has a summer and a winter rather than an unvarying climate. So is the fact that a compass works, which needs the liquid iron core that differentiation created — the next chapter takes that up.
And there is a harder one. Every heavy atom in you was made in a star. Hydrogen and helium came from the Big Bang. Carbon, nitrogen and oxygen were fused in the cores of ordinary stars. Iron was made in the final moments of massive ones. Gold, platinum and uranium needed something more violent still — the collision of two neutron stars, an event so energetic that the gravitational waves from one were detected on Earth in 2017, and the light that followed showed the freshly made heavy elements streaming outward. That is not a metaphor. The gold in a ring was manufactured in a collision between the corpses of two stars, and then sat in a cloud for billions of years waiting to be swept into a planet.
What the next page covers
The planet melted and separated, and it has been layered ever since. Chapter 1.3 goes inside — the crust, the mantle, the liquid outer core and the solid inner core — and answers the question that ought to bother you: how does anybody know what is 5,000 kilometres down, when the deepest hole ever drilled reached 12 kilometres and stopped because the rock was too hot and too soft to keep the hole open?