Free Creation by Adam Rutherford Page A

there by its own gravity. Over the course of the next few million years, this matter, mostly dust and gas, began to stick together in clumps. At first these were the size of medium-size concert halls, but over hundreds of centuries, lumps collided and stuck together in a process called accretion. Closer to the hot sun, the temperature is higher, which makes it harder for gasses to condense and accrete. It’s for this reason that the inner four planets of the solar system—Mercury, Venus, Earth, and Mars—are terrestrial, made of rock, whereas the outer four—Jupiter, Saturn, Uranus, and Neptune—are gaseous. 1
    It’s impossible to describe a whole planet in simple terms, as we know very well how our home is not a unified land mass. Much of the surface of modern Earth is solid; more is ocean. Of the grounded parts, there are extremes of temperature and geography, snow, deserts, marshes, forests, plains, mountains, and so on. The rock beneath your feet is likely to be very different from the rock underneath a reader’s feet on the other side of the planet. Similarly, it’s virtually impossible to describe simply what Earth looked like in the Hadean. The evidence is vanishingly sparse, and as a result this period unhelpfully gets called the Cryptic era. Yet we can extract meaningful models in broad terms. We know that immediately after accretion, the planet was probably largely molten. Contrary to previous thinking, however, this period of molten Earth is now thought to have been short-lived. There are no surviving rocks from that era, so in the past we had assumed that there were no rocks. Yet the absence of evidence is not the same as evidence of absence.
    Just as we look in rocks for the traces of recent life—the fossilized forms of dinosaurs, or even cells, for example—our planet’s history before life is embedded in geology. The oldest dated matter on Earth comes not from rocks, but in the form of zircon crystals, an abundant mineral found all over the world but probably best known as a cheap substitute for diamonds in jewelry.
    Zircons have two convenient characteristics. The first is that they can withstand metamorphosis—the brutal churning of rock over very long periods of time. The second is that the atoms of zircons are naturally arranged in a neat cubic structure. This molecular box can trap atoms of uranium inside them, as few as ten parts per million. A small proportion of uranium, like many elements, is radioactive, and over time will decompose into lead. Due to the precise nature of their crystal structure, when zircons form they can include uranium, but exclude lead atoms. Once imprisoned in this cage, radioactive uranium slowly mutates into lead over the course of millions of years, and because this decomposition happens at a fixed rate (what we call a half-life), the moment a zircon crystal forms, it sets a clock at zero. Lead found inside zircons must have begun its life as imprisoned uranium, so by quantifying it we can date the origin of that crystal with 99 percent accuracy. Out in Jack Hills in Western Australia, zircon crystals have been found that trapped their uranium 4.404 billion years ago.
    Dating is not all we can tell from the constituents of cheap jewelry. We can also infer from those same crystals that their formation was part of a solidifying process, the development of a crust. This means that, although there are no rocks from that period, we can know that there was land in the Hadean. We can also determine what other ingredients were present. The Jack Hills zircons also harbor a particular type of radioactive oxygen whose presence looks like that found in modern crystals, which are formed as the earth’s crust gets sucked down beneath the ocean floor. The presence of this oxygen suggests that from as little as a hundred million years after the initial molding of the earth, water was present. On Earth, there is no life without it, though