Read all seven of the biggest mysteries about Earth
Explore an interactive map of our Unknown Earth
Some 4.53 billion years ago, as the infant Earth was settling down in its orbit around the sun, disaster struck. Our young planet was dealt a glancing blow by an object the size of Mars. Debris from the impact was thrown into Earth’s orbit to form the moon, and the energy of the collision supplied enough heat to melt the Earth’s upper layers, erasing our planet’s previous geological record. This has left a yawning chasm in our knowledge of the planet’s first 500 million years, a period that has become known as the Hadean era, Earth’s darkest age. We know almost nothing about it.
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“Time zero” for the solar system is generally agreed to be 4.567 billion years ago, and by 4.55 billion years ago, about 65 per cent of the Earth had assembled. Then, 20 million or so years later, the wayward object struck, sending vaporised silicon into the atmosphere. This condensed and fell as lava rain, depositing a sea of molten rock at a rate of perhaps a metre per day. The Earth melted to its core, and the process of forming a solid surface began all over again.
The Earth’s crust today is composed almost exclusively of rocks no older than 3.6 billion years, so traces of the hellish Hadean environment that followed the impact are thin on the ground. Of the ancient rocks that remain – amounting to about one part per million of the crust – most have since been modified by heat or pressure. But thanks to tiny resilient crystals, called zircons, there are some clues.
Zircons, found in the rocks of the Jack Hills in Western Australia, are Earth’s oldest minerals. They are composed of exceptionally durable zirconium silicate crystals and contain a high concentration of uranium, which allows their age to be determined from the amount of radioactivity that remains. And even though they are found within much younger rocks, many zircons date to more than 4 billion years old (though see our new story, Discovery of world’s oldest rocks challenged).
They cannot tell us exactly what happened as the molten Earth cooled, but their oxygen content shows that they formed in water, suggesting that Earth’s oceans were in place more than 4 billion years ago. This raises new questions: oceans need to sit on a solid surface, so what was this crust like? So far there are no clear answers. Perhaps the most obvious observation about the Hadean crust is that it no longer exists. While this is frustrating, it is itself a clue: perhaps plate-tectonic action was much more vigorous back then.
There are two other ways we can learn more about the Hadean. On Earth, concerted searches for more ancient rocks or minerals, combined with ever-improving methods of microanalysis, should yield further clues about what the Earth was like as it formed for the second time.
Secondly, mineral prospecting on the moon and Mars could reveal what Earth was like before the catastrophic impact – as rocky debris from the impact is what formed the moon. Unlike Earth, neither of those worlds have remelted, so there is a much greater chance of finding truly ancient rocks on their surface. We may even hit the geological jackpot and find a piece of the Hadean Earth that was blasted into space by an asteroid impact, and which subsequently landed on the Moon or Mars. Researchers into the Hadean are nothing if not optimistic.
Read all seven of the biggest mysteries about Earth
Explore an interactive map of our Unknown Earth
