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Formative years

The Earth and Moon were born from chaos. The heat and violence of the early solar system have conspired to shroud much of Earth’s early years in mystery. What, then, do know and not know about how it developed into the dynamic planet we know today, a place fit for life to evolve?

A home unique in the universe

Very occasionally, routine events produce exceptional results. That’s what happened 4.6 billion years ago, on a minor arm of an unremarkable spiral galaxy.

A vast cloud of gas and dust began to collapse into a dense ball of matter. As gravity pulled more and more material towards it, the temperature and pressure at its core increased to the point where nuclear fusion kicked in. That released vast quantities of energy and marked the creation of a star.

What started the process off, we don’t know but it had happened

countless times before and the star itself was certainly nothing special.

As the newborn star began to spin, smaller bodies started to coalesce in orbit around it. Gas molecules and dust particles fused to form objects the size of rocks; which collided to create boulders, then ‘planetesimals’. Their increasing gravity pulled in still more matter to create hot, molten versions of the planets we know today.

Eight planets formed and on the third one from the star something truly remarkable happened. The right conditions enabled life to emerge and flourish. Eventually, intelligent life evolved in the form of beings capable of asking how their planet had formed and how it came to nurture life. They called their celestial neighbourhood the solar system, named the star ‘Sun’ and their planet ‘Earth’.

Mysterious beginnings

That’s the big picture, at least. ‘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 Earth had assembled.

The early solar system was an energetic, dynamic place. For its first few hundred million years, collisions were common and Earth was subjected to some pretty rough treatment. About 4.53 billion years ago, just as the paint was drying on the infant Earth, disaster struck. It was dealt a glancing blow

by an object the size of Mars. The impact threw debris into orbit to form the Moon, and the energy of the collision melted Earth’s upper layers, erasing any previous geological record. Vaporized silicon that didn’t make it to the Moon condensed and fell back as lava rain, depositing a sea of molten rock. Earth eventually melted to its core, and the process of forming a solid surface began all over again.

This version of the Moon’s creation is not the only one, as you will see

later. Yet it seems certain that the violence carried on, ending only with a sustained pummelling between 4.1 and 3.8 billion years ago in what is known today as the late heavy bombardment. Once again, the ferocity and length of this episode are still being debated.

The sheer violence of these events is one reason why there is a yawning chasm in our knowledge of Earth’s first 500 million years, an aeon known as the Hadean, after Hades, the ancient Greek god of the underworld. With little to go on scientists make up stories that best fit the evidence they have; evidence that comes from our knowledge of physics and chemistry, the results of hands-on experiments, observations of other astronomical objects and computer simulations.

Research to answer many of our questions is under way right now and new findings, observations and models are being made all the time. What we think we know is constantly being challenged by that new evidence. And so scientists’ stories change.

Among our unanswered questions is how Earth gained so much water. Being close to the Sun, it was probably too hot for water to simply condense out of the gas cloud as the planet formed. In any case, water that had collected would probably have evaporated during the titanic collision that formed the moon. One possible explanation is that the water arrived later, delivered by icy comets and asteroids from the outer solar system during the late heavy bombardment.

Then there’s the question of when Earth gained its crust. Today that crust is composed almost exclusively of rocks no older than 3.8 billion years, so traces of the hellish Hadean are thin on the ground. Of the ancient rocks that remain, most have been modified by heat and pressure. The good news is that tiny resilient crystals, called zircons, may be seriously old and are yielding important information. Combined with ever-improving methods of microanalysis, these may yet rewrite the story of the early Earth.

There is one other way we can learn more about the Hadean. Mineral prospecting on the Moon and Mars could also reveal what Earth was like before the great impact. Unlike Earth, neither of those worlds has re-melted, so there is a much greater chance of finding truly ancient rocks on their surfaces. 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.

With this overview of Earth’s earliest years in mind, let’s drill deeper into the issues that are keeping earth scientists, astrophysicists and palaeobiologists up at night.