Earth's Hadean Period

The Hadean Period begins at the inception of Earth itself, roughly 4.7 billion years ago until about 3.8 billion years ago. The name "Hadean" is derived from the Greek word Hades, the god of the underworld. This name was presumably taken to describe the very inhospitable geologic conditions on earth during this period. The Hadean is not technically a geological period because no rocks on Earth are as old as the Hadean time-frame except for meteorites. The ending point of this particular epoch of the Precambrian Period is 3.8 billion years ago, the point at which the oldest rocks on earth made their appearance. Thus, the Hadean Period can be considered a kind of "ghost period" of Earth's history because no solid rocks had yet formed and the planet was still in the process of cooling from a molten state.

During the beginning of the Hadean Period, Earth and the other planets of the Solar System were being formed out of space debris. It is probable that the Earth was forming out of dust and other debris that had formed around the sun after its formation, in a wide expanse called an accretion disc. This debris started to form larger lumps of matter called planetesimals, which collected more and more matter due to increased gravitational pull. As Earth was forming, it became extremely hot due to the processes of friction and fusion taking place on our young, molten planet. Thus, over the course of the Hadean Period, Earth was transforming from a mass of tiny particles and gases into a liquid, and eventually into a solid planet.

It is during the Hadean Period that the Giant Impact Hypothesis comes into play. This hypothesis covers both the formation of the moon and also changes made to Earth itself that might have made the planet more suitable for eventual life forms. The hypothesis states that a Mars-sized celestial body called Theia collided with the earth, destroying the smaller mass and also ejecting a large amount of the earth's mass out into space. This mass then condensed to form Earth's singular orbiting satellite which we call the Moon. There is evidence for this hypothesis, particularly the absence of iron on the moon and its lower density than earth. By the time Theia collided with Earth, scientists speculate that much of the molten iron on earth had sunk to its core, where it remains today.

The arrangement of elements in the Earth began to also be roughly formed during the Hadean Period. As has been noted, heavy iron began to fall toward the core of the earth while lighter elements migrated to the surface. It is during this period that the Earth's crust began to roughly form as an amalgamation of impure "scum" that was floating to the surface of the molten planet.

Unfortunately, many of these hypotheses cannot be proven due to the inability of rock specimens from the Hadean Period. Also, many scientists have begun to challenge the contention that Earth was a molten mass for much of the Hadean Period. No rocks before 3.8 million years exist, but certain crystals called zircons do. These tiny crystalline formations no bigger than a grain of sand date back as old as 4.4 billion years, or a mere 170 million years after Earth's supposed formation. Some scientists argue that zircons need water-rich magma conditions, such as those found today in our continental world in order to form. Others argue that zircons can form under different conditions and that the Hadean Period might well have been as hellish as most previous conceptions make it out to be.