The Hadean eon was the earliest geological epoch in Earth’s history, from the formation of the planet to the beginning of the Archean period. The landscape was thought to be lifeless, but some scientists believe it was an exaggeration. The Earth was a condensed part of the accretion disk and eventually formed a proto-Earth. The surface was composed of silicates, and the atmosphere was deprived of oxygen. The Moon was formed after the Earth was struck by a Mars-sized body. Early life began shortly after the Earth cooled, and the Late Heavy Bombardment occurred towards the end of the Hadean eon.
The Hadean eon refers to the earliest geological epoch in Earth’s history, from the formation of the planet about 4.5 billion years ago to the beginning of the Archean period 3.8 billion years ago. The name “Hadean eon” comes from the word Hades, the Greek word for hell or “unseen”. It is commonly thought that the landscape of Hades was a lifeless place filled with lava, bombarded by meteorites, but some scientists believe that this characterization is somewhat of an exaggeration, and that at least towards the end of the Aeon of Hades, the Earth wasn’t hellish as some might think. The fact is, we have relatively little evidence of conditions on Earth at the time, compared to any other era.
The beginning of the Hadean Aeon was certainly a tough place. It’s when the Earth was nothing more than a condensed part of the accretion disk that made up the early Solar System. Over millions of years, this condensed portion began to break apart under the mutual attraction of gravity and a primitive proto-Earth was formed. For a period of hundreds of millions of years, up to about 4.100 million years ago, the Earth was quite warm. Convection and subduction churned the planet’s surface, creating a planetary foam of magma. Heavier elements, such as iron, lead, and iridium, sank deep beneath the planet’s surface, forming its core. Lighter elements, such as gases, have risen to the top in spectacular outgassing events. The surface of the Earth was composed of silicates, which floated on top of magma seas. Somewhere around the late middle portion of the Hadean Eon, the Earth began to cool and the outlines of continents formed. The atmosphere at that point was still deprived of oxygen.
An important event happened at the beginning of the Hadean Eon, about 4.5 billion years ago. Earth was significantly smaller then, until it was struck by a Mars-sized body known as Theia. This impact vaporized a huge portion of the crust and sent many quadrillion tons of material into outer space, which hung suspended in the form of rings for a few million years, until these rings condensed into the body we now call the Moon. .
Shortly after the Earth cooled, early life began, about four billion years ago. It’s truly remarkable how quickly the first primitive cells appeared after the Earth cooled. There is also some sketchy evidence, traces of organic carbon in the strata, which suggests that photosynthetic organisms may have appeared around this time, about half a billion years earlier than previously thought. However, more evidence is needed to confirm this.
Toward the end of the Hadean Eon, Earth — and other planets in the inner Solar System — were subjected to a 300-million-year event called the Late Heavy Bombardment. During this time, the number of asteroid and comet impacts has increased dramatically. Scientists aren’t sure why. It’s worth noting that while the late heavy bombardment meant significantly more impacts than we see today, the time interval between major impacts could still be measured in centuries or millennia. During this time, life had to function in an all CO2 atmosphere, oxidizing inorganic materials for energy. Around the end of the Hadean eon, primitive prokaryotic cells developed glycosis, a process for oxidizing organic molecules that laid the foundation for modern life.
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