If you are going to talk about history, then you might as well start from the beginning. The Hadean Eon is the first defined time period in our earth’s history. While we don’t know much about this eon in a definitive sense, there have been a lot of interesting observations made based on the few items we have found that we can date back to this time period. The name for this period comes from the word Hades, because of the hellish conditions that we believe to have prevailed during this time period. As violent and ancient as this time period was, it is also where we find the first traces of several things that are important to us: water, life, and the moon.
Important Vocabulary
Accretion:
The process of collecting cosmic dust from Earth’s orbit to make the planet.
Radiometric dating:
Using the rate of radioactive decay of certain unstable elements to determine the age of an object.
Cyanobacteria:
A type of photosynthetic bacteria that is visible to the naked eye as a colorful slime, generally blue-green, but can be found in brown or red variations.
Important Dates
Beginning: 4.55 Billion Years Ago
Formation of the Moon: 4.53 Billion Years Ago
First Water: 4.4 Billion Years Ago
First Life: 4.28 Billion Years Ago
End: 4.0 Billion Years Ago
Discussion
The Hadean Beginning Of The Earth
Our Earth started as a collection of dust a particular distance away from Sol, our sun. Through forces such as gravity and static attraction, this dust clumped together and formed larger and larger rocks. These rocks then accreted together to form a planet. Note that the recent controversy over Pluto no longer being classified as a planet, because its formation did not clear its orbit. These rocks were made up of the various elements that we find in our earth like Iron, Nickel, Silicon, etc but were all mixed together.
As this accretion of rocks grew in size, the force of gravity holding it together grew stronger and the pressure on the interior caused the temperature to rise and melt the rocks. Once the rock becomes liquid, materials begin to separate and heavier materials tend to sink down to the center, which pushes lighter elements like Silicon up to the top to form a crust or outer layer. As this outer layer cools, it forms hardened plates that float on top of the more dense layers underneath. Note that it is hard to know specific details, because there are very few places on Earth where rocks have been radiometrically dated as having solidified this far back. This lends to the idea that the Hadean Eon saw a lot of upheaval and remelting of the crust as it was bombarded and melted.
The Formation Of The Moon
Speaking of bombardment, we now come to the formation of the moon. Computer simulations have show that an object colliding with the Earth about the size of Mars could have knocked a chunk of the earth away to become the moon. In analyzing moon rocks, we have found that the moon is made up of material similar to the earths crust. This suggests that the impact happened after the earth had gained a heavy liquid core. Heavier elements would have been too deep to be knocked away in significant quantities. Radiometric dating of moon rocks suggests that the moon solidified about 4.5 Billion years ago. The other interesting fact about the moon is how dry it is, being almost completely devoid of water.
Oceans Cover The Blue Planet
The idea that oceans covered our planet in a significant way during the Hadean Eon is a new idea. This comes from the recent discovery of zircon crystals with liquid water trapped inside. These crystals have been dated to about 4.4 Billion years old in places like Australia and Canada. Zircon crystals form in a similar way to diamonds and require tremendous amounts of pressure. This sort of pressure would be found at the bottom of a large ocean. About 70% of the Earth today is covered by water. The interesting thing here is that the closest planets to us, namely Venus, Mars, and our moon, not to mention the other smaller bits of rock, are comparatively bone dry. There are 2 main theories about where the water came from, but they all have their problems.
Comets and Meteors
One theory is that the Earth was bombarded by comets or meteors baring water. Several people like this theory, because it is believed that the inner solar system was too hot for water to stay on the planet if it started there. The problem with this theory is that not all water is the same, and we can measure its signature. Water can be made with different types of Hydrogen. It has isotopes called Deuterium and Tritium which have 1 and 2 extra Neutrons compared to the more common Protium, which has none. Studies of hydrous (wet) meteors and comets have shown that the type of water they carry could only account for a small percent of the water that we know on earth. It is also very strange that our neighbors wouldn’t have experienced a similar hydration.
Wet Accretion of Earth
Another theory is that the Earth accreted with water embedded in the rocks. Different studies have differing opinions as to whether or not this could have been possible unless there was significant atmospheric pressure from something like C02 to keep the water from boiling away, or if maybe the earth was at a further distance from the sun to protect its oceans while they formed. This once again begs the question as to why the moon wouldn’t have had more water. Maybe the Earth took the moons water, and the moon wants it back, which is why it keeps tugging at our oceans. I jest, but nobody really knows why we have oceans as we do. If someone tells you that they do, they might try to sell you some oceanfront property in the middle of Canada.
The First Life Appears
Fossils that have been radiometrically dated back to the Hadean Eon are the oldest records we have that show organisms like cyanobacteria. As far as we know it, these organisms appeared almost as soon as there were oceans on the planet to swim in. This is interesting because there is evidence that the atmosphere of the earth during the Hadean Eon would have been poisonous for large aerobic (Oxygen breathing) organisms. This would have been the perfect environment for cyanobacteria to step in and convert all that CO2 into oxygen over the next 200 million years so that larger animals could develop.
The End of An Eon
Now that we have arrived at 4 Billion years ago, we are at the end of the Hadean Eon. The Earth is seeing tectonic plate movement now that it has oceans, a crust, and an atmosphere. We don’t really know what it was like, because there really isn’t that much evidence left. If we keep talking about it and sharing our ideas, maybe someday something will stick. For now, lets keep on learning, and next week we will talk about the Archean Eon.