The Hadean Eon (4600-4000 Ma)
Callan Bentley, Karen Layou, Russ Kohrs, Shelley Jaye, Matt Affolter, and Brian Ricketts and Charlene Estrada
The Hadean Eon, named after the Greek god and ruler of the underworld, Hades, is the oldest eon and dates from 4.6-4.0 billion years ago. This time represents Earth’s earliest history, during which the planet was characterized by a partially molten surface, volcanism, and asteroid impacts. Several mechanisms made the newly forming Earth incredibly hot: gravitational compression, radioactive decay, and asteroid impacts. Most of this initial heat still exists inside the Earth. The Hadean was originally defined as the birth of the planet occurring 4.0 billion years ago and preceding the existence of many rocks and life forms. However, geologists have dated minerals at 4.4 billion years, with evidence that liquid water was present. There is possibly even evidence of life existing over 4.0 billion years ago. However, the most reliable record for early life, the microfossil record, starts at 3.5 billion years ago.
Origin of the Earth’s Crust
As Earth cooled from its molten state, minerals crystallized and settled, separating minerals based on density and creating the crust, mantle, and core. The earliest Earth was chiefly molten material and would have been rounded by gravitational forces, so it resembled a ball of lava floating in space. As the outer part of the Earth slowly cooled, the high melting-point minerals formed solid slabs of early crust. These slabs were probably unstable and easily reabsorbed into the liquid magma until the Earth cooled enough to allow numerous larger fragments to form a thin primitive crust.
Scientists generally assume this crust was oceanic and mafic in composition and littered with impacts, much like the Moon’s current crust. There is still some debate over when plate tectonics started, which would have led to continental and oceanic crust formation. Regardless of this, as Earth cooled and solidified, less dense felsic minerals floated to the surface of the Earth to form the crust. In contrast, the denser mafic and ultramafic materials sank to form the mantle and the highest-density iron and nickel sank into the core. This differentiated the Earth from a homogenous planet into a heterogeneous one with layers of felsic crust, mafic crust, ultramafic mantle, and iron and nickel core.
Below is an image showing the depth to the Moho, which shows the thickness of the crust.
Origin of the Moon
Several unique features of Earth’s Moon have prompted scientists to develop the current hypothesis about its formation. The Earth and Moon are tidally locked, meaning that as the Moon orbits, one side always faces the Earth and the opposite side is not visible to us. Also, most importantly, the Earth’s and Moon’s chemical compositions show nearly identical isotope ratios and volatile content. Apollo missions returned from the Moon with rocks that allowed scientists to precisely compare Moon and Earth rocks. Other bodies in the solar system and meteorites do not share the same degree of similarity and show much higher variability. If the Moon and Earth formed together, this would explain why they are so chemically similar.
Many ideas have been proposed for the origin of the Moon: The Moon could have been captured from another part of the solar system and formed together with the Earth, or the Moon could have been ripped out of the early Earth. None of the proposed explanations can account for all the evidence. The currently prevailing hypothesis is the giant-impact hypothesis. It proposes a body about half of Earth’s size must have shared at least parts of Earth’s orbit and collided with it, resulting in a violent mixing and scattering of material from both objects. Both bodies would be composed of a combination of materials, with more of the lower density splatter coalescing into the Moon. This may explain why the Earth has a higher density and thicker core than the Moon.
Origin of Water
Explanations for the origin of Earth’s water vary. The volcanic outgassing hypothesis for Earth’s water is that it originated from inside the planet and emerged via tectonic processes as vapor associated with volcanic eruptions. Since all volcanic eruptions contain some water vapor, at times more than 1% of the volume, these alone could have created Earth’s surface water. Another likely source of water was from space. Comets are a mixture of dust and ice, with some or most of that ice being frozen water. Seemingly dry meteors can contain small but measurable amounts of water, usually trapped in their mineral structures.
During heavy bombardment periods later in Earth’s history, its cooled surface was pummeled by comets and meteorites, which could be why so much water exists above ground. There isn’t a definitive answer for what process is the source of ocean water. Earth’s water isotopically matches water found in meteorites much better than that of comets. However, it is hard to know if Earth’s processes could have changed the water’s isotopic signature over the last 4-plus billion years. It is possible that all three sources contributed to the origin of Earth’s water.
