Editing Hadean Eon (section)

==== Earth ====
Early Earth was sufficiently hot to allow dense materials like iron and nickel to melt and segregate to form the planet’s core; this differentiation process was essential for establishing Earth's magnetic field, generated by the movement of molten iron in the outer core. The less dense materials, primarily silicates, floated above the core and formed the mantle. Continued cooling of the Earth led to the solidification of the surface and the formation of the primitive crust, likely initially thin and subject to frequent melting from impacts and internal heat.

Volcanic outgassing released gases from Earth's interior, contributing to the formation of the primary (original) atmosphere, likely composed of hydrogen, helium, water vapor, methane, ammonia, and carbon dioxide, significantly different from the current atmosphere as it lacked free oxygen. The young Sun emitted a much stronger solar wind, which may have stripped away much of the Earth's primary atmosphere, leading to the formation of a secondary (evolved) atmosphere through continued outgassing.

The composition of a secondary atmosphere is markedly different from that of the primary atmosphere, being richer in heavier gases like carbon dioxide (CO2), nitrogen (N2), water vapor (H2O), and, in Earth's case, eventually free oxygen (O2) due to the emergence of '''photosynthetic life'''. This composition depends significantly on the planet's volcanic activity, the presence of life, and the geochemical processes that recycle and transform atmospheric gases.

Secondary atmospheres can undergo even further evolution. On Earth, the action of living organisms (notably photosynthetic microbes and later plants) dramatically altered the secondary atmosphere, increasing its oxygen content. This process influenced Earth's ability to support a more diverse range of life forms and led to the development of the '''ozone layer''', which protects the planet from harmful solar ultraviolet radiation.

As the Earth cooled, water vapor began to condense into liquid water, falling as rain and accumulating in the planet's lowest-lying areas to form the first shallow oceans. In addition to outgassing, water and other volatile compounds may have been delivered to Earth through impacts by comets and water-rich asteroids, contributing to the growth of the oceans.

The Hadean Earth was characterized by a dynamic '''crust''', continually resurfaced by volcanic activity, and impacted by frequent collisions with other celestial bodies. The cooling of Earth's surface led to the gradual formation of more stable landmasses, or '''proto-continents''', much smaller and less stable than present-day continents.

===== Formation of the Moon =====
Very early after the accretion of the Earth, ~4.5 GYA, the <u>Moon</u> emerged. The prevailing explanation, known as the <u>Giant Impact Hypothesis,</u> suggests that the Moon formed as a result of a collision between early Earth and a Mars-sized protoplanet, often referred to as <u>Theia</u>. Prior to the impact, Earth was already a differentiated body with a core, mantle, and a primitive crust, orbiting in the habitable zone around the young Sun. Theia is hypothesized to have formed in a similar orbit, leading to its eventual collision with Earth. The proto-planets collided at a low angle and with a high velocity, an event that was neither a direct hit nor a glancing blow but something in between, maximizing '''material ejection''' into Earth's orbit. The impact generated immense heat and energy, melting large portions of the Earth's crust and mantle. It was of such magnitude that Theia's core and much of its mantle merged with Earth's core and mantle, allowing their materials to mix thoroughly. 

The collision ejected a significant amount of '''debris''' from both Theia and Earth into orbit. This debris consisted of material from the mantles of Theia and Earth, and possibly a small amount from their cores. Over time, this debris coalesced through gravity to form the Moon. The Moon likely formed relatively quickly after the impact, within a few thousand years. Its rapid accretion was essential for the Moon to capture enough material to form a body large enough to be gravitationally stable before the debris either fell back to Earth or dispersed into space. 

The impact significantly altered Earth's '''rotation''', leading to a shorter day length. It's estimated that immediately after the impact, an Earth day might have been as short as six hours. The collision also likely determined Earth's '''axial tilt''', which has profound effects on the planet's climate and seasons. Both Earth and the newly formed Moon were left in partially molten states after the impact. The energy released by the collision contributed to a global '''magma ocean''' on Earth, affecting its further differentiation and surface evolution. 

The impact could have stripped away part of Earth's existing atmosphere, leading to the formation of a new secondary atmosphere. The energy delivered by the impact and the subsequent increase in volcanic activity would have contributed to the outgassing of water vapor and other gases, influencing the development of the planet's '''hydrosphere'''. The exchange of material between Earth and Theia during the collision contributed to the chemical similarities between Earth's mantle and the Moon. This exchange is a key piece of evidence supporting the Giant Impact Hypothesis, as it explains why Moon rocks have '''isotopic compositions''' similar to Earth's mantle rather than being entirely distinct. 

The bulk of the Moon is made up of silicate materials, similar to the terrestrial planets; this includes a '''mantle''' composed primarily of olivine, pyroxene, and lesser amounts of minerals like spinel, and a '''crust''' that is rich in anorthosite, a type of rock composed mostly of plagioclase feldspar. Unlike the terrestrial planets, which have substantial iron cores, the Moon has a relatively small core, thought to comprise a small fraction of its mass (1-2%). 

The Moon shows a marked depletion in '''volatile elements''' compared to Earth and the other terrestrial planets, thought to be the result of the high-temperature environment of the Moon's formation, where much of the volatile material was lost to space. Early assumptions posited the Moon was entirely dry, but recent missions and studies have found trace amounts of water within lunar rocks and potentially significant ice deposits in permanently shadowed craters at the poles. These water levels are still far lower than on Earth. 

The Moon does not have active plate tectonics; lunar geology is characterized by the vast lunar '''maria''' (formed by ancient volcanic flows), highlands, and impact craters. Its surface is covered by '''regolith''', a layer of fragmented material produced by the impact of '''micrometeorites''' and its thermal expansion and contraction during the lunar day-night cycle. This feature, while not unique to the Moon among terrestrial bodies, is more pronounced due to the Moon's lack of atmosphere and water. 

Aside from its fiery beginnings which likely would have vaporized or expelled most volatile elements, it also has a light gravity, making the retention of necessary elements to build an atmosphere difficult. Adding to the difficulty is a lack of magnetic field, such that the solar wind would strip the Moon of any gaseous accumulation even if it had a chance to gather there.