Editing Hadean Eon (section)

== Solar System Bodily Accretion ==
Even as the densest regions of the molecular cloud collapsed and began protostars, other bodies had begun to form in the cooler, less dense regions of the '''accretion disk''' surrounding this more active center. The process of planetary accretion begins with the coalescence of dust and ice particles into larger bodies through '''electrostatic forces''' and '''gravity'''. Over time, these bodies grow into planetesimals and eventually protoplanets, influenced by the composition of the accretion disk, which varies with distance from the protostar, and the dynamic interactions within the disk itself.

By ~4.5 GYA, the rocky terrestrial planets (<u>Mercury</u>, <u>Venus</u>, <u>Earth</u>, and <u>Mars</u>) underwent differentiation; this process, driven by the decay of radioactive elements, caused these planets to develop a layered structure comprising a core, mantle, and crust. Around the tail end of the Hadean and into the early Archean (about 4.1 to 3.8 GYA), the terrestrial planets experienced the <u>Late Heavy Bombardment</u>, a period characterized by intense '''asteroid''' and '''comet''' impacts. This event is not entirely within the Hadean but is crucial for understanding the early evolutionary conditions of the terrestrial planets.

=== The Planetesimals ===
~4.6 GYA, following the gravitational collapse of part of the local giant molecular cloud, a '''solar nebula''' formed, consisting of a hot, dense disk of gas and dust orbiting the young Sun. In the inner regions of the solar nebula, closer to the proto-Sun, temperatures were high, allowing only metals and rocky materials to condense. These materials began to coalesce, through electrostatic forces and later gravity, forming increasingly larger bodies known as planetesimals.

The larger planetesimals began to grow more rapidly than smaller ones, a phase known as '''runaway growth''', due to their stronger gravitational pull which allowed them to attract more material. Eventually, the growth transitioned into an '''oligarchic phase''', where a few large bodies dominated their vicinity in the nebula, accreting much of the available material and clearing their orbital paths.

==== Mercury ====
As <u>Mercury</u> reached its final size ~4.5 GYA, the heat generated from kinetic energy of accreting material, along with radioactive decay, was enough to melt part of the planet, allowing it to differentiate into a metallic core and a silicate mantle and crust. Mercury is notable for its large '''metallic core''', which makes up a significant portion of its mass compared to other terrestrial planets. Its location close to the Sun influenced its composition, with higher temperatures likely leading to the loss of more volatile materials during its formation.

'''Differentiation''' is a process where the planet's internal structure segregated into distinct layers; it was driven by heat from radioactive decay and the kinetic energy from accreting material. During differentiation, denser materials, primarily metals, sank towards the center to form the core, while lighter silicate materials floated upwards, forming the mantle and the primitive crust.

As the planet continued to cool, contraction occurred, which is evident from the extensive system of lobate scarps observed across its surface. These are essentially cliffs resulting from the crust crunching and folding over itself as Mercury cooled and shrank. Volcanic activity contributed to the formation of the crust by extruding lava onto the surface, creating vast plains and filling in old impact basins. This volcanic resurfacing played a significant role in shaping the crust’s current composition and thickness.

Although slightly beyond the initial formation phase, the Late Heavy Bombardment (LHB; ~4.1 to 3.8 GYA) played a crucial role in shaping the planet's surface. During this time, Mercury, like other terrestrial planets, was bombarded by a large number of asteroids and comets, leading to significant cratering. These impacts further modified the crust, creating large impact basins and contributing to a secondary layer of ejecta and impact melt. 

Post-LHB, Mercury's crust continued to evolve but at a significantly slower rate. The planet's small size and proximity to the Sun limited its volcanic and tectonic activity compared to Earth, slowing down crustal renewal processes. The primary evidence for tectonic activity on Mercury comes from the presence of '''lobate scarps''' and '''thrust faults''' on its surface, which are indicative of compressional stresses. These features suggest that Mercury has experienced a global contraction as its interior cooled and solidified, causing the crust to buckle and break.  

However, the movement of large crustal plates over a convective mantle, characteristic of plate tectonics on Earth, is not observed on Mercury. Mercury's small size means it has a higher surface-area-to-volume ratio than Earth, which likely led to a faster cooling of its interior. A cooler interior reduces mantle convection, a key driver of plate tectonics. The crust and lithosphere of Mercury are thought to be thicker in relation to its size compared to Earth, making it less prone to breaking into moving plates. The global contraction of Mercury more closely resembles a planet covered by a single tectonic plate, rather than multiple interacting plates. 

There are several theories explaining Mercury's large core relative to its mantle:

* A prevailing hypothesis for Mercury’s large metallic core relative to its silicate shell suggests that Mercury might have been struck by a planetesimal or another proto-planet early in its history. This impact would have stripped away much of its original mantle and crust, leaving behind a body with a relatively large core and a thin silicate mantle and crust. The aftermath of such a colossal impact would result in the planet being largely molten. As Mercury cooled, a new crust formed from the solidification of magma on its surface. This crust would be thinner and compositionally different from the pre-impact crust, primarily made from minerals that crystallized from the magma.
* Early in the Solar System's history, the solar wind might have been much stronger, capable of eroding the lighter elements and compounds from Mercury's surface and atmosphere, enriching the metallic core component.
* Being close to the Sun, the conditions may have favored the formation of a planetesimal with a higher proportion of metallic materials due to the preferential condensation of metals at higher temperatures.

==== Venus ====
Early in its history, <u>Venus</u> underwent differentiation into a core, mantle, and crust, similar to other terrestrial planets. However, the specifics of Venus' differentiation might have been influenced by its size, composition, and the solar environment, leading to a planet that today lacks a global intrinsic magnetic field. This absence is possibly due to a slower rotation rate or differences in its core's composition or thermal dynamics. Venus' thick atmosphere, primarily composed of carbon dioxide with clouds of sulfuric acid, is likely a result of volcanic '''outgassing'''. The planet shows evidence of extensive volcanic activity in its past, which would have released large amounts of CO2 into the atmosphere. Unlike <u>Earth</u>, Venus lacks plate tectonics and significant surface water, mechanisms that on Earth recycle carbon dioxide back into the planet's interior. The inability to remove CO2 from the atmosphere led to a runaway '''greenhouse effect''' on Venus; it trapped solar radiation and dramatically increased surface temperatures, leading to conditions that evaporated any liquid water that may have existed, further exacerbating the greenhouse effect.

Though Venus does not have plate tectonics like Earth, there is evidence of tectonic activity, such as '''folding''' and '''faulting''', likely driven by mantle plumes and volcanic upwelling. This tectonic activity contributes to the creation of features like '''coronae''' and '''tesserae''', which are unique to Venus and indicate a complex interplay of crustal deformation and volcanic activity. The extreme surface temperatures and high atmospheric pressure on Venus could contribute to the crust being too soft for significant fracturing into plates and could inhibit '''subduction''', a key process in plate tectonics. Venus' lithosphere may be more rigid and thicker than <u>Earth's</u>, possibly due to its high surface temperatures. This rigidity can prevent the '''lithosphere''' from breaking into plates and moving over the mantle.

Differences in mantle temperature and the style of mantle convection may also play a role; Venus' mantle convection might occur in a <u>single-layer regime</u>, unlike the Earth's <u>multi-layer convection</u>, which is crucial for plate tectonics. Water acts as a '''lubricant''' for subduction on Earth, and the relative dryness of Venus' mantle compared to Earth's might hinder the development of plate tectonics. Venus' lack of surface water could mean that its lithosphere does not have the same level of '''plasticity''' required for plate movements.

One of the most intriguing aspects of Venus is its '''superrotating''' atmosphere, where the upper atmosphere circulates around the planet much faster than the planet itself rotates. This phenomenon is thought to be driven by solar heating and complex interactions between the planet's surface and atmosphere. The atmospheric conditions on Venus during the Hadean or its equivalent early history would have been drastically different from today, however. The early atmosphere of Venus, like that of the other terrestrial planets, was likely influenced by volcanic outgassing, comet impacts, and the composition of the solar nebula from which it formed. Over time, Venus evolved under the runaway greenhouse effect, leading to the extremely dense, CO2-rich atmosphere we observe today. This process increased surface temperatures to the current average of about 467°C (872°F), creating a thick atmosphere dominated by carbon dioxide with clouds of sulfuric acid.

==== 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.  

==== Mars ====
Young Mars quickly underwent differentiation, where its denser materials, such as iron, sank to form a metallic core, while lighter silicates floated to form its mantle and crust. This process was likely aided by the heat generated from accretion, radioactive decay, and potential early tidal heating effects if Mars had a more substantial moon or moons in its past. Volcanic activity on early Mars released gases trapped in the mantle, contributing to the formation of its primary atmosphere, which likely included water vapor, carbon dioxide, and nitrogen. Impacts from comets and asteroids are believed to have delivered additional volatiles, including water, to Mars. These events could have been particularly significant during the Late Heavy Bombardment period, enriching Mars with the necessary ingredients for a thicker atmosphere and liquid water on its surface.

Evidence from valley networks and mineralogical features suggests that Mars experienced periods of warmer and wetter conditions, particularly in its early history. These conditions might have allowed for the temporary presence of liquid '''water''' on its surface, shaping the planet's landscape through erosion and sediment deposition. Over time, Mars lost much of its atmosphere to space, a process accelerated by the solar wind and the lack of a global magnetic field to protect it. As the atmosphere thinned, Mars's ability to retain heat diminished, leading to the cold, arid conditions observed today.

Mars hosts some of the largest volcanoes in the Solar System, such as <u>Olympus Mons</u>; these volcanoes are a testament to the planet's volcanic activity, which has played a crucial role in shaping its surface and atmosphere. Unlike Earth, Mars does not show evidence of active plate tectonics. However, features such as the <u>Valles Marineris</u>, a vast canyon system, suggest that the planet has experienced significant '''crustal deformation''', possibly related to volcanic upwelling and the contraction of the planet's crust as it cooled. Mars has polar '''ice caps''' composed of water ice and frozen carbon dioxide. These caps contain valuable records of Mars' climate history and undergo seasonal changes that affect the planet's climate and atmospheric pressure.

Mars' atmosphere is thin and composed mostly of carbon dioxide, with trace amounts of nitrogen, argon, and oxygen. 

=== The Asteroid Belt ===
<u>Jupiter</u> formed from the solar nebula relatively quickly in Solar System history, accumulating a massive atmosphere of hydrogen and helium around a rocky and icy core. Its rapid growth allowed it to dominate the gravitational dynamics of the young Solar System. Models of Solar System evolution, such as the '''Grand Tack hypothesis''', suggest that Jupiter initially migrated inward towards the Sun, due to interactions with the gas and dust in the protoplanetary disk. This inward migration could have had dramatic effects on the inner Solar System, including the distribution and composition of material in the terrestrial planet region. 

According to the Grand Tack hypothesis, as Saturn formed and migrated inward, its interaction with Jupiter resulted in a gravitational tug that reversed Jupiter's direction, causing both planets to migrate outward to their current positions. This "tack" could explain the small size of Mars as well as the composition of the asteroid belt. Jupiter's outward migration, along with Saturn, would have influenced the formation and migration of the ice giants, Uranus and Neptune, potentially pushing them to their current locations and into the Kuiper belt, scattering objects in the process. 

Jupiter’s massive gravity disturbed the orbital paths of nearby planetesimals and protoplanets, preventing them from coalescing into a larger planet in the region of the asteroid belt. Its location is significantly influenced by '''gravitational resonances''' with Jupiter, which occur when the orbital period of a body in the belt is a simple fraction (like 2:1 or 3:2) of Jupiter’s orbital period, leading to gravitational interactions that can alter the body's orbit. This has led to gaps in the asteroid belt, known as '''Kirkwood gaps''', and helped prevent the formation of a planet in this region. 

The asteroid belt contains a diverse range of bodies, from rocky to icy, reflecting the compositional gradient of the solar nebula. The early Hadean would have seen significant collisional evolution in the belt, with impacts leading to fragmentation, accretion, and the ejection of material. Some of this ejected material would have contributed to the late heavy bombardment of the inner Solar System. 

The asteroid belt contains bodies ranging in size from <u>Ceres</u>, the largest asteroid and a dwarf planet with a diameter of about 940 kilometers, down to dust particles. The size distribution of asteroids follows a '''power-law distribution''', meaning there are many more small asteroids than large ones. This distribution is indicative of a '''collisional evolution''' where larger bodies are broken into smaller pieces over time. Besides Ceres, other large asteroids include <u>Vesta</u>, <u>Pallas</u>, and <u>Hygiea</u>, which have diameters of approximately 525 km, 512 km, and 430 km, respectively. These large asteroids constitute a significant portion of the belt's total mass. 

Some asteroids are grouped into families or clusters based on similarities in their orbital parameters. These families are thought to be the remnants of larger parent bodies that were shattered by collisions. Moreover, gravitational resonances with Jupiter and other planets have sculpted the distribution and dynamics of asteroids within the belt. 

=== The Giants ===
<u>Jupiter</u> and <u>Saturn</u>, the '''gas giants''', likely formed from the accretion of icy and rocky material into large cores, followed by the rapid accumulation of hydrogen and helium gas from the solar nebula. Jupiter, being closer to the Sun and forming in a region of the nebula with a higher density of materials, likely formed first, with Saturn following. This model suggests that once these planets' cores reached a critical mass, they were able to gravitationally attract and retain the surrounding gas, growing rapidly into the gas giants we recognize today.

Uranus and Neptune are often referred to as '''ice giants''' due to their large compositions of volatiles (substances that easily become gas) like water, methane, and ammonia, in addition to hydrogen and helium. These planets formed further from the Sun, where temperatures were low enough for these compounds to condense.

=== Kuiper Belt and Oort Cloud Formation ===
The <u>Kuiper Belt</u>, extending from beyond Neptune's orbit out to approximately 55 astronomical units (AU) from the Sun, began to take shape in the Hadean era. It is composed of icy bodies, including dwarf planets like <u>Pluto</u>. The formation of the Kuiper Belt was likely influenced by the gravitational perturbations of the giants, particularly <u>Neptune</u>.

The <u>Oort Cloud</u> is a distant, spherical shell of icy objects surrounding the Solar System, believed to extend up to 100,000 AU from the Sun. It is thought to have formed from the scattering of icy bodies by the gas giants' gravitational forces into distant orbits. Some of these objects were likely ejected to the cloud's outer reaches, while others were pulled into the inner Solar System as comets.