Hadean Eon

Overview

Dating

Starting ~4567.30 ± 0.16 MYA (4.6 billion years ago)

Lasting ~536 million years.

Ending ~4031.00 ± 0.16 MYA (4.0 billion years ago)

Etymology

The eon's name comes from Hades, Greek god of the Underworld; it is a poetic reference to the extreme environmental conditions of early Earth.

Other names: Priscoan Period (from priscus, 'ancient'); pre-Archean (deprecated)

Eons are the longest portions of geologic time (eras are the second-longest). Three eons are formally recognized: the Phanerozoic Eon (dating from modern times back to the beginning of the Cambrian Period), the Proterozoic Eon, and the Archean Eon - the Hadean Eon is an informal predecessor.

Informal?

The eon is characterized by an almost complete absence of preserved rock formations. The oldest known rocks on Earth date back to about 4.0 billion years ago, towards the end of the Hadean. Without significant rock records, it's challenging for geologists to define and study this eon using traditional geological techniques. Geological time scales are primarily based on significant events in Earth's history, often marked by changes in the rock record, such as mass extinctions or major shifts in life forms, sedimentation, or atmospheric composition. The Hadean lacks these clear markers in the rock record, making it harder to define using the criteria applied to later geological periods.

The Molecular Cloud

Evidence suggests that the Solar System began to form ~4.6 GYA (GYA means 'gigayears ago'/ 'billion years ago'), when a giant molecular cloud experienced gravitational collapse. Molecular clouds are sometimes referred to as stellar nurseries, as star formation often occurs within them; they are a distinct type of structure within the interstellar medium (ISM), which is composed of gas (mainly hydrogen, but also helium and trace amounts of heavier elements), plasma, and dust.

Formation of Molecular Hydrogen

The vast majority of the contents of the interstellar medium is gaseous H2, a molecule - a structure consisting of more than one atom - in this case, two hydrogen atoms bound together (H-H).  Molecular hydrogen forms when two hydrogen atoms share their electrons through a covalent bond, a chemical process that occurs at the low energies typical in cold, dense regions of the interstellar medium. The resulting molecule consists of two hydrogen nuclei (protons) and two electrons. It's the most basic molecular structure possible and is a stable, neutral molecule. In contrast with a nuclear process (for example, the creation of helium), within the hydrogen molecule each nucleus/electron pair retains its identity as a separate hydrogen atom; they are simply bonded together. The protons (nuclei) do not fuse into a different element with a heavier nucleus, as they would in a higher-energy nuclear reaction.

Electrons do not have fixed orbits in the way planets orbit a star; their positions are described by a probability distribution within electron clouds. When two hydrogen atoms come close together, their electron clouds merge to form a region of shared electron density (the covalent bond, when the 1s orbitals of each hydrogen atom overlap). This combination of electron density in the space between the nuclei holds the atoms together into a molecule of hydrogen gas.

The 'stability' of molecular hydrogen is due to the fact that both hydrogen atoms achieve a full valence shell (for hydrogen: two electrons). Most atoms have a maximum of eight electrons that can occupy their outermost (valence) shell, and they generally seek to stabilize by 'filling' that shell with electrons. For most elements this is called the octet rule - but hydrogen has its own analog, the duplet rule, as its valence shell is naturally more compact, and can hold a maximum of two valence electrons. The behavior of an atom, particularly its ability to form chemical bonds with other atoms, is largely determined by the number of electrons in this outer shell. Hydrogen is the simplest element, with just one proton and one electron; its electron resides in the 1s orbital, the closest orbital to the nucleus. Because hydrogen is in the first row of the periodic table, its 1s orbital can hold a maximum of two electrons. By sharing its single electron with another atom that also needs additional electrons to complete its own valence shell, hydrogen can effectively achieve a stable electronic arrangement resembling that of the noble gases, which are naturally stable and nonreactive due to their full valence shells.

The formation of H2 from two hydrogen atoms is energetically favorable and exothermic (it releases energy). However, in space, there's a challenge: conservation of momentum and conservation of energy. The newly formed H2 molecule must find a way to release the excess energy from the bond formation to become stable; on Earth, this energy would be dissipated quickly into the surrounding environment which is rich with atoms, but in the low-density environment of space, there's often nothing nearby to absorb it. Without a third body to take away the energy, the two hydrogen atoms would simply bounce off each other without forming a stable molecule (this is why dust often acts as a suitable 'surface' upon which these reactions can occur).

When a chemical bond forms, such as an H-H bond in an H2 molecule, energy is released because the system goes from a higher energy state (separate hydrogen atoms) to a lower energy state (the bonded molecule). This is a fundamental principle of chemistry - systems tend to move toward lower energy states, which are more stable. Separated hydrogen atoms have potential energy due to their mutual electrostatic potential and the potential of their electrons in the field of another nucleus. When they come close enough to share their electrons and form a covalent bond, they create a molecule with lower potential energy than the sum of the two atoms taken separately.

The bond formation process releases energy, primarily in the form of heat, which can be absorbed by a dust grain, causing it to vibrate more intensely. This process not only allows the dust to act as a heat sink but also to catalyze the formation of molecular hydrogen. As these interactions occur on a vast scale, they collectively facilitate the formation and thermal regulation of molecular clouds, rich in molecular hydrogen and dust, precursors to star formation. The energy transfer to dust grains and subsequent infrared radiation that results contribute to the cloud's thermal balance, essential for its evolution and the birth of new stars. This dynamic interplay of physical and chemical processes underpins the formation of molecular clouds in the interstellar medium, marking them as fertile grounds for the complex chemistry that leads to star and planet formation.

Dust

(Star) dust refers to tiny solid particles composed of elements like carbon, oxygen, silicon, magnesium, and iron, often found in the form of compounds or minerals similar to those seen on Earth (silicates, graphite, water ice, and iron oxides). These particles are remnants of previous astrophysical processes, including outflows from aging stars (when stars at the ends of their lives begin to 'leak' matter and energy), supernovas (when dying stars explode), and the collision and fragmentation of asteroids and comets with other bodies. Dust particles can range in size from a few molecules to micrometers of diameter.

Dust particles act as cooling agents in the ISM by absorbing energy from surrounding gas and re-emitting it at infrared wavelengths, facilitating the collapse of gas into denser clouds that can eventually form stars (as described above). Dust grains also provide surfaces on which chemical reactions can occur, which can lead to the formation of complex organic molecules, including those essential for life. Dust absorbs and scatters visible and ultraviolet light, affecting the appearance of astronomical objects in a process known as interstellar extinction; this makes distant stars appear dimmer and redder than they are in reality. Dust is also responsible for nebulae observed in the night sky, as it scatters light from nearby stars or emits light due to its own raised temperature.

Star dust can include graphite and complex organic molecules, some of which are similar to soot or polycyclic aromatic hydrocarbons (PAHs); others form large, interconnected structures. Silicates (particles primarily composed of silicon, oxygen, magnesium, and iron) are also common. These are similar in composition to many of the minerals found in Earth's crust but are in the form of very small grains in space. In colder regions of the ISM, dust grains can be coated with ices - water (H2O), methane (CH4), ammonia (NH3), carbon dioxide (CO2), and other molecules can freeze onto the surfaces. Tiny solid particles of metals such as iron, nickel, and magnesium also contribute to interstellar dust.

Plasma

Plasma, often referred to as the fourth state of matter, is a gas in which a certain proportion of the particles are ionized (meaning electrons have been separated from atoms) resulting in a mix of free electrons and ions. In the context of the ISM, plasma exists primarily in ionized regions such as HII clouds (described below; around hot stars where UV radiation is strong enough to ionize hydrogen). Plasma often has unique properties compared to neutral gases, such as being highly conductive to electricity and responsive to magnetic fields due to its charged particles.

Interstellar Clouds

Interstellar clouds are accumulations of gas, plasma, and dust within a galaxy. They are simply regions of space where this material is denser-than-average compared to the wider interstellar medium. The ISM is not uniform; it exists in various configurations and structures due to variations in gravity, pressure, and the remnants of earlier stellar processes.

• An HI region describes a cloud of neutral atomic hydrogen (HI), in addition to the local abundance of helium and other elements already present. H is the chemical symbol for hydrogen, and I is the Roman numeral for 'one'. It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly-ionized (HII is H+ in other sciences) III for doubly-ionized; OIII instead of O++). HI regions form where temperatures are relatively low, allowing hydrogen atoms to remain neutral rather than becoming ionized by stellar radiation. These conditions are conducive to the large-scale accumulation of hydrogen. The movement and interaction of galaxies can compress interstellar gas, leading to the formation of HI regions. Over time, these regions can become the birthplaces of new stars, but of themselves they do not emit detectable visible light.
• An HII region is a vast cosmic cloud of ionized hydrogen gas (H+), where new stars are born and illuminate their surroundings. These regions are named after the ionized state of hydrogen (HII, using the astronomical convention where 'II' indicates ionization; H+ in chemical notation). HII regions represent areas within the ISM where hydrogen gas has been ionized (its atoms have lost their electrons due to high energy) typically from ultraviolet (UV) radiation emitted by nearby young, hot stars. The theoretical model describing the ionized region around a hot star is called a Strömgren sphere, a spherical volume where the radiation from the star has ionized hydrogen gas within its boundaries. The horizon of this sphere marks where the rate of ionization balances with the rate of recombination (electrons recombining with ions to form neutral atoms once again). HII regions are known for their bright emission in optical wavelengths - when free electrons recombine with ionized protons (HII), they emit photons. The specific wavelengths of light emitted during these transitions produce the characteristic emission lines observed in the spectra of HII regions. The most prominent of these lines is the Balmer series of hydrogen, especially the H-alpha line (Hα). HII regions are not randomly distributed within galaxies but are often found in the arms of spiral galaxies and the central regions of irregular galaxies. The Orion Nebula (M42) is one of the closest and most studied HII regions, providing a wealth of information about star formation and the interaction of newly formed stars with their natal environments. The Carina Nebula and the Eagle Nebula (M16), famous for the Pillars of Creation, are other notable examples.

Gravitational Collapse

The accumulation of gas, dust, and plasma in the ISM can reach a threshold where gravitational forces overcome internal pressure, leading to collapse and the formation of new stars; this is called gravitational collapse. This process is influenced by the density and temperature of the cloud, as well as external factors such as shock waves from nearby supernovas; it relates to the Jeans instability criterion, which describes the conditions under which a cloud will begin to collapse under its own gravity.

• Jeans Mass: The minimum mass that a cloud must have for gravitational forces to overcome internal pressure and initiate collapse. If a cloud's mass is greater, it is considered gravitationally unstable and likely to collapse to form stars. The internal thermal energy of a cloud provides the outward pressure needed to resist gravitational collapse. When a cloud's mass increases (holding temperature constant), its gravitational attraction becomes stronger, making it more difficult for the thermal pressure to resist collapse. Conversely, if the cloud can lose thermal energy (cool down), the reduced pressure makes it easier for gravity to dominate, leading to collapse. During the collapse, gravitational potential energy is converted into kinetic energy (heat) as particles accelerate toward the cloud's center. This heat must be radiated away for the collapse to proceed; otherwise, it would increase the internal pressure and add resistance to further collapse.
• Jeans Radius (or Jeans Length): The critical radius of a cloud at which it becomes gravitationally unstable, and prone to collapse. It essentially marks the minimum size that a cloud must have, given its mass and temperature, for gravity to overcome the internal gas pressure and initiate collapse. Efficient cooling mechanisms (such as radiation from dust and stable molecules) lower the temperature, thereby decreasing the Jeans radius. During collapse, a cloud can fragment into smaller pieces, each potentially with its own Jeans radius based on its local conditions (mass, temperature, density). This fragmentation can lead to the formation of multiple stars or a star system, rather than a single monolithic star. This also enhances the cloud's instability by making it easier for any portions that exceed this reduced Jeans radius to collapse into star formation.

When a molecular cloud reaches the Jeans instability criterion, several key processes unfold, leading to star formation. The specific timeframe for these processes can vary widely depending on the initial conditions of the cloud, such as its mass, temperature, and composition. During the Free-fall Phase, the cloud contracts, and the density increases, particularly at the center. For a solar-mass star, the initial collapse from a diffuse cloud to a denser core might take about 100,000 years; a relatively short period compared to the total lifetime of a star.

As the cloud collapses, it may fragment into smaller clumps due to variations in density. Each clump, containing a few solar masses of material, can potentially form a star or a star system; this fragmentation is critical in determining the initial mass function of stars (the distribution of stellar masses at birth). The collapsing material forms a protostar at the center of each clump; the material from the surrounding cloud continues to accrete onto it, increasing its mass and temperature. For a solar-mass star, the Accretion Phase might last around 100,000 to several million years.

Protostars often exhibit powerful outflows and jets, which help to remove angular momentum and allow accretion to continue. These outflows can impact the surrounding cloud, influencing further star formation. As the core of the protostar becomes increasingly dense and hot (reaching about 10 million Kelvin for a star like Sol), hydrogen nuclei begin to fuse into helium, releasing energy through nuclear fusion. This marks the birth of a star and its entry into the main sequence phase of its life. The outward pressure from nuclear fusion balances the inward pull of gravity, stabilizing the star; it will remain in the main sequence phase for the majority of its lifetime, converting hydrogen to helium in its core. For the Sun, this phase will last about 10 billion years (which gives it something close to 6 billion years of life left).

Properties of the Sun

The Sun (symbol: ☉) is by far the brightest object in Earth's sky, with an apparent magnitude of −26.74. This is about 13 billion times brighter than the next brightest star, Sirius, which has an apparent magnitude of −1.46. A G-type (dwarf) main-sequence (hydrogen-to-helium fusion-powered) star, the Sun's original chemical composition was inherited from the local interstellar cloud out of which it and its neighbors formed.

The radius of the Sun (R☉) is 109 times that of Earth, but its distance from Earth is 215 R☉, so it subtends an angle of only 1/2° in the sky, roughly the same as that of the Moon. The mass of the Sun, M☉, is 743 times the total mass of all the planets in the Solar System and 330,000 times that of Earth.

The Sun is a very stable source of energy; its radiative output, called the solar constant, is 1.366 kilowatts per square metre at Earth and varies by no more than 0.1 percent; but superimposed on this stability is an 11-year cycle of magnetic activity manifested by regions of transient strong magnetic fields.

Originally the Sun would have been about 71.1% hydrogen, 27.4% helium, and 1.5% heavier elements (now 74.9% hydrogen, 23.8% helium, 1.3% heavier elements). The hydrogen and most of the helium were produced by Big Bang nucleosynthesis in the first moments of the universe, and the heavier elements were produced by previous generations of stars and spread into the interstellar medium during the final stages of their stellar lives; their remnants became dust in our cloud.

Most of the collapsing cloud (99.86% of the total mass) gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core, which formed the star we call the Sun. It has an absolute magnitude of +4.83, estimated to be brighter than about 85% of the stars in the Milky Way galaxy (most of which are red dwarfs). The Sun is a Population I, or heavy-element-rich, star. Its formation may have even been triggered by shockwaves from one or more nearby supernovas; this is suggested by a high abundance of heavy elements such as gold and uranium relative to the abundances of these elements in so-called Population II, heavy-element-poor, stars.

Apparent magnitude measures how bright a star or celestial object appears from Earth. It is a scale, traditionally inversely logarithmic, where lower (or more negative) values denote brighter objects. This scale is rooted in an ancient system devised by the Greek astronomer Hipparchus. Absolute magnitude is a measure of the intrinsic brightness of a celestial object. It is defined as the apparent magnitude the object would have if it were placed at a standard distance of 10 parsecs (about 32.6 light-years) from Earth. This allows astronomers to compare the true luminosities of celestial objects without the distortion caused by distance. The Sun's absolute magnitude is +4.83; although it appears very bright in the sky due to its proximity, in terms of the vast array of stars, the Sun is of average brightness. Sirius has an absolute magnitude of +1.4, indicating it is intrinsically brighter than the Sun if both were observed from the same distance; though it appears far less so, because of the distortion of distance. This fundamentally illustrates the difference between apparent and absolute magnitude.

Parts of the Sun

The formation of these layers and features was a gradual process that occurred over millions of years as the collapsing molecular cloud reached nuclear fusion temperatures at its core and began to differentiate based on temperature, density, and rotational dynamics.

The Sun does not have a definite boundary, but its density decreases exponentially with increasing height above the photosphere. For the purpose of measurement, the Sun's radius is considered to be the distance from its center to the edge of the photosphere (its apparent visible surface). It is a near-perfect sphere, with its polar diameter differing from its equatorial diameter by only 10 kilometers.

Core

The core is the central region of the Sun, where nuclear fusion occurs. Hydrogen nuclei fuse to form helium, releasing enormous amounts of energy that powers the star. Each second within the core of the Sun, ~600 million tons of hydrogen atoms transform into helium in a fusion process known as the proton–proton chain (4 hydrogen atoms form 1 helium atom). The proton-proton chain comprises several steps, involving the fusion of solitary protons (hydrogen nuclei) into helium nuclei. Protons are positively charged and normally repel each other due to the electrostatic force, but the extreme temperature and pressure of the star's core can push protons (hydrogen nuclei) close enough to one another for the strong nuclear force to overcome their electrostatic repulsion (known as the Coulomb barrier).

Fusion - Step 1

When two protons come very close, one of the protons can (very rarely; one in a billion such interactions) undergo a weak nuclear reaction, where it transforms into a neutron. This transformation is mediated by the weak nuclear force, which operates at very short ranges and is responsible for processes like radioactive decay. The emergent neutron can then pair with the remaining proton to form deuterium (²H), a hydrogen isotope with one proton and one neutron in its nucleus. The remaining proton (p⁺) fuses with its decayed neutron partner, forming a new deuterium nucleus (²H), but also releasing a positron (e⁺), and a neutrino (ν_e).

• 1p⁺ + [1p⁺→n + (W+→e⁺ + ν_e)]_{The proton becomes a neutron, releasing a W+ boson (which quickly becomes a positron and a neutrino).}
• p⁺ + n → ²H + e⁺ + ν_{eThe proton and the neutron fuse, creating a new deuterium nucleus; the positron and neutrino remain.}

Both protons and neutrons are nucleons (components of atomic nuclei); a proton has a positive electric charge, while a neutron has no electric charge. Despite this difference, protons and neutrons are quite similar in mass and are both fermions, meaning that they both follow the Pauli exclusion principle.

The transformation from a proton to a neutron changes the type (or flavor) of one of the nucleon's fundamental particles, called quarks. The proton (composed of two up quarks and one down quark) transforms into a neutron (composed of one up quark and two down quarks) through this process, an example of beta-plus decay (β+ decay). This transformation emits a W+ boson (a carrier particle of the weak nuclear force), which then quickly decays into a positron (e⁺) and an electron neutrino (ν_e). In the transformation, charge conservation is maintained; the initial positive charge of the proton is balanced by the creation of a positron, which carries a positive charge, ensuring that the total charge before and after the reaction remains constant.

Fusion - Step 2

The emergent positron quickly encounters another electron, leading to their mutual annihilation and an energy release as gamma rays. Electrons naturally carry a negative electrical charge; positrons have the same properties as electrons but an exactly analogous negative charge and are considered antielectrons (their antiparticle equivalent). Particles and their antiparticles annihilate one another on contact, often (as in this case) producing new particles as a result of the energy release. The newly formed deuterium nucleus then fuses with yet another proton (hydrogen nucleus), producing a helium-3 nucleus (³He) and releasing more energy in the form of gamma radiation (γ; high-energy photons; with short wavelengths / high frequency).

• ²H + p⁺ → ³He + γ

Fusion - Step 3

In the final stage of main sequence fusion, two emergent helium-3 nuclei collide and fuse to form the isotope helium-4 (⁴He), releasing two protons in the process. This step does not always follow directly after the first two steps; it requires the accumulation of sufficient helium-3 nuclei from step 2 within the star's core. Even at these high temperatures, the ³He nuclei carry a positive electrical charge, and thus repel one another. Quantum tunneling allows them to get close enough for the strong nuclear force to take over at very short ranges, facilitating fusion.

Quantum tunneling is a quantum mechanical phenomenon that allows particles to pass through a potential barrier that they classically shouldn't be able to surmount, due to insufficient energy. Protons in a star's core repel each other via the Coulomb barrier (electrostatic force). At the temperatures and pressures at a star's core, classical physics would suggest that protons do not have enough kinetic energy to overcome this barrier and get close enough to fuse.

Quantum mechanics allows for the probability that particles like protons can 'tunnel' through the Coulomb barrier and come within close enough range for the strong nuclear force to take effect in opposition to the laws of classical physics, which significantly increases the rate of fusion reactions in a star's core. When two ³He nuclei fuse, they form a ⁴He nucleus (with 2 protons and 2 neutrons), more stable due to the strong nuclear force binding its nucleons more tightly together. The reaction also releases two protons (2p⁺), which essentially are the 'excess' not needed in the formation of helium-4.

• ³He + ³He → ⁴He + ²p⁺

The energy released during the proton-proton chain reaction comes primarily from the mass difference between the initial reactants and the final products, according to Einstein's equation, E = mc². Most of the energy is carried away by gamma rays and neutrinos; the gamma rays make their way to the Sun's surface at a glacial pace and are eventually emitted as sunlight, while neutrinos escape the Sun almost immediately due to their weak interaction with matter (neutrinos respond only to the weak nuclear force and gravitational force).

The presence of the immediately expelled neutrinos is one of the primary proofs that nuclear fusion is the source of the Sun's energy. Its released gamma rays are a significant source of the star's energy output; but as they make their way out of the Sun's core, they interact with solar material, gradually losing energy and getting converted into lower-energy photons, eventually contributing to the sunlight we receive on Earth. This process of energy transfer from the core to the surface takes an incredible amount of time - thousands to millions of years - due to the dense plasma's opacity to electromagnetic radiation. The light you see left the Sun's surface eight minutes ago, but it was created in the Sun's core millennia before you were born.

The fusion rate in the Sun's core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the density and hence the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the density and increasing the fusion rate and again reverting it to its present rate.

Radiative Zone

Surrounding the core, the radiative zone is where energy produced in the core is transported outward by photons through radiative diffusion. As the collapsed cloud's central region heated up and began nuclear fusion, the surrounding material, still collapsing, became stratified due to temperature and density gradients, forming the radiative zone.

Convective Zone

Above the radiative zone, the convective zone is where energy is transported by convection. Hot plasma rises, cools as it loses energy to the outer layers, and then sinks to be reheated and rise again. The convective zone formed because the outer layers, being cooler than the core, became inefficient at radiative energy transfer, turning instead to convection.

The Sun rotates faster at its equator than at its poles, due to convective motion from heat transport, and the Coriolis force of the Sun's rotation. Convective motion refers to the movement of plasma within the Sun driven by the transfer of heat from the interior towards the surface. Heat from the core, generated via nuclear fusion, is initially transferred through the radiative zone via the emission and absorption of photons. Closer to the surface, in the convective zone, the plasma becomes cooler and less efficient at radiative heat transfer. The temperature gradient causes the plasma to become buoyant and rise toward the photosphere. As the plasma reaches the photosphere, it releases its heat into space and cools down, becomes denser, and sinks back toward the interior, creating a continuous cycle of convective motion. This process is similar to boiling water, where hot water rises, cools, and then sinks in a convection cell.

The Coriolis force affects the motion of the plasma in the Sun, particularly in the convective zone. As plasma moves up and down in the convective zone, the Coriolis force acts on it, deflecting its path. In the northern hemisphere, the motion is deflected to the right, and in the southern hemisphere, to the left. This effect is due to the Sun's rotation and affects how heat and magnetic fields are distributed across it. The combination of centrifugal force from the Sun's rotation and the Coriolis force slightly flattens the Sun at the poles and bulges it at the equator, although the difference is very small.

The Sun exhibits differential rotation because it is a fluid body, not a solid one. The Coriolis effect introduces some asymmetries in solar activities (like sunspot patterns and flows within the convective zone) but doesn't significantly alter the overall spherical shape due to the dominant balancing act of hydrostatic equilibrium. The Sun's rotation has its origins in the process of star formation from a rotating molecular cloud in the interstellar medium, which was not perfectly stationary but had some initial rotation. This rotation could have been due to gravitational interactions with nearby objects, the impact of galactic rotation, or the remnants of motion from previous supernovae explosions in the area.

Photosphere

The photosphere is the visible surface of the Sun from which light is emitted. It's where sunspots, regions cooler than their surroundings, are seen. The photosphere formed as the outermost layer of the Sun cooled and became transparent to light, making it effectively the 'surface' from which sunlight is radiated. This emission of light defines the Sun's visible diameter and features that can be observed.

Chromosphere

Just above the photosphere, the chromosphere is a layer of the Sun characterized by the reddish glow seen during solar eclipses. It contains spicules and filaments driven by the Sun's magnetic field. The chromosphere developed above the photosphere as the temperature began to increase with altitude, influenced by the Sun's magnetic field dynamics.

Corona

The corona is the outermost layer of the Sun's atmosphere, extending millions of kilometers into space. It is surprisingly hot, much hotter than the surface, and is visible during a total solar eclipse. The corona formed as the Sun's magnetic field lines extend outward into space, carrying with them solar wind and plasma. The heating mechanism of the corona is a topic of active research, with magnetic reconnection and wave heating being key theories.

Solar Wind

Beyond a distance of 5R_☉ from the Sun, the corona flows outward at a speed (near Earth) of 400 kilometres per second (km/s); this flow of charged particles is called the solar wind. The solar wind is a stream of charged particles (plasma) released from the upper atmosphere of the Sun (the corona) into space. The solar wind forms as hot coronal gas expands into space, accelerated by the Sun's intense heat and magnetic field.

Formation of the Sun

As the cloud collapsed under its own gravity to form the Sun and the solar system, it conserved its angular momentum. Angular momentum is a physical quantity that measures the amount of rotation of an object, and it is conserved in a system unless acted upon by an external torque. Since there's no external force to provide such a torque in the vacuum of space, the collapsing cloud's rotation speed increased as it shrank in size, much like a figure-skater spins faster when pulling in their arms.

L = Iω, where:

• I is the moment of inertia (a measure of an object's resistance to changes in its rotation rate), and
• ω is the angular velocity (the rate of rotation). The moment of inertia is dependent on the mass distribution relative to the axis of rotation; for a given mass, spreading it further from the rotation axis increases.

The cloud's radius decreases, significantly reducing its moment of inertia (I) by I=mr². According to the conservation of angular momentum, if I decreases and L is conserved, then ω (angular velocity) must increase. This means the cloud spins faster as it collapses.

The outcome of this conservation is that as the molecular cloud that became the solar system collapsed, it spun faster, leading to the formation of a rotating protostar - the young Sun. Over time, the Sun reached its current rotation period, which is about 25 days at the equator and longer at higher latitudes, due to its gaseous nature allowing for differential rotation. Over time, the distribution of the Sun's mass and interactions between its magnetic field and the solar wind (a stream of charged particles emitted by the Sun) have likely influenced its rotation rate, a process known as magnetic braking. This process can gradually slow down the rotation rate of stars, including the Sun.

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 (Mercury, Venus, Earth, and Mars) 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 Late Heavy Bombardment, 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 Mercury 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, Venus 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 Earth, 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 Earth's, 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 single-layer regime, unlike the Earth's multi-layer convection, 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 Moon emerged. The prevailing explanation, known as the Giant Impact Hypothesis, suggests that the Moon formed as a result of a collision between early Earth and a Mars-sized protoplanet, often referred to as Theia. 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 Olympus Mons; 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 Valles Marineris, 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

Jupiter 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 Ceres, 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 Vesta, Pallas, and Hygiea, 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

Jupiter and Saturn, 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 Kuiper Belt, 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 Pluto. The formation of the Kuiper Belt was likely influenced by the gravitational perturbations of the giants, particularly Neptune.

The Oort Cloud 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.

Emergence of Organic Molecules

Organic molecules are defined by their carbon-based structure; carbon's unique ability to form four stable bonds with other atoms, including other carbon atoms, allows for the creation of a vast diversity of complex molecular structures. This property is the cornerstone of organic chemistry and is central to the formation of the molecules that constitute living organisms.

At the simplest level, organic molecules include hydrocarbons, which are compounds made solely of carbon and hydrogen. However, the category encompasses a much broader array of molecules that contain carbon-hydrogen bonds along with other elements, notably oxygen, nitrogen, phosphorus, and sulfur. These elements are key components of life's critical molecules, such as nucleic acids (DNA and RNA), proteins, carbohydrates, and lipids. Organic molecules are often associated with complexity and the potential for participating in biochemical processes, as they can serve as building blocks for life, and are involved in key functions such as energy storage, structural integrity, information transfer, and catalysis of biochemical reactions.

Inorganic molecules are not based on carbon-hydrogen bonds; while they can contain carbon (such as carbon dioxide, CO2), they do not exhibit the complex carbon-based structures characteristic of organic molecules. Inorganic chemistry covers a broad range of substances, including metals, salts, and minerals. In general, inorganic molecules have simpler structures; they play crucial roles in various physical and chemical processes, including those relevant to geological and atmospheric phenomena, as well as in biological systems (metal ions in enzymes and electrolytes in bodily fluids, et cetera). Despite their simplicity, inorganic molecules are essential for life and the environment. They participate in Earth's geochemical cycles, act as catalysts in industrial processes, and form the basis of many pigments and structural materials.

The distinction between organic and inorganic molecules is not always clear-cut; some compounds, like carbonates (calcium carbonate) and simple carbon oxides (carbon monoxide, CO, and carbon dioxide, CO2), are traditionally classified as inorganic despite containing carbon. The key differentiation often lies in the complexity of the molecule and its potential or actual role in biological systems. Organic molecules, including simple hydrocarbons, alcohols, and more complex compounds like amino acids, have their origins in the interstellar medium (ISM). The cold, dense clouds within the ISM are sites of complex chemistry, where ultraviolet radiation, cosmic rays, and the presence of dust grains catalyze the formation of these molecules from simpler components like molecular hydrogen (H2), carbon monoxide (CO), and ammonia (NH3).

As the Solar System formed from a collapsing molecular cloud, these organic molecules were incorporated into the protoplanetary disk surrounding the young Sun; the varying temperatures and pressures within the disk influenced the complexity and distribution of organic compounds, with ices and volatile compounds condensing in the cooler outer regions.

These early organic compounds were likely delivered to the terrestrial planets, including Earth, via comets and asteroids; the carbonaceous chondrites, a type of stony meteorite rich in organic compounds, are thought to be remnants of the early Solar System's building blocks. Their impact on the young Earth could have seeded the planet with the organic precursors necessary for life. Additionally, the synthesis of organic molecules could also occur on the early Earth's surface and its oceans, driven by energy sources like UV radiation, lightning, and hydrothermal vents. The Miller-Urey experiment famously demonstrated that amino acids could be synthesized from simple gases and an energy source, mimicking the conditions of early Earth.

Organic molecules are the chemical foundation for life as we know it. Amino acids form proteins, nucleotides form RNA and DNA, and fatty acids contribute to cell membranes. The presence and availability of these organic molecules set the stage for the emergence of life, providing the necessary components for the development of self-replicating systems.

The Late Heavy Bombardment

The Late Heavy Bombardment (LHB) is a proposed period in the Solar System's history characterized by an intense increase in the rate of asteroid and comet impacts on the inner planets. This event is thought to have occurred approximately 4.1 to 3.8 billion years ago, during the tail end of the Hadean Eon and into the early Archean Eon. The prevailing theory suggests that gravitational perturbations in the outer Solar System, caused by the migration of the gas giants (Jupiter and Saturn), destabilized distant asteroid and comet orbits. This led to a significant increase in objects being sent into the inner Solar System, where they collided with the terrestrial planets.

The Grand Tack Hypothesis and the Nice model are two prominent theories explaining these gravitational perturbations. The Grand Tack Hypothesis proposes that Jupiter migrated inward before being pulled back outward by Saturn, scattering objects toward the inner Solar System. The Nice model suggests that interactions among the gas giants, and with a primordial disk of planetesimals, led to the rearrangement of the outer planets and a subsequent influx of comets and asteroids into the inner Solar System.

Regardless of the catalyst for movement, the majority of impacts during the LHB were likely caused by asteroids dislodged from the main asteroid belt. These impacts ranged from small craters to massive basins hundreds of kilometers in diameter. Comets, originating from the Kuiper Belt and Oort Cloud, also contributed to the LHB. While less frequent, cometary impacts would have delivered significant amounts of water and organic materials to the terrestrial planets.

The Moon's heavily cratered highlands provide the most direct evidence of the LHB. Lunar samples returned by the Apollo missions have been dated to this period, supporting the hypothesis. The Moon's lack of geological activity has preserved these impact craters, serving as a record of this tumultuous time. The LHB likely played a significant role in shaping Earth's surface, though evidence of specific impacts has been largely erased by active plate tectonics and erosion. The delivery of water and organic molecules via cometary impacts may have been crucial for the development of life. Mars shows evidence of massive impact basins dating back to the LHB period, such as the Hellas and Argyre basins. Mars' thinner atmosphere compared to Earth's would have offered less protection against impacts, leading to significant surface modification. Like the Moon, Mercury's surface preserves a record of intense cratering from this period. Its global magnetic field may have been affected by large impacts, which could have contributed to the planet's high iron content.

The high frequency of impacts during the LHB led to extensive resurfacing of the terrestrial planets, erasing much of the earlier geological record. Impacts would have had significant effects on the planets' atmospheres, potentially stripping away or adding gases. On Earth, this could have influenced the early climate and the conditions necessary for life to emerge. The LHB is thought to have played a crucial role in delivering water and organic molecules to the inner planets, particularly Earth, potentially seeding the conditions necessary for life.

Some theories suggest that the LHB was a consequence of the Moon's formation, with the destabilization of asteroid orbits resulting from gravitational effects of the newly formed Earth-Moon system.

Duration and Timing

The LHB is believed to have lasted approximately 300 million years, starting around 4.1 billion years ago and tapering off by about 3.8 billion years ago. This period corresponds with the end of planetary formation and the beginning of a more stable phase in Solar System history. The Late Heavy Bombardment represents a formative period in the Solar System's development, profoundly affecting the terrestrial planets' geological and potentially biological evolution. The exact causes, timing, and effects of the LHB continue to be subjects of active research and debate within the planetary science community.