The Hadean is the beginning of the history of the Earth, and is the period in which the Earth formed. It is not officially recognized by the International Commission on Stratigraphy, since no actual strata from this period now exist.
The Hadean Eon is divided informally into 4 periods: Cryptic, Basin Groups, Nectarian and Early Imbrian. Since none of Earth's early surface has survived, these divisions are based primarily on the geology of the moon. The absence of wind and water on the moon, as well as significantly lower volcanic activity, have preserved details on the moon that have been lost on Earth.
The Cryptic, or "hidden" period is the earliest part of Earth's history, roughly 4.56 - 4.50 billion years ago. This period is called "cryptic" because even the moon hasn't preserved details from this time (in fact, the moon was probably formed during the Cryptic). Our knowledge of this period comes from the study of meteorites, as well as physical models and calculations.
Artist's Impression of Accretion Disk (NASA)
The Earth was formed from the solar nebula about 4.56 billion years ago. Matter aggregated under the force of gravity into clumps, which grew in size as they attracted more material. This was a relatively rapid process. We have no direct knowledge of how long this took, but computer models seem to indicate that the process of planet formation happens very quickly, possibly within a couple of million years. This agrees with the oldest basaltic meteorites ever found, which can be dated to within a couple of million years of the formation of the solar system. Basalt only forms under great pressure and temperature, indicating that planetesimals of sufficient size to generate such temperatures and pressures at their cores existed very early.
The Earth was initially molten, due to the violence of its formation. Heat was generated by the radioactive decay of uranium, thorium, potassium, etc., and also by gravitational compression and meteorite impacts. As its temperature increased and crossed the melting point of iron (about 1500 °C), the iron and other heavier elements sank towards the center of the earth. This is known as the "iron catastrophe", and it was at least partly responsible for the formation of Earth's iron-rich core. The "falling" of the heavier elements to the core released a lot of gravitational potential energy in the form of heat, which probably raised the temperature of the Earth by another couple of thousand degrees Centigrade.
It is believed that in this same period (within the first 40 million years), a large planetesimal, about the size of Mars (sometimes called Theia), collided with the Earth. The Earth itself gained about 10% of its mass from the impactor, but much of the material from Theia was sheared off and went into orbit around the Earth. Over the next 10-15 million years, this material slowly aggregated to form the moon. Computer models show that this could well have been the case, and agree with the physical evidence so far.
The moon was formed much closer to the Earth (about 16,000 km) than it is now, and must have appeared much larger in the sky. Such large collisions were rare, even this early in the Earth's history. But other planets also show signs of large collisions. Mercury's crust has been stripped away, leaving mostly just the core. Venus rotates in the opposite direction to all the other planets. These things probably happened due to large impactors.
Radiometric dating has been used to date these early events. The dates do not always agree, and are somewhat controversial. For example, the hafnium-tungsten clock dates the "iron catastrophe" that led to the formation of the Earth's core to about 30 million years after the formation of the Earth. However, uranium-lead dating produces a date of about 80 million years after the formation of the Earth. It's possible that a second core-forming event happened due to the Theia impact, which reset the uranium-lead clock.
At any rate, about 60-80 million years after its formation, at the end of the Cryptic, the Earth was nearly complete. It had achieved about 99% of its current mass, it possessed a moon, and had a core rich in iron and nickel. Water was present, in the form of vapor, from the outgassing of rock, as well as from comets that impacted the Earth.
This is a long (550 million years), murky period in the earth's history, from about 4.50 - 3.80 billion years ago. Again, due to the lack of rocks dating back to this period on earth, we are forced to rely on lunar geology to extrapolate what was happening on earth. Unfortunately, the very early history of the moon is also not very clear.
Two sources of information exist. Even before the Apollo missions, astronomers had started dating features on the moon based on the superposition of craters (features superimposed on other features were newer). Secondly, the Apollo missions brought back moon rocks, which were radiometrically dated.
Although fragments of rock brought back from the moon have been dated back as far as the initial formation of the moon (4.5 billion years), they cannot be correlated with certainty with any physical features on the moon. The oldest rocks on the moon are typically found on the lunar highlands. The high frequency of early impact events probably melted and re-melted the lunar crust, obliterating the earliest features. Some time between 4.4 and 4.2 billion years ago, the lunar crust differentiated and stabilized, preserving features from around 4.2 billion years and on. This earliest part of the moon's history (4.5 b - 4.2 b) is therefore the moon's "cryptic period" and all we can tell about it is that impacts were frequent, the surface was either not solid, or frequently liquified by impact events, but it left older rocks scattered throughout the surface of the moon. Unlike the earth, erosion played no part on the moon, and many of these rocks have survived.
The earliest features on the moon date to a period after the stabilization of the crust. They are often divided into "basin groups" - related features that formed near the same time.
The oldest features are included in Basin Group 1, namely Mare Procellarum (Ocean of Storms) and South Pole-Aitken, which date back to about 4.15 and 4.10 billion years, respectively. Following this, the moon's surface records impacts in great profusion. Basin Groups 2 through 9 consist of 28 basins and about 3400 craters (larger than 30 km across), which were formed between 4.1 and 3.92 billion years ago.
This period is somewhat arbitrarily cut off at 3.95 billion years by the Nectaris event, which is described later. The Nectaris event (and the Imbrian, which followed) are collectively known as the lunar cataclysm, or late heavy bombardment (LHB).
So what was happening to the earth during this period? From looking at the moon, we know that impacts on earth must have continued with a high frequency. This is also the period from which the oldest rocks and zircons on earth have been found. The oldest rocks from Isua (Greenland) have problematic dating, but lead isotope ratios show that crust subduction was under way. This indicates that earth may have had a proto-crust as early as 4.3 billion years ago. This proto-crust was nothing like today's crust. It was basaltic, rather than granitic, and the basalt was unlike those we see today, perhaps representing an earlier stage before the distillation of lighter elements towards the surface.
The early atmosphere formed within the first 100 million years of earth's history. High temperatures lead to outgassing from rocks, and combined with volcanism to produce an atmosphere high in methane, with hydrogen, nitrogen, water vapor and smaller amounts of carbon dioxide and noble gases making up the remainder. The atmosphere was very dense; surface pressure being around 250 atmospheres. The hydrogen gradually leaked into space. Water vapor did not condense, as it was still too hot at this time.
At some point the earth cooled enough for liquid water to exist on the surface. We do not know precisely when this happened. It was originally thought that no water existed before the late heavy bombardment in the Nectarian, or if it did, the proto-oceans and seas were evaporated by the LHB and had to start all over again. But newer evidence is accumulating that surface water existed before LHB and persisted through it. That is, the late cataclysm was not as cataclysmic as previously thought. There is some evidence that life may have originated before LHB and survived through it.
In particular, zircons from West Australia indicate that liquid water and proto-continents may have existed as early as 4.3 to 4.4 billion years ago. Rocks that have been exposed to liquid water typically have a higher O-18/O-16 ratio than rocks that have not been so exposed. Oxygen isotope ratios in the oldest zircons indicate that they were exposed to liquid water. If liquid water existed, it may well have collected in low lying areas, so it is possible that oceans existed as early as 4.4 billion years ago. Water is also necessary for the formation of continental crusts, and specially lighter granitic rocks. Zircons are typically formed in granite, which suggests that proto-continents also existed 4.3-4.4 billion years ago.
Since no rocks of such an early age have been found, the theory is that these early rocks and continents were destroyed during the LHB. At least locally, the impacts must have evaporated any oceans that existed at this time, but it is possible that liquid water persisted somewhere on the earth's surface throughout the LHB. The presence of water gives a strong boost to the idea that abiogenesis may have also ocurred very early in the earth's history, and that life may have survived the LHB.
To summarize, during the Basin Groups period the earth continued cooling. The primordial atmosphere built up during the first 100 million years. The proto-crust was formed; there are even indications of recognizable subduction and plate tectonic events. This proto-crust was very different from the crust today, being heavier and therefore unable to "float" over the magma. Consequently, it was re-melted and recycled often and has not survived. The first liquid water appeared possibly as early as 4.4 billion years ago, and life originated soon after.
This brief era of the Hadean is named for Mare Nectaris (Sea of Nectar), a basin on the south-west lunar nearside. It lasted approximately from 3.95 billion to 3.85 billion years ago. This period marks the beginning of the phase known as "Late Heavy Bombardment", which continued past the Nectarian into the Imbrian.
Some scientists believe that the heavy outer planets were settling into their orbits around this time. In particular, computer models suggest that when Jupiter and Saturn reached their 1:2 orbital resonance, the solar system rapidly reconfigured itself for the wide Jovian system we have today. Resonances between the gas giants would sweep both inwards towards the asteroid belt, and outwards to trans-Neptunian objects, such as those in the Kuiper belt. This would affect the trajectories of a large number of objects from the asteroid and Kuiper belts, sending some of them inwards towards the inner solar system.
Another theory is that a hypothetical 5th planet between Mars and the asteroid belt became unstable and started sending asteroids towards the inner planets. This theory is based on computer simulations, which show that a planet sized body in orbit past Mars (but within the asteroid belt) could start off in a relatively circular orbit, but might be pushed into a highly eccentric orbit due to gravitational interactions with the inner planets. This would send it through the inner asteroid belt, changing the orbits of several asteroids into earth-crossing trajectories. This hypothetical planet would have been destroyed later by falling into the sun.
At any rate, a number of very large objects started impacting within the inner solar system at this time. Craters on Mercury show that it was also subjected to impacts during LHB. The Earth and Venus must also have been hit many times, but geological activity and weathering have erased all features from this time.
On the moon, the Nectaris Basin was created by perhaps a dozen impactors, all within an area only 860 km wide. The Nectaris Basin marks the beginning of the period of LHB. There are about 9 basins on the moon from the Nectarian, although some may slightly overlap the early Imbrian. Of these 9, 4 are visible on the nearside: Nectaris, Humboldtianum, Crisium and Serenitatis.
Because of the uncertainty in moon geology and the absence of significant craters earlier than the Nectarian, many texts simply list the entire history of the moon before Nectaris as "pre-Nectarian".
The Imbrian is named after Mare Imbrium, the 3rd largest basin on the moon after Procellarum and South Pole-Aitken. It lasted from approximately 3.85 billion to 3.80 billion years ago, and with it ended the Hadean eon of Earth's history.
The Imbrian has two distinct phases. The first, or Early Imbrian consists of two large craters, Imbrium and Orientale, which formed within 50 million years of each other. These are huge craters, with ejecta reaching 1000+ km from the crater rim, forming secondary craters with their own ejecta.
The period of LHB ends with the Early Imbrian, although the Imbrian Epoch on the moon continues for another 600 million years (3.8 - 3.2 billion years) with the Late Imbrian. However, impacts within this and subsequent periods were never again as heavy as during LHB, and rocks and stratigraphy from 3.8 billion years on (the Archean) survive in earth's own crust, removing the need to rely primarily on lunar geology to extrapolate events on earth.
For the sake of completeness, subsequent geological periods on the moon include the Erastothenian, which shows basalts and lava flows (2.1 - 1.1 billion years ago) and the Copernican (1.1 billion years ago - present), which shows mostly just craters, and minimal basalts.
Mare Imbrium, in the northeast quadrant of nearside, is about 1160 km across (South Pole-Aitken on the farside is more than twice as big).
To get some idea of the scale of these events, here are some basin sizes for Necatarian and Early Imbrian events (all sizes in km): Imbrium: 1160, Orientale: 930, Serenitatis: 920, Nectaris: 860, Crisium: 740, Humboldtianum: 650, Herzsprung: 570, Korolev: 440. For comparison, the pre-Nectarian South Pole-Aitken on farside is 2600 km. The "dino-killer" impact crater at Chicxulub, off the coast Yucatan, is 180 km. These were some truly massive impacts.
So what can we infer about earth during this period? Being more massive than the moon, the earth probably attracted many more impactors than the moon during the same period. One estimate is that there would have been several impact basins with a diameter of 5000 km, about 40 impact basins with diameters of around 1000 km, and around 22,000 impact craters with diamters exceeding 20 km. On average, an impact causing serious environmental damage would have ocurred every 100 years.
What could have survived this period of LHB? We know rocks didn't, at least, we haven't found any rocks so far that did. But various models show that liquid water, protocrusts, rocks, continental plates, and possibly even life did exist before the LHB. The earth cooled sufficiently for liquid water to exist on the surface, as early as 100 - 200 million years after the formation of the earth.
The traditional, and still widely held view is that LHB destroyed most of whatever existed on the surface of the earth, and everything started over again, perhaps several times. However, there are some lines of evidence that indicate that LHB might not have been as catastrophic as previously thought, at least locally.
Radiometric dating shows that the uranium-lead clock doesn't seem to have been "reset" frequently, as might be expected due to repeated heavy impacts. Some of the oldest zircons show growth layers that reflect significant accretion and buildup during LHB, but not temperatures high enough to destroy the zircons.
Newer evidence may change our understanding, but it does seem that if life originated in deep marine environments and migrated to deep subsea rock, it may well have survived LHB.
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