Understanding the conection between the Late Heavy Bombardment and the Emergance of Life.

The first RNA-based life is thought to have emerged during the Late Hadean Eon, a time when the Earth had largely accreted and differentiated into the core, mantle, and athenosphere, but was still subjected to frequent large impacts by projectiles 200-1000 km in diameter and potentially impactors as large as 3000 km in diameter. This presents problems for our understanding of the emergence of life, as such large impacts could potentially have steralised the surface of the Earth. However, it is also quite likely that these impacts played a key role in the development of life, by altering the composition of the atmosphere. The atmosphere of the Earth during the Late Hadean is thought to have been weakly reducing, comprised largely of carbon dioxide, nitrogen, and water, under which conditions RNA is unlikely to form naturally. However, an impact by a large asteroid could potentially change this, delivering substantial amounts of free iron into the atmosphere, and causing the formation of a strongly reducing atmosphere, rich in hydrogen and methane, under which conditions RNA forms readily.

Understanding these events has proven to be difficult. Early attempts at modelling Hadean impacts assumed that 25-50% of the projectile kinetic energy was partitioned towards evaporating a pre-existing ocean, and that all of the projectile iron is available to reduce pre-existing surface water, but it is far from clear how accurate this is, nor how an impact by a bolide 200-3000 km in diameter would actually affect the habitability of the Earth.

In a paper published on the arXiv database at Cornell University on 23 January 2022, and submitted to the Planetary Science Journal, Robert Citron and Sarah Stewart of the Department of Earth and Planetary Science at the University of California Davis, present the results of a study in which they modelled a set of 3D simulations of large impacts on the early Earth in order to better understand what types of impacts would either sterilize the early Earth or provide sufficient iron to sustain a post-impact reducing atmosphere.

Citron and Stewart modelled the potentiall effects of bolides between 1500 and 3400 km in diameter impacting the Earth at a range of velocities and angles, in order to estimate the degree of surface melting, and ocean vaporisation they would cause, as well as the distribution of iron they could deliver. They attempt to take into account the state of the behavior of the iron core and forsterite mantle during such impacts, and how this would affect the degree of vaporisation during a giant impact event, and therefore the subsequent composition of the post-impact atmosphere. 

They estimate that sterilising impacts would need to be larger than previously calculated, but nevertheless that they are likely to have happened several times during the late accretion phase. Previous estimates have suggested that a Ceres-sized object (473 km in diameter) would be capable of vaporising several ocean-masses of water (although this might not be completely sterilising), while a Moon-sized object (1737 km in diameter) would completely melt the Earth's surface. Furthermore, only the very largest impacts (of objects around 3400 km in diameter, would deliver enough iron to the Earth's surface to produce a strongly reducing atmosphere, at which size it is unclear how much of the iron would be absorbed by the resultant melt, making it unavailable for subsequent reactions with water. Thus, sterilising impacts are likely to have been common during the late accretion, but forming a strongly reducing atmosphere through impact-delivered iron may require additional mechanisms to prevent that iron being sequestered in the upper mantle.

 

Habitability of Earth in the aftermath of late accretion impacts depends strongly on the post-impact melt and iron distribution, and the energy deposited into the post-impact atmosphere. (a) In a nominal oblique impact a portion of the mantle is melted and ejecta heating also occurs downrange of the impact. The post-impact Earth has an ambiguous surface boundary of supercritical  fluid, and the atmosphere consists of volatiles and a mix of vaporized silicate and projectile iron. Projectile iron is deposited in the melt/mantle/atmosphere, and some fraction also escapes the system with the impact ejecta. (b) In a head-on impact the projectile iron penetrates deep into the mantle, potentially reaching the core for larger impacts, and little material escapes the system. (c) In hit-and-run impacts, less melt is generated by the impact and a significant fraction of the projectile escapes the system. Citron & Stewart (2022).

The Earth's core is believed to have formed during the impact that formed the Moon, removing a large proportion of the Earth's iron and highly siderophile elements (i.e. elements which will completely dissolve in an iron melt) from further interaction with the Earth's outer layers. Based upon the observed content of iron and highly siderophile elements in the Earth's mantle, it is assumed that about 0.5-1% of the Earth's mass (rock and iron) was accumulated after this impact, material sometimes known as the Late Veneer. This provides a minimum mass for the total material deposited onto the Earth during the late accretion, which is equivalent to a single impactor with a diameter of between 2300 and 2900 km, although if some of the material delivered is assumed to have entered the core through extensive melting, then the total deposited could be two to five times as large. If it is assumed that the material impacting the Earth came from objects with a similar size distribution to that found in the Inner Asteroid Belt, then the majority of this material is likely to have been delivered by a single large object, with a diameter in excess of 2500 km. This would also explain the greater proportion of iron seen in the Earth's upper layers than their equivalents on the Moon, as material from a small number of large impactors would be preferentially deposited on the Earth, due to its larger gravitational cross-section.

There are currently two rival models of impact events during the late accretion. The Late Heavy Bombardment suggests a spike in impact activity about 3.9 billion years ago, probably caused by a dynamical instability in the orbits of the giant planets, which scattered trans-Neptunian objects, asteroids, and leftover planetesimals into the Inner Solar System, producing the pattern of impact basins seen on the Moon and other bodies. The alternative tail-accretion scenario suggests that the level of bombardments declined steadily as the Solar System aged and the available material in the Inner Solar System was used up. Both these models could potentially deliver a similar amount of material to the Earth, and altered conditions on the planet by either sterilising its surface or donating enough iron to produce a strongly-reducing atmosphere, but the timing of the impacts is somewhat different.

Logically, the emergence of life on Earth, or at least that life from which modern organisms are descended, must have happened after the last sterilising impact. Since it is reasonable to assume that no subsequent impacts were larger than this event, it is logical to calculate the size of the minimum event that could have sterilised the Earth's surface. If early life was dependent on the presence of a liquid ocean, then the minimum size of a sterilising impact can be assumed to be equivalent to the minimum size of an impactor which could vaporise the Earth's oceans. Alternatively, if life was restricted to the surface layers of such an ocean, then an impact capable of vaporising the upper 200 m of the ocean would be enough to sterilise the Earth's surface. If, however, early life was present in subsurface environments, then vaporising the oceans might not be sufficient to sterilise the planet, particularly since large impacts tend to produce large shockwaves, which could potentially bury organisms deep enough to protect them from any transient atmospheric superheating. Under such a scenario it might be necessary to completely melt the Earth's surface to stelrilise the planet, or, more conservatively, raise it's temperature above the limit for hyperthermophilic organisms (80-110°C). It is unclear if the Earth's surface has been completely melted since the Moon-forming impact, and therefore conceivably possible that the Earth's surface might have been continuously habitable since this event.

The minimum size of an impact needed to vaporise the oceans is also unclear. While a large enough impact will vaporise any water in the region where it impacts, vaporising a global ocean would probably require the planet to be enveloped in a cloud of impact-generated silicate vapour, radiating heat down at the surface of at least 1500 K (1227°C). It has been suggested that the complete vaporisation of an ocean with the same volume as that of the modern Earth could be achieved by an object 350-440 km in diameter, if it is assumed that 25-50% of the impact kinetic energy is directed towards vaporising surface water; but this hypothesis has not been subjected to vigorous mathematical modelling.

Similarly, the size of an impactor needed to melt the Earth's surface is far from clear. Most models of the outcome of large impact events are based upon scaling up the effects of smaller events, but such scaling up cannot really account for the extra volumes of melt generated by impactor in excess of 100 km in diameter. Furthermore, melt generated by large impacts tends to remain within the impact basin, limiting its global impact. It has been suggested that impacts by objects in excess of 100 km might result in a layer of melt 3 km thick covering an area 20-30 times the diameter of the object. If this is realistic, then global surface melting would require an object 1300-2000 km in diameter. However, global melting could potentially be achieved by the deposition of ejecta from an impact, with some models suggesting an impactor as small as 500 km in diameter could result in the Earth's surface being covered by a layer of molten ejecta 200 m thick; however this has only been tested in two dimensional model, and it is not clear if it would actually be enough to cover the whole Earth's surface, or whether areas of unmelted crust might remain. Heating the entire surface of the Earth to above 80-110°C would clearly require a smaller body than completely resurfacing the planet, with some models suggesting that a body as small as 300 km might be able to achieve this, although the models involved relied on grid sizes of 50-100 km, leaving plenty of space for patches of surface to remain habitable.

As well as sterilising the surface of the Earth, large impacts during the late accretion could potentially have donated enough iron to the Earth's surface to temporarily create a strongly reducing atmosphere, suitable for the abiological formation of RNA, which in turn is hypothesized to have predated the modern DNA-based biology. This would have required the formation of a large number of RNA precursor compounds, most of which would form readily under strongly reducing conditions, but which are less likely to have formed in the weekly reducing atmosphere predicted for Late Hadean Earth. This weekly reducing atmosphere is thought to have formed rapidly after the Moon-creating impact led to the differentiation of the core and mantle, removing most of the iron from the mantle, which became oxidized to a state near the fayalite-magnetite-quartz buffer, but thus could have potentially been altered by the delivery of a large quantity of new iron to the system from an extra-terrestrial source.

Such a reducing atmosphere can occur through the reaction of iron with water, forming iron oxides and free hydrogen (a highly reducing gas). An small planetary body with a core mass fraction of 0.3 (roughly the same as that of the Earth), and a mass of approximately 0.5% of that of the Earth, would produce about 10 000 000 000 000 000 kilotons of iron, enough to reduce 2.3 ocean masses of water, generating a hydrogen atmosphere with a pressure of several bars. This hydrogen would probably react quickly with carbon to form methane, another reducing gas, and creating conditions ideal for the formation of RNA precursors. 

Previous reconstructions of large late accretion impacts have suggested that if a differentiated body 3000 km wide impacted the Earth at an angle of 45°, then 60% of the mass of the core of that body should be available to react with material present on Earth. However, these studies have not taken into account whether this material would be available to react at the Earth's surface or be sequestered into the mantle.

Citron and Stewart's model simulated impactors with masses equivalent to 0.0012, 0.003, 0.006, and 0.012 times that of the Earth, which would correspond to objects with diameters of 1500, 2000, 2700, and 3400 km, or one tenth, one quarter, one half, and the entire mass of the Moon, impacting at velocities of 1.1, 1.5 and 2 times the Earth's escape velocity (which is 11.2 km per second), impacting at angles of 0°, 30°, 45°, and 60°, where 0 corresponds to a head-on impact.

These simulations suggested that when objects impacted the Earth at an oblique angle, this resulted in the formation of a blanket of ejecta in the form of silicate vapour that encircled the Earth. The material from the core was in all cases quickly absorbed into the Earth's core, until the model was adjusted to take into account structural strength of both bodies. This resulted in the majority of the iron being retained in a melt pool at the impact site; Citron and Stewart suspect that much of this material would subsequently find its way to the core, but this was beyond the scope of the model.

 

Snapshots of a simulation of a 0.006 Earth mass projectile impacting a Earth target at 1.5 times the Earth's escape velocity, at an angle of 45°. Colours track the projectile and target core and mantle materials. Axis units are in present-day Earth radii (6371 km). Citron & Stewart (2022).

For each simulation, Citron and Stewart quantified the potential of the impact to sterilize the early Earth either through vaporization of a pre-existing ocean or globally melting the surface. In most cases the majority of the objects kinetic energy was transferred to the Earth's surface or interior, however in the case of the most high-velocity or oblique impacts, much of this energy escapes the system as unbound ejecta.

For the sake of the model, Citron and Stewart assumed that the vaporisation of any ocean would be caused by thermal radiation caused by the impact, although they acknowledge that this is unlikely to be the case with impacts large enough to melt the Earth's entire surface. Therefore, in order to calculate the ability of an impactor to vaporise all water on the Earth's surface, Citron and Stewart look at that body's ability to create a world-enveloping cloud of hot, silicate vapour. 

Citron and Stewart calculate that 1-21% of the projectile kinetic energy is directed into the impact as thermal energy. This is lower than previous estimates, and does not take into account the heating caused by hot iron ejecta on the planet's surface. They further estimate that an atmosphere heated to 2300K (2027°C) would cool within 1 to 100 years due to simple radiative cooling.

All of the modelled impacts produced enough energy to vaporise an ocean with equivalent mass to that of the modern Earth, although this did not take into account energy radiated back into space, which would reduce the energy available for ocean vaporisation. Citron and Stewart estimate that about half of the energy entering the atmosphere following a major impact would be radiated back into space, although the extent of this would depend on the composition and conditions within the atmosphere; cooler initial atmospheres would be likely to radiate more energy out into space following an impact. They also note that factors other than direct heating would contribute to ocean heating, such as the energy contained within silica drops precipitating out of the atmosphere and falling to the surface. 

The majority of the modelled impacts produced layer of melt 100s of meters to 100s of kilometres thick, which totally covered the Earth, and which therefore would presumably have sterilised its surface of all life. Only the smallest impacts, with masses equivalent to 0.003 times that of the Earth, produced only localised melting, with more oblique impacts producing additional melting downrange of the impact site. Despite this, Citron and Stewart calculate that only the very largest impactors could reliably sterilise the whole planet, and then only if they impacted it at an acute angle, more oblique impacts are less reliable, with even the largest impactors failing to sterilise the whole planet if they hit at an angle of 60°.

 

Distribution of melt thickness 24 hrs post-impact for several example simulations. In the plotted coordinates the impact direction is east along the equator. The longitude of the impact point 24 hrs post impact varies based on the planetary rotation induced by the impact. Citron & Stewart (2022).

For each simulation Citron and Stewart also calculated the proportions of iron that were deposited into the interior of the planet, deposited on the surface of the planet, remained in the atmosphere, or were ejected into space. In the case of the most acute impacts, the object was burrowed deep into the planets mantle, and were quickly covered over by the mantle material, preventing any further interact with the iron from the impactor then unavailable for further reactions. At oblique angles, much of the material is lost into space as ejecta, but more is also deposited on the surface, with the amount deposited at the surface tending to increase with larger or faster objects.

The ability of any impact to produce a reducing atmosphere depends on the amount of iron deposited in places where it can interact with large volumes of water. It has been suggested that an ocean 1-3 times the mass of the current oceans would need to be completely reduced in order to generate an atmosphere rich enough in hydrogen and methane for the RNA precursors needed to kick-start the RNA world to form. It is unlikely that all of the iron derived from any impactor would become available for reducing such an ocean. Iron which penetrated the mantle would quickly be covered-over by other material, becoming unavailable for reactions with water, and it is likely that a high proportion of material deposited at the surface would be covered over by other ejecta. In fact Citron and Stewart's simulations suggest that much of the iron deposited on the surface following an impact would be deposited in highly melted areas, where it would be easy for it to sink down to deeper layers within the Earth. Such iron might still have an impact on the Earth's atmosphere over a geological timescale, releasing some reducing gasses from ground sources for the next 10-100 million years, which might help sustain a reducing atmosphere once formed, but would be unlikely to result in one forming. 

For these reasons, Citron and Stewart suggest that the major reserve of iron for the development of a reducing atmosphere would be iron which is vapourised during any impact, and retained, at least temporarily, within the atmosphere. A post impact atmosphere would be expected to contain a significant amount of vapourised iron, rock, and water. In such an environment iron should react readily with water, either combining to form iron oxides while in a vaporous state, or precipitating out to form droplets which would react with water either while still in the atmosphere or after falling to Earth. The total mass of the iron vapourised into the atmosphere by the impact will provide an upper limit on the amount of iron available to reduce water. Thus, while larger impacts would completely melt the Earth's surface and probably lock the majority of their iron away more rapidly and thoroughly, they are also likely to contribute more iron to the atmosphere, and create a hotter post-impact atmosphere, where vapourised iron might remain for longer.

In Citron and Stewart's simulations, only large impacts at an angle of 45° delivered enough iron into the atmosphere to reduce an ocean-sized body of water. A 0.003 Earth mass object delivered a maximum of enough iron to reduce half an ocean. Most impacts delivered mor iron to the atmosphere than to the surface of the planet, with the exception being head-on or highly incident collisions (0°-30° from perpendicular). All effects taken into acount, the maximum amount of iron any impact could be expected to deliver into the atmosphere would be enough to reduce 1.5 ocean-masses of water.

Citron and Stewart's simulations suggest that large impacts during Late Accretion would have been a highly inefficient way to deliver iron to Earth-systems where it could act as a reducing agent. In most cases the majority of the iron from an impact was buried deep into the crust or mantle, from where larger volumes of iron could potentially sink further, into the core, with much of any iron not sequestered deep within the Earth being ejected into space. They further predict that a large proportion of any iron entering the atmosphere will precipitate out rapidly, falling to the surface while that is still molten, and subsequently being lost from the system. 

This presents serious problems for the idea that a large impactor might have completely reduced the Late Hadean Earth's oceans to form a strongly reducing atmosphere, thus facilitating the development of RNA. It has been calculated that almost all the iron from an impactor 2500-3000 km in diameter would be needed to reduce an ocean double the size of the Earth's current ocean (which is predicted for the Late Hadean Earth, as the warmer mantle would lack the retention ability of the modern ocean), but Citron and Stewart's calculations show that even for the very largest impacts, most of this iron would be locked away rapidly rather than becoming available for reactions.

This does not completely rule out this scenario as a possible precursor to the appearance of RNA. It is possible that the Earth could have been impacted by an object significantly larger than is predicted by the observed excess of mantle highly siderophile elements, or by a smaller object with a much larger iron core than we would typically expect.

Alternatively, if large amounts of iron were deposited into the crust or upper mantle, but remained in contact with an impact-generated melt pool, then it is possible that this could equilibrate with the atmosphere, allowing this source of iron to contribute to the reduction of atmospheric gasses. This would require the iron in the melt to remain distributed throughout the melt, rather than settling out at the base, something which might happen if the iron was broken down into droplets circulating in an emulsified melt.

Even if iron was trapped within the crust beneath a a surface fusion crust, or layer of non-ferrous melt, it could potentially react with water in the crust, forming iron oxide and releasing hydrogen, which could be released into the atmosphere either as gasseous hydrogen, or methane if it reacts with a source of carbon within the crust.

It is also possible that iron ejected from the atmosphere during an impact could play a role in reducing the atmosphere, if it formed a debris ring which fell back to Earth over time. It has been estimated that 20-30% of the material which was ejected from the atmosphere during a large impact would fall enter such a debris ring, falling back to Earth over the next few tens of millions of years. Citron and Stewart's model suggests that with an impactor hitting the Earth at an angle of 60° from the perpendicular, almost all of the iron would be ejected from the atmosphere. If 20% of this later fell back to Earth, then this would be equivalent to the amount of iron delivered to the atmosphere by a similar sized object impacting the Earth at 45°. Furthermore, because this iron would be falling back to Earth in smaller pieces, it would be far less likely to bury itself in the ground and become unavailable for reaction with the atmosphere, rather many of these objects might 'burn up' in the atmosphere, vaporising before they impact the ground.

An alternative hypothesis is that the Earth's atmosphere was repeatedly transiently reduced by much smaller (100 000 000 000 000-100 000 000 000 000 000 kiloton) impactors. Such impacts would only produce very transient reducing atmospheres, but this might still allow for the formation of nucleobase precursors and ammonia, before returning to equilibrium with the fayalite-magnetite-quartz mantle, where these compounds might further react with compounds present in such an atmosphere and produced by volcanic eruptions. Potentially, an atmosphere which swung back-and-forth between two redox states might have been more beneficial for the emergence of life, as this allows for a greater range of chemical reactions.

If the ancestor of modern life did originate in a post-impact strongly reducing environment in the Late Hadean, then the Earth must have avoided any subsequent sterilising impact event. This is difficult to explain, as Citron and Stewart's model suggests that the size of an impactor needed to produce a strongly reducing atmosphere is considerably larger than the size of an impactor needed to sterilise the Earth. Thus, sterilising impacts should have been more common than impacts producing strongly reducing atmospheres, and if, as seems likely, the scale of impacts reduced slowly over time as larger planetary bodies in the Inner Solar System were swept up, impacts capable of sterilising the Earth should have persisted after impacts with the potential to produce a reducing atmosphere had ceased.

However, Citron and Stewart's model does suggest that complete sterilisation does require a larger body than previously assumed, with a body of 2000 km being the minimum with the potential to melt the entire surface of the planet, and a body of about 2700 km needed to reliably do this. Acheiving the lesser objective of vaporising the oceans, which might sterilise the Earth, would require a body of at least 740 km according to Citron and Stewart's model, whereas previous estimates have assumed a body of 350-450 km could do this. However, Citron and Stewart do note that vaporising only a portion of the ocean could potentially turn the remaining water into a hot, dense, brine, which might be uninhabitable to early organisms, thereby achieving sterilisation. 

If an object at the upper end of this range is needed to completely sterilise the surface of the Earth, then this might leave room for an organism evolving in a post-impact reducing atmosphere to survive and leave ancestors still alive today. It is quite plausible that an impact by a body more than 3000 km in diameter could have been followed only by impacts smaller than 2000 km.

Citron and Stewart's model suggests that Late Hadean impacts could have been less likely to sterilise the Earth's surface than previously thought, but also that such impacts are likely to have delivered far less available iron than previously thought. Thus, if life did require a strongly reducing atmosphere in order to originate, then this would probably have required an object of at least 3400 km in diameter to impact the Earth. After such an origin, life would have needed to survive without any subsequent impacts sterilising the Earth, which also might have required a considerably larger impact than previously thought. 

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Understanding the influence of large bolide impacts on the Earth's carbon cycle and climate.

Many scientists suggest that the Earth has recently transitioned into a new period, the provisionally termed 'Anthropocene', this is defined to reflect the planet-wide effects of human activity. The Anthropocene could be described as a gigantic combustion experiment in which reduced, energy rich forms of carbon (e.g., coal, oil, gas, wood) are oxidised to CO₂, with additional significant atmospheric emissions from industrial and land-use activity. The cumulative atmospheric CO₂ release since 1750 AD is about 2000 gigatons. For comparison, the bolide strike that formed the Chicxulub structure in Mexico about 66 million years ago released between 425 and 1400 gigatons of CO₂. Thus, some large bolide impacts are comparable to the Anthropocene effect in terms of the rapid disruption of the carbon cycle and the potential for exceeding the currently unknown critical degree of perturbation. However, during the most intense bombardment period on the early Earth, the surface was poor in the carbon and sulphur rich sediments that exert the greatest control over climate perturbation. Time, therefore, provides a natural narrative for a review of environmental consequences.

In a paper published in the journal Elements on 1 October 2019, Balz Kamber of the School of Earth, Environmental and Biological Sciences at the Queensland University of Technology, and Joseph Petrus of the School of Earth Sciences at the University of Melbourne, and the Harquail School of Earth Sciences at Laurentian University, examine the relationship between bolide impacts, atmospheric carbon fluctuations, and the climate, as preserved in the Earth's rock record.

Large bolide impact events have become rare. At most, there is one strike of one roughly 10 km object every 100–200 million years. But the rich history of former bombardment is evident on the surfaces of inner solar system bodies, as well as from the few preserved impact features on Earth itself. Impact basins more than 1000 km across exist on our planetary neighbours and they are pockmarked with thousands of smaller impact features. Counting crater numbers and measuring the sizes of craters on images of the Moon’s surface qualitatively shows that most of the very large basins formed early in the history of the solar system. Dates for lunar samples constrain the bulk of the bombardment to have occurred within the first roughly 700 million years since planet formation. Importantly, however, significant events capable of producing basins hundreds of kilometres in diameter also happened in more recent history. It is instructive to study how these could have disrupted the complex interplay between biology and geology on Earth; that is, how they have affected the global biogeochemical carbon cycle. 

The Earth’s geological history is subdivided into eons, eras and periods. For the older eras, these subdivisions were defined with the appearance or disappearance of dominant rock types, whereas most boundaries of the younger eras and periods coincide with the rapid disappearance of organisms (mass extinctions).

Timeline of significant geologic events relative to geological age, subdivided into the four, colour-coded, geological eons (Hadean, Archaean, Proterozoic, Phanerozoic). The eons are subdivided into the geologic eras; the boundaries of the Proterozoic periods are shown but not named. (A) The cumulative age-distribution of bolides capable of creating basin sized impact structures on Earth (number as y-axis). (B) The ages of the six largest known terrestrial impact events (as solid lines during the Proterozoic and Phanerozoic) and the most prominent spherule layers (as broken lines, all during the Archaean). The Chicxulub (Mexico) impact is shown in red. (C) A summary of key events and preserved pieces of evidence relevant to planetary evolution. The End Cretaceous extinction event is shown as a red vertical bar. (D) Global data compilation of the carbon isotope compositions (δ13C as y-axis) of carbonate (blue symbols) and organic carbon (dark grey symbols). Kamber & Petrus (2019).

The Hadean Eon (4567 Ma to about 3850 Ma) is the oldest eon of Earth’s history, and it witnessed by far the largest number of impacts. Unfortunately, the Hadean geological record is very sparse and significant uncertainty exists about Earth’s evolution during that time. Nevertheless, three key questions are of great scientific importance because of their enduring legacy for the remainder of Earth history. They all relate to the Hadean impact history: (1) Did the Hadean bombardment deliver volatile elements, including water and carbon, to the early Earth? (2) What happened to the Earth’s vanished primordial crust? (3) Was life on Earth already established during the Hadean? 

The delivery of extraterrestrial matter to Earth happens with two dominant size classes of objects: the tiniest, and the largest. The fully formed Earth was struck by a few hundred bolides capable of causing very large (over 100 km) impact basins, and these objects contained at least one-third of the delivered extraterrestrial matter. At the other end of the spectrum, cosmic dust, having particle diameters in the micrometre range, constitutes the second significant source of extraterrestrial matter. Of the two types of impactors, comets and asteroids, comets are predominantly composed of volatile species (oxygen, carbon, hydrogen, nitrogen), whereas asteroids include undifferentiated chondritic bodies that also contain about 1.1% by weight hydrogen and about 1.8% by weight carbon, with organic molecules. Although intuitively, this suggests that the delivery of such matter to Earth during the Hadean might have contributed to the build-up of the hydrosphere and to the surficial carbon reservoir, this simple logic is complicated by impact physics.

Comparing the delivery of extraterrestrial elements of a refractory nature (i.e., with a high boiling temperature) to those of a volatile character (low boiling temperature) shows that most of the volatile cargo would be lost from cosmic dust upon its atmospheric entry, except for particles of less than 35 μm, which experience limited frictional heating. Impacts of larger bolides at velocities in excess of about 15 kilometres per second cause at least partial vapourisation of the target and intense heating to more than 10 000 K of the vapourised material, and this leads to the formation of a silicate vapour plume. The behaviour of the various chemical elements in these plumes, particularly atmospheric escape versus condensation and fallback to the Earth, is currently not fully understood. The isotopic systematics of light elements and noble gases suggest that late addition to the Earth from comets is unlikely to have been volumetrically important for water, nitrogen and carbon (the proportions of isotopes of each element in a body depend upon where it formed in the primordial disk from which the Solar System formed, with lighter isotopes mote proportionally abundant further from the Sun). Current evidence favours an origin of the terrestrial volatiles by early capture during planetary accretion rather than by late addition during very large impact events.

Regardless of the origin of volatiles, the rate at which the lunar surface was bombarded (and, by analogy, the Earth) can be reconstructed by combining crater density statistics with the known ages of rocks from the Moon’s surface. The largest uncertainty in this flux estimate arises from the paucity of samples returned from the older, more heavily cratered dark side of the Moon and the few direct dates for large lunar impact basins. There are two end-member models for the bombardment flux: one that envisages a spike in very large impacts between 3850 million years ago and 4200 million years ago (the Late Heavy Bombardment scenario) versus one that favours an exponentially decaying flux (the Accretion Tail Scenario'). With currently available data, modelling cannot unequivocally rule out either scenario. One of the strongest pieces of evidence in favour of the Late Heavy Bombardment remains the uranium/lead age line of lunar highland samples (uranium, decays into, amongst other things, lead at a known rate; it is possible to calculate the age of a mineral which would not have lead in in when it formed from the ratio between these elements) that was originally used to advance the concept of a late bombardment. This line is interpreted to date the timing of volatile element loss and homogenisation. The age conspicuously coincides with the more widespread preservation of terrestrial rocks, i.e., the Archaean–Hadean boundary. If future lunar data confirm the existence and timing of the Late Heavy Bombardment, one of the most significant environmental consequences of very large bolide impacts on Earth could have been the destruction of the protocrust. On Mars and Mercury, the ancient protocrusts persisted, despite bombardment, but the Late Heavy Bombardment on Earth may have been effective at crust destruction if the crust–mantle system had reached a vulnerable state, due, for example, to build-up of internal heat.

An artists impression of impacts on the Moon during the Late Heavy Bombardment. Australian National University.

With no supracrustal rocks of Hadean age preserved, the question of putative Hadean life and its effects on the carbon cycle cannot be studied directly. By contrast, the Archaean sedimentary record does contain samples with remains of organic (reduced) carbon, as well as carbonate, and there is clear evidence that the Archaean Earth was struck by very large bolides. No unequivocal Archaean impact basins have been found to date. Instead, the evidence for impacts comes from so-called spherule layers within sedimentary sequences. These tell-tale sediment layers are millimetre-to metre thick, laterally continuous, and contain spherules of various compositions, some with evidence for quench cooling, high pressure minerals, or shock features. The first important inference drawn from their distribution in time is that the Earth continued to be bombarded with large bolides well beyond 3850 million years ago and that the Archaean witnessed more large impacts than the later eons. Because many spherule beds are enriched in iron-loving (siderophile) elements, it has also been possible to incontrovertibly prove that some layers have had a contribution to their formation from a vapourized asteroid, for example via the isotope composition of Chromium.

A particular advantage of studying spherule beds is that they are preserved within a stratigraphic context. This provides additional sedimentological information and geochemical evidence of potential environmental disruption. Most of the well-preserved Archaean spherule beds from the Kaapvaal Craton of southern Africa and the Pilbara Craton of western Australia  show evidence for sedimentary redistribution caused by currents and/or waves. The consistent occurrence of spherules within reworked eroded local detritus rather than the pure deposits of constant thickness expected from fallout, strongly suggests that reworking was a consequence of the impact itself via tsunamis, impact-induced turbidity currents, or bottom return flows.

Previous studies have theorised that most of the spherule bed–forming bolides were 20–50 km in diameter and would have excavated transient craters of up to 100 km deep and final basins reaching several hundred kilometres in diameter. To date, no such basin has actually been discovered. One interesting area of future research is the question of shock-metamorphism of the lithospheric mantle during excavation and collapse of transient cavities well below the crust–mantle boundary. In terms of environmental and carbon isotope consequences, the impact that caused the 2629 million year old spherule layers in Western Australia and South Africa is particularly instructive because it is found within carbonate (mostly dolomite), which is conducive to chemical and isotopic analysis. The corresponding impact basin would have been about 100–150 km in diameter and, thus, represent a significant event. The carbon isotope values for reduced carbon across the spherule bed do not show an incontrovertible trend, but nonetheless indicate a significant general shift towards lighter carbon isotopes. By contrast, the shallow marine carbonate isotope values remain near constant across the spherule bed, but these data were obtained at a more limited spatial resolution that may not have captured the disruption of the global biogeochemical carbon cycle. More detailed isotopic studies across spherule beds are needed to explore to what extent the balance between the buried sedimentary reduced carbon and the dissolved oceanic carbon was disrupted by these impact events.

Carbonate is a less dominant sediment type in the Archaean supracrustal rock record than in the Proterozoic and the Phanerozoic. Therefore, it is impossible to produce a continuous global carbonate carbon isotope record that would cover all the 15 known Archaean spherule layers to test how representative the 2629 Ma event was. Notwithstanding this limitation, it is evident from the existing global compilation that the presently documented fluctuations in Archaean carbon isotope ratios were much less pronounced than in the Palaeoproterozoic and Neoproterozoic. Regardless of the potential of very large bolide impacts to temporarily disrupt the ancient carbon cycle, the apparent stability of the cycle itself, as well as the similarity of the predominant Archaean carbon isotope ratios with modern carbonate carbon, is astonishing. The oldest carbonates occur in the Isua Greenstone Belt of Southwest Greenland. They are between 3710 and 3810 million years old and, although not universally accepted as sedimentary in origin, some appear to have yielded carbon isotope ratios close to the modern-day value, and they have co-existing very light carbon preserved in putative biogenic graphite Due to the pervasive metamorphic overprint of the Isua rocks, some doubt remains as to whether the recorded carbon isotope values truly reflect the sedimentary system. Regardless, many more paired reduced carbon and carbon isotope values have been reported for younger Archaean sedimentary rocks, leaving little doubt as to the stability of the early terrestrial carbon cycle.

Photomicrographs of typical spherules collected from the 2130–1848 million year old spherule layer in northeast Midternæs, Greenland. (a) Spherule filled with finely crystalline radial-fibrous chalcedony and sericite in plane polarised light. Note marginal replacement by invasive dolomite and concentration of carbonaceous matter (black) along boundary between chalcedony and dolomite. (b) Same field of view as (a) between crossed polarisers. Arrows indicate planar interfaces between adjacent radial-fibrous aggregates. (c) Spherule filled with combination of coarser, equigranular quartz crystals (white) and fibrous sericite (grey) in plane polarised light. Sericite is unusually coarse and locally organized into radiating aggregates. (d) Part of spherule similar to (c) between crossed polarisers showing radiating sericite aggregates. Scale bars are 300 µm in (a), (b) and (c) and 100 µm in (d). Chadwick et al. (2001).

Of the six largest preserved terrestrial impact structures, three are Proterozoic in age: the 2023 million year old Vredefort impact structure in South Africa, the 1849 million year old Sudbury Basin in Canada, and the 580–590 million year old Acraman crater in South Australia. Due to deep erosion of the Vredefort structure and the lack of a confirmed corresponding impactite layer, it is impossible to reconstruct the environmental consequences of Earth’s largest preserved bolide impact. By contrast, both the Sudbury and Acraman events preserve remnant impact structures, as well as corresponding impactite layers in the sedimentary record. These two impact events are, therefore, more conducive to studying putative global environmental consequences.

The impact layer corresponding to the Sudbury Basin is found up to 700 km away in the iron-rich sedimentary strata of the Lake Superior region of North America. The layer is a breccia containing lithic fragments (some shocked), devitrified glasses of various kinds, as well as accretionary lapilli; this layer differs from the Archaean spherule beds. Of critical importance is that the breccia layer occurs within a Palaeoproterozoic sedimentary context. The bolide is believed to have hit a foreland basin covered by relatively shallow water, and the main excavated rocks were quartz-rich sandstones of the over 2200 million year old Huronian Supergroup and Archaean basement. These contained very little carbon. However, it has been argued that the bolide was likely a 15 km diameter comet (with a density of 0.6 g per cm³). If this body was similar in composition to comet Halley's Comet, which is estimated to be 18.4% carbon by mass, then the Sudbury object would have contained 195 gigatons of carbon and, if fully vapourised, would have released about 700 gigatons of CO₂, or about one-third of the CO₂ perturbation of the current Anthropocene experiment.

In the lead-up to the impact, the continental foreland basin of the Lake Superior region was ferruginous, with thick banded-iron formations being deposited. The Sudbury impact layer nearly always caps the iron formations and other ferruginous sediment, and subsequent deposition continues with different mud-sized detritus. No re-occurrence of the dominant deposition of iron formation after the impact has yet been observed. There is, thus, strong regional evidence that the Sudbury impact event caused a sharp change in basin water conditions over 700 km away. The disappearance of Palaeoproterozoic banded iron formation at about 1850 million years ago is a global phenomenon, and there is strong evidence for tsunami deposits within some of the impact layers at variable water depths, which has led to the proposal that that the Sudbury impact could have pervasively changed the regional, and probably the global, oceanic stratification, bringing to an end the long-lasting dominantly ferruginous state of the early Palaeoproterozoic deep oceans. The physical reasons for the inferred change remain to be established, however. It is currently unknown how an event such as the Sudbury impact could have disrupted the global oceanic iron supply and started the fickle oceanic states of the remaining Proterozoic. 

Stratigraphic relationship shown in six sedimentary logs between the Palaeoproterozoic Sudbury (Canada) impact layer (in blue) and the type of sediments that preceded and followed this event. Note the lack of deposition of banded iron formation after the impact event. The six logs relate to the following: Mesabi Iron Range (Minnesota, USA); Gunflint Iron Range (Minnesota, USA, and northwest Ontario, Canada); Gogebic Iron Range (Michigan and Wisconsin, USA); Iron River–Crystal Falls District (Michigan, USA); Marquette Iron Range (Michigan, USA); Baraga Basin and Dead River Basin (both in Michigan, USA). (Inset) Example of a lapilli-stone, one of the rock types that makes up the Sudbury impact layer. The rock abounds with millimetre-sized accretionary lapilli that formed in the impact plume. Photo width is 5 cm. Kamber & Petrus (2019).

The Sudbury Basin itself preserves the best-exposed and most accessible stratigraphy through a very large impact basin on Earth. It may originally have measured 170–200 km across but thanks to its remnant now being folded, there is an unparallelled opportunity to study transects from the shocked basement into the differentiated melt sheet and across the basin fill without the need for drilling. Of particular interest is the 1300 m thick unit that overlies the crystallised melt sheet. It consists of breccias and tuffs that collectively are far too thick to represent the fallback from the impact. Instead, the first 300 m of chaotic breccias most likely formed through a fuel-coolant interaction, when seawater flooded onto the superheated melt sheet The remaining stratigraphy is characterised by sustained deposition of subaqueous volcanic products (bombs, lapilli and ash) that are more mafic than the average target rocks. The observation of on-going igneous activity within a subaqueous impact basin has led to speculation that it could represent deeply sourced magmatism.

Numerical impact modelling has demonstrated that the depth of the transient cavity (created within less than a few seconds) nearly linearly increases with increasing bolide diameter, whereas the final depth of even a 500 km diameter basin is less than 3 km. The divergence in depth between transient cavity and final basin necessitates an ever-increasing material flow during the rebound and collapse of the original cavity. It has been proposed that the vertical component of this material flow could give rise to secondary decompression melting. In areas of unusually high continental heat flow and on weak plates (such as ocean basins), one environmental consequence of very large impact events could, therefore, be sustained, deeply sourced magmatism and the associated release of volatiles.

Regardless of this possibility, a final noteworthy aspect of the Sudbury crater fill is the progressive enrichment of the breccias and tuffs in reduced carbon. Studying the chemistry of the fine-grained ash-sized matrix of the crater fill, has led to the conclusion that the crater basin was likely cut off from the open ocean and so developed a distinctive water chemistry within it. The sustained magmatic activity within the basin supported base-metal deposition similar to volcanogenic massive sulfide ores, which otherwise occur at oceanic spreading sites. Apart from the destructive forces of very large impacts, one very different environmental consequence of subaqueous events could, thus, be the formation of enclosed 'ponds' (similar in shape to atolls), which contained chemical 'factories' (hydrothermal systems) producing organic molecules as potential building blocks for life. Whereas life had long been established by 1849 Ma, similar Hadean or early Archaean subaqueous impact basins should be considered as possible birth places of life and the kick-start of the terrestrial carbon cycle.

The structure of the Sudbury Basin. Natural Resources Canada/Wikimedia Commons.

The approximately 590 million year old Acraman impact occurred during a period of intense fluctuations in the carbon cycle in the late Neoproterozoic Era. The possibly 85–90 km diameter impact structure is now deeply eroded, but the corresponding impact layer can be traced for over 500 km. Palaeomagnetic data suggest a low latitude impact, which could potentially increase any resultant environmental effects. But in terms of the on-going Neoproterozoic fluctuations in carbon isotopes, the Acraman event seems to have been relatively minor, with only a small excursion towards a more negative reduced carbon isotope value; however, the detailed isotope stratigraphy is currently missing. The Acraman impactite layer coincides with a marked change in fossil plankton (Acritarch) successions and may have been more significant in terms of radiation than the preceding worldwide Marinoan Glacial Event of the Cryogenian Period. The main target lithology of the Acraman impact were acidic volcanic rocks poor in carbon. There may have been limited disruption of the global carbon cycle, although detailed carbon isotope stratigraphy is unavailable.

The remaining three largest terrestrial impact structures are Phanerozoic in age. The possible causal relationship between a large bolide impact and a Phanerozoic extinction event has been widely discussed, but there are two very clear observations. One is that there have been more significant extinction events during the Phanerozoic than there are very large impact structures to account for them; the second is that there were large impact events, such as the the 215 million year old Manicouagan Crater in Quebec, Canada, with no correlative mass extinction.

Against this backdrop, the exceptional coincidence between the End Cretaceous extinction event and the roughly180 km diameter, 66 million year old, Chicxulub impact structure in Yucatan stands out. It is still being debated whether the environmental effects of the bolide strike on their own were responsible for extinction or whether the Earth was struck at a time when its biology had already been pushed close to a tipping point by volcanic degassing and dropping sea-level. Regardless, it is widely agreed that the Chicxulub impact caused planet-wide climate disruption, as supported by the geological context of the impact site. In the late Cretaceous, the Yucatan Peninsula was a partially emerged platform composed of calcium carbonate and evaporite deposited on older sediments, themselves sitting on Precambrian basement. The bolide excavated through this 'fertile' stratigraphy at a site partly on land and partly submerged.

From a carbon cycle perspective, the presence of thick carbonate beds at the target site is of greatest relevance. The potential quantity of CO₂ devolatilised to high atmospheric altitude from bolides is dwarfed by that modelled to be ejected from a thick carbonate platform. On a 500–1000 year timescale, the effects of releasing 425–1400 gigatons of CO₂ into the atmosphere is climate warming, but in the case of Chicxulub, where limestone, sulfate and seawater were the target, the short term effect was dramatic SO₂-driven cooling with global annual mean surface air temperatures dropping by more than 20 °C, recovering only after 30 years. The bolide strike may also have caused massive wildfires and/or stratospheric emission of smoke from combustion of hydrocarbons within the target marine platform (oil and gas are produced to the north and west of the impact site). The nature of recovered molecules from incompletely combusted hydrocarbons preserved in the impact layer supports the idea that the bulk of the soot was released from reduced carbon contained within the impacted target rocks, amplifying SO₂-driven cooling along the equator and causing droughts. All this is consistent with the extinction patterns.

The End Cretaceous event, thus, emphasises a further aspect of impacts on the terrestrial surface, which is lithologically and geochemically highly diversified and evolved. Less than one-sixth of the current planetary surface has a suitable make-up to cause strong stratospheric cooling if hit by a large bolide; significant direct disruption of the global carbon cycle seems only likely from impacts onto thick carbonate targets.

The size and location of the Chicxulub Crater. Passant Rabie/Inverse.

Bolide impacts have affected the Earth’s carbon cycle in a multitude of ways. The widely held view that there are direct effects to the carbon cycle through environmental devastation and mass extinction, such as has been popularised with the End Cretaceous boundary event, is probably the exception rather than the rule. Most of the consequences of large impacts have been indirect. On the Hadean Earth, intense bombardment may have contributed to the destabilisation of the original crust, thereby possibly promoting plate motion that has become an integral part of carbon cycling through plate destruction. Subaqueous early impact basins may also have been self-contained production sites
of organic molecules that could have been potential cradles for life.

Throughout the Archaean, the Earth continued to be occasionally bombarded by large bolides, as inferred from thick beds of spherules that must have splashed down from giant melt and vapour plumes. The existing Archaean sedimentary carbon isotope record does not appear to show fluctuations of the magnitude seen in later times: however, the record is of limited temporal resolution. In general, partitioning of carbon between the reduced and oxidised pools has remained surprisingly constant. The much more pronounced carbon isotope excursions of the Palaeoproterozoic and Neoproterozoic do not coincide with known impact events. Instead, the two very large events, at 1849 and 590 million years ago, are traceable in the sedimentary record and are associated with the end of the deposition of banded iron formations and the radiation of Acritarch plankton, respectively. If future work demonstrates these to be causal relationships, they would illustrate the indirect influence of large impacts on the carbon cycle through the reorganisation of the ocean’s redox state and the disruption of biological evolution.

Most Phanerozoic mass extinctions are not coincident with very large impact events. The Chicxulub event, occurring at the End Cretaceous boundary, caused a moderate carbon isotope excursion and greatly disrupted the budget of climate-active gases in the atmosphere. This, in turn, led to a short-term abrupt cooling and a medium-term strong warming The lesson drawn for the Anthropocene is that the release of several thousand gigatons of CO₂ into the atmosphere may not leave a marked carbon isotope signal in the geological record. Instead, the Anthropocene is more likely to leave its legacy as a mass extinction from greenhouse-induced climate change on a biosphere already at a tipping point caused by habitat loss.

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Cooking the primordial soup; did the first life emerge in volcanic pools?

The blood plasma and lymph of modern animals is similar in chemical composition to seawater, strongly supporting the idea that animal life began in the oceans, but the liquid inside our cells has a quite different chemistry, suggesting that cells themselves first arose in a different environment, since the first cells are unlikely to have shared modern cells ability to maintain an interior chemistry very different to the liquid outside their membranes.

In a paper published online in the Proceedings of the National Academy of Sciences on 13 February 2012, a team of scientists led by Armen Mulkidjanian of the School of Physics at University of Osnabrück and the A. N. Belozersky Institute of Physico-Chemical Biology at Moscow State University, describe a review of our understanding of the chemistry of the earliest cells and the environment on the early Earth in these cells are thought to have inhabited, and the conclusions drawn from this.

The oldest known rocks on Earth are about 4 billion years old, but life is thought to have originated earlier than this (life does not pre-date rocks, just any rocks still around), in a period known at the Hadean. This makes the chances of finding direct evidence of the earliest life effectively nil, but does not stop us speculating, as we are still able to make inferences about the environment on the Hadean Earth.

An artist's impression of the Hadean Earth. The continents were still assembling from thousands of volcanic island arcs, the moon was closer, the sun dimmer, the planet was being constantly bombarded by meteors, the atmosphere rich in Carbon Dioxide but entirely lacking in Oxygen, consequently there was no Ozone layer and the planet was constantly bathed in ultra-violet radiation. From the Palaeos website.

The oldest, conserved, proteins found in all organisms tend to use Zinc and Manganese, but not Iron, which is used by more modern proteins that have evolved separately in different groups, implying that the first cells evolved in an environment rich in Zinc and Manganese, but poor in Iron. Water from hydrothermal vents tends to be rich in Zinc, Manganese and Iron, but the iron precipitates out of solution rapidly.

All known cells maintain an interior environment richer in Potassium than in Sodium; the reverse of the situation found in the modern ocean. We do not have any evidence that suggests the situation in the earliest oceans would have been any different. Hydrothermal vents in the seas have similar Sodium/Potassium ratios to seawater, since this is where the water in them derives from, and the contribution of Sodium and Potasium ions from the seawater outweighs the volcanic contribution. Terrestrial hydrothermal springs derive their water from precipitation (rain and snow), which lacks Sodium and Potassium ions. The Sodium/Potassium ratio in such pools is variable; those which are dominated by water that is emitted in a liquid tend to be rich in Sodium, but those where water is emitted as a gas and then condenses tend to be rich in potassium.

All cells maintain a high interior Phosphate concentration, but the oceans are not rich in Phosphates, and there is no reason to believe the early oceans were any different. Water from volcanic vents tends to be rich in Phosphates.

Previous theories have suggested that life may have originated around deep-sea hydrothermal vents, but these have steep chemical gradients, and chemistry unlike that found inside cells. Hydrothermal pools on-land tend to have stable chemistry, but are very acidic, and so have been discounted as a likely source of life by many studies. However this acidity is caused by the reaction of Sulphur-compounds with Oxygen in the atmosphere, something that could not have happened in an ancient environment lacking atmospheric Oxygen.

The other objection to terrestrial hydrothermal pools as a place of origin for life is the high level of ultra-violet radiation that would have bathed the early Earth and which is harmful to life. The modern world is protected from ultra violet radiation by the Ozone Layer, but Ozone (O₃) is a form of Oxygen, so this would not have existed prior to the evolution of an Oxygen rich atmosphere. Water also protects against ultra-violet radiation, but there needs to be enough of it; the sea would protect early cells, but shallow pools would probably not. However Sulphur from volcanic vents, if it was not reacting with oxygen from the atmosphere, would probably react with Manganese and Zinc if they were present in the same pools of water. Sulphur compounds of Manganese and Zinc are good at absorbing ultra violet radiation, offering protection to any life living in these pools.

See also A deep-sea hydrothermal vent community discovered in the Mariana Trench, The oldest animals - Pre-Ediacaran Sponges from Namibia(?) and Did the first land plants cause an Ordovician glaciation?