Late Heavy Bombardment Period Research Articles

Radiation-induced reactions in comet analogues

AbstractComets are a source of prebiotic molecules that likely enriched the early Earth during the Late Heavy Bombardment period. Laboratory experiments that replicate cometary conditions may facilitate understanding of the chemical reactions and supplement observational studies of these icy bodies. Prebiotic compounds, such as formic acid and formaldehyde, have been observed in comets. Furthermore, these compounds can easily be formed in experimental models using a variety of gas combinations and energy sources. We conducted experimental cometary simulations using radiation chemistry tools to obtain insight into the possible fate of formic acid and formaldehyde. The main results suggest a redundant system, signifying that the irradiation of formic acid forms formaldehyde molecules and vice versa. This phenomenon ensures the permanence of prebiotic molecules in high-radiation environments. Additionally, the potential role of forsterite and graphite was explored in cometary simulations. Our experimental results show the differential formation of aldehydes and other carbonyl-containing compounds dependent on the mineral phase present.

The lunar Dhofar 1436 meteorite: 40Ar‐39Ar chronology and volatiles, revealed by stepwise combustion and crushing methods

AbstractThe lunar meteorite Dhofar 1436 is dominated by solar wind type noble gases. Solar argon is equilibrated with “parentless” 40Ar commonly known as lunar orphan argon. Ar‐Ar isochron analyses determined the lunar trapped 40Ar/36Ar ratio to 2.51 ± 0.04, yielding a corrected plateau age of 4.1 ± 0.1 Ga, consistent with the lunar Late Heavy Bombardment period. Lunar trapped and radiogenic argon components are all released at high temperatures (1200–1400 °C). Surprisingly, solar noble gases and lunar trapped argon can largely be released by crushing. Initial crushing steps mainly release elementally fractionated solar wind gases, while in advanced crushing steps, cosmogenic components dominate. Cosmogenic noble gases indicate irradiation at the lunar surface; they are less fractionated than solar wind species. We favor a scenario in which both solar and a large fraction of cosmogenic gases were acquired before the 4.1 Ga event, which caused shock metamorphism and formation of the regolith breccia. Sintering and agglutination along grain boundaries resulted in mobilization of solar wind, reimplanted, radiogenic, and cosmogenic noble gases, and resulted in their partial homogenization, fractionation, and retrapping in voids and/or defects accessible by crushing. An alternative scenario would be complete reset of the K‐Ar system 4.1 Ga ago accompanied by loss of all previously accumulated solar and cosmogenic noble gases. Later, the precursor of Dhofar 1436 became lunar regolith and accumulated solar and cosmogenic noble gases and reimplanted 40Ar before its final formation of the polymict impact breccia. The C abundance of the step‐combusted Dhofar 1436 is 555.3 ppm, with δ13C of −28‰ to +11‰. Nitrogen contents released by crushing and combustion are 3.2 ppm and 20.8 ppm, respectively. The lightest nitrogen composition (δ15N = −79‰) is likely due to release from voids of shock metamorphic phases and is rather a result of the mobilization of nitrogen components that accumulated prior to the 4.1 Ga event.

Could Purines Be Formed from Cyanamide and Cyanoacetylene in a Prebiotic Earth Environment?

Knowledge of prebiotic nucleobase formation is important for understanding the origin of contemporary genetics. Observation of nucleobase precursor radicals in previous impact laser plasma simulations of the late heavy bombardment period (FerusProc. Natl. Acad. Sci. U.S.A.2015, 112, 65725489115) points toward possible nucleobase formation through free-radical pathways. However, previously explored radical routes to nucleobase formation involve a large number of reaction steps, repetitive addition of precursors, and a number of chemical transformations. The possibility of competing side reactions under such conditions questions the feasibility of such pathways. In view of these shortcomings, the present work employs density functional theory to explore purine formation pathways through reaction of cyanamide and cyanoacetylene with radicals via a five-membered intermediate, 4-cyanoimidazole in the presence of ammonia. Our analysis reveals that the skeletal components of 4-cyanoimidazole can be solely obtained from cyanamide and cyanoacetylene via barrierless cyclization and a small number of reaction steps. In addition, the proposed mechanisms are characterized by a small number of precursors and low energy barriers and are thus likely feasible under extreme conditions on the prebiotic earth such as meteoritic impact during late heavy bombardment period. Overall, the present study underscores the importance of cyanamide and cyanoacetylene precursors in kinetically accessible routes to purine formation.

Radical pathways for the formation of non-canonical nucleobases in prebiotic environments.

Due to the inability of canonical nucleobases (adenine, uracil, guanine and cytosine) to spontaneously form ribonucleosides and base pairs in free form in solution, RNA is believed to be preceded by a primitive information polymer (preRNA). The preRNA is proposed to contain non-canonical, heterocyclic bases that possess the above-mentioned capabilities. An extensive search for such candidate heterocycles has recently revealed that barbituric acid (BA), melamine (MM) and 2,4,6-triaminopyrimidine (TAP) have the capability to spontaneously form ribonucleosides and supramolecular assemblies that are held by Watson–Crick type hydrogen-bonded base pairs involving BA, MM, TAP and cyanuric acid (CA) heterocycles. However, despite this evidence, the prebiotic formation pathways of these heterocycles have not been fully explored. Further, for these heterocycles to interact and assemble into informational polymers under prebiotic conditions, it is expected that they should have formed in the proximity of each other. In this context, the present work employs density functional theory to propose the associated radical based formation pathways starting from cyanamide. Our pathways suggest that cyanamide, its derivatives (malonic acid and urea) and malononitrile can form BA, MM, CA and TAP in the presence of ammonia and hydroxyl radicals. In addition to originating from a common precursor, similarities in the highest reaction barriers (13 to 20 kcal mol−1) obtained for these pathways suggest that these heterocycles may likely form under similar conditions. Specifically, these pathways are relevant to high energy events such as meteoritic impact during the late heavy bombardment period on the early earth, which would have created conditions where radicals might have formed in reasonable concentrations. Overall, the present study emphasizes the importance of cyanamide in prebiotic heterocycle formation.

Lunar impact history constrained by GRAIL-derived basin relaxation measurements

GRAIL observations have been used to derive an updated catalog of lunar impact basins with diameters ≥ 450 km. In this study, we assess the age and relaxation state of these basins and find that the 13–15 stratigraphically younger basins are significantly less relaxed than the 8–16 identifiable older basins. The change in relaxation state is most likely a signature of the cooling of the base of the lunar crust below 1300–1400 K, which we infer happened between 4.21–4.45 Ga based on geochronological measurements. We compare the predicted number of basins before and after this reference point with several different modeled lunar cratering chronologies. While these models are sensitive to the ratio of 1km:450 km craters, we conclude that a model with a broad period of late heavy bombardment (a “sawtooth”) is more likely to fit both the number of relaxed and unrelaxed basins, and the mass of material delivered as a late veneer.

Ceres and the terrestrial planets impact cratering record

Dwarf planet Ceres, the largest object in the Main Asteroid Belt, has a surface that exhibits a range of crater densities for a crater diameter range of 5–300 km. In all areas the shape of the craters’ size-frequency distribution is very similar to those of the most ancient heavily cratered surfaces on the terrestrial planets. The most heavily cratered terrain on Ceres covers ∼15% of its surface and has a crater density similar to the highest crater density on <1% of the lunar highlands. This region of higher crater density on Ceres probably records the high impact rate at early times and indicates that the other 85% of Ceres was partly resurfaced after the Late Heavy Bombardment (LHB) at ∼4 Ga. The Ceres cratering record strongly indicates that the period of Late Heavy Bombardment originated from an impactor population whose size-frequency distribution resembles that of the Main Belt Asteroids.

TiO2-catalyzed synthesis of sugars from formaldehyde in extraterrestrial impacts on the early Earth.

Recent synthetic efforts aimed at reconstructing the beginning of life on our planet point at the plausibility of scenarios fueled by extraterrestrial energy sources. In the current work we show that beyond nucleobases the sugar components of the first informational polymers can be synthesized in this way. We demonstrate that a laser-induced high-energy chemistry combined with TiO2 catalysis readily produces a mixture of pentoses, among them ribose, arabinose and xylose. This chemistry might be highly relevant to the Late Heavy Bombardment period of Earth’s history about 4–3.85 billion years ago. In addition, we present an in-depth theoretical analysis of the most challenging step of the reaction pathway, i.e., the TiO2-catalyzed dimerization of formaldehyde leading to glycolaldehyde.

KIC 8462852: TRANSIT OF A LARGE COMET FAMILY

ABSTRACT We investigate the plausibility of a cometary source of the unusual transits observed in the KIC 8462852 light curve. A single comet of similar size to those in our solar system produces a transit depth of the order of 10−3 lasting less than a day which is much smaller and shorter than the largest dip observed ( &sim; 20 &percnt; ?> for ∼3 days), but a large, closely traveling cluster of comets can fit the observed depths and durations. We find that a series of large comet swarms, with all except one on the same orbit, provides a good fit for the KIC 8462852 data during Quarters 16 and 17, but does not explain the large dip observed during Quarter 8. However, the transit dips only loosely constrain the orbits and can be fit by swarms with periastrons differing by a factor of 10. To reach a transit depth of ∼0.2, the comets need to be in a close group of ∼30, if they are ∼100 km in radius or in a group of ∼300 if they are ∼10 km in radius. The total number of comets required to fit all of the dips is ∼70 ∼ 100 km or ∼700 ∼ 10 km comets. A single comet family from a tidally disrupted Ceres-sized progenitor or the start of a Late Heavy Bombardment period explains the last ∼60 days of the unusual KIC 8462852 light curve.

Corrections

February 2015, page 45—The Osirak reactor was in Iraq.February 2015, page 50—The late heavy bombardment period occurred 4 billion years ago.© 2015 American Institute of Physics.

High-energy chemistry of formamide: A unified mechanism of nucleobase formation

The coincidence of the Late Heavy Bombardment (LHB) period and the emergence of terrestrial life about 4 billion years ago suggest that extraterrestrial impacts could contribute to the synthesis of the building blocks of the first life-giving molecules. We simulated the high-energy synthesis of nucleobases from formamide during the impact of an extraterrestrial body. A high-power laser has been used to induce the dielectric breakdown of the plasma produced by the impact. The results demonstrate that the initial dissociation of the formamide molecule could produce a large amount of highly reactive CN and NH radicals, which could further react with formamide to produce adenine, guanine, cytosine, and uracil. Based on GC-MS, high-resolution FTIR spectroscopic results, as well as theoretical calculations, we present a comprehensive mechanistic model, which accounts for all steps taking place in the studied impact chemistry. Our findings thus demonstrate that extraterrestrial impacts, which were one order of magnitude more abundant during the LHB period than before and after, could not only destroy the existing ancient life forms, but could also contribute to the creation of biogenic molecules.

Impact chemistry of methanol: Implications for volatile evolution on icy satellites and dwarf planets, and cometary delivery to the Moon

Methanol (CH3OH) is one of the primordial volatiles contained within icy solids in the outer solar nebula. This paper investigates the impact chemistry of CH3OH ice through a series of impact experiments. We discuss its fate during the accretion and evolution stages of large icy bodies, and assess the possibility of intact delivery of cometary volatiles to the lunar surface. Our experimental results show that the peak shock pressures for initial and complete dissociation of CH3OH ice are approximately 9 and 28GPa, respectively. We also found that CO is more abundant than CH4 in the gas-phase products of impact-induced CH3OH dissociation. Our results further show that primordial CH3OH within icy planetesimals could have survived low-velocity impacts during accretion of icy satellites and dwarf planets. These results suggest that CH3OH may have been a source of soluble reducing carbon and that it may have acted as antifreeze in liquid interior oceans of large icy bodies. In contrast, CH3OH acquired by accretion on icy satellites and Ceres would have been dissociated efficiently by subsequent impacts, perhaps during the heavy bombardment period, owing to the expected high impact velocities. For example, if Callisto originally contained CH3OH, cometary impacts during the late heavy bombardment period would have resulted in the formation of a substantial atmosphere (ca. ⩾10−4bar) composed of CO, H2, and CH4. To account for the current CO levels in Titan’s atmosphere, the CH3OH content in its crust may have been much lower than that typical of comets. Our numerical simulations also indicate that intact delivery of cometary CH3OH to the lunar surface would not have occurred, which suggests that CH3OH found in a persistently-shadowed lunar region probably formed through low-temperature surface chemistry on regolith.

Replacement and late formation of atmospheric N2 on undifferentiated Titan by impacts

The origin of Titan’s massive nitrogen atmosphere is largely unknown. Laser-gun experiments and numerical calculations suggest that the nitrogen could have been generated by conversion from ammonia during the period of Late Heavy Bombardment.

Aeronomical evidence for higher CO 2 levels during Earth’s Hadean epoch

According to recent simulations of the Earth’s thermosphere, the exospheric temperature is not expected to rise above 7000–8000 K even under extreme solar EUV conditions anticipated for the early Earth. Rather, when the solar EUV flux exceeds some critical value, the escaping flow of the bulk upper thermosphere starts cooling it due to adiabatic expansion, which results in a decrease of the exobase temperature. Under these extreme conditions, the exobase might have expanded above the magnetopause and the magnetosphere had not been able to protect the upper atmosphere against strong non-thermal erosion by the solar wind. This study shows that a nitrogen-rich terrestrial atmosphere with a present-day composition would have been removed within a few million years during the extreme EUV and solar wind conditions that are expected to have prevailed before the late heavy bombardment period ∼3.8 Ga ago. Our results suggest that a CO 2 amount in the early nitrogen-rich terrestrial atmosphere of at least two orders of magnitude higher than the present-time level was needed to confine the upper atmosphere after the onset of the geodynamo within the shielding magnetosphere and thus might have protected it from complete destruction.

Origin of the Ganymede–Callisto dichotomy by impacts during the late heavy bombardment

Jupiter’s large moons Ganymede and Callisto are similar in size and composition, but different in surface and interior characteristics. Simulations with geophysical models of core formation indicate that the difference in impact energy received by the two satellites during the period of late heavy bombardment can explain the dichotomy. Jupiter’s large moons Ganymede1,2 and Callisto2,3 are similar in size and composition. However, Ganymede has a tectonically evolved surface1 and a large rock/metal core2, whereas Callisto’s surface shows no sign of resurfacing3 and the separation of ice and rock in its interior seems incomplete2. These differences have been difficult to explain4,5,6,7,8,9,10,11. Here we present geophysical models of impact-induced core formation to show that the Ganymede–Callisto dichotomy can be explained through differences in the energy received during a brief period of frequent planetary impacts about 700 million years after planet formation, termed the late heavy bombardment12,13,14,15. We propose that during the late heavy bombardment, impacts would have been sufficiently energetic on Ganymede to lead to a complete separation of rock and ice, but not on Callisto. In our model, a dichotomy between Ganymede and Callisto that is consistent with observations is created if the planetesimal disk that supplied the cometary impactors during the late heavy bombardment is about 5–30 times the mass of the Earth. Our findings are consistent with estimates of a disk about 20 times the mass of the Earth as used in dynamical models that recreate the present-day architecture of the outer solar system and the lunar late heavy bombardment15,16.

The Earth–Moon system during the late heavy bombardment period – Geochemical support for impacts dominated by comets

The solid planets assembled 4.57 Gyr ago during a period of less than 100 Myr, but the bulk of the impact craters we see on the inner planets formed much later, in a narrow time interval between 3.8 and 3.9 Gyr ago, during the so-called late heavy bombardment (LHB). It is not certain what caused the LHB, and it has not been well known whether the impactors were comets or asteroids, but our present study lend support to the idea that it was comets. Due to the Earth’s higher gravity, the impactors will have hit the Earth with ∼twice the energy density that they hit the Moon, and the bombardment will have continued on Earth longer than on the Moon. All solid surface of the Earth will have been completely covered with craters by the end of the LHB. However, almost nothing of the Earth’s crust from even the end of this epoch, is preserved today. One of the very few remnants, though, is exposed as the Isua greenstone belt (IGB) and nearby areas in Western Greenland. During a field expedition to Isua, we sampled three types of metasedimentary rocks, deposited ∼3.8 billion years ago, that contain information about the sedimentary river load from larger areas of surrounding land surfaces (mica-schist and turbidites) and of the contemporaneous seawater (BIF). Our samples show evidence of the LHB impacts that took place on Earth, by an average of a seven times enrichment (150 ppt) in iridium compared to present-day ocean crust (20 ppt). The clastic sediments show slightly higher enrichment than the chemical sediments, which may be due to contamination from admixtures of mafic (proto-crustal) sources. We show that this enrichment is in agreement with the lunar cratering rate and a corresponding extraterrestrial LHB contribution to the Earth’s Hadean-Eoarchean crust, provided the bulk of the influx was cometary (i.e., of high velocity and low in CI abundance), but not if the impactors were meteorites (i.e. had velocities and abundances similar to present-day Earth-crossing asteroids). Our study is a first direct indication of the nature of the LHB impactors, and the first to find an agreement between the LHB lunar cratering rate and the Earth’s early geochemical record (and the corresponding lunar record). The LHB comets that delivered the iridium we see at Isua will at the same time have delivered the equivalent of a ∼1 km deep ocean, and we explain why one should expect a cometary ocean to become roughly the size of the Earth’s present-day ocean, not only in terms of depth but also in terms of the surface area it covers. The total impacting mass on the Earth during the LHB will have been ∼1000 tons/m 2.

Search for traces of the late heavy bombardment on Earth—Results from high precision chromium isotopes

High precision mass spectrometric analyses of the chromium isotopic composition of metamorphosed turbiditic and pelagic sedimentary rocks and banded iron stones from the ∼3.7 Gyr Isua Supracrustal Belt (ISB) in West Greenland cannot be distinguished from the standard terrestrial 53Cr / 52Cr ratio at our present level of resolution. As a consequence, our search for chemical traces of possible impact-derived meteoritic components (asteroidal and/or cometary material, or accreted cosmic dust) in the Earth's oldest chemical and detrital sediments was negative. Our results, based on the 53Mn– 53Cr short-lived radionuclide system (half-life of 3.7 Myr), cannot confirm the recent findings by [1] [R. Schoenberg, B.S. Kamber, K.D. Collerson, S. Moorbath. Tungsten isotope evidence from approximately 3.8-Gyr metamorphosed sediments for early meteorite bombardment of the Earth. Nature 418 (2002) 403–405.] of tungsten isotope anomalies (based on the 182Hf– 182W short-lived radionuclide system; half-life of 9 Myr) in these sediments, which were interpreted as indicating a component derived from meteorites. Possible reasons for the failure to trace cosmic material in the ISB metasediments are various: 1. The samples studied are not representative; 2: The sedimentation period did not overlap with the period of late heavy bombardment of the Moon; and 3. The potential chromium anomalies, if present, are too small to be traceable by our present levels of detection. Unequivocal evidence of a late heavy bombardment on the early Earth therefore remains elusive and uncertain.

Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets

The petrology record on the Moon suggests that a cataclysmic spike in the cratering rate occurred approximately 700 million years after the planets formed; this event is known as the Late Heavy Bombardment (LHB). Planetary formation theories cannot naturally account for an intense period of planetesimal bombardment so late in Solar System history. Several models have been proposed to explain a late impact spike, but none of them has been set within a self-consistent framework of Solar System evolution. Here we propose that the LHB was triggered by the rapid migration of the giant planets, which occurred after a long quiescent period. During this burst of migration, the planetesimal disk outside the orbits of the planets was destabilized, causing a sudden massive delivery of planetesimals to the inner Solar System. The asteroid belt was also strongly perturbed, with these objects supplying a significant fraction of the LHB impactors in accordance with recent geochemical evidence. Our model not only naturally explains the LHB, but also reproduces the observational constraints of the outer Solar System.

Observational Constraints on Late Heavy Bombardment Episodes around Young Solar Analogs

The excess thermal infrared emission from 36 G and K stars with ages corresponding to the period of Late Heavy Bombardment on the Moon (prior to 3.8 Ga) is estimated. These stars are among 38 selected by analogy to the Sun at an age less than 0.8 Gyr.

Crater populations on Ganymede and Callisto

The discovery of heavily cratered surfaces on Ganymede and Callisto by Voyager 1 shows that like the inner Solar System, a period of heavy bombardment also occurred in the outer Solar System. Comparisons among the crater size/density curves of Ganymede, Callisto and the terrestrial planets show several striking features. The overall crater density of the most heavily cratered terrain on Ganymede is down by a factor of about 3 compared to Callisto, and when allowance is made for the difference in crater production rate due to the influence of Jupiter's gravity field it is down by a factor of nearly 6. This indicates that the oldest regions of Ganymede began recording the observed crater population at a later time than Callisto, and therefore Ganymede either experienced a large‐scale (perhaps global) diameter‐independent resurfacing event or simply developed a rigid crust capable of retaining craters later than Callisto. In either case, this process took place during the period of late heavy bombardment. Based on earlier studies of the terrestrial‐planets' cratering record, neither Ganymede nor Callisto is saturated with craters. Compared to Callisto, a diameter‐dependent loss of craters in the size range 10–40 km occurs on the grooved terrain of Ganymede and probably results from obliteration of small craters due to the formation of new ice. A similar but less severe loss also occurs on Ganymede's heavily cratered terrain and may be due to an earlier period of ice formation and/or the formation of arcuate troughs in this terrain. Seven different crater curves, in the diameter range of about 40–130 km, representing vastly different crater densities, different surface ages, different terrain types, and even different satellites all possess nearly the same distribution function. This together with other observational evidence strongly suggests that at least in this diameter range the curve basically represents its production function which is completely different from that on the terrestrial planets. This indicates that the population of bodies responsible for the period of late heavy bombardment in the inner Solar System was very different from that responsible for the late heavy bombardment in the outer Solar System. We can only speculate at this early stage that Ganymede and Callisto may principally record a population of bodies that never penetrated the inner Solar System in numbers great enough to leave a recognizable signature.