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Jan 02 2015

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Late Heavy Bombardment

The Late Heavy Bombardment and its Impact on the Terrestrial Planets

Sanuja Senanayake1, Jenna Sie1, Brendan Visser1 and Cassie Vocke1
1Geology Undergraduate Students: Fall 2014, University of Calgary.

Abstract

The Late Heavy Bombardment (LHB) is a hypothetical astrophysical event which occurred in our Solar System 4.1 to 3.8 billion years ago. At this time, an increased flux of impacting materials hit the Earth, Moon and other terrestrial planets of the inner Solar System. This has been suggested as the source for the increased number of crater impacts seen on the lunar surface, Venus, and Mars, and inferred to have struck all the inner terrestrial planets; preserved evidence has yet to be discovered on Earth. Several theories have been proposed to explain the crater formations, however the focus will be on the two most accepted theories: the Nice Model and Planet V Hypothesis. The preservation of craters from the LHB is best seen on the Moon due to the lack of plate tectonics, minimal erosion and deposition. Analyzing the surface of the Moon can help us understand the impact that the LHB had on the inner solar system. A lunar timescale is currently being modified, and when calibrated with radiometric dates from Martian samples, a timescale for Mars and other planetary bodies could be developed to verify if the LHB was a synchronous event. The LHB was early in Earth’s evolution and the contribution of extraterrestrial material to the planet is thought to have affected it in different ways; this includes the development of the atmosphere, biosphere and hydrosphere. The LHB is important not only to explain the sudden increase in crater evidence but also to help confirm the current geochemical properties of the terrestrial planetary system as seen today.

Introduction

The increased frequency in crater formation observed on the Moon, Mars and Mercury is thought to be a result of the mass bombardment of the inner Solar System 4.1 to 3.8 Ga. Several computer models have been proposed based on available data such as lunar samples from the Apollo missions. Relative ages of the crater formations were examined based on superposition and degradation of lunar craters and the geochronology on lunar samples. Lunar data has the most comprehensive record, thus the basin formation frequency on Mars and Mercury were correlated with the lunar crater count distribution to correlate the age of the LHB. Further understanding of the effects on the other terrestrial planets provides insight into the impact of LHB on Earth.

Hypotheses and Models

There is no universally accepted explanation for the Late Heavy Bombardment (Levison et al., 2001). Researchers have suggested several theories with the majority of them were based on the hypothesis that LHB was a cataclysmic event. Popular theories include the Nice Model, Terrestrial Planet V Hypothesis and Late Uranus-Neptune formation; the Nice Model is currently the most accepted.

The Nice Model was proposed by Gomes et al. in 2005, and states that the LHB was caused by the migration of the giant gas planets. According to the model, the inner most planets, Jupiter, Saturn, Uranus and Neptune were stable with highly compact orbital configurations of 5.5 to 17 astronomical units (AU) at the early stage of the Solar System (Gomes et al., 2005). Jupiter and Saturn interacted with the surrounding planetesimals while the gaseous circumsolar nebula dissipated. As a result, the two planets crossed the 1:2 Mean Motion Resonance (MMR) 700 Myr after the Solar System’s formation. This event triggered a rapid migration of the giant planets (Gomes et al., 2005), and resulted in the expansion of their orbital configurations (Fig. 1). During the migration of Jupiter and Saturn, their respective planetesimal disks were destabilized and resulted in a scattering of materials from their originally stable position. This scattering resulted in a spike of crater formation on the inner terrestrial planets, known as the LHB (Gomes et al., 2005).

The rapid migration of Jupiter and Saturn facilitated Neptune and Uranus to exchange positions to their current state. This also created small concentrations of planetary bodies, now known as Jupiter’s Trojans and Neptune’s Kuiper belts (Gomes et al., 2005). Computer simulation on four giant gas planets indicated that at the beginning of LHB, 879 Myr after the formation of the Solar System, there was a high abundance of planetesimal “particles” around each orbital axis. However, 200 Myr later, only 3% of the initial mass of the disk was left (Fig. 1).

Gomes et al., (2005) stated that the limited available data and complex mathematical extrapolation limited their ability to predict every aspect of the LHB. They were only able to demonstrate main characteristics such as the 700Myr delay between the terrestrial planet formation and the LHB. However, they did not specify the shortcomings of their model. The model was recently improved by Levison et al. (2011), known as the Nice II Model. This model postulates that the inner edge of the planetesimal disk was several AUs further beyond the orbit of the outer most planets than stated by the original researchers (Gomes et al., 2005). Levison et al. (2011) also proved that energy can be exchanged between the planets and the planetesimals without close interactions. This means that planetesimals might have been destabilized during LHB without a physical interaction (Levison et al., 2011).

The terrestrial Planet V hypothesis was introduced by NASA Scientists as an explanation for radiometric and geochemical data obtained from the Apollo Mission samples (Chambers and Lissauer, 2002). Brasser and Morbidelli (2011) further suggested the LHB was caused by instability and subsequent migration of an arbitrary planet known as Planet V. As the Planet V migrated across the asteroid and comet belts, it scattered particles within the two belts across the Solar System (Chambers and Lissauer, 2002). Because the two belts were highly populated at the time, the intensity of the scattering resulted in cataclysmic event. Brasser and Morbidelli (2011) had done a detailed analysis of the original location of these astrophysical particles, and suggested that the Planet V could only have migrated across the inner Earth-Venus (EV) belt, but not the Earth-Mars (EM) belt. Thus, the Planet V hypothesis was used to mathematically model and identify the specific location for the materials contributed to the LHB (Brasser and Morbidelli, 2011).

Another theory suggests that LHB was caused by the late formation of Uranus and Neptune (Levison et al. 2001), which destabilized Jupiter and Saturn. Subsequent migration of Jupiter and Saturn would have caused interactions with objects within populated regions Jovian Trojan swarms, and the asteroid belt (Levison et al., 2001). After being dislodged, these objects might have triggered the LHB. However, the numerical simulation suggested that objects within asteroid belt were most likely the primary source, which also agrees with the Planet V hypothesis (Brasser and Morbidelli, 2011). This is the least accepted hypothesis for the Late Heavy Bombardment.

Researchers had difficulties in developing models for LHB primarily due to limited information on lunar impact melts, which is our best evidence for the frequency in crater formations (Brasser and Morbidelli, 2011). The complexity of models increased with the large number of unknowns and variables. This problem has been compounded by the limitations of technology, which computer simulations may require 4 to 6 months to generate viable results (Brasser and Morbidelli, 2011). Brasser and Morbidelli (2011) suggested that this problem can be dramatically improved if the available data from the lunar samples and other sources increased.

Lunar Cataclysm

The Late Heavy Bombardment had a large effect on the Moon from 4.1-3.8 Ga. This event is also known as the Lunar Cataclysm when specifically talking about the Moon. The most obvious evidence of the effects from the LHB is seen by the abundance of craters on the Moon’s surface. Impact craters are found on nearly every solid body in the solar system, however, the Moon happens to have the most complete and clear impact history available (Kring and Cohen, 2002). This is because of the Moon’s exceptional surface preservation due to the lack of plate tectonics, water and atmosphere. Dating of the craters on the Moon’s surface can help to understand the timing of the event and the frequency and mass of impacting material (Ryder, 2002). Further evidence has been preserved in isotopic systems of rocks and impact melts on the Moon (Tera et al., 1974). Analyzing isotopic data of these lunar samples has shown that widespread isotopic disturbances have occurred between 4.0 – 3.85 Ga (Tera et al., 1974). Proper interpretation of impact craters and isotopic disturbances can help to further understand the LHB and its effects on the Moon and the other terrestrial planets.

The flux of material impacting the Moon has varied significantly over time. As seen in Figure 2, the majority of impacting material was before 3.8 Ga with a large spike during the LHB (Tera et al., 1974; Ryder, 2002). The mass of impacting material during a specific time period is calculated by examining the characteristics of craters with known relative age. As most of the largest impact craters have been radiometrically dated from impact melt ejecta, we can determine the relative ages of the craters on these surfaces (Ryder, 2002). Projectile masses can then be determined by the dimensions of the crater, estimations on how much energy it took to produce, and how this energy relates to the mass, velocity and angle of the impacting meteorite (Ryder, 2002). These factors are not well known and therefore analysis of smaller, more recent and well-known craters were necessary. It has been calculated that approximately 2 – 6 x 1018 kg of material from over 1700 impacts struck the Moon during the LHB (Cohen et al., 2000; Ryder, 2002; Gomes et al., 2005).

The largest impacts during the LHB that have formed craters greater than 300km in diameter have been classified as impact basins. Dating of impact basins by superposition, crater counting and radiometric dating of impact melts has shown that the majority of these basins were formed between 4.6-3.7 Ga (Ryder, 1990; Stoffler et al., 2006). As seen in Figure 2, many of these impact basins formed during the LHB. Basin forming impacts are thought to be the cause for the widespread metamorphism and element redistribution seen in lunar samples during the LHB (Tera et al., 1974).

The impacts that formed extensive basins on the Moon would have caused widespread disturbances in lunar samples. Shock metamorphism and element redistribution are some of the disturbances seen in lunar samples due to the large amount of heat generated by the impact events (Tera et al., 1974). Over 800Lbs of lunar rock has been brought back from Apollo and Luna missions for further analysis. Many of these rocks are from the ancient anorthositic lunar highlands, which are greater than 4 Ga and are considered the oldest rocks on the Moon (Dalrymple and Ryder, 1993). Radiometric dating of U-Th-Pb, K-Ar, and 40Ar-39Ar isotopic systems within lunar samples was the main method used to determine the age of the lunar highland rocks (Tera et al., 1974). Due to their age, the lunar highlands have been heavily cratered with a majority of cratering before 3.8 Ga. Disturbances can be seen in the lunar highland rocks when looking at isochrons of U-Pb and Rb-Sr systems (Tera et al., 1974). Total rock analysis showed a U-Pb isochron with a metamorphic age of ~3.9 Ga and an Rb-Sr isochron showed a metamorphic age between 4.0-3.85 Ga (Tera et al., 1974). These ages infer an event or series of events within a narrow time interval of ~200 Ma (Tera et al., 1974). The LHB is the event responsible for the disturbances seen in the lunar samples due to the amount of impacting material that hit the Moon in such a short interval of time.

Another source of evidence on the Moon that supports the theory of the LHB is present in the abundance of impact melts younger than 4.0 Ga (Ryder, 1990; Cohen et al., 2000; Stoffler et al., 2006). Impact melts are materials that have been melted from the intense amount of heat generated during an impact event. This material is then ejected onto the surface of the Moon where it is rapidly cooled to form a glass-like volcanic substance. Radiometric dating of impact melts from the Moon can help to determine the age of formation of craters (Ryder, 1990; Stoffler et al., 2006). No impact melt material has been dated older than 4.0 Ga with many of the samples dated between 4.0-3.8 Ga (Ryder, 1990; Stoffler et al., 2006). This leads to the assumption that either the evidence has not been preserved or that there was little to no impact activity before 4.0 Ga (Ryder, 1990; Cohen et al., 2000). The second assumption is more widely accepted and overall impact melt data strongly suggests a spike of impacts on the Moon during the LHB.

The lunar samples from Apollo missions were only collected from the near side of the Moon because of limitations on radio and visual contact with the astronauts. Therefore, the evidence seen in these rocks can only be conclusive for about half of the Moon (Cohen et al., 2000). To determine if the same disturbances in the rock record have occurred globally, lunar meteorites found on Earth were analyzed. These meteorites have been ejected from the Moon by large impact events and after travelling approximately 1 Ma in space they have impacted the Earth’s surface (Cohen et al., 2000). The lunar meteorites have been found in deserts and in Antarctica where alteration due to weathering is minimal and preservation of the sample is exceptional. Based on the composition of the lunar meteorites and the interpretation of satellite photos, it has been predicted that some of the meteorites have originated from the unexplored far side of the Moon (Cohen et al., 2000). Analysis of U-Pb and Rb-Sr isochrons as well as cosmic ray exposure (CRE) dating has determined that the disturbances in the lunar samples due to the LHB are in fact a global occurrence (Cohen et al., 2000).

Many different authors have introduced two conflicting ideas for the rates of lunar cataclysm. ‘Terminal lunar cataclysm’ involves the sudden spike of impacting material within a short 200 Ma interval and is generally supported by most recent papers (Tera et al., 1974; Ryder, 1990 & 2002). However, other research papers by Hartmann et al. (2007) and Baldwin (2006) have suggested that the lunar cataclysm was a gradually decreasing event. These authors introduce the theory that the Moon has been bombarded fairly consistently since its formation 4.6 Ga. The argument for gradual bombardment mentions that radiogenic impact melt data is not present before 4.0 Ga because it was constantly reset by impacts (Hartmann et al., 2007). However, recent papers have disregarded this theory because extensive bombardment would not be able to reset all impact melt data before 4.0 Ga without leaving any evidence (Ryder, 2002).

Evidence on Mars and Mercury

Correlating evidence of LHB from the terrestrial planets of Mercury and Mars can help to piece together the events which took place in the Solar System over 3 Ga. Venus will not be discussed as its surface does not have good preservation of the LHB due to its intense resurfacing processes and much denser atmosphere. Like the Moon, Mercury and Mars show a history of intense cratering impacts on their surface in the form of basins that are kilometers in diameter (Fassett et al., 2013). However, the Moon has the best preserved cratering record with the most complete and accurate time scale. This is because the surface of Mercury and Mars are much more complex than the Moon when it comes to resurfacing processes. Thanks to MESSENGER and Viking data, Greeley et al. (1981) found that there is strong evidence that LHB triggered widespread volcanism on both terrestrial planets. This can be seen through mare ridges and flow lobes which are characteristic of volcanic plains (Greeley et al., 1981). These plains have partially to completely buried impact craters which has constrained dating methods used for the terrestrial planets.

Another limit to dating methods is the lack of samples from the planets. An exception to this lack of data is the SNC group of Martian meteorites (Shergottites, Nakhlites, Chassignites). These meteorites are known to be ejected from Mars because of their correspondence with Martian atmospheric compositions available from Viking data (Geiss et al., 2013). Through K/Ar dating methods the ages of these meteorites are determined to be significantly young, ranging from 1.3-0.58 Ga. The oldest among the Martian meteorites, ALH 84001, which is not classified as an SNC meteorite due to its composition, is 4.1 Ga (Geiss et al., 2013). These meteorite dates along with references from the lunar crater count time scale are what scientists are currently using to create a timescale for Mars, which in turn will aid in the understanding of the LHB. In Figure 3, the two curves on the time scale are representative of measured dates and hypothesized dates. The hypothesized dates come from the lunar chronology curve which is calibrated to Mars through the difference in crater production rates on Mars and the Moon. The time scale at this point is incomplete; there is no data beyond 4 Ga, and has some error, shown by the shaded areas under the curves. This is where the dates hypothesized don’t correlate to measured dates. In the future with samples returned from the planet, a more accurate timescale can be constructed using radiomateric dating and cosmic ray exposure ages.

Dating on Mercury is based on models of impact rate along with global mosaic images taken by Mariner 10 and MESSENGER (Marchi et al., 2013). The surface has a less distinct record of cratering than the Moon which is related to the more intensive resurfacing processes it underwent (Marchi et al., 2013). Processes such as volcanism and erosion modified or even completely erased the ancient features (Geiss et al., 2013). It is assumed that in general the larger the crater, the less it was affected by resurfacing. Hence, large and undisturbed basins are used for the derivation of absolute ages from the measurement of crater size frequency distributions. These distributions may be used in conjunction with lunar chronology if they are calibrated to Mercury based on differences in factors affecting impact basins (Marchi et al., 2013). Presently, there are no available samples from Mercury, so dating of the terrestrial planet relies on the lunar crater count time scale and crater frequency distributions.

When dating the surface of Mercury and the Moon, basins on a global-wide scale could be taken into account. Alternatively, when dating the surface of Mars, only the highlands which make up close to half of the surface could be considered (Werner, 2014). The lowlands are thought to have been reworked by volcanism, therefore the highlands are a more reliable source of basins to date. Overall, the Moon is thought to have the oldest surface at 4.3 ± 0.05 Ga, followed by Mercury at 4.1 ± 0.05 Ga, and Mars at 4.1 ± 0.01 Ga (Werner, 2014). As seen in Figure 4, the frequency of impact varies for each planet suggesting that LHB may have not been a synchronous event. Further evidence will be needed to confirm that bombardment of the terrestrial planets may have varied, but researchers have come up with some hypothesis to explain this phenomena. One hypothesis discussed the possibility that the Moons formation process led to a cratering record different than the terrestrial planets (Werner, 2014). Some scientists suggest hindered global surface solidifcation, while others argue that widespread volcanism could have created gaps in the cratering record (Werner, 2014). Although it is difficult to accurately constrain the dates of impact basins on Mercury and Mars, extrapolation of lunar cratering along with Martian meteorites has led scientists to believe that the terrestrial planets were all subject to an intense spike in bombardment 4.1 to 3.8 Ga (Fassett et al., 2013). A more complete understanding of how the LHB affected Mercury and Mars is important because it may lead scientists to an improved understanding of what has not been preserved on Earth.

Implications for Earth

The Late Heavy Bombardment (LHB) has no direct preserved evidence on Earth due to recycling of the crust by plate tectonics. Data extrapolated from lunar craters suggests there was an average of ~2 x 1020 kg (De Niem et al., 2012; Abramov et al., 2013) accreted material added between 4.1 – 3.8 Ga to the planet during this time. The crater diameters left by impactors were up to 1000 km (Glikson, 2001), however cumulative number of craters decreased with increased diameter (Fig. 5). As diameter of impactor increased, so did the long lasting physical effects on Earth due to greater sterilization volumes and longer cooling periods. Basin counting on the Moon suggests that the Earth collected 1.3-1.5 times more objects of the same mass per unit area than the Moon (Grieve, 1980). The material was added to Earth resulted in physical and chemical alterations of the hydrosphere, biosphere, atmosphere, and lithosphere.

The hydrosphere was affected to variable degrees, depending on the diameter of the impactor to strike. Valley (2005) provides evidence of pre-bombardment liquid water 4.4-4.0 Ga, due to δ18O values in zircons. This initial water, as explained by Frey (1980) and Izawa et al. (2010), arose as a result of the degassing of Earth’s interior from a magma ocean. Impactors with a crater diameter of >500 km had the potential to boil the ocean; however, the majority of impactors boiled only the surface layer (Zahnle et al., 2007). The increased temperature of liquid water established hydrothermal vents, which had the potential to shelter pre-bombardment life or host post-bombardment life.

All necessary conditions for life were present on Earth before the LHB, including continental crust, liquid water, and a primitive atmosphere (Martin et al., 2006). During periods of extreme heating, microbes located on the bottom of the ocean and deep within the subsurface were in good positions to survive (Houtkooper, 2011). This was possible as 1.2 – 2.5 vol% of the upper 20 km crust was melted and 0.3 – 1.5 vol% in a molten state at any given time, according to the models proposed by Abramov et al. (2013). The overall calculated total habitable volume of the crust pre-bombardment is proposed to be 2.1 x 109 km3, and 1.7 x 109 km3 at the end of the LHB (Abramov and Mojzsis, 2009).

Primitive life forms could have survived extreme environmental conditions in the subsurface and hydrothermal vents; thermophiles thrive in 50-80 oC environments, hyperthermophiles in 80-110 oC (Abramov and Mojzsis, 2009). As denoted by Figure 6, the percentage of habitable volume for hyperthermophiles increased during the period of bombardment, indicating better survival rate as temperature of hydrothermal environments increased. This does not necessarily suggest that life originated under hot conditions, only that the survival of hyperthermophiles created a bottleneck effect leading to the diversification of species today (Zahnle et al., 2007), with hyperthermophiles near the roots of the Tree of Life. This life may have originated on its own in pre-bombardment conditions, later to thrive in the hydrothermal systems created by the LHB. Or, perhaps the energy of the impactors and the post-bombardment conditions established were ideal environments to sustain life. Evidence of life is observed soon after the LHB, at 3.465 Ga, as filamentous microbes preserved in the Apex Chert of Western Australia (Schopf, 1993). Thus, it has been suggested that the impactors brought extraterrestrial life to seed Earth upon impact (Houtkooper, 2011), assuming the microbes capability of surviving transport and impact (Sheldon and Hoover, 2007).

Earth’s atmosphere endured both loss and gain processes, which altered the pressure and composition of the atmosphere. Impact erosion, as described by Hamano and Abe (2010), is the process in which energetic expansion of a vapor cloud blows off a fraction of a planet’s atmosphere due to rock ejecta from impact events. Atmospheric erosion due to impact was minor, however the atmospheric pressure increased during the course of the bombardment, mainly due to buildup of CO and CO2 (De Niem et al., 2012). The pressure built up as a result of the impact-produced rock and water vapor build up in the atmosphere, which helped slow down further impactors, and retain more of the vapor-plume (De Niem et al., 2012). Elemental depletion was a result of hydrodynamic escape within the hydrogen rich primordial atmosphere of Earth. As impactors enter the atmosphere, hydrogen escaped and drew heavier constituents out with it, such as Xenon (Pepin, 2006). However, while impacts stripped away parts of the atmosphere, they also contributed volatiles, including cometary water (Pepin, 2006). Comets can contain up to 50% ice, and up to 10% ice in carbonaceous chondrites (Marty and Yokochi, 2006). Thus, the competition between volatile supply by retention of the vapor cloud, and atmospheric loss via vapor expansion, would ultimately affect the volatile budget of Earth’s atmosphere (Hamano and Abe, 2010).

Earth’s lithosphere would have endured the most physical effects brought upon by the LHB through the formation of basins. When an impactor strikes, excavation of the target area ejects rock into the surrounding area, creating topographic dichotomy 3-4 km high (Grieve 1980). Partial melting of the mantle released basaltic magma into the basin floor from decompression melting (Frey, 1980). At impact sites >1000 km, the lithosphere may have been penetrated resulting in the uplift of the asthenosphere and the distortion of the geothermal gradient, which has been suggested to be steeper in the Archean than the present day, up to 90o C/km (Grieve, 1980). These thermal gradients could be steepened further by 20% from increases in impactors, which would have stirred up convection by up to 25% in the thinned and badly fractured lithosphere (Frey, 1980). Figure 7 summarizes these processes as the formation of a multi-ring structure. The large impact basins provided sinks for early oceans to localize, which suggests dry continental masses before bombardment was complete (Frey, 1980).

Conclusion

The evidence from the Moon and terrestrial planets suggests there was an increase in crater formation between 4.1 to 3.8 Ga. Even with the lunar evidence, it has been a difficult task to postulate a suitable mechanism for the causes of the LHB. While several theories have been suggested, the Nice Model is the most widely accepted. The exceptional surface preservation of craters on the Moon holds the most clear and complete impact history for the inner Solar System. Radiometric dating of lunar rocks and impact melts has provided a time frame and magnitude for the LHB that can then be interpreted for other terrestrial planets. The evidence found on Moon can be correlated with Mars and Mercury suggesting the LHB affected the entire inner Solar System. The peak impact time varies slightly between these bodies with the Moon recording the oldest peak time followed by Mercury and Mars. This suggests that the LHB may not have been a synchronous event. The LHB also had a significant impact on the Earth, which may have had long lasting implications on the Earth’s systems. This may have established the current environment that host the unique life. Although the LHB remains hypothetical as no model has yet been proven, it is likely the best explanation for the sudden increase in cratering of the terrestrial planets.

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