Category Archives: Earth Science

Also known as Geology, Geosciences, etc. It is a science that deal with Geologic materials and processes which made them. A multidisciplinary field in which also includes Geophysics, Hydrology, Structure and Civil Engineering and many more.

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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Spudis, P. D., Wilhelms, D. E., & Robinson, M. S. 2011. The Sculptured Hills of the Taurus Highlands: Implications for the relative age of Serenitatis, basin chronologies and the cratering history of the Moon. Journal of Geophysical Research: Planets (1991-2012), v. 116, p. 1-9.

Stöffler, D., Ryder, G., Ivanov, B. A., Artemieva, N. A., Cintala, M. J., & Grieve, R. A. 2006. Cratering history and lunar chronology. Reviews in Mineralogy and Geochemistry, v. 60, p. 519-596.

Strom, R. G., Malhotra, R., Ito, T., Yoshida, F., & Kring, D. A. 2005. The origin of planetary impactors in the inner solar system. Science, v. 309, p. 1847-1850.

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Valley, J. W., 2005, A cool early Earth?. Scientific American, v. 293, p. 58-65.

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This is an online publication of the final Term Paper written by four students at the University of Calgary for Geology 535: Early Earth Evolution in Fall 2014. The text is copyrighted to all four authors. You may follow the guidelines posted under the general Site Copyright Notice. Figures used in the original paper have been removed due to copyright laws.

No part of this publication may be reproduced or redistributed in any form or format without prior permission from the authors mentioned above. Please contact Sanuja Senanayake for requests for permission to reproduce and redistribution.

Thin Section Sketches

Thin section sketches are drawings that represent what you observed. But most of us (students, researchers and professors) are not artistically inclined. Even if you are good at drawing diagrams, you still have to empathize key features when drawing a thin section sketch. Here are some tips and tricks for making a good (if not perfect) thin section sketch.

Kyanite - PPL
Photomicrograph of Kyanite – PPL

I. Microscope scale and polarization

Rarely a thin section is drawn with both PPL (plane polarized light) and XPL (cross polarized light). Thin section sketches should be drawn in one type of polarization. Depending on the mineral assemblages in the sample, choose the polarization that shows the best detail and most features. Generally the sketches are made in XPL. But this is a rule or may not work for every sample. For example, if you have a thin section with very strong differences in relief between the minerals in PPL but similar features (for example, all isotropic) under XPL, it is best to use a PPL sketch.

Do not switch between powers (lenses) on the microscope during the sketch. Choose the power that best suited for the size of minerals and features and record it beside the sketch. You should also include a scale measuring from one end of the diagram to the other. When drawing by hand I found that it is a waste of time to reduce the scale bar. I rather draw the scale bar across the entire sketch.

Key points:

1. Draw the thin sections in either PPL or XPL, not both.

2. Choose a magnification that is best suited for the size of minerals and petrologic features.

II. Drawing

It is almost impossible for a mineral to grow by itself. Minerals are attached to either the matrix or other neighboring minerals. So if you are making a sketch of a specific grain, make sure you also draw the surrounding (even if it is just glass).

In order to show relief try the following; if the grain has a strong relief compared to the surrounding, thicken the border of that grain by drawing the grain boundary over and over. If the grain has a low relief use less pressure on your pencil when drawing the grain. Use common sense to draw everything in between the highest and the lowest relief. Remember, you can have two grains with very high relief next to each other. But in order to identify that you must have lower relief minerals surrounding those grains.

Yes, you may cheat a bit on your drawing. While you should always draw only what you can see under a particular field of view, sometimes not all features can be captured in a single view. In this case you have two options; drawing two different diagrams or adding features to the diagram you have that you found in other areas of the slide. To do this, you need experience to make and educated decision so that you do not fabricate any textures or features.

Key points:

1. Draw what you see! Nature has it’s own rules such as mineral by itself in a thin section is rare.

2. Pay attention to detail such as relief.

3. Sometimes you have to add addition features from other parts of the slide to your sketches in order to show all the features in one diagram. Use educated decisions on such cases to avoid fabrication of information.

4. Use good pencils and erasers. I have seen some students just smudges their paper trying to correct diagrams.

III. Labeling

To make it neat, place the labels to one side or limit writing the labels on all sides. If your scale is for the entire field of view, makes sure the scale bar do not cut across your labels/ It is also better to use conventional abbreviations for minerals. For example, Hbl – Hornblende, Qtz – Quartz, Grt/Gt – Garnet, Cum – Cummingtonite etc. For a complete list, please consult British Geological Survey mineral list document.

Avoid labeling the same mineral more than once except when a different form of optical properties are shown. For example, you may label Hornblende both at extinction and without extinction. But do not label Hornblende in one form (non-extinct for example) more than once.

Instead of using arrow, use lines. This will avoid obscuring the what mineral or features you are highlighting.

IV. Presentation

Finally, what ever you draw make sure that the next person who look at your sketch can intemperate what you are trying communicate. There is no point of drawing a thin section sketch if no one can understand what you are drawing. If there are additional information on the sample (such as location, series number, etc) write that on the corner of your sketch. This will not only help others, but also will help you for future references.

Few tips for new Geoscience students

Fault steps
Fault steps

Even before we go into minute details on being a successful Geoscience student, let me explain why we use “Geoscience” as opposed to “Earth Science” or “Geology”. Geology can be loosely defined as the study of solid Earth and the processes which the Earth evolve. But it has grown into a multidisciplinary field with several different specializations. Geology itself has few different sub specializations such as hydrology, environmental, petroleum, engineering, mining and precious metal, geochemistry, etc. Additionally we have two major sectors; Geology and Geophysics. Hence I think the best way to describe all of these sub sections is to use the term “Geoscience”.

Classes and Labs

Unlike taking a degree in Business, Economics or Political Science, Geoscience degrees involve both theoretical lectures and practical labs. In some classes it is more important to pay attention to laboratory materials than to study lecture notes. In others, visualization skills are more important than analytical skills. When I was a first year student, even I was surprised about the amount of hands-on materials in Geology labs. To be successful, you must manage your time appropriately. Even if the lab components (including the lab exams) worth less, I would spend more time studying lab materials than lecture materials. However, each and every individual have their own system.

Here are few quick tips for labs:

  • Spend a lot of time on identification of minerals, rocks, fossils and features.
  • Ask a lot of questions from your TAs, Professors and friends.
  • Always discuss with your friends when working with lab materials (do not copy each other).
  • Try to relate lab materials to lectures.
  • Read the lab manuals carefully because they are written specifically to cover information on your course (as opposed to textbook).
  • Always pay attention to detail and create your own “tricks” to identify materials.
  • Make sure to draw diagrams with proper labels, scales and key features.
  • Lab samples are for all the students to use. It is not yours! Treat them with care and do not take any lab samples or equipment out of the lab without prior permission from an appropriate authority.

Here are few quick tips for lectures:

  • Never skip (miss) classes unless you have to. Most Professors will provide very useful hints, tricks and tips during their lectures that may not appear on lecture handouts.
  • Make useful friends (not like sitting next to a cute girl with a dumb brain) so that if you missed a class, you can get a copy of the class notes from them instead of emailing the entire 200+ students.
  • Never assume anything. Always ask questions if you are not sure about something.
  • Be prepared for the class. Some Professors may post the lecture notes in advanced. Read them or at least quickly go over the key titles/concepts. Never skip on reading recommended textbook pages or other materials.

Here are few tips for exams:

  • Always ask questions about the exam BEFORE the exam.
  • If you come across questions that you are not sure about, ask the invigilators during the exam.
  • Find old exams from exam banks such as the Geology Students Union’s exam bank. Sometimes these old exams questions can show up on your exam word to word.
  • For lab exams, spend at least few hours per each lab sample that will (may) be covered in the exam.
  • You may want to come to the labs after hours to study. If you don’t like crowded labs, I highly recommend Friday evenings or Sunday evenings.
  • Study with your friends because Geoscience is not always a clear cut science. Sometimes there is no such thing as one right answer, but rather the better answer.
  • If you have any leaning difficulties or you have any disabilities, contact the appropriate office for help as soon as possible. There is always suitable help available for most students.

Additionally there are always extra help available to students in most universities and colleges in Canada. If you do not know where to go to, ask your Professor or the Students Union.

Field Work

The best advice I can give you is to have fun. Some people may panic and overdo something. It is not very productive to be panicking. However, pay attention to the safety protocols and always ask questions before you embark on misadventures. Additionally field school will allow you to think outside the box. So be an independent thinker and try to interpret formations, rocks, minerals, etc on your own before asking for help.

Homework

I find it very irritating that some people have this fixed system of studying. For example, some say that you should spend minimum of two hours studying per each hour of lecture. While this is true for almost 90% of the students, if you happen to be that exceptional one, feel free to study less. Personally I would study as much as possible because sometimes the difference between an A- and a B+ may be just that one concept that I did not study. Also please do not get caught in procrastination. Those high school last night pull offs most likely not going to work at the university level. Additionally, I have written up on Effective Study Habits tips on a previous article.

Hope all these articles help you in someway. Good luck on your academics!

Subject to interpretation

World is full of mysteries. Science is the study of natural world through experiments and observations. Hence to solve mysteries we often rely on scientific method, logical processes, to explain the unexplainable. But what if I told you that large portion of science is also subjected to numerous interpretations?

Role of human nature

Modern science is a gradual progress of failures and accomplishments. It was not created over night nor does it have a perfect track record. Sometimes Scientists make mistakes out of limited knowledge, experience or simply due to their egos. For example, the AC-DC war (“War of Currents”) in the late 1880s both Thomas Edison and George Westinghouse did not realized the potential for both AC and DC currents. Therefore they fought for a monopoly. Today we use both types of currents; AC for home electrical systems and DC for our electronics. Another example of limited knowledge would be how we used to handle radioactive materials without the concern for radiation. In the old days Scientists like Marie Curie did not know the effects of radiation on human body. Therefore she used the same tools used for uranium salts for cooking food. She was poisoning herself without being aware of it. Unlike what most people assume, the subject of science is not perfect and it is already muddier with a so many failures and assumptions.

Public expectations

The problem with the term “science” is that majority of the general public expect it to be prefect and logical. For example, if 1+1 is equal to 2, then it must be true for everyone under any given circumstances. Public expects the Scientists like us to search for explanation to problems and find solutions the same way that 1+1=2. This is where the relationships between the general public and the scientific community fall apart.

In 2012, Geologists who did not predict the earthquake in Italy were treated with jail time. The reason the judge gave was that the Geologists downplayed a series of minor earthquakes in L’Aquila, Italy in 2009 prior to the big one. The public want accountability for our actions. But most people do not understand that science is not a perfect tool. It is a series of tools that often far from perfect. Instead of jailing the Scientists, public should have encourage them to find better ways to interpret minor earthquakes so we can predict better in the future.

Interpretations

Yes, there are many different shades of black (or any other color). You can say the “sample X was white on day 1” and “turns to dark gray on day 3”. Then someone else observing the exact same sample can say the “sample X was purple-white on day 1” and “turns to light black on day 3”. You both may be right because the color is subjected to interpretations. The ambiguity factor is everywhere. How do you do you define sea level when sea rise and fall with seasons and time of the day, how do you know the soil color index is correct, how do you separate one Geologic Formation from another, how to you measure a success of a medical trial, etc?

Scientists have spent vast amount of resources to calibrate and standardize how we record and interpret our observations. Still as individuals we have to make decisions when we observe, record and interpret data. During all three steps, we can make mistakes or we may be influenced by our superiors (CEO, Manager, Professor, “smart friend”, etc). This is not always a problem in every area of science. But it does affect a lot of Geologists. I found this is a huge problem for students in Geology and Geophysics compared to Students in Mathematics and Classical Physics.

Let’s look at the problem from a Field Geologist’s point of view. Imagine you are given the task of mapping formations in a five by five kilometer area. This is what exactly I did during field school in Carlin Canyon, Nevada in May 2014. Compared to millions of square kilometers of Earth, this is a very small area. But even with such a limited area, you can be faced with multiple interpretation issues. First you need to define lithologies. Assuming you had no outside help, it will require you to walk across the area scanning for outcrops with different properties. Then you have to define the definitions you will be using to separate one formation from another. Just because of the lithology has changed; it may not be enough to call it a new formation from the unit above/below. (There are many other features like on a fault; you must decide what type of fault and the sense of motion. The list goes on…) This is where your interpretation is important. While not everyone will agree with you, whatever the interpretation you make has to be backed up by facts; facts that you observe and recorded. These facts must be able to traced and tested. For example, providing a GPS location reading for the contact between two or more formation must be able to trace back by someone else. You may be correct or you may be completely wrong. The important thing is that you have solid evidence on what you have found.

Just because all observations are subject to interpretation, it does not mean that all interpretations and subsequent conclusions are wrong. Even if the interpretations sound too extreme, they might be right. In fact, most reputed findings are born out of interpretations that at the time might even sound crazy. For example, by analyzing a wave, scientists said we can transmit sound and images over the air wirelessly. It was a crazy idea at that time, but this lead to the development of television.

Interpretations are not wild guesses, but rather educated decisions based on previous studies and your own observations. The problem is there is a human factor in which we all think differently.

Public must be educated

An experienced Geologist cannot predict when the next earthquake or sinkhole may occur. Even with the advancement of technologies we still cannot predict when and where these would occur. We can predict, but we cannot be sure. We need to realize this is always be a problem when we try to define nature.

Perceptions that all sciences are based on clear cut observations and evidence (by the public hurts) the academic community. In my opinion the solution is to educate the public by admitting that science is not perfect. Even if it damages the controversial and politically sensitive matters like climate research, we need to admit that science is not perfect. Otherwise the trust between the science community and the public will be broken. As a result I think we might even be going back to an age where religion rules the world over the reasoning and science. That world without logic, reason and scientific method is to me, unthinkable.

Pictures from field school

The pictures are from the Carlin Canyon, Nevada Field School instructed by Dr. Charles Henderson and Dr. Benoit Beauchamp at the University of Calgary. However, this page has no affiliation to the professors or the university. This is a personal (Sanuja Senanayake) collection of images. The GPS reading are taken either from the built-in GPS locator in the camera or from field notes. I found that the location information can be highly inaccurate. One should not use the information on this page for any type of field work. The images are posted purely for entertainment.

Conglomerate

Location: 40°44’05.8″N 116°01’18.3″W
Elevation: 1478.70 m
Image direction: 100.73° (true direction)

Image: Click on the image for high resolution version.

Conglomerate formed by cyclothems.
Conglomerate formed by cyclothems.

Features: Conglomerate formed as a result of cyclothems.

Cross-bedding

Location: 40° 44′ 10.26″ N 116° 1′ 2.946″ W
Elevation: 1641.20 m
Image direction: 51.06° (true direction)

Image: Click on the image for high resolution version.

Crossbeds indicating marine environment.
Crossbeds indicating marine environment.

Features: Inclined layers with dipping indicating paleocurrents.

Paleosols

Location: 40° 44′ 6.678″ N 116° 1′ 9.48″ W
Elevation: 1653.70 m
Image direction: 66.78° (true direction)

Image: Click on the image for high resolution version.

Paleosols identified by roots and root traces.
Paleosols identified by roots and root traces.

Features: Roots and rootlets are indicative of paleosols. The Paleosols are formed under subareal exposure type environments.

Lava Rocks

Location: TBA
Elevation: TBA
Image direction: 2.05° (true direction)
Image: Click on the image for high resolution version.

Rocks formed by lava flows
Rocks formed by lava flows

Rocks formed by lava flows II
Rocks formed by lava flows II

Features: Ropy texture of this lava formation is indicative of pahoehoe lava.

Chert

Location: 40° 44′ 8.016″ N 116° 1′ 9.114″ W
Elevation: 1654.20 m
Image direction: 62.95° (true direction)

Image: Click on the image for high resolution version.

Chert sticking out of the outcrop.
Chert sticking out of the outcrop.

Chert at another location.
Chert bands sticking out of the outcrop.
Chert bands sticking out of the outcrop.

Features: Chert is more weather resistant than the outcrop surrounding them. Therefore it will stick out and easy to identify. Generally all cherts are harder than the surrounding Geologic material.

Contact between Formations

Location: 40° 43′ 46.764″ N 116° 1′ 0.006″ W
Elevation: 1663.10 m
Image direction: 89.70° (true direction)

Image: Click on the image for high resolution version.

Contact between two Formations.
Contact between two Formations.

Features: Different lithoologies will often have different weathering colours and patterns.

Misidentify a contact

Location: 40° 43′ 20.394″ N 116° 1′ 16.794″ W
Elevation: 1504.70 m
Image direction: 249.06° (true direction)

Image: Click on the image for high resolution version.

This looks like a contact, but it is not.
This looks like a contact, but it is not.

Features: This is not a contact but rather a fracture within the same formation. The difference in weathering colour may have been caused by the lower part being exposed to more fluid runoffs(?) from the fracture.

Fluvial Deposits

Location: TBA
Elevation: NA
Image direction: TBA
Image: Click on the image for high resolution version.

Fluvial deposits - Hoodoos
Fluvial deposits – Hoodoos

Features: Poorly sorted clasts from almost all Formations in the region. Lose sediments, friable and extremely poor bedding (almost no bedding). Formed as a result of weathering and erosion of other formations. May indicate a paleo river formation. Highly matrix supported with sandy size matrix particles.

Resistant (cliff forming) and Recessive

Location: 40° 43′ 40.8″ N 116° 1′ 10.1″ W
Elevation: NA
Image direction: 9.59° (true direction)

Image: Click on the image for high resolution version.

Formations : Resistant and (cliff) recessive outcrops.
Formations : Resistant and (cliff) recessive outcrops.

Features: Even from a distance we can interpret some Geological formations. In this example, we can say that at least two major formations based on the cliff forming and recessive units. Often Geologists scans the area before climbing to the outcrop of interest. In this picture we are about 1 km (or may be bit less) away from the outcrops shown.

Lose (non-outcrop type) Carbonate

Location: 40° 44′ 9.54″ N 116° 0′ 15.426″ W
Elevation: 1617.40 m
Image direction: 58.65° (true direction)

Image: Click on the image for high resolution version.

Lose sediments are an erosional feature.
Lose sediments are an erosional feature.

Features: Lose materials like these red carbonate pieces can be used to determine where the actual outcrop may be; up the slope!

Searching for outcrops like mountain goats

Location: 40° 43′ 3.3″ N 116° 0′ 9.306″ W
Elevation: 1569.70 m
Image direction: 167.06° (true direction)

Image: Click on the image for high resolution version.

Searching for outcrops.
Searching for outcrops.

Features: None that can be identified at this scale.

Miscellaneous

Chert in an unknown location (same area).
Chert in an unknown location (same area).
Crossbeds in an unknown location (same area).
Crossbeds in an unknown location (same area).