sign in sign up
<<
Home , Meteorite classification, Murchison meteorite, Winona meteorite

Chemical analysis of organic molecules in carbonaceous Torrao Pinto Martins, Zita Carla

Citation Torrao Pinto Martins, Z. C. (2007, January 24). Chemical analysis of organic molecules in carbonaceous meteorites. Retrieved from https://hdl.handle.net/1887/9450

Version: Corrected Publisher’s Version Licence agreement concerning inclusion of doctoral License: thesis in the Institutional Repository of the University of Leiden Downloaded from: https://hdl.handle.net/1887/9450

Note: To cite this publication please use the final published version (if applicable). ______

CHAPTER 1

______

Introduction

1.1 Heavenly stones-from myth to science

Ancient chronicles, from the Egyptian, Chinese, Greek, Roman and Sumerian civilizations documented the fall1 of meteorites, with Sumerian texts from around the end of the third millennium B. C. describing possibly one of the earliest words for meteoritic (Fig. 1.1 Left). Egyptian hieroglyphs meaning “heavenly iron” (Fig. 1.1 Right) found in pyramids together with the use of meteoritic iron in jewellery and artefacts show the importance of meteorites in early Egypt. Meteorites were worshiped by ancient Greeks and Romans, who struck coins to celebrate their fall, with the cult to worship meteorites prevailing for many centuries. For example, some American Indian tribes paid tribute to large iron meteorites, and even in modern days the Black Stone of the Ka´bah in Mecca is worshiped and regarded by Muslims as “an object from heaven”. The oldest preserved that was observed to fall (19th May 861) was found recently (October 1979) in a Shinto temple in Nogata, Japan. It weighted 472 g and it was stored in a wooden box. The second oldest observed fall from which the meteorite is still preserved occurred in Ensisheim in Alsace, France (at that time it was part of Germany) on the 7th November 1492, just before noon (Fig. 1.2). Soon after, the town people gathered around the place where the meteorite was lying and started chipping off pieces, thinking these were good luck charms, until stopped by the town magistrate. The meteorite was then carried into the city and placed in front of the church door. A few weeks later, the German King Maximilian travelled through the city of Ensisheim, and after examining the meteorite declared it as divine and a sign of his victory against the French enemy. The King ordered the meteorite to be preserved in the church as a reminder of the intervention from God. After the meteorite was placed inside the church, the following inscription was written next to it: “Many know about it, each know something, but no one knows enough”. The meteorite stayed there until the French Revolution, when it was moved to Colmar (France) in 1793 and fragments were taken for analysis. Years later, a 56 kg specimen of the meteorite returned to Ensisheim, being exhibited until today in the town hall.

______1”Falls” are recovered meteorites that were observed to fall, while “finds” are recovered meteorites that were not seen to fall. 1 Chapter 1

Fig. 1.1 – (Left) The Sumerian symbol Kù-an may represent the earliest word for meteoritic iron. (Right) The hieroglyph bith, meaning heavenly iron, was found in Egyptian pyramids. Taken from Bevan and de Laeter (2002).

Despite reports of meteorite falls like the one in Ensisheim, for centuries there was no scientific explanation for the “stones falling from heaven”. During the time of the Greek philosopher Aristotle (384-322 B.C.) meteorites were thought to be atmospheric phenomena. In fact, the word meteorite comes from the ancient Greek word meteoros or meteora, which means “things lifted in the air”. Aristotle thought that rocks could not fall from the sky because the heavens represented the celestial perfection. In order to explain the fall of a meteorite at Thrace near Aegospotami, Aristotle concluded that strong winds had lifted a rock formed on Earth into the atmosphere, and then dropped it again! His view was shared for many centuries.

Scientific progress was extremely slow over the following centuries. The next significant step toward the understanding of the solar system came toward the end of the sixteenth century. The work of the astronomer Copernicus (1473-1543), published in 1543, replaced the Earth (geocentric theory) by the (heliocentric theory) as the centre of the solar system. The first evidence for Copernicus’s heliocentric theory was provided by observations of the phases of and the moons of by the Italian astronomer Galileo (1564-1642). Additional evidence of a heliocentric model was presented by the German mathematician and astronomer Johannes Kepler (1571-1630), who deduced empirical laws of planetary motion, describing planetary orbits around the Sun. The English physicist and mathematician Isaac Newton (1643-1727) introduced the concept of gravity (through his theory of Universal Gravitation), which allowed the determination of the orbits of and .

By the eighteenth century scientists had a more rational view of the world, due to the influence of Enlightenment that aimed to find the truth via objective means. Many scientists refused the idea that stones could fall from the sky, with some scientists simply denying the existence of meteorites. Soon a new era would start with the work by German physicist (1756-1827). In 1794 he published a 63-page book entitled Über den Ursprung der von Pallas gefundenen und anderer ihr änlicher Eisenmassen und über einige damit in Verbindung stehende Naturerscheinungen (On the origin of the mass of iron found by Pallas and of other similar iron masses, and on a few natural phenomena connected therewith). In his book, Ernst Chladni described the stony- Pallas (that was found in Siberia in 1749) and four iron meteorites, based only on reports of fireballs and falling meteorites. He suggested that stones and iron meteorites fell from the sky and originated from cosmic space. Chladni additionally

2 Chemical analysis of organic molecules in carbonaceous meteorites

Fig. 1.2 - Drawing of the Ensisheim (1492) showing the meteorite in the air and in the wheat fields outside the city, which gives the idea of movement. Taken from http://ares.jsc.nasa.gov/Education/Activities/ExpMetMys/Lesson15.pdf.

speculated that under the influence of Earth’s gravitational force meteorites could be the observed fireballs, as friction with the terrestrial atmosphere would heat them and produce an incandescent glow. Although remarkable, especially because they were only based in few evidences, the ideas of Ernst Chladni were not accepted by the scientific community of that time. In that same year, on the 19th June 1794, a shower of stones fell at Siena (Italy). It was witnessed by such a large number of people, that its authenticity could not be denied. The simultaneous eruption of Mt Vesuvius raised the question about whether there was a possible link between these stones and volcanic activity. During the course of that year the Siena fall was documented by two eminent Italian scholars, and by the English Ambassador in Naples, Sir William Hamilton who reported simultaneously on the Siena fall and the eruption of Mt. Vesuvius, his reports being published by the Royal Society.

All these accounts led to the scientific debate around Europe about the origin of the fallen stones. Subsequent reports of meteorite falls at the World Cottage (England) in 1795, in Belaya Tserkov (Russia) and Évora (Portugal) in 1796, Salles (France) and Benares (India) in 1798, led the president of the Royal Society, Sir Joseph Banks, to think that it was time to perform a serious study on this subject. Sir Joseph Banks gave samples of the Siena and World Cottage stones to the English chemist Edward C. Howard (1774-1816). He collected two more stones and four “native ” (which according to Ernst Chladni must have fallen from the sky), and analysed them, together with the French mineralogist Jacques-Louis de Bournon (1751-1825). Bournon separated each stone into its main components, including grains of metal. In 1802, Howard showed not only similarities between the chemical composition of the four stones, but also a significant quantity of nickel (a metal that is rare in iron ores on Earth) in each of the irons and in the metal grains of the stones. This established for the first time not only a link between stony and iron meteorites that had fallen at different times and locations on Earth, but also strongly implied an origin outside the Earth. The work of Howard and Bournon rapidly convinced scientists that indeed meteorites fell from the sky, but also that they had an extraterrestrial origin. The few remaining sceptics were finally convinced by the carefully written paper of the renowned French scientist Jean-Baptiste

3 Chapter 1

Biot (1774-1862), who was commissioned by the French Minister of the Interior to investigate the fall of about three thousand stones in L’Aigle (France) on the 26th April 1803. Ernst Chladni received full credit for his hypothesis that meteorites fell from the sky. However, it took decades until his hypothesis of linking falling bodies with fireballs was generally accepted.

Until about 1860, possible origins of meteorites included condensation within the atmosphere and eruptions from lunar volcanoes. Most astronomers of the time supported the latter idea. In fact, the German-born British astronomer William Herschel (1738- 1822) reported in 1787 to have observed active volcanoes on the Moon. Ernst Chladni believed that meteorites originated from cosmic space, because of the high apparent velocities of meteorites and fireballs. However, in 1805 he accepted a probable lunar origin based on the fact that all the meteorites analysed showed no oxidation and also due to the agreement between the average density of the meteorites with calculations of the density of the Moon. On the other hand, the discovery of the first on the 1st January 1801 (Ceres) by the Italian astronomer Giuseppi Piazzi (1746-1826), and the discovery of three other between 1802 and 1807, started the discussion of a possible asteroidal origin. By 1818, and having in mind the latest asteroid discoveries, Ernst Chladni returned to his original hypothesis that meteorites originated in cosmic space. Between 1845 and 1854 twenty more asteroids were discovered. Around 1860, the lunar and the atmospheric origins were abandoned. For the next hundred years an asteroidal versus cosmic (interstellar) origin was discussed. The asteroidal hypothesis considered asteroids and therefore meteorites as debris of a that had exploded. Around 1950 there was evidence (using photographic cameras during the fall of meteorites) that the parent bodies of most meteorites were a population of objects with elliptical orbits that crossed the Earth, originating from the asteroid belt. Therefore, both hypotheses (interstellar and asteroidal) were replaced by the idea, widely accepted today, that most meteorites are fragments resulting from collisions of asteroids, and not the result of the explosion of a planet. Since 1970, comparison of the ground-based telescopic visible and near-infrared (from 0.3 to 3.3 µm) reflectance spectra of asteroids to laboratory measurements of crushed meteorite samples have clearly demonstrated that there is a generic link between the mineralogical composition of asteroids and meteorites (e.g. Hiroi et al. 1993, 1996, 2001, 2003; Luu et al. 1994; Burbine et al. 2001a, 2001b). Recently, space probes have taken a closer look at asteroids. The asteroids Gaspra and Ida were visited and photographed, in 1991 and 1993, respectively, by the American probe Galileo on its way to Jupiter (e.g. Belton et al. 1992, 1994; Veverka et al. 1994; Chapman 1996;). The asteroid Eros was orbited and probed by the American NEAR Shoemaker spacecraft in 2001 (e.g. Nittler et al. 2000, 2001; Trombka et al. 2000; Veverka et al. 2000). The Japanese mission Hayabusa to asteroid Itokawa (e.g. Abe et al. 2006; Okada et al. 2006) may return samples to Earth by 2010, which will be a major contribution to study the relation between asteroids and meteorites.

In 1982, the first was discovered in Antarctica. For the first time meteorites could be unambiguously connected to a due to their similarity in the mineralogical, chemical and isotopic composition to lunar samples collected during the Apollo missions (e.g. Bogard and Johnson 1983a; Marvin 1983; Mayeda et al. 1983).

4 Chemical analysis of organic molecules in carbonaceous meteorites

A few years later another group of rare meteorites, the SNCs, was analysed and showed a young crystallisation age (around 1.3 billion years), and a chemical and isotopic noble gas composition very similar to the atmosphere, as analysed by the Viking Landers in 1976, strongly suggesting they had originated on Mars (e.g. Bogard and Johnson 1983b; Becker and Pepin 1984).

Other possible meteorite parent bodies within the solar system are comets. Comets are dusty ice balls, composed of silicate dust and water ice, containing organic compounds such as ammonia, methane, methanol and dioxide, orbiting the Sun in highly elliptical orbits (Greenberg 1990). Recent developments in the identification of - asteroid transition objects, new information on the composition of cometary solids as well as on the collisional history of Jupiter-family comets indicates a possible cometary origin of some meteorites (Campins and Swindle 1998, and references given). Theoretical calculations suggest that many Near-Earth-Objects (NEOs) are extinct comets indicating a link between comets and asteroids (Hartmann et al. 1987). Although comets do not contain hydrous minerals, meteorites have been suggested as fragments of extinct cometary nuclei (Lodders and Osborne 1999). analysis of the CI meteorites and Ivuna also suggest that these meteorites could have come from such an extinct comet (Ehrenfreund et al. 2001). Recently, the sample return mission Stardust collected particles from the tail of the comet Wild-2. Analysis of these samples, returned to Earth on January 2006, may shed some light on the possible meteorite- asteroid link (Sandford et al. 2006).

1.2

Meteorites can be divided into two main types, depending on whether or not they have been melted. The unmelted meteorites, or , are all stony meteorites. The melted (or differentiated) meteorites include stony, stony-iron and iron meteorites. Both unmelted and melted meteorites are further sub-divided into groups and classes.

1.2.1 Iron meteorites

Iron meteorites are predominantly composed of iron-nickel alloys, and most contain only a residual percentage of non-metallic minerals. They are highly differentiated meteorites, resulting from extensive melting processes on their parent bodies. Some iron meteorites, after their surfaces are cut, polished and treated with acid show beautiful structures called the Widmanstätten patterns (Fig. 1.3). These consist of the regular intergrowth of and (iron-nickel minerals present in iron meteorites) with appropriate composition, formed during slow cooling of the parent bodies of iron meteorite. Presently, iron meteorites are classified according to a chemical classification system that divides irons into 12 different groups, each identified by letters (from A to F) and Roman numerals (from I to IV). This classification is based on the content of nickel and trace elements gallium, germanium, and iridium. Prior to the chemical classification, iron meteorites were classified in terms of their observable structural features (structural classification). For further details of the different iron meteorite groups see Buckwald (1975), Scott and Wasson (1975), Kracher et al. (1980) and Sears and Dodd (1988).

5 Chapter 1

Fig. 1.3 - The Odessa iron meteorite shows Widmanstätten patterns. Taken from http://www.minresco.com/meteor/meimages/me605a.jpg

1.2.2 Stony-iron meteorites

Stony-irons are the rarest of meteorites. Like the iron meteorites they have suffered melting. Stony-iron meteorites have approximately equal amounts of silicate minerals and iron-nickel metal. They can be divided into two big groups, and which have very different origins and histories. Pallasites are named after the German naturalist Peter Pallas, who studied the type specimen of the group, the meteorite2 (Russia). Pallasites are composed of abundant crystals enclosed in iron-nickel metal. They are assumed to have formed on the boundary region of the core-mantle of their parent bodies. Mesosiderites are named after the Greek words mesos (for middle or half) and sideros (for iron). They are thought to represent impact mixtures of metal and igneous rocks of diverse origins. For further details of the stony- iron meteorites see e.g. Buseck (1977), Scott (1977) and Haack et al. (1996).

1.2.3 Stony meteorites

Stony meteorites, as the name indicates, are largely made of rocky (silicate) materials. Additionally, they contain iron-nickel alloys and sulphides. They are the most common type of meteorite fall. Stony meteorites are divided into two groups, chondrites and , depending on whether or not they contain chondrules3.

1.2.3.1 Achondrites

Achondrites were formed by a process which involved melting and recrystallization on their parent bodies. Therefore, most achondrites resemble terrestrial igneous rocks. They ______2Meteorites are named after the town or geographic feature in which they are found. The name of Antarctic meteorites additionally contains a number. The first two digits correspond to the year in which the expedition, that found the meteorite, arrived in Antarctica and the last three digits to the specimen number. Meteorites from hot deserts take the name from the area where they where found followed by a three digit number that corresponds to the find. 3Chondrules are silicate millimetre-sized spherules, usually found in chondrites, formed by melting (or partial melting) before accretion into the meteorite parent bodies.

6 Chemical analysis of organic molecules in carbonaceous meteorites

have differentiated compositions and generally do not contain . There are many different groups of achondrites. See e.g. Clayton and Mayeda (1996) for further reading on the groups.

(i) The -- (HED) association is the most abundant class of achondrites. They represent a collection of igneous/volcanic meteorites, with similar oxygen isotopic compositions. The asteroid Vesta is thought to be the parent body of the achondrites for the HED group (Burbine et al. 2001b, and references given). are named after the 18th century British chemist Edward Howard. They are regolith containing eucrite and diogenite debris, and clasts of carbonaceous material and their crystals show damage from solar wind. are named after the Greek word eukritos meaning "easily distinguished". They closely resemble terrestrial basalts. Eucrites are composed of very small interlocking crystals, the calcium-poor pyroxene (pigeonite) and the calcium-rich plagioclase (anorthite). are named after the Greek philosopher of the 5th century B.C., Diogenes of Apollonia. Diogenites contain large crystals, meaning that they must have formed under slow cooling conditions, within the meteorite parent body.

(ii) The - association is thought to represent the residues from different degrees of partial melting of chondrites. However, their oxygen isotopic compositions show that they are different from any known . Acapulcoite took the name after a meteorite that fell near a Mexican resort in 1976. Some contain a few relics of chondrules, showing that the acapulcoites represent the transition between chondrites and achondrites. are named after the Lodran meteorite that fell in Pakistan in 1868. Their mineralogical and oxygen isotopic compositions are similar to Acapulcoites. However, they have experienced significantly higher temperatures than acapulcoites.

(iii) are igneous meteorites with approximately chondritic bulk composition. However, their trace-element pattern in addition to a unique oxygen isotopic composition indicates a distinct origin from chondrites. They are named after the meteorite Brachina that was found in Australia in 1974.

(iv) are carbon-rich igneous meteorites, whose origin and history are controversial. Some of their chemical characteristics indicate that they are highly fractionated igneous meteorites. On the other hand, their oxygen isotopic compositions suggest that they formed as residues from partial melting. Ureilites contain carbon in the low-pressure form of graphite and also in the high-pressure form of diamonds and . The ureilites are named after the Novo Urei meteorites (Russia) that fell in 1886.

(v) are closely related to the silicate inclusions in IAB iron meteorites4, showing a similar mineralogical and oxygen isotopic composition. This suggests that the ______4IAB is one of the 12 chemical groups of iron meteorites. See e.g. Scott and Wasson (1975) for further information about iron meteorite groups.

7 Chapter 1

winonaites and IAB iron meteorites have the same parent body. The (USA, 1928) is the type specimen of this group.

(vi) are basaltic igneous meteorites that have considerable amounts of calcium and aluminium. Their oxygen isotopic composition is similar to HED achondrites, but differences in chemical composition indicate that they are not related. Angrites are named after the Angra dos Reis meteorite that fell in Brazil in 1869.

(vii) , also known as enstatite achondrites, contain large amounts of the magnesium silicate, enstatite. Their highly reducing nature, mineralogy and oxygen isotopic composition are similar to those of the enstatite chondrites. Aubrites are named after Aubres, a meteorite that fell in France in 1836.

(viii) The SNC group is named after the three prototypes shergottite, and chassignite. The shergottites are named after the that fell in India (in 1865). The took their name from the shower that fell in Egypt in 1911 (famous for one of the stones hitting and killing a dog). Chassignites only have one classified member, , a meteorite that fell in France in 1815. There is an additional member of the SNC group which is the Antarctic meteorite Allan Hills (ALH) 84001. Like Chassigny, this is the only one of its type. The ALH84001 meteorite was the focus of large scientific and media attention because it was claimed to contain traces of past (fossil) Martian life (McKay et al. 1996). This led to a major controversy in the scientific community and later investigations did not support these claims (Bada et al. 1998; Jull et al. 1998; Barrat et al. 1999). All the meteorites present in the SNC group are thought to have originated from Mars. This assumption is based on their young crystallisation age (about 1.3 billion years) and noble gas composition (argon, krypton, xenon and neon) at levels similar to those detected in 1976 by the Viking Landers in the atmosphere of Mars (e.g. Bogard and Johnson 1983b; Becker and Pepin 1984).

(ix) The lunar group contains meteorites from the Moon. ALH81005, the first lunar meteorite discovery in Antarctica during the 1981-82 field season, and other members of this group show mineralogical, chemical and isotopic similarities to the lunar samples returned by the Apollo and Luna missions in the 1960's and 1970's (e.g. Bogard and Johnson, 1983a; Marvin 1983; Mayeda et al. 1983).

1.2.3.2 Chondrites

Chondrites are stony meteorites that have not been melted since their formation early in the history of the solar system, around 4.6 billion years ago. Therefore, they are the most primitive of all meteorites. Apart from the most volatile of elements, such as hydrogen and helium, chondrites have a chemical composition which is close to the composition of the Sun. Almost all chondrites contain chondrules and many chondrites also contain CAIs (calcium aluminium-rich inclusions), which are up to 1 cm irregular-shaped, white inclusions. Chondrites are divided into ordinary (O), enstatite (E), carbonaceous (C), Rumurutiite (R) and Kakangari (K) chondrites. The O, E and C chondrites are all further sub-divided, according to their chemical and mineralogical composition.

8 Chemical analysis of organic molecules in carbonaceous meteorites

(i) Ordinary chondrites (O) are the most common type of stone meteorite, and include more than half of all known meteorites. Three groups constitute the ordinary chondrites, based on the total iron and metal content: the H (high total iron content), the L (low total iron content) and LL (low total iron content; low metallic iron) chondrites. The H chondrites have the most metallic iron, but least oxidised iron, while the L and LL chondrites have less metal, but higher proportions of oxidised iron.

(ii) Enstatite chondrites (E) are so named because of their high abundance of the magnesium silicate mineral, enstatite. Enstatite chondrites have low oxygen content. Like the ordinary chondrites, they are divided into H and L type according to the total iron content. EH chondrites have high metal and high total iron content, while EL chondrites have less metal and less total iron content.

(iii) Carbonaceous chondrites (C), as the name indicates, have relatively high carbon content. Carbon is present in different forms, including , graphite, diamonds, carbonate and organic matter. The chemical compositions of carbonaceous chondrites match the chemistry of the Sun more closely than any other class of chondrites. Therefore they are considered the most primitive meteorites. Carbonaceous chondrites are thought to be formed in oxygen-rich regions of the early solar system. A further important characteristic is the presence of hydrous minerals that were formed at low temperatures by the chemical reaction of water with the original minerals. Carbonaceous chondrites contain a large variety of groups, each (apart from the CH) named after its type specimen: CI, CM, CK, CO, CR, CV, CH and CB.

- CI chondrites are named after the Ivuna meteorite that fell in Tanzania in 1938. They lack chondrules and contain a high abundance of hydrous minerals.

- CM chondrites are the most abundant carbonaceous chondrites and are typified by the Mighei meteorite that fell in Ukraine in 1889. They contain less water than the CI chondrites but their mineral composition is essentially the same. They contain some small chondrules, and minerals that were formed at relatively high temperatures. CM and CI chondrites have the richest carbon content of all carbonaceous chondrites.

- CK chondrites received their name after the meteorite Karoonda (South Africa, 1930). They are highly oxidised, contain no metal, and have iron-rich silicates.

- CO chondrites are dominated by chondrules, and have iron-nickel inclusions scattered through them. They are named after the meteorite that fell in France in 1868.

- CR chondrites were initially classified as CM chondrites, but recently the Renazzo meteorite (Italy, 1824) was considered the type specimen for this group. Chondrules are large and together with fragments make up about 50% of the meteorite. Additionally, the metal content is a distinguishable characteristic.

- CV chondrites contain large, clearly defined chondrules, and the most distinctive feature is the presence of CAIs. The meteorite Vigarano (Italy) that fell in 1910 gives its name to this group of chondrites.

9 Chapter 1

- CH chondrites were named, not from a type specimen, but from one of the properties of these meteorites. The "H" stands for "high metal", since the CH chondrites have high- iron content. As an example, the Antarctic CH meteorite ALH85085 is one of the most metal-rich chondrites. Additionally they show very fragmented chondrules.

- CB chondrites are a new group (Weisberg et al. 2001) named after the Bencubbin meteorite (Australia, 1930). They contain a large amount of metal (more than 50% nickel-iron).

Additionally, there are a few carbonaceous chondrites that cannot be classified according to their chemical and mineralogical composition, and therefore do not fit into any of these groups (ungrouped chondrites). An example of this is the Antarctic meteorite LEW85332 (e.g. Rubin and Kallemeyn 1990). More recently, the meteorite that fell in British Colombia (Canada) on the 18th January 2000 appears to be a new type of (Brown et al. 2000; Friedrich et al. 2002; Grady et al. 2002; Mittlefehldt 2002; Zolensky et al. 2002).

(iv) Rumurutiite chondrites (R) have the highest iron oxidation of all chondrites. Metals are extremely rare in this group. Rumurutiites have brecciated clasts in a fine-grained matrix. Their name comes from the only fall of the group, the Rumuruti meteorite (Kenya, 1934).

(v) Kakangari chondrites (K) are not really a group, but instead a grouplet. It is practice not to name a group until it contains at least five meteorites. Presently, there are only three known Kakangari chondrites. The type specimen of this grouplet is the Kakangari meteorite that fell in India in 1890. They are unique in their chemical composition and show an oxygen-isotopic signature that distinguishes them from all other chondrite groups (e.g. Weisberg et al. 1993, 1996).

After their accretion, chondrites experienced different degrees of thermal metamorphism5 or aqueous alteration6 processing in their parent bodies. Chondrites can be classified and divided into groups according to their primary characteristics (such as chemical and mineralogical composition). Additionally, they can also be classified by their secondary characteristics (petrologic types). These reflect the processes that occurred on the meteorite parent body and the extent. Petrologic classification of chondrites ranges from 1 to 6. A petrologic type from 4 to 6 indicates increasing thermal metamorphism. A petrologic type from 3 to 1 indicates increasing aqueous alteration. Type 3 chondrites are the least altered. Table 1.1 displays both the primary (chemical types) and secondary (petrologic types) chondrite classification.

______5Thermal metamorphism is the adjustment of the minerals in response to the increased temperatures in the meteorite parent body. 6Aqueous alteration is the transformation of the original minerals present in the meteorite parent body into a new assemblage of minerals, caused by the reaction at low temperature with water.

10 Chemical analysis of organic molecules in carbonaceous meteorites

Table 1.1 - The classification of chondrites based on their chemical types and levels of processing (petrologic types)a. Petrologic types 1 2 3 4 5 6 H Ordinary L Chondrites LL Enstatite EH Chondrites EL CI CM CK Carbonaceous CO Chondrites CR CV

Chemical types CH Rumurutiite R chondritesb Kakangari K chondrites <150ºC <200ºC 400ºC 600ºC 700ºC 750ºC 950ºC

Increasing aqueous alteration Increasing thermal metamorphism aTemperatures at which the various petrologic types were thought to be produced are shown. Black shading indicates currently known sample. Adapted from Norton (1998) and McSween (1999). bR chondrites usually have clasts of petrologic type 5-6, and matrix of petrologic type 3-4.

1.3 Primitive organic matter kits

Carbonaceous chondrites are the most primitive and unaltered type of meteorite known. It is thought that some of their types, like the CIs, have never been heated above 50°C (Leshin et al. 1997). Therefore, their composition is considered to be representative of the conditions on the early solar system. Carbonaceous chondrites contain up to 3 wt% of organic carbon (for reviews see e.g. Botta and Bada 2002; Sephton 2002; Sephton and Botta 2005). Less than 30% of it is a mixture of solvent-soluble organic compounds. The solvent fraction can be isolated by treating the meteorite sample with organic solvents of different polarities. The remaining 70% is composed of a solvent-insoluble macromolecular material (Gardinier et al. 2000; Cody et al. 2002; Cody and Alexander 2005). This material can be isolated using hydrochloric acid (to remove carbonates) and hydrofluoric acid (to remove silicates). As part of this thesis, the soluble organic matter of different carbonaceous chondrites was analysed. Therefore, we will further describe this fraction of the organic carbon present in meteorites.

The CM and CI chondrites are amongst the most analysed meteorites for soluble organic compounds, especially the CM2 chondrite Murchison. This meteorite fell in Australia on the 28th September 1969. Several pieces of the were quickly picked

11 Chapter 1

up after its fall and analysed by well-equipped laboratories. These laboratories were developed to analyse samples returned from the Moon by the Apollo 11 mission that same year. Due to the fact that a large amount of the Murchison meteorite is available and it is a relatively pristine meteorite sample, its organic content has been extensively analysed ever since. Therefore, it provides a good reference to all other carbonaceous chondrites analysed for organic compounds.

Carbonaceous chondrites, and in particular the Murchison meteorite, have a rich organic inventory. These include amino acids (e.g. Cronin and Pizzarello 1983; Cronin et al. 1988; Cronin and Chang 1993), carboxylic acids (e.g. Lawless and Yuen 1979; Yuen et al. 1984), and (e.g. Stoks and Schwartz 1979; 1981), polyols (Cooper et al. 2001), diamino acids (Meierhenrich et al. 2004), sulphonic acids (Cooper et al. 1997), hydrocarbons (e.g. Kvenvolden et al. 1970; Pering and Ponnaperuma 1971), alcohols (e.g. Jungclaus et al. 1976a), amines and amides (e.g. Jungclaus et al. 1976b; Cooper and Cronin 1995), aldehydes and ketones (Jungclaus et al. 1976a). Some of the organic compounds present in meteorites are important in terrestrial biochemistry. Namely, the amino acids are integral components of , and the are components of DNA and RNA. Table 1.2 summarizes the soluble organic compounds found in the Murchison meteorite. Carboxylic acids are the most abundant class of compounds in the soluble fraction of meteorites with a total abundance of about 330 parts per million (ppm). Amino acids are also an important component of the soluble fraction of meteorites. An amino acid total abundance of about 60 ppm was determined for the CM2 Murchison meteorite. More than 80 different amino acids have been identified in the Murchison meteorite (Cronin et al. 1988; Cronin and Chang 1993). Most of these amino acids are rare (or unknown) in the terrestrial biosphere, like α- aminoisobutyric acid (α-AIB) and isovaline.

In order to determine if the organic compounds found in carbonaceous chondrites are indigenous to the meteorites two approaches are generally applied: 1) determination of the enantiomeric ratios in chiral molecules, and 2) stable values of hydrogen, carbon and . is a useful tool for determining the origin (biotic versus abiotic) of a compound present in meteorites. For example, on Earth proteins and enzymes are made of only the L-enantiomer of chiral amino acids. On the other hand, abiotic synthesis of amino acids yields racemic mixtures. However, for non-chiral compounds, stable isotope measurements are the only mean to establish its origin (e.g. Yuen et al. 1984; Krishnamurthy et al. 1992; Cronin et al. 1993; Cooper et al. 1997).

The abundances of stable are usually expressed in δ values. These indicate the difference in per mil (‰) between the ratio in the sample and the same ratio in the standard, as shown in equation 1:

(Rsample − Rstandard ) δ()‰ = × 1000 (1) Rstandard

12 Chemical analysis of organic molecules in carbonaceous meteorites

Table 1.2 - Abundances (in ppm) of the soluble organic matter found in the Murchison meteoritea.

Compounds Concentration (ppm) Carboxylic acids (monocarboxylic) 332 Sulphonic acids 67 Amino acids 60 Dicarboximides >50 Dicarboxylic acids >30 Ketones 17 Hydrocarbons (aromatic) 15-28 Hydroxycarboxylic acids 15 Hydrocarbons (aliphatic) 12-35 Alcohols 11 Aldehydes 11 Amines 8 Pyridine carboxylic acid >7 Phosphonic acid 1.5 Purines 1.2 Diamino acids 0.4 Benzothiophenes 0.3 Pyrimidines 0.06 Basic N-heterocycles 0.05-0.5 aAdapted from Pizzarello et al. (2001); Botta and Bada (2002); Sephton (2002).

where R represents 2H/1H for hydrogen, 13C/12C for carbon and 15N/14N for nitrogen. The standards usually used are standard mean ocean water (SMOW) for hydrogen, Pee Dee Belemnite7 (PDB) for carbon, and air for nitrogen.

Meteoritic organic compounds were found to be substantially enriched in deuterium, 13C and 15N, which imply an extraterrestrial abiogenic origin (Epstein et al. 1987; Pizzarello et al. 1991; 1994; Krishnamurthy et al. 1992; Engel and Macko 1997; Pizzarello and Huang 2002; Huang et al. 2005; Busemann et al. 2006). Stable isotope measurements have been useful for establishing the origin of organic components present in carbonaceous meteorites and to show that organic compounds are delivered intact to the Earth. It was suggested that 3.8 to 4.5 Gyrs ago, during the period of heavy bombardment, extraterrestrial organic molecules were exogenously delivered to the early Earth (Chyba et al. 1990; Chyba and Sagan 1992). Therefore, carbonaceous meteorites, together with comets and interplanetary dust particles (IDPs) may have contributed the first prebiotic building blocks of life to our early planet. This may also have happened in other early planets of our solar system, namely Mars. For example, meteoritic material was shown to deliver high concentrations of carbon onto the surface of Mars (Flynn 1996). However, in 1976 the Viking mission was unable to detect any traces of organic material above the parts per billion (ppb) to ppm level on the surface of Mars (e.g. Biemann et al. 1977). The US missions Phoenix and Mars Science Laboratory, the European mission ExoMars, and additional future missions to Mars will continue to look ______7Belemnite is a Cretaceous fossil found in the Pee Dee formation, South Carolina (USA).

13 Chapter 1

for traces of extinct or extant life on that planet. Laboratory analyses of meteoritic material together with future missions (including sample return missions) to solar system planets and satellites, asteroids and comets will provide crucial insights on the early history of our solar system, the link between meteorites and their parent bodies, and the origin and evolution of life on our planet and maybe elsewhere.

1.4 Thesis outline

The research described in this thesis focuses on the analysis of the soluble organic matter of carbonaceous meteorites. These experiments seek to contribute to our understanding of the formation and evolution of extraterrestrial organic compounds, and their possible importance for the origin of life on Earth.

In Chapter 2 we analyse the free carboxylic acids present in water extracts of the CM2 chondrite Murchison and the CI1 chondrite Orgueil, using gas chromatography- (GC-MS). The analyses revealed the presence of a structurally diverse suite of both aliphatic and aromatic acids in Murchison. The first detection of free phthalic acids, methyl phthalic acids, and hydroxybenzoic acids in the Murchison meteorite is reported. Orgueil shows a much more simple distribution containing exclusively aromatic acids. Most structural isomers of the aromatic acids were identified in both Murchison and Orgueil, suggesting an origin by abiotic processes. Carboxylic acids are much more abundant in Murchison than in Orgueil. The data suggest that different degrees of aqueous alteration on the meteorite parent body (or bodies) has produced dissimilar distributions of carboxylic acids.

In Chapter 3 we present for the first time compound-specific carbon isotope measurements of and compounds present in carbonaceous meteorites. Formic acid extracts of the CM2 chondrite Murchison were analysed for the presence of nucleobases using GC-MS and gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). The formic acid extracts were purified to limit the presence of interfering compounds during chromatographic analysis. Compound specific carbon isotope ratio measurements by GC-C-IRMS indicate that the measured purine and pyrimidine compounds are indigenous components of the Murchison meteorite. Carbon isotope ratios for and of δ13C = +44.5‰ and +37.7‰, respectively, indicate a non-terrestrial origin for these compounds. These new results demonstrate the presence of genetic code components in the early solar system that may have played a key role in life’s origin.

The possible link of the amino acid composition between CM2, CM1 and CI1 carbonaceous chondrites is discussed in Chapter 4. Amino acid analyses were performed for the first time on Antarctic CM1 chondrites. MET01070, ALH88045 and LAP02277 were analysed for their amino acid content using GC-MS and high performance liquid chromatography-fluorescence detection (HPLC-FD). The concentrations of the eight most abundant amino acids in these meteorites were compared to those of CM2s and also CI1s. The total amino acid concentration in CM1 carbonaceous chondrites was much lower than the average of the CM2s. Relative amino

14 Chemical analysis of organic molecules in carbonaceous meteorites

acid abundances were also compared in order to identify synthetic relationships between the amino acid compositions in the CM2, CM1 and CI1 meteorite classes. The data indicate that amino acids in CM and CI-type meteorites were synthesized under different physical and chemical conditions.

In Chapter 5 we report on the analyses of the Oman desert CR Shisr 033. Several different analytical techniques were used to study the degree of terrestrial alteration of this meteorite and also its petrologic classification. Bulk chemical analyses confirm the CR classification. Oxygen isotope analysis on a bulk sample additionally indicates that Shisr 033 is of type CR2. The amino acid content of the Shisr 033 meteorite was analysed using HPLC-FD and liquid chromatography-time of flight-mass spectrometry (LC-ToF-MS). Amino acid analysis show that the absolute and the relative amino acid content of Shisr 033 is distinct from other carbonaceous chondrites. Oxygen isotope analysis of a phyllosilicate-rich dark inclusion shows that this inclusion is closer to CV3 or CO3 chondrites. The effects of terrestrial weathering in Shisr 033 are evident by the dark inclusion carbon isotopic data, bulk chemistry, and amino acid data. Yet, Shisr 033 contains a small fraction of indigenous components indicated by the presence of the extraterrestrial amino acid α-AIB. Finally, the terrestrial age of Shisr 033 was determined and discussed in the context of high levels of contamination.

The first ever measurements of amino acids in Antarctic CR meteorites are presented in Chapter 6. EET92042, GRA95229 and GRO95577 were analysed for their amino acid content using HPLC-FD, GC-MS and GC-C-IRMS. Fig. 1.4 displays typical HPLC-FD chromatograms of the acid hydrolysed, hot-water extracts of the Antarctic CR meteorites plus a serpentine blank. EET92042 and GRA95229 are the most amino acid-rich carbonaceous chondrites ever analysed. Their total amino acid concentrations ranged from 180 ppm to 249 ppm. However, GRO95577 is depleted in amino acids. The α- amino acids , isovaline, α-AIB, and are the most abundant amino acids present in the EET92042 and GRA95229 meteorites, with δ13C values ranging from + 31.6‰ to +50.5‰. Carbon isotope results and racemic enantiomeric ratios found for most amino acids suggest an abiotic origin of these compounds. Additionally to the exceptionally high abundance of amino acids in EET92042 and GRA95229, these meteorites show the highest L-enantiomeric excess of isovaline (33.0% and 35.9%, respectively) ever measured.

The results obtained in this thesis provide crucial insights on the indigenous organic compounds present in carbonaceous meteorites. The performed research aims to provide an important contribution to understand the formation and evolution of these compounds. Ultimately, the physical and chemical processes that occurred during and after the formation of the solar system have altered these organic compounds. The determination of their molecular and isotopic composition is essential to understand and constrain these processes. In this thesis the use of complementary analytical techniques allowed us to gain information about extraterrestrial delivery processes of organic compounds, subsequent incorporation into the carbonaceous material available on the young Earth, and their possible importance for the origin and evolution of life on Earth.

15 Chapter 1

A 6 15

10 11 12 14 4 13 12 3 5 7 89 1617 GRA95229 15

6

Relative Intensity Relative 11 13 10 12 3 4 14 17 12 5 7 8 9 16 EET92042 Blank

0 5 10 15 20 25 30 35 40 Retention time (minutes)

B

10 6 13 GRO95577 12 3 11 1415 16 12 4 5 7 89 17

Relative Intensity Relative 10 6 1 2 3 5 8 11 16 Blank

0 5 10 15 20 25 30 35 40 Retention time (minutes)

Fig. 1.4 – Amino acid analysis of Antarctic CR meteorites. The 0 to 40 min region (no peaks were observed outside this region) of the HPLC-FD chromatograms (described in chapter 6). OPA/NAC derivatisation of amino acids in the 6 M HCl-hydrolysed hot-water extracts from (A) the CR2 carbonaceous chondrite EET92042 and GRA95229, and the serpentine blank; (B) the CR1 carbonaceous chondrite GRO95577 and corresponding serpentine blank. HPLC-FD chromatograms (A) and (B) are not on the same scale. Peaks were identified by comparison of the retention time to those in the amino acid standard run on the same day: 1. D-aspartic acid; 2. L- aspartic acid; 3. L-; 4. D-glutamic acid; 5. D, L-; 6. glycine; 7. β-Alanine; 8. γ- ABA; 9. D, L-β-AIB; 10. D-alanine; 11. L-alanine; 12. D, L-β-ABA; 13. α-AIB; 14. D-isovaline; 15. L-isovaline; 16. L-valine; 17. D-valine. The EET92042 and GRA95229 meteorites show the highest abundance of amino acids as well as the highest L-enantiomeric excess of isovaline ever measured in a meteorite.

16 Chemical analysis of organic molecules in carbonaceous meteorites

References

References used in the first four pages of section 1.1 Heavenly stones-from myth to science are indicated by the symbol *.

Abe et al. 2006. Science 312, 1334-1338. Bada J. L., Glavin D. P., McDonald G. D. and Becker L. 1998. Science 279, 362. Barrat J. A., Gillet Ph., Lesourd M., Blichert-Toft J. and Poupeau G. R. 1999. Meteorit. Planet. Sci. 34, 91-97. Becker R. H. and Pepin R. O. 1984. Earth Plan. Sci. Lett. 69, 225-242. Belton M. J. S., Veverka J., Thomas P., Helfenstein P., Simonelli D., Chapman C., Davies M. E., Greeley R., Greenberg R. and Head J. 1992. Science 257, 1647- 1652. Belton M. J. S. et al. 1994. Science 265, 1543-1547. *Benest D. 2001. Little planets. In Meteorites, their impact on science and history. Edited by Zanda B. and Rotaru M.. Cambridge University Press. pp 68-75. *Bevan A. and de Laeter J. 2002. In Meteorites, a journey through space and time. Smithsonian Institution Press in association with the University of New South Wales Press. Biemann K., Oro J., Toulmin P. III, Orgel L. E., Nier A. O., Anderson D. M., Flory D., Diaz A. V., Rushneck D. R., Simmonds P. G. 1977. J. Geophys. Res. 82, 4641- 4658. Bogard D. D. and Johnson P. 1983a. Geophys. Res. Lett. 10, 801-803. Bogard D. D. and Johnson P. 1983b. Science 221, 651-654. Botta O. and Bada J. L. 2002. Surv. Geophys. 23, 411-467. Brown et al. 2000. Science 290, 320-325. Buckwald V. F. 1975. In Handbook of iron meteorites. University of California, Berkeley. Burbine T. H., Binzel R. P., Bus S. J. and Clark B. E. 2001a. Meteorit. Planet. Sci. 36, 245-253. Burbine T. H., Buchanan P. C., Binzel R. P., Bus S. J., Hiroi T., Hinrichs J. L., Meibom A. and McCoy T. J. 2001b. Meteorit. Planet. Sci. 36, 761-781. Buseck P. R. 1977. Geochim. Cosmochim. Acta 41, 711-721. Busemann H., Young A. F., Alexander C. M. O'D., Hoppe P., Mukhopadhyay S., Nittler L. R. 2006. Science 312, 727 – 730. Campins H. and Swindle T. D. 1998. Meteorit. Planet. Sci. 33, 1201-1211. Chapman C. R. 1996. Meteorit. Planet. Sci. 31, 699-725. Chyba C. F., Thomas P. J., Brookshaw L. and Sagan C. 1990. Science 249, 366-373. Chyba C. F. and Sagan C. 1992. Nature 355, 125-132. Clayton R. N. and Mayeda T. K. 1996. Geochim. et Cosmochim. Acta 60, 1999-2017. Cody G. D., Alexander C. M. O’D. and Tera F. 2002. Geochim. Cosmochim. Acta 66, 1851–1865. Cody G. D. and Alexander C. M. O’D. 2005. Geochim. Cosmochim. Acta 69, 1085– 1097. Cooper G. W. and Cronin J. R. 1995. Geochim. Cosmochim. Acta 59, 1003-1015. Cooper G. W., Thiemens M. H., Jackson T. L. and Chang S. 1997. Science 277, 1072.

17 Chapter 1

Cooper G., Kimmich N., Belisle W., Sarinana J., Brabham K. and Garrel L. 2001. Nature 414, 879-883. Cronin J. R. and Pizzarello S. 1983. Adv. Space Res. 3, 5-18. Cronin J. R., Pizzarello S. and Cruikshank D. P. 1988. Organic matter in carbonaceous chondrites, planetary satellites, asteroids and comets. In Meteorites and the early solar system. Edited by Kerridhe J. F. and Matthews M. S. University of Arizona Press. pp. 819-857. Cronin J. R. and Chang S. 1993. Organic matter in meteorites: molecular and isotopic analyses of the Murchison Meteorite. In The Chemistry of Life's Origin. Edited by Greenberg J. M., Mendoza-Gomez C. X. and Pirronello V. Kluwer Academic Publishers. pp. 209-258. Cronin J. R., Pizzarello S., Epstein S. and Krishnamurthy R. V. 1993. Geochim. Cosmochim. Acta 57, 4745-4752. Ehrenfreund P., Glavin D. P., Botta O., Cooper G. and Bada J. L. 2001. Proc. Nat. Acad. Sci. USA 98, 2138-2141. Engel M. H. and Macko S. A. 1997. Nature 389, 265-268. Epstein S., Krishnamurthy R. V., Cronin J. R., Pizzarello S. and Yuen G. U. 1987. Nature 326, 477-479. Flynn G. J. 1996. Earth, Moon and Planets 72, 469-474. Friedrich J. M., Wang M.-S. and Lipschutz M. E. 2002. Meteorit. Planet. Sci. 37, 677- 686. Gardinier A., Derenne S., Robert F., Behar F., Largeau C. and Maquet J. 2000. Earth Planet. Sci. Lett. 184, 9-21. *Grady M. 2001. In . Edited by Coyne C. Smithsonian Institution Press in association with The Natural History Museum. Grady M. M., Verchovsky A. B., Franchi I. A., Wright I. P. and Pillinger C. T. 2002. Meteorit. Planet. Sci. 37, 713-735. Greenberg J. M. and Hage J. I. 1990. Astrophys. J. 361, 260-274. Haack H, Scott E. R. D. and Rasmussen K. L. 1996. Geochim. Cosmochim. Acta 60, 2609-2619. Hartmann W. K., Tholen D. J. and Cruikshank D. P. 1987. Icarus 69, 33-50. Hiroi T., Pieters C. M., Zolensky M. E. and Lipschutz M. E. 1993. Science 261, 1016- 1018. Hiroi T., Zolensky M. E., Pieters C. M. and Lipschutz M. E. 1996. Meteorit. Planet. Sci. 31, 321-327. Hiroi T., Zolensky M. E. and Pieters C. M. 2001. Science 293, 2234-2236. Hiroi T. and Hasegawa S. 2003. Antarct. Meteorite Res. 16, 176-184. Huang Y., Wang Y., Alexandre M. R., Lee T, Rose-Petruck C., Fuller M. and Pizzarello S. 2005. Geochim. Cosmochim. Acta 69, 1073-1084. Jull A. J. T., Courtney C., Jeffrey D. A. and Beck J. W. 1998. Science 279, 366. Jungclaus G. A., Yuen G. U., Moore C. B. and Lawless J. G. 1976a. 11, 231- 237. Jungclaus G., Cronin J. R., Moore C. B. and Yuen G. U. 1976b. Nature 261, 126-128. Kracher A., Willis J. and Wasson J. T. 1980. Geochim. Cosmochim. Acta 44, 773-787. Krishnamurthy R. V., Epstein S., Cronin J. R., Pizzarello S. and Yuen G. U. 1992. Geochim. Cosmochim. Acta 56, 4045-4058.

18 Chemical analysis of organic molecules in carbonaceous meteorites

Kvenvolden K., Lawless J., Pering K., Peterson E., Flores J., Ponnamperuma C., Kaplan I. R. and Moore C. 1970. Nature 228, 923-926. Lawless J. G. and Yuen G. U. 1979. Nature 282, 396-398. Leshin L. A., Rubin A. E. and McKeegan K. D. 1997. Geochim. Cosmochim. Acta 61, 835-845. Lodders K. and Osborne R. 1999. Space Sci. Rev. 90, 289-297. Luu J., Jewitt D. and Cloutis E. 1994. Icarus 109, 133-144. Marvin U. B. 1983. Geophys. Res. Lett. 10, 775-778. *Marvin U. B. 1992. Meteoritics 27, 28-72. *Marvin U. B. 1996. Meteorit. Planet. Sci. 31, 545-588. *Marvin U. B. 2001. Stones which fell from the sky. In Meteorites, their impact on science and history. Edited by Zanda B. and Rotaru M. Cambridge University Press. pp 16-29. Mayeda T. K., Clayton R. N., and Molini-Velsko C. A. 1983. Geophys. Res. Lett. 10, 799-800. McKay D. S., Gibson E. K., Thomas-Keprta K. L., Vali H., Romanek C. S., Clemett S. J., Chiller X. D. F., Maechling C. R. and Zare R. N. 1996. Science 273, 924-930. *McSween H. Y. Jr. 1999. In Meteorites and their parent planets. Cambridge University Press. Meierhenrich U. J., Muñoz Caro G. M., Bredehöft J. H., Jessberger E. K. and Thiemann W. H.-P. 2004. Proc. Nat. Acad. Sci. USA 101, 9182-9186. Mittlefehldt D. W. 2002. Meteorit. Planet. Sci. 37, 703-712. Nittler L. R., Clark P. E., McCoy T. J., Murphy M. E. and Trombka J. I. 2000. Abstract #1711. 31st Lunar Planet. Sci. CR-ROM. Nittler L. R. et al. 2001. Meteorit. Planet. Sci. 36, 1673-1695. *Norton O. R. 1998. In Rocks from space, meteorites and meteorite hunters. Moutain Press Publishing Company. Okada T., Shirai K., Yamamoto Y., Arai T., Ogawa K., Hosono K. and Kato M. 2006. Science 312, 1338-1341. Pering K. L. and Ponnamperuma C. 1971. Science 173, 237-239. Pizzarello S., Krishnamurthy R. V., Epstein S. and Cronin J. R. 1991. Geochim. Cosmochim. Acta 55, 905-910. Pizzarello S., Feng X., Epstein S. and Cronin J. R. 1994. Geochim. Cosmochim. Acta 58, 5579-5587. Pizzarello S., Huang Y., Becker L., Poreda R. J., Nieman R. A., Cooper G. and Williams M. 2001. Science 293, 2236-2239. Pizzarello S. and Huang Y. 2002. Meteorit. Planet. Sci. 37, 687-696. Rubin A. E. and Kallemeyn G. W. 1990. Meteoritics 25, 215-225. Sandford et al. 2006. Abstract #112. 37th Lunar Planet. Sci. CR-ROM. Scott E. R. D. and Wasson J. T. 1975. Rev. Geophys. Space. Phys. 13, 527-546. Scott E. R. D. 1977. Geochim. Cosmochim. Acta 41, 693-710. Sears D. W. G. and Dodd R. T. 1988. In Meteorites and the early solar system. University of Arizona Press. Sephton M. A. 2002. Nat. Prod. Rep. 19, 292-311. Sephton M. A. and Botta O. 2005. Int. J. Astrobio. 4, 269-276. *Shima M., Murayama S., Yabuki H. and Okada A. 1980. Meteoritics 15, 365.

19 Chapter 1

Stoks P. G. and Schwartz A. W. 1979. Nature 282, 709-710. Stoks P. G. and Schwartz A. W. 1981. Geochim. Cosmochim. Acta 45, 563-569. Trombka et al. 2000. Science, 289, 2101-2105. Veverka J., Belton M., Klaasen K. and Chapman C. 1994. Icarus 107, 2-17. Veverka et al. 2000. Science 289, 2088-2097. Weisberg M. K., Prinz M., Clayton R. N., Mayeda T. K., Grady M. M. and Franchi I. 1993. Meteoritics 28, 458. Weisberg M. K., Prinz M., Clayton R. N., Mayeda T. K., Grady M. M., Franchi I., Pillinger C. T. and Kallemeyn G. W. 1996. Geochim. Cosmochim. Acta 60, 4253- 4263. Weisberg M. K., Prinz M., Clayton R. N., Mayeda T. K., Sugiura N., Zashu S. and Ebihara M. 2001. Meteorit. Planet. Sci. 36, 401-418. Yuen G., Blair N., Des Marais D. J. and Chang S. 1984. Nature 307, 252-254. Zolensky M. E., Nakamura K., Gounelle M., Mikouchi T., Kasama T., Tachikawa O. and Tonui E. 2002. Meteorit. Planet. Sci. 37, 737-761.

20