Preliminary Remarks

Petrology is the study of rocks. The prefix, petro, derives from a Greek word meaning rock, or stone. Those unacquainted with geology may be surprised to find that this subdiscipline of geology is huge. All aspects of the study of rocks are included. Not only are various rock types, their mineralogy and chemistry a part of petrology, but also the question of which rocks are where on the surface of the earth, and why. Cosmochemists, of course, must be concerned with lunar rocks and meteorites as well as their terrestrial congeners. Fortunately, lunar and meteoritic petrology are comparatively simple. A fundamental grasp of terrestrial igneous petrology will provide a good introduction to the study of extraterrestrial solids.

The three major divisions of terrestrial rocks are igneous, metamorphic, and sedimentary. Like all classifications these divisions have shortcomings. The boundaries are often ill-defined, and sometimes it is not particularly useful to place a rock in one category rather than another. Igneous rocks are those that have crystallized from a melt or magma. Geologists use the term magma to mean primarily liquid material, which may include some already solidified or not yet melted matter as well as dissolved or imprisoned gases. Lava, on the other hand, may be flowing, or long since frozen magma. Magma becomes lava when it erupts on the surface of the earth.

The signature of an igneous rock is usually evident from a freshly broken surface.

The Identification of Lines in Stellar Spectra

We start our discussion of the analysis of stellar spectra with line identifications. In chemistry, this would be called qualitative analysis. With line identification, we find out which elements and ions are present in stellar atmospheres, leaving the quantitative analysis to subsequent techniques.

The most thoroughly explored stellar spectrum is surely that of the sun. The region from λλ2935–8770, essentially the traditional spectrum available from the ground, is described in a volume by Moore, Minnaert, and Houtgast (1966). This work is a revision of a previous study that was, itself, revised from a still older work. No one at the present time needs to begin the study of any stellar spectrum from first principles. It will be possible in virtually every case to find some at least relevant identification list for a star whose spectrum is similar in nature to the one for which new identifications are desired.

Many of the classical identification studies were done from a list of measured wavelengths, for which the measurer had supplied eye estimates of the line intensities. In modern work one could always have in addition to the list of wavelengths, a tracing of the spectrum, such as the one shown in Figure 12.1. On tracings such as this, one quickly identifies some of the strongest features, such as the hydrogen lines, the resonance lines of Ca II, called H and K, or the strong lines of iron.

An Overview

We may divide the meteorites into three broad categories, the stones, the stony-irons, and the irons. Meteorite samples are described as “falls” or “finds.” If a meteorite is observed to fall, and brought to a museum curator, it is called a fall. Finds are meteorites that have not been seen to fall, or at least not by the person who discovers them.

It is now reasonable to speak of two major divisions of meteorites – those discovered in the last several decades in Antarctica, and all of the rest. The Antarctic meteorites have roughly doubled the available samples of solid cosmic debris. It is difficult to know precisely how many independent falls are represented, since all of these samples are finds. However, we must leave differences among the Antarctic and non-Antarctic samples to the references (see Koeberl and Cassidy 1991).

The non-Antarctic meteorites are named by the location of the fall or find. The names are often exotic. For the Antarctic meteorites, locations are also used for the names, but these are supplemented by alphanumerical codes. Most of the world's classified (non-Antarctic) meteorites are listed in the Catalogue of Meteorites (Graham, Bevan, and Hutchison 1985). They include listings for selected Antarctic meteorites.

Most of the meteorites in museums are irons, while the opposite is true of falls – most of the latter are stones.

Introduction

The chemical composition of matter is often the result of factors which, at first, are not at all obvious. Consider the following examples. In certain very stable stellar atmospheres a separation of the chemical elements can take place. One might think that the heavier elements would be the first to sink with respect to the abundant hydrogen, which forms the bulk of (most) stellar matter. In the earth's upper atmosphere, for example, there is a region where the heavy species sink. The number density of a given species roughly follows the law for an isothermal atmosphere, N ∝ exp(–z/H), where z is the altitude, and H the scale height, H = ℜT/gμ. (See the Index for the meaning of symbols not explained in the text.) Thus molecules such as O2 and N2 are concentrated at low altitudes relative to atomic hydrogen and helium.

For the stars in question, the situation is not so simple. There is a competing, upward force due to radiation pressure that can overwhelm gravity. Given time, exotic heavy elements such as mercury or platinum can be pushed up from the envelope of a star and concentrated in the photosphere, where they may be revealed by spectroscopy. In these stars, the abundant, light elements have a tendency to sink! We shall discuss this counterintuitive process in some detail in Chapter 13, since much of the writer's own research has been concerned with its observational consequences.

The Scope of Cosmochemistry

The cosmochemist has two basic tasks. The first is to determine the chemical composition of matter in the material universe. The form of this matter ranges from such mundane materials as terrestrial rocks to distant galaxies. The second task is to understand the reasons for the compositions that are found. While the first of these tasks is basically a matter of analytical chemistry, the second has important evolutionary aspects.

From a logical and historical point of view, cosmochemistry is an extension of the well-established discipline of geochemistry. Victor Goldschmidt, one of the pioneers of modern geochemistry, had a keen interest in abundances of the chemical elements in meteorites and the sun and stars. Goldschmidt (1937) was an early compiler of what became known as the cosmic abundances, in which the analyses of extraterrestrial sources played important roles.

The logical complement to the term geochemistry, in the astronomical domain, would be astrochemistry. This word is frequently in the literature, but typically with a more restricted meaning, such as the formation of molecules in cool interstellar clouds. Solar system astronomers have used terms such as planetary geology or lunar geochemistry rather than astrogeology or astrochemistry.

For many years, astronomers thought of cosmochemistry primarily in terms of the nuclear history of matter, and the search for a standard abundance distribution (SAD). The modern aspects of this work began with Goldschmidt, and were continued by Hans Suess and H. C. Urey (1956).

Introductory Remarks

The philosopher Auguste Comte (1798–1857) asserted that man would never know the chemical composition of the stars. It is therefore ironical that Gustav Kirchhoff (1824–1887) discovered the laws of spectroscopy at about the same time as Comte's death. With the help of the principles articulated by Kirchhoff we now claim knowledge of the composition not only of the nearby stars, but of galaxies so distant that it has taken a substantial fraction of the age of the universe for their light to reach us.

In this chapter we will review the laws of atomic and molecular spectroscopy that enable us to analyze the electromagnetic radiation from space. Naturally, we cannot give a complete account of these rather complicated topics. There is only space to highlight the nomenclature, and in some cases provide heuristic insight into the more important formulae.

The following chapters will deal with the application of atomic and molecular physics to chemical analyses of stars and stellar systems, and interstellar material.

Atomic Spectra: The Nomenclature of LS Coupling

The identification of spectral lines in a star is done with the help of certain reference volumes, the most important of which is possibly C. E. Moore's (1972) A Multiplet Table of Astrophysical Interest. While the basic work appeared as a series of the publications of the Princeton Observatory, the demand for this material was so great that it has gone through one major revision and innumerable reprints and updates.

Introduction

In 1910, the British astronomer Arthur Eddington published an influential monograph with the impressive title of Stellar Movements and the Structure of the Universe. Eddington, who became “The most distinguished astrophysicist of his time” (Chandrasekhar 1983), was only 28 when Stellar Movements was published, but his clarity of exposition and physical insight are readily seen in this small volume. Nevertheless, our present view of the Galaxy in which we live, and the universe around us, is completely different from that limned by Eddington at the end of the century's first decade. Not only were the astronomers of that time uncertain of the nature of the spiral nebulae we now call galaxies (Chapter 16), but they thought the solar system was at the center of our own system of stars.

It had been known since the time of the star gauges (counts) of William Herschel (1738–1822) that faint stars did not increase in number as one would expect, but indicated an “end” of the entire system. Today, at visual wavelengths we can in some sense detect the end of our Galaxy if we look out of the plane, toward its poles. Within the plane, starlight is significantly dimmed by interstellar material – by 1 to 2 magnitudes per kiloparsec (kpc) at visual wavelengths (§§13.3, 14.4).

If we merely count stars as a function of brightness, there is no way to distinguish between the effect of dust and an “end” of the stellar system.

An Introduction to Galactic and Extragalactic Research

In the first half of the twentieth century it became clear that our own stellar system was but one of a very large number of galaxies. To be sure, philosophers such as Immanuel Kant had speculated upon the notion of “island universes,” but it was really the work of Edwin Hubble in the mid 1920's that established the great distance of the Andromeda Nebula (galaxy), and clarified the nature of the extragalactic domain as we know it today. His marvelous book The Realm of the Nebulae (Hubble 1936) is now primarily of historical interest.

Hubble made use of certain highly luminous variable stars known as Cepheids. The Harvard astronomer Henrietta Leavitt had shown that the intrinsic brightnesses of these stars could be obtained from their periods of variation. With this relationship in hand, it was only necessary for Hubble to locate such stars in the Andromeda galaxy, and measure their periods and apparent brightnesses. Their distances followed immediately.

An interesting historical sidelight concerns the errors in the calibration of the absolute brightnesses of the Cepheid variables. By a curious combination of errors, Hubble underestimated the distance to the Andromeda galaxy by a factor between two and three. Harlo Shapley, using similar methods to determine the size of our own system, obtained results that were much more nearly correct because of a cancellation of effects of which he was unaware.

The Concept of Geochemical Classification

The Norwegian geochemist Victor Goldschmidt is the father of the notion of geochemical classifications of the chemical elements. Goldschmidt's (1954) posthumous work Geochemistry is still of great value. His basic aim was to divide the elements into groups which might be identified with the major divisions of the earth during its cooling history. He thought there might be three separate liquid phases, one metal, one silicate, and one primarily iron sulfide. These would be surrounded by a gaseous phase. He classified the elements from their association with, or preference for one or the other of these phases.

Let us begin with a consideration of the chemistry of meteorites and the earth. The earth may be divided into a core, a mantle and a crust. The chemistry of the core must be largely inferred, and this is essentially true for most of the mantle (Ringwood 1975, Pasteris 1984). The upper continental crust is relatively well sampled (Taylor and McLennan 1985), but it is not representative. Meteorites, on the other hand, have been repeatedly and thoroughly analyzed in the laboratory. Moreover, they are thought to be pieces of a broken-up planet, not unlike the earth (see, e.g., McSween 1987). Because of this they have been used to infer the chemistry of the earth as well as of much of the cosmos.

Introduction

The isotopic abundances of cosmic materials may change for a number of reasons. If a substance contains radioactive nuclei, there will be a continual decrease in the parent and a buildup of the daughter isotopes. Bombardment of materials by cosmic rays or other high-energy particles can also alter the isotopic complement of a sample. During radioactive decays or nuclear fission, particles are emitted which can affect the surrounding nuclei. Fission fragments remain in the neighborhood of the parent nuclei. A third possibility is fractionation, by either diffusion or small mass-dependent effects in chemical reactions. All three of these contingencies have been mentioned or intimated previously. We shall now take up certain aspects of these processes in detail.

It will not be possible for us to discuss most of the dating techniques. The interested reader may consult the textbooks of Faure (1986) or Durrance (1986). Richardson and McSween (1989) have an excellent chapter on radioactive dating.

Rubidium–Strontium Dating; Sample and Model Ages

One of the most straightforward methods of age determination makes use of the decay of 87Rb to 87Sr. We shall discuss this particular method here in detail, because of its pedagogical advantages. We shall have time to mention only briefly other methods, some of which are now more actively pursued than rubidium–strontium.

Both rubidium and strontium are geochemically dispersed, that is they occur primarily as impurities in major minerals.

My career as a professional astronomer was some 15 years old when it first became necessary for me to learn something of the new developments in the solar system. At that time, in the mid-1970's, I was about as ignorant of the solar system as one trained in astronomy could possibly be. Worse than that, I had an attitude typical of many astronomers today. Because the field was old, I thought it was dull and uninteresting! Nevertheless, when it became necessary for me to give an introductory course in solar system astronomy, I thought I must try to understand what all the fuss over moon rocks was all about.

Moon rocks are not so terribly different from terrestrial rocks, and so I began to read an introductory geology text. Soon, I was making trips to the building next door to visit the Geology Department. I became an amateur geologist, and a rockhound. On automobile trips I would stop at various rock formations, and bash off samples with a rock hammer. These samples were typically shown to a geologist, sometimes in a nearby university or college, sometimes back at Michigan.

The experience of becoming an amateur geologist was immensely broadening. Not only did I become a great fan of planetary science, but I began to be interested in other areas of astronomy that had never particularly appealed to me. Eventually, I began to realize that there was a single theme behind all of these endeavors – the history of matter.