2. Provenance and Cosmochemistry 12 2 Provenance and Cosmochemistry In this chapter, we consider the relationship between the composition of a planet and the origin of the material from which it was made. Since this is not a book about planet formation, there is no intent here to be comprehensive; it is merely enough background so that one can appreciate the link between these different subjects. 2.1 How should we think about a Planet? If you wanted to understand the taste of an apple or the merits of a particular wine vintage, you would surely take at least two approaches: One is to consider the biochemical processes that make an apple or a grape distinctive and cause it to develop in a particular way—in living things, this is the genetic code; the other is to consider provenance—the character of the apple or the grapes depends on the soil in which the plant grew, the climate, the particular vagaries of the seasonal weather, the tender loving care provided, and perhaps even whether the plant grew on a sun-facing slope. In many ways, this is akin to the argument in the social sciences about what determines the type of a person that a child grows up to become. The main factors can be divided into nature, the child’s genetic makeup, and nurture, how the child is raised by her parents or society. In a similar way, we should think about the two aspects of a planet: The planet as an engine, processes that operate within and change the materials from the form in which they were delivered; and the cosmic provenance of the planet, the material it was built from and how these are determined by the location and conditions where the planet formed. Unlike living things, the planetary engine is not genetically determined, but it is nonetheless strongly constrained by the laws of physics and chemistry The most striking lesson we have learned from planetary exploration is that a remarkably wide range of outcomes and behaviors is possible, and recent exoplanet discoveries provide strong confirmation of this picture of diversity. 2.2 What are Planets made of? The best way to answer this question is to “ask the planet,” which means deducing planetary composition from its observed properties. In practice, this often does not work that well even for Earth. So far as we know, there are no rocks at Earth’s surface that were delivered from Earth’s core (and maybe not even any that were delivered from the core-mantle boundary, though claims have been made). Consequently, we do not have direct observational evidence that Earth’s core is mostly iron; it could be an alloy of niobium (which can have similar density, compression properties, and electrical conductivity—indeed all of the properties attributed to Earth’s core by remote sensing). To make the argument that Earth’s core is mostly iron, we must appeal, at least in part, to cosmochemical arguments like “What is abundant as a planet forming material?” These arguments are necessarily plausible rather than rigorous, but they still make a strong case due to the large differences in cosmic abundance among materials of similar chemistry. Cosmic abundances of elements are determined by nuclear physics. Hydrogen overwhelmingly dominates because it is an elementary particle; helium is also abundant because it is stable and can be formed in the Big Bang era. Heavier elements (what2. Provenance and Cosmochemistry 13 astronomers call “metals”) are formed in stars and then ejected into the interstellar medium, where the material then becomes available to form solar systems. Combinations of alpha particles, multiples of 4 mass units with proton number equal to neutron number, are very favorable at low mass; oxygen, formed by combining four alphas, is especially favorable because of its nuclear shell structure. So oxygen is the next most abundant element followed closely by carbon. Neon (five alphas) and nitrogen (not a combination of alphas) follow somewhat behind. It is more complicated as one goes to larger masses, but magnesium, silicon and iron are particularly favored by nuclear physics. Iron is the “endpoint” of equilibrium nucleosynthesis in the sense that all more massive nuclei are less stable. Cosmic abundances can be estimated by observations of the interstellar medium and other stars. However, they are not spatially uniform because of differences in stellar activity from place to place. These abundances are continuously evolving in response to both ongoing thermonuclear synthesis and the sudden recycling of material back into the interstellar medium after a star dies; since both of these processes occur at different rates depending on the local environment, abundances themselves evolve in both space and time. This means that planets forming around other stars could have significantly different properties, even though the fundamental classes of materials (discussed in the next chapter) are surely universal. Solar system abundances are determined from the solar photosphere and correlate well with the relative abundances measured in meteorites, except for the most volatile elements. Accordingly, the ratio of hydrogen to silicon is enormously different in the Sun from the value in a meteorite or in Earth, but the ratio of Ca to Al, say, is very similar between the Sun and a meteorite. This comparison is not easy to do at high precision for many reasons, most of all that meteorites vary from one class of meteorite to another (though often only by small amounts), and determination of solar abundances from spectroscopy is not straightforward (and still debated for some important ratios, e.g., C/O, Ne/H). Meteorites are not truly primitive; they consist of materials that have been processed in some way. This processing is sometimes “planetary” in character. For example, iron meteorites are created when the parent body is large enough to have formed a core, separating its silicate and metallic iron components. Sometimes the processing involves loss of volatiles or irradiation by cosmic rays. The most primitive rocks delivered to us from space are called Type One Carbonaceous Chondrites (written CI chondrites). It is popular to suppose that meteorites provide a guide to materials in the nebula from which the solid planets formed, at least inward of the asteroid belt. It is important to understand, however, that this is really little more than a hypothesis, but it is one that has proven to have substantial