Secondary ion mass spectrometry

Secondary ion mass spectrometry (SIMS) is a widely used analytical technique in fields such as microelectronics, metallurgy, biology, geochemistry, and cosmochemistry. Major SIMS applications in cosmochemistry are measurements of the isotopic compositions of the light-to intermediate-mass elements and of minor and trace element abundances of nanometer-to micrometer-sized samples. In the context of this book, the major application of SIMS is the study of presolar dust and organics found in primitive Solar System materials. The basic principle of SIMS can be described as follows: the sample of interest is bombarded with primary ions (several keV energy), mostly oxygen or cesium. This triggers a collisional cascade in the target and secondary particles (atomic and molecular ions, neutrals) are emitted from the uppermost layers. The information depth, i.e. from where the secondary particles originate, is typically 5–20nm and depends on parameters such as primary particle energy, angle of incidence, and target composition. Typically, some permil or percent of the sputtered particles are ionized and can be analyzed in a mass spectrometer.

Secondary ion mass spectrometry is a powerful technique, which has several advantages: detection limits are ppm for most elements and ppb for favorable elements, all elements (except the noble gases) are detectable, isotopes can be distinguished, and a high lateral resolution, ranging from ≈50 nm to several μm, depending on the type of instrument and application (see below), can be achieved. Disadvantages of SIMS are its destructive nature and the fact that secondary ion yields vary by more than six orders of magnitude which makes isotope studies of certain elements very difficult or impossible.

Abstract Dust is an important constituent in the Universe. About 1% of the mass of the interstellar matter is in dust. This dust is either stardust that condensed in the winds of evolved stars and in the ejecta of supernova and nova explosions or dust that formed in dense interstellar clouds. Here, we will discuss the cycle of matter from stars to the interstellar medium and how interstellar clouds evolve to protostars and protostellar disks. We will discuss the nature and origin of interstellar dust and how it entered the Solar System. A small fraction of the stardust grains survived the earliest stages of Solar System formation and can be recognized by highly anomalous isotopic compositions as presolar grains in meteorites, interplanetary dust particles, and cometary matter, with concentrations at the subpermil level. Imprints of likely interstellar chemistry are seen as D and 15N enrichments in organic matter in primitive Solar System materials.

Dust is an important constituent in the Universe and its meaning for astrophysics is manifold. In the interstellar medium (ISM) about 1% of the mass is in dust. A major fraction of the refractory elements in the ISM is locked up in dust leading to a depletion of these elements in the gas phase. Dust is responsible for interstellar extinction (absorption and scattering of light). It was this feature that led to the first firm identification of dust in the ISM in the twentieth century. Detailed studies of interstellar extinction imply the presence of solid particles with sizes of the wavelength of visible light, i.e. in the submicrometer range.

Mineralogy of chondrite components

Calcium–aluminum-rich inclusions

Our description of meteorite mineralogy starts with the minerals characteristic of the calcium–aluminum-rich inclusions (CAIs). The mineralogy of CAIs varies systematically with their composition. The most Al-rich CAIs contain spinel, hibonite, and/or grossite. More rarely, corundum or calcium mono-aluminate is present. As the bulk composition becomes more Si-rich, the melilite solid solution becomes important. With additional Mg and Si in the bulk composition, fassaite and anorthite are present.

Inclusions that are predominantly melilite with minor spinel, perovskite, and hibonite are referred to as Type A. Most Type-A CAIs have a porous structure and are called fluffy Type-A CAIs. Some Type-A CAIs have a compact form and generally rounded shapes. These are referred to as “compact” Type-A CAIs. Type-B1 CAIs are characterized by coarse-grained, melilite-rich mantles surrounding cores composed of melilite, spinel, fassaite, and anorthite. Type-B2 inclusions have the same mineralogy, but lack the melilite-rich mantle. Type-B3 inclusions contain significant amounts of forsterite in addition tomelilite. Type-C inclusions are similar to Type B2s, but anorthite is more abundant than melilite. All Type-B and Type-C inclusions have compact morphologies.

Chondrules

Aluminum-rich chondrules

Aluminum-rich chondrules are a broad class of objects with compositions intermediate between those of CAIs and the more common ferromagnesian chondrules. Their bulk compositions are generally Mg-, Si-rich and Ca-, Al-poor relative to most CAIs.

Abstract Crystalline silicates around other stars demonstrate that protoplanetary material is often heated or processed. Similarly, primitive Solar System materials (chondrule components, IDPs, Stardust samples, comet grains) provide multiple lines of evidence for repeated dramatic heating events that affected most or all the protoplanetary materials in the first few million years. The existence of such powerful heating events is not predicted or understood from planet-formation models, yet may have had important implications on the status and composition of planetary raw materials. Here we synthesize the astronomical and meteoritic evidence for such events and discuss proposed models. By matching astronomical analogs to events in the young Solar System we attempt to reconstruct a possible scenario for the thermal processing of protoplanetary materials consistent with all evidence. We also highlight details where the astronomical and cosmochemical views are difficult to reconcile and identify key directions for future research.

Crystalline silicates are tracers of high temperatures (<1000 K), yet they are often observed in cool outer regions of protoplanetary disks (>350 K). Their origin is one of the puzzles that offer insights into the thermal evolution of protoplanetary disks. These crystals may have formed in situ in the cold disk in past heating events or may have been mixed outward from the hot inner disk; perhaps both mechanisms played a role. Similarly, once-molten silicate spherules – chondrules – and refractory inclusions delivered from the cold Asteroid Belt (∼180 K) by primitive chondritic meteorites are products of high temperatures. In many chondritic meteorites chondrules account for over 80% of the total mass, revealing that most of the primordial material in the Asteroid Belt, and perhaps elsewhere, has been heated and compacted.

Abstract In this chapter, we review the general properties of protoplanetary disks and how the gaseous and solid components contained within evolve. We focus on the models that are currently used to describe them while highlighting the successes that these models have had in explaining the properties of disks and primitive materials in our Solar System. We close with a discussion of the open issues that must be addressed by future research in order to develop fully our understanding of protoplanetary disk structures.

Protoplanetary disks are natural consequences of star formation, being composed of the molecular cloud material that had too much angular momentum to fall directly onto their central stars. While protoplanetary disks are common around young stars, observations of these objects indicate that they exhibit a range of properties, and it is unclear which, if any, of the objects that have been observed are good analogs for the solar nebula – the protoplanetary disk from which our own planetary system formed. As an example of this caveat, it is important to note that none of the over 350 (and counting) exoplanetary systems provides a good dynamical or structural analog for our own. In this chapter, we take the approach that protoplanetary disk evolution is a universal process, which can be described by a common set of models and equations, and that the variety of structures and properties that have been observed reflect a range of different starting conditions from which, and environments within which, these objects evolved.

Abstract Planet formation is a very complex process through which initially submicron-sized dust grains evolve into rocky, icy, and giant planets. The physical growth is accompanied by chemical, isotopic, and thermal evolution of the disk material, processes important to understanding how the initial conditions determine the properties of the forming planetary systems. Here we review the principal stages of planet formation and briefly introduce key concepts and evidence types available to constrain these.

Tiny solid cosmic particles – often referred to as “dust” – are the ultimate source of solids from which rocky planets, planetesimals, moons, and everything on them form. The study of the dust particles' genesis and their evolution from interstellar space through protoplanetary disks into forming planetesimals provides us with a bottom-up picture on planet formation. These studies are essential to understand what determines the bulk composition of rocky planets and, ultimately, to decipher the formation history of the Solar System. Dust in many astrophysical settings is readily observable and recent ground-and space-based observations have transformed our understanding on the physics and chemistry of these tiny particles. Dust, however, also obscures the astronomical view of forming planetary systems, limiting our knowledge. Astronomy, restricted to observe far-away systems, can only probe some disk sections and only on relatively large scales: the behavior of particles must be constrained from the observations of the whole disk.

However, planet formation is a uniquely fortunate problem, as our extensive meteorite collections abound with primitive materials left over from the young Solar System, almost as providing a perfect sample-return mission from a protoplanetary disk.