The asteroid belt was dynamically shaped during and after planet formation. Despite representing a broad ring of stable orbits, the belt contains less than one one-thousandth of an Earth mass. The asteroid orbits are dynamically excited, with a wide range in eccentricity and inclination, and their compositions are diverse (generally dry objects in the inner belt and more water-rich objects in the outer belt). The asteroid belt’s origins and dynamical history are reviewed. The classical view is that the belt was born with several Earth masses in planetesimals, then strongly depleted. However, it is possible that very few planetesimals ever formed in the asteroid region and the belt’s story is one of implantation rather than depletion. Many processes may have implanted asteroids from different regions of the Solar System, dynamically removed them, and excited their orbits. During the gaseous disk phase these include the effects of giant planet growth, migration, and sweeping secular resonances. After this phase these include scattering from resident planetary embryos, chaos in the giant planets’ orbits, giant planet instability, and long-term dynamical evolution. Different global models for Solar System formation imply contrasting dynamical histories of the asteroid belt. Vesta and Ceres may have been implanted from opposite regions of the Solar System – Ceres from the Jupiter–Saturn region and Vesta from the terrestrial planet region – and could therefore represent very different formation conditions.

The presence of ammonium on Ceres was first speculated based on telescopic data in the 1990s. Subsequent data from Dawn unambiguously confirmed the presence on Ceres’s surface. Ammonium has been identified within near-ubiquitous dark materials, and in salts in few localized bright faculae in the interiors of craters as we describe further in this chapter.

The presence of ammonium on Ceres is significant because it implies the availability of ammonia during its evolution. More broadly, understanding the processes that led to the presence of ammonium on Ceres provides important information on the aqueous environments in the early Solar System and the origins and dynamical histories of the large outer main belt asteroids. We briefly review the significance of ammonia and then describe what was known or speculated about ammoniated species on Ceres before Dawn’s arrival. We then review findings of the Dawn mission, in particular the detection and mapping of ammoniated phases by the Visible and Infrared spectrometer (VIR): which species host ammonia/ammonium, their abundance, and spatial distribution. We then discuss the potential origins and implications of ammonia, drawing on laboratory studies and modeling efforts. Finally, we summarize the key findings and the outstanding questions that remain for future investigation.

In 1992, NASA’s planetary efforts were invigorated with the launch of the Discovery Program of principal investigator-led missions. Over the next eight years, a group of planetary scientists and engineers gathered regularly to design and propose to NASA solar-electric propulsion missions targeted to various scientifically important bodies. Ultimately, Dawn, a mission to orbit and explore both Vesta and Ceres, was selected for flight in 2001. It launched in 2007, arrived at Vesta in July 2011, and departed in September 2012 for Ceres. Arrival at Ceres occurred in March 2015, where Dawn operated productively until 31 October 2018, when it exhausted its attitude control propellant. Herein, we summarize the history of Dawn and recount the observations and discoveries made by this pioneering mission.

Ceres’ composition has been a long-standing issue since the first ground-based observations because of its peculiar spectrum and lack of an established connection with meteorites. NASA’s Dawn mission acquired unprecedented measurements of the surface of the dwarf planet Ceres, bringing a breakthrough in the comprehension of the mineralogy of the surface. Ceres’ surface is a mixture of ultra-carbonaceous material, Mg-phyllosilicates, NH4-phyllosilicates, carbonates, organics, Fe-oxides, and volatiles, as determined by remote sensing instruments onboard Dawn: Visible and InfraRed imaging spectrometer (VIR), Gamma Ray and Neutron Detector (GRAND), and the Framing Camera (FC). The average mineralogy of Ceres reveals a possible past global aqueous alteration. Regional variations of such materials unveil possible processing acting on large scales, both endogenous and exogenous. Local areas of Ceres surface, on a spatial scale of a few kilometers, present significant spectral variations with respect to the measured average spectrum, and thus significant variation on the inferred mineralogy. Most imply recent or ongoing geological activity involving upwelling of subsurface carbonate-rich and salt-rich brines (Occator crater and Ahuna Mons), organic material (Ernutet crater), hydrated carbonates, and water ice (Oxo and Juling craters). Global aqueous alteration and recent hydrothermal activity place Ceres among the most interesting targets in astrobiology.

The Dawn mission revealed that Ceres’ interior underwent partial differentiation and aqueous alteration, probably in its early history. The dwarf planet also preserved brines until present, at least on a regional scale. This chapter addresses the various processes involved in shaping Ceres’ interior based on the Dawn observations and knowledge gained from the analysis of carbonaceous chondrites and from observations of other icy worlds. The Dawn results highlight the importance of better understanding the extent of the feedback between geophysical and chemical evolution in ice-rich bodies. In particular, brines produced as a consequence of aqueous alteration can drive geological activity and the transfer of material from the deep interior to the surface. The four main evolution pathways proposed to explain Ceres’ current state are assessed against observational constraints. Most of these models offer explanations for the presence of deep brines below Ceres’ crust. However, uncertainties in the density of Ceres’ mantle and the extent of the brine reservoir prevent converging on the most likely evolutionary path. Altogether, the knowledge gained at Ceres can be applied to other icy worlds, and in particular to dwarf planets and icy moons with limited tidal heating.

Chemical compositions of asteroids, comets, and their samples

This chapter provides a brief review of missions using X-ray, gamma-ray, and neutron spectroscopy to determine the chemical composition of planetary surfaces. This chapter presents the history of planetary radiation measurements, including significant discoveries. Summary tables with links to the archived data provide a resource for readers interested in working in this field. Upcoming missions and possible future directions are described.

New visible and infrared data of minor bodies, including minor planet 1 Ceres, asteroids 4 Vesta, 21 Lutetia, 2867 Steins and comet 67P/Churyumov–Gerasimenko (hereafter 67P/CG) have been collected in the last years by remote sensing instruments aboard NASA-Dawn and ESA-Rosetta missions. These minor bodies are among the most primitive bodies in the Solar System, and the understanding of their composition, surface morphology and evolution history is a fundamental step to shed light on the processes that occurred during planetary formation.By merging spatial and spectral information retrieved from the surfaces of these objects it is possible to infer their composition and physical properties and to correlate them with local morphology and geological processes. A discussion about spectral indicators, modeling, and mapping is given for both asteroids and comet 67P/CG. Given that the remote sensing observation techniques are very similar between Dawn and Rosetta missions, a comparative approach is used for the entire chapter and methods and interpretation for the results of these different objects are given together.