A search for volcanic and plutonic features on Vesta was an important driver for a geomorphological examination of the asteroid. Another goal was to determine if the asteroid was a protoplanet, one of the remnants of the material that formed the Solar System. Therefore, NASA’s Dawn spacecraft collected imaging, spectroscopic, and elemental abundance data, which were utilized to examine the asteroid’s surface. A digital terrain model was created and the asteroid’s various geomorphic features were analyzed. Large scale features include the Rheasilvia and Veneneia impact basins, the Divalia Fossae and Saturnalia Fossae trough sets, and the Vestalia Terra plateau. Small scale features include deposits of dark material, pitted terrain, pit crater chains, mass-wasting deposits, and impact craters. While these geomorphic analyses revealed no evidence of volcanism, evidence of magmatic activity on Vesta was identified. In addition, analysis of Vesta’s geomorphology suggests that it is not only a protoplanet, but also an intermediate body between asteroids and planets.

Nucleosynthetic and radiogenic isotope data from meteorites have significantly advanced the understanding of how the protoplanetary disk was structured during the accretion of planetary precursors. Meteorites exhibit an isotopic dichotomy between carbonaceous (CC) and non-carbonaceous (NC) meteorites. This NC–CC dichotomy, combined with the chronology of meteorite parent body accretion, implies a potentially strict spatial divide between the inner (NC) and outer (CC) protoplanetary disk which lasted several million years. This divide may have been facilitated by early formation of the gas giant planets, which acted as a barrier, thereby significantly influencing the chemical evolution of the disk and thus the planet building process. These meteorite-derived findings and their implications for planet evolution are discussed here, with an emphasis on the role that Vesta and Ceres play in piecing together the history of the Solar System, as these bodies may be considered as samples of the inner and outer protoplanetary disk, respectively.

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.

Vesta's surface composition provides insights on its internal structure, geological evolution, and space environment. The bulk igneous composition, the link to the howardite–eucrite–diogenite (HED) meteorites, and the differentiation into a crust and a mantle were confirmed by telescopic observations and by the Dawn mission. This chapter presents several key topics. The distribution of indigenous materials helps in understanding the structure and mineralogy of the crust and the thickness of the mantle as an insight to the geological evolution and history of the whole body. Hydroxylated, low-albedo areas indicate exogenous materials and widespread contamination of the surface by carbonaceous chondrites; this main result from the Dawn mission also has implications for the collisional history of Ceres. Finally, the characterization of surficial processes on Vesta clarifies the role of space weathering and lateral mixing. The surface composition studied from telescopic observations, geochemical measurements of the HED meteorites, and from the Dawn mission at Vesta is based on reflectance imaging spectroscopy, high-resolution imagery, and elemental data from gamma-ray and neutron spectroscopy. This chapter includes analyses of data from the Visible and InfraRed mapping spectrometer that benefited from improved instrument calibrations developed after the Dawn mission to Vesta and Ceres.

The dwarf planet Ceres has unique geomorphology, different from airless silicate objects like the Moon or asteroid Vesta, but also different from the icy outer planet satellites. All four primary planetary geologic processes [impact cratering, tectonism, volcanism, and gradation (weathering, erosion, and deposition of loose material)] are visible on Ceres’ surface.Ceres’ low albedo, heavily cratered surface displays craters <300 km in diameter, in which the lack of larger, multi-ring basins suggests resurfacing event(s) early in the dwarf planet’s history. Ejecta blankets in the youngest craters display bluish ejecta and rays, and lobate deposits in and around craters suggest impact slurries, ice-rich landslides, or cryovolcanic flows. Some landslides have exposed water ice, in less than a dozen locations on the surface. Tectonic features include impact-induced secondary crater chains and non-impact-related pit chains and fractures. Several impact craters have heavily fractured floors akin to those on the Moon. The distinctive mountain Ahuna Mons appears to be a cryovolcanic edifice, composed of a viscous, salt-rich, carbonate-bearing material. Ceres distinctive bright spots, Cerealia and Vinalia Faculae within Occator crater, are composed of salt-rich liquids containing carbonates, and were likely emplaced by some combination of deep brines extrusion and hydrothermal (shallow brines) processes.

Carbon, central to astrobiology, shaped the development of the dwarf planet Ceres, a water-rich protoplanet explored by NASA’s Dawn mission. As a candidate ocean world, Ceres has the potential to provide new insights into prebiotic chemistry and habitability. This chapter reviews observations of carbon and organic matter on Ceres by Dawn and Earth-based telescopes. The observations are placed in context with astrophysical processes that produced organic matter in nebular materials from which Ceres grew. We consider mechanisms for destruction and synthesis of organic matter with changing hydrothermal conditions within Ceres’ interior. This is supported by studies of Ceres’ closest meteorite analogs, the aqueously altered carbonaceous chondrites, and halite crystals containing organic matter that may have formed within Ceres. Ultraviolet-, infrared-, and nuclear-spectroscopy show that Ceres’ surface contains a mixture of carbonates and organic matter in concentrations higher than the meteorite analogs. Ceres carbon-rich surface results from a combination of impacts and complex processes that occurred within Ceres’ interior, including low-temperature aqueous alteration, ice-rock fractionation, and modification of the accreted carbon species during serpentinization. This chapter reviews the current state of knowledge about carbon on Ceres, including sources of carbon and organics, parent body processes, remote sensing observations, and their interpretation.

Geophysical data from Dawn’s mission revealed complex and divergent internal structure evolutionary paths for Vesta and Ceres. Dawn’s data indicated that Vesta has a differentiated internal structure with uncompensated topography and Ceres is partially differentiated with compensated topography. Vesta experienced a magma ocean state, leading to effective early shape relaxation. Vesta’s current non-hydrostatic shape is dominated by Rheasilvia and Veneneia impact basins, formed when Vesta was too rigid to relax. However, northern terrains still reflect its pre-impact, closer-to-hydrostatic shape. Ceres incorporated abundant volatile material upon its accretion and subsequently underwent ice–rock fractionation. Observed surface aqueous alteration indicates extensive past hydrothermal circulation that facilitated efficient heat transfer and preserved Ceres’ interior in a relatively cool state. Lower viscosities at depth allowed isostatic compensation of Ceres’ long-wavelength topography. The high inferred abundance of water ice, hydrated salts, and/or clathrate phases suggest previous globally significant regions of solute-rich fluids that froze from the surface inward, leading to the vertical density gradient inferred from Dawn’s Second Extended Mission (XM2) high-resolution gravity data. This, coupled with thermal modeling, indicated that Ceres could have brine reservoirs, at least regionally, which were likely mobilized by the Occator crater-forming impact, leading to long-lived brine extrusion and faculae formation.

The study of the largest (D ≳100 km) Main Belt asteroids is not only important because of the clues it delivers regarding the formation and evolution of the Main Belt itself but also because many of these bodies are likely “primordial” remnants of the early Solar System, that is their internal structures have likely remained intact since their formation. Thus, many of these bodies offer, similarly to Ceres and Vesta detailed in the present book, invaluable constraints regarding the processes of planet formation over a wide range of heliocentric distances. Here, we review the current knowledge regarding these objects derived from Earth-based spectroscopic and imaging observations, with an emphasis on D >200 km bodies including Ceres and Vesta. Our motivation is to provide a meaningful context for the two largest main belt asteroids visited by the Dawn mission and to guide future in-situ investigations to the largest asteroids.

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.

The Dawn orbiter mission has revealed the mineralogical and chemical composition of Vesta’s surface materials and constraints on its interior structure. The surface is composed of breccias of basalt and ultramafic rocks, contaminated by exogenic carbonaceous chondrite.At the center of the asteroid is a metallic core about half the diameter of the body, and gravity data provide information on the thicknesses and densities of the mantle and crust.Huge, overlapping impact basins expose rocks of the lower crust and mantle. Howardite–eucrite–diogenite (HED) meteorites are samples of Vesta, mostly excavated by the giant impacts and delivered to Earth via an orbital resonance with Jupiter.Petrologic and geochemical studies of HEDs constrain interpretations of Dawn’s spectral and geochemical data, and offer otherwise unobtainable insights into the asteroid’s origin, bulk composition, global differentiation, impact history, and geochronology.Major unresolved questions include whether Vesta had an early magma ocean, as well as the source of “missing” olivine in mantle rocks, and a possible role for fluids. As the sole surviving rocky protoplanet, Vesta provides a unique perspective on the nebular raw materials that accreted to form the terrestrial planets.