Vesta’s surface is dominated by two overlapping impact basins: the older ~400 km Veneneia basin and the younger ~500 km diameter Rheasilvia basin.Their age and nature, along with the ejecta they produced in the form of V-type asteroids, can help us probe Vesta’s evolution.By modeling the production of craters superposed on these basins or on features created by their formation, we predict Veneneia and Rheasilvia basins are 3.2–3.5 Ga and ~1 Ga, respectively. Numerical models indicate they were created by the impact of ~60–70 km projectiles. These impacts likely dredged up material formed at >50 km depths within Vesta. The evidence for the formation time of Veneneia and Rheasilvia in the eucrite and howardite meteorite record exists but is limited. The absence of an obvious spike of 40Ar/39Ar shock degassing ages may be a consequence of low Main Belt impact velocities (< 5 km/s). Most V-type asteroids in the inner main belt are ejecta from one of these two basins. The scattered and limited population of V-types in the central and outer main belt have no clear source. We postulate they are fragments from Vesta-like bodies that originally formed in the terrestrial planet region.

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.

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.

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.

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.

Within the general framework of differentiation in the early solar system, the asteroid Vesta is a particularly interesting case study. First, its size is well constrained, simplifying modeling efforts that can concentrate on bodies of relevant size. Second, the rich diversity of HED meteorites provides constraints on bulk composition and a unique opportunity to confront predictions of numerical models with petrologic reality. Finally, the Dawn mission, in addition to confirming the link between Vesta and the HED’s, also provides critical constraints on the internal density structure and composition of the asteroid. In this chapter we begin by considering petrologic and geochemical constraints on the bulk composition and differentiation time-scales of Vesta, before presenting modeling efforts to understand its chemical and physical evolution. The modeling indicates accretion within the first million years of solar system history and complex thermal and chemical retroactions linked to the redistribution of 26Al during transport of melt toward the surface. Formation of a shallow magma ocean is predicted, leading to a vertically stratified mineralogical structure with olivine sequestered at depth and protracted cooling at depth. These features are consistent with the essential features of HED petrology and chronology and observations of the Dawn mission.

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.