Ceres

The following sections are included:

• Preface
• About the Authors
• Contents

Humans’ view of the Solar System has been slowly expanding since the first day we looked up into the sky. The discovery of Ceres marked a significant milestone that opened a whole new territory of the solar system in front of us for the first time.

The study of Ceres has a long history. Early observations before the 1970s were focused on astrometry, photometry, and polarimetry that helped determine its basic properties such as orbit, albedo, size, and color. The understanding of its composition greatly benefited from the development of modern spectroscopic observations of asteroids since the 1970s. Theoretical modeling has allowed us to gain some knowledge about its interior and evolutionary history based on its shape since the start of this century. In this chapter, we provide a comprehensive review of the state of knowledge about Ceres prior to the arrival of NASA’s Dawn mission in 2015.

Dawn is one of NASA’s Discovery Class missions, which are focused, low cost, and quick in development. Dawn carries three scientific instruments, a visible camera, a visible and near-infrared mapping spectrometer, and a gamma ray and neutron detector, plus a gravity science investigation utilizing the spacecraft’s two-way radio communication system. Dawn is the first science mission that leverages the high fuel efficiency of ion propulsion systems to journey to the main asteroid belt, the first to orbit two targets with a single launch, and the first to orbit a dwarf planet…

The VIR and GRaND instruments permitted a more definitive determination of the surface composition of Ceres than ground-based observations. They also allowed the study of compositional variations and their association with surface topography and geology. Many minerals and elemental species that are key to understanding the evolution and current status of Ceres have been identified and their abundances quantified, including phyllosilicates, carbonates, organics, water ice, hydrogen, iron, and carbon…

Ceres’s surface geology has been primarily shaped by impact craters, which are responsible for its morphology on regional and local scales, as well as for partial resurfacing via blanketing by ejecta from large impacts. Mass wasting (landslides and slumping), triggered in part by micrometeorite bombardment, further acts in long-term surface modification. Ceres also displays evidence for limited tectonic features on a global scale and crater-generated fractures on a local scale. Lastly, Ceres exhibits evidence for some form of volcanic activity driven by brines in a few places (see more detail in Chapter 8). Crater morphologies as well as expressions of melting and viscous flow indicate the existence of ice in the crust (see also Chapter 6).

Many pieces of evidence from ground-based observations and theoretical modeling point to the existence of subsurface water ice on Ceres. Coordinated studies of the water ice on Ceres from its surface to the expected crust-mantle boundary have been performed with the compositional, morphological, geological, and gravitational mapping achieved by the Dawn mission, supplemented by theoretical modeling. This effort resulted in a rough picture of the current status of Ceres’s cryosphere and its history. The surface and subsurface water ice could have contributed to the production of the putative Ceres’s exosphere…

Information on the interior structure of Ceres comes from multiple sources: combined gravity and topography on a global scale, crater morphologies on a regional to local scale, and compositional constraints. This chapter reviews this state of knowledge and interpretation for Ceres’s internal evolution.

Ceres shows expressions of recent activity in a couple of places: Ahuna Mons and Occator crater (Fig. 8.1). In both cases, the newly emplaced material has been traced to a source in the upper mantle, based on geophysical observations. Hence, the study of these geological landmarks, described in turn below, can provide key information about the nature of Ceres’s interior. Ceres also displays several older sites that have been interpreted as evidence for past activity.

The Dawn mission has revolutionized our understanding of Ceres in the same decade that has also seen the rise of ocean worlds as a research focus. The most significant findings from the Dawn mission are unambiguous evidence for oceanic material exhibited on Ceres’s surface, evidence for high concentrations of organics (Ernutet region; De Sanctis et al., 2017, 2019), as well as features indicative of recent brine effusion. Dawn’s observations confirmed earlier predictions for a volatile-rich crust encompassing the bulk of the frozen ocean, and a warm and liquid-bearing interior, akin to the icy moons of the outer solar system (Fig. 9.1). The prospect that a relict ocean may have survived into the present led to Ceres’s classification as a “candidate” ocean world in the Roadmap to Ocean Worlds (Hendrix et al., 2019). Past and ongoing material transfer from the deep interior to the surface makes Ceres very interesting for astrobiology-driven exploration. The recent exposure of deep material provides a window into the conditions in Ceres’s deep brine and allows testing hypotheses of importance to ocean worlds (e.g., evolution of organics and long-term preservation of chemical gradients).

Ceres is a rich target for scientific exploration. The Dawn mission has resolved many questions about Ceres and opened new questions stemming from the discovery of the dwarf planet’s potential significance for Astrobiology. Investigations of Ceres can also inform on early solar system evolution and planetary processes. The interest in Ceres has generated several mission concepts that have in common to target the faculae in Occator crater, for in situ exploration and even sample return. Preparing for these missions will require pursuing research on various fronts: for example, continued analysis of the Dawn datasets, theoretical research on the workings of dwarf planets, experimental research on Ceres’s analogs as well as fieldwork, and observation with Earth-bound assets.

The following section is included:

• Index