INTRODUCTION: Dwarf planet Ceres’ low average-density (2162 ± 3 kg m ⁻³ ) indicates that it must contain considerable water. Water is likely a key component in the chemical evolution and internal activity of Ceres, possibly resulting in a layer of ice-rich material and perhaps liquid in the mantle. Mineral hydroxides (OH-bearing) and hydrates (H ₂ O-bearing), such as clays, carbonates, and various salts, would be created. These hypotheses were supported by the detection of hydroxyl (OH)–rich materials, OH-bearing molecule releases, H ₂ O vapor molecules, and haze. However, the presence of H ₂ O on the surface has not previously been confirmed. The detection and mapping of H ₂ O on Ceres is one objective of the Dawn spacecraft, in orbit around Ceres since March 2015. RATIONALE: The purpose of the Dawn space mission at Ceres is to study the geology, geophysics, and composition remotely by means of high-resolution imagery and spectrometry. Dawn’s Visible and InfraRed Mapping Spectrometer (VIR) measures the sunlight scattered by the surface of Ceres in a range of wavelengths between 0.25 and 5.1 μm. The position and shape of absorption features in VIR reflectance spectra are sensitive to the surface mineral and molecular composition. In spectroscopy, absorption bands at 2.0, 1.65, and 1.28 μm are characteristic of vibration overtones in the H ₂ O molecule. RESULTS: Dawn has detected water-rich surface materials in a 10-km-diameter crater named Oxo, which exhibit all absorption bands that are diagnostic of the H ₂ O molecule (see the figure). These spectra are most similar to those of H ₂ O ice, but they could also be attributable to hydrated minerals. Oxo crater appears to be geologically very young (~1 million to 10 million years); it has sharp rims and its floor is almost devoid of impacts, suggesting a recent exposure of surface H ₂ O. The high latitude and morphology of the Oxo crater protects much of the surface area from direct solar illumination for most of the cerean day, presenting favorable conditions for the stability of water ice or heavily hydrated salts. CONCLUSION: Four ways to create or transport H ₂ O on Ceres are considered: (i) Exposure of near-surface H ₂ O-rich materials by a recent impact or an active landslide seems most consistent with the presence of both mineral hydrates and water ice. (ii) Release of subsurface H ₂ O may occur on Ceres, similar to release on comet nuclei, but may never recondense on the surface. (iii) Infall of ice-bearing objects is not likely to deposit water on Ceres, because the H ₂ O molecule likely would dissociate upon impact. (iv) Implantation of protons from the solar wind on the surface is not a probable origin of OH on Ceres because of the low flux of solar wind charged particles. We therefore conclude that surface H ₂ O or hydrated minerals are the most plausible explanation. Dawn VIR infrared observations of Oxo crater on Ceres demonstrate the detection of H ₂ O at the surface.: ( A ) Reflectance spectrum collected where absorption bands of H ₂ O at 1.28, 1.65, and 2 μm are the strongest (in blue) compared with a laboratory spectrum of H ₂ O ice (black). The lab spectrum is scaled and vertically shifted for clarity. ( B ) Perspective view of Oxo crater observed by the Dawn Framing Camera (FC), where the two high-albedo areas right next to the scarps contain H ₂ O-rich materials.

Although ice in the Martian midlatitudes is typically covered by a layer of dust or regolith, it is exposed in some locations by fresh impact craters or in erosional scarps. In both cases, the exposed ice is massive or excess ice with a low lithic content. We find that erosional scarps occur between 50° and 61° north and south latitude and that they are concentrated in and near Milankovič crater in the northern hemisphere and southeast of the Hellas basin in the southern hemisphere. These may represent locations of particularly thick or clean bodies of ice. Pits created by retreat of the scarps represent sublimation‐thermokarst landforms that evolve in a manner distinct from other ice‐loss landforms on Mars. New impact craters reveal that clean subsurface ice is widespread at middle‐ and high‐latitudes in both hemispheres at depths less than 1 m. Both the depth to ice and the ice content appear to exhibit significant variability over tens to hundreds of meters. The lowest‐latitude exposed ice is near 39°N and is at the edge of a region where impact exposures between 40° and 50°N are common, consistent with other indications of a high ice content. This lowest‐latitude ice may be currently unstable and subliming. Impact craters on lineated valley fill excavate ice blocks that may represent the top of debris‐covered glacial ice. Together, these landforms indicate widespread, clean subsurface ice at middle‐latitudes on Mars. The distribution and properties of this ice could provide information about past climate conditions.

One important, almost ubiquitous, tool for understanding the surfaces of solid bodies throughout the solar system is the study of impact craters. While measuring a distribution of crater diameters and locations is an important tool for a wide variety of studies, so too is measuring a crater's “depth.” Depth can inform numerous studies including the strength of a surface and modification rates in the local environment. There is, however, no standard data set, definition, or technique to perform this data‐gathering task, and the abundance of different definitions of “depth” and methods for estimating that quantity can lead to misunderstandings in and of the literature. In this review, we describe a wide variety of data sets and methods to analyze those data sets that have been, are currently, or could be used to derive different types of crater depth measurements. We also recommend certain nomenclature in doing so to help standardize practice in the field. We present a review section of all crater depths that have been published on different solar system bodies which shows how the field has evolved through time and how some common assumptions might not be wholly accurate. We conclude with several recommendations for researchers which could help different data sets to be more easily understood and compared.