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<jats:sec> <jats:title>INTRODUCTION</jats:title> <jats:p>Thermochemical models have predicted that the dwarf planet Ceres has, to some extent, formed a mantle. Moreover, due to viscous relaxation, these models indicate that Ceres should have an icy crust with few or no impact craters. However, the Dawn spacecraft has shown that Ceres has elevation excursions of ~15 km, cliffs, graben, steep-sided mountains, and a heavily cratered surface.</jats:p> </jats:sec> <jats:sec> <jats:title>RATIONALE</jats:title> <jats:p>We used Dawn’s Framing Camera to study the morphology, size frequency, and spatial distribution of the craters on Ceres. These data allow us to infer the structure and evolution of Ceres’ outer shell.</jats:p> </jats:sec> <jats:sec> <jats:title>RESULTS</jats:title> <jats:p>A large variety of crater morphologies are present on Ceres, including bowl-shaped craters, polygonal craters, floor-fractured craters, terraces, central peaks, smooth floors, flowlike features, bright spots, secondary craters, and crater chains. The morphology of some impact craters is consistent with water ice in the subsurface. Although this might have favored relaxation, there are also large unrelaxed craters. The transition from bowl-shaped simple craters to modified complex craters occurs at diameters of about 7.5 to 12 km. Craters larger than 300 km are absent, but low-pass filtering of the digital elevation model suggests the existence of two quasi-circular depressions with diameters of ~570 km (125.56°E and 19.60°N) and ~830 km (24.76°W and 0.5°N). Craters are heterogeneously distributed across Ceres’ surface, with more craters in the northern versus the southern hemisphere. The lowest crater densities are associated with large, well-preserved southern hemisphere impact craters such as Urvara and Yalode. Because the low crater density (LCD) terrain extends across a large latitude range in some cases (e.g., Urvara and Yalode: ~18°N and 75°S; Kerwan: ~30°N and 46°S), its spatial distribution is inconsistent with simple relaxation driven by warmer equatorial temperatures. We instead propose that impact-driven resurfacing is the more likely LCD formation process, although we cannot completely rule out an internal (endogenic) origin. We applied two different methodologies to derive absolute model ages from observed crater size-frequency distributions. The lunar-derived model adapts the lunar production and chronology functions to impact conditions on Ceres, taking into account impact velocities, projectile densities, current collision probabilities, and surface gravity. The asteroid-derived model derives a production function by scaling the directly observed object size-frequency distribution from the main asteroid belt (extended to sizes &lt;5 km by a collisional model) to the resulting size-frequency distribution of cerean craters, using similar cerean target parameters as the lunar-derived model. By dating a smooth region associated with the Kerwan crater, we determined absolute model ages of 550 million and 720 million years, depending on which chronology model is applied.</jats:p> </jats:sec> <jats:sec> <jats:title>CONCLUSION</jats:title> <jats:p>Crater morphology and the simple-to-complex crater transition indicate that Ceres’ outer shell is likely neither pure ice nor pure rock but an ice-rock mixture that allows for limited relaxation. The heterogeneous crater distribution across the surface indicates crustal heterogeneities and a complex geologic evolution of Ceres. There is evidence for at least some geologic activity occurring in Ceres’ recent history.</jats:p> <jats:fig fig-type="figure" orientation="portrait" position="float"> <jats:caption> <jats:title>Spatial density of craters larger than 20 km on Ceres.</jats:title> <jats:p>Crater rims are shown as black solid circles. Blue indicates areas with LCDs; yellow and red represent more highly cratered areas. The smallest dashed ellipse denotes the idealized former rim of an extremely degraded impact crater at 48.9°E and 44.9°S, which is barely recognizable in imagery but apparent from the global digital elevation model. Also shown as dashed circles are the outlines of two large putative basins. Unambiguously recognized basins &gt;300 km in diameter are missing, and there are several areas with LCDs associated with large impact craters (e.g., Yalode, Urvara, Kerwan, Ezinu, Vinotonus, Dantu, and two unnamed craters northeast and southeast of Oxo). Areas A and B are topographic rises with central depressions that also show LCDs.</jats:p> </jats:caption> <jats:graphic xmlns:xlink="http://www.w3.org/1999/xlink" orientation="portrait" position="float" xlink:href="351_aaf4759_Fa" /> </jats:fig> </jats:sec>

<jats:sec> <jats:title>INTRODUCTION</jats:title> <jats:p> Dwarf planet Ceres’ low average-density (2162 ± 3 kg m <jats:sup>−3</jats:sup> ) 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 <jats:sub>2</jats:sub> 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 <jats:sub>2</jats:sub> O vapor molecules, and haze. However, the presence of H <jats:sub>2</jats:sub> O on the surface has not previously been confirmed. The detection and mapping of H <jats:sub>2</jats:sub> O on Ceres is one objective of the Dawn spacecraft, in orbit around Ceres since March 2015. </jats:p> </jats:sec> <jats:sec> <jats:title>RATIONALE</jats:title> <jats:p> 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 <jats:sub>2</jats:sub> O molecule. </jats:p> </jats:sec> <jats:sec> <jats:title>RESULTS</jats:title> <jats:p> 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 <jats:sub>2</jats:sub> O molecule (see the figure). These spectra are most similar to those of H <jats:sub>2</jats:sub> 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 <jats:sub>2</jats:sub> 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. </jats:p> </jats:sec> <jats:sec> <jats:title>CONCLUSION</jats:title> <jats:p> Four ways to create or transport H <jats:sub>2</jats:sub> O on Ceres are considered: (i) Exposure of near-surface H <jats:sub>2</jats:sub> 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 <jats:sub>2</jats:sub> 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 <jats:sub>2</jats:sub> 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 <jats:sub>2</jats:sub> O or hydrated minerals are the most plausible explanation. </jats:p> <jats:fig fig-type="figure" orientation="portrait" position="float"> <jats:caption> <jats:title> Dawn VIR infrared observations of Oxo crater on Ceres demonstrate the detection of H <jats:sub>2</jats:sub> O at the surface. </jats:title> <jats:p> ( <jats:bold>A</jats:bold> ) Reflectance spectrum collected where absorption bands of H <jats:sub>2</jats:sub> O at 1.28, 1.65, and 2 μm are the strongest (in blue) compared with a laboratory spectrum of H <jats:sub>2</jats:sub> O ice (black). The lab spectrum is scaled and vertically shifted for clarity. ( <jats:bold>B</jats:bold> ) 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 <jats:sub>2</jats:sub> O-rich materials. </jats:p> </jats:caption> <jats:graphic xmlns:xlink="http://www.w3.org/1999/xlink" orientation="portrait" position="float" xlink:href="353_aaf3010_Fa.jpeg" /> </jats:fig> </jats:sec>

<jats:sec> <jats:title>INTRODUCTION</jats:title> <jats:p>Classic volcanism prevalent on terrestrial planets and volatile-poor protoplanets, such as asteroid Vesta, is based on silicate chemistry and is often expressed by volcanic edifices (unless erased by impact bombardment). In ice-rich bodies with sufficiently warm interiors, cryovolcanism involving liquid brines can occur. Smooth plains on some icy satellites of the outer solar system have been suggested as possibly cryovolcanic in origin. However, evidence for cryovolcanic edifices has proven elusive. Ceres is a volatile-rich dwarf planet with an average equatorial surface temperature of ~160 K. Whether this small (~940 km diameter) body without tidal dissipation could sustain cryovolcanism has been an open question because the surface landforms and relation to internal activity were unknown.</jats:p> </jats:sec> <jats:sec> <jats:title>RATIONALE</jats:title> <jats:p>The Framing Camera onboard the Dawn spacecraft has observed &gt;99% of Ceres’ surface at a resolution of 35 m/pixel at visible wavelengths. This wide coverage and resolution were exploited for geologic mapping and age determination. Observations with a resolution of 135 m/pixel were obtained under several different viewing geometries. The stereo-photogrammetric method applied to this data set allowed the calculation of a digital terrain model, from which morphometry was investigated. The observations revealed a 4-km-high topographic relief, named Ahuna Mons, that is consistent with a cryovolcanic dome emplacement.</jats:p> </jats:sec> <jats:sec> <jats:title>RESULTS</jats:title> <jats:p>The ~17-km-wide and 4-km-high Ahuna Mons has a distinct size, shape, and morphology. Its summit topography is concave downward, and its flanks are at the angle of repose. The morphology is characterized by (i) troughs, ridges, and hummocky areas at the summit, indicating multiple phases of activity, such as extensional fracturing, and (ii) downslope lineations on the flanks, indicating rockfalls and accumulation of slope debris. These morphometric and morphologic observations are explained by the formation of a cryovolcanic dome, which is analogous to a high-viscosity silicic dome on terrestrial planets. Models indicate that extrusions of a highly viscous melt-bearing material can lead to the buildup of a brittle carapace at the summit, enclosing a ductile core. Partial fracturing and disintegration of the carapace generates slope debris, and relaxation of the dome’s ductile core due to gravity shapes the topographic profile of the summit. Modeling of this final phase of dome relaxation and reproduction of the topographic profile requires an extruded material of high viscosity, which is consistent with the mountain’s morphology. We constrained the age of the most recent activity on Ahuna Mons to be within the past 210 ± 30 million years.</jats:p> </jats:sec> <jats:sec> <jats:title>CONCLUSION</jats:title> <jats:p>Cryovolcanic activity during the geologically recent past of Ceres constrains its thermal and chemical history. We propose that hydrated salts with low eutectic temperatures and low thermal conductivities enabled the presence of cryomagmatic liquids within Ceres. These salts are the product of global aqueous alteration, a key process for Ceres’ evolution as recorded by the aqueously altered, secondary minerals observed on the surface.</jats:p> <jats:fig fig-type="figure" orientation="portrait" position="float"> <jats:caption> <jats:title>Perspective view of Ahuna Mons on Ceres from Dawn Framing Camera data (no vertical exaggeration).</jats:title> <jats:p>The mountain is 4 km high and 17 km wide in this south-looking view. Fracturing is observed on the mountain’s top, whereas streaks from rockfalls dominate the flanks.</jats:p> </jats:caption> <jats:graphic xmlns:xlink="http://www.w3.org/1999/xlink" orientation="portrait" position="float" xlink:href="353_aaf4286_fa.jpeg" /> </jats:fig> </jats:sec>

<jats:sec><jats:title>INTRODUCTION</jats:title><jats:p>The surface of the dwarf planet Ceres is known to host phyllosilicate minerals, but their distribution and origin have not previously been determined. Phyllosilicates are hydrated silicates, and their presence on the surface of Ceres is intriguing given that their structure evolves through an aqueous alteration process. In addition, some phyllosilicates are known to bear NH<jats:sub>4</jats:sub>, which places a constraint on the pH and redox conditions during the evolution of Ceres. We studied the distribution of phyllosilicates across the planet’s surface to better understand the evolutionary pathway of Ceres.</jats:p></jats:sec><jats:sec><jats:title>RATIONALE</jats:title><jats:p>Using the data acquired by the mapping spectrometer (VIR) onboard the Dawn spacecraft, we mapped the spatial distribution of different minerals on Ceres on the basis of their diagnostic absorption features in visible and infrared spectra. We studied the phyllosilicates through their OH-stretch fundamental absorption at about 2.7 µm and through the NH<jats:sub>4</jats:sub>absorption at about 3.1 µm. From our composition maps, we infer the origin of the materials identified.</jats:p></jats:sec><jats:sec><jats:title>RESULTS</jats:title><jats:p>We found that Mg- and NH<jats:sub>4</jats:sub>-bearing phyllosilicates are ubiquitous across the surface of Ceres and that their chemical composition is fairly uniform. The widespread presence of these two types of minerals is a strong indication of a global and extensive aqueous alteration—i.e., the presence of water at some point in Ceres’ geological history. Although the detected phyllosilicates are compositionally homogeneous, we found variations in the intensity of their absorption features in the 3-µm region of the reflectance spectrum. Such variations are likely due to spatial variability in relative mineral abundance (see the figure).</jats:p></jats:sec><jats:sec><jats:title>CONCLUSION</jats:title><jats:p>The large-scale regional variations evident in the figure suggest lateral heterogeneity in surficial phyllosilicate abundance on scales of several hundreds of kilometers. Terrains associated with the Kerwan crater (higher concentration of phyllosilicates) appear smooth, whereas the Yalode crater (lower concentration of phyllosilicates) is characterized by both smooth and rugged terrains. These distinct morphologies and phyllosilicate concentrations observed in two craters that are similar in size may reflect different compositions and/or rheological properties. On top of this large-scale lateral heterogeneity, small-scale variations associated with individual craters could result from different proportions of mixed materials in a stratified upper crustal layer that has been exposed by impacts. Variations associated with fresh craters, such as the 34-km-diameter Haulani, indicate the presence of crustal variations over a vertical scale of a few kilometers, whereas much larger craters, such as the 126-km-diameter Dantu, suggest that such stratification may extend for at least several tens of kilometers.</jats:p><jats:fig fig-type="figure" orientation="portrait" position="float"><jats:caption><jats:title>Abundance maps.</jats:title><jats:p>Qualitative maps of the abundances of (<jats:bold>top</jats:bold>) phyllosilicates and (<jats:bold>bottom</jats:bold>) NH<jats:sub>4</jats:sub>, based on the depth of their absorption features. The two maps have a similar global pattern, although they differ in some localized regions such as Urvara. The scale bar is valid at the equator.</jats:p></jats:caption><jats:graphic xmlns:xlink="http://www.w3.org/1999/xlink" orientation="portrait" position="float" xlink:href="353_aaf4279_Fa.jpeg" /></jats:fig></jats:sec>

<jats:sec> <jats:title>INTRODUCTION</jats:title> <jats:p>Observations of Ceres, the largest object in the asteroid belt, have suggested that the dwarf planet is a geologically differentiated body with a silicate core and an ice-rich mantle. Data acquired by the Dawn spacecraft were used to perform a three-dimensional characterization of the surface to determine if the geomorphology of Ceres is consistent with the models of an icy interior.</jats:p> </jats:sec> <jats:sec> <jats:title>RATIONALE</jats:title> <jats:p>Instruments on Dawn have collected data at a variety of resolutions, including both clear-filter and color images. Digital terrain models have been derived from stereo images. A preliminary 1:10 M scale geologic map of Ceres was constructed using images obtained during the Approach and Survey orbital phases of the mission. We used the map, along with higher-resolution imagery, to assess the geology of Ceres at the global scale, to identify geomorphic and structural features, and to determine the geologic processes that have affected Ceres globally.</jats:p> </jats:sec> <jats:sec> <jats:title>RESULTS</jats:title> <jats:p>Impact craters are the most prevalent geomorphic feature on Ceres, and several of the craters have fractured floors. Geomorphic analysis of the fracture patterns shows that they are similar to lunar Floor-Fractured Craters (FFCs), and an analysis of the depth-to-diameter ratios shows that they are anomalously shallow compared with average Ceres craters. Both of these factors are consistent with FFC floors being uplifted due to an intrusion of cryomagma.</jats:p> <jats:p>Kilometer-scale linear structures cross much of Ceres. Some of these structures are oriented radially to large craters and most likely formed due to impact processes. However, a set of linear structures present only on a topographically high region do not have any obvious relationship to impact craters. Geomorphic analysis suggests that they represent subsurface faults and might have formed due to crustal uplift by cryomagmatic intrusion.</jats:p> <jats:p>Domes identified across the Ceres surface present a wide range of sizes (&lt;10 km to &gt;100 km), basal shapes, and profiles. Whether a single formation mechanism is responsible for their formation is still an open question. Cryovolcanic extrusion is one plausible process for the larger domes, although most small mounds (&lt;10-km diameter) are more likely to be impact debris.</jats:p> <jats:p>Differences in lobate flow morphology suggest that multiple emplacement processes have operated on Ceres, where three types of flows have been identified. Type 1 flows are morphologically similar to ice-cored flows on Earth and Mars. Type 2 flows are comparable to long-runout landslides. Type 3 flows morphologically resemble the fluidized ejecta blankets of rampart craters, which are hypothesized to form by impact into ice-rich ground.</jats:p> </jats:sec> <jats:sec> <jats:title>CONCLUSION</jats:title> <jats:p>The global trend of lobate flows suggests that differences in their geomorphology could be explained by variations in ice content and temperature at the near surface. Geomorphic and topographic analyses of the FFCs suggest that cryomagmatism is active on Ceres, whereas the large domes are possibly formed by extrusions of cryolava. Although spectroscopic analysis to date has identified water ice in only one location on Ceres, the identification of these potentially ice-related features suggests that there may be more ice within localized regions of Ceres’ crust.</jats:p> <jats:fig fig-type="figure" orientation="portrait" position="float"> <jats:caption> <jats:title>Dawn high-altitude mapping orbit imagery (140 meters per pixel) of example morphologic features.</jats:title> <jats:p> ( <jats:bold>A</jats:bold> ) Occator crater; arrows point to floor fractures. ( <jats:bold>B</jats:bold> ) Linear structures, denoted by arrows. ( <jats:bold>C</jats:bold> ) A large dome at 42° N, 10° E, visible in the elevation map. ( <jats:bold>D</jats:bold> ) A small mound at 45.5° S, 295.7° E. ( <jats:bold>E</jats:bold> ) Type 1 lobate flow; arrows point to the flow front. </jats:p> </jats:caption> <jats:graphic xmlns:xlink="http://www.w3.org/1999/xlink" orientation="portrait" position="float" xlink:href="353_aaf4332_fa.jpeg" /> </jats:fig> </jats:sec>

<jats:p>How do we ensure the effective role of science in public policy-making? This well-worn, long-standing question reflects the fact that the answer is not simple. Later this month in Brussels, scientists and policy-makers will convene at the International Network for Government Science Advice (INGSA) Forum to consider the most promising ways forward.</jats:p>

<jats:p>A roundup of weekly science policy and related news.</jats:p>

<jats:p>OSIRIS-REx plans to return more than 60 grams of dust from carbon-rich Bennu.</jats:p>

<jats:p>The £650 million Francis Crick Institute aims to foster collaboration—but Brexit casts a shadow.</jats:p>