Dawn Framing Camera Research Articles

Spectral properties of bright deposits in permanently shadowed craters on Ceres

Context. Bright deposits in permanently shadowed craters on Ceres are thought to harbor water ice. However, the evidence for water ice presented thus far is indirect. Aims. We aim to directly detect the spectral characteristics of water ice in bright deposits present in permanently shadowed regions (PSRs) in polar craters on Ceres. Methods. We analyzed narrowband images of four of the largest shadowed bright deposits acquired by the Dawn Framing Camera to reconstruct their reflectance spectra, carefully considering issues such as in-field stray light correction and image compression artifacts. Results. The sunlit portion of a polar deposit known to harbor water ice has a negative (blue) spectral slope of −58 ± 12% µm−1 relative to the background in the visible wavelength range. We find that the PSR bright deposits have similarly blue spectral slopes, consistent with a water ice composition. Based on the brightness and spectral properties, we argue that the ice is likely present as particles of high purity. Other components such as phyllosilicates may be mixed in with the ice. Salts are an unlikely brightening agent given their association with cryovolcanic processes, of which we find no trace. Conclusions. Our spectral analysis strengthens the case for the presence of water ice in permanently shadowed craters on Ceres.

Bright Na‐Carbonate Exposures Reveal Recent, Widespread Mobilization of Material in Ceres' Shallow Subsurface

AbstractStriking exposures of Na‐carbonate‐bearing bright materials are found in Ahuna Mons and the central landforms of Occator and other impact craters on Ceres; however, most bright materials on its surface occur as excavated material on the rims and walls of impact craters. The source of the widespread excavated Na‐carbonate is uncertain. We map the distribution and extent of rim/wall Na‐carbonate with Dawn's Visible and Infrared Imaging Spectrometer and Dawn Framing Camera. We find a strong spatial correlation between domes and shallow subsurface Na‐carbonate deposits, suggesting that they form by the same or related processes. Using Monte Carlo impact models, we constrain the timing of emplacement of the shallow subsurface Na‐carbonate to be within the last ∼1 Ga and probably within the last few hundred Ma. We examine the likelihood that the Na‐carbonate was mobilized from Ceres' deep interior and the plausibility of brine upwelling or solid‐state processes. We demonstrate that a combination of processes, including diapirism driven by Rayleigh‐Taylor instabilities and solid‐state deformation due to differential loading, could mobilize Na‐carbonate‐bearing ices to the shallow subsurface without requiring the recent presence of brines.

Geology and colour of Kupalo crater on Ceres

Kupalo is a ∼4 ​Ma old, 26-km diameter impact crater on dwarf planet Ceres, which exhibits extensive areas of bright bluish material. Here we describe, for the first time, the geology of Kupalo on a regional and local scale in detail, based on Dawn Framing Camera (FC) imagery. We find the crater has a complex geology consistent with a brittle and heterogeneous crust in this area. Through analyses of the FC colour data, we identify a correlation between the geologic units and the spectral variations, which can be explained by a mixture of subsurface materials in response to the impact. The brightest sites of Kupalo, located at the upper west wall and the central ridge, show similar FC colour and spectral IR data, which suggest that the bright material in these locations likely has the same origin. To explain the distribution of the bright bluish material in the crater and its vicinity, we propose two scenarios for the structure of the upper Cerean crust. Both require deep-seated brine or salt reservoirs, possibly connected to a brine ocean at the crust-mantle transition. • Geology of Kupalo suggests that Ceres' crust is rather heterogeneous (69). • Mixture of impacted subsurface materials explains correlation of spectra and geology (84). • Bright material in Kupalo would have originally been deposited at a depth of ∼3 ​km (82). • The brightest exposures are linked to subsurface reservoirs of hydrated carbonates (82).

The Brittle Boulders of Dwarf Planet Ceres

Abstract We mapped all boulders larger than 105 m on the surface of dwarf planet Ceres using images of the Dawn framing camera acquired in the Low Altitude Mapping Orbit. We find that boulders on Ceres are more numerous toward high latitudes and have a maximum lifetime of 150 ± 50 Ma, based on crater counts. These characteristics are distinctly different from those of boulders on asteroid (4) Vesta, an earlier target of Dawn, which implies that Ceres’ boulders are mechanically weaker. Clues to their properties can be found in the composition of Ceres’ complex crust, which is rich in phyllosilicates and salts. As water ice is thought to be present only meters below the surface, we suggest that boulders also harbor ice. Furthermore, the boulder size–frequency distribution is best fit by a Weibull distribution rather than the customary power law, just like for Vesta boulders. This finding is robust in light of possible types of size measurement error.

Recent cryovolcanic activity at Occator crater on Ceres

NASA’s Dawn mission revealed a partially differentiated Ceres that has experienced cryovolcanic activity throughout its history up to the recent past. The Occator impact crater, which formed ~22 Myr ago, displays bright deposits (faculae) across its floor whose origins are still under debate: two competing hypotheses involve eruption of brines from the crust–mantle transition boundary (remnants of an ancient ocean) or alternatively from a shallow impact melt chamber. Here we report new constraints on the history of Occator that help in testing the hypotheses of its formation. We used high-resolution images of the Dawn Framing Camera obtained close to the end of the mission. We found a long-lasting and recent period of cryovolcanic activity, which started ≤9 Myr ago and lasted for several million years. Several resurfacing events, affecting the faculae and some (dark) solidified impact melt units, are shown to have occurred millions of years after crater formation and the dissipation of the impact-generated heat. These findings are indicative of a deep-seated brine source. Extensive volatile-driven emplacement of bright material occurred in the central floor, causing its subsidence due to mass loss at depth. Finally, a thick (extrusive) dome of bright material was raised in the central depression. The derived chronostratigraphy of Occator is consistent with a recently geologically active world, where salts play a major role in preserving liquid in a heat-starved body. High-spatial-resolution images of the bright points at Occator crater on Ceres, taken during the second extended Dawn mission, allowed reconstruction of the chronology of their formation. The area experienced extensive cryovolcanism less than nine million years ago that lasted several million years, indicating recent geological activity.

Landslides on Ceres: Diversity and Geologic Context.

Landslides are among the most widespread geologic features on Ceres. Using data from Dawn's Framing Camera, landslides were previously classified based upon geomorphologic characteristics into one of three archetypal categories, Type 1(T1), Type 2 (T2), and Type 3 (T3). Due to their geologic context, variation in age, and physical characteristics, most landslides on Ceres are, however, intermediate in their morphology and physical properties between the archetypes of each landslide class. Here we describe the varied morphology of individual intermediate landslides, identify geologic controls that contribute to this variation, and provide first‐order quantification of the physical properties of the continuum of Ceres's surface flows. These intermediate flows appear in varied settings and show a range of characteristics, including those found at contacts between craters, those having multiple trunks or lobes; showing characteristics of both T2 and T3 landslides; material slumping on crater rims; very small, ejecta‐like flows; and those appearing inside of catenae. We suggest that while their morphologies can vary, the distribution and mechanical properties of intermediate landslides do not differ significantly from that of archetypal landslides, confirming a link between landslides and subsurface ice. We also find that most intermediate landslides are similar to Type 2 landslides and formed by shallow failure. Clusters of these features suggest ice enhancement near Juling, Kupalo and Urvara craters. Since the majority of Ceres's landslides fall in the intermediate landslide category, placing their attributes in context contributes to a better understanding of Ceres's shallow subsurface and the nature of ground ice.

Spectrophotometric modeling and mapping of Ceres

We report a comprehensive analysis of the global spectrophotometric properties of Ceres using the images collected by the Dawn Framing Camera through seven color filters from April to June 2015 during the RC3 (rotational characterization 3) and Survey mission phases. We derived the Hapke model parameters for all color filters. The single-scattering albedo of Ceres at 555 nm wavelength is 0.14 ± 0.04, the geometric albedo is 0.096 ± 0.006, and the bolometric Bond albedo is 0.037 ± 0.002. The asymmetry factors calculated from the best-fit two-term Henyey-Greenstein (HG) single-particle phase functions (SPPFs) show a weak wavelength dependence from −0.04 at 438 nm increasing to 0.002 at >900 nm, suggesting that the phase reddening of Ceres is dominated by single-particle scattering rather than multiple scattering or small-scale surface roughness. The Hapke roughness parameter of Ceres is derived to be 20° ± 6°, with no wavelength dependence. The phase function of Ceres presents appreciably strong scattering around 90° phase angle that cannot be fitted with a single-term HG SPPF, suggesting possible stronger forward scattering component than other asteroids previously analyzed with spacecraft data. We speculate that such a scattering characteristic of Ceres might be related to its ubiquitous distribution of phyllosilicates and high abundance of carbonates on the surface. We further grouped the reflectance data into a 1° latitude-longitude grid over the surface of Ceres, and fitted each grid independently with both empirical models and the Hapke model to study the spatial variations of photometric properties. Our derived albedo maps and color maps are consistent with previous studies [Nathues, A., et al., 2016, Planet. Space Sci. 134, 122–127; Schröder, S.E., et al., 2017, Icarus 288, 201–225]. The SPPF over the surface of Ceres shows an overall correlation with albedo distribution, where lower albedo is mostly associated with stronger backscattering and vice versa, consistent with the general trend among asteroids. On the other hand, the Hapke roughness parameter does not vary much across the surface of Ceres, except for the ancient Vendimia Planitia region that is associated with a slightly higher roughness. Furthermore, the spatial distributions of the SPPF and the Hapke roughness do not depend on wavelength. Based on the wavelength dependence of the SPPF of Ceres, we hypothesize that the regolith grains on Ceres either contain a considerable fraction of μm-sized or smaller particles, or are strongly affected by internal scatterers of this size.

Surface Temperatures and Water Ice Sublimation Rate of Oxo Crater: A Comparison With Juling Crater

AbstractDwarf planet Ceres is characterized by several sites hosting (or have hosted) ice‐rich patches as revealed by the Dawn's Visible and InfraRed spectrometer. The study of the illumination conditions including the effects of the local topography become critical in the estimation of the ice lifetime as well as the water vapor production rate. In this work we applied a 3‐D thermophysical model in order to study the illumination conditions on the shape model, derived on the basis of the images acquired by the Dawn's Framing Camera during the Survey mission phase, and to calculate the surface temperatures and water sublimation rates. We are interested in a crater in the northern hemisphere (42°), Oxo, which hosts water ice in its southern wall. A comparison with the surface temperatures and water sublimation rates of another Ceres' crater, Juling, is carried out. Water ice sublimation rate of its ice‐rich patch suggests that the Oxo crater probably is not the source of the emission detected by Herschel, a source that could be represented instead by the Juling crater.

Ceres’ opposition effect observed by the Dawn framing camera

Context. The surface reflectance of planetary regoliths may increase dramatically towards zero phase angle, a phenomenon known as the opposition effect (OE). Two physical processes that are thought to be the dominant contributors to the brightness surge are shadow hiding (SH) and coherent backscatter (CB). The occurrence of shadow hiding in planetary regoliths is self-evident, but it has proved difficult to unambiguously demonstrate CB from remote sensing observations. One prediction of CB theory is the wavelength dependence of the OE angular width. Aims. The Dawn spacecraft observed the OE on the surface of dwarf planet Ceres. We aim to characterize the OE over the resolved surface, including the bright Cerealia Facula, and to find evidence for SH and/or CB. It is presently not clear if the latter can contribute substantially to the OE for surfaces as dark as that of Ceres. Methods. We analyze images of the Dawn framing camera by means of photometric modeling of the phase curve. Results. We find that the OE of most of the investigated surface has very similar characteristics, with an enhancement factor of 1.4 and a full width at half maximum of 3° (“broad OE”). A notable exception are the fresh ejecta of the Azacca crater, which display a very narrow brightness enhancement that is restricted to phase angles <0.5° (“narrow OE”); suggestively, this is in the range in which CB is thought to dominate. We do not find a wavelength dependence for the width of the broad OE, and lack the data to investigate the dependence for the narrow OE. The prediction of a wavelength-dependent CB width is rather ambiguous, and we suggest that dedicated modeling of the Dawn observations with a physically based theory is necessary to better understand the Ceres OE. The zero-phase observations allow us to determine Ceres’ visible geometric albedo as pV = 0.094 ± 0.005. A comparison with other asteroids suggests that Ceres’ broad OE is typical for an asteroid of its spectral type, with characteristics that are primarily linked to surface albedo. Conclusions. Our analysis suggests that CB may occur on the dark surface of Ceres in a highly localized fashion. While the results are inconclusive, they provide a piece to the puzzle that is the OE of planetary surfaces.

High-resolution shape model of Ceres from stereophotoclinometry using Dawn Imaging Data

We present a high-resolution global shape model of Ceres determined using the stereophotoclinometry technique developed at the Jet Propulsion Laboratory by processing Dawn's Framing Camera data acquired during Approach to post-Low Altitude Mapping Orbit (LAMO) phases of the mission. A total of about 38,000 images were processed with pixel resolutions ranging from 35.6 km/pixel to 35 m/pixel and the final global shape model was produced with 100-m grid spacing. The final SPC-derived topography was computed relative to the (482 km, 482 km, 446 km) mean ellipsoid, which ranges from −7.3 km to 9.5 km. For the purpose of validation, we performed various error analyses to assess and quantify realistic uncertainties in the derived topography, such as dividing the data into different subsets and re-computing the entire topography. Based on these studies, we show that the average total height error of the final global topography model is 10.2 m and 88.9% of the surface has the total height error below 20 m. We also provide improved estimates of several physical parameters of Ceres. The resulting GM estimate is (62.62905 ± 0.00035) km3/s2, or the mass value of (938.392 ± 0.005) × 1018 kg. The volume estimate is (434.13 ± 0.50) × 106 km3 with a volumetric mean radius of 469.72 km. Combined with the mass estimate, the resulting bulk density is (2161.6 ± 2.5) kg/m3. Other improved parameters include the pole right ascension, α0 = (291.42763 ± 0.0002)°, pole declination, δ0 = (66.76033 ± 0.0002)°, and prime meridian and rotation rate of (W0 = 170.309 ± 0.011)° and (dW/dt = 952.1532635 ± 0.000002) deg/day, respectively. Also, for geophysical and geological studies, we provide spherical harmonic coefficients and a gravitational slope map derived from the global shape model.

The geology of the Nawish quadrangle of Ceres: The rim of an ancient basin

Herein we present the geology of the Nawish quadrangle, located in the equatorial region of dwarf planet Ceres, named after one of the most prominent craters of the area. Geologic mapping was based on the image mosaics and digital terrain models derived from Dawn Framing Camera data. Interpretation of geologic units was supported by supplemental data, such as the multi spectral color images from the Framing Camera, and the spectral parameters derived from the Visible and Infrared Spectrometer (VIR) data, as well as Dawn gravity data. There is not a primary feature that dominates the geology of Nawish quadrangle, but rather several terrains overlap, and their relations explain the geology of the area. Crater size frequency distributions show that Nawish quadrangle is dominated by two distinct time domains. The central and eastern part of the quadrangle is topographically elevated, which we define as cratered highlands, and contains the older domain. The western lowlands show two younger domains related to impact craters Kerwan and Dantu, including the Kerwan smooth material and Dantu ejecta. This variation of elevation within the Nawish quadrangle is more than the half of the global topographic altitude variation on Ceres. Analysis and comparison of the topography of the Nawish quadrangle with surrounding ones shows that this quadrangle is dominated by the topography of the rim sector of a large, >800 km ancient impact basin, most likely the putative Vendimia Planitia. The Nawish quadrangle thus represents a sector of Ceres which has not undergone large-scale, post-Kerwan, intermediate age-events, but rather represents a place on Ceres where a well-preserved relict of old cerean crust can be studied, together with ejecta from more recent impact events.

Ceres’ spectral link to carbonaceous chondrites—Analysis of the dark background materials

AbstractCeres’ surface has commonly been linked with carbonaceous chondrites (CCs) by ground‐based telescopic observations, because of its low albedo, flat to red‐sloped spectra in the visible and near‐infrared (VIS/NIR) wavelength region, and the absence of distinct absorption bands, though no currently known meteorites provide complete spectral matches to Ceres. Spatially resolved data of the Dawn Framing Camera (FC) reveal a generally dark surface covered with bright spots exhibiting reflectance values several times higher than Ceres’ background. In this work, we investigated FC data from High Altitude Mapping Orbit (HAMO) and Ceres eXtended Juling (CXJ) orbit (~140 m/pixel) for global spectral variations. We found that the cerean surface mainly differs by spectral slope over the whole FC wavelength region (0.4–1.0 μm). Areas exhibiting slopes <−10% μm−1 constitute only ~3% of the cerean surface and mainly occur in the bright material in and around young craters, whereas slopes ≥−10% μm−1 occur on more than 90% of the cerean surface; the latter being denoted as Ceres’ background material in this work. FC and Visible and Infrared Spectrometer (VIR) spectra of this background material were compared to the suite of CCs spectrally investigated so far regarding their VIS/NIR region and 2.7 μm absorption, as well as their reflectance at 0.653 μm. This resulted in a good match to heated CI Ivuna (heated to 200–300 °C) and a better match for CM1 meteorites, especially Moapa Valley. This possibly indicates that the alteration of CM2 to CM1 took place on Ceres.

Mineralogy and temperature of crater Haulani on Ceres

AbstractWe investigate the region of crater Haulani on Ceres with an emphasis on mineralogy as inferred from data obtained by Dawn's Visible InfraRed mapping spectrometer (VIR), combined with multispectral image products from the Dawn Framing Camera (FC) so as to enable a clear correlation with specific geologic features. Haulani, which is one of the youngest craters on Ceres, exhibits a peculiar “blue” visible to near‐infrared spectral slope, and has distinct color properties as seen in multispectral composite images. In this paper, we investigate a number of spectral indices: reflectance; spectral slopes; abundance of Mg‐bearing and NH4‐bearing phyllosilicates; nature and abundance of carbonates, which are diagnostic of the overall crater mineralogy; plus a temperature map that highlights the major thermal anomaly found on Ceres. In addition, for the first time we quantify the abundances of several spectral endmembers by using VIR data obtained at the highest pixel resolution (~0.1 km). The overall picture we get from all these evidences, in particular the abundance of Na‐ and hydrous Na‐carbonates at specific locations, confirms the young age of Haulani from a mineralogical viewpoint, and suggests that the dehydration of Na‐carbonates in the anhydrous form Na2CO3 may be still ongoing.

Bright carbonate surfaces on Ceres as remnants of salt-rich water fountains

Vinalia and Cerealia Faculae are bright and salt-rich localized areas in Occator crater on Ceres. The predominance of the near-infrared signature of sodium carbonate on these surfaces suggests their original material was a brine. Here we analyze Dawn Framing Camera's images and characterize the surfaces as composed of a central structure, either a possible depression (Vinalia) or a central dome (Cerealia), and a discontinuous mantling. We consider three materials enabling the ascent and formation of the faculae: ice ascent with sublimation and carbonate particle lofting, pure gas emission entraining carbonate particles, and brine extrusion. We find that a mechanism explaining the entire range of morphologies, topographies, as well as the common composition of the deposits is brine fountaining. This process consists of briny liquid extrusion, followed by flash freezing of carbonate and ice particles, particle fallback, and sublimation. Subsequent increase in briny liquid viscosity leads to doming. Dawn observations did not detect currently active water plumes, indicating the frequency of such extrusions is longer than years.

Spectral properties and geology of bright and dark material on dwarf planet Ceres

AbstractVariations and spatial distributions of bright and dark material on dwarf planet Ceres play a key role in understanding the processes that have led to its present surface composition. We define limits for “bright” and “dark” material in order to distinguish them consistently, based on the reflectance of the average surface using Dawn Framing Camera data. A systematic classification of four types of bright material is presented based on their spectral properties, composition, spatial distribution, and association with specific geomorphological features. We found obvious correlations of reflectance with spectral shape (slopes) and age; however, this is not unique throughout the bright spots. Although impact features show generally more extreme reflectance variations, several areas can only be understood in terms of inhomogeneous distribution of composition as inferred from Dawn Visible and Infrared Spectrometer data. Additional material with anomalous composition and spectral properties are rare. The identification of the composition and origin of the dark, particularly the darkest material, remains to be explored. The spectral properties and the morphology of the dark sites suggest an endogenic origin, but it is not clear whether they are more or less primitive surficial exposures or excavated subsurface but localized material. The reflectance, spectral properties, inferred composition, and geologic context collectively suggest that the bright and dark material tends to gradually change toward the average surface over time. This could be because of multiple processes, i.e., impact gardening/space weathering, and lateral mixing, including thermal and aqueous alteration, accompanied by changes in composition and physical properties such as grain size, surface temperature, and porosity (compaction).

Geology of Ceres’ North Pole quadrangle with Dawn FC imaging data

The Dawn Framing Camera repeatedly imaged Ceres’ North Pole quadrangle (Ac-1 Asari, latitudes >66°N) at a resolution of ∼35 m/pixel through a panchromatic filter, enabling the derivation of a digital terrain model (DTM) and an ortho-rectified mosaic. Using this dataset, a photo-geologic map and stratigraphy, complemented with absolute model ages of impact craters, were produced. We identified the following key surface features: an ancient 4.5 km high isolated dome with a non-impact origin; recent lobate materials on crater interiors possibly formed as high-speed flows of collapsed rim material; and recent bright areas in permanently shadowed regions (PSRs), which we interpret as ice accumulation mostly by infall of exogenic material. Crater morphologies and dimensions do not differ from those in other quadrangles, suggesting the widespread influence of a rheologically weak target during the crater formation process. There is a paucity of lobate materials associated with impact cratering, in contrast to previous identifications with lower spatial resolution imagery.

Orbital bistatic radar observations of asteroid Vesta by the Dawn mission

We present orbital bistatic radar observations of a small-body, acquired during occultation by the Dawn spacecraft at asteroid Vesta. The radar forward-scattering properties of different reflection sites are used to assess the textural properties of Vesta’s surface at centimeter-to-decimeter scales and are compared to subsurface hydrogen concentrations observed by Dawnʼs Gamma Ray and Neutron Detector to assess potential volatile occurrence in the surface and shallow subsurface. We observe significant differences in surface radar reflectivity, implying substantial spatial variations in centimeter-to-decimeter-scale surface roughness. Our results suggest that unlike the Moon, Vesta’s surface roughness variations cannot be explained by cratering processes only. In particular, the occurrence of heightened hydrogen concentrations within large smoother terrains (over hundreds of square kilometers) suggests that potential ground-ice presence may have contributed to the formation of Vesta’s current surface texture. Our observations are consistent with geomorphological evidence of transient water flow from Dawn Framing Camera images.

Mineralogical analysis of the Ac-H-6 Haulani quadrangle of the dwarf planet Ceres

Ac-H-6 ‘Haulani’ is one of five quadrangles that cover the equatorial region of the dwarf planet Ceres. This quadrangle is notable for the broad, spectrally distinct ejecta that originate from the crater Haulani, which gives the name to the quadrangle. These ejecta exhibit one of the most negative (‘bluest’) visible to near infrared spectral slope observed across the entire body and have distinct color properties as seen in multispectral composite images. Besides Haulani, here we investigate a broader area that includes other surface features of interest, with an emphasis on mineralogy as inferred from data obtained by Dawn's Visible InfraRed mapping spectrometer (VIR), combined with multispectral image products from the Dawn Framing Camera (FC) so as to enable a clear correlation with specific geologic features.Our analysis shows that crater Haulani stands out compared to other surface features of the quadrangle. Albedo maps obtained in the near infrared range at 1.2 µm and 1.9 µm reveal that the floor and ejecta of Haulani are indeed a patchwork of bright and dark material units. Visible to near-infrared spectral slopes display negative values in crater Haulani's floor and ejecta, highlighting bluish, younger terrains. Diagnostic spectral features centered at ∼2.7 µm and ∼3.1 µm respectively indicate a substantial decrease in the abundances of magnesium-bearing phyllosilicates and ammoniated phyllosilicates in crater Haulani's floor and bright ejecta. Similar, but less prominent, spectral behavior is observed in other geologic features of this quadrangle, while the general trend in quadrangle Ac-H-6 for these two mineral species is to increase from the northwest to the southeast. However, it is worth noting that the correlation between the ∼2.7 µm and ∼3.1 µm spectral parameters is generally strong in the Haulani crater's area, but much weaker elsewhere, which indicates a variable degree of mixing between these two major mineral phases in moving away from the crater. Finally, the region of crater Haulani displays a distinct thermal signature and a local enhancement in calcium and possibly sodium carbonate minerals, which is hardly found in the rest of the quadrangle and is likely the result of intense hydrothermal processes following the impact event. These evidences all together confirm the young age of crater Haulani, as they have not been erased or made elusive by space weathering processes.

The geology of the Kerwan quadrangle of dwarf planet Ceres: Investigating Ceres’ oldest, largest impact basin

We conducted a geologic mapping investigation of Dawn spacecraft data to determine the geologic history of the Kerwan impact basin region of dwarf planet Ceres, which is mostly located in the Ac-7 Kerwan Quadrangle. Geological mapping was applied to Dawn Framing Camera images from the Low Altitude Mapping Orbit (LAMO, 35 m/pixel) and supplemented by digital terrain models and color images from the High Altitude Mapping Orbit (HAMO, 135 m/pixel), as well as preliminary Visible and Infrared Spectrometer (VIR) and gravity data. The 284-km diameter Kerwan impact basin is the oldest unequivocal impact crater on Ceres, and has a highly discontinuous, polygonal, degraded rim and contains a ‘smooth’ unit that both fills the basin floor and surrounds the degraded rim to the west, south, and east. Although there are some subtle topographic features in the Kerwan basin that could be interpreted as flow boundaries, there is no indisputable evidence of cryovolcanic features in or around the basin (however if such features existed they could be buried). Nevertheless, all data point to impact-induced melting of a cerean crust enriched in a volatile, likely water ice, to produce the Kerwan smooth material. Subsequent geologic activity in this region includes emplacement of impact craters such as Dantu, which produced a variety of colorful deposits, and rayed craters such as Rao and Cacaguat. Based on the crater size-frequency distribution absolute model ages of the Kerwan smooth material in and around the basin, marking a minimum age for the Kerwan basin, our mapping defines this as the oldest boundary within the cerean geologic timescale, separating the Pre-Kerwanan and Kerwanan Periods at > 1.3 Ga (Lunar-derived chronology model) or > 230–850 Ma (Asteroid-derived chronology model, depending on strength of target material).

Geologic mapping of the Urvara and Yalode Quadrangles of Ceres

We conducted geologic mapping of the Urvara (Ac-13) and Yalode (Ac-14) Quadrangles (21–66°S, 180–360°E) of the dwarf planet Ceres utilizing morphologic, topographic, and compositional information acquired by NASA's Dawn mission. The geologic characteristics of the two large impact basins Urvara (170 km diameter) and Yalode (260 km diameter) and their surroundings were investigated using Dawn Framing Camera datasets, including Survey (415 m/pixel), HAMO (140 m/pixel), and LAMO (35 m/pixel) images and mosaics, color and color ratio images, and DTMs derived from stereo-photogrammetry. Geologic mapping demonstrates that impact cratering has dominated the geologic history of the Urvara and Yalode Quadrangles, with early cratered terrain formation followed by formation of the large basins and widespread emplacement of basin-related smooth material. Impact craters display a wide range of preservation states from nearly completely buried/degraded forms to more recent pristine craters with terraced inner walls and lobate ejecta deposits. Cross-cutting relationships and morphologic signatures show that the Urvara impact followed the Yalode impact, consistent with ages derived from crater size-frequency distributions (580 ± 40 Ma for Yalode and 550 ± 50 Ma for Urvara). Observed differences in basin materials and rim morphology suggest heterogeneities in the substrate excavated by impact. Smooth deposits that cover large areas of the quadrangles, including the basin floors, rims, and exterior zones, are interpreted to be dominated by Urvara ejecta but Yalode ejecta and localized ice-rich flow material may be minor components. Geologic mapping results and simulations of ejecta emplacement suggest that Urvara and Yalode ejecta deposits extend for large distances (more than two crater diameters from the basin centers) and may serve as important stratigraphic markers for the geologic record of Ceres.