Dawn Spacecraft Research Articles

History of Ceres’s Cold Traps Based on Refined Shape Models

Permanently shadowed regions (PSRs) in the north polar region of Ceres have been previously mapped by the Dawn spacecraft. Putative ice deposits are found in some of these PSRs, whereas most PSRs host no bright deposits, which is thought to be due to oscillations of the axis tilt with a ∼25 ka period. We use stereophotoclinometry to construct refined topographic models of PSR-hosting craters. Ray-tracing calculations reveal that no PSRs remain at the maximum axis tilt, which implies that the ice deposits are remarkably young. The bright ice deposits do not extend beyond PSRs at an axis tilt of 10°, which last occurred about 6 ka ago. This suggests that water is delivered to the polar regions or exposed within the craters by frequent and short-lived events. Surface temperatures are calculated with a terrain irradiance model to delineate cold traps. Based on maximum equilibrium temperatures, Cerean PSRs are too warm to trap supervolatiles.

The capture probability of Dawn into ground-track resonances with Vesta

The Dawn spacecraft approached the asteroid Vesta and descended from a high-altitude mission orbit to a low-altitude mission orbit using low-thrust propulsion. During this descent, the spacecraft crossed the 2:3 and 1:1 ground-track resonances with Vesta, which posed a risk of capture that might strongly perturb the spacecraft’s orbit. This study analyzes the effects of these resonances on the spacecraft’s orbital elements and estimates the probability of capture into it through Monte Carlo simulations. Specifically, a comprehensive investigation is performed to assess the effects of 1:1 and 2:3 ground-track resonances on the semimajor axis, eccentricity, and inclination. The dynamical model includes the gravitational field of Vesta using a spherical harmonics approximation up to the fourth degree and order and the low-thrust acceleration that is assumed to be opposite to the spacecraft’s velocity vector direction. It is observed that the eccentricity evolution is mostly influenced by the 2:3 ground-track resonance which results in a large variation when the spacecraft crosses that ground-track resonance, while the semimajor axis and inclination are mostly influenced by the 1:1 ground-track resonance. Then, the probability of capture into 1:1 ground-track resonance is found to have a negative correlation with the spacecraft’s thrust magnitude and the probability of capture into 2:3 ground-track resonance is found to arise as the spacecraft’s mass increases. It is found that for circular orbits below a certain inclination value the spacecraft’s trajectory is subject to the “automatic entry into libration” phenomenon, due to the singularity in the Hamiltonian function. This research contributes to the design of successful transfer strategies when crossing resonance for future missions.

A Geologic Map of Vesta Produced Using a Hybrid Method for Incorporating Spectroscopic and Morphologic Data

We have constructed a global geologic map of the minor planet Vesta at 1:300,000-scale using Dawn spacecraft imaging, spectroscopic, topographic, and elemental data. In this effort, we used a mapping method that requires creating two maps independently: the first map uses morphology and topography to define map units, while the second map relies on multispectral data (“color”) to define units. The two are then combined into a hybrid product that retains the maximum amount of unique information from both maps in a readable format. This effort has revealed that for bodies where cratering is the dominant unit-forming process, and where there is not a close correlation between morphological feature types and multispectral signal, a hybrid mapping method better retains unique information carried by multispectral data during the mapping process than traditional morphology-based methods alone. Conversely, relying too heavily on color data risks placing too much emphasis on information drawn from the top few microns of the surface. To ensure both consistency and retention of unique information, we created a decision tree for determining which data would be primary in choosing where to draw unit boundaries. Also due to the significant amount of information borne by spectral data, we repurposed traditional mapping nomenclature so that subscripts carry color information. We recommend using this mapping methodology on bodies where (a) morphologic feature boundaries are commonly subtle, gradational, or both, and (b) spectral data carries a significant amount of unique data for identifying, characterizing, and interpreting geologic units.

A Cartographic Perspective on the Planetary Geologic Mapping Investigation of Ceres

The NASA Dawn spacecraft visited asteroid 4 Vesta between 2011 and 2012 and dwarf planet 1 Ceres between 2015 and 2018 to investigate their surfaces through optical and hyperspectral imaging and their composition through gamma-ray and neutron spectroscopy. For the global mapping investigation of both proto-planets, geologic mappers employed Geographic Information System (GIS) software to map 15 quadrangles using optical and hyperspectral data and to produce views of the geologic evolution through individual maps and research papers. While geologic mapping was the core motivation of the mapping investigation, the project never aimed to produce homogeneous and consistent map representations. The chosen mapping approach and its implementation led to a number of inconsistencies regarding cartographic representation, including differential generalization through varying mapping scales, topologic inconsistencies, lack of semantic integrity, and scale consistency, and ultimately, to the management of reusable research data. Ongoing data acquisition during the mapping phase created additional challenges for the homogenization of mapping results and a potential derivation of a global map. This contribution reviews cartographic and data perspectives on the mapping investigation of Ceres and highlights (a) data sources, (b) the cartographic concept, (c) mapping conduct, and (d) dissemination as well as research-data management arrangements. It furthermore discusses decisions and experiences made during mapping and finishes with a set of recommendations from the viewpoint of the cartographic sciences.

Pyroxene and Hydroxyl Signatures in Vesta Newly Calibrated Data from Dawn Mission

Thanks to the VIR spectrometer onboard NASA’s Dawn spacecraft, which orbited Vesta in 2011–2012, thousands of hyperspectral images of its surface have been collected. The mission confirmed the HED (Howardite–Eucrite–Diogenite) meteorite composition of Vesta. Moreover, the VIR spectrometer detected the 2.8 µm absorption band, due to the presence of the OH molecule. In this work, we took advantage of the newly calibrated data of the VIR spectrometer by characterizing new spectral features thanks to the improved signal-to-noise (S/N) ratio for these spectra. The main goals of this work are as follows: (1) to characterize Vesta’s surface in the visible range and (2) to confirm, reinforce and characterize the OH distribution on Vesta by studying the 2.8 µm band and looking for OH combination bands around 2.2–2.4 µm. A possible relation between the 1.9 µm absorption band due to the presence of pyroxenes and the one at 0.5 µm was analyzed. Finally, the analysis of hydroxyl absorption bands evidenced an anti-correlation between the abundance of hydroxyl-bearing molecules and the surface reflectance. This confirms that the hydroxyl presence is linked to the dark units on Vesta.

A Remote Sensing Perspective on Mass Wasting in Contrasting Planetary Environments: Cases of the Moon and Ceres

Mass wasting, as one of the most significant geomorphological processes, contributes immensely to planetary landscape evolution. The frequency and diversity of mass wasting features on any planetary body also put engineering constraints on its robotic exploration. Mass wasting on other Solar System bodies shares similar, although not identical, morphological characteristics with its terrestrial counterpart, indicating a possible common nature for their formation. Thus, planetary bodies with contrasting environmental conditions might help reveal the effects of the atmosphere, subsurface fluids, mass accumulation/precipitation, and seismicity on mass wasting, and vice versa. Their relative positions within our Solar System and the environmental and geophysical conditions on the Moon and the dwarf planet Ceres are not only extremely different from Earth’s but from each other too. Their smaller sizes coupled with the availability of global-scale remote sensing datasets make them ideal candidates to understand mass wasting processes in widely contrasting planetary environments. Through this concept article, we highlight several recent advances in and prospects of using remote sensing datasets to reveal unprecedented details on lunar and Cerean mass wasting processes. We start with briefly discussing several recent studies on mass wasting using Lunar Reconnaissance Orbiter Camera (LROC) data for the Moon and Dawn spacecraft data for Ceres. We further identify the prospects of available remote sensing data in advancing our understanding of mass wasting processes under reduced gravity and in a scant (or absent) atmosphere, and we conclude the article by suggesting future research directions.

Spectroscopic study of Ceres’ collisional family candidates

Context. Despite the observed signs of large impacts on the surface of Ceres, there is no confirmed collisional family associated with this dwarf planet. After a dynamical and photometric study, a sample of 156 asteroids were proposed as candidate members of a Ceres collisional family. Aims. Our main objective is to study the connection between Ceres and a total of 14 observed asteroids among the candidates sample to explore their genetic relationships with Ceres. Methods. We obtained visible spectra of these 14 asteroids using the OSIRIS spectrograph at the 10.4 m Gran Telescopio Canarias. We computed spectral slopes in two different wavelength ranges, from 0.49 to 0.80 μm and from 0.80 to 0.92 μm, to compare the values obtained with those on Ceres’ surface previously computed using the Visible and Infrared Spectrometer instrument on board the NASA Dawn spacecraft. We also calculated the spectral slopes in the same range for ground-based observations of Ceres collected from the literature. Results. We present the visible spectra and the taxonomy of 14 observed asteroids. We found that only two of the asteroids are spectrally compatible with Ceres’ surface. Further analysis of those two asteroids indicates that they are spectrally young and thus less likely to be members of the Ceres family. Conclusions. All in all, our results indicate that most of the 14 observed asteroids are not likely to belong to a Ceres collisional family. Despite two of them being spectrally compatible with the young surface of Ceres, further evaluation is needed to confirm or reject their origin from Ceres.

The surface of (4) Vesta in visible light as seen by Dawn/VIR

Aims.We analyzed the surface of Vesta at visible wavelengths, using the data of the Visible and InfraRed mapping spectrometer (VIR) on board the Dawn spacecraft. We mapped the variations of various spectral parameters on the entire surface of the asteroid, and also derived a map of the lithology.Methods.We took advantage of the recent corrected VIR visible data to map the radiance factor at 550 nm, three color composites, two spectral slopes, and a band area parameter relative to the 930 nm crystal field signature in pyroxene. Using the howardite-eucrite-diogenite meteorites data as a reference, we derived the lithology of Vesta using the variations of the 930 and 506 nm (spin-forbidden) band centers observed in the VIR dataset.Results.Our spectral parameters highlight a significant spectral diversity at the surface of Vesta. This diversity is mainly evidenced by impact craters and illustrates the heterogeneous subsurface and upper crust of Vesta. Impact craters also participate directly in this spectral diversity by bringing dark exogenous material to an almost entire hemisphere. Our derived lithology agrees with previous results obtained using a combination of infrared and visible data. We therefore demonstrate that it is possible to obtain crucial mineralogical information from visible wavelengths alone. In addition to the 506 nm band, we identified the 550 nm spin-forbidden one. As reported by a laboratory study for synthetic pyroxenes, we also do not observe any shift of the band center of this feature across the surface of Vesta, and thus across different mineralogies, preventing use of the 550 nm spin-forbidden band for the lithology derivation. Finally, the largest previously identified olivine rich-spot shows a peculiar behavior in two color composites but not in the other spectral parameters.

Impacts on Ceres and Vesta: Source regions, cratering, and fragmentation

Context. Ceres and Vesta are the largest members of the main belt (MB). They were visited by the NASA Dawn spacecraft between 2011 and 2018, which provided a great sample of detailed observations of the surface of both bodies. Aims. We perform a study on the impacts on Ceres and Vesta. We aim to determine the size-frequency distribution (SFD) of impactors and to identify and quantify the contribution of each source region, as well as the craters produced and fragments ejected in these impact events. Methods. We used a multipart collisional evolution model of the MB called ACDC (Asteroid Collisions and Dynamic Computation) that simulates the collisional evolution of the MB, which is split into six regions (namely Inner, Middle, Pristine, Outer, Cybele, and High-Inclination belts), according to the positions of the major resonances present there (ν6, 3:1J, 5:2J, 7:3J, and 2:1J). Furthermore, it includes the Yarkovsky effect as a dynamical remotion mechanism. We applied ACDC to Ceres and Vesta by keeping a record of all the bodies larger than 100 m that hit them during 4 Gyr. We performed 1600 simulations and, for our analysis, selected the runs that provide the best fits with the SFD of the six regions of the MB and also those that are able to form the Rheasilva and Veneneia, the two large basins on Vesta. Results. The six regions of the MB provide, to a greater or lesser extent, impactors on Ceres and Vesta. The Outer belt is the main source of impactors smaller than 10 km on Ceres, providing more than half of the impacts, while the Middle belt is the secondary source. On Vesta, the relative impactor contribution of the Inner, Middle, and Outer belts is almost even. We are able to reproduce the craters larger than 100 km in Vesta and identify two large depressions identified in Ceres as impact craters: one called Vendimia Planitia of ~900 km and a second one of ~570 km. As an outcome of these impacts, Ceres and Vesta eject fragments into the MB. We obtain fragmentation rates of tens of fragments larger than 1 m per year for both bodies, to tens of fragments larger than 100 m per million years for Vesta and a factor of ~4 greater for Ceres. We find that hundreds of bodies larger than 10 km should have been ejected from Ceres and Vesta during their history.

Erosion of Volatiles by Micrometeoroid Bombardment on Ceres and Comparison to the Moon and Mercury

Abstract Ceres, the largest reservoir of water in the main belt, was recently visited by the Dawn spacecraft, which revealed several areas bearing H2O-ice features. Independent telescopic observations showed a water exosphere of currently unknown origin. We explore the effects of meteoroid impacts on Ceres by considering the topography obtained from the Dawn mission using a widely used micrometeoroid model and ray-tracing techniques. Meteoroid populations with 0.01–2 mm diameters are considered. We analyze the short-term effects Ceres experiences during its current orbit, as well as long-term effects over the entire precession cycle. We find that the entire surface is subject to meteoroid bombardment, leaving no areas in permanent shadow with respect to meteoroid influx. The equatorial parts of Ceres produce 80% more ejecta than the polar regions due to the large impact velocity of long-period comets. Mass flux, energy flux, and ejecta production vary seasonally by a factor of 3–7 due to the inclined eccentric orbit. Compared to Mercury and the Moon, Ceres experiences significantly smaller effects of micrometeoroid bombardment, with a total mass flux of 4.5 ± 1.2 × 10−17 kg m−2 s−1. On average, Mercury is subjected to a 50× larger mass flux and generates 700× more ejecta than Ceres, while the lunar mass flux is 10× larger and the ejecta generation is 30× larger than on Ceres. For these reasons, we find that meteoroid impacts are an unlikely candidate for the production of a water exosphere or significant excavation of surface features. The surface turnover rate from the micrometeoroid populations considered is estimated to be 1.25 Myr on Ceres.

The morphologic and morphometric characteristics of craters on Ceres and implications for the crust

The Dawn spacecraft revealed a heavily cratered surface of the dwarf planet Ceres. These impact craters can be used to uncover the target properties of Ceres by analyzing the morphologic and morphometric characteristics. A near-global crater database containing 44,594 craters with diameters (D) ≥ 1.0 km in the latitude range 84.66°S–89.62°N over all longitudes has been developed to further understand the target properties of Ceres. For this purpose, various interior morphologies were classified such as central peaks, central pits, and lobate flows. Central floor pit craters were only observed in the northern hemisphere on Ceres indicating a weaker crustal strength. Investigations into the peak-to-crater diameter ratio (Dpk/Dc) implies the crust of Ceres has strength more similar to rocky Mercury than to Mars which exhibits localized areas of the crust containing large amounts of ice, or the volatile-rich crust of Ganymede. The Dpk/Dc, however, does display regional variations, with the northern hemisphere having a larger Dpk/Dc consistent with a more volatile-rich crust. Analysis of the depth-diameter ratio for fresh impact craters suggesting the simple-to-complex transition diameter (Dsc) of ~10.0 km and the onset diameters for central peaks indicate a Dsc of ~17.0 km. This is higher than expected if Ceres is a purely icy body and lower than expected if Ceres is a completely rocky body. These results indicate that Ceres has a mixed rock/ice composition, as proposed by other studies.

Calathus: A sample-return mission to Ceres

Ceres, as revealed by NASA's Dawn spacecraft, is an ancient, crater-saturated body dominated by low-albedo clays. Yet, localised sites display a bright, carbonate mineralogy that may be as young as 2 Myr. The largest of these bright regions (faculae) are found in the 92 km Occator Crater, and would have formed by the eruption of alkaline brines from a subsurface reservoir of fluids. The internal structure and surface chemistry suggest that Ceres is an extant host for a number of the known prerequisites for terrestrial biota, and as such, represents an accessible insight into a potentially habitable “ocean world”. In this paper, the case and the means for a return mission to Ceres are outlined, presenting the Calathus mission to return to Earth a sample of the Occator Crater faculae for high-precision laboratory analyses. Calathus consists of an orbiter and a lander with an ascent module: the orbiter is equipped with a high-resolution camera, a thermal imager, and a radar; the lander contains a sampling arm, a camera, and an on-board gas chromatograph mass spectrometer; and the ascent module contains vessels for four cerean samples, collectively amounting to a maximum 40 g. Upon return to Earth, the samples would be characterised via high-precision analyses to understand the salt and organic composition of the Occator faculae, and from there to assess both the habitability and the evolution of a relict ocean world from the dawn of the Solar System.

A Probabilistic Approach to Determination of Ceres' Average Surface Composition From Dawn Visible‐Infrared Mapping Spectrometer and Gamma Ray and Neutron Detector Data

AbstractThe Visible‐Infrared Mapping Spectrometer (VIR) on board the Dawn spacecraft revealed that aqueous secondary minerals—Mg‐phyllosilicates, NH4‐bearing phases, and Mg/Ca carbonates—are ubiquitous on Ceres. Ceres' low reflectance requires dark phases, which were assumed to be amorphous carbon and/or magnetite (∼80 wt.%). In contrast, the Gamma Ray and Neutron Detector (GRaND) constrained the abundances of C (8–14 wt.%) and Fe (15–17 wt.%). Here, we reconcile the VIR‐derived mineral composition with the GRaND‐derived elemental composition. First, we model mineral abundances from VIR data, including either meteorite‐derived insoluble organic matter (IOM), amorphous carbon, magnetite, or combination as the darkening agent and provide statistically rigorous error bars from a Bayesian algorithm combined with a radiative‐transfer model. Elemental abundances of C and Fe are much higher than is suggested by the GRaND observations for all models satisfying VIR data. We then show that radiative transfer modeling predicts higher reflectance from a carbonaceous chondrite of known composition than its measured reflectance. Consequently, our second models use multiple carbonaceous chondrite endmembers, allowing for the possibility that their specific textures or minerals other than carbon or magnetite act as darkening agents, including sulfides and tochilinite. Unmixing models with carbonaceous chondrites eliminate the discrepancy in elemental abundances of C and Fe. Ceres' average reflectance spectrum and elemental abundances are best reproduced by carbonaceous‐chondrite‐like materials (40–70 wt.%), IOM or amorphous carbon (10 wt.%), magnetite (3–8 wt.%), serpentine (10–25 wt.%), carbonates (4–12 wt.%), and NH4‐bearing phyllosilicates (1–11 wt.%).

The surface of (1) Ceres in visible light as seen by Dawn/VIR

Aims.We study the surface of Ceres at visible wavelengths, as observed by the Visible and InfraRed mapping spectrometer (VIR) onboard the Dawn spacecraft, and analyze the variations of various spectral parameters across the whole surface. We also focus on several noteworthy areas of the surface of this dwarf planet.Methods.We made use of the newly corrected VIR visible data to build global maps of a calibrated radiance factor at 550 nm, with two color composites and three spectral slopes between 400 and 950 nm. We have made these maps available for the community via the Aladin Desktop software.Results.Ceres’ surface shows diverse spectral behaviors in the visible range. The color composite and the spectral slope between 480 and 800 nm highlight fresh impact craters and young geologic formations of endogenous origin, which appear bluer than the rest of the surface. The steep slope before 465 nm displays very distinct variations and may be a proxy for the absorptions caused by the O2− → Fe3+or the 2Fe3+→ Fe2++ Fe4+charge transfers, if the latter are found to be responsible for the drop in this spectral range. We notice several similarities between the spectral slopes and the abundance of phyllosilicates detected in the infrared by the VIR, whereas no correlation can be clearly established with carbonate species. The region of the Dantu impact crater presents a peculiar spectral behavior – especially through the color and the spectral slope before 465 nm – suggesting a change in composition or in the surface physical properties that is not observed elsewhere on Ceres.

Evidence of non-uniform crust of Ceres from Dawn’s high-resolution gravity data

The gravity and shape data acquired by the Dawn spacecraft during its primary mission revealed that Ceres is partially differentiated with an interior structure consistent with a volatile-rich crust, a mantle of hydrated rock and isostatically compensated topography1–3. Detailed analyses showed that the mechanically strong crust overlays a weak, fluid-bearing upper mantle4. Previous studies, however, assumed that Ceres’s crust is a uniform layer. Here, we report findings from the new high-resolution gravity data from Dawn’s second extended mission (XM2), which reveal a complex crustal structure of Ceres. In the low-altitude regions probed by the Dawn spacecraft during the XM2 phase, we observe that gravity–topography admittance progressively shifts to a lower density solution at higher degrees, implying a radial density gradient across Ceres’s crust that is consistent with decreasing porosity with depth and/or increasing content of dense phases, such as rock and salts. That gradient brings a critical new constraint on the crustal freezing history, suggesting that the salts and silicates concentrated in the liquid phase while the crust was growing. Localized spectral analysis of the new data also shows evidence for a lower crustal density in the north polar region than in the south or near the equator, supporting impact-driven porosity variations for the observed latitudinal density differences5. On the local scale, the new data show evidence for density or rheological variations within the crust, in association with lobate landslides and ejecta deposits that were inferred to be ice-rich6,7 as well as an extensional fault system8. These inferences provide geophysical context for geological features on the surface and help us advance our understanding of the evolution of an ice-rich but heat-starved body, whose evolution was in part shaped by impacts. High-resolution gravity data from Dawn’s second extended mission could probe the global and local structure of Ceres’s crust. The results show significant spatial and vertical variations of crustal density and porosity, associated with ice features and ice-related processes driven from the interior, and impacts.

Impact-driven mobilization of deep crustal brines on dwarf planet Ceres

Ceres, the only dwarf planet in the inner Solar System, appears to be a relict ocean world. Data collected by NASA’s Dawn spacecraft provided evidence that global aqueous alteration within Ceres resulted in a chemically evolved body that remains volatile-rich1. Recent emplacement of bright deposits sourced from brines attests to Ceres being a persistently geologically active world2,3, but the surprising longevity of this activity at the 92-km Occator crater has yet to be explained. Here, we use new high-resolution Dawn gravity data to study the subsurface architecture of the region surrounding Occator crater, which hosts extensive young bright carbonate deposits (faculae). Gravity data and thermal modelling imply an extensive deep brine reservoir beneath Occator, which we argue could have been mobilized by the heating and deep fracturing associated with the Occator impact, leading to long-lived extrusion of brines and formation of the faculae. Moreover, we find that pre-existing tectonic cracks may provide pathways for deep brines to migrate within the crust, extending the regions affected by impacts and creating compositional heterogeneity. The long-lived hydrological system resulting from the impact might also occur for large impacts in icy moons, with implications for creation of transient habitable niches over time. High-resolution data of Ceres’s bright spots (faculae), obtained by Dawn’s second extended mission, suggest the existence of a deep brine-rich reservoir that emerged to the surface through long-lived cryovolcanic activity as a consequence of the impact that created Occator crater.

A bountiful harvest on Ceres

In its second extended mission at Ceres, the Dawn spacecraft returned a harvest of high-resolution data on the intriguing Occator crater, a landmark for understanding the role of impacts in shaping ice-rich bodies, explain Project Scientist Julie Castillo-Rogez and Chief Engineer Marc Rayman.

Aspects of Pulsar Navigation for Deep Space Mission Applications

This paper investigates the performance of pulsar-based navigation in deep space mission applications. The noise properties of X-ray based and radio-baspulsar measurements are examined and compared. A closed form parametric covariance analysis tool was developed in this study. It provides a rough estimate of the navigation performance associated with a deep space cruise that makes use of ion thrusters and sequential pulsar observations. In addition, the flight trajectory of the Dawn spacecraft was used to form a hypothetical deep space mission scenario that utilizes pulsars as navigation beacons. This simulated scenario accounts for clock uncertainty, pulsar timing noise, maneuver execution errors, sequential observation and interruptions between pulsar observations. A particle filter was implemented to reduce the large initial position uncertainty by resolving the number of pulsar wavelengths between the spacecraft and the Solar System Barycenter. The resulting position and velocity uncertainties from the particle filter can be used to initialize an Extended Kalman Filter, which estimates the spacecraft position and velocity for steady state operations.

Dawn Spacecraft Performance: Resource Utilization and Environmental Effects During an 11-Year Mission

Dawn was a deep space science mission to the asteroid belt, using solar electric propulsion and a spacecraft derived from an earth-orbiting communications satellite. Although much of the operational history of the mission has been captured in the literature, this work is intended to gather and highlight underreported issues and lessons that would be of specific interest to future electric propulsion and/or asteroid missions. This paper describes the most significant engineering resources consumed over the course of the mission and the use of redundant hardware and software. Also detailed are thermal, micrometeoroid, radiation, and other environmental effects and anomalies that the Dawn spacecraft experienced. A brief description of the mission and spacecraft hardware is also provided.

Correction of the VIR-visible data set from the Dawn mission.

Data acquired at Ceres by the visible channel of the Visible and InfraRed mapping spectrometer (VIR) on board the NASA Dawn spacecraft are affected by the temperatures of both the visible (VIS) and the infrared (IR) sensors, which are, respectively, a charged coupled device and a HgCdTe array. The variations of the visible channel temperatures measured during the sessions of acquisitions are correlated with the variations in the spectral slope and shape for all the mission phases. The IR channel temperature is more stable during the acquisitions; nonetheless, it is characterized by a bimodal distribution whether the cryocooler (and, therefore, the IR channel) is used or not during the visible channel operations. When the infrared channel temperature is high (175 K, i.e., not in use and with the cryocooler off), an additional negative slope and a distortion are observed in the spectra of the visible channel. We developed an empirical correction based on a reference spectrum for the whole dataset; it is designed to correct the two issues related to the sensor temperatures that we have identified. The reference spectrum is calculated to be representative of the global Ceres' surface. It is also made of the data acquired when the visible and infrared channel temperatures are equal to the ones measured during an observation of the Arcturus star by VIR, which is consistent with several ground-based observations. The developed correction allows reliable analysis and mapping to be performed by minimizing the artifacts induced by fluctuations of the VIS temperature. Thanks to this correction, a direct comparison between different mission phases during which the VIR experienced different visible and infrared channel temperatures is now possible.