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1.
As the Dawn mission approaches a successful conclusion at Ceres, it seems time to assess how its findings have sharpened the picture of Ceres’s evolution. Before Dawn, we inferred from Ceres's bulk density of about 2100 kg m?3 that Ceres contained about 25% water by mass. Thermodynamic modeling of the interior evolution suggested that the original accreted ice had to melt even if only long‐lived radionuclides were present, leading to the aqueous alteration of the original chondritic silicates and differentiation of the altered silicates from any remaining water, consistent with telescopic detection of aqueously altered silicates (serpentine and clay minerals) on Ceres’s surface. Earth‐based observations of Ceres’s shape were not accurate enough to constrain the extent of differentiation of its interior. Dawn's results confirm these early findings and extend them dramatically to reveal an evolved and active small planet, probably even today, due to water/ice‐driven processes. A nearly uniform global distribution of surface mineralogy, which includes Mg‐serpentines, ammoniated clays, and salts including carbonates, suggests extensive, endogenous, planet‐wide aqueous alteration. Local exceptions show salt‐rich deposits of varied composition, which suggests subsurface heterogeneities. Concentration of Fe below carbonaceous chondrite levels suggests chemical fractionation, leading to Ceres being chemically differentiated. The high spatial uniformity of element abundance measurements of equatorial regolith also indicates that some ice‐rock fractionation occurred on a global scale. Even some local exposures of ice are seen, especially in higher latitudes and in low‐illumination regions that must be very young, as surface water ice is unstable on time scales of 1–1000 years under Ceres’s surface temperatures. Subsurface ice is also likely in abundance at higher latitudes in at least the upper few meters of the surface, as suggested by near‐surface H‐rich polar deposits. Observations of bright ice deposits in permanently shadowed regions suggest cold‐trapping of migrating H2O across the surface. Gravity field measurements indicate a concentration of mass toward the center and near isostatic equilibrium, consistent with at least some mass differentiation driven by water‐related processes. Abundant small and midsize craters but relaxed or missing large craters suggest a stiff upper crust with water abundance lower than 30 vol%. A sharp decrease in viscosity at ~40 km depth suggests the occurrence of a small fraction of liquid, consistent with earlier thermophysical models. Surface cryogenic features, such as flows, extrusions, and domes, some geologically very recent, are evidence of active water/ice‐driven subsurface processes. Ceres experienced extensive water‐related processes and at least some mass and chemical fractionation and is probably active today, consistent with previous moderate heating thermodynamic models. Clearly, Ceres is a “wet,” evolved planet at the edge of the inner solar system, as described in this special issue. We conclude with a list of questions suggested by the Dawn findings; they especially regard the state and fate of water and its role in driving past and possibly current chemical and physical activity in this dwarf planet.  相似文献   

2.
High‐energy gamma rays (HEGRs) from Ceres’s surface were measured using Dawn's Gamma Ray and Neutron Detector (GRaND). Models of cosmic‐ray‐initiated gamma ray production predict that the HEGR flux will inversely vary with single‐layer hydrogen concentrations for Ceres‐like compositions. The measured data confirm this prediction. The hydrogen‐induced variations in HEGR rates were decoupled from the measurements by detrending the HEGR data with Ceres single‐layer hydrogen concentrations determined by GRaND neutron measurements. Models indicate that hydrogen‐detrended HEGR counting rates correlate with water‐free average atomic mass, which is denoted as <A>*. HEGR variations across Ceres’s surface are consistent with <A>* variations of ±0.5 atomic mass units. Chemical variations in the CM and CI chondrites, our closest analogs to Ceres’s surface, suggest that <A>* variations on Ceres are primarily driven by variations in the concentration of Fe, although other elements such as Mg and S could contribute. Dawn observations have shown that Ceres’s interior structure and surface composition have been modified by some combination of physical (i.e., ice‐rock fractionation) and/or chemical (i.e., alteration) processes that has led to variations in bulk surface chemistry. Locations of the highest inferred <A>* values, and thus possibly the highest Fe and least altered materials, tend to be younger, less cratered surfaces that are broadly associated with the impact ejecta of Ceres’s largest craters.  相似文献   

3.
Impact crater Dantu not only exhibits a very complex geological history but also shows an exceptional heterogeneity of its surface composition. Because of its location within a low‐lying region named Vendimia Planitia, which has been proposed to represent an ancient impact basin, Dantu possibly offers a window into the composition of Ceres’s deeper crust, which apparently is enriched in ammonia. Local concentration of carbonates within Dantu or its ejecta blanket may be either exposed or their emplacement induced by the Dantu impact event. Because carbonates can be seen along Dantu's crater walls, exposed due to recent slumping, but also as fresh spots or clusters of spots scattered across the surface, the deposition/formation of carbonates took place over a long time period. The association of several bright spots enriched in carbonates with sets of fractures on Dantu's floor might be accidental. Nevertheless, its morphological and compositional similarity to the faculae in Ceres’s prominent impact crater Occator including its hydrated state does not exclude a cryo‐volcanic origin, i.e., upwelling of carbonate‐enriched brines influenced by H2O ice in the subsurface. Indeed, an isolated H2O ice spot can be identified near Dantu, which shows that ice still exists in Ceres’s subsurface at midlatitudes and that it can exist on the surface for a longer period of time.  相似文献   

4.
The mineralogy and geochemistry of Ceres, as constrained by Dawn's instruments, are broadly consistent with a carbonaceous chondrite (CM/CI) bulk composition. Differences explainable by Ceres’s more advanced alteration include the formation of Mg‐rich serpentine and ammoniated clay; a greater proportion of carbonate and lesser organic matter; amounts of magnetite, sulfide, and carbon that could act as spectral darkening agents; and partial fractionation of water ice and silicates in the interior and regolith. Ceres is not spectrally unique, but is similar to a few other C‐class asteroids, which may also have suffered extensive alteration. All these bodies are among the largest carbonaceous chondrite asteroids, and they orbit in the same part of the Main Belt. Thus, the degree of alteration is apparently related to the size of the body. Although the ammonia now incorporated into clay likely condensed in the outer nebula, we cannot presently determine whether Ceres itself formed in the outer solar system and migrated inward or was assembled within the Main Belt, along with other carbonaceous chondrite bodies.  相似文献   

5.
The Visible and Infrared Spectrometer (VIR) instrument on the Dawn mission observed Ceres’s surface at different spatial resolutions, revealing a nearly uniform global distribution of surface mineralogy. Clearly, Ceres experienced extensive water‐related processes and chemical differentiation. The surface is mainly composed of a dark component (carbon, magnetite?), Mg‐phyllosilicates, ammoniated clays, carbonates, and salts. The observed species suggest endogenous, global‐scale aqueous alteration. While mostly uniform at regional scale, Ceres’s surface shows small localized areas with different species and/or variations in abundances. Few local exposures of water ice are seen, especially at higher latitudes. Sodium carbonates have been identified in several areas on the surface, notably in Occator bright faculae. Organic matter has also been discovered in several places, most conspicuously in a large area close to the Ernutet crater. The observed mineralogies, with the presence of ammoniated species and sodium salts, have a strong resemblance to materials found on other bodies of the outer solar system, such as Enceladus. This poses some questions about the original material from which Ceres accreted, suggesting a colder environment for such material with respect to Ceres’s present position.  相似文献   

6.
A 2‐D numerical study of the evolution of Ceres from a “frozen mudball” to the present era emphasizes the importance of hydrothermal processes. Particulates released as the “frozen mudball” thaws settle to form a roughly 290 km radius core. Hydrothermal flow is driven by radiogenic heating and serpentinization. Both salt‐free and brine fluids are considered. Our modeling suggests that Ceres’s core has been warm over most of its history and is still above freezing, and convective processes are active in core and mantle to the present. The addition of low eutectic solutes greatly expands the region of active convection. A global muddy ocean persists for the first 3 Gyr, and at present, there may be several regional mud seas buried under a frozen crust. Transport of interior material to the near surface occurs throughout our model's history. Eutectic brines drive convective flow to near the surface, even breaching the surface in isolated regions, on the order of 30 km in width, similar in size to some mounds detected using the Dawn visible imaging camera (Sizemore et al. 2015). Surface features such as the bright spot in Occator crater and Ahuna Mons could be the result of eutectic plumes. The CM‐based model density profile is within 10% of Ermakov et al.'s ( 2017 ) results. The model mud mantle has a roughly 42:58 volumetric partitioning of H2O to rock. Our mud model is consistent with the absence of large craters (Marchi et al. 2016 ) and an internal viscosity decreasing with depth (Fu et al. 2017 ).  相似文献   

7.
The highly hydrated, petrologic type 1 CM and CI carbonaceous chondrites likely derived from primitive, water‐rich asteroids, two of which are the targets for JAXA's Hayabusa2 and NASA's OSIRIS‐REx missions. We have collected visible and near‐infrared (VNIR) and mid infrared (MIR) reflectance spectra from well‐characterized CM1/2, CM1, and CI1 chondrites and identified trends related to their mineralogy and degree of secondary processing. The spectral slope between 0.65 and 1.05 μm decreases with increasing total phyllosilicate abundance and increasing magnetite abundance, both of which are associated with more extensive aqueous alteration. Furthermore, features at ~3 μm shift from centers near 2.80 μm in the intermediately altered CM1/2 chondrites to near 2.73 μm in the highly altered CM1 chondrites. The Christiansen features (CF) and the transparency features shift to shorter wavelengths as the phyllosilicate composition of the meteorites becomes more Mg‐rich, which occurs as aqueous alteration proceeds. Spectra also show a feature near 6 μm, which is related to the presence of phyllosilicates, but is not a reliable parameter for estimating the degree of aqueous alteration. The observed trends can be used to estimate the surface mineralogy and the degree of aqueous alteration in remote observations of asteroids. For example, (1) Ceres has a sharp feature near 2.72 μm, which is similar in both position and shape to the same feature in the spectra of the highly altered CM1 MIL 05137, suggesting abundant Mg‐rich phyllosilicates on the surface. Notably, both OSIRIS‐REx and Hayabusa2 have onboard instruments which cover the VNIR and MIR wavelength ranges, so the results presented here will help in corroborating initial results from Bennu and Ryugu.  相似文献   

8.
Thermal evolution modeling has yielded a variety of interior structures for Ceres, ranging from a modestly differentiated interior to more advanced evolution with a dry silicate core, a hydrated silicate mantle, and a volatile‐rich crust. Here we compute the mass and hydrostatic flattening from more than one hundred billion three‐layer density models for Ceres and describe the characteristics of the population of density structures that are consistent with the Dawn observations. We show that the mass and hydrostatic flattening constraints from Ceres indicate the presence of a high‐density core with greater than a 1σ probability, but provide little constraint on the density, allowing for core compositions that range from hydrous and/or anhydrous silicates to a mixture of metal and silicates. The crustal densities are consistent with surface observations of salts, water ice, carbonates, and ammoniated clays, which indicate hydrothermal alteration, partial fractionation, and the possible settling of heavy sulfide and metallic particles, which provide a potential process for increasing mass with depth.  相似文献   

9.
Ceres’ 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 ?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.  相似文献   

10.
Abstract— This study serves as a proof‐of‐concept for the technique of using visible‐near infrared (VNIR), short‐wavelength infrared (SWIR), and thermal infrared (TIR) spectroscopic observations to map impact‐exposed subsurface lithologies and stratigraphy on Earth or Mars. The topmost layer, three subsurface layers and undisturbed outcrops of the target sequence exposed just 10 km to the northeast of the 23 km diameter Haughton impact structure (Devon Island, Nunavut, Canada) were mapped as distinct spectral units using Landsat 7 ETM+ (VNIR/SWIR) and ASTER (VNIR/SWIR/TIR) multispectral images. Spectral mapping was accomplished by using standard image contrast‐stretching algorithms. Both spectral matching and deconvolution algorithms were applied to image‐derived ASTER TIR emissivity spectra using spectra from a library of laboratory‐measured spectra of minerals (Arizona State University) and whole‐rocks (Ward's). These identifications were made without the use of a priori knowledge from the field (i.e., a “blind” analysis). The results from this analysis suggest a sequence of dolomitic rock (in the crater rim), limestone (wall), gypsum‐rich carbonate (floor), and limestone again (central uplift). These matched compositions agree with the lithologic units and the pre‐impact stratigraphic sequence as mapped during recent field studies of the Haughton impact structure by Osinski et al. (2005a). Further conformation of the identity of image‐derived spectra was confirmed by matching these spectra with laboratory‐measured spectra of samples collected from Haughton. The results from the “blind” remote sensing methods used here suggest that these techniques can also be used to understand subsurface lithologies on Mars, where ground truth knowledge may not be generally available.  相似文献   

11.
We model Ceres’ thermo-physical-chemical evolution by considering a large range of initial conditions as well as various evolutionary scenarios. Models are constrained by available shape measurements, which point to a differentiated interior for Ceres. We address the role played by hydrothermal activity in the long-term evolution of Ceres and especially the evolution of its hydrosphere. We suggest that models with times of formation shorter than about 5 My after the production of calcium-aluminum inclusions are more likely to undergo hydrothermal activity in their early history, which affects Ceres’ long-term thermal evolution. We evaluate the conditions for preserving liquid water inside Ceres, a possibility enhanced by its warm surface temperature and the enrichment of its hydrosphere in a variety of chemical species. However, thermal modeling of the hydrosphere needs to be further investigated. We show that shape data can help constrain the amount of hydrated silicate in the core, and thus the extent of hydrothermal activity in Ceres. We discuss the importance of these results for the Dawn mission’s arrival at Ceres in 2015.  相似文献   

12.
Haughton is a ~24 Myr old midsize (apparent diameter 23 km) complex impact structure located on Devon Island in Nunavut, Canada. The center of the structure shows a negative gravity anomaly of ?12 mGal coupled to a localized positive magnetic field anomaly of ~900 nT. A field expedition in 2013 led to the acquisition of new ground magnetic field mapping and electrical resistivity data sets, as well as the first subsurface drill cores down to 13 m depth at the top of the magnetic field anomaly. Petrography, rock magnetic, and petrophysical measurements were performed on the cores and revealed two different types of clast‐rich polymict impactites: (1) a white hydrothermally altered impact melt rock, not previously observed at Haughton, and (2) a gray impact melt rock with no macroscopic sign of alteration. In the altered core, gypsum is present in macroscopic veins and in the form of intergranular selenite associated with colored and zoned carbonate clasts. This altered core has a natural remanent magnetization (NRM) four to five times higher than materials from the other core but the same magnetic susceptibility. Their magnetization is still higher than the surrounding crater‐fill impact melt rocks. X‐ray fluorescence data indicate a similar proportion of iron‐rich phases in both cores and an enrichment in silicates within the altered core. In addition, alternating‐field demagnetization results show that one main process remagnetized the rocks. These results support the hypothesis that intense and possibly localized post‐impact hydrothermal alteration enhanced the magnetization of the clast‐rich impact melt rocks by crystallization of magnetite within the center of the Haughton impact structure. Subsequent erosion was followed by in situ concentration in the subsurface leading to large magnetic gradient on surface.  相似文献   

13.
The formation of hydrated salts is an expected consequence of aqueous alteration of Main Belt objects, particularly for large, volatile‐rich protoplanets like Ceres. Sulfates, present on water‐bearing planetary bodies (e.g., Earth, Mars, and carbonaceous chondrite parent bodies) across the inner solar system, may contribute to Ceres’ UV and IR spectral signature along with phyllosilicates and carbonates. We investigate the presence and stability of hydrated sulfates under Ceres’ cryogenic, low‐pressure environment and the consequent spectral effects, using UV–Vis–IR reflectance spectroscopy. H2O loss begins instantaneously with vacuum exposure, measured by the attenuation of spectral water absorption bands, and a phase transition from crystalline to amorphous is observed for MgSO4·6H2O by X‐ray powder diffraction. Long‐term (>40 h), continuous exposure of MgSO4·nH2O (n = 0, 6, 7) to low pressure (10?3–10?6 Torr) causes material decomposition and strong UV absorption below 0.5 μm. Our measurements suggest that MgSO4·6H2O grains (45–83 μm) dehydrate to 2% of the original 1.9 μm water band area over ~0.3 Ma at 200 K on Ceres and after ~42 Ma for 147 K. These rates, inferred from an Avrami dehydration model, preclude MgSO4·6H2O as a component of Ceres’ surface, although anhydrous and minimally hydrated sulfates may be present. A comparison between Ceres emissivity spectra and laboratory reflectance measurements over the infrared range (5–17 μm) suggests sulfates cannot be excluded from Ceres’ mineralogy.  相似文献   

14.
Abstract— The 50,000 year old, 1.8 km diameter Lonar crater is one of only two known terrestrial craters to be emplaced in basaltic target rock (the 65 million year old Deccan Traps). The composition of the Lonar basalts is similar to martian basaltic meteorites, which establishes Lonar as an excellent analogue for similarly sized craters on the surface of Mars. Samples from cores drilled into the Lonar crater floor show that there are basaltic impact breccias that have been altered by post‐impact hydrothermal processes to produce an assemblage of secondary alteration minerals. Microprobe data and X‐ray diffraction analyses show that the alteration mineral assemblage consists primarily of saponite, with minor celadonite, and carbonate. Thermodynamic modeling and terrestrial volcanic analogues were used to demonstrate that these clay minerals formed at temperatures between 130°C and 200°C. By comparing the Lonar alteration assemblage with alteration at other terrestrial craters, we conclude that the Lonar crater represents a lower size limit for impact‐induced hydrothermal activity. Based on these results, we suggest that similarly sized craters on Mars have the potential to form hydrothermal systems, as long as liquid water was present on or near the martian surface. Furthermore, the Fe‐rich alteration minerals produced by post‐impact hydrothermal processes could contribute to the minor iron enrichment associated with the formation of the martian soil.  相似文献   

15.
SHARAD (SHAllow RADar) is a nadir-looking Synthetic Aperture Ground Penetrating Radar on board NASA's 2005 Mars Reconnaissance Orbiter. There are three main characteristics that define the performance of this instrument: ground penetration (due to the operational frequency, the observed echoes can be related to reflections from surface or subsurface), spaceborne operation (the first reflection does not necessarily correspond to the nadir reflection), and nadir looking SAR (there will always be left/right ambiguities). All this implies that there will be surface/subsurface range ambiguity and the geological interpretation of the radargrams cannot be straightforward. In order to avoid data misinterpretation, a simulator of SHARAD’s expected response for a given observation geometry and topography is needed. Simulations can take into account all surface/subsurface reflections in order to identify common families of ambiguities and facilitate the interpretation. In this work we present SHARSIM (SHARAD Radargram SIMulator), a software tool designed to simulate SHARAD radargrams taking as inputs Mars surface information and hypothetical subsurface structure. Its performance is analyzed by investigating typical artifacts and by a direct comparison with real radargrams. We show that SHARSIM simulations can help to discern between artifacts and real subsurface features in order to make accurate geological interpretations.  相似文献   

16.
Dwarf-planet (1) Ceres is one of the two targets, along with (4) Vesta, that will be studied by the NASA Dawn spacecraft via imaging, visible and near-infrared spectroscopy, and gamma-ray and neutron spectroscopy. While Ceres’ visible and near-infrared disk-integrated spectra have been well characterized, little has been done about quantifying spectral variations over the surface. Any spectral variation would give us insights on the geographical variation of the composition and/or the surface age. The only work so far was that of Rivkin and Volquardsen ([2010], Icarus 206, 327) who reported rotationally-resolved spectroscopic (disk-integrated) observations in the 2.2–4.0 μm range; their observations showed evidence for a relatively uniform surface.Here, we report disk-resolved observations of Ceres with SINFONI (ESO VLT) in the 1.17–1.32 μm and 1.45–2.35 μm wavelength ranges. The observations were made under excellent seeing conditions (0.6″), allowing us to reach a spatial resolution of ～75 km on Ceres’ surface. We do not find any spectral variation above a 3% level, suggesting a homogeneous surface at our spatial resolution. Slight variations (about 2%) of the spectral slope are detected, geographically correlated with the albedo markings reported from the analysis of the HST and Keck disk-resolved images of Ceres (Li et al. [2006], Icarus 182, 143; Carry et al. [2008], Astron. Astrophys. 478, 235). Given the lack of constraints on the surface composition of Ceres, however, we cannot assert the causes of these variations.  相似文献   

17.
Abstract— In the framework of international planetary exploration programs, several space missions are planned to search for organics and bio‐signatures on Mars. Previous attempts have not detected any organic compounds in the Martian regolith. It is therefore critical to investigate the processes that may affect organic molecules on and below the planet's surface. Laboratory simulations can provide useful data about the reaction pathways of organic material at Mars' surface. We have studied the stability of amino acid thin films against ultraviolet (UV) irradiation and use those data to predict the survival time of these compounds on and in the Martian regolith. We show that thin films of glycine and D‐alanine are expected to have half‐lives of 22 ± 5 hr and of 3 ± 1 hr, respectively, when irradiated with Mars‐like UV flux levels. Modelling shows that the half‐lives of the amino acids are extended to the order of 107 years when embedded in regolith. These data suggest that subsurface sampling must be a key component of future missions to Mars dedicated to organic detection.  相似文献   

18.
Abstract– A model is presented in which the aqueous conditions needed to generate phyllosilicate minerals in the absence of carbonates found in the ancient Noachian crust are maintained by an early CO2‐rich atmosphere, that, together with iron (II) oxidation, would prevent carbonate formation at the surface. After cessation of the internal magnetic dynamo, a CO2‐rich primordial atmosphere was stripped by interactions with the solar wind and surface conditions evolved from humid to arid, with ground waters partially dissolving subsurface carbonate and sulfide minerals to produce acid‐sulfate evaporitic deposits in areas with upwelling ground water. In a subsequent geochemical state (Late Noachian to Hesperian), surface and subsurface acidic solutions were neutralized in the subsurface through interaction with basaltic crust, allowing the precipitation of secondary carbonates. This model suggests that, in the early Noachian, the surface waters of Mars maintained acidity because of a drop in temperature. This would have favored increased dissolution of CO2 and a reduction in atmospheric pressure. In this scenario, physicochemical conditions precluded the formation of surface carbonates, but induced the precipitation of carbonates in the subsurface.  相似文献   

19.
Variations 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).  相似文献   

20.
Abstract Petrographical and chemical analysis of melt particles and alteration minerals of the about 100 m‐thick suevitic sequence at the Chicxulub Yax‐1 drill core was performed. The aim of this study is to determine the composition of the impact melt, the variation between different types of melt particles, and the effects of post‐impact hydrothermal alteration. We demonstrate that the compositional variation between melt particles of the suevitic rocks is the result of both incomplete homogenization of the target lithologies during impact and subsequent post‐impact hydrothermal alteration. Most melt particles are andesitic in composition. Clinopyroxene‐rich melt particles possess lower SiO2 and higher CaO contents. These are interpreted by mixing of melts from the silicate basement with overlying carbonate rocks. Multi‐stage post‐impact hydrothermal alteration involved significant mass transfer of most major elements and caused further compositional heterogeneity between melt particles. Following backwash of seawater into the crater, palagonitization of glassy melt particles likely caused depletion of SiO2, Al2O3, CaO, Na2O, and enrichment of K2O and FeOtot during an early alteration stage. Since glass is very susceptible to fluid‐rock interaction, the state of primary crystallization of the melt particles had a significant influence on the intensity of the post‐impact hydrothermal mass transfer and was more pronounced in glassy melt particles than in well‐crystallized particles. In contrast to other occurrences of Chicxulub impactites, the Yax‐1 suevitic rocks show strong potassium metasomatism with hydrothermal K‐feldspar formation and whole rock K20 enrichment, especially in the lower unit of the suevitic sequence. A late stage of hydrothermal alteration is characterized by precipitation of silica, analcime, and Na‐bearing Mg‐rich smectite, among other minerals. This indicates a general evolution from a silica‐undersaturated fluid at relatively high potassium activities at an early stage toward a silica‐oversaturated fluid at relatively high sodium activities at later stages in the course of fluid rock interaction.  相似文献   

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