Ceres – VIS-IR Surface Composition Analysis: A Review In Advance Of The DAWN Mission

Ceres has been heavily investigated during the last years prior to the DAWN mission. Although it is the largest object in the Main Asteroid Belt, its properties, especially the surface composition, are not well understood. Studies of Ceres surface composition and texture are of particular importance to generally analyze the interior and evolution of Solar System objects as well as the surface processes that are/were active on those bodies. VIS-IR spectroscopy is an effective method to detect characteristic absorption bands in the spectra of surface materials which can be related to the surface composition of planets and asteroids. The primary aim of this work is to review the previous visible and infrared earth-based observations and the supporting laboratory work that have been done so far to get an overview on the possible surface composition of Ceres prior to DAWN’s arrival. These data will be compared with complementary spectral measurements in the wavelength range of the VIR instrument onboard the DAWN spacecraft between 0.5 to 5 µm. Measured analogue materials include meteorites (CM, CO, and CV chondrites) and minerals (brucite, cronstedtite, tochilinite, buddingtonite). Additional spectra were collected from databases like Relab to increase the range of data. These data include spectra of meteorites, especially CM, CO, CV and CI chondrites, and of terrestrial analogue materials, e.g. montmorillonite, carbonates, water ice and frost, pyrite, magnesite. Diagnostic spectral characteristics, like the wavelength of slope change, the spectral slopes in the VIS and NIR, and absorption bands, have been defined and analyzed in the available spectra. They are a useful tool to identify Ceres’ surface materials and to draw implications for the DAWN composition analysis

Berlin Reflectance Spectral Library (BRSL)

The Berlin Reflectance Spectral Library (BRSL) provides a collection of reflectance spectra between 0.3 and 17 µm. It was originally dedicated to support space missions to small solar system bodies. Meanwhile the library includes selections of biconical reflectance spectra for spectral data analysis of other planetary bodies as well. The library provides reference spectra of well-characterized terrestrial analogue materials and meteorites for interpretation of remote sensing reflectance spectra of planetary surfaces. We introduce the BRSL, summarize the data available, and access to use them for further relevant applications

Reflectance spectra of synthetic Fe-free ortho- and clinoenstatites in the UV/ VIS/IR and implications for remote sensing detection of Fe-free pyroxenes on planetary surfaces

For a better spectral characterization of planetary bodies with enstatite-rich surfaces like Mercury or E-type asteroids, we synthesized two different enstatite (Mg2Si2O6) polymorphs: Orthoenstatite and clinoenstatite. Both enstatite polymorphs are known from the meteorite record and are commonly observed in aubrites and enstatite chondrites. The synthesized enstatites are particulate samples suitable for laboratory reflectance measurements and can be used for compositional modelling by preparing mixtures of samples in the laboratory or by using the sample's spectra in mathematical models. We report on the synthesis process, chemical composition, grain size distribution, and reflectance spectra of these synthetic enstatites covering the wavelength range from 0.25 to 17 μm, compare them to other pyroxenes (meteoritic enstatite and other synthetic enstatites and diopside), and discuss the implications of retrieving surface compositions of planetary bodies like E-type asteroids, comets, or Mercury. Both enstatite spectra are very bright in the VIS and NIR and show almost neutral to slightly bluish spectral slopes with a steep absorption in the UV. Very low iron in the enstatites (below ~0.04 wt% FeO) already results in weak albeit noticeable absorptions in the VNIR between 0.4 and 0.9 μm. Orthoenstatite and clinoenstatite are not distinguishable based only on their spectra in the VIS and NIR. At the Reststrahlen bands in the MIR a systematic difference in the number and exact position of local minima at ~10 μm between clinoenstatite and orthoenstatite is evident. This can be used to discern between the polymorphs in this wavelength range. Additionally, we can distinguish between Fe-free low- and high-Ca pyroxenes in the MIR

Laboratory Measurements of Synthetic Pyroxenes and their Mixtures with Iron Sulfides as Inorganic Refractory Analogues for Rosetta/VIRTIS' Surface Composition Analysis of 67P/CG

The Visible and InfraRed Thermal Imaging Spectrometer VIRTIS on board Rosetta provided 0.25-5.1 µm spectra of 67P/CG's surface (Capaccioni et al., 2015). Thermally corrected reflectance spectra display a low albedo of 0.06 at 0.65 µm, different red VIS and IR spectral slopes, and a broad 3.2 µm band. This absorption feature is due to refractory surface constituents attributed to organic components, but other refractory constituents influence albedo and spectral slopes. Possible contributions of inorganic components to spectral characteristics and spectral variations across the surface should be understood based on laboratory studies and spectral modeling. Although a wide range of silicate compositions was found in "cometary" anhydrous IDPs and cometary dust, Mg-rich crystalline mafic minerals are dominant silicate components. A large fraction of silicate grains are Fe-free enstatites and forsterites that are not found in terrestrial rocks but can be synthesized in order to provide a basis for laboratory studies and comparison with VIRTIS data. We report the results of the synthesis, analyses, and spectral reflectance measurements of Fe-free low-Ca pyroxenes (ortho- and clinoenstatites). These minerals are generally very bright and almost spectrally featureless. However, even trace amounts of Fe-ions produce a significant decrease in the near-UV reflectance and hence can contribute to slope variations. Iron sulfides (troilite, pyrrhotite) are among the most plausible phases responsible for the low reflectance of 67P's surface from the VIS to the NIR. The darkening efficiency of these opaque phases is strongly particle-size dependent. Here we present a series of reflectance spectra of fine-grained synthetic enstatite powders mixed in various proportions with iron sulfide powders. The influence of dark sulfides on reflectance in the near-UV to near-IR spectral ranges is investigated. This study can contribute to understand the shape of reflectance spectra of 67P's surface at different spectral ranges. Implications for the VIRTIS data analysis are discussed

Analysis of reflectance spectra of enstatite-oldhamite mixtures for comparison with 2867 Šteins.

We present 0.3-17 µm reflectance spectra of synthetic orthoenstatite (Mg2Si2O6), synthetic oldhamite (CaS), and their mixtures that can be used as analogue materials for comparison with E[II]-type asteroids like 2867 Šteins. We investigate the spectral behavior of the mixtures in regard to their oldhamite content and especially the changes in band depth in the VIS

Laboratory spectral VNIR studies supporting VIRTIS’ nucleus surface composition analysis of 67P/CG and prospects for future observations

The Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) onboard Rosetta provided 0.25-5.2 μm spectra of 67P/CG (Capaccioni et al., 2015) enabling a nucleus surface composition analysis. Thermally and photometrically corrected reflectance spectra display low geometric albedo, different red VIS and IR spectral slopes, and a complex broad absorption band centered at 3.2 μm. The low albedo and the structure of the ubiquitous 3.2 μm band suggest that the cometary surface is homogeneously enriched in refractory organic materials and darkening phases (Capaccioni et al., 2015). Detailed studies of laboratory analogue materials are necessary to disentangle different interfering factors that influence the appearance of reflectance spectra. Thus, such investigations are indispensable for reliable compositional analysis of remote sensing data. Here we present 0.25-5 μm laboratory reflectance spectra of well-characterized terrestrial hydrocarbon materials (solid oil bitumens, coals) and discuss their relevance as spectral analogues for 67P/CG refractory organics. In addition, the possible contribution of inorganic matter to spectral characteristics of 67P/CG should be understood based on laboratory reflectance measurements. Although a wide range of silicate compositions was found in cometary dust and anhydrous IDPs of presumably cometary origin, Mg-rich crystalline mafic minerals are dominant silicate components. A large fraction of silicate grains are Fe-free pyroxenes and olivines (forsterites) that are not found in terrestrial rocks but can be synthesized in order to provide a basis for laboratory studies and comparison with VIRTIS data. We report the results of the synthesis, analyses, and/or spectral reflectance measurements of Fe-free low-Ca pyroxenes (ortho- and clinoenstatites), forsterites, and a high-Ca pyroxene (diopside). Finally, fine-grained opaque refractory phases (e.g. iron sulfides, Fe-Ni alloys) are likely responsible for the low IR reflectance and low contrast of the 3.2 μm absorption band. Therefore, we present and discuss 0.25–5 μm reflectance spectra of pure Fe-sulfides (troilites, pyrrhotites) and their intimate mixtures with organic and inorganic analogue materials. Based on the reported laboratory data, we discuss the ability of iron sulfides to suppress absorption bands of other cometary refractory components and to affect the spectral slopes and reflectance values of the 67P/CG surface at different wavelengths from the near-UV to the IR. Summarizing these results, we discuss some prospects for future spectral studies

Laboratory spectral reflectance studies aimed at providing clues to composition of refractory phases of comet 67P/CG's nucleus

We present 0.3-5 micron reflectance spectra of well-characterized powdered crystalline materials (Fe-sulfides, Mg-silicates), natural complex hydrocarbons and their mixtures that can serve as spectral analogues of comet 67P/CG's refractory phases. We study the ability of Fe-sulfides to suppress absorption bands of other cometary refractory components and to affect spectral slopes and reflectance values of the 67P/CG surface at different wavelengths from the near-UV to the IR. We investigate the evolution of organic absorption bands as a function of sulfide content in the mixtures and the possibility for detection of individual C-H stretching bands in reflectance spectra of 67P/CG

Spectral analysis of selected sediment core samples from the Chew Bahir Basin, Ethiopian Rift in the spectral range from 0.3 to 17 µm: support for climate proxy information

Investigations on short (≤18.8 m) sediment cores retrieved along a NW-SE transect across the Chew Bahir (CB) basin, Southern Ethiopian Rift, have shown that they can provide valuable climate information (Förster et al., 2012). The relationship between mineralogical and geochemical properties of the core samples is closely linked to the hydroclimate history of the region. During dry climate episodes both the illitization of the smectites and the octahedral Al-to-Mg substitution in the phyllosilicate materials has been documented. An enhanced potassium fixation during dry intervals is also linked to the increase in layer charge caused by the authigenic changes in octahedral composition (Förster et al., 2018

Reflectance spectroscopy of natural organic solids, iron sulfides and their mixtures as refractory analogues for Rosetta/VIRTIS' surface composition analysis of 67P/CG

Analysis of 0.25-5 µm reflectance spectra provided by the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) onboard Rosetta orbiter revealed that the surface of 67P/CG is dark from the near-UV to the IR and is enriched in refractory phases such as organic and opaque components. The broadness and complexity of the ubiquitous absorption feature around 3.2 µm suggest a variety of cometary organic constituents. For example, complex hydrocarbons (aliphatic and polycyclic aromatic) can contribute to the feature between 3.3 and 3.5 µm and to the low reflectance of the surface in the visible. Here we present the 0.25-5 µm reflectance spectra of well-characterized terrestrial hydrocarbon materials (solid oil bitumens, coals) and discuss their relevance as spectral analogues for a hydrocarbon part of 67P/CG's complex organics. However, the expected low degree of thermal processing of cometary hydrocarbons (high (H+O+N+S)/C ratios and low carbon aromaticities) suggests high IR reflectance, intense 3.3-3.5 µm absorption bands and steep red IR slopes that are not observed in the VIRTIS spectra. Fine-grained opaque refractory phases (e.g., iron sulfides, Fe-Ni alloys) intimately mixed with other surface components are likely responsible for the low IR reflectance and low intensities of absorption bands in the VIRTIS spectra of the 67P/CG surface. In particular, iron sulfides are common constituents of cometary dust, "cometary" chondritic IDPs, and efficient darkening agents in primitive carbonaceous chondrites. Their effect on reflectance spectra of an intimate mixture is strongly affected by grain size. We report and discuss the 0.25-5 µm reflectance spectra of iron sulfides (meteoritic troilite and several terrestrial pyrrhotites) ground and sieved to various particle sizes. In addition, we present reflectance spectra of several intimate mixtures of powdered iron sulfides and solid oil bitumens. Based on the reported laboratory data, we discuss the ability of iron sulfides to suppress absorption bands of other cometary refractory components and to affect the spectral slopes and reflectance values of the 67P/CG surface at different wavelengths from the near-UV to the IR

Correlation of comet 67P/CG's morphology with the occurrence of exposed water ice patches

Comet 67P's surface is quite homogeneously covered by dark refractory materials rich in organics. Rare water ice exposures on the surface, most likely originating from sub-surface layers, have recently been discovered. Such H2O ice patches on 67P's Imhotep region in the pre-perihelion phase were examined and related to the local morphology to understand the exposure mechanisms