Mercury Radiometer And Thermal Infrared Spectrometer Research Articles

Laboratory reflectance spectra of enstatite and oldhamite mixtures for comparison with Earth-based reflectance spectra of asteroid 2867 Šteins and Mercury

The reflectance spectra of synthetic oldhamite (CaS), synthetic enstatite (Mg2Si2O6), and their mixtures have been studied in the spectral range from 0.3 μm to 16 μm. The spectrum of enstatite is very bright, with a steep slope in the ultraviolet (UV) and an almost neutral slope in the visible (VIS) and near-infrared (NIR). The mid-infrared (MIR) region is characterized by the Christiansen feature, Reststrahlen bands and the Transparency feature. The oldhamite spectrum shows a red slope in the UV and VIS and an absorption band at 0.41 μm. The absorption band has a relative depth of 11.4%. In the MIR, the oldhamite spectrum is much brighter than the enstatite spectrum and shows several broad absorption bands. The spectra of the mixtures show an intermediate behavior between the two endmembers. The absorption band at 0.41 μm is visible in the spectra of all mixtures, even in the spectrum of the mixture with only 1 vol% oldhamite. In the MIR, the spectra of the mixtures with ≤10 vol% oldhamite are very similar to the spectrum of pure enstatite. Changes in the spectral characteristics such as reflectance or band depths do not follow simple linear trends but show two distinct trends: One for mixtures with ≤10 vol% oldhamite and one for mixtures with ≥20 vol% oldhamite. Changes occur significantly faster in the spectra of mixtures with ≤10 vol% than in those with ≥20 vol% oldhamite.Reflectance spectra of the E[II]-type asteroid 2867 Šteins are flat and almost featureless but show an absorption band at 0.49 μm, which is attributed to oldhamite. Comparison of the laboratory spectra in the VIS and MIR with spectra of Šteins gives an upper limit for the oldhamite content on its surface of 40 %vol. Hence, Šteins probably consists of aubrite-like material with virtually FeO-free enstatite as the major constituent and an oldhamite abundance of <40 vol%. Šteins and other E[II]-type asteroids likely formed through igneous processes on a larger now destroyed body, where an immiscible CaS-melt formed within a silicate melt.The surface of Mercury is also linked to FeO-poor silicates like enstatite. Oldhamite and other sulfides are linked to the formation of hollows on Mercury. Mixtures of enstatite and oldhamite could therefore serve as suitable model analog for the surface of Mercury. Contrasting trends at ∼7–8.5 μm in the MIR reflectance spectra of oldhamite and enstatite could be used as an indicator for the presence of oldhamite in spectral data that will be collected by the MERTIS (Mercury Radiometer and Thermal Infrared Spectrometer) instrument onboard the ESA/JAXA BepiColombo mission to Mercury.

The second Venus flyby of BepiColombo mission reveals stable atmosphere over decades

Studies of the Venusian mesosphere provide important information about the current state of the entire Venusian atmosphere. This includes information about the dense cloud structure, its vertical thermal profile, temperature fields, and the resulting dynamical and meteorological processes that contribute to a deeper understanding of the climatologically different evolutionary paths of Earth and Venus. However, the last measurements were acquired in 1983 during Venera-15 mission. In this paper, results of mid-infrared spectral measurements of the Venusian atmosphere are presented. Here we show Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) measurements of the Venusian atmosphere during the second flyby of BepiColombo mission on its way to Mercury. Our Venus measurements provide reliable retrievals of mesospheric temperature profiles and cloud parameters between 60 and 75 km altitude, although MERTIS was only designed to operate in Mercury environment. Our results are in good agreement with the Venera-15 mission findings. This indicates the stability of the Venusian atmosphere on time scales of decades.

Mid-IR spectral properties of different surfaces of silicate mixtures before and after excimer laser irradiation

The behaviour of the Christiansen feature (CF) and the Reststrahlen bands (RBs) in mid-infrared (IR) reflectance spectra on various silicate mixtures as pressed pellets and powders was investigated in high-vacuum. In addition, the influence of micrometeorite bombardment simulated with an excimer laser was studied. The mixtures cover a wide range of possible hermean surface compositions and include the minerals olivine, pyroxene (enstatite and diopside), and plagioclase.For the laser experiments, silicates were pressed into pellets and examined by reflectance infrared spectroscopy to identify changes caused by micrometeorite impacts as one tracer of space weathering on airless bodies such as Mercury. For comparison, measurements were also performed on loose powders with the same compositions under the same conditions. As a result, it can be shown that the RBs of olivine are rather affected by laser irradiation although SEM investigations show the destruction mainly of plagioclase, indicating that the RBs of plagioclase are masked by the “stronger” RBs of olivine and pyroxene. Furthermore, we found that the CF in mixtures with a plagioclase content of >50% does not shift significantly towards the CF of pyroxene or olivine. On the other hand, the CF of a mixture containing 50% olivine shifts significantly to shorter wavelengths when pyroxene or plagioclase are present in the mixture. Therefore, care is required when interpreting remote sensing data using the CF alone. We also found that the CF shifts to longer wavelengths in rough (regolithic) samples.Our work demonstrates large dependencies of the CF and the RBs positions on the composition of the silicates as well as on the nature of the surface, which is important for space missions, e.g., data acquired by the MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS) experiment onboard BepiColombo.

A mid-infrared study of synthetic glass and crystal mixtures analog to the geochemical terranes on mercury

The MERTIS (MErcury Radiometer and Thermal Infrared Spectrometer) onboard of the BepiColombo ESA/JAXA mission to Mercury will map the surface of Mercury in the wavelength range of 7–14 μm and for the interpretation of these spectra a database of analog materials is needed. We analyzed bulk grain size fractions of a series of analog materials relevant to the distinct terranes of Mercury in diffuse reflectance in the mid-infrared (2.5 μm to 18 μm). Mineral mixtures cover a wide range of modal amounts of forsterite, enstatite, diopside and plagioclase, the resulting spectra can be divided into three distinct groups: (1) is dominated by a single glass feature, (2) by forsterite bands, and (3) by pyroxene bands. Despite often high contents, plagioclase features, are usually ‘overprinted’ by forsterite and pyroxene bands.Spectral parameter CF, an easy obtainable proxy for chemistry (SiO2) and polymerization (SCFM) places the hermean mixtures mostly in the intermediate and basaltic range. The correlation of parameters easily obtainable in remote sensing, Mg/Si ratio, and CF, allows differing materials from high-energy evaporation processes in impacts from such formed in igneous processes.Preliminary comparison with a spectrum covering most of the hermean surface shows some similarity with band positions of the Inter Crater Plain and Heavily Cratered Terrains (IcP-HCT) and High-Mg Northern Volcanic Plains (High-Mg NVP) mixtures, but none of our spectra is able to reproduce the remote sensing data entirely.

Mid-infrared reflectance spectroscopy of synthetic glass analogs for Mercury surface studies

We have synthesized and analyzed by mid-infrared reflectance spectroscopy silicate glasses that are representative for the glasses on the surface of Mercury. The glass compositions are based on high-pressure laboratory experiments and the resulting compositions of the glass phase. The spectra are of interest for investigating the surface of Mercury using the MERTIS (Mercury Radiometer and Thermal Infrared Spectrometer) instrument on board of the ESA/JAXA BepiColombo mission.Both powdered fractions and polished blocks have been analyzed. Powdered size fractions of 0–25 μm, 25-63 μm, 63-125 μm, and 125–250 μm were measured in reflectance in the thermal infrared (2–20 μm). Spectra for powdered bulk glasses (1.6 wt% - 19.0 wt% MgO) show a single, dominating Reststrahlenband (RB, 9.3–9.8 μm), a Christiansen Feature (CF; 7.6 μm - 8.1 μm), and a size dependent Transparency Feature (TF; 11.6–11.9 μm). Micro-FTIR analyses of polished blocks of glasses (3.4–26.5 wt% MgO) have characteristic bands at 7.8–8.2 μm (CF), and 9.3–9.9 μm (RB). Only few olivine crystalline features were observed.Spectral features correlate with compositional characteristics, e.g. SiO2 content or SCFM (SiO2/(SiO2 + CaO + FeO + MgO) index. The strongest correlation between band features CF and the strong RB are with Mg/Si. No simple mixture of glass spectra from this study is able to reproduce the entire ground based spectrum of the surface of Mercury. However, Mg-rich glasses reproduce identified features at 8.5 μm, 9.9 μm and 12.4 μm.

Correction to: Studying the Composition and Mineralogy of the Hermean Surface with the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) for the BepiColombo Mission: An Update

A Correction to this paper has been published: https://doi.org/10.1007/s11214-020-00780-w

Mid‐infrared reflectance spectroscopy of aubrite components

AbstractAubrites Peña Blanca Spring and Norton County were studied in the mid‐infrared reflectance as part of a database for the MERTIS (Mercury Radiometer and Thermal Infrared Spectrometer) instrument on the ESA/JAXA BepiColombo mission to Mercury. Spectra of bulk powder size fractions from Peña Blanca Spring show enstatite Reststrahlen bands (RB) at 9 µm, 9.3 µm, 9.9 µm, 10.4 µm, and 11.6 µm. The transparency feature (TF) is at 12.7 µm, the Christiansen feature (CF) at 8.1–8.4 µm. Micro‐FTIR of spots with enstatite composition in Norton County and Peña Blanca Spring shows four types: Types I and II are similar to the bulk powder spectra but vary in band shape and probably display axis orientation. Type III has characteristic strong RB at 9.2 µm, 10.4 µm, and 10.5 µm, and at 11.3 µm. Type IV is characterized by a strong RB at 10.8−11.1 µm. Types III and IV could show signs of incipient shock metamorphism. Bulk results of this study confirm earlier spectral studies of aubrites that indicate a high degree of homogeneity and probably make the results of this study representative for spectral studies of an aubrite parent body. Spectral types I and II occur in all mineralogical settings (mineral clasts, matrix, melt, fragments in melt vein), while spectral type III was only observed among the clasts, and type IV in the melt. Comparison with surface spectra of Mercury does not obtain a suitable fit, only type IV spectra from quenched impact glass show similarity, in particular the 11 µm feature. Results of this study will be available upon request or via the IRIS database (Münster) and the Berlin Emissivity Database (BED).

Estimation of surface temperatures on Mercury in preparation of the MERTIS experiment onboard BepiColombo

The Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) is part of the payload on the joint ESA-JAXA BepiColombo Mission, launched in October 2018. The spectrometer is designed to map surface composition, identify rock-forming minerals, map surface mineralogy, and study surface temperature variations. The surface of Mercury undergoes large temperature variations depending on solar irradiation. Mineral spectra show significant changes in spectral features with changing temperature. In preparation of the experiment, we developed a thermal model that calculates surface temperatures based on appropriate insolation conditions and thermo-physical properties. This model has been validated with lunar parameters and hence been applied to the conditions of Mercury. Here we present surface temperature maps based on MESSENGER albedo and topography data. The results have been compared to previous models and measurements of Mercury's surface temperatures.The surface of Mercury undergoes large temperature variations each diurnal period. At the equator temperatures vary between less than 100 K during the night up to 700 K at local noon at longitudes 0° W and 180° W, and up to 570 K at longitudes 90° W and 270° W. Due to the 3:2 spin orbit resonance local noon at longitudes 0° W and 180° W coincide with perihelion, while longitudes 90° W and 270° W experience local noon at aphelion, which results in “hot” and “warm” poles around the equator. At 45° N surface temperatures at local noon reach 645 K (0° W and 180° W) and 510 K (90° W and 270° W).

Mid-infrared spectroscopy of planetary analogs: A database for planetary remote sensing

The MERTIS (MErcury Radiometer and Thermal Infrared Spectrometer) instrument onboard the ESA/JAXA BepiColombo mission will provide mid-infrared data, which will be crucial to characterize the surface mineralogy of Mercury. In order to interpret the results, we are creating a database of mid-infrared spectra. As part of a study of synthetic glasses which are to serve as analog materials for the interpretation of remote sensing and modeling data, we present mid-infrared data for analog materials of Mercury regolith, surface and mantle compositions. In addition, we provide data for similar analogs of Earth, Moon, Venus, and Mars rocks for a coherent picture.The analog samples have been first characterized by optical microscopy, Raman spectroscopy and EMPA. Powdered size fractions (0–25 μm, 25–63 μm, 63–125 μm, and 125–250 μm) were studied in reflectance in the mid-infrared range from 2.5 to 18 μm (550 to 2000 cm−1), additional micro-FTIR analyses were also obtained.Results for the size fractions of the surface and regolith analogs for Mercury show typical features for amorphous material with Christiansen Features (CF) at 8–8.1 μm, Reststrahlen Bands (RB) at 9.8–9.9 μm, and the Transparency Feature (TF) at 12 μm. The six bulk silicate Mercury analogs have varying CF positions from 8.1 to 9 μm, with RB crystalline features of various olivines dominating in most samples. Similarly, bulk silicate analogs of the other planetary bodies show glassy features for the surface analogs with CF from 7.9 μm (Earth Continental Crust) to 8.3 μm (Lunar Mare), strong RB from 9.5 μm (Earth Continental Crust) to 10.6 μm (Lunar Mare and Highlands). TF are usually very weak for the glassy analogs.Bulk silicate analogs for the other planetary bodies are again dominated by crystalline olivine features. Trends between SCFM (SiO2/(SiO2 + CaO + FeO + MgO)) index reflecting polymerization, CF and RB positions, and the SiO2 contents in previous studies are basically confirmed, but there is indication that several samples (Moon Mare and Highlands) do not follow the trend observed for low SiO2 samples and the RB position in earlier studies. Comparison with a ground based mid-IR spectrum of Mercury demonstrates general similarity in band positions with glassy Mercury surface and regolith material, and a chondrite based model of the planet. However, no mix so far can explain all spectral features.

Elmar K. Jessberger (1943–2017)

Elmar K. Jessberger (1943–2017)

IR spectroscopy of synthetic glasses with Mercury surface composition: Analogs for remote sensing

In a study to provide ground-truth data for mid-infrared observations of the surface of Mercury with the MERTIS (Mercury Radiometer and Thermal Infrared Spectrometer) instrument onboard the ESA/JAXA BepiColombo mission, we have studied 17 synthetic glasses. These samples have the chemical compositions of characteristic Hermean surface areas based on MESSENGER data.The samples have been characterized using optical microscopy, EMPA and Raman spectroscopy. Mid-infrared spectra have been obtained from polished thin sections using Micro-FTIR, and of powdered size fractions of bulk material (0–25, 25–63, 93–125 and 125–250µm) in the 2.5–18µm range.The synthetic glasses display mostly spectra typical for amorphous materials with a dominating, single Reststrahlen Band (RB) at 9.5–10.7µm. RB Features of crystalline forsterite are found in some cases at 9.5–10.2µm, 10.4–11.2µm, and at 11.9µm. Dendritic crystallization starts at a MgO content higher than 23wt.% MgO.The Reststrahlen Bands, Christiansen Features (CF), and Transparency Features (TF) shift depending on the SiO2 and MgO contents. Also a shift of the Christiansen Feature of the glasses compared with the SCFM (SiO2/(SiO2+CaO+FeO+MgO)) index is observed. This shift could potentially help distinguish crystalline and amorphous material in remote sensing data. A comparison between the degree of polymerization of the glass and the width of the characteristic strong silicate feature shows a weak positive correlation.A comparison with a high-quality mid-IR spectrum of Mercury shows some moderate similarity to the results of this study, but does not explain all features.

Mid-infrared bi-directional reflectance spectroscopy of impact melt glasses and tektites

We have analyzed 14 impact melt glass samples, covering the compositional range from highly felsic to mafic/basaltic, as part of our effort to provide mid-infrared spectra (7–14µm) for MERTIS (Mercury Radiometer and Thermal Infrared Spectrometer), an instrument onboard of the ESA/JAXA BepiColombo mission.Since Mercury was exposed to many impacts in its history, and impact glasses are also common on other bodies, powders of tektites (Irghizite, Libyan Desert Glass, Moldavite, Muong Nong, Thailandite) and impact glasses (from the Dellen, El'gygytgyn, Lonar, Mien, Mistastin, and Popigai impact structures) were analyzed in four size fractions of (0–25, 25–63, 93–125 and 125–250µm) from 2.5 to 19µm in bi-directional reflectance. The characteristic Christiansen Feature (CF) is identified between 7.3µm (Libyan Desert Glass) and 8.2µm (Dellen). Most samples show mid-infrared spectra typical of highly amorphous material, dominated by a strong Reststrahlen Band (RB) between 8.9µm (Libyan Desert Glass) and 10.3µm (Dellen). Even substantial amounts of mineral fragments hardly affect this general band shape.Comparisons of the SiO2 content representing the felsic/mafic composition of the samples with the CF shows felsic/intermediate glass and tektites forming a big group, and comparatively mafic samples a second one. An additional sign of a highly amorphous state is the lack of features at wavelengths longer than ∼15µm. The tektites and two impact glasses, Irghizite and El'gygytgyn respectively, have much weaker water features than most of the other impact glasses.For the application in remote sensing, spectral features have to be correlated with compositional characteristics of the materials. The dominating RB in the 7–14µm range correlates well with the SiO2 content, the Christiansen Feature shows similar dependencies. To distinguish between glass and crystalline phases of the same chemical composition, a comparison between CF the SCFM index (SiO2/(SiO2+CaO+FeO+MgO)) (Walter and Salisbury [1989] J. Geophys. Res., 94, 9203–9213) is useful, if chemical compositional data are also available.

Mid-infrared spectroscopy of impactites from the Nördlinger Ries impact crater

This study is part of an effort to build a mid-infrared database (7–14μm) of spectra for MERTIS (Mercury Radiometer and Thermal Infrared Spectrometer), an instrument onboard of the ESA/JAXA BepiColombo space probe to be launched to Mercury in 2017.Mercury was exposed to abundant impacts throughout its history. This study of terrestrial impactites can provide estimates of the effects of shock metamorphism on the mid-infrared spectral properties of planetary materials.In this study, we focus on the Nördlinger Ries crater in Southern Germany, a well preserved and easily accessible impact crater with abundant suevite impactites. Suevite and melt glass bulk samples from Otting and Aumühle, as well as red suevite from Polsingen were characterized and their reflectance spectra in mid-infrared range obtained. In addition, in-situ mid-infrared spectra were made from glasses and matrix areas in thin sections. The results show similar, but distinguishable spectra for both bulk suevite and melt glass samples, as well as in-situ measurements.Impact melt glass from Aumühle and Otting have spectra dominated by a Reststrahlen band at 9.3–9.6μm. Bulk melt rock from Polsingen and bulk suevite and fine-grained matrix have their strongest band between 9.4 and 9.6μm. There are also features between 8.5 and 9μm, and 12.5–12.8μm associated with crystalline phases. There is evidence of weathering products in the fine-grained matrix, such as smectites. Mercury endured many impacts with impactors of all sizes over its history. So spectral characteristics observed for impactites formed only in a single impact like in the Ries impact event can be expected to be very common on planetary bodies exposed to many more impacts in their past. We conclude that in mid-infrared remote sensing data the surface of Mercury can be expected to be dominated by features of amorphous materials.

Estimation of lunar surface temperatures and thermophysical properties: test of a thermal model in preparation of the MERTIS experiment onboard BepiColombo

The Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) is part of the payload on the joint ESA-JAXA BepiColombo Mission, scheduled for launch in 2016. The spectrometer is designed to map surface compositions, to identify rock-forming minerals, to map surface mineralogy, and to study surface temperature variations. In preparation of the experiment we developed a thermal model that calculates surface temperatures based on appropriate insolation conditions and thermophysical properties. In the absence of thermal measurements on Mercury, we validate the model with lunar parameters. The results show good agreement with Apollo 17, Clementine and LRO-Diviner data. With appropriate changes of the orbital parameters and ephemeris data this model can be applied to the conditions of Mercury.

In-situ high-temperature emissivity spectra and thermal expansion of C2/c pyroxenes: Implications for the surface of Mercury

This work was carried out within the framework of the European Space Agency and Japanese Aerospace Exploration Agency BepiColombo space mission to Mercury and intends to provide valid tools for the interpretation of spectra acquired by the MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS) on board of BepiColombo. Two C 2/ c augitic pyroxenes, with different Mg/Fe ratios and constant Ca contents, were investigated by in situ high-temperature thermal infrared spectroscopy and in situ high-temperature single-crystal X-ray diffraction up to temperatures of about 750 and 770 K, respectively. The emissivity spectra of the two samples show similar band center shifts of the main three bands toward lower wavenumbers with increasing temperature. In detail, with increasing temperature bands 1 and 2 of both samples show a much stronger shift with respect to band 3, which remains almost unchanged. Our results indicate that the center positions of bands 1 and 2 are strong functions of the temperature, whereas the center position of band 3 is a strong function of the Mg# [with Mg# = Mg/ (Mg + Fe 2+ ) atomic ratio]. The analysis of the thermal behavior gives similar thermal expansion volume coefficients, α V, for the Mg-rich and Fe-rich samples, with α V = 2.72(8) and 2.72(7) × 10 -5 K -1 , respectively, using the Berman (1988) equation. This correspondence totally explains the band center shifts similarity between the two samples. Our data suggest that MERTIS spectra will be able to provide indications of C 2/ c augitic pyroxene Mg# and will allow a correct interpretation that is independent on the spectra acquisition temperature.

Characterization of the 300 K and 700 K Calibration Sources for Space Application with the Bepicolombo Mission to Mercury

The Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) onboard the European–Japanese space mission BepiColombo to Mercury will be launched in 2014. The MERTIS scientific objective is to identify rock-forming minerals and measure surface temperatures by infrared spectroscopy (7 μm to 14 μm) and spectrally unresolved infrared radiometry (7 μm to 40 μm). To achieve this goal, MERTIS utilizes two onboard infrared calibration sources, the MERTIS blackbody at 700 K (MBB7) and the MERTIS blackbody at 300 K (MBB3), together with deep space observations corresponding to 3 K. All three sources can be observed one after the other using a rotating mirror system. The leaders of the project MERTIS are the Westfälische University of Münster, institute for planetary investigation, Mr. Prof. Dr. H. Hiesinger (PI) and the DLR, Institute of Planetary Research Berlin-Adlershof, Mr. Dr. J. Helbert (CoPI). Both blackbody radiators have to fulfill the severe mass, volume, and power restrictions of MERTIS. The radiating area of the MBB3 is based on a structured surface with a high-emissivity space qualified coating. The relatively high emissivity of the coating was further enhanced by a pyramidal surface structure to values over 0.99 in the wavelength range from 5 μm to 10 μm and over 0.95 in the wavelength range from 10 μm to 30 μm. The MBB7 is based on a small commercially available surface emitter in a standard housing. The windowless emitter is an electrically heated resistor, which consists of a platinum structure with a blackened surface on a ceramic body. The radiation of the emitter is expanded and collimated through use of a parabolic mirror. The design requirements and the radiometric and thermometric characterization of these two blackbodies are described in this paper.

The emissivity of a fine-grained labradorite sample at typical Mercury dayside temperatures

Analyzing the surface composition of Mercury's regolith from remote-sensing measurements is a challenging task. In support of the National Aeronautics and Space Agency's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission and especially in preparation for the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) instrument on the BepiColombo mission of the European Space Agency and the Japan Aerospace Exploration Agency, we are developing a Planetary Emissivity Laboratory at Deutsches Zentrum für Luft-und Rahmfahrt (DLR) in Berlin. Here we present the first measurements of labradorite as a Mercury surface analog obtained with a test setup at a temperature of 420 °C, appropriate for the dayside of Mercury. Because of limitations on the current test apparatus, we present here data only for mid-infrared wavelengths. The spectra show strong indications of significant changes in spectral features in the mid-infrared with changing temperature. This result introduces an additional complication to the analysis of spectra obtained from the surface of Mercury, as the surface temperature and the thermal history of an observed area must be taken into account in the interpretation of the spectra. This inference implies that the latitude, and possibly also the time of day, must be considered when analyzing a spectrum. In particular, the surface at the equator might exhibit different spectral characteristics than the surface at high latitudes even for the same composition. These observations, while still preliminary, underscore strongly the need for temperature-dependent measurements of emissivity of candidate planetary surface materials.

The Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) for the BepiColombo mission

Scheduled for launch on board the BepiColombo Mercury Planetary Orbiter (MPO) in 2014, the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) is an innovative instrument for studying the surface composition and mineralogy of planet Mercury. MERTIS combines an uncooled grating push broom IR-spectrometer (TIS) with a radiometer (TIR), which will operate in the wavelength region of 7–14 and 7–40 μm, respectively. The spatial resolution of the MERTIS observations will be about 500 m globally and better than 500 m for approximately 5–10% of the surface. The thermal infrared range offers unique diagnostic capabilities to study the surface composition of Mercury. In particular, feldspars can easily be detected and characterized, because they show several diagnostic spectral signatures in the 7–14 μm range: the Christiansen feature, reststrahlen bands, and the transparency feature. In addition, MERTIS will allow the identification and mapping of elemental sulfur, pyroxenes, olivines, and other complex minerals. The scientific objectives of MERTIS include: (1) characterization of Mercury's surface composition, (2) identification of rock-forming minerals, (3) mapping of the surface mineralogy, and (4) study of surface temperature variations and the thermal inertia. In preparation for the MERTIS data interpretation, we are performing spectral measurements of appropriate analogue materials in the Planetary Emissivity Laboratory (PEL) and are building a spectral library (Berlin Emissivity Database (BED)) of these materials for a variety of grain sizes.

Emissivity measurements of analogue materials for the interpretation of data from PFS on Mars Express and MERTIS on Bepi-Colombo

The Planetary Fourier Spectrometer (PFS) onboard the Mars Express Mission provides thermal infrared, hyperspectral images of Mars and in the future the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS), part of the selected payload for the Bepi-Colombo Mission, will collect analogous data for Mercury. To interpret these remote sensing data it is essential to understand the spectral emittance of planetary analogue materials and a spectral library of emissivity measurements is needed. Here we introduce the emissivity device built at DLR (Berlin). The device is coupled to a Fourier transform infrared spectrometer (Bruker IFS 88), purged with dry air and equipped with a cooled MCT-detector. We discuss theoretical background of our thermal emission measurements and describe our standard experimental procedures, which are being used to create the Berlin emissivity database (BED). This study presents and discuss the 6.3–22 μm thermal emission spectra of fine-grained feldspar separates ranging from <25 to 90–125 μm. We discuss diagnostic potential of the features present in the emission spectra of plagioclase and alkali feldspars and the particle size effects.