Mid-Infrared Reflectance Spectroscopy of Analogs for the BepiColombo Mission

Introduction: The purpose of the IRIS (Infrared and Raman for Interplanetary Spectroscopy) laboratory is to produce laboratory spectra for the  mid-infrared spectrometer MERTIS (Mercury Radiometer and Thermal Infrared Spectrometer) on the ESA/JAXA BepiColombo mission to Mercury. This device will map the mineralogy of the hermean surface spectral features in the 7-14 µm range, with a spatial resolution of ~ 500 meters [1,2]. 

For the interpretation the data from MERTIS, we need laboratory spectra for comparison. A wide range of natural mineral and rock samples such as terrestrial impact rocks and meteorites [e.g., 3-5] was studied for this purpose. Since we do not have natural samples from surface of Mercury, we produced synthetic analogs based on MESSENGER data and laboratory experiments [6-9].  A central component for such mixtures will be glass to replicate material formed by impact events and lava extrusion and explosive volcanism (i.e., pyroclastic debris) [e.g. 10]. We present results of our ongoing study of synthetic glasses. These synthetic glasses will be studied in the mid-infrared to obtain spectra for the IRIS database, the material will also used for future mixtures and experiments.

Samples and Techniques: 

Sample Production: In order to simulate the petrologic evolution of magmas on early Mercury, we synthesized analog material under controlled temperature, pressure and oxidation state [7-9]. The glass was produced following a procedure described in [11] with the oxidation state controlled by exposing the sample to a CO-CO₂ gas-mixture equivalent to four orders of magnitude below the iron-wüstite buffer (IW-4). For in-situ studies we selected run products from earlier, similar laboratory experiments [9].

Infrared Spectroscopy: For the bulk powder FTIR diffuse reflectance analyses, powder size fractions 0-25 µm, 25-63 µm, 63-125 µm, and 125-250 µm were measured, in addition to a polished sample. We used a Bruker Vertex 70 v infrared system with a MCT detector at the IRIS laboratories at the Institut für Planetologie in Münster. Analyses were conducted under low pressure to reduce atmospheric bands, analyses were made in reflectance from 2-20 µm.

FTIR microscope analyses for in-situ studies in polished blocks and thin sections were conducted using a Bruker Hyperion 1000/2000 System at the Hochschule Emden/Leer. We used a 250×250 µm sized aperture. In addition, a Perkin-Elmer Spotlight-400 FTIR spectrometer at the University of Manchester was used to map samples using an adjoining Focal Plane Array (FPA) mapping unit with a resolution of 6.25 µm × 6.25 µm.

Results: Figure 1 gives an example of an area mapped using micro-FTIR. Sample 131_1 is based on the MESSENGER derived composition of the Mercurian High Magnesium Regions (HMg) [9]. Spectra in Fig.2 are of the Mg-poor (5.2 wt%) glassy component in this sample. Compared with spectra of glasses based on the Inter Crater Planes (ICP) (21.5 wt% MgO), 123_3 (17.7 wt% MgO) and 126_3 (12.2 wt% MgO), we see generally spectra typical for glassy materials. The single Reststrahlenband (RB) shifts with increasing MgO from 9.3 µm to 9.9 µm, and the CF from 7.9 µm to 8.2 µm.

The two examples for bulk powders (Fig.3) have low MgO contents: 1.6 wt% (Low Mg C) and 4.7 wt % (Low Mg B) [9]. Different grain size fractions show  intensity correlated with increasing grain size (Fig. 3). The RB and CF features are basically similar for all size fractions, but an characteristical Transparency Feature (TF) appears in the finest fraction (0-25 µm). The Low Mg B sample has the TF at 11.7 µm, the CF at 7.8 µm and the RB at 9.4 µm. Low-Mg C shows the CF from 7.6 µm to 7.7 µm, the RB at 9.2 µm – 9.3 µm and a TF at 11.6 µm.

Summary & Conclusions: The sieved size fractions of the bulk glass material show typical features for highly crystalline materials. They follow a trend of band shifts for CF and RB towards longer wavelengths with increasing MgO contents [3,4]. We will present further glasses cover higher MgO contents (>6 wt%) to provide material for the whole range of expected Mercurian regolith glass compositions [7-9].

However, for a complete picture of the hermean surface, we expect mixtures of glassy and crystalline material. This will be the next step in our study, where we will obtain spectra of mixtures representing various regions on Mercury [e.g.7,8]. Furthermore, the impact of space weathering, which changes the structural and thus spectroscopic properties of grain surfaces will be taken into account [12,13,14].

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