Infrared (IR) spectroscopy



Infrared (IR) spectroscopy takes advantage from vibrational spectroscopy, whose basic principle is that vibrational motions occur in a crystal lattice at particular frequencies, associated to the crystal structure and elemental composition. For most geologic materials (except for hydrated phases and organics), the fundamental vibrations occur at frequencies between 1000 and 2000 cm-1 (10 to 5 µm). The unique spectral bands produced from the fundamental vibrations of anions like CO32-, SO42-, PO43-, SiO44-, and of metal-O bonds allow for identification of carbonates, phosphates, silicates, sulfates, oxides, and hydroxides. On the other hand, these stretching and bending modes are affected by cations like Mg2+, Fe2+;3+, Ca2+, and Na+, which is useful for discriminating various minerals within the silicate and carbonate classes.

Band shapes and band depths are not only affected by composition and structure, but also by various physical/textural properties of the material, including grain size distribution, surface roughness, packing density, crystallinity, and preferential orientation of crystals.

In the Planetary Emissivity Laboratory (PEL) we identify mineral structures by measuring emissivity, reflectance or transmittance in the IR spectral region.

The mineral molecules are formed by atoms, acting like balls at the end of molecular springs: when energy (light) of just the right wavelength hits a particular molecule, it start vibrating (bending and stretching), re-radiating the same wavelength of light. The wavelengths of light that cause molecular vibrations occur in the infrared region. Every unique molecule has its own characteristic frequency of vibration.

The emissivity (e) of a material is the relative ability of its surface to emit energy by radiation, and is expressed as the ratio of energy radiated by a particular material to energy radiated by a black body (by definition, having e=1) at the same temperature. Thus, e < 1 for all the materials. Reflectance generally refers to the fraction of incident electromagnetic power that is reflected at an interface. Reflection can be specular, like for mirrors or polished metals, when is nearly zero for all angles except for an appropriate reflected angle; or diffuse, like for all the mineral surfaces, where radiation is reflected near-equally in all the directions.

Transmittance is the fraction of incident light (electromagnetic radiation) at a specified wavelength that passes through a sample.

Berlin Emissivity Database

Remote sensing infrared spectroscopy is the principal field of investigation for planetary surfaces composition. Past, present and future missions to the solar system bodies include in their payload instruments measuring the emerging radiation in the infrared range. For the interpretation of the measured data an emissivity spectral library of planetary analogue materials is needed.

 BED Berlin Emissivity Database
zum Bild BED Berlin Emissivity Database

The Berlin Emissivity Database (BED) is focused on relatively fine-grained size separates, providing a realistic basis for interpretation of thermal emission spectra of planetary regoliths. The BED is therefore complimentary to existing thermal emission libraries, like the ASU library for example. The Berlin Emissivity Database (BED) currently contains emissivity spectra of plagioclase and potassium feldspars, low Ca and high Ca pyroxenes, olivine, elemental sulphur, Martian analogue minerals and volcanic soils, and a lunar highland soil sample measured in the wavelength range from 3 to 16 µm as a function of particle size. For each sample we measured the spectra of four particle size separates: <25, 25-63, 63-125 and 125-250 µm. These size separates have been selected to truly represent most of the planetary surfaces.

The spectral measurements are performed with a Fourier transform infrared spectrometer Bruker IFS 88, purged with dry air and equipped with a liquid-nitrogen-cooled HgCdTe (MCT) detector. The spectrometer is attached to an external emissivity chamber, a double-walled box with three apertures:

  • a 15 cm squared door used to insert the cup in the chamber,
  • a 5 cm rounded opening through which the beam is directed to the spectrometer
  • and a 5 cm opening facing the attached blackbody unit.

A heater is installed in the chamber and is used to heat the cup with the samples from the bottom. The thermal radiation emitted normal to the surface by the sample or the blackbody is collected by an rotatable Au-coated parabolic off-axis mirror and reflected to the entrance port of the spectrometer. A pump circulates water at a constant temperature (20° C or lower) in the volume between the inner and outer walls of the chamber. The surfaces of the box are painted with black high emissivity paint. The chamber is purged with dry air to remove particulates, water vapour and CO2. All spectra were acquired with a spectral resolution of 4 cm-1.

Advanced data handling and processing techniques

The MASCS instrument on the MESSENGER mission has so far collected more than 3 million spectra. PFS on MarsExpress has collected more than 3 million spectra. MERTIS on BepiColombo will collected several orders of magnitude more data. To analyze such large datasets developed an advanced database management system. This system allows the extraction and simultaneous analysis of large amounts of data, transparent to the underlying data structure.

 MESSENGER/MASCS database extraction over user defined region of interest (Waters crater on Mercury).
zum Bild MESSENGER/MASCS database extraction over user defined region of interest (Waters crater on Mercury).

The most recent version of our data analysis procedure uses PostgreSQL, a type of database management that controls the creation, integrity, maintenance, and use of a database. It embeds a high-level query language, which greatly simplifies database organization as well as retrieval and presentation of database information. For the application to MESSENGER we have set up a data pipeline to update automatically the MASCS data, read them from the NASA Planetary Data System format, regrid the data to a common grid length, and store all information in the database. All data are then readily available to any authorized user in our network. We are working on a library to access the data directly from within our analysis software, and some preliminary functions have been implemented. It is thus straightforward to create and analyze rapidly the data, as for example the distribution of normalized radiance at a fixed wavelength. The new methodology provides facilities for controlling data access, enforcing data integrity, managing concurrency control, and recovering the database after a failure and restoring it from backup files, as well as maintaining database security. As an example, a simple query on the volume of data results in 2,476,048 spectra in 700 ms. All parameters can be searched in combination with customized ranges, and the search returns pointers to the relevant spectra.

Moreover, we use PostGIS to add support for geographic objects in a geographic information system (GIS) and to extend the database language with functions to create and manipulate geographic objects. A typical application is the definition of a large number of regions of interest (ROIs) and the search for all data points falling within each ROI. This facility may be used to extract spectral signatures specific to user-defined geological units in a few seconds and to explore the properties of the data from the different ROIs allowing quick analysis of the spectral characteristics of Mercury. We have successfully tested remote access to the database with a GIS visualization system, and we have created data visualization products that layer camera data and real-time-queried MASCS data.

Sample preparation and sample collection

The institute has a collection of several hundred planetary analog materials, including terrestrial minerals and rocks, synthetic materials and meteoritic samples. The spectral library measured at PEL contains entries for solar system analogue materials, separated in well-defined grain size ranges: <25 μm, 25-63 μm, 63-125 μm, and 125-250 μm. These size separates have been selected to truly represent most of the planetary surfaces. The importance of having the fine-grained separates has been stressed in several papers in the recent literature.

 Part of the PEL sample collection
zum Bild Part of the PEL sample collection

The minerals are fragmented with a laboratory jaw crusher, magnetic impurities are removed when needed, and then the samples are ground in a centrifugal ball mill by means of corundum jar and balls. Where the process is non-destructive, the resulting particles are wet-sieved, using distilled water, with a mechanical shaker, to obtain the four standard size fractions. In the other cases (e.g., for salts and clays), a dry-sieving procedure (using a tower of sieves for different dimensions) is applied. For most of the samples, X-ray fluorescence analyses are performed to infer the chemical composition of the mineral. In some cases, X-ray diffractometry is applied to identify impurities.
Each newly prepared sample gets an ID (8-digit identification number), unique for each sample, also one for each produced size fraction. In case the sample is thermally shocked or its structure is altered, the sample gets a new ID and in the sample description file its origin and modifications made to the parent body are described.

For emissivity measurements under purging air, the produced size fractions are placed into aluminium cups, having 50 mm internal diameter, 5 mm thick bottom and 3 mm depth and then heated in an oven at appropriate temperature for at least 1 hour, to reduce the amount of adsorbed water from the samples and to bring them to the measuring temperature. The powder is poured in the cups and their surfaces gently flatten to maximise the energy emitted in direction of the parabolic mirrors (the same for both external chambers) that conveys the radiation to the spectrometers.

For reflectance measurements, small cups with 10 mm internal diameter and 2 mm depth made from aluminium or black plastic are used. Aluminium cups allow heating the samples to moderate temperature (max. 180° C) in air if needed.

For emissivity measurements in vacuum at high temperatures (70° C to above 800° C), the produced size fractions are placed into stainless steel cups, with 5 mm thick bottom, having 50 mm internal diameter and 20 mm depth. Our studies show that high rims allow reducing thermal gradients in the sample, being the latter a great source of disturbances for emissivity spectra in vacuum. For each sample, 3 mm of material is gently flattened in the cup, leaving so 12 mm of rim to heat up and contrast the thermal gradients.
For reflectance measurements in vacuum the same cups as for purging are used. If the experiment foreseen to thermally process the samples and then measure them again in reflectance (at T ambient), stainless steel cups the same dimensions of the other cups are used.

For transmission measurements, pellets made of 99% pulverized KBr and 1% analogue material are produced.


Contact
Dr.rer.nat. Jörn Helbert
German Aerospace Center

Institute of Planetary Research
, Experimental Planetary Physics
Tel: +49 30 67055-319

Fax: +49 30 67055-384

E-Mail: Joern.Helbert@dlr.de
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