Helbert, Jörn

Hiesinger, H.

Maturilli, Alessandro

Varatharajan, Indhu

Institute of Transport Research:Publications

EMISSIVITY SPECTRA OF ANALOGUE MATERIALS AT MERCURY T-P CONDITIONS. A. Maturilli1, J. Helbert1, I. Varatharajan1, H. Hiesinger2 1Institute for Planetary Research, German Aerospace Center DLR, Ruth-erfordstr. 2, 12489 Berlin, Germany – alessandro.maturilli@dlr.de, 2University of Münster, Germany.  Introduction:  The MERTIS (MErcury Radiome-ter and Thermal infrared Imaging Spectrometer) in-strument onboard the ESA/JAXA BepiColombo mis-sion (launch is scheduled for 2018) is designed to identify rock-forming minerals, to map the surface composition, and to study the surface temperature variations with an uncooled microbolometer detector in the hot environment of Mercury. MERTIS is an advanced IR instrument combining a push-broom IR grating spectrometer (TIS) with a radiometer (TIR) sharing the same optics, instrument electronics and in-fight calibration components for a wavelength range of 7-14 and 7-40 µm, respectively. At the Planetary Spectroscopy Laboratory (PSL) we measured emissivity spectra in vacuum (0.7 mbar) for a large suite of Mercury surface analogue materials in the MERTIS spectral range (7-14 µm) for sample temperatures from 100°C to above 400°C.  Set-up description:  Figure 1 shows the optical ta-ble at PSL, where two identical FTIR instruments (Bruker Vertex 80V) are operating in an air-conditioned room.  Figure 1. Laboratory set-up at the PSL. One spectrometer is equipped with aluminum mir-rors optimized for the UV, visible and near-IR, the second features gold-coated mirrors for the near to far IR spectral range. An external chamber is attached to the second spectrometer to measure the emissivity of solid samples. A shutter allows separating the spec-trometer from the external chamber, that can be evacu-ated to the same pressure as the spectrometer. If need-ed, an optical window (vacuum tight) can be mounted at the entrance of the emissivity chamber to operate while keeping the external chamber at ambient pres-sure under purged air or under inert gases. Sample targets are brought to measuring temperature using an induction heating system. For this reason our sample cups and heating surfaces are made of stainless steel. Our high efficiency induction system heats the samples to temperatures from 320K up to 900K. A sample caroussel driven by a very precise stepper motor (com-puter controlled) allows measuring several consecutive samples without breaking the vacuum. A large number of temperature sensors in the emissivity chamber allow monitoring the temperature of the sample as well as of several points at the equipment and chamber. A webcam is mounted in the emissivity chamber to monitor the heated sample and its vicinity. For more details, see [1, 2, 3, 4]. The same set-up, slightly modi-fied is also used in [11]. Mercury analogues collection:  Most recent re-sults from MESSENGER‘s X-Ray Spectrometer, Gamma-Ray Spectrometer, and Neutron Spectrometer [5] have been used to compile the list of Mercury ana-logues. Average surface composition falls very close to the komatiite–boninite boundary. The surface of Mer-cury is mainly composed of Mg-rich orthopyroxene and plagioclase; plagioclase and orthopyroxene domi-nate the surface materials, with lesser amounts of cli-nopyroxene, sulfides, olivine, and silica (at lower abundances than olivine). Our collection of analogues contains olivine, enstatite, labradorite, augite, ko-matiite (see [6]), tektite, anorthoclase, bytownite, L-chondrite, albite, hypersthene, diopside, quartz, nephe-line, graphite, a lunar simulant JSC-1A, and many sulfides. Emissivity spectra of sulfides in particular are described in [7].  Emissivity Measurements:  Mercury regolith is mostly composed of very fine particles, therefore we selected the samples to be measured from our collec-tion in the smaller fraction, being < 25 µm. Each sam-ple was poured in a stainless steel cup and placed in the external emissivity chamber, in a position ready for measurement. Temperature sensors were put in contact with the sample emitting layer and with the sample cup, to monitor its heating properties. The emissivity chamber was then evacuated and when stable vacuum was reached (0.7 mbar), the induction system was switched on for heating the samples. Each sample was carefully and slowly heated to reach 100°C, 200°C, 300°C, and 400°C, with different sample surface tem-perature simulating different insulation properties (time of day and/or latitude) on Mercury surface. Spectral Collection:  In the wavelength range (7-14 µm) which will be investigated by the MERTIS spectrometer, several very diagnostic spectral features 1427.pdfLunar and Planetary Science XLVIII (2017)can be identified. Their evolution with surface temper-ature (that can also be predicted and modeled) have been already described in many scientific publications [3, 8, 9, 10, 12]. In this paper we show the emissivity spectra measured for 3 endmembers of the Mercury analogue collection, at the 4 already described increas-ing surface temperatures: komatiite from Barberton (Fig.2), tektite (Fig. 3), and olivine (Fig. 4), all of them in the size fraction <  25 µm.  Figure 2. Emissivity spectra for a Barberton komatiite < 25 µm, measured at PSL.  Figure 3. Emissivity spectra for a tektite < 25 µm, measured at PSL.  Figure 4. Emissivity spectra for an olivine (Fo#89) < 25 µm, measured at PSL. These 3 entries show the strong diagnostic capabilities of the spectral signatures characteristic for the MERTIS spectral range: one can see variations in the position and shape on the CF (emissivity maxima) below 9 µm, in the shape of the Reststrahlen band (9-11 µm), and in that of the Transparency region (around 12 µm) for the 3 different materials, and the tempera-ture dependence of some of them (c.f. the Reststrahlen bands for olivine [9, 10]). Conclusions and Outlook:  MERTIS on Bepicolombo will be the first spectrometer to observe the hot surface of Mercury in the thermal infrared wavelength range (7-14 µm), where very diagnostic features will help the detection of minerals composing the Hermean surface. These features are temperature dependent to a varying degree, therefore their spectral behavior is influenced by local insolation conditions. To prepare an adequate spectral database for MERTIS, at the Planetary Spec-troscopy Laboratory (PSL) we measured emissivity spectra in vacuum for a large suite of Mercury surface analogue materials for sample temperatures from 100°C to above 400°C. The spectral library for Mercu-ry analogues is completed by reflectance measure-ments on the same samples, fresh and after been heated in vacuum, covering the whole spectral range from 0.2 to 200 µm. See more information on the PSL website: http://www.dlr.de/pf/desktopdefault.aspx/tabid-10866/19013_read-44267/ References:  [1] Maturilli, A. and Helbert, J.: PSS, Vol. 54, pp. 1057-1064, 2006. [2] Maturilli, A., Hel-bert, J., and Moroz L., PSS, Vol. 56, pp. 420-425, 2008, spectral library now available at http://figshare.com/articles/BED_Emissivity_Spectral_Library/1536469. [3] Helbert, J. and Maturilli, A., EPSL, Vol. 285, pp. 347-354, 2009. [4] Maturilli A, Helbert J., Journal of Applied Remote Sensing, Vol 8, 2014. [5] Vander Kaaden, K. et al. (2016) doi: 10.1016/j.icarus.2016.11.041 [6] Maturilli A., Helbert J., St. John J.M., Head III J.W., Vaughan W.M., D’Amore M., Gottschalk M., Ferrari S., EPSL, Vol. 398, pp. 58-65, 2014. [7] Varatharajan et al. (2017) this meeting. [8] Logan, L. M., and Hunt, G. R. Hunt (1970), J. Geophys. Res., 75, 6539-6548. [9] Hender-son, B.G., Jakosky, B.M. (1997) doi:10.1029/96JE03781. [10] Helbert, J. et al. (2013) doi:10.1016/j.epsl.2013.03.038. [11] Helbert et al. (2017) this meeting. [12] Ferrari, S., F. Nestola, M. Massironi, A. Maturilli, J. Helbert, M. Alvaro, M. C. Domeneghetti and F. Zorzi (2014) American Mineral-ogist 99(4): 786-792.  1427.pdfLunar and Planetary Science XLVIII (2017)

Emissivity Spectra of Analogue Materials at Mercury P-T Conditions

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