Oral presentations and abstracts

The Apollo 15 & 16 missions were the first to explore the Lunar surface chemistry by investigating about 10 percent of the Lunar surface using a remote sensing X-ray fluorescence spectrometer experiment. The data obtained has been extensively used to study Lunar formation history and geological evolution. In this work a re-evaluation of the Apollo 15 & 16 X-ray fluorescence experiment is conducted with the aim to obtain up-to-date empirical values for aluminum (Al) and magnesium (Mg) concentrations relative to silicon (Si) of the upper Lunar surface. An up-to-date orbit reconstruction, updated instrument response, and improved intensity ratio calculations are used to obtain new intensity ratio maps.

How to cite: Kuulkers, E. and Gloudemans, A.: Re-evaluation of Lunar X-ray observations by Apollo 15 & 16, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-210, https://doi.org/10.5194/epsc2020-210, 2020.

The Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) is part of the ESA BepiColombo Mercury Planetary Orbiter (MPO) payload and consists of a push-broom IR-spectrometer (-TIS) and a radiometer (-TIR) [1].

During the long cruise to Mercury, and before its arrival on December 5th 2025, BepiColombo will perform 9 flybys: among them, the Earth/Moon flyby on April 10^th 2020. Due to the flight configuration, not all the instruments onboard BepiColombo are able to operate during cruise and flybys. Among the instruments that can operate is MERTIS, providing the first hyperspectral observation of the Moon in the thermal infrared (TIR) wavelength range from space.

At the Planetary Spectroscopy Laboratory (PSL) of DLR in Berlin, a spectral library for lunar analog rocks in the TIR spectral range, measured under simulated Moon surface conditions, has been built to help the interpretation of MERTIS’ Moon spectra.

Shortly after launch, MERTIS underwent a Near-Earth Commissioning Phase on Nov. 13-14 2018 during which the instrument was turned on for the first time in space. MERTIS was found to be fully operational [4], and the radiometer showed an excellent correspondence of the 2013 preflight sensitivity measurements and the 2018 in-flight measurements.

Although most instruments on the BepiColombo MPO are blocked by the Mercury Transfer Module (MTM) during cruise and flyby operations, MERTIS is able to acquire data through its space baffle. We adapted the MERTIS operations software to allow for this unique opportunity. Especially the Earth/Moon fly-by is of interest, as the surface composition of the Moon and Mercury have been frequently compared in the literature [5-10]. Observing the Apollo and Luna landing sites with MERTIS, in combination with laboratory studies, provides extremely valuable ground truth for our MERTIS measurements.

The time allocated for MERTIS pointing to the Moon was 4 hours and started 1 day before closest approach. During this slot the Moon was in the FoV of MERTIS. The 4 hours visibility slot was divided in 4 segments of 1 hour approximately connected by short slews. The attitude in each segment was quasi inertial (no tracking, keeping the Sun within illumination constraints) with the Moon slowly drifting in the FoV such that it is aligned with the boresight right in the middle of the segment. Within the 4 hours allocated for observations the Moon was nearly fully illuminated.

In the last decades orbital spectroscopic observations of the lunar surface have greatly advanced our understanding of the global distribution of different rock types and their chemical compositions. This vast dataset is complemented by the first in situ reflectance spectra from the lunar surface obtained by the recent Chang’E 3 and current Chang’E 4 missions, which provide more detailed information about the mineralogy of local surface materials and the geological context of the landing sites [11].

A reliable quantification of mineral modal abundances from measured reflectance spectra requires the availability of laboratory spectra of comparable samples. Current spectral databases primarily contain spectra measured on powder samples, lacking spectra of coarse grained rock samples. Reflectance spectra are sensitive to grain size and surface roughness [12], the available powder spectra might not be sufficient for a quantitative interpretation of measured rock spectra.

Rock samples obtained during the Apollo missions indicate that lunar anorthosites are typically coarse grained and can reach grain sizes of more than 1 cm. Hence, the global abundance of anorthosite as the dominant rock type of the lunar surface suggests that such coarse grained rocks are ubiquitous.

Therefore the extension of the current spectral databases by new spectral data of whole rock samples is crucial for the interpretation of current remote and in-situ measurements.

The Planetary Spectroscopy Laboratory (PSL) of DLR in Berlin is a spectroscopy facility providing spectral measurements of planetary analogues from the visible to the far-infrared range for comparison with remote sensing spacecraft/telescopic measurements of extraterrestrial surfaces [13-17]. External simulation chambers are attached to the FTIR spectrometer to measure the emissivity of solid samples.

The samples selected for this work includes: - slabs and stone chunks of plagioclases bearing rocks such as anorthosite, diorite, monzodiorite, gabbro and diabas; several basalts, rhyolite, olivine, granite, andesite, labradorite, obsidian.

Samples are heated in vacuum slowly and gradually up to 400° C. Measurements were taken at 100° C, 200° C, 300° C and 400° C in the MIR and FIR spectral ranges.

Thermally processed samples are measured in hemispherical and bi-directional reflectance in the full spectral range from UV to FIR.

A sample of graphite measured in emissivity at increasing T, adopting the same configuration and procedure used for the samples was used as blackbody for emissivity calibration.

MERTIS on ESA BepiColombo will be the first instrument to obtain hyperspectral measurements of the Moon in the TIR spectral range from space. Here we present the first results combined with a spectral library of emissivity for lunar analog rocks measured under simulated Moon conditions.

How to cite: Maturilli, A., Helbert, J., Hiesinger, H., Alemanno, G., Schwinger, S., and D'Amore, M.: MERTIS seeing the Moon in the TIR: results from the first Bepicolombo flyby, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-271, https://doi.org/10.5194/epsc2020-271, 2020.

We present a thermophysical model (TPM) of the Moon which matches the observed, global, disk-integrated thermal flux densities of the Moon in the mid-infrared wavelength range for a phase angle range from -90° to +90°.
The model was tested and verified against serendipitous multi-channel HIRS measurements of the Moon obtained by different meteorological satellites (NOAA-11, NOAA-14, NOAA-15, NOAA-17, NOAA-18, NOAA-19, MetOp-A, MetOp-B). The sporadic intrusions of the Moon in the deep space view of these instruments have been extracted in cases where the entire Moon was within the instruments' field of view. The HIRS long-wavelengths channels 1-12 cover the range from 6.5 to 15 μm, the short-wavelengths channels 13-19 are in the 3.7 to 4.6 μm range.

The model is based on an asteroid TPM concept (Lagerros 1996, 1997, 1998; Müller & Lagerros 1998, 2002), using the known global properties of the Moon (like size, shape, spin properties, geometric albedo, thermal inertia, surface roughness, see Keihm 1984; Racca 1995; Rozitis & Green 2011; Hayne et al. 2017), combined with a model for the spectral hemispherical emissivity which varies between 0.6 and 1.0 in the HIRS wavelength range (Shaw 1998; ECOSTRESS data base: https://ecostress.jpl.nasa.gov/). The spectral emissivity as well as characteristics of the surface roughness are crucial to explain the well-calibrated measurements.

Our Moon model fits the flux densities for the currently available 22 epochs (each time up to 19 channels) with an absolute accuracy of 5-10%. The phase curves at the different wavelengths are well explained. The spectral energy distributions are very sensitive to emissivity and roughness properties. Here, we see minor variations in the model fits, depending on the origin (phase and aspect angle related) of the thermal emission. We also investigated the influence of reflected sunlight at short wavelengths.

Our TPM of the Moon has a wide range of applications: (i) for Earth-observing weather satellites in the context of field of view and photometric calibration (e.g., Burgdorf et al. 2020); (ii) for interplanetary space missions (e.g., Hayabusa2, OSIRIS-REx or BepiColombo) with infrared instruments on board for an in-space characterization of instrument properites (e.g., Okada et al. 2018); (iii) to shed light on the thermal mid-infrared properties of the lunar surface on a global scale; and, (iv) to benchmark thermophysical model techniques for asteroids in the regime below 10 μm (e.g., observed by WISE in the W1 and W2 bands at 3.4 and 4.6 μm, by Spitzer-IRAC at 3.55 and 4.49 μm or from ground in M band at around 5 μm).

References:
Burgdorf M., et al. 2020, Remote Sens. 12, 1488; Hayne, P. et al. 2017, JGRE 122, 237; Keihm, S.J. 1984, Icarus 60, 568; Lagerros 1996,  A&A 310, 1011; Lagerros 1997, A&A 325, 1226; Lagerros 1998, A&A 332, 1123; Müller & Lagerros 1998, A&A 338, 340; Müller & Lagerros 2002, A&A 381, 324; Okada T. et al. 2018, P&SS 158, 46; Racca G. 1995, P&SS 43, 835; Rozitis & Green 2011, MNRAS 415, 2042.

How to cite: Müller, T. G., Burgdorf, M. J., Buehler, S. A., and Prange, M.: Thermophysical model of the Moon from 3.7 to 15 μm, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-586, https://doi.org/10.5194/epsc2020-586, 2020.

1. Introduction

One of the Moon’s most surprising characteristics is its crustal asymmetry; the farside hemisphere, dominated by ancient highlands and covered with a few mare deposits, has a different volcanic history than the (sampled) nearside [1]. Surface dating of the lunar mare basalts revealed that the volcanism on the Moon lasted between ~3.9-4.0 and ~1.2 Ga, with a peak in the volcanic activity ~3.6-3.8 Ga ago [2]. A recent study further demonstrated that the mineralogy of the nearside mare basalts reveals a late stage volcanism with high titanium and olivine contents [3]. More recent spectroscopic analyses focused on the farside’s crustal rocks composition (e.g. [4] for South Pole-Aitken [SPA] basin), but no comprehensive study of the farside mare compositions has been conducted up to now.

2. Data and methods

In this work, we used spectroscopic data of the Moon Mineralogy Mapper (M3) onboard Chandrayaan-1 to survey the mineralogy of all mare deposits in the VNIR domain (0.4-3 μm). Each mare unit was mosaicked separately and processed using the method of [5]: an IDL routine automatically computes and removes the continuum of each spectrum, and determines its spectral parameters (e.g. band centers, band areas, band depths, spectral slope). To compare with the mineralogy of the nearside [3], we also mosaicked three selected mare regions, each representative of one spectral unit. Finally, we applied statistics on spectral parameters of interest to survey the variation in mineral signatures and compositions of farside maria.

3. Preliminary results

We found that spectra of mare basalts are dominated by pyroxene signatures (Figure 1).

However, some mare units (e.g. Apollo, see Figure 2) showed considerable differences in band characteristics in which cases we computed statistics on each mare subunits.

Major differences were also observed in SPA mare units when they were compared to Mare Australe deposits and other maria outside of the SPA basin (Figure 3).

This suggests different pyroxene compositions within SPA. Moreover, the nearside mare units we defined based on [3] are clearly distinct from all farside mare deposits since they are the richest in clinopyroxenes (Figure 3).

4. Conclusion and perspectives

The results we have so far indicate slightly different mineralogies of mare basalts between the nearside and the farside of the Moon and, if confirmed by looking at the other spectral parameters, could mirror its crustal dichotomy. We plan to investigate possible evidence of olivine and possible correlations with age and location, as well as intra-SPA diversity with our spectral parameters set.

Acknowledgements

The authors want to thank Jan Hendrik Pasckert and Harald Hiesinger for having kindly shared with us their shapefiles of SPA mare deposits [8]. Fundings from the CNRS Momentum, LUE Future Leader and CNES APR are much appreciated.

References

[1] Head III, J. W., and Wilson, L.: Lunar mare volcanism: Stratigraphy, eruption conditions, and the evolution of secondary crusts, Geochimica et Cosmochimica Acta, Vol. 56, 1992.

[2] Hiesinger, H., et al.: Ages of mare basalts on the lunar nearside, Journal of Geophysical Research, Vol. 105, 2000.

[3] Staid, M. I., et al.: The mineralogy of late stage lunar volcanism as observed by the Moon Mineralogy Mapper on Chandrayaan-1, Journal of Geophysical Research, Vol. 116, 2011.

[4] Moriarty, D. P., and Pieters, C. M.: The Character of South Pole-Aitken Basin: Patterns of Surface and Subsurface Composition, Journal of Geophysical Research, Vol. 123, 2018.

[5] Martinot, M., et al.: Mineralogical Diversity and Geology of Humboldt Crater Derived Using Moon Mineralogy Mapper Data, Journal of Geophysical Research, Vol. 123, 2018.

[6] Klima, R. L., et al.: Spectroscopy of synthetic Mg-Fe pyroxenes I: Spin-allowed and spin-forbidden crystal field bands in the visible and near-infrared, Meteoritics & Planetary Science, Vol. 42, 2007.

[7] Adams, J. B.: Visible and near-infrared diffuse reflectance spectra of pyroxenes as applied to remote sensing of solid objects in the solar system, Journal of Geophysical Research, Vol. 79, 1974.

[8] Pasckert, J. H., Hiesinger, H., Van der Bogert, C. H.: Lunar farside volcanism in and around the South Pole-Aitken basin, Icarus, Vol. 299, 2018.

How to cite: Bott, N., Flahaut, J., Martinot, M., and Ito, G.: Unveiling the mineralogical composition of lunar farside mare basalts, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-561, https://doi.org/10.5194/epsc2020-561, 2020.

Introduction:  The ability to discriminate between different mineral phases in lunar materials is critical in reconstructing the origin and evolution of the moon and understanding its relationship to Earth.  Resource identification is also critical to develop a viable long-term lunar exploration program enabling a continued human presence. Mineral-bound metals are important resource targets in regolith and mare basalts [1]. Minerals in lunar basalts and regolith such as ilmenite and pyroxene are known hosts of metals like Cr, Ni, Co, and Mn, and ilmenite in particular has been considered for Fe and O₂ extraction [2][3].  

     Raman spectroscopy non-destructively provides structural information to identify trace compounds and minerals in a matter of seconds. Raman spectroscopy has been used for decades to measure the composition of returned lunar samples and analog materials ([4][5][6] and references therein). Raman measurements can be undertaken with multiple excitation wavelengths, (UV ~244nm, VIS ~532nm, NIR ~785nm, and IR ~1064nm), with advantages to using one excitation wavelength over another. The 532 nm wavelength provides high-energy excitation with good sensitivity and low noise for most materials covering a wide spectral range (~0 – 4000 cm^-1). This wide range is essential for the identification of volatiles, in which spectral shift for functional groups like OH occur within ~3000 - 3700 cm^-1, and minerals with peak shifts ~100-200 cm^-1, such as sulfides, feldspars, and quartz polymorphs. NIR Raman has been shown to exhibit better signal-to-noise, particularly for opaque minerals, like sulfides and oxides, and is the typical wavelength utilized in portable Raman instruments for terrestrial resource prospecting ([7][8]).  The two wavelengths used together ensure near-comprehensive identification and accurate characterization of lunar materials suitable for resource extraction.

Materials and Methods:  This study looks at a suite of lunar analog mineral samples (olivine, plagioclase, pyroxene, ilmenite, apatite) and relevant terrestrial analog rocks using portable and benchtop VIS (532nm) and NIR (780nm) Raman spectroscopy and Fourier transform infrared spectroscopy for comprehensive characterization of composition. Samples were probed either in tact or as powders at room temperature. A subset of powdered samples was heated  (T= 298K, 325K, and 373K) and characterized using NIR Raman to study temperature dependence of Raman spectra.

Initial Results: The mineral samples included in the study were all identifiable using both VIS and NIR Raman spectroscopy, and in some cases the portable instruments performed better than the high-resolution benchtop systems. Basalt and ilmenite samples were better characterized under NIR excitation, while the silicate minerals had a better response under VIS excitation (Fig. 1). FTIR measurements of the silicate minerals were informative, but the opaque nature of ilmenite results in low reflectance under higher wavelengths. For both Raman and FTIR measurements, better results were achieved with the pure minerals in comparison to the rocks composed of multiple minerals. The NIR Raman temperature experiments showed an enhancement of spectral peak intensity for minerals like plagioclase as the temperature increased up to 373K.