MERTIS seeing the Moon in the TIR: results from the first Bepicolombo flyby

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.