The Mercury Radiometer and Thermal Infrared imaging Spectrometer (mertis) onboard Bepi Colombo: 2020 Moon flybys data results.

Introduction: The MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS) is an instrument to study the mineralogy and temperature distribution of Mercury’s surface in unprecedented detail [1]. During the nominal mission, MERTIS will map the whole surface at 500 m scale, combining a push-broom IR grating spectrometer (TIS) with a radiometer (TIR) sharing the same optics, instrument electronics, and in-flight calibration components for the whole wavelength range of 7-14µm (TIS) and 7-40µm (TIR) [1].  MERTIS successfully completed its planned tests of the Near-Earth Commissioning Phase (NECP) in November 2018 and several checkouts, collecting thousands of measurements of its internal calibration bodies and deep space. Those data show a performance comparable with ground-based measurements. Scientific data will arrive well before the 2025 arrival at Mercury. MERTIS observed the Moon on in April 2020 producing ~12k TIS observation with ~3k measurements of the Moon at various angles. Even if the Moon is 7000 farther away and half maximum temperature than Mercury during nominal operation, the signal is clearly visible and shows the excellent MERTIS TIS sensor. MERTIS archival data are stored in Planetary Data System v4 format (PDS4) [2] format, that actually describe 2 physical formats for each MERTIS channel. Each channel will be stored in Flexible Image Transport System (FITS) [3] and in pure ASCII.

The Mission and the Instrument: BepiColombo [2] is a dual spacecraft mission to Mercury that has been launched in October 2018 and is jointly carried out by the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA). The spacecraft comprises two separate orbiters: the Mercury Planetary Orbiter (MPO), focused on observations of the surface and internal composition, and the Mercury Magnetospheric Orbiter (MMO), which will study the particle science in the extreme thermal environment. In addition to a suite of instruments complementary to the NASA MESSENGER mission, BepiColombo will be able to observe both the northern and southern hemispheres at high spatial resolution.  BepiColombo uses an innovative solar electric propulsion system and its trajectory toward Mercury is a combination of low-thrust arcs and flybys at Earth, Venus, and Mercury. This will allow us to reach Mercury with low relative velocity. The spacecraft was successfully launched on the 20^th of October 2018, 01:45 UTC, from the ESA Guiana Space Centre using an Ariane 5 rocket and will reach its mappings orbit at Mercury in 2026. The MERTIS instrument was proposed in 2003 as payload of the Mercury Planetary Orbiter spacecraft of the ESA-JAXA BepiColombo mission and the final Flight Model (FM) was delivered in 2013. MERTIS is an innovative and compact spectrometer, that combines a push-broom IR grating spectrometer (TIS) with a radiometer (TIR) with only 3kg of mass and an average 10 W power consumption [1,4]. TIS operates between 7 and 14 μm and will record the day-side emissivity spectra from Mercury, whereas TIR is going to measure the surface temperature at the day- and night side in the spectral range from 7-40 μm corresponding to temperatures from 80- 700 K. TIR is implemented by an in-plane separation arrangement, while TIS is an imaging spectrometer with an uncooled micro-bolometer array. The optical design of MERTIS combines a three-mirror anastigmatic lens (TMA) with a modified Offner grating spectrometer. A pointing device allows viewing the planet (through the planet-baffle), deep space (through the space-baffle), and two internal black bodies at 300 K and 700 K temperature, respectively. MERTIS was developed at DLR in collaboration with the Westfälische Wilhel-Universität Münster and industry partners. The MPO operational plan foreseen a 2.3 hour low eccentricity orbit that allows MERTIS to achieve its 500 meters global mapping scientific goal. As confirmed by the NASA MESSENGER mission, due to the iron-poor nature of the surface the thermal infrared is the most useful wavelength range to study Mercury’s surface composition. Silicates as well as sulfides have characteristic spectral features in this range that MERTIS can map with high signal-to-noise ratio. The MERTIS spectrometer aims to capture data on the mineralogy whereas the radiometer surveys the thermal inertia of the planet.

Flyby Results: MERTIS nominal nadir viewing port (Planet View) is obstructed by the MTM (Mercury Transfer Module) during the whole cruise to Mercury, while the Space View port is unobstructed and normally used for calibration purpose. We trick MERTIS nominal observation cycle (Internal reference Blackbodies, Space, multiple Space) to acquire only from the Space port. We collect ~12k TIS observation, with more than 3000 spectra from the Moon. The acquisition campaign was split in 8 “observation”, that describe different modes of the instrument. During the Flyby the only TIS sensor binning was change to test different acquisition modes. A typical observation crossing the Moon is in Fig.1. The spectra radiance values are sharply decaying towards the Moon edges. The Moon is a much colder target than Mercury (around 400K max vs. 700K) and is it farther than the planned target distance during the nominal mission at Mercury (700.000 km vs ~1000 km ). The signal from the Moon is nevertheless clearly discernible from the background. The PI Team at DLR and Münster University is currently testing the calibration procedures and geometrical registration and on the verge to deliver the first calibrated data acquired in space to the CoI team. The final data format is still to be defined, but the data container will be the same as in the RAW level data [6] available on ESA PSA website[7].

References: [1] Hiesinger, H. and Helbert, J., Planet. Space Sci. 58, (2010). [2] NASA Planetary Data System, pds.nasa.gov [3] FITS Support Office, fits.gsfc.nasa.gov [4] Instrument User Manual (FM), MER-DLR-MA-001 (2017). [5] Python PDS4 Tools, SBN sbndev.astro.umd.edu/wiki  [6] D’Amore et al, LPSC 2020, 2020LPI....51.1438D [7] European Space Agency's Planetary Science Archive (PSA) archives.esac.esa.int/psa

Fig.1 left: Calibrated spectra radiance in W/(m² µm sr). right: Same spectra as in the first panel, as produced from TIS sensor (y is spatial direction , x is reversed spectral direction).