MERTIS/BepiColombo: the challenges of observing the Moon with an instrument designed for Mercury

The Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) is part of the ESA-JAXA BepiColombo science payload en-route to Mercury. On April 10th 2020, BepiColombo flyby the Earth and obtained data from the Moon surface at a distance of around 700,000 km. The MERTIS thermal-infrared spectrometer (TIS) operating between 7 μm and 14 μm recorded more than 9,600 single hyperspectral observations of the Moon through its space baffle built for deep-space calibration. MERTIS has been designed to observe the surface of Mercury at a spatial resolution more than 1000 times better and at temperatures up to 2 times greater than that of the Moon. Therefore, lunar observations present a significant challenge for the instrument and the team. The standard acquisition procedure and the operations software were adapted to obtain data during the BepiColombo cruise phase. A specific calibration procedure and spatial binning have been developed to obtain the best radiometric data from the lunar surface. The calibrated data demonstrate the exceptional performance of the instrument, developed for a very different planetary object, comparable with ground-based measurements. The observations made during the cruise phase are very promising for future observations of Mercury with the space baffle during the fifth Mercury flyby scheduled for December 2024 and the nominal nadir viewing port (planet baffle) in orbit around Mercury.