Returning to Mars with BEBOP (Broadband Exploration with Bolometric Optics)

In preparation for the human exploration of Mars, several orbital assets will need to be in place. ESA is exploring concepts for very-high-altitude platforms which will facilitate communications links to the Martian surface amongst other duties. Such platforms would have the capacity to monitor most of the Martian surface simultaneously, while also providing mission critical services including global navigation and Earth-Mars data relay. Part of a possible scientific payload will be instrumentation to monitor the Martian climate over the whole planet to enable future weather forecasting. These observations will eventually provide a better understanding of the formation and evolution of Martian dust storms, and therefore their prediction which is critical for the safety of human exploration. Here, we present the Broadband Exploration with Bolometric Optics (BEBOP) concept for these missions. This instrument is a thermal imaging system that combines a filter array with heritage from Mars Reconnaissance Orbiters Mars Climate Sounder (MCS; McCleese et al., 2007) and Lunar Reconnaissance Orbiters Lunar Diviner (Paige et al., 2010) with a fast, wide field of view, compact freeform optical system and uncooled microbolometer detector. The optical configuration, detector, and electronics have heritage from the Lunar Trailblazer Lunar Thermal Mapper (LTM; Shirley et al., 2020; Bowles et al., 2020) and Comet Interceptors Modular InfraRed Molecules and Ices Sensor (MIRMIS; Jones et al., 2024) instruments. Fig. 1 shows the completed LTM assembly, which was delivered in 2023 and is awaitinga November 2024 launch. The filter assembly will have 15-19 channels covering a spectral range of 6-25 m, including the 15 m CO2 feature. The instrument is compact, low mass and power, and does not require cryogenic cooling for the detectors. On-board time delay and integration (TDI) leads to high sensitivity and low noise. A scan mechanism and internal black-body target calibrates the entire optical chain between observations. Fig. 1 The fully assembled LTM instrument, now mated to Lunar Diviner and ready for a November 2024 launch. The spectral range of BEBOP and the necessarily wide field of view at high orbit will allow the measurement of atmospheric parameters across nearly the entire Martian disk. From MCS heritage, we will have spectral bands covering the 15 m CO2 band, allowing the retrieval of temperature and pressure of the lower Martian atmosphere (Kleinbhl et al., 2009; Smith et al., 2022; Vlasov et al., 2023). To either side of this band, dust and water ice aerosol opacity can be retrieved, providing column opacities over the Martian disk. Other spectral channels that will be included will be thermal bands, providing Martian surface temperatures with high precision, and a series of mineralogical bands over the 7-10 m region to determine crustal composition via Christiansen feature mapping. We will also be able to monitor surface ice and frost coverage, identify clouds and dust storms, trace the movement of clouds and dust, and extract wind fields. The ability to include a bandpass covering the emission and absorption of water vapour and other gases is under consideration. Expected spatial resolution is 1.5-2.6 km from a 5700 km altitude orbit. The field of view extends across the entire Martian disc, additionally facilitating limb sounding to retrieve vertical profiles of temperature, pressure, dust extinction, water ice extinction, and possibly water vapour with a vertical resolution of ~5 km. With three spacecraft, this will be done pole-to-pole at six longitudes at high cadence, having nearly global coverage each Martian day. This will lead to a better understanding of the dust and water cycles on Mars, providing insights into contemporary and past climate. A key question is how do dust storms form and how do they transform into global events? The instrument will also provide valuable information about the surface mineralogy, accessing longer wavelengths than contemporary instruments such as CRISM and OMEGA. This will allow us to address the crucial scientific question: what is the crustal history of Mars? The formation of the Martian crust was a complex process and the origins of its magmatic and volcanic content are unknown, and their study will lead to better understanding of the history and formation process of Mars and, therefore, Earth. The 6-25 m range includes emission peaks for silicate mineral Christiansen features and silicate minima within the Reststrahlen bands. These allow the differentiation between plagioclase, olivine, and pyroxene. The surface spectra will inform about mineralogy and help answer the outstanding question of whether phyllosilicates (Fe/Mg) are smectites or the intermediate material in the diagenetic sequence from smectite to chlorite, illite, and other higher-temperature clays.