mertis

mercury radiometer and thermal infrared spectrometer

science goals

Among the terrestrial planets, Mercury plays a special role. It is the smallest, most dense and least explored terrestrial planet. Mercury has probably the oldest surface – which is heavily gardened and altered by space weathering. It also shows the most extreme surface temperature variations, ranging from 170°C in the permanently shadowed regions to +430°C on the day side. Understanding Mercury is crucial to develop a better understanding of the early processes in the inner Solar System, how our Earth formed and evolved, and how planets interact with the Sun.

The science goals of MERTIS are to:

• obtain the first global map of the surface mineralogy
• study surface composition and rock-forming minerals
• study surface temperature and thermal inertia
• investigate the permanently shadowed regions

MERTIS will achieve these goals by measuring the emitted light of Mercury at high spectral resolution during day and night in the thermal infrared (IR) domain. On Mercury, the spectral radiance at day side shows that the thermal emission starts to dominate the radiance already at wavelengths larger than 1.2 µm (at 725 K) depending on the surface albedo. The range between 0.8 and 2.8 µm is a transition region characterized by the overlapping of the reflected solar radiation and the thermal emission. However, in the thermal IR Mercury's thermal flux exceeds the flux reflected from its surface. This enables emittance spectroscopy in the thermal IR range where there is high potential for mineral identification because it is in this region where the major rock-forming minerals (e.g. feldspar) have their fundamental vibration bands. The flexibility of the instrumental setup and the high performance of the instrument will allow the study of the composition of the radar bright polar deposits in the permanently shadowed regions with a S/N ratio of >50 for an assumed surface temperature of 200 K.

measurement principle

MERTIS is a thermal infrared (IR) imaging instrument composed of two channels: a spectrometer (TIS) and a radiometer (TIR). It will use micro-bolometer technology, where no cooling is required allowing the instrument to operate in Mercury’s extreme thermal environment. The TIS channel covers the 7–14 µm spectral range, enabling detection of main features in this spectral region, such as Christiansen feature (emissivity maxima), Reststrahlen bands (emissivity minima), and transparency features. TIS has a spectral resolution of up to 90 nm, which can be adapted based on surface properties to optimize the signal-to-noise ratio (S/N). Its two-dimensional microbolometer array provides spectral separation and spatial resolution, with a field of view (FOV) of 4 degrees. The spectrometer’s grating combines diffractive and imaging tasks in one element. The TIR radiometer, covering 7–40 µm, complements TIS by enabling the measurement of thermo-physical surface properties such as thermal inertia and surface heat flux. MERTIS will globally map Mercury’s surface with a spatial resolution better than 500 meters and a signal-to-noise ratio of at least 100.

Passing a highly reflective planet baffle in front of the instrument the IR radiation is guided via the pointing mirror through an IR-window (filter) to the entrance optics. This part ends up with a slit at the intermediate focus which is the interface to the spectrometer optics. Diffracted by a curved grating the beam is focused onto the spectrometer sensor. Thermal links to radiators as well as specific couplings are foreseen to minimise the temperature fluctuations within the calibration cycle of the instrument. This cycle corresponds to a 270° revolution of the pointing unit within >20 sec with >65% duration at planet position. To get control over the instrument’s self-induced radiation (i.e. sensor head background) and for noise depression a shutter (placed after the slit) is foreseen for reference/background signal generation.

Both sensor channels, the spectrometer and the radiometer, are highly integrated sensors and use the same entrance optics and the same calibration sources according an in-plane separation arrangement. Both work in a push-broom mode synchronously with the same FOV.

The operation concept for both channels is characterized by intermediate scanning of the planet view and 3 different calibration targets by the pointing unit/mirror:

• the free space view represents a cold zero radiance reference allows determining the instrument energy and noise
• the 300K on-board black body (BB3) represents the instrument temperature and
• the 700K on-board black body (BB7) represents the hot Mercury’s dayside surface temperature.

This is called calibration/operation cycle.

The surface temperature of Mercury has the largest diurnal variations of all terrestrial planets. While the daytime temperature can reach up to 700K, the night-time temperature can drop below 100K. the TIR radiometer channel of MERTIS can measure temperatures over the whole diurnal cycle with a high accuracy (< 1K at 100 K). To determine the thermal inertia and related thermo-physical properties a large range of thermal surface condition has to be measured on both day and night side, during several phases of Mercury´s orbit around the Sun.

First Results
 

MERTIS became the first instrument to observe Mercury in the thermal infrared spectral range, during BepiColombo’s fifth flyby on 1 December 2024. MERTIS was reprogrammed to use its space port optics—since the planet-view port is blocked until orbit—allowing it to capture these early data. It captured new mid-infrared images of the Caloris Basin with a resolution of ~26–30 km. These first hyperspectral images reveal variations in surface temperature, mineral composition, and roughness, with temperatures reaching up to ~420 °C on sunlit terrain. A standout feature is the Bashō impact crater, previously imaged in visible to near-infrared light by Mariner 10 and MESSENGER. In thermal-infrared, Bashō appears as a thermal anomaly, offering fresh insight into its dark and bright materials. These early findings suggest MERTIS will greatly improve our understanding of Mercury’s mineralogy and thermo-physical properties. This success builds on decades of planning, including laboratory spectral measurements of Mercury's analogues and exotic minerals at DLR and the University of Münster to prepare for the interpretation.

BepiColombo reveals Mercury in a New Light

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