Advanced mid-IR remote sensing for planetary exploration

The Mercury Radiometer and Thermal-infrared Imaging Spectrometer (MERTIS) is part of the European Space Agency (ESA)'s Mercury Planetary Orbiter mission BepiColombo to the innermost planet of the solar system. Flybys by the ongoing Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) mission have shown that the features on Mercury's surface are more complex and variable than expected. Volcanoes were involved in the formation of plains, and a magnetic field appears to be actively produced in the planet's core.^1,2 Reflectance, color variation, and high-resolution imaging at different wavelengths indicate variations in surface chemistry and mineralogy.

Detailed studies of the surface mineralogy, which would support a better understanding of planetary genesis, require global, high-resolution, mid-IR spectral and temperature data.³ MERTIS is an advanced mid-IR imaging spectrometer designed to provide these essential observations. Its goals are to search for rock-forming minerals, map the surface composition, and study surface-temperature variation in the hot environment of Mercury. Figure 1 shows a structural and thermal model of MERTIS.

Figure 1. Structural and thermal model of the Mercury Radiometer and Thermal-infrared Imaging Spectrometer (MERTIS).

MERTIS combines a pushbroom IR grating spectrometer (TIS) with a microradiometer (TIR). These instruments share the same optics, instrument electronics, and in-flight calibration components for the entire wavelength range of 7–14μm (TIS) and 7–40μm (TIR).^4–7 The mission uses a modular concept for the sensor head, electronic units, and power/calibration systems within a mass budget of 3.4kg and a nominal power consumption of less than 13W (less than 19W under cold environmental conditions). The MERTIS optical design includes a telescope with a focal length of 50mm and an F-number of 2. Its field of view will be 4°(diameter). The TIS will map the entire planet with a spatial resolution of 500m in the nominal mode and 5–10% of the surface with a spatial resolution of better than 500m.

The TIS is an imaging spectrometer with an uncooled microbolometer array. It combines a three-mirror anastigmate (TMA) with a modified Offner grating spectrometer. The TMA consists of three off-axis mirrors with an additional one serving as aperture stop. The Offner spectrometer uses two concentric spherical elements, a small convex one for the grating opposed by a large, concave mirror. The grating is placed midway between the slit and the concave mirror. The TIR detector elements are situated on both sides of the entrance slit, using the same TMA optics (see Figure 2).

Figure 2. Functional configuration of MERTIS.

To avoid thermal influence on the detector core by the main electronics, the sensor head, proximity electronics, and main electronic units, as well as the interfaces and power units, will be thermally separate. The instruments' functional and in-flight calibration concept is based on sequential observation of four targets, including a planet, deep space, and two onboard blackbody sources. A single rotary pointing device with a 45° tilted mirror between baffles and optics enables frequent views of the in-flight calibration targets (at 300 and 700K), as well as cold space and the planet, viewed through two separate baffles oriented perpendicular to each other. A cylinder shields the mirror and closes the other ports of the instrument against incoming radiation. The pointing device is shielded by an aluminum cover that is thermally decoupled from the optical structure.

The instruments' electronics will ensure data acquisition and processing, independent control of MERTIS's subsystems (pointing device, blackbodies, and shutters), provision of internal voltages, telecommand and control management of the instrument, housekeeping, detector control, and temperature stabilization of the bolometer. The electronics architecture combines the instrument subsystems with cold redundancies for the instrument controller and power electronics. The detectors, their front-to-end electronics, and the blackbody drivers are unique. Cross-strapping electronics were added to establish an internal connection between all electronics subsystems. This approach is based on a strategy for implementing a highly integrated digital logic using a single field-programmable gate array.

The model shown in Figure 1 was recently delivered to ESA. The engineering model is under development and will be shipped this year. The qualification model is in the development phase, which will result in a refurbishment that will provide the flight spare. Finally, the delivery of the flight model to ESA is planned for December 2011.

The team of collaborators contributing to the MERTIS development includes the University of Münster (Münster, Germany), the German Aerospace Center (Berlin), Kayser-Threde GmbH (Munich, Germany), the Fraunhofer Institute for Applied Optics and Precision Engineering (Jena, Germany), IB Ulmer (Germany), Astro- und Feinwerktechnik Adlerhof (Germany), and the Polish Academy of Science.