The Atmospheric Chemistry Suite (ACS) package is an element of the Russian contribution to the ESA-Roscosmos ExoMars 2016 Trace Gas Orbiter (TGO) mission. ACS consists of three separate infrared spectrometers, sharing common mechanical, electrical, and thermal interfaces. This ensemble of spectrometers has been designed and developed in response to the Trace Gas Orbiter mission objectives that specifically address the requirement of high sensitivity instruments to enable the unambiguous detection of trace gases of potential geophysical or biological interest. For this reason, ACS embarks a set of instruments achieving simultaneously very high accuracy (ppt level), very high resolving power (>10,000) and large spectral coverage (0.7 to 17 µm-the visible to thermal infrared range). The near-infrared (NIR) channel is a versatile spectrometer covering the 0.7-1.6 µm spectral range with a resolving power of ∼20,000. NIR employs the combination of an echelle grating with an AOTF (Acousto-Optical Tunable Filter) as diffraction order selector. This channel will be mainly operated in solar occultation and nadir, and can also perform limb observations. The scientific goals of NIR are the measurements of water vapor, aerosols, and dayside or night side airglows. The mid-infrared (MIR) channel is a cross-dispersion echelle instrument dedicated to solar occultation measurements in the 2.2-4.4 µm range. MIR achieves a resolving power of >50,000. It has been designed to accomplish the most sensitive measurements ever of the trace gases present in the Martian atmosphere. The thermal-infrared channel (TIRVIM) is a 2-inch double pendulum Fourier-transform spectrometer encompassing the spectral range of 1.7-17 µm with apodized resolution varying from 0.2 to 1.3 cm −1 . TIRVIM is primarily dedicated to profiling temperature from the surface up to ∼60 km and to monitor aerosol abundance in nadir. TIRVIM also has a limb and solar occultation capability. The technical concept of the instrument, its accommodation on the spacecraft, the optical designs as well as some of the calibrations, and the expected performances for its three channels are described.

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Infrared Fourier-transform spectrometers (FTIR) onboard of the planetary missions are commonly used for the thermal sounding of the atmosphere and retrieval of aerosol profiles. To derive a calibrated spectrum of the target source, one needs three separate measurements: the target source itself and two calibration measurements of sources with known emissivity and temperature. An overview of the design of a compact in-built calibration source (a blackbody) emitting at 210-330 K for a spaceborne FTIR instrument is presented. Mechanically it is an aluminum structure matching the aperture of the instrument. The emissivity depends on its surface relief and finish. Four different types of surface shape are considered. The best-achieved emissivity is better than 0.99 (at 15 μm). The optimal placement of heaters allowing for minimal thermal non-uniformity (0.1 K) across the aperture is found. The accuracy of the thermal control is also ~0.1 K. We discuss the thermal control system and its characteristics (accuracy and drift). The proposed design accounts for a minimum mass applicable to the space instrumentation. For a one-inch aperture, the mass is 0.12 kg. The expected accuracy of the instrument calibrated with the designed blackbody is estimated.

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