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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An interferometer is an essential subsystem of the Fourier-transform spectrometer (FTS). We describe an FTS instrument to operate at the surface of Mars based on a Michelson interferometer with hollow retroreflectors. The instrument will operate in two different regimes, observing the solar disc through the atmosphere to measure trace gases, and measuring the thermal emission from the atmosphere to study the planetary boundary layer (PBL). The interferometer has an aperture of 1 inch, operates in the spectral range 1.7-17 μm, and features low mass and volume (≤1 kg with all necessary subsystems). Beam splitter and compensator are made of potassium bromide (KBr). A single-axis robot with stepper motor drive provides a linear movement of the retroreflector (the speed stability is about 2%) and enables a maximal optical path difference (MOPD) of 15 cm. A reference channel with a distributed-feedback laser diode (0.76 μm) and a photodiode (Si) supports the interferogram sampling and the speed stabilization loop. The time to measure one interferogram with a best spectral resolution of about 0.05 cm-1 is 500 s (the sun tracking regime). In the thermal sounding regime, one measurement of a two-side interferogram (with the spectral resolution of ~1 cm-1) takes less than 1 min. Laboratory calibrations with a black body and a laser confirm the design parameters of the instrument.

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.