Trace gas retrievals for the ExoMars Trace Gas Orbiter Atmospheric Chemistry Suite mid-infrared solar occultation spectrometer

Abstract : ExoMars is a two-part mission to Mars jointly led by ESA and Roscosmos. The first phase was launched in March 2016 and consisted of the Trace Gas Or-biter (TGO) and Schiaparelli lander. The TGO successfully entered orbit around Mars in October 2016 and has since begun a crucial aerobreaking campaign to circularize its orbit with a nominal 400 km altitude and 2 hr period. There are four scientific instruments on TGO: the Atmospheric Chemistry Suite (ACS), the Nadir and Occultation for Mars Discovery (NOMAD) spectrometer, the Colour and Stereo Surface Imaging System (CaSSIS), and the Fine-Resolution Epithermal Neutron Detector (FREND). This presentation will focus on trace gas retrievals for the mid-infrared (MIR) channel of the ACS instrument operating in solar oc-cultation mode. ACS is a set of three spectrometers that are designed to better characterize the atmosphere of Mars with unprecedented accuracy. It aims to detect and quantify unknown trace gases diagnostic of active geological or biological processes, to map their distribution and attempt to identify sources, and to refine our knowledge of the vertical distribution of major and minor atmospheric gases. It has three channels: near-infrared (NIR), thermal-infrared (TIRVIM) and MIR. The NIR channel is combination of an echelle grating and an acousto-optical tunable filter (AOTF), and is similar to the Ultraviolet and Infrared Atmospheric Spectrometers for Mars and Venus (SPICAM/V) on Mars Express and Venus Express (Korablev et al., 2006; Bertaux et al., 2007). It has a spectral range of 0.73–1.6 µm and operates in nadir mode. It is intended to provide mapping support to solar occulta-tion measurements. TIRVIM is a small Fourier transform spectrometer with a spectral range of 2–17 µm and resolution of 0.2 cm−1. It has heritage from the Mars Express Planetary Fourier Spectrometer (PFS), operates in both nadir and solar occultation mode, and will be able to measure the physical state of the atmosphere (vertical profiles of temperature, pressure and dust opacity). NOMAD is also a multi-channel spectrometer with complimentary objectives to ACS. It consists of a pair of combination echelle-AOTF spectrometers , much like SPICAM/V and ACS NIR, that operate in both nadir and solar occultation mode. In its original configuration, TGO carried a high-resolution Fourier transform spectrometer (FTS) covering a wide spectral range to detect trace gases (Wennberg et al., 2011), supported by the nadir-viewing NOMAD instrument capable of carrying out trace gas mapping studies. The ACS MIR channel aims to reproduce the capabilities of the FTS using a novel concept for atmospheric studies: a cross-dispersion spectrometer combining an echelle grating with a wide blaze angle and secondary, steerable diffraction grating (Korablev et al., 2017). It is capable of finer resolution than its echelle-AOTF counterparts, but is limited in its instantaneous spectral range compared to its FTS predecessor. The ACS MIR block is thermally isolated from TIRVIM and coupled to NIR, but shares a common electronics block. It consists of an entry telescope and collimator, a large echelle grating (107×240 mm, 3.03 grooves per mm), a steerable pair of secondary grating mirrors, and a Sofradir MCT array detector The low-density echelle grating at a high blaze angle (63.43 °) provides overlapping spectra at high orders. The secondary grating separates the orders and the resulting spectra are recorded by the detector with 640 pixels in the x direction corresponding to wavelength, and 512 pixels in the y direction corresponding to order. Several spectra are recorded for each order on sequential pixel rows. The secondary grating has two reflective gratings mounted side-by-side that can rotated.
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K. Olsen, Franck Montmessin, A. Fedorova, Alexander Trokhimovskiy, Oleg Korablev. Trace gas retrievals for the ExoMars Trace Gas Orbiter Atmospheric Chemistry Suite mid-infrared solar occultation spectrometer. European Planetary Science Congress 2017, Sep 2017, Riga, Latvia. pp.EPSC2017-938. ⟨insu-01591430⟩



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