Observations and ongoing calibration of the infrared channels of NOMAD on ExoMars Trace Gas Orbiter

<p>NOMAD is a three-channel spectrometer on the ExoMars 2016 Trace Gas Orbiter, which has been operating around Mars since April 2018 [1]. NOMAD contains two infrared channels: SO, which operates in solar occultation mode; and LNO, which operates primarily in nadir mode, but can also measure limbs, solar occultations, Phobos and Deimos.</p> <p>&#160;</p> <p>In the past four years of science operations, SO has taken over 35 million solar occultation spectra, whilst LNO has taken 2.8 million nadir spectra of Mars. Recently, we have started observing Phobos and Deimos in the infrared, and work is currently ongoing to calibrate the spectra and the geometry [2] of the observations. Work is also ongoing to improve the radiometric calibration of LNO, using the latest data, and also to calibrate the Acousto-Optic Tuneable Filter (AOTF) and diffraction grating in the same manner as was done for SO [3,4]. We are investigating the removal of systematic noise and binning multiple spectra to improve the SNR of the SO channel, which will allow us to improve retrievals of temperature/pressure from CO₂ [5,6], H₂O [7,8], HDO [3] CO [9,10], HCl [11] and the detection limits of trace gases [12] in Mars&#8217; atmosphere.</p> <p>&#160;</p> <p>References</p> <p>[1] Vandaele, Ann Carine, et al. "NOMAD, an integrated suite of three spectrometers for the ExoMars Trace Gas mission: technical description, science objectives and expected performance&#8221;. Space Sc. Rev. (2018): 214, 80, https://doi.org/10.1007/s11214-018-0517-2</p> <p>[2] Thomas, Ian, et al. "Calibration of NOMAD on ESA's ExoMars Trace Gas Orbiter: Part 2&#8211;The Limb, Nadir and Occultation (LNO) channel." Planetary and Space Science (2021): 105410. https://doi.org/10.1016/j.pss.2021.105410</p> <p>[3] Villanueva, Geronimo, et al. &#8220;The deuterium isotopic ratio of water released from the Martian caps as measured with TGO/NOMAD&#8221;, submitted to Geophysical Research Letters</p> <p>[4] Thomas, Ian, et al. "Calibration of NOMAD on ESA's ExoMars Trace Gas Orbiter: Part 1&#8211;The Solar Occultation channel." Planetary and Space Science (2021): 105411. https://doi.org/10.1016/j.pss.2021.105411</p> <p>[5] Trompet, Loic., et al. &#8220;Carbon dioxide retrievals from NOMAD-SO on ESA&#8217;s ExoMars Trace Gas Orbiter and temperature profiles retrievals with the hydrostatic equilibrium equation. II. Temperature variabilities in the mesosphere at Mars terminator.&#8221; Journal of Geophysical Research: Planets, Submitted.</p> <p>[6] Lopez Valverde, Miguel, et al., &#8220;Martian atmospheric temperature and density profiles during the 1st year of NOMAD/TGO solar occultation measurements&#8221;. Journal of Geophysical Research: Planets, Submitted.</p> <p>[7] Aoki, Shohei, et al., &#8220;Global vertical distribution of water vapor on Mars: Results from 3.5 years of ExoMars-TGO/NOMAD science operations&#8221;, Journal of Geophysical Research: Planets, Submitted.</p> <p>[8] Brines, Adrian, et al., &#8220;Water vapor vertical distribution on Mars during perihelion season of MY34 and MY35 with ExoMars-TGO/NOMAD observations.&#8221; Journal of Geophysical Research: Planets, Submitted.</p> <p>[9] Yoshida, Nao et al. "Variations in vertical CO/CO2 profiles in the Martian mesosphere and lower thermosphere measured by the ExoMars TGO/NOMAD: Implications of variations in eddy diffusion coefficient", Geophysical Research Letters, accepted</p> <p>[10] Modak, Ashim, et al., Retrieval of Martian atmospheric CO vertical profiles from NOMAD observations during the 1st year of TGO operations. Journal of Geophysical Research: Planets, Submitted.</p> <p>[11] Aoki, Shohei, et al., &#8220;Annual Appearance of Hydrogen Chloride on Mars and a Striking Similarity With the Water Vapor Vertical Distribution Observed by TGO/NOMAD&#8221;, Geophysical Research Letters (2021) https://doi.org/10.1029/2021GL092506</p> <p>[12] Knutsen, Elise, et al. &#8220;Comprehensive investigation of Mars methane and organics with ExoMars/NOMAD&#8221;, Icarus (2021): 357. https://doi.org/10.1016/j.icarus.2020.114266</p> <p>&#160;</p>