Reader Response Draft 2.0

The article “NOMAD, an Integrated Suite of Three Spectrometers for the ExoMars Trace Gas Mission: Technical Description, Science Objectives and Expected Performance”, discusses a spectrometer suite incorporated into the ExoMars Trace Gas Orbiter, NOMAD (“Nadir and Occultation for Martian Discovery”).

NOMAD comprises of three spectrometers: Solar Occultation (SO), Limb and Nadir Occultation (LNO) and Ultraviolet/Visible Spectrometer (UVIS). The SO spectrometer only performs solar occultation and works in the infrared range(2.3-4.3um). The LNO spectrometer is capable of performing solar and nadir occultation and limb observations and also operates in the infrared range(2.3-3.8um. The UVIS spectrometer operates in the ultraviolet and visible light range(200-650nm) and can be used to detect carbon dioxide ice grains at high altitudes. These spectrometers work with each other to collect data which allows us to understand Mars’ “atmospheric composition and the related physical and chemical processes”(Vandaele et al., 2017).


NOMAD is the only instrument that is highly sensitive available on the market that can provide vertical profiles and can map trace gases, main constituents, temperature and dust/clouds present on Mars’ atmosphere which will expand our knowledge of the Martian atmosphere. 


By using the spectrometers which cover ultraviolet, visible light and infrared ranges, NOMAD can detect a greater number of gases like carbon dioxide (CO₂), nitrogen dioxide(NO₂), water vapour(H₂O) and hydroperoxyl(HO₂). The high resolution of the SO and LNO spectrometer allows for greater sensitivity when detecting the trace gases and their different isotopes. Table 2: The LNO and UVIS detection limits for nadir and solar occultation geometry from the article “NOMAD, an Integrated Suite of Three Spectrometers for the ExoMars Trace Gas Mission: Technical Description, Science Objectives and Expected Performance” provides evidence for this. 


The article “NOMAD, an Integrated Suite of Three Spectrometers for the ExoMars Trace Gas Mission: Technical Description, Science Objectives and Expected Performance” states that hydroperoxyl compound has never been measured before and that NOMAD can detect it when it is the solar occultation mode and also provide high-resolution vertical profiles of the said compound along with other gases. It is the first time that very detailed vertical profiles of carbon monoxide (CO) were produced. It is also stated in the article that NOMAD is the first instrument that can produce “high vertical resolution observations of the key species (H2O, O3, HO2, …) in combination with ice clouds” using its three spectrometers. 


Methane is another trace gas that is widely searched for on Mars’ to prove that there was life on Mars’ as on Earth methane is generated in large amounts by all living things. Methane also has a lifetime of several hundred years before it is decomposed by the Sun, therefore it would be possible to detect methane in Mars’ atmosphere. NOMAD was unable to detect any trace of methane but NASA’s Curiosity rover had observed the “highest burst of methane recorded yet”. This leads to a lot of confusion regarding why a highly sensitive instrument capable of detecting a totally new gas like hydrochloride could not detect methane when it was flying over the same location where the Curiosity rover flew at a similar time.


The European Space Agency in their article “ESA’s Mars orbiters did not see latest Curiosity methane burst” (ESA,2019) stated that the measurement varies according to how dusty the atmosphere is and this could have contributed to the lack of methane detection. It also stated that Curiosity took the measurements during might and if methane was released at that time, then it would be trapped under the surface till morning and it would have been mixed with other gases and hence it would be impossible for NOMAD to detect it.


Hence, NOMAD is a highly sensitive instrument capable of measuring various gases including new ones (HCl) and the lack of methane detection is probably due to Mars’ atmosphere. NOMAD is one of the best-suited instruments in the market as its ability lies in detection and mapping trace gases.



References


Vandaele, A. C., Lopez-Moreno, J.-J., Patel, M. R., Bellucci, G., Daerden, F., Ristic, B., Robert, S., Thomas, I. R., Wilquet, V., Allen, M., Alonso-Rodrigo, G., Altieri, F., Aoki, S., Bolsée, D., Clancy, T., Cloutis, E., Depiesse, C., Drummond, R., Fedorova, A., … Wolff, M. (2018). Nomad, an integrated suite of three spectrometers for the ExoMars Trace Gas Mission: Technical Description, science objectives and expected performance. Space Science Reviews, 214(5). https://doi.org/10.1007/s11214-018-0517-2 


ESA's Mars Orbiters did not see latest curiosity methane burst. ESA. (2019, November 13). Retrieved October 10, 2021, from https://www.esa.int/Science_Exploration/Human_and_Robotic_Exploration/Exploration/ExoMars/ESA_s_Mars_orbiters_did_not_see_latest_Curiosity_methane_burst. 

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