Remote sensing in the IR
Scientists in the Planetary Aeronomy group are experts in remote sensing. They analyze the data obtained by different instruments on Earth or on spacecraft in order to gain insights concerning the atmosphere of Mars (or Venus). To retrieve information from the data, they use a radiative transfer model.
Measuring spectra in the infrared spectral region allows them to characterize different aspects of the atmosphere. The main topics of interest are:
To perform these studies, several "tools" are used. First of all, data are obtained from an instrument, called a spectrometer, for instance SOIR or one of the channels of NOMAD. This spectrometer points to the Sun (solar occultation viewing geometry) or to the planet (nadir viewing geometry) and measures the incoming radiation. The raw data is then downlinked to Earth and calibrated (dark current, spectral calibration, non linearity, ...). This leads to the creation of a dataset of spectra. One simulated spectrum of the Martian atmosphere as should be measured by NOMAD LNO is shown:
The Radiative Transfer model will be then applied to retrieve physical quantities. At BIRA-IASB, we use an home-made Radiative Transfer model algorithm called ASIMUT-ALVL. For instance, from the spectrum shown above, scientists will be able to infer the abundances of molecular species.
Based upon the characteristics of the spectrometers and the values of Signal-to-Noise Ratio obtained from radiometric models discussed in Vandaele et al., Planet. Space Sci. 2015, Vandaele et al., Opt. Express 2015 and Thomas et al., Opt. Express 2016, the expected performances of the instrument in terms of sensitivity to detection have been investigated. The analysis led to the determination of detection limits for 18 molecules, namely CO, H2O, HDO, C2H2, C2H4, C2H6, H2CO, CH4, SO2, H2S, HCl, HCN, HO2, NH3, N2O, NO2, OCS, O3. NOMAD should be able to measure methane concentrations < 25 parts per trillion (ppt) in solar occultation geometry, and 11 parts per billion (ppb) in nadir geometry. (from Robert et al., Planet. Space Sci. 2016)