ExoMars NOMAD instrument: 3 channels
NOMAD is a 3-channel spectrometer. 2 channels (SO and LNO) work in the infra-red and build upon the expertise of BIRA-IASB’s successful SOIR The 3rd channel (UVIS) work in the ultraviolet-visible range and builds upon the UVIS instrument that was developed for the ExoMars Lander.
The infra-red SO channel operates by observing up to 6 small slices of the full spectra range each second. This allows them to observe several different target molecules that absorb at different wavelengths, whilst maximising the signal to noise ratio for each. A solar occultation lasts about 5 minutes, allowing us to take 300 spectra of each wavelength and profile the atmospheric composition from the top of the atmosphere down to almost the surface (depending on dust levels).
The LNO channel is the limb, nadir and occultation infra-red channel. It is an improved version of SOIR, with changes to make it more sensitive to the lower light levels we will see during nadir observations of the planet Mars.
The changes to be made to SOIR include a new AOTF that will allow more light through, a wider slit for the front end optics, redevelopped electronics to include more computing power and a change in cooling. SOIR uses a cyrocooler to chill the detector and increase the instrument's sensitivity to the infra-red light under study. This cooler is used for about 20 minutes per day at Venus, but for nadir observations at Mars it will be on far longer. This means we need to find a cooler with a longer lifetime. We will also cool a larger volume of the instrument, including some of the optics, to minimise thermal background. This work will be carried out in collaboration with thermal experts in Madrid, Spain.
The infra-red LNO channel can also observe up to 6 spectral regions per measurement but may have to integrate longer than 1 s in order to have enough signal to detect trace gases. Nadir measurements can only determine the total column of gas between the instrument and the reflecting surface, but can map variations across the planet, along the ground-track of the orbit. This way we can observe differences between the pole and the equator, and over the lifetime of the mission, also differences with the Martian seasons.
The additional UVIS channel, supplied by our UK partners, will extend the wavelength range from IR to UV and visible wavelengths, giving the NOMAD suite access to ozone, sulphuric acid and aerosol studies. This ultra-light and very capable instrument had been selected for the Exomars lander, but was cancelled when the lander was cancelled in favour of a double rover mission in 2018.
UVIS will be adapted for use in orbit and will image its entire spectral range in each observation of 1 second, covering several interesting molecules and giving more information about aerosols in the atmosphere.