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) works in the ultraviolet-visible range and builds upon the UVIS instrument that was developed for the ExoMars Lander.
The solar occultation channel (SO) is a copy of SOIR/VEX. The Limb, Nadir and Occultation channel (LNO) is an improved version of SOIR, more adapted for fainter light sources as it measures in solar occultation but also in nadir mode, i.e. looking directly at the sunlight reflected from the surface and atmosphere of Mars. The modified design includes a larger entrance slit to increase the amount of light entering the optics and a new acousto-optical tunable filter to maximise throughput. The UV-Visible channel (UVIS) is based on a design for a previous Mars lander made by the Open University, UK. It measures in both solar occultation and nadir modes.
Three models of the instrument were built, as required by ESA:
- Structural-Thermal Model (STM): This is a representative model of NOMAD, having the same size, mass, centre of gravity of the real instrument, but without any optical components. This was mounted to the spacecraft before the real flight model was completed, so that the satellite could undergo thermal and structural tests with a representative load without having to wait for the real instrument
- proto-Flight Model (pFM): This is the instrument that will fly to Mars. The model underwent a wide range of tests to ensure that it will survive the journey to Mars and will work as expected once there.
- Flight Spare Model (FS): This is an identical copy of the pFM, which is built in case there is a problem with the flight model that cannot be easily solved. The window for launching to Mars only opens every 2 years, and so if a problem occurs a replacement is needed immediately. In the case of NOMAD, we discovered that the detector in the LNO channel was not working correctly, so it was replaced by the detector from the flight spare model. Without such a backup, NOMAD would have likely been sent to Mars without a functioning LNO channel!
NOMAD STM (left), pFM (centre), and FS (right)
NOMAD development and construction was carried out by the prime contractor, OIP, Oudenaarde, in collaboration with expert partners in Spain, the UK and Italy. The central electronics unit and firmware were designed by Spanish colleagues who worked on Rosetta, Mars Express and Venus Express, and were built by Thales Alenia Space Belgium (also known as ETCA). UVIS was designed and built by Lambda-X, Nivelles; its electronics were designed and built by the Mullard Space Science Laboratory, UK.
The Royal Belgian Institute for Space Aeronomy (BIRA-IASB) designed much of the SO and LNO channels, including the electronics and ground support equipment. The institute also managed all of the tasks carried out across Europe and maintained the documentation necessary for ESA. They also took care of the planetary protection aspects necessary for missions going to Mars, alongside their project management responsibilities. Testing and science performance were also the responsibility of the institute.
The infra-red SO channel operates by observing up to 6 small slices of the full spectra range each second. This allows it to observe several different target molecules that absorb at different wavelengths, whilst maximising the signal to noise ratio for each. A solar occultation lasts just a couple of minutes, allowing us to take around 500 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 observe during nadir observations of the planet Mars.
The changes to be made to SOIR included a new AOTF that will allow more light through, a wider slit for the front end optics, redeveloped 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 switched on much more. An external radiator also cools the channels, including some of the optics, to minimise noise from the thermal background. This work was carried out in collaboration with thermal experts in Madrid, Spain.
The infrared LNO channel can also observe up to 6 spectral regions per measurement but typically measures 2 or 3 to increase the 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, extends the wavelength range from the infrared to ultraviolet 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 original Exomars lander, but was cancelled when the lander was replaced in favour of a double rover mission in 2020.
UVIS is adapted for use in orbit and can image either its entire spectral range or a subset of the spectral range in regular intervals, covering several interesting molecules and giving more information about aerosols in the atmosphere.