RESUMO
We present water vapor vertical distributions on Mars retrieved from 3.5 years of solar occultation measurements by Nadir and Occultation for Mars Discovery onboard the ExoMars Trace Gas Orbiter, which reveal a strong contrast between aphelion and perihelion water climates. In equinox periods, most of water vapor is confined into the low-middle latitudes. In aphelion periods, water vapor sublimated from the northern polar cap is confined into very low altitudes-water vapor mixing ratios observed at the 0-5 km lower boundary of measurement decrease by an order of magnitude at the approximate altitudes of 15 and 30 km for the latitudes higher than 50°N and 30-50°N, respectively. The vertical confinement of water vapor at northern middle latitudes around aphelion is more pronounced in the morning terminators than evening, perhaps controlled by the diurnal cycle of cloud formation. Water vapor is also observed over the low latitude regions in the aphelion southern hemisphere (0-30°S) mostly below 10-20 km, which suggests north-south transport of water still occurs. In perihelion periods, water vapor sublimated from the southern polar cap directly reaches high altitudes (>80 km) over high southern latitudes, suggesting more effective transport by the meridional circulation without condensation. We show that heating during perihelion, sporadic global dust storms, and regional dust storms occurring annually around 330° of solar longitude (L S) are the main events to supply water vapor to the upper atmosphere above 70 km.
RESUMO
NOMAD is a suite of three spectrometers that will be launched in 2016 as part of the joint ESA-Roscosmos ExoMars Trace Gas Orbiter mission. The instrument contains three channels that cover the IR and UV spectral ranges and can perform solar occultation, nadir and limb observations, to detect and map a wide variety of Martian atmospheric gases and trace species. Part I of this work described the models of the UVIS channel; in this second part, we present the optical models representing the two IR channels, SO (Solar Occultation) and LNO (Limb, Nadir and Occultation), and use them to determine signal to noise ratios (SNRs) for many expected observational cases. In solar occultation mode, both the SO and LNO channel exhibit very high SNRs >5000. SNRs of around 100 were found for the LNO channel in nadir mode, depending on the atmospheric conditions, Martian surface properties, and observation geometry.
