2:30 PM - 2:45 PM
[PPS06-14] High resolution mapping of water vapor over the southern high latitude of Mars observed by MEx/OMEGA: Preliminary results
Keywords:Mars, atmosphere, spectroscopy, water
It is well known that Mars has a seasonal water cycle. Sublimation of the summer polar cap provides water vapor into the atmosphere, which transports to the winter hemisphere by meridional circulation (e.g., Montmessin et al., 2004; Aoki et al., 2022). Such a global picture of water cycle has been measured, while local transport of water vapor has not been well constrained. For that, mapping of water vapor with high spatial resolution is indispensable. Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activite (OMEGA) instrument onboard Mars Express is able to measure water vapor column abundance with high spatial resolution (better than a few km) at different local solar time. Indeed, OMEGA was used to study water vapor distribution over the northern and southern high latitudes (Melchiorri et al., 2007, 2009). In these previous studies, it was suggested that the water vapor abundances in the southern high latitudes had local time variability, enhanced in the late afternoon than early morning. Moreover, unexpected local enhancement of water vapor (so-called “snake” features) was found outside the ice cap in a high spatial resolution map of water vapor. However, more detailed studies have not been performed.
In this study, we analyze the OMEGA spectra in order to better understand the local time variation and unexpected local enhancement of water vapor (“snake” features) by newly comparing water vapor maps with those of surface ices and atmospheric aerosols. We employ a look-up-table method to derive water vapor column abundance from the observed OMEGA spectra at 2,6 µm water band. In this method, a large number of synthetic spectra are calculated in advance for tabulated grid values of physical parameters (such as water vapor column density, surface pressure, atmospheric temperature, observing geometry, aerosols abundances, etc. e.g., see Forget et al., 2007). It allows us to instantaneously obtain the water vapor column abundances from observed spectra by multi-dimensional interpolation of the synthetic spectra in the look-up-table. With this method, we process the OMEGA data taken in Mars Year 27 between Ls = 250 to 274 (southern summer period) at southern high latitudes (60-90S). In addition to the retrievals of water vapor, we investigate maps of surface water ice (with 1,5 µm feature), surface CO2 ice (with 1,429 µm feature), atmospheric dust (with 2,0 µm and 2,7 µm CO2 bands), atmospheric water ice clouds (with 3.4 and 3.52 µm feature), and regolith water (with 3 µm feature).
We confirm that the retrieved column density of water vapor is indeed larger in the later afternoon than early morning as suggested by Melchiorri et al. (2009). In the previous study, it was interpretated as the result of the stronger sublimation of the polar cap in the later afternoon. In contrast, our preliminary results suggest differently. We have tentatively identified very faint features of CO2ice in the spectra taken in the late afternoon. These features are widely present at the southern high latitudes, which is consistent with the recent finding with CRISM (Cartwright et al., 2023). The CO2 ice features are found only in the late afternoon OMEGA spectra (i.e., no features identified in the early morning OMEGA spectra). While, the data taken in the early morning shows higher abundance of dust in the atmosphere. It suggests that atmospheric dust may prevent faint surface CO2 ice detection. As presence of the surface CO2 ice contaminates the retrieval of water vapor, the obtained large abundances of water vapor in the late afternoon may be an artefact related to the faint presence of surface CO2 ice.
We have also identified several other local enhancements of water vapor abundances (“snake” features) in early morning data. We have found such a feature at multiple orbits but over different locations. We will discuss possible origins of these features in the presentation.
In this study, we analyze the OMEGA spectra in order to better understand the local time variation and unexpected local enhancement of water vapor (“snake” features) by newly comparing water vapor maps with those of surface ices and atmospheric aerosols. We employ a look-up-table method to derive water vapor column abundance from the observed OMEGA spectra at 2,6 µm water band. In this method, a large number of synthetic spectra are calculated in advance for tabulated grid values of physical parameters (such as water vapor column density, surface pressure, atmospheric temperature, observing geometry, aerosols abundances, etc. e.g., see Forget et al., 2007). It allows us to instantaneously obtain the water vapor column abundances from observed spectra by multi-dimensional interpolation of the synthetic spectra in the look-up-table. With this method, we process the OMEGA data taken in Mars Year 27 between Ls = 250 to 274 (southern summer period) at southern high latitudes (60-90S). In addition to the retrievals of water vapor, we investigate maps of surface water ice (with 1,5 µm feature), surface CO2 ice (with 1,429 µm feature), atmospheric dust (with 2,0 µm and 2,7 µm CO2 bands), atmospheric water ice clouds (with 3.4 and 3.52 µm feature), and regolith water (with 3 µm feature).
We confirm that the retrieved column density of water vapor is indeed larger in the later afternoon than early morning as suggested by Melchiorri et al. (2009). In the previous study, it was interpretated as the result of the stronger sublimation of the polar cap in the later afternoon. In contrast, our preliminary results suggest differently. We have tentatively identified very faint features of CO2ice in the spectra taken in the late afternoon. These features are widely present at the southern high latitudes, which is consistent with the recent finding with CRISM (Cartwright et al., 2023). The CO2 ice features are found only in the late afternoon OMEGA spectra (i.e., no features identified in the early morning OMEGA spectra). While, the data taken in the early morning shows higher abundance of dust in the atmosphere. It suggests that atmospheric dust may prevent faint surface CO2 ice detection. As presence of the surface CO2 ice contaminates the retrieval of water vapor, the obtained large abundances of water vapor in the late afternoon may be an artefact related to the faint presence of surface CO2 ice.
We have also identified several other local enhancements of water vapor abundances (“snake” features) in early morning data. We have found such a feature at multiple orbits but over different locations. We will discuss possible origins of these features in the presentation.