Spectroscopic Characterization and Evolution of Martian Nighttime CO2 Frost at Equatorial Latitudes with EMM/EMIRS
Authors: Stcherbinine Aurélien , Edwards Christopher , Haberle Christopher , Smith Michael , Lange Lucas , Pilorget Cédric 
Published: July 2024

IntroductionThe Emirates Mars InfraRed Spectrometer (EMIRS) instrument onboard the Emirates Mars Mission (EMM) &#8220;Hope&#8221; probe is a Fourier Transform Infrared spectrometer that has been observing the Martian surface and atmosphere between 6 and 100&#160;&#956;m since February 2021 [1, 2]. The unique orbit of EMM allows EMIRS to observe the entire Martian disk at each observation, covering all the surface of the planet in ~4&#160;orbits, which corresponds to ~5&#176; of Ls, or 10 Earth days.[3] used the surface temperature retrievals from EMIRS to detect and monitor the presence of H2O and CO2 ice on the surface of the planet, including the temporal and local time evolution of the CO2 frost that can appear at Martian equatorial latitudes. This frost has been observed in the second half of the night around the equinoxes. Small crystals and optically thin layers are expected from the work of [4] using THEMIS data. However, as EMIRS provides fully resolved spectra whereas THEMIS is a multi-band instrument, we will be able to strengthen the constraints on the physical properties of these deposits. And monitor for the first time the evolution of the ice properties during its condensation phase from 12&#160;a.m. to 6&#160;a.m. thanks to the unique temporal coverage of EMM instruments.Data &amp; MethodsBased on the first identification of the EMIRS pixels mostly covered by CO2 ice presented in [3], we use here the full spectroscopic power of the instrument to characterize and constrain the physical properties of these icy deposits (crystal size, thickness) and their temporal evolution. First, we identify areas where CO2 surface frost has been detected at different local times over a few degrees of Ls(to have a sufficient spatial and temporal coverage), then we bin the data over temporal bins of 1-hr and compare the averaged spectra for each bin. The spatial extent of the EMIRS pixel footprints is computed using the SPiP Python module [5].ResultsFirst, we observe that CO2 ice spectral signatures are observed in the EMIRS spectra when predicted by the temperature criterion, between midnight and 6&#160;a.m., but mostly from 3&#160;a.m. Comparison of spectra presented in Figure&#160;1a with models from [4] may suggest optically thin layers of CO2 ice, with a thickness of ~&#160;100&#160;&#956;m, when models can predict condensation of up to a few tens of microns of ice just before sunrise.As the surface temperature remains below the freezing point of CO2 during the second half of the night, and CO2 is always available at the surface from the atmosphere, condensation is expected to occur over the night hours. However, no significant temporal variations of the spectra have been observed over the night. The differences between spectra that can be seen in Figure&#160;1 are mostly associated with spatial variability of the surface emissivity, as can be seen on panels c &amp; d. This is not in favor of the scenario of an accumulation of ice over the night which would increase progressively the frost thickness and/or the size of the crystals. Thus, the lack of variability over the night may suggest that the condensation may occur within the surface regolith and/or the subsurface, as suggested by [6].Figure 1:&#160;EMIRS spectra from EMM orbits 171 to 180 where surface CO2 frost has been detected between 150&#176;W and 90&#176;W and between 0&#176;N and 60&#176;N (North of Tharsis), averaged over temporal bins of 1-hr (a) and footprints of the considered EMIRS pixels (b). The colors of the spectra and pixels indicate the local time range for the observations. (c &amp; d) EMIRS average spectrum and pixel footprints over the same region for observations between 11&#160;a.m. and 1&#160;p.m. to provide a daytime emissivity reference to be compared with the nighttime spectra.Conclusion &amp; PerspectivesIn this work, we present the first spectroscopic monitoring of surface nighttime CO2 frost under equatorial latitudes as a function of the local time. We do not observe significant variation of the spectra over the night but a spatial variability is present. This suggests that the frost condensation likely occurs within the regolith and/or close subsurface rather than a simple accumulation of ice at the surface overnight. Further spectral modeling of the emissivity will be needed to assess the physical properties of these deposits considering different scenarios including notably: &#8220;dirty&#8221; CO2 ice, subsurface CO2 ice, or slab of CO2 ice.AcknowledgmentsThis work was funded by the Emirates Mars Mission project under the Emirates Mars Infrared Spectrometer instrument via The United Arab Emirates Space Agency (UAESA) and the Mohammed Bin Rashid Space Centre (MBRSC).A. S. also acknowledges funding by CNES.References[1] Amiri, H. E. S. et al. (2022), SSR, 218, 4. [2] Edwards, C. S. et al. (2021), SSR, 217, 77. [3] Stcherbinine, A. et al. (2023), GRL, 50, e2023GL103629. [4] Piqueux, S. et al. (2016), JGR: Planets, 121, 1174-1189. [5] Stcherbinine, A. (2023), Zenodo, doi:10.5281/zenodo.7714204. [6] Lange, L. et al. (2022), JGR: Planets, 127, e2021JE006988.&#160;