Homogeneous nucleation on Mars. An unexpected process that deciphers mysterious elongated clouds

Homogeneous nucleation has not been considered a possibility in cloud formation processes in the atmosphere of Mars (e.g. Clancy et al., 2017), since Määttänen et al. (2005) made a careful analysis that indicated that extreme supersaturations in the order of 10⁵ were required. Such extreme supersaturations were considered unlikely, especially because the abundant dust in the atmosphere of Mars was expected to deplete water in excess of saturation very quickly by heterogeneous nucleation.The Arsia Mons Elongated Cloud (AMEC) is an eye-catching and mysterious cloud occurring recurrently every morning during the perihelion season over the Arsia Mons volcano on Mars (Hernández-Bernal et al., 2021). It shows a peculiar elongated shape that in only 3 hours expands up to 1800 km from its origin point. Hernández-Bernal et al. (2022) investigated this cloud based on the LMD Mars Mesoscale model (Spiga and Forget, 2009). The tail of the cloud was not reproduced in the model, but a cold pocket with temperatures down to 30K below the environment and supersaturation up to 105 appeared next to Arsia Mons, in a position, altitude, and local time and season coincident with the origin point of the AMEC in observations. In this work we show that these are conditions conducive to homogeneous nucleation, and when we introduce this process as a new cloud formation process in the LMD Mars Mesoscale model, we obtain a good representation of the AMEC, and its long tail.This provides an excellent explanation for this mysterious cloud and shows that homogeneous nucleation is possible and can have significant effects in the atmosphere of Mars, contrary to the widespread assumptions during the last twenty years of Mars exploration. The finding of supersaturations up to 108 in the surveys performed by Fedorova et al. (2020; 2023) observationally supports that these extreme supersaturations can indeed happen in the atmosphere of Mars and homogeneous nucleation could be happening in other clouds. As a first example, we find that the Perihelion Cloud Trails (Clancy et al., 2009; 2021) could be the result of homogeneous nucleation, as our mesoscale model also predicts cold pockets spatially coincident with locations where Clancy et al. observed cloud trails. We intend to explore these and other clouds on Mars possibly involving homogeneous nucleation. References:Clancy, R. T., Wolff, M. J., Cantor, B. A., Malin, M. C., & Michaels, T. I. (2009). Valles Marineris cloud trails. Journal of Geophysical Research: Planets, 114(E11). https://doi.org/10.1029/2008JE003323  Clancy, R., Montmessin, F., Benson, J., Daerden, F., Colaprete, A., & Wolff, M. (2017). Mars Clouds. In R. Haberle, R. Clancy, F. Forget, M. Smith, & R. Zurek (Eds.), The Atmosphere and Climate of Mars (Cambridge planetary science (pp. 76–105). Cambridge: Cambridge University Press. https://doi.org/10.1017/9781139060172.005  Clancy, R. T., Wolff, M. J., Heavens, N. G., James, P. B., Lee, S. W., Sandor, B. J., ... & Spiga, A. (2021). Mars perihelion cloud trails as revealed by MARCI: Mesoscale topographically focused updrafts and gravity wave forcing of high altitude clouds. Icarus, 362, 114411. https://doi.org/10.1016/j.icarus.2021.114411  Määttänen, A., Vehkamäki, H., Lauri, A., Merikallio, S., Kauhanen, J., Savijärvi, H., & Kulmala, M. (2005). Nucleation studies in the Martian atmosphere. Journal of Geophysical Research: Planets, 110(E2). https://doi.org/10.1029/2004JE002308  Fedorova, A. A., Montmessin, F., Korablev, O., Luginin, M., Trokhimovskiy, A., Belyaev, D. A., ... & Wilson, C. F. (2020). Stormy water on Mars: The distribution and saturation of atmospheric water during the dusty season. Science, 367(6475), 297-300. https://doi.org/10.1126/science.aay9522  Fedorova, A., Montmessin, F., Trokhimovskiy, A., Luginin, M., Korablev, O., Alday, J., ... & Shakun, A. (2023). A two‐Martian years survey of the water vapor saturation state on Mars based on ACS NIR/TGO occultations. Journal of Geophysical Research: Planets, 128(1), e2022JE007348. https://doi.org/10.1029/2022JE007348  Hernández‐Bernal, J., Sánchez‐Lavega, A., del Río‐Gaztelurrutia, T., Ravanis, E., Cardesín‐Moinelo, A., Connour, K., ... & Hauber, E. (2021). An extremely elongated cloud over Arsia Mons volcano on Mars: I. Life cycle. Journal of Geophysical Research: Planets, 126(3), e2020JE006517. https://doi.org/10.1029/2020JE006517  Hernández‐Bernal, J., Spiga, A., Sánchez‐Lavega, A., del Río‐Gaztelurrutia, T., Forget, F., & Millour, E. (2022). An extremely elongated cloud over Arsia Mons volcano on Mars: 2. Mesoscale modeling. Journal of Geophysical Research: Planets, 127(10), e2022JE007352. https://doi.org/10.1029/2022JE007352  Spiga, A., & Forget, F. (2009). A new model to simulate the Martian mesoscale and microscale atmospheric circulation: Validation and first results. Journal of Geophysical Research: Planets, 114(E2). https://doi.org/10.1029/2008JE003242