Arsia Mons is the southernmost of the three large volcanoes on the Tharsis Plateau on Mars, rising to about 11 miles above the average martian surface. Winds impinging on such mountains create updrafts, typically at the base of the mountains, and water in those updrafts then freezes at higher elevations, forming clouds. One such cloud is the 1100-mile-long Arsia Mons Elongated Cloud (AMEC), which forms every morning between 5:40 and 8:30 local time around the southern summer solstice when clouds are typically rare near the martian equator.
To further understand the mechanism of formation of the AMEC, Jorge Hernández-Bernal from the University of the Basque Country and colleagues used the Mars Mesoscale Model to simulate the cloud. This model applies the primitive equations of meteorology, which describe the motion of air in thin planetary atmospheres, to Mars and incorporates radiative and cloud microphysical phenomena that are important on Mars. Around the southern summer solstice, westward winds are strongest compared to the rest of the martian year (about 180 miles per hour), and water concentrations are highest (because temperatures are the highest), all of which are necessary for the AMEC to form. Hernández-Bernal and colleagues found that the AMEC is formed by gravity waves generated by the strong winds impinging on Arsia Mons, which temporarily squeezes the passing air. The air then oscillates, creating updrafts with wind speeds as great as 45 miles per hour that cool the atmosphere by over 54°F, allowing water at about 28 miles above the average martian surface to freeze. Hernández-Bernal and colleagues posited that the resulting ice and associated cold air are then blown westward by the strong winds, though the tail of the AMEC was not present in their model, likely due to a lack of ice nuclei. This study highlights both the model’s success in finding the cause of the AMEC and the need to improve microphysics in Mars models. READ MORE