Singular Clouds in Mars Southern Hemisphere around solstice and aphelion season

We present a study of singular systems of clouds seen as single “dot” clouds, clusters of dot clouds and ring-shaped clouds that form every Martian Year (MY) around solstice and aphelion period (from solar longitudes L_s ~ 40° to ~ 120°) in the Southern Hemisphere of Mars. The study is based on images taken with the VMC and HRSC cameras onboard Mars Express from 2008 (MY29) to 2025 (MY38). These clouds mainly concentrate in a sector between longitudes ~ 120°E to 180°E and latitudes ~ 35°S to 50°S in Terra Cimmeria (around Kepler and Cruls craters), with a second much less dense concentration west of the Argyre basin (280°E - 310°E). The isolated bright and compact clouds (dot-shaped clouds) occur in early morning hours (~ 8-11 hr LTST) and have sizes of ~ 100 km. An analysis of their projected shadows indicates cloud bases at heights of ~ 49 km and tops at ~ 55 km. The spots have maximum optical depths of ~ 0.5 (at visual wavelengths) and lifetimes of ~ 1 hr. At the same location and season, but a few hours earlier (LTST ~ 6-7 hr), clusters of bright dots are observed at dawn in twilight, and in some cases projected onto the sky above the Martian limb. They consist of ~ 15 bright spots each with a size of ~ 125 km, separated by ~ 200 km and tops at 65-70 km height. On some cases, the clusters appear to be organized in a ring-like morphology, with projected size of ~ 700-1000 km and tops in twilight at ~ 75-80 km. These clouds are most likely made of H₂O ice and probably form when the dominant eastward winds flow on the craters walls and force a vigorous ascent. However, the mechanism leading to the formation of clusters and the ring-like organization, and the possible role of the magnetic crust anomaly at the region of their occurrence, remain to be explored.