Seasonal southern circum-polar spots and araneiforms observed with the colour and stereo surface imaging system (CaSSIS)

Abstract

The southern polar area of Mars is home to various seasonal activity commonly explained by the Kieffer model. During southern spring, the ice covering the polar area sublimates and leaves distinct features (spiders, spots, fans) observable from orbit. The Colour and Stereo Surface Imaging System (CaSSIS) onboard the ExoMars Trace Gas Orbiter (TGO), provides high-resolution multi-filter images of the Martian surface offering high sensitivity to colour contrasts. Its stereo capability is pivotal for momentary processes and offers a unique perspective for studying surface sublimation processes and their relation to atmospheric features. For the first time, we identify clouds well correlated with surface features (araneiforms and spots at southern circum-polar latitudes) hence motivating a new campaign to refine these observations over time periods where CO 2 sublimation processes occur. We focus here on the structure of spot deposits and their evolution through time. We identify and describe seven structures: dark spot, bright-haloed spot, ringed spot, inverted spot, dark-haloed spot, banded spot, and bright spot. By morphological and spectral analyses, we hypothesize a new chronology of events that characterize the origin, formation and evolution of these features. • We analysed dune spots and araneiform terrain during southern spring on Mars. • Various structures of spots are observed with the Colour and Stereo Surface Imaging System through time and provide grounds for a classification of spots. • We hypothesize a new chronology of events that characterize the origin, formation and evolution of these features. • Diffuse atmospheric features (clouds and haze) have been observed correlating with surface features (spot, araneiform terrains).