Atmospheric drivers of interannual variability from the northern hemisphere seasonal cap on Mars

The seasonal exchange of volatiles (CO₂ and H₂O) between the atmosphere and surface on Mars drives seasonal changes in the extent of ice at the poles. Interannual variability, or year to year changes, in the extent of seasonal ice have been observed, suggesting fluctuations in the processes that drive volatile exchange. In the northern hemisphere, previous studies have mapped seasonal frosts during the winter and spring months. However, a critical knowledge gap persists, namely multi-year records of seasonal frost extent that distinguish composition. In this work, we fill this gap, quantifying the interannual variability of seasonal frost extent for CO₂ and H₂O across 6 Mars years. We identify the edges of seasonal CO₂ and H₂O frost caps with binary ice mask products, derived from CRISM mapping-style observations taken during Mars Year 28–33. We find that the extent of CO₂ seasonal frost is fairly consistent between years, changing by ∼1.5° of global equivalent latitude, but the extent of H₂O seasonal frost can change by ∼3.9° of global equivalent latitude. To identify potential drivers of the observed variations, we further compare our results with atmospheric measurements of dust, the position of the polar vortex, and water vapor availability. We find that neither the timing of atmospheric dust loading nor of polar vortex retreat fully explain the observed seasonal cap variability. However, the amount of available water vapor appears to be proportional to the extent of the H₂O seasonal cap.