TO EVERYTHING THERE IS A SEASON AND A TIME TO EVERY PURPOSE UNDER MARS’ ATMOSPHERE

The martian polar regions, both south and north, have now been continuously monitored for approximately one martian decade. Several missions have contributed to this effort that was effectively kick-started by Mars Global Surveyor (MGS) in the late 1990s. The Mars Orbiting Camera (MOC) on MGS observed the polar regions getting brighter with seasonal snow deposition in fall and winter and darker during spring while ice sublimed, as expected. However, it also discovered various decameter-scale features were changing through seasons with – unexplained at the time – repeatability from year to year [Kieffer, H.H. 2003]. This motivated following missions to dedicate whole observational campaigns to monitoring the polar areas. The most recent advances have been brought by missions with high-resolution cameras, namely Mars Reconnaissance Orbiter with the High Resolution Imaging Science Experiment (HiRISE) and Trace Gas Orbiter with the Colour and Stereo Surface Imaging System (CaSSIS). These missions have monitored how seasonal frost deposits at sub-meter scales appear and evolve, as well as how these deposits modify the underlying permanent substrate and interact with the martian atmosphere. HiRISE is continuing its uninterrupted record of seasonal monitoring of the martian polar regions from Martian Year (MY) 28 (2006) [Piqueux, et. al, 2015]. During this span, HiRISE has monitored how fan-shaped deposits from cold CO₂ jets appear and wane; has detected furrows and dendritic troughs being dug out in the substrate by the CO₂ movement below the seasonal ice; and has revealed connections between small scale seasonal activity and martian weather and potentially global climate [Portyankina, et. al., 2022]. CaSSIS started observing Mars in 2018 producing images with exceptional color depth that highlight seasonal frosts [Cesar, et. al, 2022] as well as in-orbit DTMs. While coverage of CaSSIS in the polar regions is restricted to latitudes up to 74 degrees, it adds information about seasonal processes by observing at local times inaccessible to HiRISE. In this presentation we will summarize our understanding of seasonally active processes after monitoring them for the past martian decade. These processes are a crucial piece of the puzzle to comprehending Mars as an active planet.