SPIDERS AND PLUMES ON MARS AND IN THE LABORATORY: EXPERIMENTS TO TEST THE KIEFFER MODEL UNDER MARS POLAR CONDITIONS

Spiders are striking dendritic, negative topography networks of troughs native to the Martian South Polar Layered Deposits and surrounds (Piqueux et al., 2003). Coined `araneiforms’ by Candy Hansen, a key leader in detecting and analyzing them, these features have no direct Earth analog and have been proposed to form due to seasonal CO<sub>2</sub> ice sublimation (Kieffer et al., 2006, Piqueux et al., 2003). The `Kieffer Model’ proposes that springtime sunlight penetrates seasonal translucent slab ice, heating the regolith beneath and inducing a gas pressure buildup at the base of the ice from sublimating it via the heated regolith, eventually causing it to crack forming a vent. High velocity CO₂ gas escapes through the vent in the form of a plume, scouring the substrate to carve dendritic channels and depositing the material on top of the ice as dark fans and spots. Spiders and their fans and spots have been monitored for the last two decades with MRO’s HiRISE and TGO’s CaSSIS instruments (e.g. Hansen et al., 2011, Portyankina et al., 2011, Cesar et al., 2022). However, plume activity and the direct formation of non-dune high south polar latitude spiders, have not been directly observed.

We present on experiments designed to test the Kieffer Model on the laboratory scale in JPL’s DUSTIE (Dirty Under-vacuum Simulation Testbed for Icy Environments) vacuum chamber. We condensed translucent CO₂ ice on cooled Mojave Mars Simulant under Mars polar temperature and pressure regimes identified by Portyankina et al., 2019. We then activated strip heaters located just below the regolith surface for a variety of timespans and locations and recorded the dynamics observed. We report that CO₂ condensed into the substrate and filled the pore spaces, and also formed a thin layer on top of the material. On heater activation, we report (i) plume activity, (ii) white `halo’ formation surrounding the original plume location and (iii) an alternative process, where sublimation from within the substrate formed `cracked’ spider-like morphologies. We discuss these dynamics in the context of the environments where spiders are detected on Mars, as well as their relative activity or lack thereof, in the present day.

We thank Candy for her leadership and inspiration in this field and feel honored to have had her input in running these experiments.