Spectral Characterization of CO ₂ ‐H ₂ O Ice Layering Under Controlled Mars Polar Conditions as Laboratory Analogs for Seasonal Cap Activity

Seasonal CO 2 and H 2 O ice blanket the Martian polar caps, and their spectral contributions mix during seasonal evolution. Particularly in late northern winter, the spectral signature of H 2 O ice obscures that of CO 2 ice. Using the MARVIN chamber at York University, we produced controlled analogs of layered polar ice under Mars‐like pressure (∼6 mbar) by depositing H 2 O frost atop ∼1 cm CO 2 ice. The CO 2 surfaces were grown under two temperature regimes that generated either frosty (<140 K) or translucent slab (>140 K) morphologies, each tested with either an underlying H 2 O frost layer or a black‐painted copper plate as a substrate. Short‐wave infrared (1.20–2.20 μm) reflectance spectra were collected incrementally during H 2 O deposition from 0 to 10 mL (0–0.87 mm thickness) to quantify how thin frost mantles modify CO 2 spectral band features between 1.4 and 2.0 μm. Across all experiments, only 0.08–0.25 mm of H 2 O frost was required to substantially suppress major CO 2 absorption bands, with exact thresholds dependent on CO 2 morphology and substrate. Averaging over all cases, we identify a representative spectral masking depth of ∼0.15 mm H 2 O, suggesting that even a submillimeter water frost veneer can cause CO 2 ‐rich surfaces to appear H 2 O‐dominated.