Formation of coarse sediment lags in ice-sediment mixtures: A geomorphic signature of sublimation on regolith surfaces

In many high-latitude locations on Mars, boulder sized clasts are present at the ground surface atop finer-grained sediments. These areas are concentrated in the latitude dependent mantle (LDM), an ice-rich surface unit that is globally distributed polewards of similar to 30 degrees latitude. Here, we test the hypothesis that sublimation of buried ice can drive grain size sorting in elastic deposits. We conducted pilot laboratory experiments to determine how buried and interstitial dry ice (CO2) sublimation modifies the grain size distribution in overlying and intermingled lithic sediments. We found that sublimation can sort grains into horizons with distinct grain size distributions by allowing fines to winnow into space freed by sublimation of underlying ice. On sloped surfaces, this mechanism enables lateral sorting by concentrating large grains on the surface and allowing them to slide or roll downslope. We infer that sublimation-driven sorting may affect the surface grain size distribution of solar system objects for which solid-vapor phase transitions dominate (e.g., modern Mars, comet 67P/Churyumov-Gerasimenko, asteroids, etc.).