CO <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si54.svg" display="inline" id="d1e723"> <mml:msub> <mml:mrow/> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:math> condensation and sublimation on a range of substrates

The winter/springtime CO 2 condensation and sublimation cycle is recognized as a cardinal agent of present-day surface change on Mars, and was likely also instrumental in modifying the surface during the recent past. The Kieffer Model postulates that slab ice condenses in winter and sublimates in spring, causing pressurized CO 2 gas beneath the ice to rush to the surface, forming a ‘zoo’ of features ranging from seasonal plumes, dark fans and spots and the mysterious ‘spiders’ or araneiforms surrounding the Martian south pole. However, the lack of terrestrial analogs or empirical observations of this conceptual process hamper our understanding of how the Martian surface is modified in this way today. In Mc Keown et al. (2024), we presented experiments that simulated all three main stages of the Kieffer model on a ∼ 1 cm layer of Mars Mojave (regolith) Simulant (MMS) < 150 μ m : (i) CO 2 condensation, (ii) sublimation of CO 2 ice, and plume, spot and halo formation and (iii) the resultant formation of ‘cracked’ spiders, where interstitial pore ice is sublimated and cracks, preserving patterns in the surrounding regolith. In this paper, we present experiments where CO 2 condenses on different discrete grain size ranges of regolith: < 53 μ m , 75–150 μ m , and 180–500 μ m , for both ‘dry’ regolith and a water-mixed ‘permafrost’ simulant, and on glass beads 250–355 μ m , but forms and sublimates in different ways. We find that CO 2 diffuses deeper and across a greater area within the regolith pore spaces for finer grain sizes, and the top ice layer grows inward from the sample edges for coarser grains, resulting in finer grains being more prone to ‘cracked’ spider morphologies than coarser grains. Condensation of CO 2 appears to be affected by thermal properties and circularity of the grains, with rate of ice accumulation on the surface slower on the surface of glass beads and final patterns of ice on the surface differing in appearance from the MMS simulant. Water ice within the pore spaces of the regolith encourages the growth of a thick CO 2 surface layer, but sublimation of that CO 2 is significantly hampered. We also find that plume activity is more vigorous and lasts longer for finer grain sizes than coarser grains. Using our laboratory observations, we discuss how deposition of CO 2 across different substrates may reflect varying sublimation activity levels and morphologies on Mars.