Climate-driven deposition of water ice and the formation of mounds in craters in Mars’ north polar region

Abstract

This paper explores the origins and evolution of ice-rich interior mounds found within craters of the north polar region of Mars. We present a systematic study of impact craters above 65°N, and identify 18 craters that have interior mounds. At least eleven of these mounds are composed of water ice and geometric similarities suggest that dune-covered mounds may also have a water ice core. The mounds are found in the deeper craters in the north polar area and we suggest that these form a specific microclimate favorable for mound initiation and growth. It is likely that at least seven of the mounds have evolved as individual outliers, rather than conterminous with the main polar cap. Our observations suggest that the mounds are built up by atmospheric deposition, similar to that of the north polar layered deposits. Using a combination of remote sensing techniques enabling topographic, spectral, radar and image data analyses, we have documented the morphology, composition and stratigraphy of selected mounds. We advance and test four hypotheses for formation of these mounds: artesian outpouring from a deep aquifer, hydrothermal activation of ground ice, remnants of a more extensive polar cap, and atmospheric deposition on ice caps in meteorologically isolated locations. We propose that during periods when the perihelion was located in northern summer (most recently 10-25 ka before present) the microclimate in these craters retarded the sublimation of CO 2 and water ice in northern spring, thus creating a cold trap for volatiles released as the seasonal cap retreated. This created a thick enough deposit of water ice to withstand sublimation over the summer and initiate a positive feedback leading to mound-building. Mounds without complete dune-cover may be in dynamic equilibrium with the ambient climate and show evidence of both present-day and past periods of erosion and aggradation. We conclude that the water ice mounds formed in deep impact craters in Mars" north polar region may contain sensitive records of past polar climate that may enhance our understanding of the CO 2-H 2 O system in the polar regions.

Three‐dimensional structure and origin of a 1.8 km thick ice dome within Korolev Crater, Mars

“…The asymmetrical deposition and domed reflector shape support the second hypothesis that Korolev's central mound was formed by in‐place deposition, a finding consistent with optical layer mapping results and a single radar profile [ Conway et al ., ]. However, this alone does not resolve the question of age.…”

“…The unconformities are truncations of radar reflectors as pinchouts, downlap, or angular unconformities (Figure d). Both Korolev Crater and the NPLD share these shallow unconformities [ Tanaka , ; Tanaka et al ., ; Conway et al ., ]. In addition to similar reflector density and stratigraphic patterns, the overall thickness of Korolev's ice is comparable to that of the NPLD.…”

A New Analysis of Mars “Special Regions”: Findings of the Second MEPAG Special Regions Science Analysis Group (SR-SAG2)

“…As the seasonal caps retreat in the spring, frost outliers composed of both CO 2 and H 2 O ice are left behind, often within topographic depressions such as impact craters (Kieffer et al, 2000;Armstrong et al, 2005Armstrong et al, , 2007Titus, 2005;Conway et al, 2012). The region surrounding the north polar cap largely comprises the Vastitas Borealis Formation, which is interpreted as being composed of icerich fine-grained (dust) deposits and ice-rich sediments from ancient fluvial activity .…”

Evolution of major sedimentary mounds on Mars: Buildup via anticompensational stacking modulated by climate change

“…If this is correct, then mounds grew in place by net deposition of layers on preexisting topographic highs (anticompensational stacking) [e.g., Michalski and Niles, 2012;Kite et al, 2013a]. This distinctively Martian mechanism is suggested by (i) growth of polar ice + dust + sand mounds by anticompensational stacking [Holt et al, 2010;Conway et al, 2012;Brothers et al, 2013;Brothers and Holt, 2016]; (ii) the importance of aeolian sediment transport and slope winds on modern Mars Spiga, 2011;Kok et al, 2012;Bridges et al, 2013;Silvestro et al, 2013;Kite et al, 2013a]; and (iii) the strong inference of layered-sediment accumulation via anticompensational stacking for some Mars equatorial layered sediments (the Medusae Fossae Formation) [Bradley et al, 2002;Zimbelman and Scheidt, 2012;Kite et al, 2015]. Dry conditions bring aeolian processes to the fore, whereas vigorous and sustained fluvial erosion would inhibit mound construction.…”