Estimating the age of ice in a Martian mid-latitude debris-covered glacier from numerical modelling and particle tracking

Mars' mid latitudes contain thousands of ‘viscous flow features’ (VFFs), akin to debris-covered glaciers on Earth. They are thought to have formed during martian ‘ice ages’, driven by variations in Mars' spin-axis obliquity. Key to understanding the nature and timing of such glacial cycles, and the palaeoclimate histories they reflect, is knowledge of the emplacement age of ice within VFFs. Current methods to estimate VFF surface ages, which place VFF formation broadly within the last few Myr to 100 s Myr, predominantly rely on the size-frequency distributions of impact craters across their surfaces. However, these ‘impact crater retention ages’ likely reflect the time since the emplacement or last major modification of the surficial debris layer; they implicitly assume a uniform age across the sampled area. They also provide no direct information about the emplacement ages of the underlying ice layers, the configurations (and hence age distributions) of which are likely to have been modified during transit by ice flow. Here, we develop a new, physically-based method to reconstruct the flow paths and transit times of ice within VFFs, and hence estimate variations in the minimum age of ice across their (now debris-covered) surfaces, and with depth. We use 3-dimensional ice flow modelling and particle tracking, and apply our method to a small VFF in Mars' southern mid-latitudes. Our method produces spatially-variable near-surface ice age estimates which range from very young (< 1Myr) in the upper parts of the VFF to ~500 Myr close to the VFF terminus, assuming current martian temperatures and a conventional ice rheology. Toward the terminus, the calculated surface ages increase rapidly over short distances as compressional ice flow transports old, deep ice upwards toward the surface. The distributed 3D age estimates provided by our method also allow prediction of the depths and configurations of isochronous layers within the VFF. The spatial patterns we find are insensitive to the assumed ice deformation mechanism, but the specific calculated ages are highly sensitive (> 2 orders of magnitude) to ice temperature and grain size, which emerge as the main controls on modelled ice flow velocities, and hence the estimated ages. Our results have significant implications for identifying landing sites and ice sampling strategies for future missions which could extract climate records potentially hosted within glacial ice layers on Mars. The significant variations we find in the age of ice across the VFF surface, arising from the flow-induced deflection of ice layers up to the surface, suggest that such missions could access ice with a large range of ages (and hence potentially longer-timespan climate records) by sampling from shallow depths across the surface a single VFF. • We develop a physically-based method to estimate age of ice within martian glaciers. • We apply our method to a kilometre-scale glacier in Mars' southern hemisphere. • Modelled flow patterns match well with visible surface and englacial structures. • Estimated ice age increases toward the downstream VFF margin and with depth. • The method complements crater retention age estimates, at smaller spatial scales. • Shallow ice sampling by possible future missions should allow access to old ice.