Martian dunes indicative of wind regime shift in line with end of ice age

The Martian climate history remains complex and elusive due to the interplay of atmospheric conditions, weather patterns, and orbital parameters. While forward modeling attempts exist, limitations in the early geological record hinder comprehensive understanding. Inverse modeling, utilizing current conditions and recent geological records (polar deposits), offers a more robust approach, particularly given the predictable orbital and spin-axis conditions over the last 20 million years. This approach has identified a recent ice age characterized by increased obliquity, mobilizing polar ice and depositing a thick ice-dust mantle to mid-latitudes. A decrease in obliquity around 0.4 million years ago is believed to have initiated an interglacial period, returning marginal ice to the polar layered terrain. However, in-situ surface observations to support this glacial-interglacial transition at lower to mid-latitudes, reconcilable with atmospheric circulation models, have been lacking. The Zhurong rover, landing in the critical LDM transition region (25.066°N) of Utopia Planitia, offers the opportunity to address this gap through direct observation of aeolian features. Its traverse across 1921 meters, perpendicular to the LDM margin, allows in-situ analysis of aeolian features, providing data crucial for validating and refining atmospheric model predictions and understanding recent climate change hypotheses.

Decades of research have investigated Martian aeolian features, particularly transverse aeolian ridges (TARs). However, their formation mechanisms, ages, and contribution to the global sediment cycle remain poorly understood, hindered by the lack of in-situ meteorological and environmental data. General circulation models (GCMs) have struggled to reconcile predicted wind fields with TAR observations, underscoring the need for detailed rover-scale data to improve model accuracy. Previous rover missions have primarily focused on low-latitude sites, failing to cover the most dynamic regions (southern LDM margin). The existing literature on Mars's aeolian features demonstrates a need for high-resolution, in-situ data to understand the formation and evolution of these features, particularly in relation to climate change and orbital variations. Past work suggests that changes in Mars's orbital parameters, particularly obliquity, have played a significant role in shaping its climate history, and studies on polar layered deposits offer insights into past climate cycles.

The Zhurong rover, equipped with the Navigation Terrain Camera (NaTeCam), Mars Surface Composition Detection Package (MarSCoDe), and Multispectral Camera (MSCam), traversed the Utopia Planitia region. The study focused on analyzing five bright barchan dunes and their associated dark longitudinal dunes. High-resolution imagery from NaTeCam and MSCam provided morphological information on dune structure, surface features (cracks, crusts), and particle size distribution. MarSCoDe, using Laser-Induced Breakdown Spectroscopy (LIBS), determined the chemical composition of bright and dark sands. Shortwave infrared (SWIR) spectroscopy was employed to identify hydrated minerals (sulfates, chlorides). The orientations of bright barchans, longitudinal dunes, wind erosion traces, and ventifacts were measured to infer wind directions during different periods. Crater counting on the bright barchans, using size-frequency distribution methods, was employed to estimate their age. This detailed multi-faceted approach combining in-situ imagery, spectroscopy, and crater counting enabled a comprehensive reconstruction of the region's aeolian history.

The Zhurong rover's observations revealed a two-stage aeolian regime in the Utopia Planitia region. The earlier stage featured isolated, bright barchan dunes oriented to indicate a predominant northeast wind (12.5° azimuth). The bright barchan surfaces were encrusted, suggesting a period of relative stability and low sediment supply. The later stage involved the erosion and partial covering of the bright barchans by dark longitudinal dunes indicating a shift to a predominant northwest wind (300° azimuth). The dark sands contained coarser agglomerated particles, suggesting cementation of finer grains possibly due to brine involvement. The wind direction shift between the two regimes (approximately 70°) indicates a significant change in atmospheric circulation. Crater counting estimated the age of the bright barchan formation to between 2 and 0.4 million years ago, aligning with the end of the proposed Martian ice age. The superimposition of the dark longitudinal dunes on the bright barchans demonstrates the subsequent activity of the second regime within the modern climate.

The findings strongly support the hypothesis of a significant climate transition on Mars related to obliquity changes. The shift from northeast to northwest winds, the distinct morphology of the bright barchans and dark longitudinal dunes, and their estimated ages are consistent with the predicted glacial-interglacial transition based on orbital data and polar layered deposits. The observed cementation of dune materials by brines and the changes in wind patterns are likely linked to variations in temperature, humidity, and atmospheric dust concentrations associated with obliquity changes. These results demonstrate the value of in-situ rover observations in providing fine-scale details on climate change that complement and enhance information from global climate models (GCMs). While GCMs may not capture the seasonal variations in wind direction, this study highlights the importance of those finer-scale details.

The Zhurong rover's observations in Utopia Planitia provide compelling evidence of a significant wind regime shift on Mars coinciding with the end of a recent ice age. The two-stage aeolian sequence, with its distinct dune morphology and inferred wind directions, aligns with predictions from climate models and orbital data. This study highlights the value of in-situ, multi-disciplinary rover investigations in refining our understanding of Mars's climate history. Future research could focus on higher-resolution analysis of dune features, extending these observations to other regions to assess the global extent of this climatic transition, and on more detailed investigations into the role of brines and dust in the cementation and albedo changes of the dunes.

The age determination using crater counting on a limited area of barchan dunes has inherent uncertainties. While the results strongly suggest a correlation between the wind regime shift and the end of the ice age, further research with more extensive crater counting across a larger area is needed to strengthen this conclusion. The interpretation of the cementation processes relies on indirect evidence from particle size analysis and spectral data. Further investigations are necessary to confirm the specific mechanisms involved and the role of brines and other factors.