• The initial loess deposit has an extremely loose particle packing structure. • The loose packing governs by strong inter-particle forces during aeolian deposition. • Initially loose aggregates and overhead pores crucially shaped today’s porous loess. Aeolian deposition is fundamental to the formation of the porous structure of present-day loess. However, the true initial packing structure of loess dust remains poorly understood. In this study, we reconstructed initial loess deposits by simulating the dust deposition process, using Malan loess, which is compositionally analogous to ancient dust, as the raw material. The microstructure of the simulated deposits was characterised using micro-computed tomography (μ-CT) scanning and compared with that of natural Malan loess. The results indicate that the initial loess deposits possessed an exceptionally loose particle packing, with void ratios ranging from 2.79 to 3.75. Loose clay or silt–clay aggregates formed extensively due to inter-particle forces (such as van der Waals force and electrostatic attraction) during deposition. These aggregates, along with isolated detrital particles, acted as the primary skeletal components. Surface clays on skeletal particles play a critical role in bonding adjacent particles, which is essential for establishing and stabilising the loose framework. Notably, the initial deposits with an intermediate clay content (23%) among the samples studied exhibited the loosest microstructure, featuring chain-like particle packing and abundant large overhead pores. The initially loose aggregates and open packings compact over geological time while retaining structural inheritance, resulting in the reduced but partially preserved pore space that shapes the microstructure of present-day loess. This study provides the first direct 3D visualisation and quantification of the initial particle-scale microstructure, offering a key reference for understanding subsequent post-depositional processes and associated geomechanical properties.

• A new air mill device simulates the effects of aeolian transport on sand grains. • Design is optimized for aeolian research, resolving limitations of previous devices. • The device is validated by similitude measurements and microtexture experiments. • Applications include sand abrasion/dust production experiments for Earth and Mars. On Earth and Mars, aeolian transport causes sand grains to become abraded, resulting in mineralogic and textural changes. Understanding how sands evolve, or mature, with transport via experimental studies is important for understanding the origins, geologic history, and cycling of sediments, as well as dust production. Previous experimental works have used a variety of methods to simulate aeolian transport in the laboratory, but practical limitations and similitude concerns have limited such research. Here, we present and validate the Sand AbrasioN Device for Aeolian Research (SANDAR), a modified air mill that uses pressurized air to circulate sand around a small abrasion chamber, simulating the effects of aeolian transport. This device is re-circulating to simulate long-distance transport, and it allows for repeated analyses of well-constrained sediment samples, revealing their evolution over time. It is compatible with the grain sizes (74–500 μm) and grain impact velocities (∼0.6–3.7 m/s) typically expected for natural aeolian environments, and is also adaptable for diverse applications simulating different wind conditions. We show that the SANDAR achieves similitude of kinetic energy with respect to saltating sand on both Earth and Mars. SEM and optical microscope imaging reveal that the SANDAR produces microtextures on the surfaces of sand grains similar to those found with natural aeolian transport, demonstrating that it effectively simulates the mechanical effects of aeolian processes. Thus, the SANDAR is a valid tool for use in experimental research to improve our understanding of sedimentary processes across the Solar System.