The choice of test conditions in this study reflects the extreme environment of the lunar polar regions. Permanently shadowed regions at the poles never receive direct sunlight, and surface temperatures there remain low enough that water-ice can survive for billions of years even under ultrahigh vacuum, sublimating only very slowly. To replicate these conditions, samples were held at approximately 93 K while the probe itself was varied across a range from 83 K to 163 K. Maintaining such temperatures during mechanical testing is nontrivial, and the experimental design addressed this with careful thermal management, including thermally isolating components so that sensitive instrumentation such as the load cell remained near room temperature even while the probe tip was immersed in cryogenic conditions. This attention to thermal boundary conditions matters because the mechanical behavior of ice is strongly temperature dependent, and uncontrolled warming during a test could substantially distort the measured strength.

A central contribution of the work is its systematic comparison of four distinct icy regolith morphologies prepared from the same lunar highlands simulant. Ice-cemented samples are made by freezing liquid water into a granular mineral matrix, producing a material in which ice acts as a cement between particles, analogous to the way cement binds concrete. Pressure sintered samples instead combine granular ice and dry regolith compacted together, yielding a more granular, less cemented texture. Unsintered samples represent loosely mixed material, while vapor deposited samples simulate ice that has condensed directly from vapor onto cold regolith surfaces, a process thought to be relevant in permanently shadowed regions where ice may be deposited from transient atmospheres or impact-generated vapor. Because no in-situ observations have yet been made within a lunar PSR, the actual form of lunar ice remains unknown, and each of these morphologies represents a plausible candidate for what future missions may encounter.

The finding that microstructure dominates geotechnical behavior has significant implications for how the community interprets prior simulant studies. Previous research had already hinted at enormous variability: ice-cemented simulants showed sharp strength increases at critical ice contents of roughly 1 and 3 weight percent, and ice-enriched material above about 12 weight percent was reported to behave like hard rock or concrete with strengths exceeding 100 megapascals. In contrast, pressure sintered mixtures at 10 weight percent ice were far weaker, below 14 megapascals, and showed no abrupt strength transitions. The new controlled measurements confirm that even under identical ice contents, densities, and temperatures, different preparation methods can produce materials whose strengths differ by orders of magnitude. This means that the preparation recipe, not merely the ice content, controls mechanical response.

The quantitative trafficability results are particularly striking. When cone index gradients were corrected for lunar gravity and normalized to a common penetration depth, a vapor deposited sample containing 6.88 weight percent ice proved 3.92 times stronger than a pressure sintered sample with 8 weight percent ice, and even 1.23 times stronger than the upper bound of in-situ cone index values measured in the Descartes Highlands during Apollo 16. This comparison anchors the laboratory measurements to the only direct penetrometer data ever collected on the lunar surface, providing a useful reality check. It suggests that some icy morphologies could produce surfaces that are at least as firm as the dry regolith traversed by Apollo astronauts, which is encouraging for rover mobility planning in polar terrain.

The excavation energy results reveal a more nuanced picture, with opposing trends depending on morphology. For pressure sintered samples, the energy required per unit mass of excavated water fell from 0.35 joules per gram at 1 weight percent ice to 0.11 joules per gram at 30 weight percent, a 3.2-fold improvement in efficiency. This counterintuitive result arises because richer ice deposits contain proportionally more water per unit of material excavated, so the energy cost attributable to each gram of recovered water declines even if the material itself is somewhat harder to dig. For ice-cemented samples the trend runs in the opposite direction, with excavation energy climbing from 2.74 joules per gram at 1 weight percent to 143.03 joules per gram at 12 weight percent, a 52-fold increase. The cementing action of ice between mineral grains makes the material progressively harder to break apart, overwhelming the benefit of higher water content.

These opposing trends carry direct consequences for the economics of lunar water extraction. In-situ resource utilization concepts that envision mining polar ice to produce propellant, drinking water, and oxygen depend on favorable energy budgets for excavation and processing. If lunar ice occurs predominantly in sintered, granular, or vapor deposited forms, the study’s results suggest that excavation may be considerably more economical than many hardware development programs have assumed, since tests based on ice-cemented simulants would have systematically overestimated the resistance that mining equipment must overcome. Conversely, if substantial fractions of polar ice are genuinely cement-like, excavation energy demands could be far higher than optimistic projections. Because the true morphology is unknown, mission planners face a wide envelope of possible outcomes, and this work helps bound that envelope with quantitative data.

The implications extend to the validation of flight hardware. Robotic excavators, drills, and mobility systems destined for the lunar poles are typically qualified against simulant beds in terrestrial laboratories. If those beds are made with ice-cemented recipes, the hardware will be tested against a material that may be far stronger than what it actually encounters, leading to over-engineered systems with unnecessary mass and cost. On the other hand, hardware validated only against weak sintered simulants could fail catastrophically if it encounters cemented ice. The authors’ observation that ice-cemented simulants, rocks, and concrete analogues may be unsuitable as general-purpose proxies for all icy regolith morphologies therefore challenges a widespread practice in the field and argues for morphology-specific testing campaigns matched to the best available scientific estimates of how lunar ice actually forms.

The study also addresses strategic knowledge gaps formally identified by the International Space Exploration Coordination Group, including the geotechnical characteristics and accessibility of permanently shadowed regions, the form and distribution of frozen polar volatiles, and trafficability across the lunar surface. Missions such as the Volatiles Investigating Polar Exploration Rover and the crewed Artemis program will operate in precisely the environments where these uncertainties matter most, traversing and sampling terrain whose mechanical properties cannot yet be observed directly. Quantitative laboratory measurements of the kind reported here provide a bridge between remote sensing indications of ice and the engineering parameters, such as bearing capacity and excavation resistance, that determine whether wheels will sink, slip, or find firm purchase.

Methodologically, the work demonstrates the value of miniature cone penetration testing as a compact, information-rich technique for cryogenic simulant characterization. The apparatus described, combining a small stainless steel cone with a low-capacity load cell mounted on a universal test machine, achieves sufficient precision to resolve differences between morphologies while remaining simple enough for routine laboratory use. The inclusion of a thermocouple within the hollow cone tip allows probe temperature to be monitored and controlled independently of sample temperature, an important capability given that probe-sample friction and ice deformation behavior both depend on temperature. Calibration against a certified reference load cell further strengthens confidence in the absolute values reported.

Looking forward, the results suggest several productive avenues for the field. First, confirming the morphology of lunar ice through in-situ measurements, whether by penetrometers on future rovers or by sample return from polar regions, would dramatically narrow the uncertainty in excavation and mobility predictions. Second, extending this experimental framework to other volatiles, such as carbon dioxide ice, and to mixtures of multiple ices, would broaden its applicability to other cold solar system bodies including Mars, comets, and the moons of the outer planets. Third, relating cone index measurements to full-scale excavation forces and wheel-soil interaction models would help translate these laboratory parameters into mission-level engineering requirements. In the meantime, the demonstration that icy lunar regolith may be easier to excavate and traverse than previously thought offers a cautiously optimistic outlook for the emerging economy built around lunar polar resources, while reminding the community that the ground truth beneath the shadows remains one of the most important open questions in planetary exploration.

Subject of Research: Trafficability and excavatability of icy lunar regolith simulants quantified using cone penetration

Article Title: Trafficability and excavatability of icy lunar regolith simulants quantified using cone penetration

Article References: Ricardo, D., Hodgkinson, J., Rhamdhani, M. A., & Brooks, G. (2026). Trafficability and excavatability of icy lunar regolith simulants quantified using cone penetration. Space and Planetary Resources, 2(1), Article 9. https://doi.org/10.1007/s44461-026-00015-w

Image Credits: AI Generated

DOI: 10.1007/s44461-026-00015-w

Keywords: Trafficability, excavatability, lunar, regolith, simulants, quantified, cone, penetration, scientific research