Research progress and prospects on piezocone penetration testing mechanisms and applicability under environmental constraints of Moon and Mars

Abstract

[Objective] Cone penetration testing(CPT) and its pore-pressure extension, piezocone penetration testing(CPTU), have formed a relatively mature observation-interpretation framework in terrestrial geotechnical engineering, with cone tip resistance(q_c), sleeve friction(f_s), and pore pressure(u₂) as the core measured parameters. When terrestrial CPTU is extended to shallow subsurface exploration on the Moon and Mars, factors such as low gravity, near-vacuum or rarefied atmospheric conditions, intense thermal cycling, and ice-bearing, saline, cemented, and unsaturated regolith can systematically alter stress paths, boundary conditions, measurement-chain stability, and the interpretability of measurement responses during penetration. To meet the demands of lunar water resource exploration and Martian shallow subsurface engineering investigation, the response mechanisms, applicability boundaries, and technical development directions of CPTU-like techniques under lunar and Martian environmental constraints are systematically reviewed. [Methods]Taking the terrestrial CPTU interpretation framework as the reference baseline, the analysis follows the main line of “environmental constraints-measurement chain-response indicators-interpretation boundaries. ” It examines three representative mission scenarios: ice-bearing regolith in the lunar polar regions, near-surface engineering conditions in non-polar lunar regions, and sampling/drilling scenarios in candidate Martian plains. On this basis, the impacts of low gravity, vacuum or rarefied atmosphere, ice-/salt-bearing and unsaturated media, and existing mission observations on the formation mechanisms and interpretation framework of q_c, f_s, and u₂ are synthesized. Additionally, possible directions for system improvement, multi-sensor joint constraints, ground-equivalent calibration, and probabilistic interpretation under small-sample conditions are discussed. [Results] The impact of low gravity is not merely limited to reducing the in situ stress level. It also alters the reaction force supply, attitude stability, and penetration boundary conditions, making q_c and f_s more prone to exhibiting abrupt or threshold-type responses. This undermines the direct applicability of terrestrial empirical charts and continuous parameter inversion relationships. Near-vacuum or rarefied atmospheric conditions and thermal cycling significantly affect the stability of the pore-pressure measurement chain, making u₂ more suitable as a conditional auxiliary observation parameter in extraterrestrial scenarios that can be used only after state diagnosis. Under the near-vacuum conditions on the Moon, when its physical attribution remains unclear, u₂ should not be directly used for the inversion of soil mechanical parameters. The presence of ice, salts, cementation, and layered structures further enhances the non-uniqueness of penetration responses. Given the current evidence, layer identification, interface identification, and event-type response analysis are generally more appropriate as priority interpretation approaches under complex media conditions. Existing mission data can provide external constraints on bearing capacity, shallow structure, and penetration boundary conditions, but they cannot directly replace the standard CPTU calibration system. [Conclusion] For lunar and Martian applications, the key to CPTU-like investigation does not lie in simply replicating the terrestrial three-channel configuration, but in reconstructing a mission-oriented framework that integrates the measurement chain, penetration boundary conditions, multi-source constraints, and interpretation/evaluation. Future efforts should focus on ground-equivalent calibration under low-gravity conditions, validation of pore-pressure measurement stability under vacuum and thermal cycling, investigation of penetration mechanisms and applicability boundaries in ice-/salt-bearing regolith, and development of ground-equivalent databases and tiered uncertainty-based interpretation schemes, so as to support the advancement of CPTU-like techniques for lunar water resource exploration and extraterrestrial shallow subsurface engineering investigations.

Keywords

CPTU / lunar/Martian environment / penetration mechanism / in situ testing / lunar regolith / Martian soil / shallow subsurface exploration / influencing factors