High-fidelity lunar regolith simulants are essential for in situ resource utilization (ISRU) construction and technology development including, microwave sintering, melt-based processing, and 3D printing. However, existing simulants often fail to capture key compositional and thermophysical features of actual lunar soils, particularly the abundance of glass and agglutinates that govern many of their geotechnical and thermodynamic behaviors. These limitations are compounded in highland simulants, many of which include a mostly crystalline Merriam Crater basalt component to supplement their glass fraction, despite the near absence of crystalline mare basalt in Apollo 16 regolith (Korotev, 1997, MAPS 32, 447–478).

To evaluate the thermodynamic fidelity of existing simulants, we conducted high-temperature differential scanning calorimetry (DSC) measurements on both mare and highland simulants to generate a comparative library of melting and crystallization behaviors. These results were benchmarked against newly acquired DSC data from Apollo soil samples (Patridge et al., 2025, GSA abs.). Most highland simulants exhibit a melting behavior that differs from Apollo 16 soils. This discrepancy stems from the widespread use of Merriam Crater basalt (CPX dominant) that melts ~110 °C earlier than actual noritic highland soils and their impact melt components (OPX dominant). In contrast to most highland simulants the NU/NUW-LHT simulant series does not include basalt, and because of this, more closely replicate the melting behavior of Apollo 16 soils underscoring the importance of not only elemental, but mineralogic, and petrologic fidelity in simulants. Based on the results, we identify the NU-LHT series, NUW-LHT-5M (and its prototypes), as the most thermodynamically representative highland simulants. For mare soils, JSC-1A approximates the melting behavior of Apollo 11 10084, but underestimates the enthalpy of fusion, likely due to its lower TiO₂ content. MLS-1 provides a closer match to the lunar enthalpy of fusion, but is holocrystalline.

To improve fidelity, future simulants should: avoid the use of volcanic basalts like Merriam Crater, which melt too early in sintering systems and fail to replicate the mafic impact melt component of highland regolith; incorporate a higher fraction of manufactured glass to better represent the amorphous content of lunar soils; and, for the highest realism, undergo artificial space weathering and irradiation. These modifications would enhance their susceptibility to microwave sintering and better replicate the thermal response and sintering behavior of genuine lunar regolith.