A Flexible Geophysical Payload Suite for Human and Robotic Lunar Exploration

Moon's subsurface holds critical clues about the history of the Solar System and the formation of our natural satellite. Beneath the lunar surface lies a complex, highly fractured layer of regolith and megaregolith, typically extending to few kilometers in depth before giving way to more solid crustal material. This layered underground structure is the product of billions of years of impact cratering and volcanic activity. Hidden within it are geological features such as volcanic strata, ancient craters, voids, fractures, and particularly in the polar regions potentially deposits of water ice.

Investigating this subsurface environment non-invasively and with high resolution is one of the central challenges of contemporary lunar science, and it is precisely the challenge that the INAF-ASI Multi-scale Geophysical Imaging of Lunar Subsurface payload concept aims to address.

Developed in-house at the INAF-IAPS laboratories in Rome, Italy, this payload suite is conceived as a flexible, reconfigurable instrument system designed to operate across a wide range of lunar environments, from mid-latitude terrains to the scientifically and strategically important polar regions. The suite is intended to be deployed either by a robotic platform or by a human crew, reflecting the evolving and diverse landscape of modern lunar exploration. Its design philosophy centers on adaptability: the system is engineered to support multiple operational scenarios, accommodate different spacecraft interfaces, and scale to varying mission requirements.

The payload suite combines two complementary geophysical investigation methods. The first is a Ground Penetrating Radar (GPR), which provides active subsurface imaging, and the second is a magnetometer (MAG), enabling passive geophysical measurements. Together, these instruments cover a depth range from a few meters down to the first hundred meters of the lunar subsurface, with spatial resolutions ranging from tens of centimeters to several meters depending on the specific instrument and mode of operation. This multi-scale, multi-method approach is a key strength of the concept, enabling scientists to characterize underground across different scales improving our interpretive confidence of the lunar geology more than either instrument could provide alone.

The GPR component is being developed using Software Defined Radio (SDR) technology, implementing a Frequency Modulated (FM) radar signal spanning a broad frequency range from 200 to 1200 MHz. This wide bandwidth enables high-resolution imaging of shallow subsurface features while still providing sufficient penetration depth for meaningful geological investigation. The adoption of SDR technology is particularly advantageous in the context of technological and scientific research in space instrumentation: it offers a compact and lightweight hardware implementation, with modest power consumption. The current prototype for the GPR electronic unit is expected to weigh less than 500 grams, occupy a volume of approximately 300 cubic centimeters, and consume around 10 watts or less depending on the operational mode. The instrument's software is written in ANSI C and runs on a GNU/Linux-based embedded real-time platform. Crucially, the software stack leverages state-of-the-art Free and Open Source Software (FOSS) libraries and implementing open protocols, ensuring that the system can be readily adapted to different spacecraft payload interfaces.  This is a significant practical advantage given the diversity of platforms involved in current and planned lunar missions and interoperability is key.

The MAG component complements the radar by enabling the detection and characterization of magnetic anomalies in the lunar subsurface. It is based on a compact design specifically optimized for identifying the shape and depth of complex, three-dimensional underground structures. Testing of the MAG prototype will make use of INAF-IAPS's dedicated magnetic chamber facility, which is capable of nulling the ambient magnetic field to simulate the near-zero magnetic environment found at the lunar surface — an essential step in validating the instrument's performance under realistic conditions.

Both instrument prototypes are being built using commercially available, off-the-shelf (COTS) components wherever possible. This approach significantly accelerates the prototyping and testing cycle while reducing development costs, making efficient use of the project's 24-month timeline. By the end of our initial efforts, both components are expected to reach Technology Readiness Levels (TRL) 3 to 4, tentatively ready for terrestrial analog testing and providing a cost-effective precursor to the higher-fidelity validation required for TRL 5 and 6.

The scientific objectives driving this payload concept are organized around three main priorities. The first is to advance understanding of Solar System history and lunar formation by characterizing the megaregolith structure at the scale relevant to surface exploration activities. The second is to address specific geological questions about the lunar underground, including the origin and nature of magnetic anomalies and geologic features as layers or and cavities. The third, and perhaps most operationally significant, is to support future human and robotic exploration by enabling the identification of subsurface resources, particularly water ice, and the detection of potential hazards such as voids or unstable layers, ideally in coordination with orbital reconnaissance missions that can guide ground-level surveys toward the most promising areas.

Our project wants to contribute to the new era of lunar exploration providing both the instruments and the methodological framework needed to probe the Moon's hidden underground in ways that will benefit science and human presence on the lunar surface alike.

Acknowledgments: This study is supported by ASI-INAF agreement no. 2025-20-HH0.