The ESA/DLR LUNA Habitat as geophysical experimentation facility

Abstract

Recently, NASA’s InSight mission has shown the value of geophysical landers by greatly increasing our knowledge of the interior of Mars. Correspondingly, geophysical experiments are also of great relevance to lunar exploration: a number of geophysical experiments were proposed in response to the ESA's 2020 call for ideas for a scientific utilization of the large logistics lander (Argonaut). Geophysical payloads are already planned for the Moon, e.g. the Farside Seismic Suite will land a broad-band seismometer in 2025. We here present how the LUNA Habitat training facility under construction in Cologne, Germany, can contribute to the development and testing of lunar geophysical instrumentation.The about 700 square meters of the LUNA Habitat will be covered by 60 cm of EAC-1 regolith simulant on most of the area. On an area of 140 square meters, regolith depth increases to 3 m along a sloping bottom (25° and 40°). This part of LUNA provides an invisible, but explorable underground structure suitable for seismic profiling, ground penetrating radar, geoelectrics, geomagnetics and other techniques, as well as sufficient depth for drilling, subsurface sampling, and deployment of heat flow probes. Sculpting craters and even caves in the regolith, as well as cooling small portions of it, is envisioned. Support by the facility will include personnel with experience in geophysical measurements and data analysis, an end-to-end operational environment including a remote control center with standard communication technology, and, last but not least, training of astronauts in co-operation with robotic units to operate the equipment in lunar surface suits and under gravity offloading.A four-element, Y-shaped array of short period seismometers, based on the layout of the Apollo 17 seismic experiment, will be deployed on the LUNA construction site before erecting the building to record seismic noise sources (car traffic on the DLR campus, the ENVIHAB short arm centrifuge, wind tunnel discharges, air traffic on the nearby CGN international airport etc.). It will also allow for ambient noise analysis aimed at the underground structure, which is expected to consist of Rhine sediments. An active refraction seismic experiment and the deployment of 12 nodal sensors will further aid in site characterization. LUNA will have a concrete floor of up to 60 cm thickness, but with a structured underside for static reasons. The array will be re-deployed on the concrete once the hall is erected to characterize in how far the new high-velocity layer hides the underlying sediments from seismic observation. After completion of LUNA, the effect of the regolith cover on seismic recordings will be characterized by a third array deployment. Documentation of construction details, especially steel enforcing in the concrete, is foreseen.  A broad-band seismometer will be installed in the LUNA Habitat permanently, once construction is finished, to support the identification of artificial noise sources and local seismicity in the recordings of customer instruments, and monitor possible changes in the background e.g. due to new buildings or other large-scale research facilities on the DLR campus.