Development of an active seismic experiment for lunar exploration

Kurzfassung

The subject of this thesis was the evaluation of historical Apollo 17 Lunar Seismic Profiling data in combination with the design and operation of a new active seismic experiment for planetary applications. During the Apollo program, running between 1961 and 1972, there were a total of six manned landings. Astronauts deployed a number of scientific instruments on the Lunar surface, some of which continued operation long after the Apollo missions. Among these was the Lunar Seismic Profiling Experiment (LSPE) set up by astronauts on Apollo 17, which consisted of four geophones in a Y-shaped array and eight explosive packages. The setup was used to register the signals of the eight remotely detonated explosions as well as to passively listen for natural seismic signals. To date, this setup represents the largest seismic experiment conducted outside of the Earth. In 2009, the Lunar Reconnaissance Orbiter (LRO) was launched with the task of mapping the lunar surface in high resolution. Special attention was paid to a precision mapping of the Apollo landing sites. It turned out that the positions of the Apollo instruments differed substantially from the previously determined positions, thus requiring updates of the previously determined seismic velocity-depth profiles. The first part of this work was to go back to the original bitstreams of the Apollo data to carry out new arrival time readings for the seismic P-waves. Using the new positional data of the seismic sources and receivers, these were inverted to derive a new velocity-depth profiles. The comparison with historical results showed that the use of the corrected distance data led to a significant reduction in layer thicknesses and a stronger increase in P-wave velocity with depth. Overall, this resulted in a stronger compaction of the subsurface material than previously assumed being indicated. In the second part of this thesis, an active seismic experiment was developed and operated, which was largely based on the idea of the Apollo 17's LSPE. The experiment scenario was now to be set up and carried out by autonomous robotic systems. DLR Bremen developed two autonomous measurement systems, what we called “Remote Units” (RU), for this scenario. The Mascot design of the Japanese Hayabusa2 probe proved to be a suitable basis for this development. Following a number of laboratory tests, the RUs were brought to application in the context of the demo mission “Space” of the Helmholtz Alliance "ROBEX", for which a terrain on Mount Etna in Italy was chosen as the experiment site. Seismic data were successfully obtained, and the evaluation of the data confirmed earlier results on the geology and subsurface structure of the test area determined with standard methods. Thus, the evaluation of the seismic data could not only show that the developed experimental scenario and equipment were suitable to explore near-surface stratifications by means of refraction seismic experiments, but also that the selected test area on Mount Etna, featuring strata of lava flows in the subsurface, actually qualified as a lunar analogue terrain.