Development of an active seismic experiment for lunar exploration

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

Re-evaluation of Apollo 17 Lunar Seismic Profiling Experiment data

We re-analyzed Apollo 17 Lunar Seismic Profiling Experiment (LSPE) data to improve our knowledge of the subsurface structure of this landing site. We use new geometrically accurate 3-D positions of the seismic equipment deployed by the astronauts, which were previously derived using high-resolution images by Lunar Reconnaissance Orbiter (LRO) in combination with Apollo astronaut photography. These include coordinates of six Explosive Packages (EPs) and four geophone stations. Re-identified P-wave arrival times are used to calculate two- and three-layer seismic velocity models. A strong increase of seismic velocity with depth can be confirmed, in particular, we suggest a more drastic increase than previously thought. For the three-layer model the P-wave velocities were calculated to 285, 580, and 1825 m/s for the uppermost, second, and third layer, respectively, with the boundaries between the layers being at 96 and 773 m depth. When compared with results obtained with previously published coordinates, we find (1) a slightly higher velocity (+4%) for the uppermost layer, and (2) lower P-wave velocities for the second and third layers, representing a decrease of 34% and 12% for second and third layer, respectively. Using P-wave arrival time readings of previous studies, we confirm that velocities increase when changing over from old to new coordinates. In the three-layer case, this means using new coordinates alone leads to thinned layers, velocities rise slightly for the uppermost layer and decrease significantly for the layers below

Re-evaluation of Apollo 17 Lunar Seismic Profiling Experiment data including new LROC-derived coordinates for explosive packages 1 and 7, at Taurus-Littrow, Moon

We re-analyze data from the Apollo 17 Lunar Seismic Profiling Experiment (LSPE) using updated locations of the applied explosive sources. Specifically, we complement our models with the previously missing coordinates and P-wave arrival times of Explosive Packages EP1 and EP7. We read new P-arrival times for all eight EP events, and solve for two- and three-layer seismic velocity models. We confirm a strong increase of seismic velocity with depth. In particular, we suggest a more drastic increase than was previously thought from post-mission coordinate information. For the three-layer model we find P-wave velocities of 315, 580, and 2680 ​m/s for the uppermost, second, and third layers respectively, with the transitions between the layers being at depths of 110 and 855 ​m. When compared with previous results, we find (1) a slightly higher velocity (+10.5%) for the uppermost layer, (2) no differences in velocity for the second layer, and a (3) significantly higher P-wave velocity for the third layer (+46.9%)

System design and laboratory tests of an autonomous seismic station for space applications

We introduce a reference concept for a small and lightweight instrument carrier system for operation in isolated areas on Earth (or even on other celestial bodies) that autonomously records data after deployment at a site remote from the main station, thus called Remote Unit (RU). In particular, we present here concepts for realizing an autonomously operating seismometer, including support functions provided by encapsulating the actual instrument into the RU carrier. The conceptualization of the RU is based on the design of MASCOT, and intended for evolving into an instrument carrier for lunar exploration. Still in this paper we focus on the functionality needed for realizing the remote and autonomous aspects of the concept. Evolutionary steps needed for the system design to survive on a planetary body are briefly discussed, but not intensively covered in this paper, but elsewhere. We developed two prototypes of this RU including three seismic sensors each, either fixed, the other one with a built-in self-leveling mechanism. We used standard seismic sensors, which were integrated into a lightweight instrument carrier equipped with all required support structures for remote terrestrial operation, including power, thermal control, and data acquisition (total mass of prototypes not exceeding 3 ​kg and 10 ​kg, respectively). We have carried out laboratory tests and evaluated seismic data from these two types of RU to evaluate noise levels, spectral response, and overall performance of the systems. We demonstrate that the systems provide reproducible data at high signal levels, which warrant comfortable scientific interpretation of seismic data from active and passive experiments. Noise level and detected spectral anomalies (due to mechanical structure and associated Eigenfrequencies) are well within expectations

A seismic-network mission proposal as an example for modular robotic lunar exploration missions

In this paper it is intended to discuss an approach to reduce design costs for subsequent missions by introducing modularity, commonality and multi-mission capability and thereby reuse of mission individual investments into the design of lunar exploration infrastructural systems. The presented approach has been developed within the German Helmholtz-Alliance on Robotic Exploration of Extreme Environments (ROBEX), a research alliance bringing together deep-sea and space research to jointly develop technologies and investigate problems for the exploration of highly inaccessible terrain – be it in the deep sea and polar regions or on the Moon and other planets. Although overall costs are much smaller for deep sea missions as compared to lunar missions, a lot can be learned from modularity approaches in deep sea research infrastructure design, which allows a high operational flexibility in the planning phase of a mission as well as during its implementation. The research presented here is based on a review of existing modular solutions in Earth orbiting satellites as well as science and exploration systems. This is followed by an investigation of lunar exploration scenarios from which we derive requirements for a multi-mission modular architecture. After analyzing possible options, an approach using a bus modular architecture for dedicated subsystems is presented. The approach is based on exchangeable modules e.g. incorporating instruments, which are added to the baseline system platform according to the demands of the specific scenario. It will be described in more detail, including arising problems e.g. in the power or thermal domain. Finally, technological building blocks to put the architecture into practical use will be described more in detail

The ROBEX Lunar Analogue Mission on Mt. Etna, Sicily

The joint development of exploration technologies for highly inaccessible terrains has been central to the Helmholtz Alliance "Robotic Exploration of Extreme Environments" (ROBEX). Within its nominal five-year funding period, an intensive exchange between Germany's Helmholtz centers for space, marine, and polar research, universities and industrial partners has been accomplished. In summer 2017, technology demonstrations were conducted in the Arctic Sea (Expedition PS108 of R/V "Polarstern") and on Mt. Etna, Sicily, the latter aimed at the future installation of an active lunar seismic network (ASN). The partly autonomous experiment scenario consists of a lander, rover and payload boxes, termed "remote units". A seismometer network was used for reference, and the technological goal was to demonstrate the recording of seismic data in scientific quality. In cooperation with Parco dell'Etna and INGV Catania, the field site for the demonstration mission was chosen based on geological and geophysical reasoning. First scouting activities in 2015 revealed similarities between regolith-covered mare regions on the Moon and the Piano del Lago plain on the southern flank of Mt. Etna. Therefore, the latter was considered as lunar analogue test site in terms of expected scientific gain, communication infrastructure, and accessibility. Basaltic lava flows covered by thick tephra layers were deposited during the 2001 flank eruption when the Laghetto ash cone formed. Evaluation of the recorded seismograms revealed that the uppermost, about 10-meter thick layer is composed of material with a very low P-wave velocity and underlain by more consolidated materials with a much higher velocity. Both thickness and velocities are compatible with previous geological studies. In parallel, the annual ROBEX International Summer School for Planetary and Ocean Exploration was held on Vulcano, Sicily, combining lectures and extensive field training for graduate students (MSc, PhD), and using geophysical and oceanographic sensor-packages and robots inside Grand Cratere, Vulcano, and in the coastal waters around

A robotically deployable lunar surface science station and its validation in a Moon-analogue environment

This article presents a system design and a surface operations concept for a robotically deployable, small scientific observatory on the lunar surface. The design reference mission scenario considers its implementation as part of larger international exploration mission such as the European Exploration Envelope Programme. The underlying science case particularly addresses scientific objectives for long term observations as part of scientific networks. Considered strawman payload for this surface station focuses on instruments which are – or could be – candidates for geophysical and astronomical observation networks. A seismometer, a radio experiment and an IR telescope have been taken as sizing case to assess the station’s system budgets. First part of the article looks at the station design from engineering perspective whether a small modular station can address common needs of such instruments such as a sustained operation in the lunar environment. Focus is given to design features to enable the station’s deployment by robotic assets. Secondly, the core unit of the conceptualized station has been built as engineering model including its basic system functions, interfaces to neighboring mission elements such as the lander vehicle and the rover and a set of geophones as representative for a science instrument. This hardware realization was used in a functional end-to-end demonstration from robotic deployment to delivery of geo-scientific data. The mission demonstration has been carried out in a Moon analogue field test on Mt. Etna, Sicily/Italy and confirmed the general feasibility of the proposed concept for lunar scientific exploration. Particularly, the evaluation of the acquired seismic data confirmed its suitability for sub-surface exploration. Results from the Moon analogue test are presented together with the design details of the surface station and the necessary conditions for its implementation and use in a robotic exploration scenario

The Network Infrastructure for the ROBEX Demomission Space

The demonstration mission space of the alliance Robotic Exploration of Extreme Environments (ROBEX) was a campaign on Mt. Etna in summer 2017. The network infrastructure and parts of the ground segment for this demonstration mission space were set up by the Mobile Rocket Base (MORABA) of the DLR. The ground segment included a control center which was based near Catania, Italy. Various terminals allowed for controlling a lander, a robotic rover and several experiment carriers which have been placed in 23 km airline distance on Mt. Etna. The distance was bridged by a radio link between the control center and a base camp at the demonstration site. From the base camp a shorter radio link of several hundreds of meters to the lander was established, and from there, the signal was distributed using several access points