Shining a light on the subsurface of the Moon and Mars: extraterrestrial seismic exploration – and beyond

Geophysical exploration techniques and in particular seismic methods have played a fundamental role in studying the interior structure of Earth at all scales. Likewise, observational and exploration seismology proved to be critical to investigate the interior of planetary bodies like Moon and Mars. First extraterrestrial seismic experiments date back to the Apollo area. Active and passive seismic measurements were an instrumental part of the scientific activities carried out by the astronauts on Moon. Observations of moonquakes allowed to study the interior of Moon, and active-source seismic experiments were carried out to the investigation of the shallow lunar subsurface at the Apollo landing sites.

The Viking missions mark the beginning of seismology on Mars. Unfortunately, these first attempts were much less successful than the experiments on Moon due to less sensitive instruments operated on the lander, which primarily recorded wind-induced noise. It was not until NASA’s InSight mission that a seismometer package could successfully be brought to Mars and placed, for the first time, on the Martian surface in November 2018. The InSight seismometers have since their deployment measured a wealth of seismic signals.

While the core seismic investigations of the Apollo and InSight missions were planned, there have been very valuable opportunistic seismic experiments in space exploration as well. Sensitive ground motion sensors placed next to penetrometers with self-hammering thermal probes to sample the subsurface heat offered unique opportunities for ‘active source’ seismic measurements. Such ‘un-planned’ measurements were conducted on Mars during the InSight mission and on comet 67P/Churyumov–Gerasimenko during the Rosetta mission with lander Philae.

The topic of this webinar is a review of recent developments in active source exploration of planetary bodies with a focus on Moon and Mars. Seismic experiments using explosive sources carried out by the astronauts on Moon during the Apollo missions provided first insights into the structure of the shallow lunar subsurface. Because of the enigmatic nature of the recorded data characterized by a long-duration coda, only the P-wave first arrivals could be interpreted at that time. It was not until the development of seismic gradiometry around 40 years after the Apollo missions that the data from a Y-shaped geophone array deployed during Apollo 17 could be exploited to compute spatial seismic wavefield gradients and identify shear waves. This application of a new seismic processing techniques enabled even decades after acquisition to construct, for the first time, an elastic model of the shallow lunar subsurface.

The InSight seismic instrument package SEIS was designed to observe Marsquakes with the goal to resolve the deep interior of Mars and eventually improve our understanding of the formation of solid planets. On Mars, SEIS was placed next to the self-hammering HP3 probe designed to observe the Martian heat flow at depth. Listening to the seismic waves generated by the hammering was not part of the mission’s high-level experiments. However, once the potential for characterizing the elastic properties of the shallow Martian subsurface with the hammering signals was recognized, a series of adaptations were implemented to extract the maximum possible scientific value from this opportunistic experiments. For example, to extend the frequency bandwidth of the seismic data acquisition chain, temporally aliased hammering signals were recorded and the full-bandwidth data were reconstructed post-acquisition. The hammering signals then allowed determining the elastic parameters of the shallowest subsurface.

As seismological instrumentation on Earth advances, these new sensing technologies are also evaluated for space applications. The recent development of rotational seismometers, for example, shows great promise to advance seismology, in particular for single-station applications. The potential of improved wavefield characterization and wavefield separation, tilt noise corrections as well as the determination of local physical parameters from a single seismological station could be of significant benefit for future planetary seismology missions.

Beyond studying elastic waves on planetary objects, seismological signal processing techniques are promising tools to analyze other classes of wave-type phenomena observed in space. The goal of the ESA-led LISA mission is to bring three satellites into orbit in early 2030 to observe gravitational waves by constructing a giant interferometer. Despite a scale difference, such a gravitational waves detector resembles a seismometer. Data conditioning and analysis of LISA data could therefore be thought of as a new branch of seismology.

Speaker Bio

Cedric Schmelzbach

Graduate of University of Zurich, Switzerland, (M.Sc.), and Uppsala University, Sweden (Ph.D.), including a visit to Rice University, Houston. After graduating, Dr. Schmelzbach worked as a postdoc first at University of Potsdam, Germany, and later at Freie Universität Berlin, Germany. He joined ETH Zurich in 2012, and currently holds a position as senior scientist and lecturer. Dr. Schmelzbach’s research is centered on seismic exploration at different scales including near-surface seismic methods, acquisition and processing of seismic multicomponent data, seismic spatial-gradient and rotational-motion measurements, and multi-disciplinary investigations of the shallow subsurface. More recently, he became involved in space exploration and is a member of the Science Team of NASA’s InSight mission to Mars as well as the LISA consortium. Dr. Schmelzbach is member of EGU, AGU, EAGE, SEG and DFG. He holds offices with the EGU and is editor of Advances in Geophysics.