Optical seismometer for the Lunar Geophysical Network

Seismic studies provide definitive knowledge of internal planetary structure. Reexamination of seismic data from the Moon has provided an important glimpse of its mantle and core structure, which has bearing on its thermal, petrological, and rotational history. Data suggest the presence of a fluid-like transition layer between the lunar core and mantle. The Moon may therefore still be undergoing chemical segregation and thermal layering. The seismologist's challenge is substantiating this view.

Further investigation of internal structure hinges on innovative development of a global Lunar Geophysical Network (LGN) that includes advanced seismic sensors and deployment systems. This network will need to overcome the limitations of an initially sparse grid and improve upon the quality of Apollo-era seismograms. The best design for such instruments is currently underdeveloped.

The Seismometer for a Lunar Network (SLN) is a project funded by NASA's Development and Advancement of Lunar Instrumentation (DALI) Program to develop the instrument for a lunar seismological network. The seismometer design is based on a commercial-off-the-shelf device manufactured by Silicon Audio Inc., and is a novel combination of a classic seismic geophone and a laser interferometer. Submicron-scale motions are recorded by the laser interferometer system. This allows for a small (<300gm), sensitive (1x10-8 m/s2/Hz1/2) broadband (0.01-100Hz) seismic instrument that is competitive with state of the art planetary seismometers. The 3-axis instrument is insensitive to tilt over 180°.

SLN will be deployed using a pneumatic drilling technique developed by Honeybee Robotics. Drilling is accomplished by compressed gas jetting through a cylindrical, 3-axis, seismic probe, stirring up the soil underneath and lofting the regolith out of the hole. Burial will enable lower mass and power by automatically creating an isothermal environment, will improve seismic coupling, and reduce noise from the lander spacecraft and scattering in the lunar regolith. This drilling mechanism follows the design of one recently selected to fly aboard one of the upcoming 2020/2021 commercial lunar payloads as part of the LISTER heat flow probe.

Here we report on the progress underway to advance the Silicon Audio seismometer from TRL 4 to 6, in preparation for LGN.