A Microseismometer for Venus

The recent investigation of Mars by InSight [1, 2] has confirmed that seismology is the most powerful way of investigating the interior of any planet, critical for understanding its structure and evolution.  However, Venus, with its high surface temperature and pressure present considerable challenges to developing equivalent instrumentation for a similar surface investigation [3]. While this has motivated the development of airborne approaches to detect seismic activity on Venus using infrasound [4], ground truth from the deployment of a seismic package on the surface still provides the most direct path to study the interior.

We describe here the development of a micromachined silicon seismometer, based on the instrument successfully deployed as part of InSight’s seismic payload [5], and the first tests under simulated Venus conditions in the Glenn Extreme Environments Rig (GEER).

The silicon sensor of InSight’s microseismometer offers the possibility of adapting a proven approach of a through-wafer-etched single-crystal-silicon suspension die incorporating gold sputtered electrodes, an insulating oxide and electroplated coils. We have adapted this design by adopting open-loop sensing and using thermocompressive bonding between the die frame and the glass. We demonstrated that open-loop operation does not compromise the noise floor of the device, with a floor below 5x10^-9 m/s²/rtHz, and are modifying the capacitance topology to allow operation over an enhanced tilt range.

To investigate the viability of this approach, prototype sensors were exposed to a Venus environment in GEER. Samples used the same wafer fabrication processes as used for InSight.  These results suggest the viability of a sensor fabrication flow based on sputtered gold under a protective oxide and thermocompressive bonding. In parallel, we are undertaking the coupled development of electronics to drive the sensor’s capacitance transducer, with all operations based in the analog domain, and the packaging approach to allow integration of the sensor and its proximity electronics in a robust package.

References: [1] Banerdt W. B. et al. (2020) Nature Geoscience, 13 (3), 183–189. [2] Stahler S. C. et al. (2021) Science, 373 (6553) 443-448. [3] Kremic T. et al., (2020) Planet. Space Sci. 190 (104961). [4] Krishnamoorthy, S., GRL (2022), e2022GL100978. [5] Pike W. T. et al., 2018 IEEE Micro Electro Mechanical Systems (MEMS), Belfast, UK, 2018, 113-116.