Robust Passive Mechanical Filter for Sub-Hertz Seismic Detection on Venus

This study presents a passive mechanical filter designed to enhance sub-Hertz Venusquake detection by shaping the seismic transfer path with a tunable, high-Q pendulum mounted inside a cylindrical enclosure on a three-ring gimbal. The gimbal provides self-leveling on uneven terrain, while the housing–gimbal assembly remains broadband-stiff (<1–1000 Hz), limiting platform-induced motion and preventing spurious high-frequency amplification. Unlike approaches that rely on broadband digitization followed by digital filtering, which require large dynamic range, high bandwidth, and thermally stable electronics yet not feasible on Venus, the proposed mechanism performs pre-filtering at the mechanical level that can be energy-saving, reducing the required analog-to-digital conversion (ADC) range while amplifying the target band. Response spectrum analysis shows a clear low-pass behavior with peak sensitivity in the 0.5–0.8 Hz range. When tuned to 50-55 mm pendulum length and assumed undamping, the pendulum-mount mechanism improves detectability at best by 10-100 relative to a bare sensor for moderate magnitude (M_s = 3-6) in a 12-h observation window, with signal-to-noise (SNR) ratio of 3, and amplitude spectrum density (ASD) of 10⁻⁸ m/s²/√Hz. Furthermore, we extrapolate that the predicted minimum detectable event rates follow Nmmin∝(SNR)^1.2(ASD)^1.2f_s^0.6, where f_s is the quake wave frequency. A limitation is the quasi-static regime (0.05 Hz or below), where rigid-body motion overrides the benefit. Overall, the passive, power-free architecture offers a robust alternative to existing Venus Lander designs, enabling sub-Hz detection even during short-duration surface operations while adhering to mission constraints.