Mars’ Background Free Oscillations

Abstract

Observations and inversion of the eigenfrequencies of free oscillations constitute powerful tools to investigate the internal structure of a planet. On Mars, such free oscillations can be excited by atmospheric pressure and wind stresses from the Martian atmosphere, analogous to what occurs on Earth. Over long periods and on a global scale, this phenomenon may continuously excite Mars' background free oscillations (MBFs), which constitute the so-called Martian hum. However, the source exciting MBFs is related both to the global-scale atmospheric circulation on Mars and to the variations in pressure and wind at the planetary boundary layer, for which no data are available. To overcome this drawback, we focus herein on a global-scale source and use results of simulations based on General Circular Models (GCMs). GCMs can predict and reproduce long-term, global-scale Martian pressure and wind variations and suggest that, contrary to what happens on Earth, daily correlations in the Martian hum might be generated by the solar-driven GCM. After recalling the excitation terms, we calculate MBFs by using GCM computations and estimate the contribution to the hum made by the global atmospheric circulation. Although we work at the lower limit of MBF signals, the results indicate that the signal is likely to be periodic, which would allow us to use more efficient stacking theories than can be applied to Earth's hum. We conclude by discussing the perspectives for the InSight SEIS instrument to detect the Martian hum. The amplitude of the MBF signal is on the order of nanogals and is therefore hidden by instrumental and thermal noise, which implies that, provided the predicted daily coherence in hum excitation is present, the InSight SEIS seismometer should be capable of detecting the Martian hum after monthly to yearly stacks.

The Polarization of Ambient Noise on Mars

“…The LVL is the leveling system of the SA capable of tilting the SA for centering and calibration purposes. The lowest and more damped mode frequencies of the LSA are about 5 and 8 Hz with low Q under Earth gravity and zero-slope condition (Lognonné et al, 2019). The mode frequencies of the LVL are much higher, 40 Hz or more and with larger Q of about 10 (Fayon et al, 2018).…”

“…(2017). If Mars seismic noise contains propagating waves, the noise might be useful for imaging the planet interior, from local scale (Berbellini et al., 2019; Romero & Schimmel, 2018), to global scale (Nishikawa et al., 2019; Schimmel, Stutzmann, & Gallart, 2011).…”

“…The origin of these local effects was extensively studied and modeled prior the mission launch and might be related to pressure ground deformation (Lognonné & Mosser, 1993), thermal effects (Van Hoolst et al, 2003), lander induced noise (Murdoch et al, 2017) as reviewed by Mimoun et al (2017). If Mars seismic noise contains propagating waves, the noise might be useful for imaging the planet interior, from local scale (Berbellini et al, 2019;Romero & Schimmel, 2018), to global scale (Nishikawa et al, 2019;.…”

SEIS: Insight’s Seismic Experiment for Internal Structure of Mars

“…Due to the size of the planet, however, the frequencies of the modes, for a given angular order, are typically twice those on Earth.Similar techniques can be applied to the background noise generated globally by atmospheric dynamics (Kobayashi and Nishida 1998) the so-called seismic “hum.” Calculations based on excitation by turbulence in the boundary layer that do not take in account resonance effects, non-turbulent wind and pressure variations associated with atmospheric circulation (Tanimoto 1999, 2001) yield amplitudes for Mars , a factor of 2–3 smaller than on Earth. See more in Lognonné and Johnson (2015), Lognonné et al (2016), Panning et al (2017), Schimmel et al (2018), Nishikawa et al (2019, this issue), Bissig et al (2018).…”

Subsurface Structure at the InSight Landing Site From Compliance Measurements by Seismic and Meteorological Experiments

“…Although many techniques used in terrestrial studies, such as the extremely powerful micro-seismic noise tomography Shapiro and Campillo (2004), rely on seismic networks, a variety of methods work for single station and can thus be applied in the framework of InSight. These methods include the study of the long-period hum of the planet (Kobayashi & Nishida, 1998), developed in more detail for Mars by Nishikawa et al (2019), the motion induced by oceanic or atmospheric pressure fluctuations (Crawford et al, 1991;Sorrells, 1971;Tanimoto & Wang, 2018), high-frequency resonances related to the very local structure (Bonnefoy-Claudet et al, 2008;Nakamura, 1989), and seismic noise autocorrelations (Tibuleac & von Seggern, 2012).…”