Martian Infrasound: Numerical Modeling and Analysis of InSight's Data. Issue 6 (12th June 2020)

• Record Type: Journal Article
• Authors: Martire, Léo; Garcia, R. F.; Rolland, Lucie; Spiga, Aymeric; Lognonné, Philippe Henri; Banfield, Don; Banerdt, William Bruce; Martin, Roland
• Is Part Of: Journal of geophysical research. Volume 125:Issue 6(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-06-12
• Subjects: infrasound -- Mars -- insight -- surface waves -- guided waves -- boundary layer; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2020JE006376
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: Acoustic waves in planetary atmospheres couple into the solid surface, producing ground displacements that can be measured using seismometers. On 26 November 2018, the InSight mission successfully landed on Mars. Its objectives include studying Mars' interior using the seismometer SEIS (Seismic Experiment for Interior Structures) and the atmosphere through the weather station APSS (Auxiliary Payload Sensor Suite). Because InSight is the first mission capable of studying infrasound on Mars, we investigate the signature of infrasound both in terms of air pressure and ground velocities. Using numerical simulations, we characterize (1) the acoustic propagation pattern in Martian dusk and (2) the mechanical atmosphere‐to‐ground coupling under acoustic waves. Then, using SEIS data, we demonstrate that two low‐frequency monotone events (S0133a and S0189a) are in fact infrasound trapped in the atmospheric nocturnal surface waveguide. We base our demonstration on the following facts. (1) Seismic signals rarely produce, at a given station, a single frequency varying from one event to the other. (2) No clear seismic phases have been identified for such events. (3) The observed SEIS signals present the characteristics expected for trapped infrasound observed through their compliance effects (specific frequency response, more energy on the vertical component, ±90° phase shift between vertical and horizontal components, and no detection on pressure sensor at these low amplitude levels). Our simulations of the nocturnal waveguide's response are however subject to uncertainties because (1) it relies on the sol‐to‐sol variability of the atmosphere, and (2) subsurface properties are not properly known at this time. Plain Language Summary: The InSight mission landed on Mars the 26 November 2018 and was designed to study the interior of the Red Planet. The noise level of its seismometer SEIS routinely stays under 1 nm/s 2, a precision unmatched by other planetary seismometers. InSight also features a meteorological station, APSS, able to measure the atmosphere's absolute pressure, wind, and temperature. In this work, we use the seismometer and meteorological station to investigate acoustic waves (sound) in the Martian atmosphere. Those acoustic waves are pressure perturbations and consequently cause the ground to move: Positive perturbations push the soil down, whereas negative ones lift it up. InSight's SEIS and APSS constitute the perfect instruments for investigating ground‐coupled acoustic waves. We start by performing numerical simulations of the coupled solid‐atmosphere system. In particular, we investigate how low‐frequency sound (infrasound) travels in Mars' atmosphere. We then derive key properties of the ground deformations due to passing infrasound. These arguments are then applied to InSight's data: ground motion measurements from SEIS and pressure records from APSS. Finally, we show that 3‐axis ground motion can help discriminate acoustic waves from meteorological perturbations. We also demonstrate that some SEIS events are caused by infrasound trapped close to the surface by nocturnal winds. Key Points: Our numerical modeling predicts both pressure variations and ground movements induced by Martian acoustic waves Regional propagation of dispersed infrasound is expected in the Martian nocturnal surface waveguide Some monotone events detected by SEIS are consistent with acoustic waves trapped in this waveguide