On the detection of normal modes using InSight and their use in constraining Mars’ seismic structure

Normal modes or whole Earth oscillations serve as vital tools in the investigation of the Earth and other planets, providing large-scale seismic constrains on their globally averaged interior structure. Recently, P. Lognonné et al. reported their detection on Mars for the largest recorded event S1222a. However, the low signal-to-noise ratio in the normal mode frequency domain makes detection of normal modes very difficult, necessitating comprehensive data processing to extract the normal mode frequencies including deglitching, phase cross-correlation, multitapering and phasor-walkout techniques. Here, weinsight show that normal mode spectra for event S1222a depend on the details of deglitching technique, producing different results using either instrument response deconvolution (MPS) or the novel time-frequency polarization filtering. Furthermore, the plethora of Martian seismic models published since the InSight mission also generate strongly varying synthetic spectra, which complicates the identification of individual modes. Here, we developed a different approach in which we use spectra for the S1222a event and data for the seismically ‘quiet’ day prior to the event. We compare these observed spectra to synthetic spectra for existing seismic models for Mars in order to verify the detection of normal modes and identify models which best fit the spectra. Our research incorporates a range of different filtering and deglitching sequences and eleven post-InSight Martian seismic model families to assess their effects on both observed and synthetic spectra. We find that, the synthetic spectra consistently yield more overlapping peaks with the S1222a data spectra, than with spectra from the quiet day before S1222a. Through overlap analysis, we identify a preferred crustal structure, with an average V_s of 3.25 km s⁻¹, that aligns well with current geophysical and seismic estimates of the global Martian crust. We also find that the detection of normal modes is improved by stacking the data of S1222a with the two additional events S1000a and S1094b. On the other hand, stacking of all available long period seismic data of InSight to detect the continuous activation of normal modes by Martian atmosphere proved less effective. Nevertheless, we find consistent spectral peaks across the various data sets and stacking methods. These findings indicate that normal modes are detectable on Mars. It highlights the need for future deployment of seismometers on Mars, the moon and other planets to continue the hunt for normal modes and improve our understanding of the internal structure of terrestrial planets and moons.