In Situ Regolith Seismic Velocity Measurement at the InSight Landing Site on Mars

Abstract

Interior exploration using Seismic Investigations, Geodesy and Heat Transport's (InSight) seismometer package Seismic Experiment for Interior Structure (SEIS) was placed on the surface of Mars at about 1.2 m distance from the thermal properties instrument Heat flow and Physical Properties Package (HP3) that includes a self‐hammering probe. Recording the hammering noise with SEIS provided a unique opportunity to estimate the seismic wave velocities of the shallow regolith at the landing site. However, the value of studying the seismic signals of the hammering was only realized after critical hardware decisions were already taken. Furthermore, the design and nominal operation of both SEIS and HP3 are nonideal for such high‐resolution seismic measurements. Therefore, a series of adaptations had to be implemented to operate the self‐hammering probe as a controlled seismic source and SEIS as a high‐frequency seismic receiver including the design of a high‐precision timing and an innovative high‐frequency sampling workflow. By interpreting the first‐arriving seismic waves as a P‐wave and identifying first‐arriving S‐waves by polarization analysis, we determined effective P‐ and S‐wave velocities of vP=119−21+45 ${v}_{P}=11{9}_{-21}^{+45}$ m/s and vS=63−7+11 ${v}_{S}=6{3}_{-7}^{+11}$ m/s, respectively, from around 2,000 hammer stroke recordings. These velocities likely represent bulk estimates for the uppermost several 10s of cm of regolith. An analysis of the P‐wave incidence angles provided an independent vP/vS ratio estimate of 1.84−0.35+0.89 $1.8{4}_{-0.35}^{+0.89}$ that compares well with the traveltime based estimate of 1.86−0.25+0.42 $1.8{6}_{-0.25}^{+0.42}$. The low seismic velocities are consistent with those observed for low‐density unconsolidated sands and are in agreement with estimates obtained by other methods.

Wave Velocities and Poisson Ratio in a Loose Sandy Martian Regolith Simulant Under Low Stresses: 1. Laboratory Investigation

“…Wave measurements were performed under low stresses by using bender elements on loose samples of Fontainebleau sand used as a Martian regolith simulant. This was made to better constrain the elastic properties of the regolith at the surface of the InSight landing site and to help interpret in situ elastic data obtained at the surface through wave measurements (Brinkman et al., 2022) of an indentation experiment carried out with the IDA (Golombek et al., 2023). Current knowledge on wave transfer in sands was extended to low stresses (<10 kPa) by using a horizontal cylindrical sample submitted to changes in vacuum (between 2 and 80 kPa).…”

Results from InSight Robotic Arm Activities

“…Together with a representative density of 1211 +149 −113 kg/m 3 (Grott et al 2021), the elastic moduli were calculated from the seismic velocities. The shear, bulk and Young's moduli were found to be 4.47 +2.0 −0.83 MPa, 7.79 +1.60 −1.55 MPa, and 11.48 +5.91 −2.23 MPa, respectively, and the Poisson ratio to be 0.29 +0.12 −2.23 (Brinkman et al 2022).…”

“…After deploying the instruments on the martian surface, it was important to determine an accurate estimate of their locations (Bailey et al 2020). From a science perspective, precisely knowing the relative position between the instruments (e.g., Brinkman et al 2022) and the lander was necessary to be able to interpret science results and identify noise in the data captured by the instruments (Golombek et al 2020b). Besides the science benefit, instrument localization also was required to continue with the rest of instrument deployment (and eventual moving of the HP 3 ).…”

“…The shallow stratigraphy consists of surficial dust, over thin unconsolidated sand, underlain by a variable thickness duricrust, with poorly sorted, unconsolidated sand with rocks beneath (Golombek et al 2020a;Warner et al 2022). Surface observations and shallow seismic results (Lognonné et al 2020;Brinkman et al 2022) are consistent with expectations made from remote sensing data prior to landing (Golombek et al 2020c), indicating an impactfragmented regolith 3-10 m thick overlying coarse ejecta blocks that grade into fractured basaltic lava flows (Golombek et al 2017(Golombek et al , 2018a(Golombek et al , 2020cWarner et al 2017Warner et al , 2022. The basalts, ∼170 m thick beneath the lander (Pan et al 2020), are Hesperian to Early Amazonian in age (Warner et al 2017(Warner et al , 2022, with a low-velocity zone at 30 to 75 m depth that may be a sedimentary layer sandwiched within the basalts (Hobiger et al 2021).…”

“…The hammerings of the mole were recorded by SEIS and the signals could be used to derive a P-wave velocity of 119 +40 −19 m/s and a S-wave velocity of 63 +11 −7 m/s (Brinkman et al 2022) representative of the entire regolith layer to 40 cm depth. These values are in reasonable agreement with previous lab estimations conducted on Earth on various regolith simulants (Delage et al 2017(Delage et al , 2022 and initial seismic results (Lognonné et al 2020).…”