Numerical modelling of impact-generated seismic waves on Mars

Seismology is a powerful tool for understanding planetary interiors. On Earth, the seismicity is dominated by tectonic earthquakes, with the atmosphere protecting us from meteorite impacts. However, on Mars, impacts are one of the key processes generating seismic waves. Despite nominal pre-landing estimates of 1–3 impacts detected per Earth year, recognising impacts in the InSight seismic data has proven challenging. This thesis focuses on filling the current gaps in knowledge of impact-generated seismic waves, specifically in the context of the InSight mission, using numerical modelling. Firstly, 13 small impacts onto martian regolith, forming craters 1–30 m, are simulated using iSALE-2D. Their seismic source properties are characterised in terms of impactor properties. In this size range scalar seismic moment increases almost linearly with impact momentum. Seismic efficiencies are ∼10−6, dependent on target material properties and impact velocity. The relatively low seismic efficiency and moment suggest that impact detectability on Mars is lower than previously assumed. Secondly, a momentum-based amplitude-distance scaling relationship is derived based on a dataset containing the artificial impacts on the Moon, the Carancas impact on Earth, and impacts detected by InSight on Mars. Impacts producing craters <30m in diameter are less detectable that pre-landing estimates suggested, whilst larger impacts are more detectable. A subset of marsquakes recorded by InSight is used to derive a new estimate of the impact rate on Mars to be ∼3–5 times higher than the estimates based on orbital observations, but consistent with crater chronology models. Finally, a further suite of iSALE-2D simulations is used to record displacement seismograms generated by impacts and compute their power spectra. The frequency content of the signals decreases with increasing impactor size and velocity, and target porosity. The low-frequency content of seismic waves generated scales well with impactor momentum, but is also sensitive to target material properties.Open Acces

Seismic constraints from a Mars impact experiment using InSight and Perseverance

NASA’s InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission has operated a sophisticated suite of seismology and geophysics instruments on the surface of Mars since its arrival in 2018. On 18 February 2021, we attempted to detect the seismic and acoustic waves produced by the entry, descent and landing of the Perseverance rover using the sensors onboard the InSight lander. Similar observations have been made on Earth using data from both crewed1,2 and uncrewed3,4 spacecraft, and on the Moon during the Apollo era5, but never before on Mars or another planet. This was the only seismic event to occur on Mars since InSight began operations that had an a priori known and independently constrained timing and location. It therefore had the potential to be used as a calibration for other marsquakes recorded by InSight. Here we report that no signal from Perseverance’s entry, descent and landing is identifiable in the InSight data. Nonetheless, measurements made during the landing window enable us to place constraints on the distance–amplitude relationships used to predict the amplitude of seismic waves produced by planetary impacts and place in situ constraints on Martian impact seismic efficiency (the fraction of the impactor kinetic energy converted into seismic energy)