DI024-0013
The seismic moment and seismic efficiency of small impacts on Mars
Tuesday, 15 December 2020
Poster
Natalia Wojcicka, Imperial College London, London, SW7, United Kingdom, Gareth S Collins, Imperial College London, London, United Kingdom, Ian D Bastow, Imperial College London, Department of Earth Science and Engineering, London, SW7, United Kingdom, Nicholas A Teanby, University of Bristol, Bristol, United Kingdom, Katarina Miljkovic, Curtin University, Perth, Australia, Andrea Rajsic, Curtin University, School of Earth and Planetary Sciences, Perth, Australia, Ingrid Daubar, Brown University, Department of Earth, Environmental and Planetary Sciences, Providence, United States and Philippe Henri Lognonné, Université de Paris, Institut de physique du globe de Paris, CNRS, Paris, France
Abstract:
Since early 2019, the InSight lander has been recording seismic signals on the surface of Mars. So far, no impacts were identified in the seismic data, despite nominal pre-landing predictions of 1-3 meteorite impacts detected per year. In order to help understand this discrepancy and inform revised detectability estimates, we used iSALE shock physics code to simulate a suite of small impacts onto Martian regolith and characterized their seismic source properties. We focused on crater diameters most relevant for InSight, 1 - 30 m. These correspond to impactor radii of 3.5 - 50 cm and velocity ranging 1- 6 km/s. The velocities are a consequence of atmospheric deceleration of impactors with pre-entry velocity of 10 km/s (mean impact speed on Mars).
Our simulations showed that in this range the seismic moment fell between 106 - 1010 Nm. We also found this value increased almost linearly with impactor momentum and we produced a least squares scaling relation. We calculated the ratio of horizontal to vertical seismic moment tensor components to be very close to 1, implying an almost isotropic P-wave source.
We calculated the seismic efficiencies of each modelled scenario to be of the order of 10-6, which are similar to those computed for artificial lunar impacts, but significantly lower than pre-landing predictions for InSight suggested. We also found a strong dependence of seismic efficiency on target crush curve properties. Our results of relatively low seismic efficiency and seismic moment suggest that meteorite impact detectability on Mars is lower than previously assumed. We conclude that impacts forming craters larger than 10 m offer the most promising scenario for seismic detection.
