Seismic detection of meteorite impacts on Mars

International audienceMeteorite impacts provide a potentially important seismic source for probing Mars' interior. It has recently been shown that new craters can be detected from orbit using high resolution imaging, which means the location of any impact-related seismic event could be accurately determined thus improving the constraints that could be placed on internal structure using a single seismic station. This is not true of other seismic sources on Mars such as sub-surface faulting, which require location using multiple seismic stations. This study aims to determine the seismic detectability of meteorite impacts and assess whether they are a viable means of probing deep internal structure. First, we derive a relation between crater diameter and equivalent seismic moment based on observational data compiled from impact tests, controlled explosions, and earthquake seismology. Second, this relation was combined with updated cratering rates based on newly observed craters to derive the impact induced seismicity on Mars, which we estimate to total 1013-1014 Nm per year. Finally, seismic waveform modelling was used to determine the detectability of these impacts based on reasonable assumptions about likely seismometer performance and background noise levels. For our nominal noise/instrument case we find that detectable impacts at teleseismic distances (source-receiver offsets greater than 60∘) are very rare and occur approximately once every 10years. For our most optimistic noise/instrument case, approximately one such event occurs each year. This suggests that using solely meteorite impacts is not a reliable way of probing the Martian interior, although local impacts are more frequently detectable and could provide important constraints on near surface seismic properties

SEIS: Insight’s Seismic Experiment for Internal Structure of Mars

By the end of 2018, 42 years after the landing of the two Viking seismometers on Mars, InSight will deploy onto Mars’ surface the SEIS (Seismic Experiment for Internal Structure) instrument; a six-axes seismometer equipped with both a long-period three-axes Very Broad Band (VBB) instrument and a three-axes short-period (SP) instrument. These six sensors will cover a broad range of the seismic bandwidth, from 0.01 Hz to 50 Hz, with possible extension to longer periods. Data will be transmitted in the form of three continuous VBB components at 2 sample per second (sps), an estimation of the short period energy content from the SP at 1 sps and a continuous compound VBB/SP vertical axis at 10 sps. The continuous streams will be augmented by requested event data with sample rates from 20 to 100 sps. SEIS will improve upon the existing resolution of Viking’s Mars seismic monitoring by a factor of ∼ 2500 at 1 Hz and ∼ 200 000 at 0.1 Hz. An additional major improvement is that, contrary to Viking, the seismometers will be deployed via a robotic arm directly onto Mars’ surface and will be protected against temperature and wind by highly efficient thermal and wind shielding. Based on existing knowledge of Mars, it is reasonable to infer a moment magnitude detection threshold of Mw ∼ 3 at 40◦ epicentral distance and a potential to detect several tens of quakes and about five impacts per year. In this paper, we first describe the science goals of the experiment and the rationale used to define its requirements. We then provide a detailed description of the hardware, from the sensors to the deployment system and associated performance, including transfer functions of the seismic sensors and temperature sensors. We conclude by describing the experiment ground segment, including data processing services, outreach and education networks and provide a description of the format to be used for future data distribution

Large impacts detected by the Apollo seismometers: Impactor mass and source cutoff frequency estimations

International audienceMeteoroid impacts are important seismic sources on the Moon. As they continuously impact the Moon, they are a significant contribution to the lunar micro-seismic background noise. They also were associated with the most powerful seismic sources recorded by the Apollo seismic network. We study in this paper the largest impacts. We show that their masses can be estimated with a rather simple modeling technique and that high frequency seismic signals have reduced amplitudes due to a relatively low (about 1 s) corner frequency resulting from the duration of the impact process and the crater formation. If synthetic seismograms computed for a spherical model of the Moon are unable to match the waveforms of the observations, they nevertheless provide an approximate measure of the energy of seismic waves in the coda. The latter can then be used for an estimation of the mass of the impactors, when the velocity of the impactor is known. This method, for the artificial impacts of the LM and SIVB Apollo upper stages, allows us to retrieve the mass within 20% of relative error. The estimated mass of the largest impacts observed during the 7 years of activity of the Apollo seismic network provides an explanation for the non-detection of surface waves on the seismograms. The specifications of future Moon seismometers, in order to provide the detection of surface waves, are given in conclusion

Impact cutoff frequency: Momentum scaling law inverted from Apollo seismic data

© 2015 Elsevier B.V. We perform the analysis of both long and short period data for 40 large meteoroid impacts event gathered by the Apollo lunar seismic network. We extract the linear momentum released by the impact and the cutoff frequency of the recorded seismic spectrum, related to the radiation process of the shock wave generated by the impact. By using a proxy to the local porosity, based on the density of surface craters and well correlated to the most recent GRAIL observations, we demonstrate that the seismic cutoff frequencies for 40 selected impacts correlate with this proxy and therefore likely with the porosity at the impacted areas. Our finding shows that lunar seismic records of meteoroid impacts represent unique geophysical data documenting medium to high-energy (0.1-1 kt TNT yield) impact processes, including the interaction of shock waves with porous media. This work can be applied to the analysis of the seismic data to be obtained by the InSight mission in 2016 and the investigation of the lateral variations in the Martian regolith

Relationship between deep structure and seismicity in the western Pyrenees

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