Very High Frequency Events
Very High Frequency (VF) events are seismic signals detected by InSight’s seismometer that have the majority of their energy at high frequencies (>4Hz). They are a unique group of marsquakes because they appear to be uniformly distributed across Mars. They have a distance dependent detection threshold - therefore only larger VF events can be detected at greater distances. This is what would be expected of impact-generated seismic events.
To date, 8 impacts were confirmed to be detected by InSight, with 6 of those belonging to the VF events family. So far a key characteristic that allowed identification of impact generated signals as well as location of the corresponding craters was the atmospheric ‘chirp’ signal, caused by the passage of the meteorite through the atmosphere (Garcia et al., 2022). Such signals are however only detectable at very short distances. Besides the atmospheric part of the signal, the confirmed impact-generated quakes share the majority of their characteristics with other VF events, suggesting that more/all of those events can be impact generated. In our work we examined the properties of VF events and concluded that this family of signals is consistent with an impact origin.
Constraining the impact rate on Mars
The two key elements required for converting the seismic event distribution are:
- A method for converting seismic magnitudes into crater sizes.
- A detectability curve, acting as the Area Time Function (ATF). This factor is defined by the ‘area searched’ over which the impacts were detected, and the time that the search covered.
In our work we used the confirmed impacts recorded by InSight to calibrate the relationship between seismic magnitude and crater size. We also used previous insights from impact modelling to inform this step. This allows us to convert the seismic event distribution into a crater size - frequency distribution for events that we haven’t observed a corresponding crater. We do this in two ways: by converting a magnitude-frequency distribution into crater size-frequency distribution as a whole, and by converting each individual event into its predicted crater size.
In order to determine the ATF we use two approaches. In the first, ‘Seismology’, we calculate the area factor by using the seismic detection curve defined by the VF event distribution. In the second approach, ‘Cratering’, we combine previous studies in numerical modelling and semi-empirical scaling relationships for impacts across the Solar System to define the ‘area searched’ during our study. The time portion was determined by the total time that InSight’s seismometer operated.
Both our approaches yield consistent results, with the yearly impact rate on Mars 5 times higher than that predicted by only orbital imaging. Our results predict that Mars’ surface is impacted by 280 to 360 meteorite forming craters 8 m wide and larger.
Conclusions and outlook
Our results show that the present impact rate on Mars is higher than previously estimated by using orbital images, but consistent with chronology model predictions. The higher rate shows that meteorite impacts should be considered in hazard assessments for future robotic and crewed missions, both from the direct risk perspective but also from the seismic hazard perspective.
Our results will also have wider implications for studies of the geological history of Mars. Higher impact rate suggests younger surface ages, which will have consequences for example for our understanding of Mars' history of volcanism. Our work shows that seismology is an effective tool for measuring impact rates and compliments other methods such as orbital imaging.
References:
Garcia, Raphael F. et. al., 2022. “Newly Formed Craters on Mars Located Using Seismic and Acoustic Wave Data from InSight.” Nature Geoscience 2022 10(11):1–7.
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