A New Crater Near InSight: Implications for Seismic Impact Detectability on Mars

Abstract

A new 1.5 m diameter impact crater was discovered on Mars only ~40 km from the InSight lander. Context camera images constrained its formation between 21 February and 6 April 2019; follow‐up High Resolution Imaging Science Experiment images resolved the crater. During this time period, three seismic events were identified in InSight data. We derive expected seismic signal characteristics and use them to evaluate each of the seismic events. However, none of them can definitively be associated with this source. Atmospheric perturbations are generally expected to be generated during impacts; however, in this case, no signal could be identified as related to the known impact. Using scaling relationships based on the terrestrial and lunar analogs and numerical modeling, we predict the amplitude, peak frequency, and duration of the seismic signal that would have emanated from this impact. The predicted amplitude falls near the lowest levels of the measured seismometer noise for the predicted frequency. Hence, it is not surprising this impact event was not positively identified in the seismic data. Finding this crater was a lucky event as its formation this close to InSight has a probability of only ~0.2, and the odds of capturing it in before and after images are extremely low. We revisit impact‐seismic discriminators in light of real experience with a seismometer on the Martian surface. Using measured noise of the instrument, we revise our previous prediction of seismic impact detections downward, from ~a few to tens, to just ~2 per Earth year, still with an order of magnitude uncertainty.

The Seismic Moment and Seismic Efficiency of Small Impacts on Mars

“…Following prelanding impact detection estimates (Teanby, 2015) we focused on impacts most likely to be detected during the InSight mission: those that produce craters of diameter 1–30 m, which occur across Mars ∼1,000–10 times per Earth year. Accounting for uncertainty in target strength (Daubar et al, 2020) and applying well‐established impactor crater scaling equations (Holsapple, 1993), this corresponds to a range in vertical impactor momentum, radius, and mass of 10 3 –10 7 kg m s −1 , 0.03–0.5 m, and 0.5–10 3 kg, respectively (Figure 1).…”

“…The expected crater size, vertical impact momentum, and vertical component of the impact velocity are shown as a function of impactor radius in Figure 1 and the impactor size, vertical impact speed at the ground, and crater size estimates are given in Table 1. Of the scenarios we consider, Scenarios 1 and 2 represent plausible candidates for the newly discovered crater on Mars (Daubar et al, 2020).…”

“…We therefore modified the approach of Teanby (2015) to develop two empirical relationships between P wave peak amplitude v , range x , and crater size D , based on vertical impactor momentum, rather than impact energy; one for impacts into a dry cohesionless sand and one for impacts into a more robust and strong soil (Figure 9). These relationships use π group scaling equations to relate impactor momentum (and other properties) to crater size (Daubar et al, 2020) combined with separate fits to ground motion versus range data.…”

“…The measured noise at the InSight landing site varies significantly over the course of a day. Following Daubar et al (2020) and Teanby (2015) we consider seismic noise in the 1–16 Hz frequency range as being most appropriate for small impact craters. The measured seismic noise at the 4 Hz geometric center of the range varies from m s −2 Hz −1/2 during quiet periods to 1 × 10 −7 m s −2 Hz −1/2 during noisy periods (Lognonné et al, 2019, 2020).…”

“…Detection of an impact‐generated seismic signal, whose location is determined by spacecraft images, would therefore be particularly valuable (Daubar et al, 2018). In the spring of 2019 one such small impact (1.5 m crater diameter) occurred nearby (Daubar et al, 2020). Despite close proximity to the lander (∼40 km), it could not be connected definitively with a specific seismic signal during the constrained time period.…”