Impact-Seismic Investigations of the InSight Mission

Impact investigations will be an important aspect of the InSight mission. One
of the scientific goals of the mission is a measurement of the current impact rate at Mars. Impacts will additionally inform the major goal of investigating the interior structure of
Mars.
In this paper, we review the current state of knowledge about seismic signals from impacts
on the Earth, Moon, and laboratory experiments. We describe the generalized physical
models that can be used to explain these signals. A discussion of the appropriate source time
function for impacts is presented, along with spectral characteristics including the cutoff frequency
and its dependence on impact momentum. Estimates of the seismic efficiency (ratio
between seismic and impact energies) vary widely. Our preferred value for the seismic efficiency
at Mars is 5 × 10−4, which we recommend using until we can measure it during the
InSight mission, when seismic moments are not used directly. Effects of the material properties
at the impact point and at the seismometer location are considered. We also discuss the
processes by which airbursts and acoustic waves emanate from bolides, and the feasibility
of detecting such signals.
We then consider the case of impacts on Mars. A review is given of the current knowledge
of present-day cratering on Mars: the current impact rate, characteristics of those impactors
such as velocity and directions, and the morphologies of the craters those impactors
create. Several methods of scaling crater size to impact energy are presented. The Martian
atmosphere, although thin, will cause fragmentation of impactors, with implications for the
resulting seismic signals.
We also benchmark several different seismic modeling codes to be used in analysis of
impact detections, and those codes are used to explore the seismic amplitude of impactinduced
signals as a function of distance from the impact site. We predict a measurement
of the current impact flux will be possible within the timeframe of the prime mission (one
Mars year) with the detection of ∼ a few to several tens of impacts. However, the error bars
on these predictions are large.
Specific to the InSight mission, we list discriminators of seismic signals from impacts
that will be used to distinguish them from marsquakes. We describe the role of the InSight
Impacts Science Theme Group during mission operations, including a plan for possible
night-time meteor imaging. The impacts detected by these methods during the InSight mission
will be used to improve interior structure models, measure the seismic efficiency, and
calculate the size frequency distribution of current impacts.