Determining the characteristics of icy moons' impactors

Abstract

Crater counting is the main method used to determine the age of a planetary surface. This method relies on knowing the cratering rate to estimate the absolute age of a surface. However, two theories currently exist for impact cratering of the moons in the Saturnian system, one suggesting that the majority of the impactors are of heliocentric origin (i.e. they come from orbits around the Sun), and the other suggesting that the majority of impactors are of planetocentric origin (i.e. they originate from orbits around Saturn). The different theories result in very different surface ages of the icy moons. According to the heliocentric model, the surface of Titan would date to ~3 Ga, while according to the planetocentric model the surface of Titan could be dated between ~15 Ma and ~4 Ga, allowing for much more erosion of the surface according to Bell (2020). Given that these two orbits result in different impact velocities, this work attempts to discover if the impact velocity can be determined for impactors on icy moons based on crater characteristics, such as crater depth and diameter, by recreating impacts in the lab.

Previous impact experiments have been performed with different impactors and target materials. This study aims at expanding previous research by performing tests on icy particles, something not done before. The data obtained from experiments are then used to obtain the estimated impactor diameter that created craters on the surface of these icy moons based on the expected impact velocities. This data can then be used to predict the likelihood of an impactor originating from either a heliocentric or planetocentric orbit.

This work developed several methods to produce impact craters using different instruments and different surfaces. The best results were obtained either with the gas gun and ice blocks or with the drop tower and icy particles. Using the scaling relationships found in this work, it was shown that the required impactor diameter can be obtained from craters on icy moons and that a distinction can be made between planetocentric and heliocentric impactors. However, more work is needed to narrow down the scaling relationship to obtain a reasonable impactor diameter given a certain impact crater. More information about the crater, such as the transition of ice type during an impact, should be used in the future to further constrain the velocity ranges.