Ballistic Shadows that Affect Impact Ejecta Distribution in the Moon's South Polar Region

BALLISTIC SHADOWS THAT AFFECT IMPACT EJECTA DISTRIBUTION IN THE MOON’S SOUTH POLAR REGION. David A. Kring, Center for Lunar Science and Exploration, Lunar and Planetary Institute, Universities Space Research Association, Houston TX 77058 ([email protected]). Introduction: The lunar south polar region is an impact-cratered terrain. Numerous analytical models have been developed to assess the consequences of impact cratering processes on the geology that will be accessible to Artemis astronauts (e.g., [1,2]). There is a need, however, to balance those mathematical models with geologic reality. In the case of impact cratering and impact ejecta, that geologic reality is the complex topography of the lunar south polar region. The application of impact ejecta thickness models, such as that of McGetchin et al. [3] and subsequent permutations of it (e.g., [4,5]), usually assume the ejecta lands on a flat surface. The Moon is, in contrast, covered with topographic highs (e.g., massifs) and lows (e.g., craters, catenas, rilles, and valles). Here I examine the consequences of topographic highs on the distribution of impact ejecta and the phenomenon of ballistic shadowing. Two Case Studies: To explore the phenomenon of ballistic shadowing, we examine two impact events on the lunar surface. The first impact event produced an unnamed 30 km-diameter crater (-70.1° latitude, 259.7° longitude) adjacent to an unnamed 6 km-elevation massif (-70.2° latitude, 266.2° longitude). When the impact occurred, material was excavated from the crater, producing a cone-shaped ejecta curtain (e.g., [6]). Discrete masses within that curtain were traveling on ballistic trajectories and, thus, ballistic formulae can be used to model the transport and deposition of the ejecta. Because the Moon’s gravity is only 1/6th that of Earth’s gravity, ballistic flight times and range are 6× greater than on Earth; and because the Moon is an airless body, no adjustments are needed for atmospheric drag and friction. Most analytical models assume ejecta thickness decreases radially from transient crater rims on a flat surface. In our first case study, however, a massif rises adjacent to a source crater (Fig. 1, top). Crater material ejected towards the massif collided with the flank of the massif. Material within the continuous ejecta blanket (e.g., within a radial distance of 33.9 ± 7.4 km [7]) hit the flank of the massif and may have slumped towards the base of the massif. None of that continuous ejecta covers the summit of the massif, as would be implied by an analytical calculation that ignored topography. Even discontinuous ejecta that might otherwise have landed at a distance of 4.5 crater radii was unable to pass over the massif (Fig. 1, top). The massif casts a ballistic shadow over the surrounding terrain. Let’s now consider the case of Haworth crater, which is adjacent to several candidate Artemis landing sites, including one on the summit of Mons Malapert (Fig. 1, bottom). In this case, most of the material that would compose the continuous ejecta blanket (a–d in Fig. 1, bottom) is intercepted by the south-facing flank of Mons Malapert at distances several kilometers short of where it would have landed if the terrain were flat. Results and Discussion: Ballistic shadows imposed by topography modify the deposition and distribution of impact ejecta. Ejecta impact velocity and kinetic energy. Because ejecta is being intercepted before returning to the same elevation as its launch point, the material is still accelerating towards the ground and hits the surface at a lower velocity than it would if the surface were flat. That, in turn, means material is deposited with less kinetic energy. In our first case (Fig. 1, top), material on flight path c hits the massif with only 90% of the terminal velocity and roughly 80% of the kinetic energy that it would have if it completed a path to a flat surface. If topography intercepts ejecta near the top of a ballistic arc, then the difference in impact velocity and kinetic energy will be even greater, potentially affecting ballistic sedimentation mixing with the underlying regolith [8]. Topography also affects the temporal spacing of material hitting the surface as a function of radial distance. Uphill slopes can compress, and downhill slopes can extend, the time over which ejecta lands. Ejecta on ballistic trajectories have a horizontal velocity component that will create flow across the surface. The flow of ejecta landing on uphill slopes will be impeded while that on downhill slopes will be enhanced. In the Haworth-Mons Malapert case, radially outward (and downward) flow of material on flight path e will likely be enhanced. Ejecta thickness. Because topographic highs, like massifs, intercept ejecta, they modify the thicknesses of ejecta as a function of radial distance. In the first case study (Fig. 1, top), ejecta that would have landed at a distance of 68 km on a flat surface instead lands at a distance of only 58 km. The thickness of that 58 km deposit is about 2/3 the thickness it would have otherwise been (i.e., 1.9+2.0-1.0 m rather than 3.0+2.9-1.5 m). Because the thickness is less, the deposited mass is less, although – in this case at that distance – the mass difference is not large. Conclusions: Ballistic shadowing like that described here occurred in the Artemis exploration region (e.g., Nobile crater ejecta was affected by Mons Mouton) and was possible along other portions of the South Pole-Aitken basin margin where massifs rise dramatically above the surrounding landscape. Topography that intercepts ejecta reduces time of flight, impact velocity, impact kinetic energy, and mass deposited as a function of radial distance. Models of ejecta distribution could be modified to account for the topographically-complex lunar surface. However, as this analysis has shown, the consequences for depositional thicknesses of ejecta may be modest unless ejection angles are <45°. Perhaps more important are slopes on the topographic features that enhance downhill movement of ejecta after it has landed. References: [1] Krasilnikov A. S. et al. (2022) LPS LIII, Abstract #1988. [2] Tai Udovicic C. J. et al. (2023) JGR: Planets, 128, e2022JE007567. [3] McGetchin T. R. et al. (1973) EPSL, 20, 226–236. [4] Kring D. A. (1995) JGR, 100, 16979–16986. [5] Fassett, C. I. et al. (2011) GRL, 38, L17201. [6] Gault D. E. et al. (1963) NASA TN-D-1767. [7] Moore H. J. et al. (1974) Proc. Lunar Sci. Conf. 5th, 71–100. [8] Oberbeck V. R. (1975) Rev. Geophys., 13, 337–362. Figure 1. (top) LOLA-calibrated and QUICKMAP-derived cross-section of massif and crater. Representative ballistic flight paths assume 45° launch angles. Calculated distances of nominal and maximum edges of the continuous ejecta blanket per [7] are labeled a and b. Path c represents material that lands on the summit. Vertical exaggeration = 3×. (bottom) Cross-section of Haworth crater and Mons Malapert with representative ballistic flight paths. Nominal and maximum edges of the continuous ejecta blanket were calculated using transient crater (a,b) and final crater (c,d) dimensions per [7]. Path e illustrates the downhill flow opportunity of ejecta landing beyond the summit of the massif. Vertical exaggeration = 3×.