Patchy secondary illumination at the shackleton crater floor permanently

Introduction: The lunar south pole is located on the rim of Shackleton crater (diameter (D) = 21 km), a wellpreserved ancient (3.6Gy [1]) simple crater. Due to low obliquity of the Moon and its depth (4000 m), the floor and most of the walls of Shackleton crater are in permanent shadow resulting in temperatures constantly below 120K (summer high) and reaching as low as 40K (winter low) where volatile molecules deposited from various sources (e.g. exogenic and endogenic) can remain cold trapped over geologic time. [2, 3]. The cold trap nature of Shackleton crater and the apparent high reflectance of the floor and walls make it a prime target for volatile exploration. While average temperatures below 90K are conducive for volatile retention, albedo signatures obtained at Shackleton PSR (wall and floor) are not conclusive for exposed ice but may exist as mixture with soil [4]. An alternate hypothesis for the bright walls is ongoing down-slope creep resulting in exposure of fresh crustal materials [5]. The floor consists masses of hummocky deposits interpreted as material slumped from the walls – a morphology seen in other lunar craters of similar size and age (e.g. Hipparchus G, D = 15 km, Imbrian age; 5.03° S, 7.40° E.). Topographic characterization of the Shackleton floor was obtained from Lunar Reconnaissance Orbiter Lunar Orbiter Laser Altimeter [6]. The largest floor mound (south) exhibits ∼200m of relief and is hypothesized to be a combined result of ejecta fallback and wall slumping [6]. North of this large mound is a more spreadout deposit of shorter relief (∼100m). Compared to the depth of Shackleton, the relief of floor features is small, yet, we find that local topographic features exert strong influence on received secondary radiance. In this abstract, we discuss the possible variation in floor temperatures, and thus cold trap behavior, based on the dynamic secondary illumination of the crater interior. Methods: Inside PSRs, secondary illumination received from scattered light reflected from nearby topographic facets controls the temperature and lighting conditions for imaging over time. The dynamic secondary lighting can be modeled simply based on the sub-solar point and topography [7]. The secondary illumination is computed only for the topographic facets within the PSR boundary based on the calculations of viewfactors that represent the path between the primary illuminated facets (areas outside the PSR) and the facets inside the PSR. For simplicity, We assumed a Lambertian photometric function and uniform albedo for the simulations. Further details of the simulation and examples of usage Figure 1: Comparison of Kaguya TC image (DTMTCO 0 3 00504S894E0575PS) and simulated image at same subsolar point (Lon=58.769, Lat=-1.514): A-Primary illumination with no stretch, BStretched image showing inside the PSR, CSimulated primary and secondary illumination superposed, DSecondary illumination at compared at crater floor