Lack of Exposed Ice Inside Lunar South Pole Shackleton Crater

Abstract

The inside of Shackleton Crater at the lunar south pole is permanently shadowed; it has been inferred to hold water-ice deposits. The Terrain Camera (TC), a 10-meter-resolution stereo camera onboard the Selenological and Engineering Explorer (SELENE) spacecraft, succeeded in imaging the inside of the crater, which was faintly lit by sunlight scattered from the upper inner wall near the rim. The estimated temperature of the crater floor, based on the crater shape model derived from the TC data, is less than approximately 90 kelvin, cold enough to hold water-ice. However, at the TC's spatial resolution, the derived albedo indicates that exposed relatively pure water-ice deposits are not on the crater floor. Water-ice may be disseminated and mixed with soil over a small percentage of the area or may not exist at all.

An upper limit for ice in Shackleton crater as revealed by LRO Mini‐RF orbital radar

“…However, detailed follow‐up investigations with high resolution, Earth‐based radar revealed evidence for high CPR (circular polarization ratio) values present high on Shackleton's walls, suggesting a source other than accumulations of slab‐like ice [ Campbell et al , 2003, 2006]. The maximum albedo of the crater floor as measured by scattered sunlight is also inconsistent with abundant surficial ice [ Haruyama et al , 2008].…”

Effects of orbital evolution on lunar ice stability

“…Shackleton was chosen for its proximity to the lunar south pole, but with an estimated age of 3.6 Gyr [ Spudis et al , 2008] it may not even have formed until after the Cassini transition. Shackleton was found by the Selene mission to be deeper than the average crater (with D = 5) and was found to have an atypical truncated cone shape [ Haruyama et al , 2008]. However, as discussed in section 4.1, an analytical bowl shaped model with D = 8 (or roughly 2.6 km depth) was found to agree more closely with temperature measurements of Shackleton from the Diviner Lunar Radiometer.…”

“…Explicitly, the surface heat balance can be written with F as the absorbed insolation (, F , or 14, F c ), ɛ the infrared emissivity, σ Boltzmann's constant, and k thermal conductivity of the surface layer. Calculations here use a 23% solar albedo [ Haruyama et al , 2008] and emissivity of 0.95 [ Vasavada et al , 1999].…”

“…Modeled current temperatures at the center of Shackleton crater as compared to brightness temperature measurements (which assume unit emissivity) for a 500 m box at the center of Shackleton from the Diviner Lunar Radiometer 100–400 micron channel. The D = 5 model uses observed crater geometry and albedo [ Haruyama et al , 2008]. D = 8 was found to be a better fit to Diviner data, likely owing to the conical nature of Shackleton.…”

“…Craters with greater diameter to depth ratios are shallower, meaning that the floors see the walls at larger angles and therefore remain cooler [ Paige et al , 1992; Vasavada et al , 1999]. Though Shackleton is flat floored with conical walls and relatively deep for a crater its size (D = 5) [ Haruyama et al , 2008], we model it as a shallow bowl‐shaped crater (D = 8). This assumption better approximates the true scattering geometry within Shackleton while still allowing us to use the analytical solution for bowl‐shaped craters.…”