Lroc Observations of Permanently Shadowed Regions: Seeing into the Dark

Permanently shadowed regions (PSRs) near the lunar poles that receive secondary illumination from nearby Sun facing slopes were imaged by the Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Cameras (NAC). Typically secondary lighting is optimal in polar areas around respective solstices and when the LRO orbit is nearly coincident with the sub-solar point (low spacecraft beta angles). NAC PSR images provide the means to search for evidence of surface frosts and unusual morphologies from ice rich regolith, and aid in planning potential landing sites for future in-situ exploration. Secondary illumination imaging in PSRs requires NAC integration times typically more than ten times greater than nominal imaging. The increased exposure time results in downtrack smear that decreases the spatial resolution of the NAC PSR images. Most long exposure NAC images of PSRs were acquired with exposure times of 24.2-ms (1-m by 40-m pixels, sampled to 20-m) and 12-ms (1-m by 20-m, sampled to 10-m). The initial campaign to acquire long exposure NAC images of PSRs in the north pole region ran from February 2013 to April 2013. Relative to the south polar region, PSRs near the north pole are generally smaller (D<24-km) and located in simple craters. Long exposure NAC images of PSRs in simple craters are often well illuminated by secondary light reflected from Sun-facing crater slopes during the northern summer solstice, allowing many PSRs to be imaged with the shorter exposure time of 12-ms (resampled to 10-m). With the exception of some craters in Peary crater, most northern PSRs with diameters >6-km were successfully imaged (ex. Whipple, Hermite A, and Rozhestvenskiy U). The third PSR south polar campaign began in April 2013 and will continue until October 2013. The third campaign will expand previous NAC coverage of PSRs and follow up on discoveries with new images of higher signal to noise ratio (SNR), higher resolution, and varying secondary illumination conditions. Utilizing previous campaign images and Sun's position, an individualized approach for targeting each crater drives this campaign. Secondary lighting within the PSRs, though somewhat diffuse, is at low incidence angles and coupled with nadir NAC imaging results in large phase angles. Such conditions tend to reduce albedo contrasts, complicating identification of patchy frost or ice deposits. Within the long exposure PSR images, a few small craters (D<200-m) with highly reflective ejecta blankets have been identified and interpreted as small fresh impact craters. Sylvester N and Main L are Copernican-age craters with PSRs; NAC images reveal debris flows, boulders, and morphologically fresh interior walls indicative of their young age. The identifications of albedo anomalies associated with these fresh craters and debris flows indicate that strong albedo contrasts (~2x) associated with small fresh impact craters can be distinguished in PSRs. Lunar highland material has an albedo of ~0.2, while pure water frost has an albedo of ~0.9. If features in PSRs have an albedo similar to lunar highlands, significant surface frost deposits could result in detectable reflective anomalies in the NAC images. However, no reflective anomalies have thus far been identified in PSRs attributable to frost.