CPR Images of unusual craterNASA Radar returns first high-resolution view of an unusual crater near Moon’s north pole.

Mini-RF, a synthetic aperture radar on board NASA’s Lunar Reconnaissance Orbiter, recently imaged a potentially ice-rich crater near the north pole of the Moon. Located at 84°N, 157°W, this permanently shadowed crater, about 5 miles (8 kilometers) in diameter, lies on the floor of the larger, more degraded crater Rozhdestvensky (110 miles, or 177 kilometers in diameter). With no sunlight to warm the crater floor and walls, ice brought to the Moon by comets or formed through interactions with the solar wind could potentially collect here.

Location map

The crater was first identified as a region of interest during the Indian Chandrayaan-1 mission last year, when it was seen to exhibit unusual radar properties consistent with the presence of ice. But with a resolution 10 times better than the radar aboard the Chandrayaan-1 spacecraft, Mini-RF allows us to see details of the crater’s interior. In particular, the circular polarization ratio (CPR) measures the polarization characteristics of the radar echoes, which give clues to the nature of the surface materials.

Same Sense radar (left) and colorized CPR (right).

The inset figure shows a “same-sense” radar image of the crater (left) next to a colorized CPR image of the crater. Red pixels have CPR values greater than 1.2. The CPR values inside the crater are almost all greater than 1, whereas the CPR values outside the crater are generally low (much less than 1). Regions with CPR greater than 1 are relatively rare in nature, but are commonly seen in regions with thick deposits of ice (such as Martian polar caps, or the icy Galilean satellites). They are also seen in rough, blocky ejecta around fresh, young craters, but in that occurrence, scientists also observe high CPR outside the crater rim. This feature has high CPR inside its rim, but low CPR outside. The Mini-RF team plans to examine data from the other LRO instruments, particularly temperature and topographic measurements, to better characterize the environment and setting of these unusual features near the poles of the Moon.

Unusual North Pole Crater
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