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Lunar and Planetary Institute

The Poles of the Moon

Dr. Paul Spudis

Dr. Paul D. Spudis
Planetary Geology and Remote Sensing

The poles of the Moon are unique environments and preserve a surprising record of water and other volatile substances on the lunar surface. The spin axis of the Moon is nearly perpendicular to the ecliptic (the plan in which the Moon orbits the sun) so that the sun always appears just at the horizon at the poles. NASA's Lunar Reconnaissance Orbiter (LRO) spacecraft has mapped the terrain and topography of the Moon, showing us the landforms and locations of ridges, mountains, and holes. Because of the craters and mountains of the Moon, some areas near the pole are in near-permanent sunlight while others are permanently dark. The DIVINER instrument on LRO has measured the surface temperatures of the polar regions and find these dark areas to be extremely cold, never reaching surface temperatures greater than about 25° to 30° above absolute zero (–272° C). Consequently, the extremely cold temperatures in these areas permit any volatile substance, like water, to accumulate there.

 

Water is present at the poles of the Moon in several different forms.  The Moon Mineralogy Mapper experiment on the Indian Chandrayaan-1 spacecraft mapped the 2800 nm spectral channel, a band where a significant water absorption feature is expected. Surprisingly, water is detected above about 65° latitude, consistent with a monolayer of water (H2O) or hydroxyl (OH) on dust grains in the regolith.  The strength of the absorption increases poleward (with decreasing mean surface temperature.) Additionally, the Mini-RF experiment on LRO mapped the poles with radar. Results show that some craters near the pole contain material with high RF backscatter, known as high ratios of circular polarization. These anomalous craters, which all have permanently dark floors, show high CPR in their interiors but low CPR outside the crater rim. These results are consistent with the presence of relatively pure water ice, meters thick. The nature, distribution and concentration of water at the poles of the Moon must be verified and mapped in detail by a properly equipped surface rover to determine the nature and extent of lunar polar volatiles.

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