EARTH-BASED RADAR OBSERVATIONS OF THE LUNAR POLAR REGIONS

The suggestion that volatiles might exist in permanently shadowed areas near the poles of the Moon generated several observational tests in the past few years. Despite a number of ground-based and spacecraft measurements, the presence or absence of ice is still not established in a decisive manner. This situation is at odds with the case of Mercury for which there is increasingly compelling evidence for the presence of volatile deposits in cold traps near the poles. Establishing the nature of the lunar polar anomalies and the reasons for the dramatic differences between the Moon and Mercury's polar deposits remain outstanding problems in planetary science. The neutron spectrometer aboard the lunar prospector spacecraft provided direct evidence for the presence of hydrogen near the lunar poles. If interpreted in terms of water ice, the abundance near the south pole represents a 1.5% weight fraction of H₂O. The enhanced hydrogen signatures appear to be correlated with regions of permanent shadow identified by Margot et al., indicating that a cold trap mechanism is involved. It is still unclear whether the trapped hydrogen is in the form of water ice, hydrated minerals, or was implanted by the solar-wind. A study of Clementine radar data resulted in a report of a possible detection of water ice on the Moon, but a separate analysis of the same data did not exhibit a distinctive ice signature. Earth-based radar observations of the lunar poles did not show any extensive anomalies similar to Mercury, but they did result in the identification of 26 bright sub-kilometer regions with anomalous radar signatures near the lunar south pole. Earth-based radar systems provide critical data to address the existence of ice at the lunar poles. We will review the most recent results obtained from the ground, including Goldstone radar interferometric observations and Arecibo-Goldstone bistatic radar observations.