Clementine: a New Beginning
The Clementine spacecraft orbited the Moon and collected science data for 10 weeks in the Spring of 1994. During this time, it collected global multispectral images and near-global altimetry. Select areas of the Moon were imaged at 25 m/pixel in visible light and 60 m/pixel in thermal wavelengths. From these data, a new paradigm for the evolution of the lunar crust emerged. the Moon is no longer viewed as a two-terrane planet, the Apollo samples were found not to represent the lunar crust as a whole, and the complexity of lunar crustal stratigraphy was further revealed.
NASA's Lunar Prospector Mission orbited the Moon from 1998-1999 and provided important new insights into the origin and evolution of the lunar crust. It carried no cameras, but rather an array of instruments to produce the first global maps of the Moon's gravity, magnetic field, and surface composition. Two instruments in particular produced vital results: the Neutron Spectrometer, which was designed to detect the water hypothesized to exist in some places on the Moon's surface , and the Gamma-Ray Spectrometer (LP-GRS), which directly measured the global distribution of key elements such as iron, potassium, and thorium on the Moon's surface for the first time. The LP-GRS was particularly sensitive to thorium, which is naturally radioactive. This gave lunar scientists their first look at the global distribution of an important thorium-rich lunar material known as KREEP (for Potassium, Rare Earth Elements, and Phosphorous).
Before the Lunar Prospector mission, it was assumed that KREEP materials were a component in a nearly global layer beneath the lunar crust, but the Lunar Prospector results showed that the KREEP materials were concentrated on the lunar nearside, a fact which could be related to the distribution and longevity of lunar mare volcanism.
Lunar Polar Ice?
Although the Moon rocks brought back to Earth by the Apollo astronauts contain no water or hydrous minerals, there are permanently shadowed regions near the north and south poles where sunlight never penetrates. These shadowed regions act as cold traps for volatiles deposited by cometary impacts. Lunar Prospector's Neutron Spectrometer instrument detected high levels of hydrogen (presumably, the H in H2O) at both lunar poles. This has been interpreted to reflect the possible presence of water ice in these craters. Water can be used for life support, agriculture, and propellant, so ice would be a vital resource for future lunar settlers. Understanding how much water exists at the lunar poles remains an important goal of the LRO mission and future lunar exploration.