"NEW VIEWS OF THE MOON"
Two unmanned orbital missions to the Moon in the 1990’s
have revolutionized our understanding of the Moon and its history and environment.
The Defense Department’s Clementine mission mapped the Moon in 11 wavelengths
in the visible and near-IR and made the first global topographic map with
laser ranging altimetry. NASA’s Lunar Prospector mission mapped the
elemental content of the surface, measured the hydrogen in the lunar soil,
and detected a small metallic core. Together with the legacy from
studies of the Apollo lunar samples, these missions have given us a detailed
picture of lunar evolution, even if some stages of that history remain
obscure. The bulk Moon appears to be enriched in refractory elements,
although it does contain a component of volatile elements; both of these
relations are consistent with the giant-impact hypothesis of lunar origin.
The early Moon melted globally, differentiating into a plagioclase-rich
crust and mafic mantle that was very heterogeneous, laterally and vertically.
Volcanism was extensive on the Moon prior to the end of heavy bombardment
(around 3.8 billion years (Ga) ago) and continued until well after 3 Ga
ago, with small eruptions possibly as late as 1 Ga. Minor bombardment
by asteroids and comets since then have made and gardened the regolith,
occasionally creating spectacular rayed craters, such as Tycho 100 ma ago.
Although the Apollo samples are bone-dry, we now know that water ice deposits
occur in the permanently shadowed regions near both poles of the Moon;
a conservative estimate suggest over 1010 metric tons of water
occur in these permanently dark regions. This resource could support
a human return to the Moon by providing both life support and rocket propellant
for an Earth-Moon transportation infrastructure. Thus, we have determined
that the Moon is both an important scientific object for study as well
as being the stepping stone to future human expansion into the Solar System.