DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

lunar libration
mainly in their concentrations of the metal oxides,
which are richer in calcium (Ca) and aluminum
(Al) than are the lowlands.
Highlands geology consists of craters 1 m
to more than 1000 km across, and overlapping
layers of material ejected from craters. Rocks
brought back from the highlands vary in age (the
time since it last solidified from molten liquid)
between 3.84 and 4.48 billion years old. The
highlands are the oldest parts of the moon, not
having been resurfaced by mare lava flows.
lunar libration The apparent rocking of
the orientation of the moon due to geometrical
and orbital relations between the Earth and the
moon. These arise from (a) the ellipticity of the
moon’s orbit (e = 0.055), (b) the slight nonalignments
of the lunar orbit: the lunar equator
istiltedfromtheeclipticby1 ◦ 32 ′ fromtheecliptic
and the lunar orbital plane is tilted 5 ◦ 9 ′ from
ecliptic; thus the lunar equator is tilted from its
orbital plane by 6 ◦ 41 ′ , (c) parallax from the
Earth (different viewpoints between moonrise
and moonset). This leads to longitudinal librations
of about 7.7 ◦ due to the ellipticity, latitudinal
librations of about 6.7 ◦ due to the tilt of
the lunar equator from the ecliptic, and about 2 ◦
total due to parallax from the Earth. As a result,
about 15 ◦ additional surface of the moon is
visible at various times from the Earth.
Lunar Maria Darker low lying areas on the
moon which are large basaltic lava flows. The
maria (latin plural of mare, “sea”) are younger
(they have fewer impact craters), and are richer
in magnesium (Mg) and iron (Fe) than are the
highlands. The maria were apparently formed
by very large impacts and clearly represent flow
into low lying regions. They solidified near or
after the end of the cratering epoch because little
subsequent cratering occurred. Although the
impacts forming the maria may have occurred
about 4 billion years ago, the maria were apparently
kept molten by heating from heavier radioactive
elements which flowed up to the surface
following the impacts that formed them.
Mare rocks have been measured to be between
3.15 and 3.77 years old, which provides an estimate
of the time of the solidification of the mare.
The maria occur essentially only on the side of
© 2001 by CRC Press LLC
290
the moon facing the Earth; the far side is essentially
all highlands.
lunar mascons The Lunar Maria are generally
associated with strong positive gravity
anomalies. Because the maria are low-lying, the
gravity anomalies are attributed to buried positive
density anomalies or loads. These loads are
known as mascons.
lunar meteorites Currently there are 13 meteorites
believed to be from the moon. After
the discovery of young meteorites believed to
be from Mars (see martian meteorites), scientists
began searching for lunar meteorites. Since
Martian meteorites are believed to be ejected
from Mars by meteorite impact, it was argued
that the lower escape velocity from the moon
should allow lunar meteorites to be common as
well. The first meteorite recognized to be from
the moon was discovered in 1981–1982. The lunar
meteorites are similar to lunar rocks returned
by the Apollo and Luna missions, reflecting both
basaltic (from the Lunar Maria) and anorthositic
(from the lunar highlands) compositions. However,
none of the lunar meteorites is exactly identical
to the chemical composition of the returned
lunar samples, which indicates the lunar meteorites
are from locations on the moon not visited
by the human and robotic explorers.
Lunar Prospector A NASA Discovery Mission
launched to the moon on January 6, 1998,
that marked the first NASA Moon mission in 25
years. It uses a complement of five instruments
that address questions concerning the moon’s
resources, its structure, and its origins. During
its primary 1 year polar orbiting mission, data
returned suggesting that the craters at the north
and south poles contain up to 300 million metric
tons of frozen water, as implied by the detection
of hydrogen. The ice is probably in the form of
frost mixed with lunar soil. The general consensus
is that the water was supplied over billions of
years via cometary bombardment. Because the
sun makes a shallow angle at the moon’s poles,
the bottoms of the polar craters never see sunlight
and so trap the frozen water. The amount
of ice may indicate the frequency of cometary
hits and how long the poles have been in their
present orientation.