The Physical Moon and its History
Looming some 384.400
km (239,000 miles) from the
Earth, with a diameter of 3476 km (2,160 miles) and a mass of
7.35e22 kg. shines the Moon. The Moon is the brightest object
in the night sky and only second in brightness to that of the
Sun. Its mean density is only 3.34 times that of water. It has
no real atmosphere and no magnetic field of its own and is the
only natural satellite of our planet Earth. In fact, the Moon is
next to the largest moon in our solar system; the Earth and Moon can
almost be said to be a double planet. The Moon undergoes extremes in
temperature: it is alternately scorched to +110 degrees celsius during
the day and frozen to -180 degrees celesius at night. Many of us have
seen its eclipses and occultations. The
Moon shines by reflected sunlight as it passes
through each of its familiar phases, and
the Moon (with a little help from the Sun) also controls the ocean's tides.
The Moon's Origins
The Moon's origins still remain
unsettled. A similarity in the composition of the Moon with the Earths
rules out any sort of capture theory. One of the most probable theories
is that at one time, a giant object, conceivably the size of Mars,
collided with the primordial Earth, shattering what crust the earth
had. The sheer force of an impact this size is believed to have forged
an immense gorge, melting the Earth's crust, and ejected a shower of
molten rock and part of the Earth's mantle into orbit. It is believed
that the material then condensed to form a massive ring of orbiting
debris. The Moon then formed from this substance.
The Moon's Composition
The Moon possesses a thick outer
crust (60 km) and below the crust
a mantle (60-1000 km), and a partly liquid core (1000-1740 km). Much of
the surface is fractured by massive impact craters formed
by meteorites, flooded by molten lava, and carved out from volcanic
explosions. The Moon's surface shows the scars of more than 3
billion years of meteorite impacts, most of which developed between
3000 and 4000 million years ago. The youngest Moon rocks are virtually
as old as the oldest Earth rocks. The largest craters are approximately
200 km in diameter, while the smallest are only about a meter
across. Impact features include crater clusters, dark halo craters,
rays, and crater chains. Recent data acquired by the Clementine
spacecraft indicates that there is also ice in the bottom of a crater
located on the Moon's South pole.
(Photo above is of the crater Copernicus,
courtesy of NASA)
The Moon's surface is covered by a rocky material called the lunar
Regolith. The Regolith consists of fine dust particles, glass spheres and a jumble of large boulders and rocky debris, produced
by constant meteor bombardments occurring throughout geological
time. Dust particles were created by the Moon's surface melting
by the heat of the impacts.
Some of the rarer moon rocks include granite, pyroxenite, norite,
green glass, tractolite and denite.
terrain is divided into two sharply contrasting areas,
the light rugged and very ancient mountainous regions (terrae)
and the dark smooth and younger lowlands (Maria). The Maria comprises
about 16% of the Moon's surface and consists of huge impact basins
flooded by molten lava some 3000 million years ago. The Maria,
for some unknown reason, are concentrated on the near side of the
Moon. The Mare Imbrium is the largest crater with a diameter
of 420 miles (676km). It is rimmed by a mountain range, and was created
by the imapct of an asteroid with an estimated diameter of 80 miles.
The Lunar Highlands are topographically higher then the
and are characterized by elevated rims of craters, volcanic domes
and plateaus, although few have slopes that exceed a 12% incline.
These are surrounded by the rolling hills of basin ejecta blankets,
consisting of crushed rock blasted out of the crater by meteorites.
The dark lunar plains show wrinkled ridges a few hundred feet
high and an unusual type of valley known as a "rille". The rille
has a flat floor and steep parallel walls.
following topics now have thier very own pages, as this page was
getting a bit lengthy.
Eclipses | Phases, | Tides