|How Our Moon was Born
Our Moon was Created by the Largest Explosion in Earths History
|The Moon is Earth's only natural satellite
and is the fifth largest satellite in the Solar System. It is the
largest natural satellite in the Solar System relative to the size of
its planet, a quarter the diameter of Earth and 1/81 its mass, and is
the second densest satellite after Io. |
is in synchronous rotation with Earth, always showing the same face;
the near side is marked with dark volcanic maria among the bright
ancient crustal highlands and prominent impact craters.
the brightest object in the sky after the Sun, although its surface is
actually very dark, with a similar reflectance to coal.
prominence in the sky and its regular cycle of phases have since
ancient times made the Moon an important cultural influence on language,
the calendar, art and mythology. The Moon's gravitational influence
produces the ocean tides and the minute lengthening of the day.
Moon's current orbital distance, about thirty times the diameter of the
Earth, causes it to be the same size in the sky as the Sun—allowing the
Moon to cover the Sun precisely in total solar eclipses. The Moon is
the only celestial body on which humans have made a manned landing.
the Soviet Union's Luna programme was the first to reach the Moon with
unmanned spacecraft, the United States' NASA Apollo program achieved the
only manned missions to date, beginning with the first manned lunar
orbiting mission by Apollo 8 in 1968, and six manned lunar landings
between 1969 and 1972—the first being Apollo 11 in 1969.
missions returned over 380 kg of lunar rocks, which have been used to
develop a detailed geological understanding of the Moon's origins (it is
thought to have formed some 4.5 billion years ago in a giant impact),
the formation of its internal structure, and its subsequent history.
The Moon is a differentiated body: it has a geochemically distinct
crust, mantle, and core. This structure is thought to have developed
through the fractional crystallization of a global magma ocean shortly
after the Moon's formation 4.5 billion years ago.
Moon is a differentiated body: it has a geochemically distinct crust,
mantle, and core.
The moon has a solid iron-rich inner core with a
radius of 240 kilometers and a fluid outer core primarily made of liquid
iron with a radius of roughly 300 kilometers.
Around the core is a partially molten
boundary layer with a radius of about 500 kilometers.
This structure is
thought to have developed through the fractional crystallization of a
global magma ocean shortly after the Moon's formation 4.5 billion years
Crystallization of this magma ocean would have created a mafic mantle
from the precipitation and sinking of the minerals olivine,
clinopyroxene, and orthopyroxene; after about three-quarters of the
magma ocean had crystallised, lower-density plagioclase minerals could
form and float into a crust on top.
The final liquids to crystallise would have been
initially sandwiched between the crust and mantle, with a high abundance
of incompatible and heat-producing elements.
Consistent with this,
geochemical mapping from orbit shows the crust is mostly anorthosite,
and moon rock samples of the flood lavas erupted on the surface from
partial melting in the mantle confirm the mafic mantle composition,
which is more iron rich than that of Earth.
Geophysical techniques suggest
that the crust is on average ~50 km thick. The Moon is the second
densest satellite in the Solar System after Io. However, the core of the
Moon is small, with a radius of about 350 km or less; this is only ~20%
the size of the Moon, in contrast to the ~50% of most other terrestrial
Its composition is not well constrained, but it is probably metallic
iron alloyed with a small amount of sulphur and nickel; analyses of the
Moon's time-variable rotation indicate that it is at least partly
The prevailing hypothesis today is that the Earth–Moon system formed as a result of a giant impact: a Mars-sized body
hit the nearly formed proto-Earth, blasting material into orbit around the proto-Earth, which accreted to form the Moon.
Since the Apollo 17 mission in 1972, the Moon has been visited only by
unmanned spacecraft, notably by Soviet Lunokhod rovers. Since 2004,
Japan, China, India, the United States, and the European Space Agency
have each sent lunar orbiters.
The Universe - The Moon
This series takes a fascinating new look at a very old universe.
Fifty years after man
first ventured into outer space, we examine the greatest secrets of the
Each episode outlines how humans have explored the universe,
and scrutinizes the discoveries they have made.
We look at hi-tech space telescopes which record the violent birth of
stars, robotic rovers which glimpse the red surface of Mars, and
sophisticated NASA probes which delve into the mysterious make-up of
As the earth churns ominously with the effects of global
warming, this is a revealing and prescient journey into the heavens.
From the planets to the stars and out to the edge of the unknown,
history and science collide in this epic exploration of the Universe and
In this installment of The Universe, we scrutinize the
craterous surface of our moon. For thousands of years, mankind has found comfort in its presence. It
has been a lantern for nocturnal travelers, a timekeeper for farmer and a
location finder for sailors at sea.
For some cultures, it has even been
a god. It is also the only cosmic body ever visited by human beings.
Today, NASA is planning a permanent outpost there. We investigate the
scientific origins of the moon, asking how it came to be. The answer is
more astounding and spectacular than most residents of earth have ever
These spacecraft have contributed to confirming the discovery of lunar
water ice in permanently shadowed craters at the poles and bound into
the lunar regolith.
Future manned missions to the Moon are planned but not yet underway; the
Moon remains, under the Outer Space Treaty, free to all nations to
explore for peaceful purposes.
Several mechanisms have
been proposed for the Moon's formation 4.5 billion years ago, some 30–50
million years after the origin of the Solar System.
These include the fission of the Moon from the Earth's crust through
centrifugal forces, which would require too great an initial spin of the
Earth, the gravitational capture of a pre-formed Moon, which would
require an unfeasibly extended atmosphere of the Earth to dissipate the
energy of the passing Moon, and the co-formation of the Earth and the
Moon together in the primordial accretion disk, which does not explain
the depletion of metallic iron in the Moon.
These hypotheses also cannot account for the high angular momentum of the Earth–Moon system.
Orbit of the Moon
Why does the same side of the Moon always face the Earth? The Moon has
synchronous rotation: it's rotation period is the same as its period of
The Moon completes its orbit
around the Earth in approximately 27.3 days (a sidereal month). The
Earth and Moon orbit about their barycentre (common centre of mass),
which lies about 4700 kilometres from Earth's centre (about three
quarters of the Earth's radius).
On average, the Moon is at a
distance of about 385000 km from the centre of the Earth, which
corresponds to about 60 Earth radii.
With a mean orbital
velocity of 1.023 km/s, the Moon moves relative to the stars each hour
by an amount roughly equal to its angular diameter, or by about 0.5°.
Moon differs from most satellites of other planets in that its orbit is
close to the plane of the ecliptic, and not to the Earth's equatorial
plane. The lunar orbit plane is inclined to the ecliptic by about 5.1°,
whereas the Moon's spin axis is inclined by only 1.5°.
is in synchronous rotation, meaning that it keeps the same face turned
toward the Earth at all times. This synchronous rotation is only true on
average, because the Moon's orbit has a definite eccentricity. As a
result, the angular velocity of the Moon varies as it moves around the
Earth, and is hence not always equal to the Moon's rotational velocity.
the Moon is at its perigee, its rotation is slower than its orbital
motion, and this allows us to see up to eight degrees of longitude of
its eastern (right) far side. Conversely, when the Moon reaches its
apogee, its rotation is faster than its orbital motion and this reveals
eight degrees of longitude of its western (left) far side.
is referred to as longitudinal libration. Because the lunar orbit is also
inclined to the Earth's ecliptic plane by 5.1°, the rotation axis of
the Moon seems to rotate towards and away from us during one complete
orbit. This is referred to as latitudinal libration, which allows one to
see almost 7° of latitude beyond the pole on the far side.
because the Moon is only about 60 Earth radii away from the Earth's
centre of mass, an observer at the equator who observes the Moon
throughout the night moves laterally by one Earth diameter.
gives rise to a diurnal libration, which allows one to view an
additional one degree's worth of lunar longitude. For the same reason,
observers at both geographical poles of the Earth would be able to see
one additional degree's worth of libration in latitude.
The prevailing hypothesis today is that
the Earth–Moon system formed as a result of a giant impact: a Mars-sized
body hit the nearly formed proto-Earth, blasting material into orbit
around the proto-Earth, which accreted to form the Moon.
Giant impacts are thought to have been common in the early Solar System.
Computer simulations modelling a giant impact are consistent with
measurements of the angular momentum of the Earth–Moon system, and the
small size of the lunar core; they also show that most of the Moon came
from the impactor, not from the proto-Earth.
However, meteorites show that other inner Solar System bodies such as
Mars and Vesta have very different oxygen and tungsten isotopic
compositions to the Earth, while the Earth and Moon have near-identical
isotopic compositions. Post-impact mixing of the vaporized material
between the forming Earth and Moon could have equalized their isotopic
compositions, although this is debated.
The large amount of energy released in the giant impact event and the
subsequent reaccretion of material in Earth orbit would have melted the
outer shell of the Earth, forming a magma ocean. The newly formed Moon
would also have had its own lunar magma ocean; estimates for its depth
range from about 500 km to the entire radius of the Moon.