How Our Moon was Born
Our Moon was Created by the Largest Explosion in Earths History



 
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.

It 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.

It is the brightest object in the sky after the Sun, although its surface is actually very dark, with a similar reflectance to coal.


Its 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.

The 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.

While 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.

These 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.
 
The 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 ago.



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 bodies.


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 molten.


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.


 
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 heavens.

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 comets.

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 its mysteries.

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 imagined.


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.

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 revolution.

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°.

The 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°.

The Moon 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.

When 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.

This 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.

Finally, 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.

This 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.