The moon: explanation
MOONS In Depth Introduction The brightest and largest object in our night sky,
Earth's Moon is the only place beyond Earth where humans have set
foot, so far. Earth's only natural satellite is simply called "the Moon" because
people didn't know other moons existed until Galileo Galilei discovered four
moons orbiting Jupiter in 1610. In Latin, the Moon is called Luna, which is the
main adjective for all things Moon-related: lunar.
Size and Distance Size and
Distance With a radius of about 1,080 miles (1,740 kilometers), the Moon is less
than a third of the width of Earth. If Earth were the size of a nickel, the Moon
would be about as big as a coffee bean. The Moon is an average of 238,855 miles
(384,400 kilometers) away. That means 30 Earth-sized planets could fit in
between Earth and the Moon.
The Moon is slowly moving away from Earth, getting
about an inch farther away each year. Orbit and Rotation Orbit and Rotation The
Moon is rotating at the same rate that it revolves around Earth (called
synchronous rotation), so the same hemisphere faces Earth all the time. Some
people call the far side – the hemisphere we never see from Earth – the "dark
side" but that's misleading. As the Moon orbits Earth, different parts are in
sunlight or darkness at different times. The changing illumination is why, from
our perspective, the Moon goes through phases.
During a "full moon," the
hemisphere of the Moon we can see from Earth is fully illuminated by the Sun.
And a "new moon" occurs when the far side of the Moon has full sunlight, and the
side facing us is having its night. The Moon makes a complete orbit around Earth
in 27 Earth days and rotates or spins at that same rate, or in that same amount
of time. Because Earth is moving as well – rotating on its axis as it orbits the
Sun – from our perspective, the Moon appears to orbit us every 29 days.
Structure Structure Earth's Moon has a core, mantle, and crust. The Moon’s core
is proportionally smaller than other terrestrial bodies' cores. The solid,
iron-rich inner core is 149 miles (240 kilometers) in radius. It is surrounded
by a liquid iron shell 56 miles (90 kilometers) thick.
A partially molten layer
with a thickness of 93 miles (150 kilometers) surrounds the iron core. The
mantle extends from the top of the partially molten layer to the bottom of the
Moon's crust. It is most likely made of minerals like olivine and pyroxene,
which are made up of magnesium, iron, silicon, and oxygen atoms. The crust has a
thickness of about 43 miles (70 kilometers) on the Moon’s near-side hemisphere
and 93 miles (150 kilometers) on the far-side. It is made of oxygen, silicon,
magnesium, iron, calcium, and aluminum, with small amounts of titanium, uranium,
thorium, potassium, and hydrogen. Long ago the Moon had active volcanoes, but
today they are all dormant and have not erupted for millions of years.
Formation
Formation The leading theory of the Moon's origin is that a Mars-sized body
collided with Earth about 4.5 billion years ago. The resulting debris from both
Earth and the impactor accumulated to form our natural satellite 239,000 miles
(384,000 kilometers) away. The newly formed Moon was in a molten state, but
within about 100 million years, most of the global "magma ocean" had
crystallized, with less-dense rocks floating upward and eventually forming the
lunar crust.
Surface Surface With too sparse an atmosphere to impede impacts, a
steady rain of asteroids, meteoroids, and comets strikes the surface of the
Moon, leaving numerous craters behind. Tycho Crater is more than 52 miles (85
kilometers) wide. Over billions of years, these impacts have ground up the
surface of the Moon into fragments ranging from huge boulders to powder. Nearly
the entire Moon is covered by a rubble pile of charcoal-gray, powdery dust, and
rocky debris called the lunar regolith. Beneath is a region of fractured bedrock
referred to as the megaregolith. The light areas of the Moon are known as the
highlands. The dark features, called maria (Latin for seas), are impact basins
that were filled with lava between 4.2 and 1.2 billion years ago. These light
and dark areas represent rocks of different compositions and ages, which provide
evidence for how the early crust may have crystallized from a lunar magma ocean.
The craters themselves, which have been preserved for billions of years, provide
an impact history for the Moon and other bodies in the inner solar system. If
you looked in the right places on the Moon, you would find pieces of equipment,
American flags, and even a camera left behind by astronauts. While you were
there, you'd notice that the gravity on the surface of the Moon is one-sixth of
Earth's, which is why in footage of moonwalks, astronauts appear to almost
bounce across the surface.
The temperature on the Moon reaches about 260 degrees
Fahrenheit (127 degrees Celsius) when in full Sun, but in darkness, the
temperatures plummet to about -280 degrees Fahrenheit (-173 degrees Celsius).
Water on the Moon During the initial exploration of the Moon, and the analysis
of all the returned samples from the Apollo and the Luna missions, we thought
that the surface of the Moon was dry. The first definitive discovery of water
was made in 2008 by the Indian mission Chandrayaan-1, which detected hydroxyl
molecules spread across the lunar surface and concentrated at the poles.
Missions such as Lunar Prospector, LCROSS, and Lunar Reconnaissance Orbiter,
have not only shown that the surface of the Moon has global hydration but there
are actually high concentrations of ice water in the permanently shadowed
regions of the lunar poles. Scientists have discovered that water is being
released from the Moon during meteor showers.
Credit: NASA's Goddard Space
Flight Center Scientists also found the lunar surface releases its water when
the Moon is bombarded by micrometeoroids. The surface is protected by a layer, a
few centimeters of dry soil that can only be breached by large micrometeoroids.
When micrometeoroids impact the surface of the Moon, most of the material in the
crater is vaporized.
The shock wave carries enough energy to release the water
that’s coating the grains of the soil. Most of that water is released into
space. In October 2020, NASA’s Stratospheric Observatory for Infrared Astronomy
(SOFIA) confirmed, for the first time, water on the sunlit surface of the Moon.
This discovery indicates that water may be distributed across the lunar surface,
and not limited to cold, shadowed places. SOFIA detected water molecules (H2O)
in Clavius Crater, one of the largest craters visible from Earth, located in the
Moon’s southern hemisphere. Atmosphere Atmosphere The Moon has a very thin and
weak atmosphere, called an exosphere. It does not provide any protection from
the Sun's radiation or impacts from meteoroids.
Magnetosphere Magnetosphere The
early Moon may have developed an internal dynamo, the mechanism for generating
global magnetic fields for terrestrial planets, but today, the Moon has a very
weak magnetic field. The magnetic field here on Earth is many thousands of times
stronger than the Moon's magnetic field. Rings Rings The Moon has no rings.
Moons Moons Earth's Moon has no moons of its own. Potential for Life Potential
for Life The many missions that have explored the Moon have found no evidence to
suggest it has its own living things. However, the Moon could be the site of
future colonization by humans. The discovery that the Moon harbors water ice,
and that the highest concentrations occur within darkened craters at the poles,
makes the Moon a little more hospitable for future human colonists
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