Download Earth and Moon

Earth looks very different today than it did when it first formed more than 4.5 billion years ago.
Historical geologists study Earth’s past to understand what happened and when it happened.
Paleontologists do the same thing, but with an emphasis on the history of life, especially as it is
understood from fossils. Despite having very little material from those days, scientists have
many ways of learning about the early Earth.
Formation of Earth
Earth came together (accreted) from the cloud of dust and gas known as the solar nebula nearly
4.6 billion years ago, the same time the Sun and the rest of the solar system formed. Gravity
caused small bodies of rock and metal orbiting the proto-Sun to smash together to create larger
bodies. Over time, the planetoids got larger and larger until they became planets.
There is little hard evidence for scientists to study from Earth’s earliest days. Much of what
scientists know about the early Earth come from three sources:
(1) zircon crystals, the oldest materials found on Earth, which show that the age of the earliest
crust formed at least 4.4 billion years ago
(2) meteorites that date from the beginning of the solar system, to nearly 4.6 billion years ago
and (3) lunar rocks, which represent the early days of the Earth-Moon system as far back as 4.5
billion years ago.
https://vimeo.com/34548966
Molten Earth
When Earth first came together it was really hot, hot enough to melt the metal elements that it
contained. Why was the early Earth so hot?
1. Gravitational contraction: As small bodies of rock and metal accreted, the planet grew
larger and more massive. Gravity within such an enormous body squeezes the material
in its interior so hard that the pressure swells. As Earth’s internal pressure grew, its
temperature also rose.
2. Radioactive decay: Radioactive decay releases heat, and early in the planet’s history
there were many radioactive elements with short half-lives. These elements long ago
decayed into stable materials, but they were responsible for the release of enormous
amounts of heat in the beginning.
3. Bombardment: Ancient impact craters found on the Moon and inner planets indicate
that asteroid impacts were common in the early solar system. Earth was struck so much
in its first 500 million years that the heat was intense. Very few large objects have struck
the planet in the past many hundreds of millions of year.
Differentiation
When Earth was forming and was nearly entirely molten, gravity drew denser elements to the
center and lighter elements rose to the surface. The separation of Earth into layers based on
density is known as differentiation. The densest material moved to the center to create the
planet’s dense metallic core. Materials that are intermediate in density became part of the
mantle.
Lighter materials accumulated at the surface of the mantle to become the earliest crust. The
first crust was probably basaltic, like the oceanic crust is today. Intense heat from the early core
drove rapid and vigorous mantle convection so that crust quickly recycled into the mantle. The
recycling of basaltic crust was so effective that no remnants of it are found today.
https://www.youtube.com/watch?v=n6rOM8c7loQ
https://www.youtube.com/watch?v=WwiiOjyfvAU
You can’t see it, but there’s an invisible force field around the Earth. Okay, not a force field,
exactly, but a gigantic magnetic field surrounding the Earth, and it acts like a force field,
protecting the planet – and all the life – from space radiation. Let’s take a look at the Earth’s
magnetic field. The Earth is like a great big magnet. The north pole of the magnet is near the
top of the planet, near the geographic north pole, and the south pole is near the geographic
south pole. Magnetic field lines extend from these poles for tens of thousands of kilometers
into space; this is the Earth’s magnetosphere. The geographic poles and the magnetic poles are
far enough apart that scientists distinguish them differently. If you could draw a line between
the magnetic north and south poles, you would get a magnetic axis that’s tilted 11.3 degrees
away from the Earth’s axis of rotation. And these magnetic poles are known to move around
the surface, wandering as much as 15 km every year.
Scientists think that the Earth’s magnetic field is generated by electrical currents flowing in the
liquid outer core deep inside the Earth. Although it’s liquid metal, it moves around through a
process called convection. And the movements of metal in the core sets up the currents and
magnetic field.
The magnetic field of the Earth protects the planet from space radiation. The biggest culprit is
the Sun’s solar wind. These are highly charged particles blasted out from the Sun like a steady
wind. The Earth’s magnetosphere channels the solar wind around the planet, so that it doesn’t
impact us. Without the magnetic field, the solar wind would strip away our atmosphere – this is
what probably happened to Mars. The Sun also releases enormous amounts of energy and
material in coronal mass ejections. These CMEs send a hail of radioactive particles into space.
Once again, the Earth’s magnetic field protects us, channeling the particles away from the
planet, and sparing us from getting irradiated.
The Earth’s magnetic field reverses itself every 250,000 years or so. The north magnetic pole
becomes the south pole, and vice versa. Scientists have no clear theory about why the reversals
happen. One interesting note is that we’re long overdue for a reversal. The last one happened
about 780,000 years ago.
How does its magnetic field protect the Earth?
QUICK ANSWER
Earth's magnetic field acts as a shield that diverts charged particles from the solar wind away
from tropical and temperate latitudes thus preventing the loss of atmosphere due to impacting
particles from the sun. Planets without strong magnetic fields tend to lose their atmospheres to
space.
FULL ANSWER
The Earth generates a magnetic field from the convection of molten metals, primarily iron and
nickel, near its core. The twisting effect of these metals relative to the planet's spin generates a
magnetic field that is usually strong and consistent. While the field is active, charged particles
from the sun are deflected around the Earth along the field's lines of force. Some particles from
the solar wind are carried along the What field's lines to the north and south magnetic poles,
where they are accelerated into the atmosphere and produce the aurora borealis and aurora
australialis. Without the protection of the magnetic field, the steady pressure of the solar wind
would gradually ablate the upper reaches of the Earth's atmosphere until little air is left. This
process takes a long time, however, and the Earth's magnetic field has weakened and shifted
polarity many times in the past without noted ill effects on life on the surface.
https://www.youtube.com/watch?v=_037xabLLAI
We cannot see the deep interior of Earth, but we know from a variety of observations that it is
in continuous motion. This is because the mantle convects. This fundamental planetary process
has profoundly influenced the character and evolution of Earth.
What is Convection?
Convection is the process by which less dense material rises and more dense material sinks. The
former is said to be more “buoyant” than the latter, and the vertical forces due to density
difference are referred to as buoyancy forces. Rocks, water, and air—indeed, most materials—
expand and thus become less dense as temperature increases, so convection is typically driven
by temperature differences. In Earth’s mantle, hot rock rises and slightly cooler rock sinks.
Convection drives our dynamic planet. The planet’s liquid outer core convects, creating the
Earth’s magnetic field; the ocean convects, enabling exchange of CO2 with the atmosphere and
transporting nutrients from depth that support important fisheries; and the atmosphere
convects, acting in concert with the ocean to transport heat and moisture around the planet to
create climate.
How Does the Mantle (asthenosphere) Convect?
Everyday examples of convection in liquids include lava lamps or water heating on a stove. But
the mantle is, in general, solid. It turns out that rocks, along with most other solids, flow by a
solid-state, creeping motion, especially when they are hot and given enough time. This is what
happens in the asthenosphere (upper mantle). Based on observations of the rates at which the
surface of Earth moves, geologists estimate the asthenosphere convectively flows at rates of
several centimeters a year.
Convection cannot take place without a source of heat. The heat driving asthenosphere
convection has three sources. "Primordial" heat (left over from the accretion and
differentiation that led to the formation of Earth’s core) contributes 20 to 50% of the heat.
Heating due to the decay of radioactive isotopes (mainly potassium, thorium, and uranium)
contributes 50 to 80%. Thirdly, tidal friction from the Moon’s pull on the Earth contributes
perhaps 10%. Mantle convection is the main mechanism by which this heat escapes from the
interior of Earth.
How is Asthenosphere Convection Related to Plate Tectonics?
Plate tectonics refers to the movement of the rigid plates around the surface of Earth. The
outer portion of the planet, or lithosphere, is relatively rigid because it is relatively cold. The
lithosphere varies in thickness but is typically a hundred or so kilometers thick. It includes the
upper mantle and both the continental and oceanic crust. The asthenosphere’s convective
motions break the lithosphere into plates and move them around the surface of the planet.
These plates may move away from, move by, or collide with each other. This process forms
ocean basins, shifts continents, and pushes up mountains.
Tectonic plates break apart and diverge where the asthenosphere beneath is upwelling. In such
regions, mid-ocean ridges develop, and new lithosphere and crust form to replace the material
that is moving away. Where plates converge, usually where the asthenosphere is downwelling,
one plate is forced beneath another. When this involves plates with embedded continental
crust, mountain belts such as the Alps and Himalayas form. If the collision involves plates with
oceanic crust, subduction zones form where one plate descends into the mantle beneath the
other plate. Above these subduction zone chains of volcanoes and island arcs like the Aleutians,
develop.
Below the Aleutian island arc, at depths of 100 to 120 kilometers, water is forced out of the
subducted Pacific plate. This water lowers the melting point of the overlying mantle, causing it
to melt. The melting forms magma, which rises to feed the 55 currently active volcanoes that
make up the island arc. Where plates move by each other, enormous fault systems form.
Hundreds to thousands of kilometers long, these fault systems are responsible for many of the
world’s earthquakes, which occur when a fault ruptures and the accumulated strain is abruptly
released. California's San Andreas fault system is an example. Therefore, asthenosphere
convection not only accounts for ocean basins, continents, and mountains, it is also the
ultimate reason for nearly all earthquakes and volcanoes.
https://www.khanacademy.org/partner-content/amnh/earthquakes-and-volcanoes/platetectonics/v/computer-model-of-mantle-convection
How the Moon Formed
One of the most unique features of planet Earth is its large Moon. Unlike the only other natural
satellites orbiting an inner planet, those of Mars, the Moon is not a captured asteroid.
Understanding the Moon’s birth and early history reveals a great deal about Earth’s early days.
To determine how the Moon formed, scientists had to account for several lines of evidence:
 The Moon is large; not much smaller than the smallest planet, Mercury.
 Earth and Moon are very similar in composition.
 Moon’s surface is 4.5 billion years old, about the same as the age of the solar system.
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For a body its size and distance from the Sun, the Moon has very little core; Earth has a
fairly large core.
The oxygen isotope ratios of Earth and Moon indicate that they originated in the same
part of the solar system.
Earth has a faster spin than it should have for a planet of its size and distance from the
Sun.
Astronomers have carried out computer simulations that are consistent with these facts and
have detailed a birth story for the Moon. A little more than 4.5 billion years ago, roughly 70
million years after Earth formed, planetary bodies were being pummeled by asteroids and
planetoids of all kinds. Earth was struck by a Mars-sized asteroid. The tremendous energy from
the impact melted both bodies. The molten material mixed up. The dense metals remained on
Earth but some of the molten, rocky material was flung into an orbit around Earth. It eventually
accreted into a single body, the Moon. Since both planetary bodies were molten, material could
differentiate out of the magma ocean into core, mantle, and crust as they cooled. Earth’s fast
spin is from energy imparted to it by the impact.
https://www.youtube.com/watch?v=mQAdYWcA7ig