4.6 billion years ago,
the Terrestrial planets formed through the coalescence of
small solid particles into planetesimals and then
through the accretion of
the planetesimals into planet-sized objects. The planetesimals
were unable to capture and hold gases because of their small masses
and the high temperature of the inner Solar Nebula. The formation process
led to an initially homogeneous set of bodies, that is,
objects where the chemical elements were mixed together. After formation,
the interiors of the planets melted (or at least became soft) through:
Chemical
differentiation then took
place during the first few 100 million years of the planets' lifetimes.
Chemical differentiation
Chemical differentiation is
easy to understand; dense material
sinks while less dense material floats. For example, in a glass of water,
most metals sink because they are denser than water. Things like cork and
most wood float because they are less dense than water.
For the Earth, this means that denser
elements such as iron and nickel sink to the center of the Earth
while less dense materials like the silicates rise (float) to the
surface.
This effect leads to a three-layered structure for the planet; the
crust, mantle, and
core
(where these layers are defined by their chemical composition).
Interior of the Earth (and Moon)
To demonstrate the rich range of structures and behaviors possible
for Terrestrial planets,
consider the current Earth:
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COMMENTS
- The temperature of the Earth increases as you
move inward. This is due to
the fact that there is
residual heat left over from the formation of the
Earth (accretion heat) and there is current heat input
from the decay of the
long-lived radioactive nuclei. At the core, based on models
of the interior of the Earth, the temperature has been inferred to
be as high as 7,500 C or roughly
13,500 F.
- The material which makes up the Earth goes from solid
to plastic to liquid
to solid as one moves from the crust to the mantle to the core.
- The separation of the Earth into the
crust, the mantle, and the core (although natural because
it represents a
separation by composition) is not the best way
to look at the structure of
the Earth from a mechanical standpoint.
It turns out that the crust and
the outer part of the mantle form a rigid unit,
the lithosphere, which
sits upon a plastic layer (which can "flow")
known as the asthenosphere.
Below the asthenosphere sits the core material.
- Since the Earth is hottest at the center and
coolest at the surface, heat
must flow from the center outward in the Earth.
In general, there are three ways
for heat to flow:
The fact that energy is carried by convection
in the mantle turns out to be very
important as these large-scale mass motions give rise to
tectonic activity.
More on this important topic later.
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