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Their common features: mostly rocky (though perhaps with a metallic core).
Features that vary from planet to planet: different amounts of craters,
volcanoes, plate tectonics,
These planets got to be the way they are through a history that includes
accidents but also cuases and effects. The way a planet formed affected
how it ended up through its: mass
and radius, distance from the Sun, rotation rate,
and composition.
Actually the most useful way to think about the layers inside the Earth is
in terms of what is most solid and what can be most easily deformed. The
solid layer, which includes both some "crust" and "mantle" is called the
"lithosphere." Closer to the center of the Earth, the temperature rises,
and hot rock becomes more pliable, more easily stretched. This layer we
call the "asthenosphere". Continental drift (plate tectonics) happens
because the more solid lithosphere "floats" on the asthenosphere.
What cools off a planet? Well, volcanoes carrying hot lava to the
surface can bring heat from the inside to the outside. Also,
convection brings heat up. Convection happens whenever something
heats up, expands, and rises. Then cool stuff sinks. So convection leads
to a "churning"--whether it is in an atmosphere or in molten rock.
But a planet can only cool off through its surface. A small planet has
more surface relative to its volume than a large planet. So small planets
cool off fastest. It's like baked potatoes--a large baked potato will
stay hot longer.
This is why Mercury and the Moon no longer have active volcanoes or
continental drift. The insides of these terrestrial worlds have cooled
off.
The Terrestrial Planets
The terrestrial planets include Mercury, Venus, Earth, and Mars
(going further out from the Sun), and we'll also include Earth's Moon. Inside a Terrestrial
Planet
As the Earth and other terrestrial planets formed, heavy things fell
towards the center (this is called differentiation). So we tend to
have metals near the "core" of the Earth and less dense rocks further out
(in the "mantle") and finally, a layer of scum called the "crust"--that's
where we live. Why Cores are Hot
The core of the Earth and other terrestrial planets is hot for several
reasons. Here are 3 that play roles at some time in a planet's history:
All of these things were more effective in the past. Over time,
radioactivity gets used up. (You don't have to study in detail the
parts in the book about radioactive half-life.) So the terrestrial
planets have been cooling down over time.
