The Formation of The Earth and The Moon


Professor Dave here, let’s look at the Earth. The inner planets of the solar system are
terrestrial planets, which means that they are rocky worlds, and the third rock from
the sun is Earth, our home planet. Although it seems that we are extremely close
to achieving manned missions to other planets, at the time of the writing of this series,
every human being that has ever existed was born on Earth, lived on Earth, and died on
Earth, never to set foot on any other planet. The dawning of the space age and the potential
for future colonization efforts will be discussed later in this series, for now let’s just
go over what we know regarding the formation of the earth, as well as its large moon, our
first encounter with such an object, as Mercury and Venus do not have any natural satellites. When we talked about the formation of the
solar system, we envisioned a protoplanetary disk, full of heavy elements ejected into
the interstellar medium during the death of one or more stars. We also saw how the matter in this disk collected
into pebble-sized and then boulder-sized chunks, and eventually into large planetesimals a
few miles across, all due to gravity. Over hundreds of thousands of years, these
combined further to form rocky objects so large that their shapes were crushed into
spherical form by the immense gravitational field that had been generated. One of these rocky objects was the early earth,
which was in a largely molten state due to all the heat generated by these incredible
collisions. Once most of the matter within earth’s orbital
radius had been collected, collisions became more rare, giving the earth the opportunity
to cool down and form a layer of crust, with the last round of major collisions producing
craters, and releasing huge quantities of gas trapped in the rock. This gas remained gravitationally bound to
the planet, along with other gas ejected during volcanic eruptions, thus generating the early
atmosphere. Comets eventually delivered more of the atmosphere
as well as lots of water, during the late heavy bombardment we mentioned before, but
we will talk about comets later in the series. After continued cooling, water vapor condensed
to form all the oceans, and thus, the primitive earth was born. We’ve already discussed how the size of
the earth was determined, which is around thirteen thousand kilometers in diameter. The composition of the earth is obviously
much better understood than any other object in the solar system, since we are living on
it, and have perpetual direct access to it. The core of the earth is made mainly of iron,
with some nickel, and is divided into a solid inner core, and a liquid outer core. This core is surrounded by a rocky mantle,
comprised largely of different combinations of silicon, aluminum, and oxygen, which we
call silicates, although there are lots of other metals and minerals as well. And all the way at the surface is the crust,
a thin layer of rock that we walk around on every day. We will notice that just like with other planets,
density determines the distribution of matter, with the heaviest elements found at the center,
mainly iron, which is eight grams per cubic centimeter, and lighter compounds distributed
upwards towards the crust, like most rocks, which are around three grams per cubic centimeter. Liquid water, being one gram per cubic centimeter,
sits upon the surface, filling in the nooks and crannies. Lastly, extremely light gases float above
the surface in our atmosphere, warming the planet and shielding us from UV radiation. These gases include nitrogen, comprising 78%
of the atmosphere, oxygen, which is most of the rest, and then some argon, and just a
little bit of water vapor, carbon dioxide, neon, ozone, methane, and a few other things. The very lightest of them, hydrogen and helium,
are so light that they can escape earth entirely, floating off into space. So we can see that density really is the determining
factor in terms of the distribution of substances. This phenomenon regarding the separation of
materials by density is called differentiation, and it happens in large bodies of molten material,
like newly formed planets, just the way that substances separate by density in a centrifuge. Going back to the interior, as we said, the
outer core is liquid, with the mantle above also capable of very gradual flow, and for
this reason, sections of the crust drift over time, which is called continental drift. It was through collisions of these tectonic
plates over millions of years that has produced the major mountain ranges we see today. The circulation happening in this molten core,
when combined with the rotation of the earth, allows for the alignment of unpaired electrons,
which generates an incredible magnetic field. This magnetic field shields the earth from
cosmic rays, deflecting high-energy charged particles to the poles, which then collide
with atmospheric particles to form the dazzling Aurora Borealis, also known as the Northern Lights. So those are the basics regarding the Earth. We could talk for days and days about its
composition, but that discussion belongs to the field of geology, so we will have to pick
things back up in the upcoming geology series. To get back to astronomy, we have to deal
with Earth’s moon. Where did this come from? First, let’s make the distinction between
a planet and a moon. A planet orbits a star, while a moon orbits
a planet, meaning it is a natural satellite of a planet. Earth has just one moon, visible as the most
prominent feature of the night sky. The evidence is very clear in suggesting that
the moon is the result of a collision between two huge planetesimals in the early stages
of earth’s formation. One of these was to become the earth, and
the other must have been a Mars sized object, and when these two objects crashed into one
another, enormous amounts of rock were splashed up into space. This debris, gravitationally bound to the
earth, eventually collected to become the moon just the way all the planets initially
formed. This model also explains the composition of
the moon, rich in silicates and poor in iron, since only Earth’s mantle was blasted apart
from the collision. It also explains the tilt in earth’s rotational
axis, from the impact, and so many other features about this two-body system. The moon was able to cool very quickly upon
forming, due to its small size, resulting in a rocky crust, covered with craters as
well as dark patches called maria. With such a small size and cool interior,
there is no mechanism by which the moon’s features can change, so it has looked pretty
much like it does now since very soon after its formation. There is no atmosphere, as any gas can easily
escape the moon’s gravity, and therefore no wind, which is why the footprints made
by the men who have been on the moon will likely remain intact for millions of years. The moon orbits the earth once every twenty-seven
days or so, and its rotational period is exactly the same as its orbital period, meaning it
rotates on its own axis once in the same time that it takes to orbit once. This is called synchronous rotation, and it
is the reason that we are always looking at the same side of the moon. This phenomenon is a result of tidal forces
between the earth and the moon, and this gravitational force is also responsible for the tides on
earth. The moon tugs very slightly on the earth,
and all the water in the oceans, being liquid, will actually bulge out slightly towards the
moon, causing high tide in these areas and low tide in these. The moon will tug in this manner constantly,
but as the earth rotates, the locations where the bulges can be found will change, and thus
in any particular coastal location, the tides come in and out over time. There are solar tides as well, although they
are much less significant than lunar tides. In this way, the moon and the sun are the
only two objects in the universe that have significant gravitational influence on the
earth. As it happens, we believe that the tides may
have been a crucial component in the development of early life, so it’s possible that without
the moon, life on earth would not exist. Speaking of life on earth, we’ve now really
tied everything together, from the birth of the universe, to the formation of our galaxy,
and then our solar system, and even our planet, all the way until conditions were such that
life could begin. Phytoplankton and eventually plants began
to generate all the oxygen we have today by photosynthesis, setting the stage for complex
animal life. Over millions of years, that life produced
humans, who were then free to look up at the sky, and realize that we are on a spinning
rock, close to a star, which is part of the Milky Way galaxy, which is sitting in the
Virgo Supercluster, among billions of other galaxies in the visible universe. This might make you feel very small, but in
fact, it should make you feel powerful. The fact that we can decipher all of these
details about the universe may be an indication of how much more we will soon learn, and perhaps
all the places we will some day visit. Speaking of visiting distant places, the other
planet that humans will set foot on first is definitely going to be Mars, so let’s
learn about that planet next.

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