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.