Earth is the third planet from the Sun and largest of the terrestrial
planets. Suprisingly, while it is only the fifth largest planet in
terms of size and mass, it is the densest (
5,513 kg/m3)
of all the planets. Regarding its name, Earth is the only planet not
named after a mythological being. Instead, its name is derived from the
Old English word "ertha," which means ground.
Throughout human history we have sought to understand our home
planet. However, the learning curve has been steep, with many errors
having been made along the way. For example, it was not until the time
of the ancient Romans that the world was understood to be spherical
rather than flat. A second example is the belief that the Sun revolved
around the Earth. Only in the sixteenth-century, through the work of
Copernicus, did we accept that, in fact, the Earth was merely a planet
orbiting the Sun.
Perhaps most importantly, it is during the last two centuries that
science has allowed us to see that the Earth is both an ordinary and
unique place in the Solar System. On one hand, many of its
characterisitcs are rather unexceptional. Take, for example, its size,
interior, and geological processes—being the fifth largest out of the
eight planets, it is close to the median in terms of size; its interior
structure is almost identical to the three other terrestrial planets;
and the same geological processes that shape its surface can be found
not only on other planets, but also on planetary moons. However, the
Earth is special in one very important regard—in all of the solar
system, the Earth is the only world known to foster life.
Atmosphere
The ability for Earth to possess life is dependent in many ways
on its atmosphere. The composition of the atmosphere is roughly 78%
nitrogen (N
2), 21% oxygen (O
2), 1% argon, with trace amounts of carbon dioxide (CO
2)
and other gases. Nitrogen and oxygen are essential to DNA and
biological energy production, respectively, without which life could not
be sustained. Additionally, the oxygen found in what is known as the
ozone layer of the atmosphere protects the surface of the planet by
absorbing harmful solar radiation.
Remarkably, the significant amount of oxygen present in the
atmosphere is due to the life found on Earth. As a byproduct of making
sugars, plants convert the carbon dioxide in the atmosphere into oxygen.
Essentially, this means that without plants the amount of carbon
dioxide in the atmosphere would be much greater and the oxygen levels
much lower. On one hand, if carbon dioxide levels were much higher, it
is likely the Earth would suffer from a runaway greenhouse effect like
that on Venus. On the other hand, if the percentage of carbon dioxide
were any lower there would not be a greenhouse effect at all, thus
making temperatures far colder. Therefore, the carbon dioxide levels are
just right to maintain hospitable temeperatures ranging from
-88° C to 58° C.
Oceans
When viewing Earth from space, there is one overwhelming
feature—the oceans of liquid water. In terms of surface area, these
oceans cover approximately 70% of the Earth. What is even more amazing
than this percentage is that a single drop of liquid water is yet to be
found on any other planet in the Solar System. In this regard, the Earth
is truly unique.
Like the Earth's atmosphere, the presence of liquid water is
vital for life. In fact, life is believed to have first developed 3.8
billion years ago in the oceans, only later evolving the ability to
survive on land.
The existence of the oceans is attributed to two sources. The
first of these is the Earth itself. It is conjectured that large amounts
of water vapor were trapped wintin the Earth during its formation. Over
time, the planet's geological mechanisms, primarily its volcanic
activity, released this water vapor into the atmosphere. Once in the
atmosphere, this vapor condensed and fell to the planet's surface as
liquid water. The second source is theorized ro have originated from the
ancient comets that struck the Earth. Upon impact, they deposited
substantial amounts of water ice on the planet.
Surface
Although most of the Earth's surface lies beneath its oceans, the
remaining "dry" surface is quite remarkable. When comparing the Earth
to other solid bodies in the Solar System, its surface stands out due to
its lacking impact craters. It is not that the Earth has been spared
the numerous impacts by small bodies; rather, it is because the evidence
of these impacts has been erased. Although there are many geological
processes responsible for this, the two most important are weathering
and erosion. In many ways these two mechanisms can be thought of as
working in tandem.
Weathering is the breaking down of surface structures into
smaller pieces by the atmosphere.
Moreover, there are two types of
weathering: chemical and physical. An example of chemical weathering is
acid rain. An example of physical weathering is abrasion of river beds
caused by rocks suspended in flowing water. The second mechanism,
erosion, is simply the movement of weathered particles by water, ice,
wind or gravity.
Thus, impact craters have been "smoothed out" through weathering
and erosion by being broken apart and redistributed to other areas on
the Earth's surface.
Two other geological mechanisms have helped to shape the Earth's
surface. The first is volcanic activity. This process consists of the
releasing of magma (molten rock) from the Earth's interior through a
rupture in the its crust. Some effects of volcanic activity can be the
resurfacing of Earth's crust or formation of islands (think of the
Hawaiian Islands). The second mechanism is orogeny, or the formation of
mountains through the compression of tectonic plates. An example of
mountains created through this process is the Rocky Mountains.
Interior
Similar to the other terrestrial planets, Earth's interior is
believed to consist of three components: a core, a mantle, and a crust.
At present, the core is thought to be comprised of two separate
layers—an inner core composed of
solid nickel and iron, and an outer core composed of
molten nickel and iron.
The mantle is very dense and almost entirely solid silicate rock; its thickness is roughly
2,850 km. Finally, the crust is also composed of silicate rock and varies in thickness. While the continental crust ranges from
30 to 40 km in thickness, the oceanic crust is much thinner at only
6 to 11 km.
Yet another distinguishing feature of the Earth when compared to
the other terrestrial planets is that its crust is divided into cool,
rigid plates that rest upon the hotter mantle below. Furthermore, these
plates are in constant motion. Along the boundaries of these plates two
processes, known as subduction and spreading, can occur. During
subduction two plates come into contact (sometimes violently, producing
earthquakes) and one plate is forced under the other. Separation, on the
other hand, is when two plates are moving away from each other.
Orbit & Rotation
At roughly 365 days, the Earth's orbit around the Sun is familiar
to us. The length of our year is due in large part to the Earth's
average orbital distance of
1.50 x 108 km.
What many people are not familiar with is that at this orbital distance
it takes sunlight, on average, about eight minutes and twenty seconds
to reach the Earth.
With an orbital eccentricity of .0167, the Earth's orbit is one
of the most circular in all the Solar System. This means that the
difference between Earth's perihelion and aphelion is relatively small.
As a result of this small difference, the intensity of the sunlight
Earth receives remains almost constant year-round. However, the Earth's
position in its orbit is responsible, in part, for the varying seasons
it experiences.
The Earth's axial tilt is approximately 23.45°. That is, the axis
the Earth rotates about is tilted slightly with respect to the plane in
which the Earth orbits the Sun. The effect of this tilt, along with
position of the Earth in its orbit, means that at certain times the
amount of sunlight the northern hemisphere receives is greater than that
of the southern hemisphere, and vice versa. This variation in intensity
is what produces the warmer temperatures during the summer and colder
temperatures during the winter.
A second commonly know charecteristic is that the Earth takes
approximately twenty-four hours to complete one rotation. This is
fastest among the terrestrial planets, but easily slower than that of
all the gas planets.
Earth Facts
Number of Moons: 1
Rings: No
Time it takes light to reach Earth from the Sun:
8 minutes 17 seconds
Mass:
5.98 x 1024 kg
Mean Diameter:
12,742 km
Mean Circumference:
40,030 km
Volume:
1.08 x 1012 km3
Density:
5,513 kg/m3
Surface Gravity:
9.81 m/s2
Length of Day:
23 hours 56 minutes
Length of Year:
365.26 days
Orbital Eccentricity:
0.0167
Mean Orbital Distance from the Sun:
1.50 x 108 km
Perihelion:
1.47 x 108 km
Aphelion:
1.52 x 108 km
Axial Tilt: 23.44°
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