Download Earth

Earth
Earth is the third planet from the Sun and largest of the terrestrial planets. Surprisingly, 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
characteristics 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 (N2), 21% oxygen (O2), 1% argon, with trace amounts of
carbon dioxide (CO2) 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 temperatures 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 within 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 to 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.
ORBIT & ROTATION
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 characteristic 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
Volume: 1.08 x 1012 km3
Rings: No
Density: 5,513 kg/m3
Time it takes light to reach
Earth from the Sun:
8 minutes 17 seconds
24
Mass: 5.98 x 10 kg
Mean Orbital Distance from the
Sun: 1.50 x 108 km
Perihelion: 1.47 x 108 km
Surface Gravity: 9.81 m/s2
Aphelion: 1.52 x 108 km
Length of Day:
23 hours 56 minutes
Mean Diameter: 12,742 km
Length of Year:
365.26 days
Mean Circumference:
40,030 km
Orbital Eccentricity: 0.0167
Axial Tilt: 23.44°
Planet Earth
Earth and moon
Tilt of the Earth and the seasons that are produced.