Statistics:
- Planet Diameter: 12,756 kilometers, 1 Earth diameter
- Planetary Mass: 6x1024 kg, 1 Earth mass
- Average Distance from Sun: 150 million kilometers, 1 AU*
- Length of Day: 23 hours 57 min, 1 Earth day
- Length of Year: 365.242 Earth days
- Atmosphere: 21% Oxygen, 78% Nitrogen, other trace elements making up the final 1%
- Day/Night Surface Temperature: -100 to +100 oF
Early Views of Earth
Earth's Atmosphere
The Internal Structure of Earth
Earth's Moon
The Origins of Life?
Early Views of the Earth:
In the early history of human civilization, and even up until the 16th to 17th centuries A.D., it was widely believed that the Earth was the center of the universe; this is also called the geocentric view of the solar system. In this viewpoint, the Earth was stationary and all other objects in the universe were thought to move around the Earth. Understandably, at first glance, this explanation of the motion that we observe in the heaven's seems accurate enough. However, when one looks with closer scrutiny at the motions observed in the sky, then the validity of a geocentric universe begins to fade.One such motion is that of retrograde motion. This shows up as the apparent motion of a planet in our solar system when said planet seems to go backwards over the period of many days along its usual path across the night sky. The figure below shows what the retrograde motion of the planet Mars might look like from the surface of the Earth over many nights of observation.
Note that in the geocentric theory of the universe, an explanation of the retrograde motion of the planets that we observe is not easily accounted for; in fact, in order to effectively take retrograde motion into account, the geocentric view must become ludicrously complex in nature! Thus, Nicolaus Copernicus postulated a heliocentric view of the universe, where the Sun and not the Earth was at the center with all objects (including Earth) in motion around it. This theory became more accepted over the old ones because it could effectively explain anomalies such as retrograde motion. Go to the History of Exploration section for more information about these and related topics.
In the last few decades, scientists have begun to learn more about the other planetary bodies that occupy our solar system. The understanding of our own home world helps us to determine unknown factors about the other eight; likewise, a study of other worlds has assisted scientists in understanding how our own world works. This subject is called comparative planetology, and a perfect example of it is how the study of Venus led scientists to the knowledge of Earth's greenhouse effect.
The Atmosphere of the Earth:
Moving inward from outer space, the first layer of the Earth that we'd encounter is the atmosphere that envelops the entire planet. Even though the atmosphere of the Earth appears very substantial to those of us inhabiting the surface, it only accounts for about 7% of the total radius of the planet. Similarly, the mass of the atmosphere makes up only about one-millionth the entire mass of the planet. Indeed, astronauts and cosmonauts have noted the fragile nature of our atmosphere when they are finally released from its confines. Our atmosphere is fragile in nature; current evidence to support this claim includes not only the depletion of the ozone layerbut Earth's greenhouse effect and global warming.
Earth's Atmosphere as Seen from Low Earth Orbit
Ozone Layer Depletion is the thinning of a crucial sliver of our atmosphere. Ozone (O3) is a heavier version of molecular oxygen. The ozone layer exists in the Earth's stratosphere (about 80 kilometers above the surface), and ozone is very good at absorbing ultraviolet radiation from the Sun; ultraviolet rays (UV-rays) are responsible for giving you a bad sunburn when you're outside. Higher levels of UV-radiation reaching the surface of our planet would have severe consequences for the life forms living there. Synthetic chemicals known as chlorofluorocarbons (CFCs) are known to attack ozone molecules and break them down; the result is that more CFCs cause the destruction of more ozone, which in turn leads to the greater amount of UV-rays that reach the surface of the Earth. Currently, there is an international effort on the part of the scientific community to have a worldwide ban on CFCs put into effect.
Global Warming is the gradual warming up of our planetary surface via a greenhouse effect. What happens is that incident solar radiation penetrates our atmosphere, hits & bounces off of the surface partially as infrared radiation (IR-rays), and the IR-rays are unable to escape the atmosphere because they are absorbed by what are called greenhouse gasses; the end result is that the atmosphere of the planet warms up. A major greenhouse gas is carbon dioxide (CO2). Thus, the emission of greater amounts of CO2 into our atmosphere through the burning of fossil fuels will lead to an increased global warming effect which could have disastrous results (like the raising of ocean levels due to melting polar caps); it is debated as to whether or not there are other natural processes that contribute to the higher levels of CO2, such as volcanic activity. However, it is much easier for humans to control how much fossil fuels they burn than regulating the volcanic activity of our planet.
Data Obtained from the Mauna Loa NOAA Observatory
The following table shows the relative abundance of the substances that make up the atmosphere of our planet...
| Element or Compound | Amount (%) |
| Nitrogen (N2) | 78.1 |
| Oxygen (O2) | 21.0 |
| Argon (Ar) | 0.93 |
| Carbon Dioxide (CO2) | 0.03 |
| Neon (Ne) | 0.002 |
There also exist trace amounts of water vapor and other gases not shown on the table above in the atmosphere of the Earth. The presence of gaseous oxygen allows for animal life to live on our planet, while the carbon dioxide present in our atmosphere allows for the proliferation of plant life.
The origin of the Earth's primordial atmosphere is still a mystery to scientists. There are currently three theories that address the origin of our atmosphere and oceans:
- The atmosphere formed along with the rest of the planet when cosmic debris accumulated after the formation of our own Sun.
- The atmosphere was released from inside the Earth's interior through volcanic processes. This would have taken place after the initial formation of the planet.
- Our atmosphere may have been derived from the materials of comets or icy meteorites that impacted with the planet during its early formation and cooling.
Earth's Internal Structure:
Like all of the inner planets of our solar system, Earth is a terrestrial world; that is, its insides are composed primarily of rocks and metals. The surface rocks of the Earth have densities of roughly 2.5 to 3.0 grams per cubic centimeter, whereas the overall density of the Earth has been calculated to be roughly 5.5 grams per cubic centimeter. This means that the internal structure of our world is very dense by comparison.Through the use of seismic waves (which are essentially strong sound waves), much of the internal structure of our planet has been mapped out indirectly. The table below shows the internal layers of the Earth (moving inward from outside) and the corresponding depths, pressures, temperatures, and component materials...
| Layer | Depth (km) | Pressure (Pascals) | Temperature (K) | Composition |
| Crust | down to ~70 | ? | ? | basalt, granite |
| Mantle | 2900 | ? | up to 4500 | liquid iron, nickel, sulfur |
| Outer Core | 5200 | 1.3x1011 | 4500 | liquid iron, nickel, sulfur |
| Inner Core | 6400 | 4x1011 | 5000 | solid iron, nickel, sulfur |
It is believed by geologists that the semi-liquid and liquid nature of the metallic materials within the Earth are responsible for such features as the Earth's magnetic field and plate tectonics.
Luna, Earth's Moon:
The Earth has only one natural satellite, Luna or more commonly known as the Moon. Here are some of the basic statistics that are adhered to our moon:
- Diameter: 3476 km, 0.272 Earth Diameters
- Average Distance from the Earth: 380,000 km, 0.0025 AU
- Period of Orbit: 27.3 Earth Days
- Atmosphere: extremely thin layer, usually escaping due to little gravity
The Moon is perhaps one of the objects that humanity is most intimately related. The exploration of this natural satellite of Earth began with numerous telescopic observations performed by ground-based telescopes, infrared detectors, and spectroscopes. These preliminary studies showed that the Moon has no atmosphere and a lack of water upon the surface (however, this question will be explored in greater detail).
It was not until 1959 that the former Soviet Union sent the first spacecraft to the Moon; Luna 2 became the first robot craft to land on the surface of the Moon. Between the USSR and United States, by 1966, they had mapped the entire surface of the Moon and transmitted pictures from the lunar surface back to Earth. However, the climax of lunar exploration came with the manned Apollo Moon missions. The first manned lunar mission took place in December of 1968, and it was a simple circumlunar flight around the Moon. The first manned lunar landing took place on July 20th, 1969; Niel Armstrong was the first member of the human species to set foot on another world, and he epitomized this in the famous statement: "That's one small step for... man, and one giant leap for mankind." Six successful manned missions were to follow, with astronauts returning lunar samples, exploring the surface via foot travel and lunar rover, and setting up remote experiment packages known as Apollo Lunar Surface Experiment Packages (ASLEPs).
Astronaut Buzz Aldrin poses with the American Flag
It has been said by scientists that the 400 kilograms of lunar material returned by the Apollo astronauts has taught humanity more about the Moon and its history than all other lunar studies combined. The sad fact of the Apollo Moon missions is this... since ending in December of 1972, no humans have ever set foot upon another world. This seems to make no sense, seeing as how the cost of the Apollo Lunar missions was very economical indeed, having cost each American citizen and average of $100 over a ten-year period of time. There is, however, hope that in the future humanity will return to the Moon. For example, some private organizations, such as the Artemis Society, are hoping to finance a return to the Moon.
The Moon shares in its appearance a great deal with the planet Mercury; that is, it is an apparently dead, airless world that is covered with impact craters. There also exist on the lunar surface a series of flat, gray plains that were once mistaken for oceans of liquid water. These are called the lunar maria, which translates literally from Latin as lunar "seas". It ends up that the maria are actually old dried out lava plains that occupy the lowlands of the Moon. There is a theory that states that billions of years ago the Moon was hit by a large object, cracking the surface like the shell of an egg. Hot, liquid magma poured up through the crack and filled in all of the low-lying areas.
Water on the Moon? In recent years, there has been a debate within the scientific community as to whether or not there exists any water on the surface of the Moon in the form of water ice. Scientists are not interested in whether or not this could provide a habitat suitable for life, as the conditions of the lunar surface are far too harsh, but rather they're wondering if a supply of water ice on the Moon would be useful as fuel for rockets exploring the solar system. A few years ago, a spacecraft named Clementine returned information from lunar radar echoes that some NASA scientists claim are signs of frozen water in the southern polar regions of the Moon. Many in the scientific community are still skeptical however, and there is a mission currently underway to address this question. The probe is called the Lunar Prospector, and to learn more about this brand new mission, click here.
Origins of Life?
As most everyone knows, the Earth is the only place in the universe where life is known to exist. However, there are hopes that life could exist elsewhere in the universe or even in our own solar system (on Mars, the Jovian moon Europa, or on Saturn's moon Titan). However, for now we'll deal with how life evolved on our home world.According to geological records, life had already existed on the Earth before the oldest known rocks were formed. By 3.5 billion years ago, life had already evolved to the point of being capable of photosynthesis -- the process of being able to derive life-giving energy from sunlight; the microorganisms are known as stromatolites and can be found in the most ancient of rocks. It is now known that the oxygen-rich atmosphere of the Earth today would not have been suitable for the formation and proliferation of the early Earthlings. However, less oxygen in the atmosphere and sufficient amounts of water appear to have been more favorable, as laboratory tests have shown in the synthesis of complex proteins and amino acids, the basic building blocks of carbon-based life. In addition, more ingredients such as methane (CH4) and ammonia (NH3) must have also been present in the evolution of early life forms on our planet. Possibly these materials existed at one time in our atmosphere, or perhaps they were deposited here from elsewhere, such as from passing comets and/or asteroids that impacted with the Earth.
