SOLAR SYSTEM: THE EARTH
Earth is the third planet, and 93,000,000 miles (150,000,000 km.) from the sun. It is estimated to be over 4.5 billion years old.
The planet rotates once every 23 hours, 56 minutes, and 4.09 seconds. It makes one full revolution around the sun every 365 days, 6 hours, 9 minutes, and 9.45 seconds. Earth's axis is tilted at a 23.5° angle.
Earth has a total surface area of 196,800,000 square miles. Approximately 57,300,000 square miles, or 29% of the total surface area is land. Water covers approximately 139,500,000 square miles, or 71% of the total surface area.
The highest temperatures on Earth have reached 136° F (58° C) at Al Asisiyah, Libya. Temperatures of - 128° F (-89° C) have been recorded at Vostok station in Antarctica.
The atmosphere is a thin, gaseous layer of air that envelops the planet. Its inner layer is called the troposphere and reaches only 11 miles above sea level. It contains most of the planet's air, which consists of nitrogen (78%) and oxygen (21%). The stratosphere, or outer layer, stretches 11-30 miles above sea level and contains ozone (O3). Ozone filters out most of the sun's harmful ultraviolet radiation.
More than 99% of earth's atmosphere is less than 50 miles (80 km.) high. However, particles of the atmosphere are found 1,000 miles (1,600 km.) in space above the planet's surface.
Earth is truly a remarkable planet. It is the only planet in our solar system that has the components necessary to support life as we recognize it. The planet is only a tiny part of the universe, but it is the home of human beings and many other organisms. Animals and plants live almost everywhere on the surface of Earth.
These organisms can live on Earth because it has an atmosphere. The atmosphere moderates daytime and nighttime temperature swings. The atmosphere filters radiant energy during the day, preventing the surface from overheating. At night the atmosphere prevents most of the radiant heat from escaping back into space, keeping the surface warm.
Most organisms - both plants and animals - must also have water to live. Earth has plenty. Seventy-one percent of its surface is covered by water.
Living things also need nitrogen, oxygen, and carbon dioxide. Earth's thin layer of atmosphere provides all of these elements.
The atmosphere also screens out lethal levels of the sun's ultraviolet radiation. The atmosphere, however, could not exist if Earth were not at the exact distance it is from the sun.
Forming Life on Earth
Scientists have studied fossils and made chemical analysis of rocks to find out how life on Earth evolved to its present system. Several theories have been suggested. It is theorized by some scientists that life developed in two phases over billions of years.
In the first phase explosions of dying stars shattered through our galaxy and created swirling clouds of dust particles and hot gases. These extended trillions of miles across space. As the cloud cooled, bits of matter began to cling to each other. Over 4 billion years ago the cloud had formed into a flattened, slowly rotating disk. Our sun was born in the center of this disk. Farther out on the disk, Earth and the other planets formed as bits of matter were drawn together. Earth started out as a molten mass that did not cool for millions of years. As it cooled it formed a thin, hard crust with no atmosphere or oceans.
Molten rock frequently erupted through the crust. Water vapor was released from the breakdown of rocks during volcanic eruptions. Eventually the crust cooled enough for this vapor to condense and come down as rain to form the oceans that covered most of Earth.
In the second phase scientists have recently hypothesized that bubbles floating on the ancient ocean trapped carbon-containing molecules and the other chemicals necessary for life. These bubbles may have popped and released these chemicals into the atmosphere. Organic compounds formed and dissolved in the early atmosphere, collecting in the shallow waters of the earth. Although no one knows how, the first living cells developed between 3.6 and 3.8 billion years ago. Over time these protocells developed into cells having the properties we describe as life.
These single-celled bacteria multiplied in the warm, shallow waters for billions of years. Here they mutated and developed into a variety of protists, fungi and, about 600 million years ago, plants and animals.
Life could not develop on land since there was no ozone layer to shield early life from damaging ultraviolet radiation. Then about 2.3-2.5 billion years ago photosynthetic bacteria emerged. These cells could remove carbon dioxide (CO2) from the atmosphere and, using sunlight, combine it with water to make carbohydrates. In the process they created oxygen (O2) and released it into the ocean. Some of the oxygen escaped into the atmosphere.
Our atmosphere was created over a span of 2 billion years. Some of the oxygen was converted into ozone (O3), which formed in the lower stratosphere and protected life forms from UV radiation. This allowed green plants to live closer to the surface of the ocean, making it easier for oxygen to escape into the atmosphere. About 400-500 million years ago the first plants began to exist on land. Over the following millions of years a variety of land plants and animals evolved.
Earth is the third planet from the Sun and the fifth largest:
Earth is the only planet whose English name does not derive from Greek/Roman mythology. The name derives from Old English and Germanic. There are, of course, hundreds of other names for the planet in other languages. In Roman Mythology, the goddess of the Earth was Tellus - the fertile soil (Greek: Gaia, terra mater - Mother Earth).
It was not until the time of Copernicus (the sixteenth century) that it was understood that the Earth is just another planet.
Earth, of course, can be studied without the aid of spacecraft. Nevertheless it was not until the twentieth century that we had maps of the entire planet. Pictures of the planet taken from space are of considerable importance; for example, they are an enormous help in weather prediction and especially in tracking and predicting hurricanes. And they are extraordinarily beautiful.
The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km):
0- 40 Crust 40- 400 Upper mantle 400- 650 Transition region 650-2700 Lower mantle 2700-2890 D'' layer 2890-5150 Outer core 5150-6378 Inner core
The crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents. The inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid. The various layers are separated by discontinuities which are evident in seismic data; the best known of these is the Mohorovicic discontinuity between the crust and upper mantle.
Most of the mass of the Earth is in the mantle, most of the rest in the core; the part we inhabit is a tiny fraction of the whole (values below x10^24 kilograms):
atmosphere = 0.0000051 oceans = 0.0014 crust = 0.026 mantle = 4.043 outer core = 1.835 inner core = 0.09675
The core is probably composed mostly of iron (or nickel/iron) though it is possible that some lighter elements may be present, too. Temperatures at the center of the core may be as high as 7500 K, hotter than the surface of the Sun. The lower mantle is probably mostly silicon, magnesium and oxygen with some iron, calcium and aluminum. The upper mantle is mostly olivene and pyroxene (iron/magnesium silicates), calcium and aluminum. We know most of this only from seismic techniques; samples from the upper mantle arrive at the surface as lava from volcanoes but the majority of the Earth is inaccessible. The crust is primarily quartz (silicon dioxide) and other silicates like feldspar. Taken as a whole, the Earth's chemical composition (by mass) is:
The Earth is the densest major body in the solar system.
The other terrestrial planets probably have similar structures and compositions with some differences: the Moon has at most a small core; Mercury has an extra large core (relative to its diameter); the mantles of Mars and the Moon are much thicker; the Moon and Mercury may not have chemically distinct crusts; Earth may be the only one with distinct inner and outer cores. Note, however, that our knowledge of planetary interiors is mostly theoretical even for the Earth.
Earth has only one natural satellite, the Moon. But
- thousands of small artificial satellites have also been placed in orbit around the Earth.
- Asteroids 3753 Cruithne and 2002 AA29 have complicated orbital relationships with the Earth; they're not really moons, the term "companion" is being used. It is somewhat similar to the situation with Saturn's moonsJanus and Epimetheus.
- Lilith doesn't exist but it's an interesting story.
Distance Radius Mass Satellite (000 km) (km) (kg) --------- -------- ------ ------- Moon 384 1738 7.35e22
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