Humans have always wondered about the origin of the Earth and the universe that surrounds it. What kinds of matter and energy are in the universe? How did the universe begin? How has the Earth evolved? This article will answer these fundamental questions and review the key concepts of Earth and space science.
Earth and space science are concerned with the formation of the Earth, the solar system and the universe, the history of Earth (its mountains, continents, and ocean floors), the weather and seasons on Earth, the energy in the Earth system, and the chemical cycles on Earth.
Energy in the Earth Systems
Energy and matter can't be created or destroyed. But energy can change form and can travel great distances.
Solar Energy
The Sun's energy reaches our planet in the form of light radiation. Plants use this light to synthesize sugar molecules, which we consume when we eat the plants. We obtain energy from the sugar molecules and our bodies use it. Ultimately our energy comes from the Sun. The Sun also drives the Earth's geochemical cycles, which will be discussed in the next section.
The Sun heats the Earth's surface and drives convection within the atmosphere and oceans, producing winds and ocean currents. The winds cause waves on the surface of oceans and lakes. The wind transfers some of its energy to the water, through friction between the air molecules and the water molecules. Strong winds cause large waves. Tsunamis, or tidal waves, are different. They result from underwater earthquakes, volcanic eruptions, or landslides, not wind.
Energy from the Core
Another source of Earth's energy comes from Earth's core. We distinguish four main layers of Earth: the inner core, the outer core, the rocky mantle, and the crust. The inner core is a solid mass of iron with a temperature of about 7,000°F.Most likely, the high temperature is caused by radioactive decay of uranium and other radioactive elements. The inner core is approximately 1,500 miles in diameter. The outer core is a mass of molten iron that surrounds the solid inner core. Electrical currents generated from this area produce the Earth's magnetic field. The rocky mantle is composed of silicon, oxygen, magnesium, iron, aluminum, and calcium and is about 1,750 miles thick. This mantle accounts for most of the Earth's mass. When parts of this layer become hot enough, they turn to slow moving molten rock or magma. The Earth's crust is a layer from four to 25 miles thick consisting of sand and rock.
The upper mantle is rigid and is part of the lithosphere (together with the crust). The lower mantle flows slowly, at a rate of a few centimeters per year. The crust is divided into plates that drift slowly (only a few centimeters each year) on the less rigid mantle. Oceanic crust is thinner than continental crust.
This motion of the plates is caused by convection (heat) currents, which carry heat from the hot inner mantle to the cooler outer mantle. The motion results in earthquakes and volcanic eruptions. This process is called plate tectonics.
Tectonics
Evidence suggests that about 200 million years ago, all continents were a part of one landmass, named Pangaea. Over the years, the continents slowly separated through the movement of plates in a process called continental drift. The movement of the plates is attributed to convection currents in the mantle. The theory of plate tectonics says that there are now 12 large plates that slowly move on the mantle. According to this theory, earthquakes and volcanic eruptions occur along the lines where plates collide. Dramatic changes on Earth's landscape and ocean floor are caused by collision of plates. These changes include the formation of mountains and valleys.
Geochemical Cycles
Water, carbon, and nitrogen are recycled in the biosphere. A water molecule in the cell of your eye could have been at some point in the ocean, in the atmosphere, in a leaf of a tree, or in the cell of a bear's foot. The circulation of elements in the biosphere is called a geochemical cycle.
Water
Oceans cover 70% of the Earth's surface and contain more than 97% of all water on Earth. Sunlight evaporates the water from the oceans, rivers, and lakes.
Living beings need water for both the outside and the inside of their cells. In fact, vertebrates (you included) are about 70% water. Plants contain even more water. Most of the water passes through a plant unaltered. Plants draw on water from the soil and release it as vapor through pores in their leaves; this process is called transpiration.
Our atmosphere can't hold a lot of water. Evaporated water condenses to form clouds that produce rain or snow onto the Earth's surface. Overall, water moves from the oceans to the land because more rainfall reaches the land than is evaporated from the land.
Carbon
Carbon is found in the oceans in the form of bicarbonate ions (HCO3–), the atmosphere, in the form of carbon dioxide, in living organisms, and in fossil fuels (such as coal, oil, and natural gas). Plants remove carbon dioxide in the atmosphere and convert it to sugars through photosynthesis. The sugar in plants enters the food chain first reaching herbivores, then carnivores, and finally scavengers and decomposers. All of these organisms release carbon dioxide back into the atmosphere when they breathe. The oceans contain 500 times more carbon than the atmosphere. Bicarbonate ions (HCO3–) settle to the bottoms of oceans and form sedimentary rocks. Fossil fuels represent the largest reserve of carbon on Earth. Fossil fuels come from the carbon of organisms that had lived millions of years ago. Burning fossil fuels releases energy, which is why these fuels are used to power human contraptions. When fossil fuels burn, carbon dioxide is released into the atmosphere.
Since the Industrial Revolution, people have increased the concentration of carbon dioxide in the atmosphere by 30% by burning fossil fuels and cutting down forests, which reduce the concentration of carbon dioxide. Carbon dioxide in the atmosphere can trap solar energy—a process known as the greenhouse effect. By trapping solar energy, carbon dioxide and other green house gases can cause global warming—an increase of temperatures on Earth. In the last 100 years, the temperatures have increased by 1°C. This doesn't seem like much, but the temperature increase is already creating noticeable climate changes and problems. Many species are migrating to colder areas, and regions that normally have ample rainfall have experienced droughts. Perhaps the most dangerous consequence of global warming is the melting of polar ice. Glaciers all over the world are already melting, and the polar ice caps have begun to break up at the edges. If enough of this ice melts, coastal cities could experience severe flooding.
Reducing carbon dioxide concentrations in the atmosphere, either by finding new energy sources or by actively removing the carbon dioxide that forms, is a challenge to today's scientists.
Nitrogen
The main component of air in the atmosphere is nitrogen gas (N2). Nitrogen accounts for about 78% of the atmosphere. However, very few organisms can use the form of nitrogen obtained directly from the atmosphere. The reason for this is that the bond between two atoms in the nitrogen gas molecule is tough to break, and only a few bacteria have enzymes that can make it happen. These bacteria can convert the nitrogen gas into ammonium ions (NH4+). Bacteria that do this are called nitrifying or nitrogen-fixing bacteria.
Another source of nitrogen for the non-nitrogenfixing organisms is lightning. Lightning carries tremendous energy, which is able to cause nitrogen gas to convert to ammonium ions (NH4+) and nitrate ions (NO3–)—fixed nitrogen.
Plants, animals, and most other organisms can use only fixed nitrogen. Plants obtain fixed nitrogen from soil and use it to synthesize amino acids and proteins. Animals obtain fixed nitrogen by eating plants or other animals. When they break up proteins, animals lose nitrogen in the form of ammonia (fish), urea (mammals), or uric acid (birds, reptiles, and insects). Decomposers obtain energy from urea and uric acid by converting them back into ammonia, which can be used again by plants.
The amount of fixed nitrogen in the soil is low, because bacteria break down most of the ammonium ion into another set of molecules (nitrite and nitrate), through a process called nitrification. Other bacteria convert the nitrite and nitrate back into nitrogen gas, which is released into the atmosphere. This process is called denitrification.
This limited amount of nitrogen has kept organisms in balance for millions of years. However, the growing human population presents a threat to this stability. In order to increase the growth rate of crops, humans manufacture and use huge amounts of fertilizer, increasing the amount of nitrogen in the soil. This has disrupted whole ecosystems, since, with extra nitrogen present, some organisms thrive and displace others. In the long run, too much nitrogen decreases the fertility of soil by depriving it of essential minerals, such as calcium.
Burning fossil fuels and forests also releases nitrogen. All forms of fixed nitrogen are greenhouse gases that are causing global warming. In addition, nitric oxide, a gas released when fossil fuels are burned, can convert into nitric acid, a main component of acid rain. Acid rain destroys habitats.
People are already suffering the consequences of the pollution they have caused. Preventing further damage to the ecosystems and fixing the damage that has been done is another challenge for today's scientists.
Origin and Evolution of the Earth System
Earth Basics
Most people know that the Earth is round and revolves around its axis in about 24 hours. It is a part of the solar system, with the Sun in its center. Other planets and their moons orbit the Sun as well. Among the planets are Mercury and Venus, which are closer to the Sun than the Earth is, and Mars, Jupiter, Saturn, Uranus, and Neptune, which are farther away from the Sun. And even farther from the Sun is the dwarf planet Pluto.
It takes about one year for the Earth to complete its orbit around the Sun. The rotation of the Earth around its axis causes the change between day and night. The tilt in the Earth's axis gives rise to seasons.
Rocks and Rock Cycles
Rocks are made up of one or more minerals, homogeneous inorganic materials. Three types of rocks are igneous, sedimentary, and metamorphic. Igneous rocks result from cooling of molten rock. If the cooling from molten rock occurred quickly on or near the Earth's surface, it is called volcanic igneous rock. If the cooling took place slowly, deep beneath the surface, it is called plutonic igneous rock. Sedimentary rocks are formed in layers in response to pressure on accumulated sediments. Metamorphic rocks are formed when either igneous or sedimentary rocks are under intense heat and pressure deep beneath the Earth's surface.
The rock cycle is the transformation of one rock type into another. Molten rock material cools and solidifies either at or below the surface of the Earth to form igneous rocks. Weathering and erosion break the rocks down into smaller grains, producing soil. The soil is carried by wind, water, and gravity and is eventually deposited as sediment. The sediments are deposited in layers and become pressed firmly together and cemented or lithified, forming sedimentary rocks. Variations in temperature and pressure can cause chemical and physical changes in igneous and sedimentary rocks to form metamorphic rocks. When exposed to higher temperatures, metamorphic rocks may be partially melted, resulting in the creation once again of igneous rocks, starting the cycle all over again.
Molten material from inside the Earth often breaks through the floor of the ocean and flows from fissures where it is cooled by the water, resulting in the formation of igneous rocks. As the molten material flows from the fissure, it forms ridges adjacent to it.
Origin of the Earth and the Solar System
The Sun, the Earth, and the rest of the solar system formed 4.6 billion years ago, according to the solar nebula theory. This theory states that the solar system was initially a large cloud of gas and dust, which most likely originated from the explosions of nearby stars. This cloud is named the solar nebula. The Sun formed at the central, densest point of the nebula. One argument that supports this hypothesis is that planets closer to the Sun are composed of heavier elements, while light, gaseous planets are farthest from the Sun. The solar nebula theory also states that planets formed in conjunction with stars. This component of the theory is supported by the fact that other stars have planets and that the age of moon rocks is comparable to the age of the Earth.
Origin and Evolution of the Universe
Nobody knows for sure how the universe originated. According to the Big Bang theory, the universe started off in a hot dense state under high pressure between ten and 20 billion years ago. The Big Bang theory also postulates that the universe has been expanding since its origination. The universe is still expanding and cooling. Some data suggest that the rate of expansion of the universe is increasing.
Whether the universe will continue to expand forever, eventually reach an equilibrium size, or shrink back into a small, dense, hot mass is unknown.
Stars are formed by the gravitational attraction of countless hydrogen and helium molecules. The stars became gravitationally bound to other stars, forming galaxies. The solar system is part of the Milky Way galaxy, which in addition to the Sun, contains about 200 billion other stars.
The energy of stars stems from nuclear reactions, mainly the fusion of hydrogen atoms to form helium. Nuclear processes in stars lead to the formation of elements.
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