Were an alien intelligence to visit our solar system, they could conceivably call our planet "Aqua" (or "Water") due to Earth's striking presence of abundant liquid water. In fact, about 72% of Earth's surface is covered by the oceans, with freshwater making up only about 1% of the total supply by volume. Among the planets in the solar system, only Earth has an abundance of liquid water at the surface, because of its fortuitous orbital distance from the Sun. (The planet Uranus and some of the moons of Jupiter, such as Callisto, are thought to have abundant liquid water, but this water exists below a frozen surface.)

    It may be said that the oceans are the last frontier for exploration. Ocean floor mapping surveys were first conducted only about five decades ago. Detailed knowledge of the deep ocean and the ocean bottom has been hindered by past technological barriers to exploration. The ocean can be a forbidding and hostile environment to human beings, requiring specialized equipment and vehicles, some of which have been invented only in recent times. To survive the cold, icy waters and the tremendous pressure of the overlying water column, humans have relied on ocean surveying vessels, submarines, specialized submersible diving craft, and robotic cameras to conduct modern deep sea exploration.

    Oceanography studies a wide diversity of topics, including, but not limited to: the composition of sea water, geology of the ocean basins, aquatic life, the physics of ocean waves, dynamics of hurricanes, coastal geological processes, and marine mineral resources. As one of the earth sciences, oceanography is interdisciplinary in nature, combining geology, geophysics, chemistry, biology, meteorology, and engineering. Specialists may have titles reflecting their area of concentration in oceanography, such as marine geologist, marine biologist, chemical oceanographer, coastal engineer, etc. A limnologist applies the principles of oceanography to freshwater lakes. It may be said that a marine geologist is often "a jack of all geological trades and master of none."

Earth's water probably came from a combination of 2 sources very early in its history: outgassing (condensation of steam from early volcanism), and comets (extraterrestrial bombardment). Exactly how much of Earth's water each source contributed can only be speculated, because geological evidence from that distant time was long ago destroyed, and we have not found rocks on Earth older than 3.8 billion years. Previously, geologists had believed that most of Earth's water originated internally as steam, due to tremendous volcanic activity early in the planet's history, through a process known as outgassing. As you recall, magma contains volatile gases as well as molten rock; water in the form of steam escapes from magma as it reaches the surface.

    Recent evidence suggests that comets may have contributed a significant amount of the Earth's water. Comets are large balls of ice and dust leftover from the formation of the solar system along with asteroids and meteorites. Comets can travel tremendous distances, driven by gravity into long-distance orbits around the Sun and possibly by other stars. When the Earth was just formed, a period of intense bombardment by comets, meteorites, and asteroids (known collectively as planetesimals) occurred for a period of hundreds of millions of years. The tremendous heat generated by these impacts probably kept the Earth's surface molten, as the planetesimals were absorbed by and mixed into the molten Earth. Eventually, the sheer number of incoming planetesimals was reduced through absorption by the Earth and other planets, and volcanism brought steam to the surface; the Earth began to cool, and the crust began to stabilize by about 3.8 billion years ago. As the Earth cooled, the steam condensed into liquid water, which began to collect in basins and low areas to form the first oceans. There may have been tremendous rain storms that lasted for hundreds or even thousands of years in order to fill the ocean basins. The salt in the oceans came from the immediate leaching of minerals by rain water as it drained off the continents and washed into the oceans, so this salt influx probably occurred in a geological instant. By this time, a stable envelope of gases had surrounded Earth to form the atmosphere, and this acted as a shield against most future impacts from comets, meteorites, and asteroids by burning up all but the largest incoming planetesimals via heat friction.

The oceans serve a number of important functions in regulating earth processes:

1. Interaction with the atmosphere creates and influences weather patterns.

A. Waves in water bodies are generated by winds that blow across the surface.

B. Oceans provide humidity and moisture to the air, greatly influencing weather. The most extreme example of this interaction is found in hurricanes, which are born over warm, tropical oceans. (The great lakes sometimes cause "lake effect" snow that is localized.)

C. Water "smoothes outs" drastic changes in air temperature because of its large heat absorbing capacity. This explains why Lake Michigan is cooler than the surrounding land during summer and warmer during winter.

A. Marine algae (microscopic aquatic plants) living in the oceans are the base of the Earth's food chain. Through a biological process called photosynthesis, these algae create their own food by taking energy (sunlight), nutrients from minerals dissolved in seawater, and excrete oxygen as a waste product, which directly supports most animal life. Collectively, algae and other microscopic creatures that make their own food are called photoplankton. Small aquatic animals that feed on photoplankton are called phytoplankton (small crustaceans and shrimp, etc.), while slightly larger creatures such as baby fish may be called zooplankton. Most species of whales, the largest creatures in the oceans, live on a diet of phytoplankton and zooplankton.

B. Ocean water provides a natural barrier against deadly ultraviolet radiation from the sun. For this reason, life evolved first in the oceans rather than on land, because there was no ozone layer until the atmosphere had been transformed from anaerobic to aerobic (more on this below).

4. Oceans were responsible for creation of the atmosphere as we know it. Earth's atmosphere wasn't always composed of nitrogen and oxygen. The early atmosphere was probably composed of a very different mixture of gases that would be poisonous to most forms of life: methane, ammonia, carbon dioxide, and water vapor. Due to the lack of free oxygen, this was called an anaerobic atmosphere.

    The earliest living things on earth were probably primitive microscopic plants that lived in the oceans, such as algae and bacteria. Using a process called photosynthesis, algae make their own food supply (sugar) by using the energy from sunlight, carbon dioxide and nutrient elements dissolved in sea water, and give off free oxygen as a waste product. At some time about 1.8 billion years ago, the algae bloomed (multiplied in great numbers) in the oceans, giving off so much excess oxygen that it could not longer be contained in the oceans, and it escaped into the atmosphere. The geological evidence of this formation of free oxygen are the red beds, widespread deposits of reddish (rusted) iron minerals found in sediments eroded from rocks on land. The transformation resulted in an aerobic (oxygen-rich) atmosphere.

Because free oxygen is chemically active, this waste product of the marine algae bubbled out of the oceans and converted the original gases in the atmosphere, as follows:
 

Original Anaerobic Atmosphere	Chemical Changes by Free Oxygen	Present Aerobic Atmosphere
methane (CH4)	CH4 + O2 = CO2 + H2O	carbon dioxide + water vapor
ammonia (NH3)	NH3 + O2 = N2 + H2O	nitrogen + water vapor
carbon dioxide (CO2)	CO2 --> CaCO3 (calcite) (biological processes; limestone formation)	CO2 is far less abundant today due to presence of limestones
water vapor (H20)	no change	no change

Formation of the Ozone Layer: One of the by-products of the excess oxygen in the newly transformed atmosphere was the formation of ozone (O₃), which is made up of a trio of oxygen atoms (O₃). In the upper atmosphere, ultraviolet rays in sunlight break apart some of the "normal" oxygen molecules (O₂) into chemically active, single oxygen atoms (O). These single oxygen atoms combine with O₂ to form ozone, O₃:

    The ozone layer in the upper atmosphere acts as a shield which filters out much of the deadly ultraviolet rays from sunlight, making the Earth's surface safe for living things. Before the ozone layer was formed, the only safe environment for living things was the ocean, whose waters also act as a filter for ultraviolet rays. So, life on land could not have evolved until the ozone layer was formed.

    It may surprise you to learn that life is much more abundant in cold ocean waters than in warm tropical waters. In fact, warm tropical waters are considered aquatic "deserts" as far as life is concerned. The reason for this is due to the relationship between gas solubility and temperature. Most aquatic organisms need dissolved oxygen in water in order to survive. Gas molecules, including oxygen, vibrate more quickly in warmer water than in cold, and are more likely to escape into the atmosphere. For this reason, cold water usually contains more dissolved oxygen than warm water. Thus, cold ocean water supports a larger and more diverse food chain. Whales usually frequent polar waters in order to feed on huge swarms of phytoplankton and zooplankton, which are mostly tiny shrimp-like creatures.

    An upwelling is a vertical movement of cold, nutrient-rich water brought up from the depths to mix with warm, surface ocean water. Winds blowing away from coastal areas creates ocean currents that can induce upwellings. Upwellings are important to the life cycles of fish and other sea creatures that need the periodic influx of food and nutrients.

    Wind drives ocean currents, which are large-scale circulation patterns of the water. As you may seen in maps shown in the textbook, clockwise circulation of ocean currents is seen north of the equator, while counter-clockwise circulation is seen south of the equator. Ocean currents are also influenced by differences in temperature, salinity, and density of sea water. In general, cold, saltier water is denser and found at greater depths than warm, less salty water. Sometimes, the differences in temperature, salinity, and density are so dramatic that the ocean water actually separates into two layers; the boundary layer resists immediate mixing between the two types of sea water. Salinity is usually controlled by the amount of evaporation vs. precipitation, as reflected by climate at particular latitudes.

Life zones by ocean depth:

• Photic Zone (0 - 100' depth, penetrable by sunlight)
• Dimly-Lit Zone (100' - 2,000')
• Twilight Zone (total darkness, below 2,000')
• Hadal Zone (extreme depths below 15,000' - such as marine trenches)

"Swimmers" are called pelagic.

"Crawlers" who live on top of or burrow into sediments are called benthic.

Deep-sea creatures are specially adapted to a dark, cold, high-pressure environment, where little food is available. They tend to have:

• Large, light-sensitive eyes (or they may not need eyes, and are blind)
• Disproportionately large jaws or teeth as compared to their body size
• Red or black color

Ocean resources include: aquaculture (commercial cultivation of sea animals and plants), mineral resources (metals, salt, etc.), energy resources (petroleum, tidal energy, etc.), and even fresh water (icebergs).