why the earth is called blue planet
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Answer:
Planet Earth has been called the "Blue Planet" due to the abundant water on its surface. Here on Earth, we take liquid water for granted; after all, our bodies are mostly made of water. However, liquid water is a rare commodity in our solar system. ... Liquid water covers most of the surface of our planet.
Answer:
Planet Earth has been called the "Blue Planet" due to the abundant water on its surface. Here on Earth, we take liquid water for granted; after all, our bodies are mostly made of water. However, liquid water is a rare commodity in our solar system. No liquid water has been confirmed in our solar system, but it is likely that Jupiter's moon Europa and Saturn's moon Enceladus have liquid oceans under a frozen crust. Nor has a drop of water been observed yet in interstellar space. Only a planet of the right mass, chemical composition, and location can support liquid water. And only on such planets could life as we know it flourish.
Liquid water covers most of the surface of our planet. This water comes in many forms, each with it's own special properties. Rain is essentially pure water (consisting only of H2O with trace amounts of elements and other compounds picked up from atmospheric dust), while fresh spring water and most lakes and rivers contain more dissolved salts (~0.02-0.4 percent %, or parts per hundred,). Ocean water is much more salty and has on average ~3.5% salt content, also referred to as "salinity". Note that 3.5% equals 35 parts per thousand (ppt) which is the typical unit for which salinity is reported, and this value equals 35,000 parts per million (ppm) - a ppt is equal to 1 gram of salt dissolved into 1000 grams of water, or a gram per kilogram (g/kg). Some spring and lake waters are even saltier than the ocean, and they are often called "brines" which can become saturated with salts. These different types of water are found in many different environments on Earth.
The largest and most dramatic bodies of water are our oceans. The oceans cover ~71% of the globe and have an average depth of 3,729 meters. As mentioned the average salinity is 35 ppt, but this ranges from ~33-37 ppt. Although the salinity of seawater is made up mostly of major elements like sodium (Na), chloride (Cl), sulfate (SO4), and magnesium (Mg), seawater also contains many dissolved elements in smaller quantities such as calcium (Ca), the nutrients nitrogen and phosphorus important for plant growth, and dissolved gases like CO2 and methane (CH4). There are two important points here: (1) these differences in chemical composition contribute to the diversity of ecosystems and the diversity of life that has adapted to different water bodies, and (2) the amounts of salt contribute to the weight or density of the water which is important for how water bodies are structured or layered, as discussed more fully below.
How did the ocean get so salty? The answer is that the supply of salts comes from weathering of rocks on land, and then transport to the sea in rivers, plus inputs of salts from hydrothermal or volcanic vents (these topics were covered in earlier lectures in this class). One of the very first scientific attempts to estimate the age of the Earth was proposed by Sir Edmund Halley (of Halley's comet) in 1715, who reasoned by mass balance that if you measured the rate of input of salts from rivers (X tons per year), and the total salt in the ocean (Y tons), you could calculate how many years it took to accumulate the salts now found. The first application of this idea in the 1800s found that the ocean was ~90 million years old, which was in great contrast to the common idea at the time that the ocean (and the Earth) was ~ 6,000 years old.