The hydrosphere (Greek word hydor (vSup) means water), also called the water sphere, includes all water on the planet Earth. The Earth is indeed a blue planet, since the oceans cover nearly 71% of its surface, that is, over 361 million km2, while the continents and islands -the solid surface of the Earth - make up only 29% of the total Earth area.

Water is the most widespread substance in the natural environment of our planet. It is available everywhere on Earth, albeit its abundance largely differs in space and time. Water exists on Earth in three states: liquid, solid, and gaseous (vapor). Liquid phase, being intermediary between solid and gaseous states, is dominant on Earth, in the form of oceans, seas, lakes, rivers on the ground, soil moisture, and aquifers under ground. In a solid state, water exists as ice and snow cover during winter in higher latitudes and during all the seasons in polar and alpine regions. Some amount of water is contained in the air as water vapor, water droplets, and ice crystals. Water is a constituent ofthe biosphere, the habitat of life, extending up to 10 km height into the high troposphere (migratory bird routes over the Himalaya) and down in the oceans to the depth of 10 km and deeper. Plant and animal tissues contain large proportion ofwater. Huge amounts ofbonded water are present in the composition of different minerals of the Earth.

The abundance of liquid water on Earth distinctly distinguishes our unique planet from other planets in the solar system, where no liquid water can be found. The Earth is the only planet in the solar system with the right distance from the Sun, the right composition of the atmosphere, the right mass (gravity) and chemical composition, permitting water to exist in all three phases, but predominantly in the liquid form. The Venus is too warm for presence of liquid water. It is too near to the Sun and it has too strong greenhouse effect due to dense atmosphere. The Mars, being further to the Sun, is too cold. However, dendritic channels and deep canyons on Mars's surface were probably sculpted in the past by free-flowing water, when the climate was warmer.

We cannot satisfactorily explain the origin of the Earth's hydrosphere. One of the important processes was the outgassing of water vapor from the interior of the Earth, which took place as extrusion of material in volcanoes and ocean upwellings. Furthermore, the early Earth was bombarded by 'snowballs' of comets and asteroids, which were rich in water. Much of the Earth's water is likely to have originated from the outer parts of the solar system.

Water is the basic element of the life-support system of the planet, being essential for self-reproducing life. Water cannot be substituted by any other substance. By its capacity to dissolve and carry substances, water plays an essential role in the chemistry of life. Most life on the planet takes place in the saltwater of the oceans. It is estimated that the oldest life on Earth started in oceanic waters already 3.5 billion years ago. Most evolution has taken place in water. However, it is freshwater that is indispensable for much life on Earth, including the life of humans. Humans need freshwater and salt, but separately rather than together, as contained in salty water. The humans depend on regular availability of freshwater for drinking. Water is indispensable, in large quantities, virtually in every human activity, in particular in agricultural production. Water and solar radiation is the driving source behind the plants' primary productivity. Water is indispensable for plant growth. Some water is incorporated in plant tissues and much is transpired.

When looking for possibilities of extraterrestrial life, the focus is on the search for liquid water. Existence of liquid water on a celestial body now, or in the past, is interpreted as a necessary condition of life. Moreover, existence of water on other planets and moons is important in human's search for habitable places, where spacemen could live without having to bring large volumes of water with them. It is hypothesized that a hydrosphere may exist on Europa and Ganymede, two of the four large moons of the Jupiter, where the water is frozen on the surface, but may remain liquid under the surface.

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