Earth

== Hydrosphere ==
{{Main|Hydrosphere}}
[[File:Ocean world Earth.jpg|thumb|A view of Earth with its [[global ocean]] and [[cloud cover]], which dominate Earth's surface and [[hydrosphere]]; at Earth's [[Polar regions of Earth|polar]] regions, its hydrosphere forms larger areas of ice cover.]]
Earth's hydrosphere is the sum of Earth's water and its distribution. Most of Earth's hydrosphere consists of Earth's global ocean. Earth's hydrosphere also consists of water in the atmosphere and on land, including clouds, inland seas, lakes, rivers, and underground waters down to a depth of {{convert|2000|m|ft|abbr=on}}.

The mass of the oceans is approximately 1.35{{e|18}}&nbsp;[[metric ton]]s or about 1/4400 of Earth's total mass. The oceans cover an area of {{convert|361.8|e6km2|e6mi2|abbr=unit}} with a mean depth of {{convert|3682|m|ft|abbr=on}}, resulting in an estimated volume of {{convert|1.332|e9km3|e6cumi|abbr=unit}}.<ref name="ocean23_2_112" /> If all of Earth's crustal surface were at the same elevation as a smooth sphere, the depth of the resulting world ocean would be {{convert|2.7|to|2.8|km|mi|2|abbr=on}}.<ref>{{cite web|title=Third rock from the Sun&nbsp;– restless Earth|url=https://ase.tufts.edu/cosmos/print_chapter.asp?id=4 |access-date=12 April 2015|work=NASA's Cosmos}}</ref> About 97.5% of the water is [[saline water|saline]]; the remaining 2.5% is [[fresh water]].<ref>{{Cite book|title=On Water|url=https://www.eib.org/en/publications/eib-big-ideas-on-water|access-date=7 December 2020|year=2019 |doi=10.2867/509830 |language=en|author1=European Investment Bank|publisher=Publications Office |isbn=9789286143199}}</ref><ref>{{Cite web|title=Chart: Globally, 70% of Freshwater is Used for Agriculture|url=https://blogs.worldbank.org/opendata/chart-globally-70-freshwater-used-agriculture|access-date=7 December 2020|website=World Bank Blogs |date=22 March 2017|last1=Khokhar|first1=Tariq|language=en}}</ref> Most fresh water, about 68.7%, is present as ice in [[ice cap]]s and [[glacier]]s.<ref>{{cite web |last=Perlman |first=Howard|date=17 March 2014|title=The World's Water|url=https://water.usgs.gov/edu/earthwherewater.html|access-date=12 April 2015|work=USGS Water-Science School}}</ref> The remaining 30% is [[ground water]], 1% [[surface water]] (covering only 2.8% of Earth's land)<ref name="Lake Scientist 2016">{{cite web | title=Where Are Lakes? | website=Lake Scientist | date=2016-02-28 | url=https://www.lakescientist.com/where-are-lakes/ | access-date=2023-02-28}}</ref> and other small forms of fresh water deposits such as [[permafrost]], [[water vapor]] in the atmosphere, biological binding, etc.<ref name="School 2019">{{cite web | last=School | first=Water Science | title=How Much Water is There on Earth? – U.S. Geological Survey | website=USGS.gov | date=2019-11-13 | url=https://www.usgs.gov/special-topics/water-science-school/science/how-much-water-there-earth#science | access-date=2023-03-03}}</ref><ref name="Education 2022">{{cite web | title=Freshwater Resources | website=Education | date=2022-08-18 | url=https://education.nationalgeographic.org/resource/freshwater-resources/ | access-date=2023-02-28}}</ref>

In Earth's coldest regions, snow survives over the summer and [[Ice formation|changes into ice]]. This accumulated snow and ice eventually forms into [[glacier]]s, bodies of ice that flow under the influence of their own gravity. [[Alpine glaciers]] form in mountainous areas, whereas vast [[ice sheets]] form over land in polar regions. The flow of glaciers erodes the surface, changing it dramatically, with the formation of [[U-shaped valley]]s and other landforms.<ref>{{Cite book|last=Hendrix|first=Mark|title=Earth Science: An Introduction|publisher=Cengage|year=2019|isbn=978-0-357-11656-2|location=Boston|page=330}}</ref> [[Sea ice]] in the Arctic covers an area about as big as the United States, although it is quickly retreating as a consequence of climate change.<ref>{{Cite book|last=Hendrix|first=Mark|title=Earth Science: An Introduction |publisher=Cengage |year=2019 |isbn=978-0-357-11656-2|location=Boston|page=329}}</ref>

The average [[salinity]] of Earth's oceans is about 35&nbsp;grams of salt per kilogram of seawater (3.5% salt).<ref name=kennish2001 /> Most of this salt was released from volcanic activity or extracted from cool igneous rocks.<ref name="mullen2002" /> The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.<ref name="natsci_oxy4" /> Sea water has an important influence on the world's climate, with the oceans acting as a large [[heat reservoir]].<ref name="michon2006" /> Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the [[El Niño–Southern Oscillation]].<ref name="sample2005" />

The abundance of water, particularly liquid water, on Earth's surface is a unique feature that distinguishes it from other planets in the [[Solar System]]. Solar System planets with considerable atmospheres do partly host atmospheric water vapor, but they lack surface conditions for stable surface water.<ref name="Center 2021">{{cite web |last=Center |first=Astrogeology Science |title=Tour of Water in the Solar System – U.S. Geological Survey |website=USGS.gov |date=14 October 2021 |url=https://www.usgs.gov/news/tour-water-solar-system |access-date=19 January 2022}}</ref> Despite some [[Natural satellite|moons]] showing signs of large reservoirs of [[extraterrestrial liquid water]], with possibly even more volume than Earth's ocean, all of them are [[List of largest lakes and seas in the Solar System|large bodies of water]] under a kilometers thick frozen surface layer.<ref>{{cite web |title=Are there oceans on other planets? |website=NOAA's National Ocean Service |date=1 June 2013 |url=https://oceanservice.noaa.gov/facts/et-oceans.html |access-date=19 January 2022}}</ref>