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A polar ice cap or polar cap is a high-
latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...
region of a
planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
,
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to ...
, or
natural satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
that is covered in
ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaqu ...
. There are no requirements with respect to size or composition for a body of ice to be termed a polar ice cap, nor any geological requirement for it to be over land, but only that it must be a body of solid phase matter in the polar region. This causes the term "polar ice cap" to be something of a misnomer, as the term ice cap itself is applied more narrowly to bodies that are over land, and cover less than 50,000 km2: larger bodies are referred to as
ice sheets In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at La ...
. The composition of the ice will vary. For example, Earth's polar caps are mainly water ice, whereas
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
's polar ice caps are a mixture of solid carbon dioxide and water ice. Polar ice caps form because high-latitude regions receive less energy in the form of
solar radiation Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre ( ...
from the Sun than equatorial regions, resulting in lower surface temperatures. Earth's polar caps have changed dramatically over the last 12,000 years.
Season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and ...
al variations of the ice caps takes place due to varied solar energy absorption as the planet or moon revolves around the Sun. Additionally, in geologic time scales, the ice caps may grow or shrink due to
climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
change.


Earth

File:North pole september ice-pack 1978-2002.png, Extent of the Arctic sea-ice in September 1978 – 2002 File:North pole february ice-pack 1978-2002.png, Extent of the Arctic sea-ice in February 1978 – 2002 File:The Earth seen from Apollo 17.jpg, '' The Blue Marble'', Earth as seen from Apollo 17 with the southern polar ice cap visible (courtesy
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
)


North Polar ice cap melting

Earth's
North Pole The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True North Pole to distingu ...
is covered by floating
pack ice Drift ice, also called brash ice, is sea ice that is not attached to the shoreline or any other fixed object (shoals, grounded icebergs, etc.).Leppäranta, M. 2011. The Drift of Sea Ice. Berlin: Springer-Verlag. Unlike fast ice, which is "fasten ...
( sea ice) over the
Arctic Ocean The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately and is known as the coldest of all the oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, a ...
. Portions of the ice that do not melt seasonally can get very thick, up to 3–4 meters thick over large areas, with ridges up to 20 meters thick. One-year ice is usually about 1 meter thick. The area covered by sea ice ranges between 9 and 12 million km2. In addition, the Greenland ice sheet covers about 1.71 million km2 and contains about 2.6 million km3 of ice. When the ice breaks off (calves) it forms icebergs scattered around the northern Atlantic. According to the National Snow and Ice Data Center, "since 1979, winter Arctic ice extent has decreased about 4.2 percent per decade". Both 2008 and 2009 had a minimum Arctic sea ice extent somewhat above that of 2007. At other times of the year the ice extent is still sometimes near the 1979–2000 average, as in April 2010, by the data from the National Snow and Ice Data Center. Still, between these same years, the overall average ice coverage appears to have declined from 8 million km2 to 5 million km2.


South Pole

Earth's south polar land mass,
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...
, is covered by the Antarctic ice sheet. It covers an area of about 14.6 million km2 and contains between 25 and 30 million km3 of ice. Around 70% of the fresh water on Earth is contained in this ice sheet. Data from the National Snow and Ice Data Center shows that the sea ice coverage of Antarctica has a slightly positive trend over the last three decades (1979–2009).


Historical cases

Over the past several decades, Earth's polar ice caps have gained significant attention because of the alarming decrease in land and sea ice.
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
reports that since the late 1970's, the
Arctic The Arctic ( or ) is a polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenland), Finland, Iceland, N ...
has lost an average of 20,800 square miles (53,900 square kilometres) of sea ice per year while the Antarctic has gained an average of 7,300 square miles (18,900 km2) of sea ice per year. At the same time, the Arctic has been losing around 50 cubic kilometres (gigatons) of land ice per year, almost entirely from Greenland's 2.6 million gigaton sheet. On 19 September 2014, for the first time since 1979, Antarctic sea ice extent exceeded 7.72 million square miles (20 million square kilometres), according to the National Snow and Ice Data Center. The ice extent stayed above this benchmark extent for several days. The average maximum extent between 1981 and 2010 was 7.23 million square miles (18.72 million square kilometres). The single-day maximum extent in 2014 was reached on 20 Sep, according to
NSIDC The National Snow and Ice Data Center (NSIDC) is a United States information and referral center in support of polar and cryospheric research. NSIDC archives and distributes digital and analog snow and ice data and also maintains information abo ...
data, when the sea ice covered 7.78 million square miles (20.14 million square kilometres). The 2014 five-day average maximum was reached on 22 Sep, when sea ice covered 7.76 million square miles (20.11 million square kilometres), according to
NSIDC The National Snow and Ice Data Center (NSIDC) is a United States information and referral center in support of polar and cryospheric research. NSIDC archives and distributes digital and analog snow and ice data and also maintains information abo ...
. This increase could be due to the reduction in the salinity of the
Antarctic Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-small ...
as a result of the previous melting of the ice sheet, by increasing the
freezing point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depend ...
of the seawater. The current rate of decline of the ice caps has caused many investigations and discoveries on glacier dynamics and their influence on the world's climate. In the early 1950s, scientists and engineers from the US Army began drilling into polar ice caps for geological insight. These studies resulted in "nearly forty years of research experience and achievements in deep polar ice core drillings... and established the fundamental drilling technology for retrieving deep ice cores for climatologic archives." Polar ice caps have been used to track current climate patterns but also patterns over the past several thousands years from the traces of and found trapped in the ice. In the past decade, polar ice caps have shown their most rapid decline in size with no true sign of recovery. Josefino Comiso, a senior research scientist at NASA, found that the "rate of warming in the Arctic over the last 20 years is eight times the rate of warming over the last 100 years." In September 2012, sea ice reached its smallest size ever. Journalist John Vidal stated that sea ice is "700,000 sq km below the previous minimum of 4.17m sq km set in 2007". In August 2013, Arctic sea ice extent averaged 6.09m km2, which represents 1.13 million km2 below the 1981–2010 average for that month.


Mars

In addition to Earth, the planet
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
also has polar ice caps. They consist of primarily water-ice with a few percent dust. Frozen carbon dioxide makes up a small permanent portion of the
Planum Australe Planum Australe (Latin: "the southern plain") is the southern polar plain on Mars. It extends southward of roughly 75°S and is centered at . The geology of this region was to be explored by the failed NASA mission Mars Polar Lander, which lost co ...
or the South Polar Layered Deposits. In both hemispheres a seasonal
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
frost deposits in the winter and sublimates during the spring. Data collected in 2001 from NASA missions to Mars show that the southern residual ice cap undergoes sublimation inter-annually. The most widely accepted explanation is that fluctuations in the planet's orbit are causing the changes.


Pluto

On 29 April 2015, NASA stated that its New Horizons missions had discovered a feature thought to be a polar ice cap on the dwarf planet
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest ...
. The probe's flyby of Pluto in July 2015 allowed the ''Alice''
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
imaging spectrometer An imaging spectrometer is an instrument used in hyperspectral imaging and imaging spectroscopy to acquire a spectrally-resolved image of an object or scene, often referred to as a datacube due to the three-dimensional representation of the data. ...
to confirm that the feature was in fact an ice cap composed of
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
and
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
ices.


See also

*
Cryosphere ] The cryosphere (from the Ancient Greek, Greek ''kryos'', "cold", "frost" or "ice" and ''sphaira'', "globe, ball") is an all-encompassing term for those portions of Earth's surface where water is in solid form, including sea ice, lake ice, ri ...
*
Ice age An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gre ...
* Polar climate


References


External links

* {{commons-inline Geography of the Arctic Ice caps Polar regions