Earth's oceans
Arctic
Atlantic
Indian
Pacific
Southern
World Oceanvt
The Arctic Ocean is the smallest and shallowest of the world's five major oceans.[1] The International Hydrographic Organization (IHO) recognizes it as an ocean, although some oceanographers call it the Arctic Mediterranean Sea or simply the Arctic Sea, classifying it a mediterranean sea or an estuary of the Atlantic Ocean.[2][3] It is also seen as the northernmost part of the all-encompassing World Ocean.
Located mostly in the Arctic north polar region in the middle of the Northern Hemisphere, the Arctic Ocean is almost completely surrounded by Eurasia and North America.  It is partly covered by sea ice throughout the year and almost completely in winter. The Arctic Ocean's surface temperature and salinity vary seasonally as the ice cover melts and freezes;[4] its salinity is the lowest on average of the five major oceans, due to low evaporation, heavy fresh water inflow from rivers and streams, and limited connection and outflow to surrounding oceanic waters with higher salinities. The summer shrinking of the ice has been quoted at 50%.[1] The US National Snow and Ice Data Center (NSIDC) uses satellite data to provide a daily record of Arctic sea ice cover and the rate of melting compared to an average period and specific past years.
History
Human habitation in the North American polar region goes back at least 50,000–17,000 years ago, during the Wisconsin glaciation. At this time, falling sea levels allowed people to move across the Bering land bridge that joined Siberia to north west North America (Alaska), leading to the Settlement of the Americas.[5]

Paleo-Eskimo groups included the Pre-Dorset (c. 3200 – 850 B.C.); the Saqqaq culture of Greenland (2500 – 800 B.C.); the Independence I and Independence II cultures of northeastern Canada and Greenland (c. 2400 – 1800 B.C. and c. 800 – 1 B.C.); the Groswater of Labrador and Nunavik, and the Dorset culture (500 B.C. to 1500 A.D.), which spread across Arctic North America. The Dorset were the last major Paleo-Eskimo culture in the Arctic before the migration east from present-day Alaska of the Thule, the ancestors of the modern Inuit.[6]
The Thule Tradition lasted from about 200 B.C. to 1600 A.D. around the Bering Strait, the Thule people being the prehistoric ancestors of the Inuit who now live in Northern Labrador.[7]
For much of European history, the north polar regions remained largely unexplored and their geography conjectural. Pytheas of Massilia recorded an account of a journey northward in 325 BC, to a land he called "Eschate Thule", where the Sun only set for three hours each day and the water was replaced by a congealed substance "on which one can neither walk nor sail". He was probably describing loose sea ice known today as "growlers" or "bergy bits"; his "Thule" was probably Norway, though the Faroe Islands or Shetland have also been suggested.[8]

Early cartographers were unsure whether to draw the region around the North Pole as land (as in Johannes Ruysch's , or Gerardus Mercator's ) or water (as with Martin Waldseemüller's ). The fervent desire of European merchants for a northern passage, the Northern Sea Route or the Northwest Passage, to "Cathay" (China) caused water to win out, and by 1723 mapmakers such as Johann Homann featured an extensive "Oceanus Septentrionalis" at the northern edge of their charts.
The few expeditions to penetrate much beyond the Arctic Circle in this era added only small islands, such as Novaya Zemlya (11th century) and Spitzbergen (1596), though since these were often surrounded by pack-ice, their northern limits were not so clear. The makers of navigational charts, more conservative than some of the more fanciful cartographers, tended to leave the region blank, with only fragments of known coastline sketched in.
This lack of knowledge of what lay north of the shifting barrier of ice gave rise to a number of conjectures. In England and other European nations, the myth of an "Open Polar Sea" was persistent. John Barrow, longtime Second Secretary of the British Admiralty, promoted exploration of the region from 1818 to 1845 in search of this.
In the United States in the 1850s and 1860s, the explorers Elisha Kane and Isaac Israel Hayes both claimed to have seen part of this elusive body of water. Even quite late in the century, the eminent authority Matthew Fontaine Maury included a description of the Open Polar Sea in his textbook The Physical Geography of the Sea (1883). Nevertheless, as all the explorers who travelled closer and closer to the pole reported, the polar ice cap is quite thick, and persists year-round.
Fridtjof Nansen was the first to make a nautical crossing of the Arctic Ocean, in 1896. The first surface crossing of the ocean was led by Wally Herbert in 1969, in a dog sled expedition from Alaska to Svalbard, with air support.[9] The first nautical transit of the north pole was made in 1958 by the submarine USS Nautilus, and the first surface nautical transit occurred in 1977 by the icebreaker NS Arktika.
Since 1937, Soviet and Russian manned drifting ice stations have extensively monitored the Arctic Ocean. Scientific settlements were established on the drift ice and carried thousands of kilometers by ice floes.[10]
In World War II, the European region of the Arctic Ocean was heavily contested: the Allied commitment to resupply the Soviet Union via its northern ports was opposed by German naval and air forces.
Since 1954 commercial airlines have flown over the Arctic Ocean (see Polar route).
Geography

The Arctic Ocean occupies a roughly circular basin and covers an area of about 14,056,000km2 (5,427,000sqmi), almost the size of Antarctica.[11][12] The coastline is 45,390km (28,200mi) long.[11][13] It is surrounded by the land masses of Eurasia, North America, Greenland, and by several islands.
It is generally taken to include Baffin Bay, Barents Sea, Beaufort Sea, Chukchi Sea, East Siberian Sea, Greenland Sea, Hudson Bay, Hudson Strait, Kara Sea, Laptev Sea, White Sea and other tributary bodies of water. It is connected to the Pacific Ocean by the Bering Strait and to the Atlantic Ocean through the Greenland Sea and Labrador Sea.[1]
Countries bordering the Arctic Ocean are: Russia, Norway, Iceland, Greenland, Canada and the United States.
Extent and major ports

There are several ports and harbors around the Arctic Ocean[14]
United States
In Alaska, the main ports are Barrow () and Prudhoe Bay ().
Canada
In Canada, ships may anchor at Churchill (Port of Churchill) () in Manitoba, Nanisivik (Nanisivik Naval Facility) () in Nunavut,[15] Tuktoyaktuk () or Inuvik () in the Northwest Territories.
Greenland
In Greenland, the main port is at Nuuk (Nuuk Port and Harbour) ().
Norway
In Norway, Kirkenes () and Vardø () are ports on the mainland. Also, there is Longyearbyen () on Svalbard, a Norwegian archipelago, next to Fram Strait.
Russia
In Russia, major ports sorted by the different sea areas are:
Murmansk () in the Barents Sea
Arkhangelsk () in the White Sea
Labytnangi () Salekhard (), Dudinka (), Igarka () and Dikson () in the Kara Sea
Tiksi () in the Laptev Sea
Pevek () in the East Siberian Sea
Arctic shelves

The ocean's Arctic shelf comprises a number of continental shelves, including the Canadian Arctic shelf, underlying the Canadian Arctic Archipelago, and the Russian continental shelf, which is sometimes simply called the "Arctic Shelf" because it is greater in extent. The Russian continental shelf consists of three separate, smaller shelves, the Barents Shelf, Chukchi Sea Shelf and Siberian Shelf. Of these three, the Siberian Shelf is the largest such shelf in the world. The Siberian Shelf holds large oil and gas reserves, and the Chukchi shelf forms the border between Russian and the United States as stated in the USSR–USA Maritime Boundary Agreement. The whole area is subject to international territorial claims.
Underwater features
An underwater ridge, the Lomonosov Ridge, divides the deep sea North Polar Basin into two oceanic basins: the Eurasian Basin, which is between 4,000 and 4,500m (13,100 and 14,800ft) deep, and the Amerasian Basin (sometimes called the North American, or Hyperborean Basin), which is about 4,000m (13,000ft) deep. The bathymetry of the ocean bottom is marked by fault block ridges, abyssal plains, ocean deeps, and basins. The average depth of the Arctic Ocean is 1,038m (3,406ft).[16] The deepest point is Litke Deep in the Eurasian Basin, at 5,450m (17,880ft).
The two major basins are further subdivided by ridges into the Canada Basin (between Alaska/Canada and the Alpha Ridge), Makarov Basin (between the Alpha and Lomonosov Ridges), Amundsen Basin (between Lomonosov and Gakkel ridges), and Nansen Basin (between the Gakkel Ridge and the continental shelf that includes the Franz Josef Land).
Oceanography
Water flow

In large parts of the Arctic Ocean, the top layer (about 50m (160ft)) is of lower salinity and lower temperature than the rest. It remains relatively stable, because the salinity effect on density is bigger than the temperature effect. It is fed by the freshwater input of the big Siberian and Canadian streams (Ob, Yenisei, Lena, Mackenzie), the water of which quasi floats on the saltier, denser, deeper ocean water. Between this lower salinity layer and the bulk of the ocean lies the so-called halocline, in which both salinity and temperature are rising with increasing depth.

Because of its relative isolation from other oceans, the Arctic Ocean has a uniquely complex system of water flow. It is classified as a mediterranean sea, which as “a part of the world ocean which has only limited communication with the major ocean basins (these being the Pacific, Atlantic, and Indian Oceans) and where the circulation is dominated by thermohaline forcing”.[17]  The Arctic Ocean has a total volume of 18.07×106 km3, equal to about 1.3% of the World Ocean.  Mean surface circulation is predominately cyclonic on the Eurasian side and anticyclonic in the Canadian Basin.[18]
Water enters from both the Pacific and Atlantic Oceans and can be divided into three unique water masses. The deepest water mass is called Arctic Bottom Water and begins around 900 metres (3,000 feet) depth.[17] It is composed of the densest water in the World Ocean and has two main sources: Arctic shelf water and Greenland Sea Deep Water. Water in the shelf region that begins as inflow from the Pacific passes through the narrow Bering Strait at an average rate of 0.8 Sverdrups and reaches the Chukchi Sea.[19]  During the winter, cold Alaskan winds blow over the Chukchi Sea, freezing the surface water and pushing this newly formed ice out to the Pacific. The speed of the ice drift is roughly 1–4cm/s.[18] This process leaves dense, salty waters in the sea that sink over the continental shelf into the western Arctic Ocean and create a halocline.[20]

This water is met by Greenland Sea Deep Water, which forms during the passage of winter storms. As temperatures cool dramatically in the winter, ice forms and intense vertical convection allows the water to become dense enough to sink below the warm saline water below.[17]  Arctic Bottom Water is critically important because of its outflow, which contributes to the formation of Atlantic Deep Water. The overturning of this water plays a key role in global circulation and the moderation of climate.
In the depth range of 150–900 metres (490–2,950 feet) is a water mass referred to as Atlantic Water. Inflow from the North Atlantic Current enters through the Fram Strait, cooling and sinking to form the deepest layer of the halocline, where it circles the Arctic Basin counter-clockwise.  This is the highest volumetric inflow to the Arctic Ocean, equalling about 10 times that of the Pacific inflow, and it creates the Arctic Ocean Boundary Current.[19] It flows slowly, at about 0.02m/s.[17] Atlantic Water has the same salinity as Arctic Bottom Water but is much warmer (up to 3°C). In fact, this water mass is actually warmer than the surface water, and remains submerged only due to the role of salinity in density.[17]  When water reaches the basin it is pushed by strong winds into a large circular current called the Beaufort Gyre. Water in the Beaufort Gyre is far less saline than that of the Chukchi Sea due to inflow from large Canadian and Siberian rivers.[20]
The final defined water mass in the Arctic Ocean is called Arctic Surface Water and is found from 150–200 metres (490–660 feet). The most important feature of this water mass is a section referred to as the sub-surface layer. It is a product of Atlantic water that enters through canyons and is subjected to intense mixing on the Siberian Shelf.[17] As it is entrained, it cools and acts a heat shield for the surface layer. This insulation keeps the warm Atlantic Water from melting the surface ice. Additionally, this water forms the swiftest currents of the Arctic, with speed of around 0.3–0.6m/s.[17] Complementing the water from the canyons, some Pacific water that does not sink to the shelf region after passing through the Bering Strait also contributes to this water mass.
Waters originating in the Pacific and Atlantic both exit through the Fram Strait between Greenland and Svalbard Island, which is about 2,700 metres (8,900 feet) deep and 350 kilometres (220 miles) wide. This outflow is about 9 Sv.[19] The width of the Fram Strait is what allows for both inflow and outflow on the Atlantic side of the Arctic Ocean. Because of this, it is influenced by the Coriolis force, which concentrates outflow to the East Greenland Current on the western side and inflow to the Norwegian Current on the eastern side.[17] Pacific water also exits along the west coast of Greenland and the Hudson Strait (1–2 Sv), providing nutrients to the Canadian Archipelago.[19]
As noted, the process of ice formation and movement is a key driver in Arctic Ocean circulation and the formation of water masses.  With this dependence, the Arctic Ocean experiences variations due to seasonal changes in sea ice cover. Sea ice movement is the result of wind forcing, which is related to a number of meteorological conditions that the Arctic experiences throughout the year.  For example, the Beaufort High—an extension of the Siberian High system—is a pressure system that drives the anticyclonic motion of the Beaufort Gyre.[18] During the summer, this area of high pressure is pushed out closer to its Siberian and Canadian sides. In addition, there is a sea level pressure (SLP) ridge over Greenland that drives strong northerly winds through the Fram Strait, facilitating ice export. In the summer, the SLP contrast is smaller, producing weaker winds. A final example of seasonal pressure system movement is the low pressure system that exists over the Nordic and Barents Seas. It is an extension of the Icelandic Low, which creates cyclonic ocean circulation in this area. The low shifts to center over the North Pole in the summer.  These variations in the Arctic all contribute to ice drift reaching its weakest point during the summer months.  There is also evidence that the drift is associated with the phase of the Arctic Oscillation and Atlantic Multidecadal Oscillation.[18]
Sea ice

Much of the Arctic Ocean is covered by sea ice that varies in extent and thickness seasonally. The mean extent of the ice has been decreasing since 1980 from the average winter value of 15,600,000km2 (6,023,200sqmi) at a rate of 3% per decade. The seasonal variations are about 7,000,000km2 (2,702,700sqmi) with the maximum in April and minimum in September. The sea ice is affected by wind and ocean currents, which can move and rotate very large areas of ice. Zones of compression also arise, where the ice piles up to form pack ice.[22][23][24]
Icebergs occasionally break away from northern Ellesmere Island, and icebergs are formed from glaciers in western Greenland and extreme northeastern Canada. Icebergs are not sea ice but may becoming embedded in the pack ice. Icebergs pose a hazard to ships, of which the Titanic is one of the most famous. The ocean is virtually icelocked from October to June, and the superstructure of ships are subject to icing from October to May.[14] Before the advent of modern icebreakers, ships sailing the Arctic Ocean risked being trapped or crushed by sea ice (although the Baychimo drifted through the Arctic Ocean untended for decades despite these hazards).
Climate

Under the influence of the Quaternary glaciation, the Arctic Ocean is contained in a polar climate characterized by persistent cold and relatively narrow annual temperature ranges. Winters are characterized by the polar night, extreme cold, frequent low-level temperature inversions, and stable weather conditions.[25] Cyclones are only common on the Atlantic side.[26] Summers are characterized by continuous daylight (midnight sun), and temperatures can rise above the melting point (0°C (32°F). Cyclones are more frequent in summer and may bring rain or snow.[26]  It is cloudy year-round, with mean cloud cover ranging from 60% in winter to over 80% in summer.[27]
The temperature of the surface of the Arctic Ocean is fairly constant, near the freezing point of seawater. Because the Arctic Ocean consists of saltwater, the temperature must reach −1.8°C (28.8°F) before freezing occurs.
The density of sea water, in contrast to fresh water, increases as it nears the freezing point and thus it tends to sink. It is generally necessary that the upper 100–150m (330–490ft) of ocean water cools to the freezing point for sea ice to form.[28] In the winter the relatively warm ocean water exerts a moderating influence, even when covered by ice. This is one reason why the Arctic does not experience the extreme temperatures seen on the Antarctic continent.
There is considerable seasonal variation in how much pack ice of the Arctic ice pack covers the Arctic Ocean. Much of the Arctic ice pack is also covered in snow for about 10 months of the year. The maximum snow cover is in March or April — about 20 to 50cm (7.9 to 19.7in) over the frozen ocean.
The climate of the Arctic region has varied significantly in the past. As recently as 55 million years ago, during the Paleocene–Eocene Thermal Maximum, the region reached an average annual temperature of 10–20°C (50–68°F).[29] The surface waters of the northernmost[30] Arctic Ocean warmed, seasonally at least, enough to support tropical lifeforms (the dinoflagellates Apectodinium augustum) requiring surface temperatures of over 22°C (72°F).[31]
Animal and plant life

Endangered marine species in the Arctic Ocean include walruses and whales. The area has a fragile ecosystem which is slow to change and slow to recover from disruptions or damage.[14] Lion's mane jellyfish are abundant in the waters of the Arctic, and the banded gunnel is the only species of gunnel that lives in the ocean.
The Arctic Ocean has relatively little plant life except for phytoplankton.[32] Phytoplankton are a crucial part of the ocean and there are massive amounts of them in the Arctic, where they feed on nutrients from rivers and the currents of the Atlantic and Pacific oceans.[33] During summer, the sun is out day and night, thus enabling the phytoplankton to photosynthesize for long periods of time and reproduce quickly. However, the reverse is true in winter when they struggle to get enough light to survive.[33]
Natural resources

Petroleum and natural gas fields, placer deposits, polymetallic nodules, sand and gravel aggregates, fish, seals and whales can all be found in abundance in the region.[14][24]
The political dead zone near the center of the sea is also the focus of a mounting dispute between the United States, Russia, Canada, Norway, and Denmark.[34] It is significant for the global energy market because it may hold 25% or more of the world's undiscovered oil and gas resources.[35]
Environmental concerns

Arctic ice melting
The Arctic ice pack is thinning, and in many years there is also a seasonal hole in the ozone layer.[36]
Reduction of the area of Arctic sea ice reduces the planet's average albedo, possibly resulting in global warming in a positive feedback mechanism.[24][37] Research shows that the Arctic may become ice free in the summer for the first time in human history within by 2040.[38][39] Estimates vary for when the last time the Arctic was ice free: 65 million years ago when fossils indicate that plants existed there to as few as 5,500 years ago; ice and ocean cores going back 8000 years to the last warm period or 125,000 during the last intraglacial period.[40]
Warming temperatures in the Arctic may cause large amounts of fresh meltwater to enter the north Atlantic, possibly disrupting global ocean current patterns. Potentially severe changes in the Earth's climate might then ensue.[37]
As the extent of sea ice diminishes and sea level rises, the effect of storms such as the Great Arctic Cyclone of 2012 on open water increases, as does possible salt-water damage to vegetation on shore at locations such as the Mackenzie's river delta as stronger storm surges become more likely.[41]
Clathrate breakdown

Sea ice, and the cold conditions it sustains, serves to stabilize methane deposits on and near the shoreline,[42] preventing the clathrate breaking down and outgassing methane into the atmosphere, causing further warming.  Melting of this ice may release large quantities of methane, a powerful greenhouse gas into the atmosphere, causing further warming in a strong positive feedback cycle and; marine genus and species to become extinct.[42][43]
Other concerns
Other environmental concerns relate to the radioactive contamination of the Arctic Ocean from, for example, Russian radioactive waste dump sites in the Kara Sea[44] and Cold War nuclear test sites such as Novaya Zemlya.[45] In addition, Shell planned to drill exploratory wells in the Chukchi and Beaufort seas during the summer of 2012, which environmental groups filed a lawsuit about in an attempt to protect native communities, endangered wildlife, and the Arctic Ocean in the event of a major oil spill.[46]
On 16 July 2015, five nations (United States of America, Russia, Canada, Norway, Denmark/Greenland) signed a declaration committing to keep their fishing vessels out of a 1.1 million square mile zone in the central Arctic Ocean near the North Pole. The agreement calls for those nations to refrain from fishing there until there is better scientific knowledge about the marine resources and until a regulatory system is in place to protect those resources.[47][48]
See also

Arctic Bridge
Arctic cooperation and politics
Arctic sea ice ecology and history
Chukchi Plateau
Explorers of the Arctic
Extreme points of the Arctic
Fauna of the Arctic
International Arctic Science Committee
Nordicity
Seven Seas
Subarctic

Further reading
Neatby, Leslie H., Discovery in Russian and Siberian Waters 1973 ISBN0-8214-0124-6
Ray, L., and Bacon, B., eds., The Arctic Ocean 1982 ISBN0-333-31017-9
Thorén, Ragnar V.A., Picture Atlas of the Arctic 1969 ISBN0-8214-0124-6

 Daily logs, photos and video from exploration mission.
, from the Woods Hole Oceanographic Institution

 Interactive map

 at NSIDC
 Images from Web Cams deployed in spring on an ice floe
 Data from instruments deployed on an ice floe

Coordinates:

What sea life lives in the Arctic ocean?


A dome is a self-supporting structural element of architecture that resembles the hollow upper half of a sphere. This list excludes dome-shaped structures that are not self-supporting such as The O2 (formerly the Millennium Dome) in London which is 365m (1,198ft) in diameter and supported by masts.
This article tries to list every dome in the world which was the largest dome of its time. The defining criterion is in each case the inner diameter of the largest circular cross-section of the dome.
Worldwide
Below is a list of buildings that have held the title of the largest dome in the world.

By continent
Below is a list of buildings that have held the title of the largest dome on their continent.
Europe

In the first half of the 1980s a dome with a diameter of 234 metres was built near Istra, Russia. It collapsed on January 25, 1986 .
In 1990, the 129 m in diameter building named "Kupolen" (the Dome) was completed in Borlänge, Sweden. Originally an exposition hall with a few stores at ground level, today a mall in three levels.
North America

South America

Asia

Africa

Australia

By structure
Below is a list of buildings that have held the title of the largest dome in terms of their structure.
Stone

Concrete

Masonry

Clay hollowware

Wood

Cast iron

Steel

Reinforced concrete

Famous large domes
Below is a list of large domes which are considered particularly important for various reasons but have never held the title of the largest dome in the world.

See also
List of Roman domes
List of celebrated domes (in chronological order)

Sources


The Winchester Model 1897, also known as the Model 97, M97, or Trench Gun, was a pump-action shotgun
with an external hammer and tube magazine manufactured by the Winchester Repeating Arms Company. The Model 1897 was an evolution of the Winchester Model 1893 designed by John Browning. From 1897 until 1957, over one million of these shotguns were produced. The Model 1897 was offered in numerous barrel lengths and grades, chambered in 12 and 16 gauge, and as a solid frame or takedown. The 16-gauge guns had a standard barrel length of 28inches, while 12-gauge guns were furnished with 30-inch length barrels. Special length barrels could be ordered in lengths as short as 20inches, and as long as 36inches. Since the time the Model 1897 was first manufactured it has been used by American soldiers,[1] police departments,[2] and hunters.[2]
History
The Winchester Model 1897 was designed by American firearms inventor John Moses Browning. The Model 1897 was first listed for sale in the November 1897  as a 12 gauge solid frame. The 12 gauge takedown was added in October 1898, and the 16 gauge takedown in February 1900.[3] Originally produced as a tougher, stronger and more improved version of the Winchester 1893, itself an improvement on the early Spencer pump gun, the 1897 was identical to its forerunner, except that the receiver was thicker and allowed for use of smokeless powder shells, which were not common at the time.  The 1897 introduced a "take down" design, where the barrel could be taken off – a standard in pump shotguns made today, like the Remington 870 and Mossberg 500 series. Over time, "the model 97 became the most popular shotgun on the American market and established a standard of performance by which other kinds and makes of shotguns were judged, including the most expensive imported articles".[2] The Winchester Model 1897 was in production from 1897 until 1957. It was in this time frame that the "modern" hammerless designs became common, like the Winchester Model 1912 and the Remington 870. The Model 1897 was superseded by the Winchester Model 1912.[4] However, the gun can still be found today in regular use.
Improvements from the 1893
While designing the new Model 1897, many of the weaknesses present in the earlier Model 1893 were taken into account and remedied.[3] These improvements included:
The frame was strengthened and made longer to handle a 12 gauge 23⁄4-inch shell, as well as the 25⁄8-inch shell.[3]
The top of the frame was covered so that the ejection of the fired shell was entirely from the side.[3] This added a great amount of strength to the frame of the gun and it allowed the use of a 23⁄4 inch shell without the danger of the gun constantly jamming.[5]
The action could not be opened until a slight forward movement of the slide handle released the action slide lock. In firing, the recoil of the shotgun gave a slight forward motion to the slide handle and released the action slide lock which enabled immediate opening of the action. In the absence of any recoil, the slide handle had to be pushed forward manually in order to release the action slide lock.[3]
A movable cartridge guide was placed on the right side of the carrier block to prevent the escape of the shell when the shotgun was turned sideways in the act of loading.[3]
The stock was made longer and with less drop.[3]
Of these improvements, the slide lock is the one that made the Model 1897 into a safe firearm. This improved slide lock kept the shotgun locked until actual firing occurred which prevented it from jamming in the case of a misfire. The slide lock "stands in such a relation to the body of the firing pin as will prevent the firing pin reaching the primer until the pin has moved forward a sufficient distance to insure locking of the breech bolt".[6] This prevents the action sleeve "from being retracted by the hand of the gunner until after firing, and hence rendering the firearm more safe".[7]
Description

The Winchester Model 1897 and the Winchester Model 1893 were both designed by John Browning. The Model 1897 is an external hammer shotgun lacking a trigger disconnector. This means that the user can hold the trigger down while cycling the shotgun and once the action is returned to battery the shotgun fires.[8] The firearm itself is classified as a slide action pump shotgun. It was the first truly successful pump-action shotgun produced. Throughout the time period the Model 1897 was in production, over a million of the type were produced in various grades and barrel lengths. 16-gauge guns had a standard barrel length of 28inches, while 12-gauge guns were furnished with 30-inch length barrels. Special length barrels could be ordered in lengths as short as 20inches, and as long as 36inches. Along with various grades and barrel lengths, the Model 1897 came in two different chamberings. One was the 12 gauge and the other was the 16 gauge.[8] The shells should be of the 23⁄4 inch or 25⁄8 inch model.[3] Any shells larger are not recommended. An average Model 1897 can hold 6 shotgun shells in the magazine tube.[9] When working the action of the Model 1897 the forend (fore grip) is pulled back, forcing the breech bolt to the rear which extracts and then ejects the spent shell while simultaneously cocking the external hammer by pushing it to the rear. When the forend is slid forward again, the breech bolt pushes a fresh shell into the gun's chamber and locks into place.

The Chinese company Norinco has made an effort to reproduce this firearm. The Norinco 97 is an almost exact copy of the Winchester 1897, produced in both Trench and Riot grades, yet lacking in the fit and finish of the originals.[1]
Grades of the Model 97

Original prices
When the Model 1897 was first introduced, the price depended upon what grade was being purchased and what features were being added to that specific shotgun. To purchase a plain finished shotgun would cost the buyer $25, whereas an engraved receiver with checkered and finer wood included cost $100.[8] The more expensive grades of the Model 1897 were the standard, trap, pigeon, and tournament grades. These were the grades that were normally equipped with an engraved receiver and with checkered, finer wood.[4][14]  The less expensive and plainer grades were the Brush, Brush Takedown, Riot, and Trench. These grades were not given the higher valued wood or special designs.[4][14] This is because these guns were designed and built for hard abuse. These grades stood a higher chance of being badly damaged so there was no need to put extra money into them for appearance purposes. As the functions that were performed with these grades required them to be lightweight, it was not beneficial to use heavy and expensive wood when designing them. Most often, when these grades were purchased, they were purchased in high numbers. By designing these grades with standard wood and finish, it kept the prices at a lower level.[4][14]
Military use

The Model 1897 was issued to American soldiers during the Philippine-American War of 1898. This first major use of issued shotguns by the United States military involved 200 weapons procured and sent to the Philippines in 1900. They were employed in countering Moro tribesmen who engaged the Americans in close-quarter combat using knives and swords.[15] (See: juramentado)

During the Punitive Expedition in Mexico, some US soldier were also equipped with M97s.[16] Already popular before World War I, sales of the Model 1897 picked up after the war broke. This was because many were produced to meet the demands of the military. When the United States entered World War I, there was a need for more service weapons to be issued to the troops. It became clear to the United States just how brutal trench warfare was, and how great the need was for a large amount of close-range firepower while fighting in a trench, after they had observed the war for the first three years.[1] The Model 1897 Trench grade was an evolution of this idea. The pre-existing Winchester Model 1897 was modified by adding a perforated steel heat shield over the barrel which protected the hand of the user from the barrel when it became over-heated,[17] and an adapter with bayonet lug for affixing an M1917 bayonet.[1]  This model was ideal for close combat and was efficient in trench warfare due to its 20-inch cylinder bore barrel. Buckshot ammunition was issued with the trench grade during the war. Each round of this ammunition contained nine 00 (.33-caliber) buckshot pellets. This gave considerable firepower to the individual soldier by each round that was fired.[2]  This shorter barrel and large amount of firepower is what made this grade ideal for trench warfare.
The Model 1897 was used by American troops for purposes in World War I other than a force multiplier. American soldiers who were skilled at trap shooting were armed with these guns and stationed where they could fire at enemy hand grenades in midair.[2] This would deflect the grenades from falling into the American trenches and therefore protect American soldiers.[2]
Unlike most modern pump-action shotguns, the Winchester Model 1897 (versions of which were type classified as the Model 97 or M97 for short) fired each time the action closed with the trigger depressed (that is, it lacks a trigger disconnector). Coupled with its six-shot capacity, this made it effective for close combat, such that troops referred to it as a "trench sweeper". This characteristic allowed troops to fire the whole magazine with great speed. The Model 1897 was so effective, and feared, that the German government protested (in vain) to have it outlawed in combat.[18] The Model 1897 was used again in World War II by the United States Army and Marine Corps, where it was used alongside the similarly militarized version of the hammerless Model 1912.[19] Some were still in service during the Korean War[20] and the Vietnam War.[21]
Other military uses of the shotgun included "the execution of security/interior guard operations, rear area security operations, guarding prisoners of war, raids, ambushes, military operations in urban terrain, and selected special operations". Despite protesting them, Germans did not listen to Ludendorff and decided to use and unofficially adopt the M1897 for their own use with modifications and named it "trench mouser" and mainly place them with stormtroopers.[18]
World War I protests
Although the Model 1897 was popular with American troops in World War I, the Germans soon began to protest its use in combat. "On 19 September 1918, the German government under Ludendorff  issued a diplomatic protest against the American use of shotguns, alleging that the shotgun was prohibited by the law of war."[18] A part of the German protest read that "[i]t is especially forbidden to employ arms, projections, or materials calculated to cause unnecessary suffering" as defined in the 1907 Hague Convention on Land Warfare.[2] This is the only known occasion in which the legality of actual combat use of the shotgun has been raised.[18] However, the United States interpreted their use of the shotgun differently than Germany. The Judge Advocate General of the Army, Secretary of State Robert Lansing carefully considered and reviewed the applicable law and promptly rejected the German protest.[18]  France and Britain did use shotguns in trench warfare weapons during World War I.  The shotgun in question was a double-barreled shotgun, which was not used because they were unable to obtain high powered ammunition and that type of gun is slow to reload in close combat.[18]
German response
The rejection of their protest greatly upset the German forces, because they believed they were treated unjustly in the war. Shortly after the protest was rejected, Germany issued threats that they would punish all captured American soldiers that were found to be armed with a shotgun.[2] This led to the United States issuing a retaliation threat, stating that any measures unjustly taken against captured American soldiers would lead to reprisals by the United States on captured German soldiers.[22]
Other uses
After the war, a shorter-barrelled version of the Model 1897 was marketed by Winchester as a riot gun. Messengers of The American Express Company were armed with this weapon as were various police departments throughout the US.[2]  The differences between this riot version and the trench version were the riot version lacked the heat shield and bayonet lug,[1] and all trench guns were equipped with sling swivels, whereas most riot guns were not.[8]
Users
Iran[23]
Philippines[24]
United States
Vietnam
See also
List of individual weapons of the U.S. Armed Forces
List of shotguns

Bibliography


Uranus (from the Latin name "Ūranus" for the Greek god Οὐρανός) is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Uranus is similar in composition to Neptune, and both have bulk chemical compositions which differ from that of the larger gas giants Jupiter and Saturn. For this reason, scientists often classify Uranus and Neptune as "ice giants" to distinguish them from the gas giants. Uranus' atmosphere is similar to Jupiter's and Saturn's in its primary composition of hydrogen and helium, but it contains more "ices" such as water, ammonia, and methane, along with traces of other hydrocarbons.[14] It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49K (−224°C; −371°F), and has a complex, layered cloud structure with water thought to make up the lowest clouds and methane the uppermost layer of clouds.[14] The interior of Uranus is mainly composed of ices and rock.[13]
Like the other giant planets, Uranus has a ring system, a magnetosphere, and numerous moons. The Uranian system has a unique configuration because its axis of rotation is tilted sideways, nearly into the plane of its solar orbit. Its north and south poles, therefore, lie where most other planets have their equators.[19] In 1986, images from Voyager 2 showed Uranus as an almost featureless planet in visible light, without the cloud bands or storms associated with the other giant planets.[19] Observations from Earth have shown seasonal change and increased weather activity as Uranus approached its equinox in 2007. Wind speeds can reach 250 metres per second (900km/h; 560mph).[20]
Uranus is the only planet whose name is derived directly from a figure from Greek mythology, from the Latinised version of the Greek god of the sky Ouranos.
History
Like the classical planets, Uranus is visible to the naked eye, but it was never recognised as a planet by ancient observers because of its dimness and slow orbit.[21] Sir William Herschel announced its discovery on 13 March 1781, expanding the known boundaries of the Solar System for the first time in history and making Uranus the first planet discovered with a telescope.
Discovery

Uranus had been observed on many occasions before its recognition as a planet, but it was generally mistaken for a star. Possibly the earliest known observation was by Hipparchos, who in 128 BC might have recorded it as a star for his star catalogue that was later incorporated into Ptolemy's Almagest.[22] The earliest definite sighting was in 1690, when John Flamsteed observed it at least six times, cataloguing it as 34 Tauri. The French astronomer Pierre Charles Le Monnier observed Uranus at least twelve times between 1750 and 1769,[23] including on four consecutive nights.
Sir William Herschel observed Uranus on 13 March 1781 from the garden of his house at 19 New King Street in Bath, Somerset, England (now the Herschel Museum of Astronomy),[24] and initially reported it (on 26 April 1781) as a comet.[25] Herschel "engaged in a series of observations on the parallax of the fixed stars",[26] using a telescope of his own design.
Herschel recorded in his journal: "In the quartile near ζ Tauri... either [a] Nebulous star or perhaps a comet."[27] On 17 March he noted: "I looked for the Comet or Nebulous Star and found that it is a Comet, for it has changed its place."[28] When he presented his discovery to the Royal Society, he continued to assert that he had found a comet, but also implicitly compared it to a planet:[26]
The power I had on when I first saw the comet was 227. From experience I know that the diameters of the fixed stars are not proportionally magnified with higher powers, as planets are; therefore I now put the powers at 460 and 932, and found that the diameter of the comet increased in proportion to the power, as it ought to be, on the supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed.[26]

Herschel notified the Astronomer Royal Nevil Maskelyne of his discovery and received this flummoxed reply from him on 23 April 1781: "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it."[29]
Although Herschel continued to describe his new object as a comet, other astronomers had already begun to suspect otherwise. Finnish-Swedish astronomer Anders Johan Lexell, working in Russia, was the first to compute the orbit of the new object.[30] Its nearly circular orbit led him to a conclusion that it was a planet rather than a comet. Berlin astronomer Johann Elert Bode described Herschel's discovery as "a moving star that can be deemed a hitherto unknown planet-like object circulating beyond the orbit of Saturn".[31] Bode concluded that its near-circular orbit was more like a planet than a comet.[32]
The object was soon universally accepted as a new planet. By 1783, Herschel acknowledged this to Royal Society president Joseph Banks: "By the observation of the most eminent Astronomers in Europe it appears that the new star, which I had the honour of pointing out to them in March 1781, is a Primary Planet of our Solar System."[33] In recognition of his achievement, King George III gave Herschel an annual stipend of £200 on condition that he move to Windsor so that the Royal Family could look through his telescopes.[34]
Name
The name of Uranus references the ancient Greek deity of the sky Uranus (Ancient Greek: Οὐρανός), the father of Cronus (Saturn) and grandfather of Zeus (Jupiter), which in Latin became "Ūranus" (Latin pronunciation:[ˈuːranʊs]).[1] It is the only planet whose name is derived directly from a figure of Greek mythology. The adjectival form of Uranus is "Uranian".[35] The pronunciation of the name Uranus preferred among astronomers is /ˈjʊərənəs/,[2] with stress on the first syllable as in Latin Ūranus, in contrast to /jʊəˈreɪnəs/, with stress on the second syllable and a long a, though both are considered acceptable.[lower-alpha 6]
Consensus on the name was not reached until almost 70 years after the planet's discovery. During the original discussions following discovery, Maskelyne asked Herschel to "do the astronomical world the faver [sic] to give a name to your planet, which is entirely your own, [and] which we are so much obliged to you for the discovery of".[37] In response to Maskelyne's request, Herschel decided to name the object Georgium Sidus (George's Star), or the "Georgian Planet" in honour of his new patron, King George III.[38] He explained this decision in a letter to Joseph Banks:[33]
In the fabulous ages of ancient times the appellations of Mercury, Venus, Mars, Jupiter and Saturn were given to the Planets, as being the names of their principal heroes and divinities. In the present more philosophical era it would hardly be allowable to have recourse to the same method and call it Juno, Pallas, Apollo or Minerva, for a name to our new heavenly body. The first consideration of any particular event, or remarkable incident, seems to be its chronology: if in any future age it should be asked, when this last-found Planet was discovered? It would be a very satisfactory answer to say, 'In the reign of King George the Third'.

Herschel's proposed name was not popular outside Britain, and alternatives were soon proposed. Astronomer Jérôme Lalande proposed that it be named Herschel in honour of its discoverer.[39] Swedish astronomer Erik Prosperin proposed the name Neptune, which was supported by other astronomers who liked the idea to commemorate the victories of the British Royal Naval fleet in the course of the American Revolutionary War by calling the new planet even Neptune George III or Neptune Great Britain.[30]
In a March 1782 treatise, Bode proposed Uranus, the Latinised version of the Greek god of the sky, Ouranos.[40] Bode argued that the name should follow the mythology so as not to stand out as different from the other planets, and that Uranus was an appropriate name as the father of the first generation of the Titans.[40] He also noted that elegance of the name in that just as Saturn was the father of Jupiter, the new planet should be named after the father of Saturn.[34][40][41][42] In 1789, Bode's Royal Academy colleague Martin Klaproth named his newly discovered element uranium in support of Bode's choice.[43] Ultimately, Bode's suggestion became the most widely used, and became universal in 1850 when HM Nautical Almanac Office, the final holdout, switched from using Georgium Sidus to Uranus.[41]
Uranus has two astronomical symbols. The first to be proposed, ♅,[lower-alpha 7] was suggested by Lalande in 1784. In a letter to Herschel, Lalande described it as "un globe surmonté par la première lettre de votre nom" ("a globe surmounted by the first letter of your surname").[39] A later proposal, ⛢,[lower-alpha 8] is a hybrid of the symbols for Mars and the Sun because Uranus was the Sky in Greek mythology, which was thought to be dominated by the combined powers of the Sun and Mars.[44]
Uranus is called by a variety of translations in other languages. In Chinese, Japanese, Korean, and Vietnamese, its name is literally translated as the "sky king star" (天王星).[45][46][47][48] In Thai, its official name is Dao Yurenat (ดาวยูเรนัส), as in English. Its other name in Thai is Dao Maritayu (ดาวมฤตยู, Star of Mṛtyu), after the Sanskrit word for "death", Mrtyu (मृत्यु). In Mongolian, its name is Tengeriin Van (Тэнгэрийн ван), translated as "King of the Sky", reflecting its namesake god's role as the ruler of the heavens. In Hawaiian, its name is Hele‘ekala, a loanword for the discoverer Herschel.[49] In Māori, its name is Whērangi.[50][51]
Orbit and rotation

Uranus orbits the Sun once every 84 years. Its average distance from the Sun is roughly 20AU (3billionkm; 2billionmi). The difference between its minimum and maximum distance from the Sun is 1.8 AU, larger than that of any other planet, though not as large as that of dwarf planet Pluto.[52] The intensity of sunlight varies inversely with the square of distance, and so on Uranus (at about 20 times the distance from the Sun compared to Earth) it is about 1/400 the intensity of light on Earth.[53] Its orbital elements were first calculated in 1783 by Pierre-Simon Laplace.[54] With time, discrepancies began to appear between the predicted and observed orbits, and in 1841, John Couch Adams first proposed that the differences might be due to the gravitational tug of an unseen planet. In 1845, Urbain Le Verrier began his own independent research into Uranus's orbit. On 23 September 1846, Johann Gottfried Galle located a new planet, later named Neptune, at nearly the position predicted by Le Verrier.[55]
The rotational period of the interior of Uranus is 17hours, 14minutes. As on all the giant planets, its upper atmosphere experiences strong winds in the direction of rotation. At some latitudes, such as about 60 degrees south, visible features of the atmosphere move much faster, making a full rotation in as little as 14 hours.[56]
Axial tilt

The Uranian axis of rotation is approximately parallel with the plane of the Solar System, with an axial tilt of 97.77° (as defined by prograde rotation). This gives it seasonal changes completely unlike those of the other planets. Near the solstice, one pole faces the Sun continuously and the other faces away. Only a narrow strip around the equator experiences a rapid day–night cycle, but with the Sun low over the horizon. At the other side of Uranus's orbit the orientation of the poles towards the Sun is reversed. Each pole gets around 42years of continuous sunlight, followed by 42years of darkness.[57] Near the time of the equinoxes, the Sun faces the equator of Uranus giving a period of day–night cycles similar to those seen on most of the other planets.
Uranus reached its most recent equinox on 7 December 2007.[58][59]

One result of this axis orientation is that, averaged over the Uranian year, the polar regions of Uranus receive a greater energy input from the Sun than its equatorial regions. Nevertheless, Uranus is hotter at its equator than at its poles. The underlying mechanism that causes this is unknown. The reason for Uranus's unusual axial tilt is also not known with certainty, but the usual speculation is that during the formation of the Solar System, an Earth-sized protoplanet collided with Uranus, causing the skewed orientation.[60] Uranus's south pole was pointed almost directly at the Sun at the time of Voyager2's flyby in 1986. The labelling of this pole as "south" uses the definition currently endorsed by the International Astronomical Union, namely that the north pole of a planet or satellite is the pole that points above the invariable plane of the Solar System, regardless of the direction the planet is spinning.[61][62] A different convention is sometimes used, in which a body's north and south poles are defined according to the right-hand rule in relation to the direction of rotation.[63]
Research by Jacob Kegerreis of Durham University suggests that the tilt resulted from a rock larger than the Earth crashing into the planet 3 to 4 billion years ago.[64]
Visibility
The mean apparent magnitude of Uranus is 5.68 with a standard deviation of 0.17, while the extremes are 5.38 and +6.03[15]. This range of brightness is near the limit of naked eye visibility. Much of the variability is dependent upon the planetary latitudes being illuminated from the Sun and viewed from the Earth[65]. Its angular diameter is between 3.4 and 3.7arcseconds, compared with 16 to 20arcseconds for Saturn and 32 to 45arcseconds for Jupiter.[66] At opposition, Uranus is visible to the naked eye in dark skies, and becomes an easy target even in urban conditions with binoculars.[6] In larger amateur telescopes with an objective diameter of between 15 and 23cm, Uranus appears as a pale cyan disk with distinct limb darkening. With a large telescope of 25cm or wider, cloud patterns, as well as some of the larger satellites, such as Titania and Oberon, may be visible.[67]
Physical characteristics
Internal structure

Uranus's mass is roughly 14.5 times that of Earth, making it the least massive of the giant planets. Its diameter is slightly larger than Neptune's at roughly four times that of Earth. A resulting density of 1.27g/cm3 makes Uranus the second least dense planet, after Saturn.[8][9] This value indicates that it is made primarily of various ices, such as water, ammonia, and methane.[13] The total mass of ice in Uranus's interior is not precisely known, because different figures emerge depending on the model chosen; it must be between 9.3and 13.5Earth masses.[13][68] Hydrogen and helium constitute only a small part of the total, with between 0.5 and 1.5Earth masses.[13] The remainder of the non-ice mass (0.5 to 3.7Earth masses) is accounted for by rocky material.[13]
The standard model of Uranus's structure is that it consists of three layers: a rocky (silicate/iron–nickel) core in the centre, an icy mantle in the middle and an outer gaseous hydrogen/helium envelope.[13][69] The core is relatively small, with a mass of only 0.55Earth masses and a radius less than 20% of Uranus's; the mantle comprises its bulk, with around 13.4Earth masses, and the upper atmosphere is relatively insubstantial, weighing about 0.5Earth masses and extending for the last 20% of Uranus's radius.[13][69] Uranus's core density is around 9g/cm3, with a pressure in the centre of 8millionbars (800 GPa) and a temperature of about 5000K.[68][69] The ice mantle is not in fact composed of ice in the conventional sense, but of a hot and dense fluid consisting of water, ammonia and other volatiles.[13][69] This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean.[70]
The extreme pressure and temperature deep within Uranus may break up the methane molecules, with the carbon atoms condensing into crystals of diamond that rain down through the mantle like hailstones.[71][72][73] Very-high-pressure experiments at the Lawrence Livermore National Laboratory suggest that the base of the mantle may comprise an ocean of liquid diamond, with floating solid 'diamond-bergs'.[74][75]
The bulk compositions of Uranus and Neptune are different from those of Jupiter and Saturn, with ice dominating over gases, hence justifying their separate classification as ice giants. There may be a layer of ionic water where the water molecules break down into a soup of hydrogen and oxygen ions, and deeper down superionic water in which the oxygen crystallises but the hydrogen ions move freely within the oxygen lattice.[76]
Although the model considered above is reasonably standard, it is not unique; other models also satisfy observations. For instance, if substantial amounts of hydrogen and rocky material are mixed in the ice mantle, the total mass of ices in the interior will be lower, and, correspondingly, the total mass of rocks and hydrogen will be higher. Presently available data does not allow a scientific determination which model is correct.[68] The fluid interior structure of Uranus means that it has no solid surface. The gaseous atmosphere gradually transitions into the internal liquid layers.[13] For the sake of convenience, a revolving oblate spheroid set at the point at which atmospheric pressure equals 1bar (100 kPa) is conditionally designated as a "surface". It has equatorial and polar radii of 25,559±4km (15,881.6±2.5mi) and 24,973±20km (15,518±12mi), respectively.[8] This surface is used throughout this article as a zero point for altitudes.
Internal heat
Uranus's internal heat appears markedly lower than that of the other giant planets; in astronomical terms, it has a low thermal flux.[20][77] Why Uranus's internal temperature is so low is still not understood. Neptune, which is Uranus's near twin in size and composition, radiates 2.61 times as much energy into space as it receives from the Sun,[20] but Uranus radiates hardly any excess heat at all. The total power radiated by Uranus in the far infrared (i.e. heat) part of the spectrum is 1.06±0.08 times the solar energy absorbed in its atmosphere.[14][78] Uranus's heat flux is only 0.042±0.047W/m2, which is lower than the internal heat flux of Earth of about 0.075W/m2.[78] The lowest temperature recorded in Uranus's tropopause is 49K (−224.2°C; −371.5°F), making Uranus the coldest planet in the Solar System.[14][78]
One of the hypotheses for this discrepancy suggests that when Uranus was hit by a supermassive impactor, which caused it to expel most of its primordial heat, it was left with a depleted core temperature.[79] This impact hypothesis is also used in some attempts to explain the planet's axial tilt. Another hypothesis is that some form of barrier exists in Uranus's upper layers that prevents the core's heat from reaching the surface.[13] For example, convection may take place in a set of compositionally different layers, which may inhibit the upward heat transport;[14][78] perhaps double diffusive convection is a limiting factor.[13]
Atmosphere

Although there is no well-defined solid surface within Uranus's interior, the outermost part of Uranus's gaseous envelope that is accessible to remote sensing is called its atmosphere.[14] Remote-sensing capability extends down to roughly 300km below the 1bar (100kPa) level, with a corresponding pressure around 100bar (10MPa) and temperature of 320K (47°C; 116°F).[80] The tenuous thermosphere extends over two planetary radii from the nominal surface, which is defined to lie at a pressure of 1 bar.[81] The Uranian atmosphere can be divided into three layers: the troposphere, between altitudes of −300 and 50km (−186 and 31mi) and pressures from 100 to 0.1bar (10MPa to 10kPa); the stratosphere, spanning altitudes between 50 and 4,000km (31 and 2,485mi) and pressures of between 0.1 and 10−10bar (10kPa to 10µPa); and the thermosphere extending from 4,000km to as high as 50,000km from the surface.[14] There is no mesosphere.
Composition
The composition of Uranus's atmosphere is different from its bulk, consisting mainly of molecular hydrogen and helium.[14] The helium molar fraction, i.e. the number of helium atoms per molecule of gas, is 0.15±0.03[17] in the upper troposphere, which corresponds to a mass fraction 0.26±0.05.[14][78] This value is close to the protosolar helium mass fraction of 0.275±0.01,[82] indicating that helium has not settled in its centre as it has in the gas giants.[14] The third-most-abundant component of Uranus's atmosphere is methane (CH
4).[14] Methane has prominent absorption bands in the visible and near-infrared (IR), making Uranus aquamarine or cyan in colour.[14] Methane molecules account for 2.3% of the atmosphere by molar fraction below the methane cloud deck at the pressure level of 1.3bar (130kPa); this represents about 20 to 30times the carbon abundance found in the Sun.[14][16][83] The mixing ratio[lower-alpha 9] is much lower in the upper atmosphere due to its extremely low temperature, which lowers the saturation level and causes excess methane to freeze out.[84] The abundances of less volatile compounds such as ammonia, water, and hydrogen sulfide in the deep atmosphere are poorly known. They are probably also higher than solar values.[14][85] Along with methane, trace amounts of various hydrocarbons are found in the stratosphere of Uranus, which are thought to be produced from methane by photolysis induced by the solar ultraviolet (UV) radiation.[86] They include ethane (C
2H
6), acetylene (C
2H
2), methylacetylene (CH
3C
2H), and diacetylene (C
2HC
2H).[84][87][88] Spectroscopy has also uncovered traces of water vapour, carbon monoxide and carbon dioxide in the upper atmosphere, which can only originate from an external source such as infalling dust and comets.[87][88][89]
Troposphere
The troposphere is the lowest and densest part of the atmosphere and is characterised by a decrease in temperature with altitude.[14] The temperature falls from about 320K (47°C; 116°F) at the base of the nominal troposphere at −300km to 53K (−220°C; −364°F) at 50km.[80][83] The temperatures in the coldest upper region of the troposphere (the tropopause) actually vary in the range between 49 and 57K (−224 and −216°C; −371 and −357°F) depending on planetary latitude.[14][77] The tropopause region is responsible for the vast majority of Uranus's thermal far infrared emissions, thus determining its effective temperature of 59.1±0.3K (−214.1±0.3°C; −353.3±0.5°F).[77][78]
The troposphere is thought to have a highly complex cloud structure; water clouds are hypothesised to lie in the pressure range of 50 to 100bar (5 to 10MPa), ammonium hydrosulfide clouds in the range of 20 to 40bar (2 to 4MPa), ammonia or hydrogen sulfide clouds at between 3 and 10bar (0.3 and 1MPa) and finally directly detected thin methane clouds at 1 to 2bar (0.1 to 0.2MPa).[14][16][80][90] The troposphere is a dynamic part of the atmosphere, exhibiting strong winds, bright clouds and seasonal changes.[20]
Upper atmosphere

The middle layer of the Uranian atmosphere is the stratosphere, where temperature generally increases with altitude from 53K (−220°C; −364°F) in the tropopause to between 800 and 850K (527 and 577°C; 980 and 1,070°F) at the base of the thermosphere.[81] The heating of the stratosphere is caused by absorption of solar UV and IR radiation by methane and other hydrocarbons,[92] which form in this part of the atmosphere as a result of methane photolysis.[86] Heat is also conducted from the hot thermosphere.[92] The hydrocarbons occupy a relatively narrow layer at altitudes of between 100 and 300km corresponding to a pressure range of 10 to 0.1mbar (10.00 to 0.10hPa) and temperatures of between 75 and 170K (−198 and −103°C; −325 and −154°F).[84][87] The most abundant hydrocarbons are methane, acetylene and ethane with mixing ratios of around 10−7 relative to hydrogen. The mixing ratio of carbon monoxide is similar at these altitudes.[84][87][89] Heavier hydrocarbons and carbon dioxide have mixing ratios three orders of magnitude lower.[87] The abundance ratio of water is around 7×10−9.[88] Ethane and acetylene tend to condense in the colder lower part of stratosphere and tropopause (below 10mBar level) forming haze layers,[86] which may be partly responsible for the bland appearance of Uranus. The concentration of hydrocarbons in the Uranian stratosphere above the haze is significantly lower than in the stratospheres of the other giant planets.[84][93]
The outermost layer of the Uranian atmosphere is the thermosphere and corona, which has a uniform temperature around 800 to 850K.[14][93] The heat sources necessary to sustain such a high level are not understood, as neither the solar UV nor the auroral activity can provide the necessary energy to maintain these temperatures. The weak cooling efficiency due to the lack of hydrocarbons in the stratosphere above 0.1mBar pressure level may contribute too.[81][93] In addition to molecular hydrogen, the thermosphere-corona contains many free hydrogen atoms. Their small mass and high temperatures explain why the corona extends as far as 50,000km (31,000mi), or two Uranian radii, from its surface.[81][93] This extended corona is a unique feature of Uranus.[93] Its effects include a drag on small particles orbiting Uranus, causing a general depletion of dust in the Uranian rings.[81] The Uranian thermosphere, together with the upper part of the stratosphere, corresponds to the ionosphere of Uranus.[83] Observations show that the ionosphere occupies altitudes from 2,000 to 10,000km (1,200 to 6,200mi).[83] The Uranian ionosphere is denser than that of either Saturn or Neptune, which may arise from the low concentration of hydrocarbons in the stratosphere.[93][94] The ionosphere is mainly sustained by solar UV radiation and its density depends on the solar activity.[95] Auroral activity is insignificant as compared to Jupiter and Saturn.[93][96]

Uranus's atmosphere
Temperature profile of the Uranian troposphere and lower stratosphere. Cloud and haze layers are also indicated.
Zonal wind speeds on Uranus. Shaded areas show the southern collar and its future northern counterpart. The red curve is a symmetrical fit to the data.

Magnetosphere

Before the arrival of Voyager 2, no measurements of the Uranian magnetosphere had been taken, so its nature remained a mystery. Before 1986, scientists had expected the magnetic field of Uranus to be in line with the solar wind, because it would then align with Uranus's poles that lie in the ecliptic.[97]
Voyager's observations revealed that Uranus's magnetic field is peculiar, both because it does not originate from its geometric centre, and because it is tilted at 59° from the axis of rotation.[97][98] In fact the magnetic dipole is shifted from the Uranus's centre towards the south rotational pole by as much as one third of the planetary radius.[97] This unusual geometry results in a highly asymmetric magnetosphere, where the magnetic field strength on the surface in the southern hemisphere can be as low as 0.1gauss (10µT), whereas in the northern hemisphere it can be as high as 1.1gauss (110µT).[97] The average field at the surface is 0.23gauss (23µT).[97] Studies of Voyager 2 data in 2017 suggest that this asymmetry causes Uranus's magnetosphere to connect with the solar wind once a Uranian day, opening the planet to the Sun's particles.[99] In comparison, the magnetic field of Earth is roughly as strong at either pole, and its "magnetic equator" is roughly parallel with its geographical equator.[98] The dipole moment of Uranus is 50times that of Earth.[97][98] Neptune has a similarly displaced and tilted magnetic field, suggesting that this may be a common feature of ice giants.[98] One hypothesis is that, unlike the magnetic fields of the terrestrial and gas giants, which are generated within their cores, the ice giants' magnetic fields are generated by motion at relatively shallow depths, for instance, in the water–ammonia ocean.[70][100] Another possible explanation for the magnetosphere's alignment is that there are oceans of liquid diamond in Uranus's interior that would deter the magnetic field.[74]
Despite its curious alignment, in other respects the Uranian magnetosphere is like those of other planets: it has a bow shock at about 23 Uranian radii ahead of it, a magnetopause at 18 Uranian radii, a fully developed magnetotail, and radiation belts.[97][98][101] Overall, the structure of Uranus's magnetosphere is different from Jupiter's and more similar to Saturn's.[97][98] Uranus's magnetotail trails behind it into space for millions of kilometres and is twisted by its sideways rotation into a long corkscrew.[97][102]
Uranus's magnetosphere contains charged particles: mainly protons and electrons, with a small amount of H2+ ions.[98][101] No heavier ions have been detected. Many of these particles probably derive from the thermosphere.[101] The ion and electron energies can be as high as 4 and 1.2megaelectronvolts, respectively.[101] The density of low-energy (below 1kiloelectronvolt) ions in the inner magnetosphere is about 2cm−3.[103] The particle population is strongly affected by the Uranian moons, which sweep through the magnetosphere, leaving noticeable gaps.[101] The particle flux is high enough to cause darkening or space weathering of their surfaces on an astronomically rapid timescale of 100,000years.[101] This may be the cause of the uniformly dark colouration of the Uranian satellites and rings.[104] Uranus has relatively well developed aurorae, which are seen as bright arcs around both magnetic poles.[93] Unlike Jupiter's, Uranus's aurorae seem to be insignificant for the energy balance of the planetary thermosphere.[96]
Climate

At ultraviolet and visible wavelengths, Uranus's atmosphere is bland in comparison to the other giant planets, even to Neptune, which it otherwise closely resembles.[20] When Voyager 2 flew by Uranus in 1986, it observed a total of ten cloud features across the entire planet.[19][105] One proposed explanation for this dearth of features is that Uranus's internal heat appears markedly lower than that of the other giant planets. The lowest temperature recorded in Uranus's tropopause is 49K (−224°C; −371°F), making Uranus the coldest planet in the Solar System.[14][78]
Banded structure, winds and clouds
In 1986, Voyager 2 found that the visible southern hemisphere of Uranus can be subdivided into two regions: a bright polar cap and dark equatorial bands.[19] Their boundary is located at about −45° of latitude. A narrow band straddling the latitudinal range from −45 to −50° is the brightest large feature on its visible surface.[19][106] It is called a southern "collar". The cap and collar are thought to be a dense region of methane clouds located within the pressure range of 1.3 to 2bar (see above).[107] Besides the large-scale banded structure, Voyager2 observed ten small bright clouds, most lying several degrees to the north from the collar.[19] In all other respects Uranus looked like a dynamically dead planet in 1986. Voyager2 arrived during the height of Uranus's southern summer and could not observe the northern hemisphere. At the beginning of the 21st century, when the northern polar region came into view, the Hubble Space Telescope (HST) and Keck telescope initially observed neither a collar nor a polar cap in the northern hemisphere.[106] So Uranus appeared to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar.[106] In 2007, when Uranus passed its equinox, the southern collar almost disappeared, and a faint northern collar emerged near 45° of latitude.[108]

In the 1990s, the number of the observed bright cloud features grew considerably partly because new high-resolution imaging techniques became available.[20] Most were found in the northern hemisphere as it started to become visible.[20] An early explanation—that bright clouds are easier to identify in its dark part, whereas in the southern hemisphere the bright collar masks them – was shown to be incorrect.[109][110] Nevertheless there are differences between the clouds of each hemisphere. The northern clouds are smaller, sharper and brighter.[110] They appear to lie at a higher altitude.[110] The lifetime of clouds spans several orders of magnitude. Some small clouds live for hours; at least one southern cloud may have persisted since the Voyager 2 flyby.[20][105] Recent observation also discovered that cloud features on Uranus have a lot in common with those on Neptune.[20] For example, the dark spots common on Neptune had never been observed on Uranus before 2006, when the first such feature dubbed Uranus Dark Spot was imaged.[111] The speculation is that Uranus is becoming more Neptune-like during its equinoctial season.[112]
The tracking of numerous cloud features allowed determination of zonal winds blowing in the upper troposphere of Uranus.[20] At the equator winds are retrograde, which means that they blow in the reverse direction to the planetary rotation. Their speeds are from −360 to −180km/h (−220 to −110mph).[20][106] Wind speeds increase with the distance from the equator, reaching zero values near ±20° latitude, where the troposphere's temperature minimum is located.[20][77] Closer to the poles, the winds shift to a prograde direction, flowing with Uranus's rotation. Wind speeds continue to increase reaching maxima at ±60° latitude before falling to zero at the poles.[20] Wind speeds at −40° latitude range from 540 to 720km/h (340 to 450mph). Because the collar obscures all clouds below that parallel, speeds between it and the southern pole are impossible to measure.[20] In contrast, in the northern hemisphere maximum speeds as high as 860km/h (540mph) are observed near +50° latitude.[20][106][113]
Seasonal variation

For a short period from March to May 2004, large clouds appeared in the Uranian atmosphere, giving it a Neptune-like appearance.[110][114] Observations included record-breaking wind speeds of 820km/h (510mph) and a persistent thunderstorm referred to as "Fourth of July fireworks".[105] On 23 August 2006, researchers at the Space Science Institute (Boulder, Colorado) and the University of Wisconsin observed a dark spot on Uranus's surface, giving scientists more insight into Uranus's atmospheric activity.[111] Why this sudden upsurge in activity occurred is not fully known, but it appears that Uranus's extreme axial tilt results in extreme seasonal variations in its weather.[59][112] Determining the nature of this seasonal variation is difficult because good data on Uranus's atmosphere have existed for less than 84 years, or one full Uranian year. Photometry over the course of half a Uranian year (beginning in the 1950s) has shown regular variation in the brightness in two spectral bands, with maxima occurring at the solstices and minima occurring at the equinoxes.[115] A similar periodic variation, with maxima at the solstices, has been noted in microwave measurements of the deep troposphere begun in the 1960s.[116] Stratospheric temperature measurements beginning in the 1970s also showed maximum values near the 1986 solstice.[92] The majority of this variability is thought to occur owing to changes in the viewing geometry.[109]
There are some indications that physical seasonal changes are happening in Uranus. Although Uranus is known to have a bright south polar region, the north pole is fairly dim, which is incompatible with the model of the seasonal change outlined above.[112] During its previous northern solstice in 1944, Uranus displayed elevated levels of brightness, which suggests that the north pole was not always so dim.[115] This information implies that the visible pole brightens some time before the solstice and darkens after the equinox.[112] Detailed analysis of the visible and microwave data revealed that the periodical changes of brightness are not completely symmetrical around the solstices, which also indicates a change in the meridional albedo patterns.[112] In the 1990s, as Uranus moved away from its solstice, Hubble and ground-based telescopes revealed that the south polar cap darkened noticeably (except the southern collar, which remained bright),[107] whereas the northern hemisphere demonstrated increasing activity,[105] such as cloud formations and stronger winds, bolstering expectations that it should brighten soon.[110] This indeed happened in 2007 when it passed an equinox: a faint northern polar collar arose, and the southern collar became nearly invisible, although the zonal wind profile remained slightly asymmetric, with northern winds being somewhat slower than southern.[108]
The mechanism of these physical changes is still not clear.[112] Near the summer and winter solstices, Uranus's hemispheres lie alternately either in full glare of the Sun's rays or facing deep space. The brightening of the sunlit hemisphere is thought to result from the local thickening of the methane clouds and haze layers located in the troposphere.[107] The bright collar at −45° latitude is also connected with methane clouds.[107] Other changes in the southern polar region can be explained by changes in the lower cloud layers.[107] The variation of the microwave emission from Uranus is probably caused by changes in the deep tropospheric circulation, because thick polar clouds and haze may inhibit convection.[117] Now that the spring and autumn equinoxes are arriving on Uranus, the dynamics are changing and convection can occur again.[105][117]
Formation

Many argue that the differences between the ice giants and the gas giants extend to their formation.[118][119] The Solar System is hypothesised  to have formed from a giant rotating ball of gas and dust known as the presolar nebula. Much of the nebula's gas, primarily hydrogen and helium, formed the Sun, and the dust grains collected together to form the first protoplanets. As the planets grew, some of them eventually accreted enough matter for their gravity to hold on to the nebula's leftover gas.[118][119] The more gas they held onto, the larger they became; the larger they became, the more gas they held onto until a critical point was reached, and their size began to increase exponentially. The ice giants, with only a few Earth masses of nebular gas, never reached that critical point.[118][119][120] Recent simulations of planetary migration have suggested that both ice giants formed closer to the Sun than their present positions, and moved outwards after formation (the Nice model).[118]
Moons

Uranus has 27 known natural satellites.[120] The names of these satellites are chosen from characters in the works of Shakespeare and Alexander Pope.[69][121] The five main satellites are Miranda, Ariel, Umbriel, Titania, and Oberon.[69] The Uranian satellite system is the least massive among those of the giant planets; the combined mass of the five major satellites would be less than half that of Triton (largest moon of Neptune) alone.[9] The largest of Uranus's satellites, Titania, has a radius of only 788.9km (490.2mi), or less than half that of the Moon, but slightly more than Rhea, the second-largest satellite of Saturn, making Titania the eighth-largest moon in the Solar System. Uranus's satellites have relatively low albedos; ranging from 0.20 for Umbriel to 0.35 for Ariel (in green light).[19] They are ice–rock conglomerates composed of roughly 50% ice and 50% rock. The ice may include ammonia and carbon dioxide.[104][122]
Among the Uranian satellites, Ariel appears to have the youngest surface with the fewest impact craters and Umbriel's the oldest.[19][104] Miranda has fault canyons 20km (12mi) deep, terraced layers, and a chaotic variation in surface ages and features.[19] Miranda's past geologic activity is thought to have been driven by tidal heating at a time when its orbit was more eccentric than currently, probably as a result of a former 3:1 orbital resonance with Umbriel.[123] Extensional processes associated with upwelling diapirs are the likely origin of Miranda's 'racetrack'-like coronae.[124][125] Ariel is thought to have once been held in a 4:1 resonance with Titania.[126]
Uranus has at least one horseshoe orbiter occupying the Sun–Uranus L3 Lagrangian point—a gravitationally unstable region at 180° in its orbit, 83982 Crantor.[127][128] Crantor moves inside Uranus's co-orbital region on a complex, temporary horseshoe orbit.
2010 EU65 is also a promising Uranus horseshoe librator candidate.[128]
Planetary rings

The Uranian rings are composed of extremely dark particles, which vary in size from micrometres to a fraction of a metre.[19] Thirteen distinct rings are presently known, the brightest being the ε ring. All except two rings of Uranus are extremely narrow – they are usually a few kilometres wide. The rings are probably quite young; the dynamics considerations indicate that they did not form with Uranus. The matter in the rings may once have been part of a moon (or moons) that was shattered by high-speed impacts. From numerous pieces of debris that formed as a result of those impacts, only a few particles survived, in stable zones corresponding to the locations of the present rings.[104][129]
William Herschel described a possible ring around Uranus in 1789. This sighting is generally considered doubtful, because the rings are quite faint, and in the two following centuries none were noted by other observers. Still, Herschel made an accurate description of the epsilon ring's size, its angle relative to Earth, its red colour, and its apparent changes as Uranus travelled around the Sun.[130][131] The ring system was definitively discovered on 10 March 1977 by James L. Elliot, Edward W. Dunham, and Jessica Mink using the Kuiper Airborne Observatory. The discovery was serendipitous; they planned to use the occultation of the star SAO 158687 (also known as HD 128598) by Uranus to study its atmosphere. When their observations were analysed, they found that the star had disappeared briefly from view five times both before and after it disappeared behind Uranus. They concluded that there must be a ring system around Uranus.[132] Later they detected four additional rings.[132] The rings were directly imaged when Voyager2 passed Uranus in 1986.[19] Voyager2 also discovered two additional faint rings, bringing the total number to eleven.[19]
In December 2005, the Hubble Space Telescope detected a pair of previously unknown rings. The largest is located twice as far from Uranus as the previously known rings. These new rings are so far from Uranus that they are called the "outer" ring system. Hubble also spotted two small satellites, one of which, Mab, shares its orbit with the outermost newly discovered ring. The new rings bring the total number of Uranian rings to 13.[133] In April 2006, images of the new rings from the Keck Observatory yielded the colours of the outer rings: the outermost is blue and the other one red.[134][135]
One hypothesis concerning the outer ring's blue colour is that it is composed of minute particles of water ice from the surface of Mab that are small enough to scatter blue light.[134][136] In contrast, Uranus's inner rings appear grey.[134]

Uranus's rings
Animation about the discovering occultation in 1977. (Click on it to start)
Uranus has a complicated planetary ring system, which was the second such system to be discovered in the Solar System after Saturn's.[129]
Uranus's aurorae against its equatorial rings, imaged by the Hubble telescope. Unlike the aurorae of Earth and Jupiter, those of Uranus are not in line with its poles, due to its lopsided magnetic field.

Exploration

In 1986, NASA's Voyager 2 interplanetary probe encountered Uranus. This flyby remains the only investigation of Uranus carried out from a short distance and no other visits are planned. Launched in 1977, Voyager 2 made its closest approach to Uranus on 24 January 1986, coming within 81,500km (50,600mi) of the cloudtops, before continuing its journey to Neptune. The spacecraft studied the structure and chemical composition of Uranus's atmosphere,[83] including its unique weather, caused by its axial tilt of 97.77°. It made the first detailed investigations of its five largest moons and discovered 10 new ones. It examined all nine of the system's known rings and discovered two more.[19][104][137] It also studied the magnetic field, its irregular structure, its tilt and its unique corkscrew magnetotail caused by Uranus's sideways orientation.[97]
Voyager 1 was unable to visit Uranus because investigation of Saturn's moon Titan was considered a priority. This trajectory took Voyager 1 out of the plane of the ecliptic, ending its planetary science mission.[138]:118
The possibility of sending the Cassini spacecraft from Saturn to Uranus was evaluated during a mission extension planning phase in 2009, but was ultimately rejected in favour of destroying it in the Saturnian atmosphere.[139] It would have taken about twenty years to get to the Uranian system after departing Saturn.[139] A Uranus orbiter and probe was recommended by the 2013–2022 Planetary Science Decadal Survey published in 2011; the proposal envisages launch during 2020–2023 and a 13-year cruise to Uranus.[140] A Uranus entry probe could use Pioneer Venus Multiprobe heritage and descend to 1–5 atmospheres.[140] The ESA evaluated a "medium-class" mission called Uranus Pathfinder.[141] A New Frontiers Uranus Orbiter has been evaluated and recommended in the study, The Case for a Uranus Orbiter.[142] Such a mission is aided by the ease with which a relatively big mass can be sent to the system—over 1500kg with an Atlas 521 and 12-year journey.[143] For more concepts see Proposed Uranus missions.
In culture
In astrology, the planet Uranus () is the ruling planet of Aquarius. Because Uranus is cyan and Uranus is associated with electricity, the colour electric blue, which is close to cyan, is associated with the sign Aquarius[144] (see Uranus in astrology).
The chemical element uranium, discovered in 1789 by the German chemist Martin Heinrich Klaproth, was named after the newly discovered planet Uranus.[145]
"Uranus, the Magician" is a movement in Gustav Holst's orchestral suite The Planets, written between 1914 and 1916.
Operation Uranus was the successful military operation in World War II by the Red Army to take back Stalingrad and marked the turning point in the land war against the Wehrmacht.
The lines "Then felt I like some watcher of the skies/When a new planet swims into his ken", from John Keats's "On First Looking Into Chapman's Homer", are a reference to Herschel's discovery of Uranus.[146]
Many references to Uranus in English language popular culture and news involve humour about one pronunciation of its name resembling that of the phrase "your anus".[147]
See also

Outline of Uranus
2011 QF99 and 2014 YX49, the only two known Uranus trojans
Colonisation of Uranus
Uranus in astrology
Uranus in fiction
Extraterrestrial diamonds (thought to be abundant in Uranus)

Notes

Further reading

 at the Encyclopædia Britannica
 at European Space Agency

 at
 A kid's guide to Uranus.
 at Jet Propulsion Laboratory's planetary photojournal. (photos)
 (photos)
 (blog)
 (photo)

 by CGP Grey