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Hugh Dowding

Air Chief Marshal Hugh Caswall Tremenheere Dowding, 1st Baron Dowding, (24 April 1882 – 15 February 1970) was a senior officer in the Royal Air Force. He was Air Officer Commanding RAF Fighter Command during the Battle of Britain and is generally credited with playing a crucial role in Britain's defence, and hence, the defeat of Operation Sea Lion, Adolf Hitler's plan to invade Britain.

Born in Moffat, Scotland, Dowding was an officer in the British Army in the 1900s and early 1910s. He joined the Royal Flying Corps at the start of the First World War and went on to serve as a fighter pilot and then as commanding officer of No. 16 Squadron. During the inter-war years he became Air Officer Commanding Fighting Area, Air Defence of Great Britain and then joined the Air Council as Air Member for Supply and Research. In July 1936, Dowding was appointed chief of the newly created RAF Fighter Command.

During the Battle of Britain in the Second World War, Dowding's Fighter Command successfully defended the UK against the attacks of the Luftwaffe, thanks to his prudent management of RAF resources and detailed preparation of Britain's air defences (the Dowding system). He subsequently came into conflict with proponents of the Big Wing tactic, most notably Trafford Leigh-Mallory and Douglas Bader, which along with the inadaquecies of RAF's nighttime defence during the Blitz led to his eventual downfall. In November 1940, Dowding was replaced in command against his wishes by Sholto Douglas, another Big Wing advocate.

Dowding retired from the Royal Air Force in July 1942 and was made a peer in June 1943. Upon retirement, Dowding subsequently became an influential member of the British spiritualist, theosophical, and animal rights movements. He died on 15 February 1970, aged 87.

Early life
Dowding was born at St. Ninian's Boys' Preparatory School in Moffat, Dumfriesshire, the son of Arthur John Caswall Dowding and Maud Caroline Dowding (née Tremenheere). His father had taught at Fettes College in Edinburgh before moving to Moffat. Dowding was educated at St Ninian's School and Winchester College. He trained at the Royal Military Academy, Woolwich before being commissioned as a second lieutenant in the Royal Garrison Artillery on 18 August 1900.

Military career
Promoted to lieutenant on 8 May 1902, Dowding served with the Royal Garrison Artillery at Gibraltar, in Ceylon and in Hong Kong before being posted to No. 7 Mountain Artillery Battery in India in 1904. After returning to the United Kingdom, he attended the Army Staff College 1912 before being promoted to captain on 18 August 1913 and being posted with the Royal Garrison Artillery on the Isle of Wight later that year. After becoming interested in aviation, Dowding gained Aviator's Certificate no. 711 on 19 December 1913 in a Vickers biplane at the Vickers School of Flying, Brooklands. He then attended the Central Flying School, where he was awarded his wings. Although added to the Reserve List of the Royal Flying Corps (RFC), Dowding returned to the Isle of Wight to resume his Royal Garrison Artillery duties. However, this arrangement was short lived and in August 1914, he joined the RFC as a pilot on No. 7 Squadron.

First World War
Dowding transferred to No. 6 Squadron in October 1914 and then, after two weeks as a staff officer in France, became a Flight Commander, first with No. 9 Squadron and then with No. 6 Squadron. He became commanding officer of the Wireless Experimental Establishment at Brooklands in March 1915 and went on to be commanding officer of No. 16 Squadron in July 1915, which was based at La Gorgue in northern France. After the Battle of the Somme, Dowding clashed with General Hugh Trenchard, the commander of the RFC, over the need to give pilots some rest and recuperation. In September 1915 Duncan Grinnell-Milne joined No 16 squadron as a junior pilot. Years later he published an account of his time in the squadron, in which he criticises Dowding as being "too reserved and aloof from his juniors", although efficient. Promoted to major on 30 December 1915, Dowding was recalled to England in January 1916, and, having been promoted to temporary lieutenant colonel on 1 February 1916 was given command of 7 Wing at Farnborough later that month. He transferred to the command of 9 Wing at Fienvillers in June 1916. Returning to England, he was promoted to temporary colonel on 1 January 1917 on appointment as commander of the Southern Group Command and promoted to temporary brigadier-general on 23 June 1917 before being given command of the southern training brigade in August 1917. He was sent to York as chief staff officer to the RAF's senior administrative officer in the area in April 1918. He was appointed a Companion of the Order of St Michael and St George on 1 January 1919.

Inter-war years
Dowding was given a permanent commission in the RAF on 1 August 1919 with the rank of group captain. He commanded No. 16 Group from October 1919 and then No. 1 Group from February 1920 where he was responsible for organising two of the annual air displays at Hendon. He was promoted to air commodore on 1 January 1922, and served as chief staff officer at Inland Area headquarters at Uxbridge from February 1922 before being appointed Chief Staff Officer for RAF Iraq Command in August 1924.

Dowding was an accomplished skier, winner of the first ever National Slalom Championship, and president of the Ski Club of Great Britain from 1924 to 1925.

In May 1926 Dowding was appointed director of training at the Air Ministry. He was appointed a Companion of the Order of the Bath on 2 January 1928 and promoted to air vice-marshal on 1 January 1929. Trenchard sent him to Palestine and Transjordan to study security problems caused by Arab–Jewish unrest: his reports, which gained Trenchard's approval, were a cause of further career advancement. Dowding became Air Officer Commanding Fighting Area, Air Defence of Great Britain in December 1929 and then joined the Air Council as Air Member for Supply and Research in September 1930. One of his first responsibilities in this post was the approval of the granting of a certificate of airworthiness to the R101 airship shortly before it set off on its ill-fated voyage to India; he later said "I think I was wrong not to insist on much more extensive trials and tests" and that his decision had been based on optimistic technical advice. Dowding's time in this office coincided with a period of rapid development in aircraft design and a growing fear that another major war was on the horizon. Although without scientific or technical training, he displayed a great capacity for understanding technical matters. He was promoted to air marshal on 1 January 1933 and advanced to Knight Commander of the Order of the Bath on 3 June 1933.

In July 1936 Dowding was appointed commanding officer of the newly created RAF Fighter Command, and was perhaps the one important person in Britain, and perhaps the world, who did not agree with British Prime Minister Stanley Baldwin's 1932 declaration that "The bomber will always get through". He conceived and oversaw the development of the "Dowding system". This consisted of an integrated air defence system which included (i) radar (whose potential Dowding was among the first to appreciate), (ii) human observers (including the Royal Observer Corps), who filled crucial gaps in what radar was capable of detecting at the time (the early radar systems, for example, did not provide accurate information on the altitude of incoming German aircraft), (iii) raid plotting, and (iv) radio control of aircraft. The whole network was linked in many cases by dedicated telephone cables buried sufficiently deeply to provide protection against bombing. The network had its centre at RAF Bentley Priory, a converted country house on the outskirts of London. The system as a whole later became known as Ground-controlled interception (GCI).

Dowding also brought modern aircraft into service during the pre-war period, including the eight gun Spitfire and Hurricane. He is also credited with having fought the Air Ministry so that fighters were equipped with bullet-proof wind shields. He was promoted to air chief marshal on 1 January 1937 and appointed a Knight Grand Cross of the Royal Victorian Order on 23 January 1937.

Battle of Britain
At the time of his retirement in June 1939, Dowding was asked to stay on until March 1940 because of the tense international situation. He was again permitted to continue serving through the Battle of Britain, first until July and finally until November 1940. In 1940, Dowding, nicknamed "Stuffy" by his men for his alleged lack of humour, proved unwilling to sacrifice aircraft and pilots in the attempt to aid Allied troops during the Battle of France. He, along with his immediate superior Sir Cyril Newall, then Chief of the Air Staff, resisted repeated requests from Winston Churchill to weaken the home defence by sending precious squadrons to France. When the Allied resistance in France collapsed, he worked closely with Air Vice-Marshal Keith Park, the commander of 11 Fighter Group, in organising cover for the evacuation of the British Expeditionary Force at Dunkirk.

Through the summer and autumn of 1940 in the Battle of Britain, Dowding's Fighter Command resisted the attacks of the Luftwaffe. Beyond the critical importance of the overall system of integrated air defence which he had developed for Fighter Command, his major contribution was to marshal resources behind the scenes (including replacement aircraft and air crew) and to maintain a significant fighter reserve, while leaving his subordinate commanders' hands largely free to run the battle in detail.

Dowding was known for his humility and great sincerity. Fighter Command pilots came to characterise Dowding as one who cared for his men and had their best interests at heart. Dowding often referred to his "dear fighter boys" as his "chicks": indeed his son Derek was one of them. Because of his brilliant detailed preparation of Britain's air defences for the German assault, and his prudent management of his resources during the battle, Dowding is today generally given the credit for Britain's victory in the Battle of Britain.

Dowding's subsequent downfall has been attributed by some to his single-mindedness and perceived lack of diplomacy and political savoir faire in dealing with intra-RAF challenges and intrigues, most obviously the still, even now, hotly debated Big Wing controversy in which a number of senior and active service officers had argued in favour of large set-piece air battles with the Luftwaffe as an alternative to Dowding's successful Fabian strategy. Another reason often cited for his removal, but characterised by some contemporary commentators more as a pretext, was the difficulty of countering German nighttime bombing raids on British cities.

The account of radar pioneer, E. G. Bowen in Radar Days (1987) rebuts the claim that Dowding's grasp of the problems of British night fighters was inadequate. He suggests that if Dowding had been left to follow his own path, the ultimately effective British response to night bombing (which depended completely on developments in air-borne radar) would have come somewhat sooner.

Dowding himself showed that he had a good grasp of night fighter defence and was planning a defence system against night bombing in a letter he wrote some time after the Battle of Britain. However, there was great political and public pressure during the Blitz for something to be done, and Fighter Command's existing resources without, as yet, airborne radar, proved woefully inadequate. A committee of enquiry chaired by Sir John Salmond produced a long list of recommendations to improve night air defence; when Dowding approved only some of them, his erstwhile supporters, Lord Beaverbrook and Churchill, decided that it was time for him to step down.

On 8 October 1940, Dowding was advanced to Knight Grand Cross of the Order of the Bath. He unwillingly relinquished command on 24 November 1940 and was replaced by Big Wing advocate Sholto Douglas. Churchill tried to soften the blow by putting him in charge of the British Air Mission to the United States, responsible for the procurement of new aircraft types.

Publication of his book Twelve Legions of Angels was suppressed in November 1941. The British Government considered that it contained information which might be of use to the Germans. The book was finally published in 1946, soon after the war ended.

Ministry of Aircraft Production
After leaving Fighter Command, Dowding was sent on special duty to the United States for the Ministry of Aircraft Production, but there he made himself unpopular with his outspokenness. On his return he headed a study into economies of RAF manpower before retiring from the Royal Air Force in July 1942. He was elevated to the peerage, as Baron Dowding of Bentley Priory on 2 June 1943.

Later life
Later in life, because of his belief that he was unjustly treated by the RAF, Dowding became increasingly bitter. The RAF passed him over for promotion to Marshal of the Royal Air Force.

He approved Robert Wright's book Dowding and the Battle of Britain, which argued that a conspiracy of Big Wing proponents, including Trafford Leigh-Mallory and Douglas Bader, had engineered his sacking from Fighter Command.

In 1951, Dowding laid the foundation stone of the Chapel of St George at RAF Biggin Hill, now London Biggin Hill Airport, in memory of fallen airmen.

Dowding and his second wife Baroness Dowding were both anti-vivisectionists, and in 1973 Britain's National Anti-Vivisection Society founded the Lord Dowding Fund for Humane Research in his honour.

Spiritualism
In his retirement, Dowding became actively interested in spiritualism, both as a writer and speaker. His first book on the subject, Many Mansions, was written in 1943, followed by Lychgate (1945), The Dark Star and God's Magic. Rejecting conventional Christianity, he joined the Theosophical Society which advocated belief in reincarnation. He wrote of meeting dead "RAF boys" in his sleep – spirits who flew fighters from mountain-top runways made of light. Dowding became a vegetarian, based on his beliefs as a theosophist and spiritualist. Although he was a vegetarian, he believed that "animals will be killed to satisfy human needs for many a long day to come", and he made several appeals in the House of Lords for the humane killing of animals intended for food. He was also a member of the Fairy Investigation Society. Although he knew that people considered him a crank for his belief in fairies, Dowding believed that fairies "are essential to the growth of plants and the welfare of the vegetable kingdom".

Death
Dowding died at his home in Royal Tunbridge Wells, Kent, on 15 February 1970. His body was cremated and his ashes were placed below the Battle of Britain Memorial Window in the Royal Air Force chapel in Westminster Abbey. Dowding's son Derek (1919–1992) inherited the title of Baron Dowding.

Personal life
Dowding married Clarice Maud Vancourt, the daughter of an officer in the Indian Army, on 16 February 1918. She had one child from a previous marriage, Marjorie Brenda Williams (1911–2003) and they had one child together, Derek Hugh Tremenheere (1919–1992). Clarice died in 1920, and Dowding's sister Hilda helped Dowding look after the two children.

Dowding married Muriel Whiting (Albino) on 25 September 1951; they had no children.

Media portrayals
In the 1940s and 1950s, H.D. wrote several novels, memoirs, and book-length poems about Dowding.

In the 1956 film Reach for the Sky, Dowding was played by Charles Carson.

In the 1969 film Battle of Britain, Dowding was played by Laurence Olivier. Olivier had himself served as a pilot in the Royal Navy's Fleet Air Arm during the Second World War. During filming in summer 1968, 86 years old and wheelchair-bound by severe arthritis, Dowding visited the set at Hawkinge airfield in Kent. Olivier told Dowding he had sat behind the latter's desk all day "pretending to be you" and was "making an awful mess of it too", to which Dowding replied, "Oh, I'm sure you are." The crew and Olivier broke into laughter. Footage of this can be seen in the special features section of the film's Special Edition DVD.

In the 2017 film Darkest Hour, Dowding was played by Adrian Rawlins.

In the 2018 film Hurricane: 303 Squadron, Dowding was played by Nicholas Farrell.

Honours and tributes
A statue of Dowding stands outside St Clement Danes church on the Strand, London. The inscription reads: "Air Chief Marshal Lord Dowding was commander-in-chief of Fighter Command, Royal Air Force, from its formation in 1936 until November 1940. He was thus responsible for the preparation for and the conduct of the Battle of Britain. With remarkable foresight, he ensured the equipment of his command with monoplane fighters, the Hurricane and the Spitfire. He was among the first to appreciate the vital importance of R.D.F. (radar) and an effective command and control system for his squadrons. They were ready when war came. In the preliminary stages of that war, he thoroughly trained his minimal forces and conserved them against strong political pressure to disperse and misuse them. His wise and prudent judgement and leadership helped to ensure victory against overwhelming odds and thus prevented the loss of the Battle of Britain and probably the whole war. To him, the people of Britain and of the Free World owe largely the way of life and the liberties they enjoy today."

Other monuments to Dowding can be found in Station Park in Moffat, the town of his birth, and in Calverley Gardens in Tunbridge Wells where he died. The RAF Association in conjunction with the RAF Benevolent Fund, purchased his birthplace, the former St Ninian's School, Moffat; the building was renamed Dowding House and restored to provide sheltered housing for former members of the Royal Air Force or their dependents.

The Dowding Centre at the School of Aerospace Battle Management (formerly the School of Fighter Control) at RAF Boulmer is named after Dowding.

A green ceramic commemorative plaque was unveiled at his former residence (1951–1967) in Darnley Drive, Southborough on 6 May 2012.

Dowding Place, Stanmore, the site of former RAF Stanmore Park is named after him.

The 1946-built Southern Railway Battle of Britain pacific (4-6-2) locomotive 21C152 was named Lord Dowding in his honour.

title: Hugh Dowding


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Callisto (moon)

Callisto, or Jupiter IV, is the second-largest moon of Jupiter, after Ganymede. In the Solar System it is the third-largest moon after Ganymede and Saturn's largest moon Titan, and as large as the smallest planet Mercury. Callisto is, with a diameter of $0.007$, roughly a third larger than Earth's Moon and orbits Jupiter on average at a distance of $1,869,000 km$, which is about six times further out than the Moon orbiting Earth. It is the outermost of the four large Galilean moons of Jupiter, which were discovered in 1610 with one of the first telescopes, being visible from Earth with common binoculars.

The surface of Callisto is the oldest and most heavily cratered in the Solar System. Its surface is completely covered with impact craters. It does not show any signatures of subsurface processes such as plate tectonics or volcanism, with no signs that geological activity in general has ever occurred, and is thought to have evolved predominantly under the influence of impacts. Prominent surface features include multi-ring structures, variously shaped impact craters, and chains of craters (catenae) and associated scarps, ridges and deposits. At a small scale, the surface is varied and made up of small, sparkly frost deposits at the tips of high spots, surrounded by a low-lying, smooth blanket of dark material. This is thought to result from the sublimation-driven degradation of small landforms, which is supported by the general deficit of small impact craters and the presence of numerous small knobs, considered to be their remnants. The absolute ages of the landforms are not known. Callisto is composed of approximately equal amounts of rock and ice, with a density of about $1,897,000 km$, the lowest density and surface gravity of Jupiter's major moons. Compounds detected spectroscopically on the surface include water ice, carbon dioxide, silicates and organic compounds. Investigation by the Galileo spacecraft revealed that Callisto may have a small silicate core and possibly a subsurface ocean of liquid water at depths greater than $16.689 d$.

It is not in an orbital resonance like the three other Galilean satellites—Io, Europa and Ganymede—and is thus not appreciably tidally heated. Callisto's rotation is tidally locked to its orbit around Jupiter, so that it always faces the same direction, making Jupiter appear to hang directly overhead over its near-side. It is less affected by Jupiter's magnetosphere than the other inner satellites because of its more remote orbit, located just outside Jupiter's main radiation belt. Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen, as well as by a rather intense ionosphere. Callisto is thought to have formed by slow accretion from the disk of the gas and dust that surrounded Jupiter after its formation. Callisto's gradual accretion and the lack of tidal heating meant that not enough heat was available for rapid differentiation. The slow convection in the interior of Callisto, which commenced soon after formation, led to partial differentiation and possibly to the formation of a subsurface ocean at a depth of 100–150 km and a small, rocky core.

The likely presence of an ocean within Callisto leaves open the possibility that it could harbor life. However, conditions are thought to be less favorable than on nearby Europa. Various space probes from Pioneers 10 and 11 to Galileo and Cassini have studied Callisto. Because of its low radiation levels, Callisto has long been considered the most suitable to base possible future crewed missions on to study the Jovian system.

Discovery
Callisto was discovered independently by Simon Marius and Galileo Galilei in 1610, along with the three other large Jovian moons—Ganymede, Io and Europa.

Name
Callisto, like all of Jupiter's moons, is named after one of Zeus's many lovers or other sexual partners in Greek mythology. Callisto was a nymph (or, according to some sources, the daughter of Lycaon) who was associated with the goddess of the hunt, Artemis. The name was suggested by Simon Marius soon after Callisto's discovery. Marius attributed the suggestion to Johannes Kepler.

"Jupiter is much blamed by the poets on account of his irregular loves. Three maidens are especially mentioned as having been clandestinely courted by Jupiter with success. Io, daughter of the River Inachus, Callisto of Lycaon, Europa of Agenor. Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell... I think, therefore, that I shall not have done amiss if the First is called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth Callisto..."

However, the names of the Galilean satellites fell into disfavor for a considerable time, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, Callisto is referred to by its Roman numeral designation, a system introduced by Galileo, as Jupiter IV or as "the fourth satellite of Jupiter".

There's no established English adjectival form of the name. The adjectival form of Greek Καλλιστῴ Kallistōi is Καλλιστῴος Kallistōi-os, from which one might expect Latin Callistōius and English *Callistóian (with 5 syllables), parallel to Sapphóian (4 syllables) for Sapphōi and Letóian for Lētōi. However, the iota subscript is often omitted from such Greek names (cf. Inóan from Īnōi and Argóan from Argōi ), and indeed the analogous form Callistoan is found. In Virgil, a second oblique stem appears in Latin: Callistōn-, but the corresponding Callistonian has rarely appeared in English. One also sees ad hoc forms, such as Callistan, Callistian and Callistean.

Orbit and rotation
Callisto is the outermost of the four Galilean moons of Jupiter. It orbits at a distance of approximately 1,880,000 km (26.3 times the 71,492 km radius of Jupiter itself). This is significantly larger than the orbital radius—1,070,000 km—of the next-closest Galilean satellite, Ganymede. As a result of this relatively distant orbit, Callisto does not participate in mean-motion resonance—in which the three inner Galilean satellites are locked—and probably never has. Callisto is expected to be captured into the resonance in about 1.5 billion years, completing the 1:2:4:8 chain.

Like most other regular planetary moons, Callisto's rotation is locked to be synchronous with its orbit. The length of Callisto's day, simultaneously its orbital period, is about 16.7 Earth days. Its orbit is very slightly eccentric and inclined to the Jovian equator, with the eccentricity and inclination changing quasi-periodically due to solar and planetary gravitational perturbations on a timescale of centuries. The ranges of change are 0.0072–0.0076 and 0.20–0.60°, respectively. These orbital variations cause the axial tilt (the angle between the rotational and orbital axes) to vary between 0.4 and 1.6°.

The dynamical isolation of Callisto means that it has never been appreciably tidally heated, which has important consequences for its internal structure and evolution. Its distance from Jupiter also means that the charged-particle flux from Jupiter's magnetosphere at its surface is relatively low—about 300 times lower than, for example, that at Europa. Hence, unlike the other Galilean moons, charged-particle irradiation has had a relatively minor effect on Callisto's surface. The radiation level at Callisto's surface is equivalent to a dose of about 0.01 rem (0.1 mSv) per day, which is just over ten times higher than Earth's average background radiation, but less than in Low Earth Orbit or on Mars.

Composition
The average density of Callisto, 1.83 g/cm3, suggests a composition of approximately equal parts of rocky material and water ice, with some additional volatile ices such as ammonia. The mass fraction of ices is 49–55%. The exact composition of Callisto's rock component is not known, but is probably close to the composition of L/LL type ordinary chondrites, which are characterized by less total iron, less metallic iron and more iron oxide than H chondrites. The weight ratio of iron to silicon is 0.9–1.3 in Callisto, whereas the solar ratio is around 1:8.

Callisto's surface has an albedo of about 20%. Its surface composition is thought to be broadly similar to its composition as a whole. Near-infrared spectroscopy has revealed the presence of water ice absorption bands at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 micrometers. Water ice seems to be ubiquitous on the surface of Callisto, with a mass fraction of 25–50%. The analysis of high-resolution, near-infrared and UV spectra obtained by the Galileo spacecraft and from the ground has revealed various non-ice materials: magnesium- and iron-bearing hydrated silicates, carbon dioxide, sulfur dioxide, and possibly ammonia and various organic compounds. Spectral data indicate that Callisto's surface is extremely heterogeneous at the small scale. Small, bright patches of pure water ice are intermixed with patches of a rock–ice mixture and extended dark areas made of a non-ice material.

The Callistoan surface is asymmetric: the leading hemisphere is darker than the trailing one. This is different from other Galilean satellites, where the reverse is true. The trailing hemisphere of Callisto appears to be enriched in carbon dioxide, whereas the leading hemisphere has more sulfur dioxide. Many fresh impact craters like Lofn also show enrichment in carbon dioxide. Overall, the chemical composition of the surface, especially in the dark areas, may be close to that seen on D-type asteroids, whose surfaces are made of carbonaceous material.

Internal structure
Callisto's battered surface lies on top of a cold, stiff and icy lithosphere that is between 80 and 150 km thick. A salty ocean 150–200 km deep may lie beneath the crust, indicated by studies of the magnetic fields around Jupiter and its moons. It was found that Callisto responds to Jupiter's varying background magnetic field like a perfectly conducting sphere; that is, the field cannot penetrate inside Callisto, suggesting a layer of highly conductive fluid within it with a thickness of at least 10 km. The existence of an ocean is more likely if water contains a small amount of ammonia or other antifreeze, up to 5% by weight. In this case the water+ice layer can be as thick as 250–300 km. Failing an ocean, the icy lithosphere may be somewhat thicker, up to about 300 km.

Beneath the lithosphere and putative ocean, Callisto's interior appears to be neither entirely uniform nor particularly variable. Galileo orbiter data (especially the dimensionless moment of inertia —0.3549 ± 0.0042—determined during close flybys) suggest that, if Callisto is in hydrostatic equilibrium, its interior is composed of compressed rocks and ices, with the amount of rock increasing with depth due to partial settling of its constituents. In other words, Callisto may be only partially differentiated. The density and moment of inertia for an equilibrium Callisto are compatible with the existence of a small silicate core in the center of Callisto. The radius of any such core cannot exceed 600 km, and the density may lie between 3.1 and 3.6 g/cm3. In this case, Callisto's interior would be in stark contrast to that of Ganymede, which appears to be fully differentiated.

However, a 2011 reanalysis of Galileo data suggests that Callisto is not in hydrostatic equilibrium. In that case, the gravity data may be more consistent with a more thoroughly differentiated Callisto with a hydrated silicate core.

Surface features
The ancient surface of Callisto is one of the most heavily cratered in the Solar System. In fact, the crater density is close to saturation: any new crater will tend to erase an older one. The large-scale geology is relatively simple; on Callisto there are no large mountains, volcanoes or other endogenic tectonic features. The impact craters and multi-ring structures—together with associated fractures, scarps and deposits—are the only large features to be found on the surface.

Callisto's surface can be divided into several geologically different parts: cratered plains, light plains, bright and dark smooth plains, and various units associated with particular multi-ring structures and impact craters. The cratered plains make up most of the surface area and represent the ancient lithosphere, a mixture of ice and rocky material. The light plains include bright impact craters like Burr and Lofn, as well as the effaced remnants of old large craters called palimpsests, the central parts of multi-ring structures, and isolated patches in the cratered plains. These light plains are thought to be icy impact deposits. The bright, smooth plains make up a small fraction of Callisto's surface and are found in the ridge and trough zones of the Valhalla and Asgard formations and as isolated spots in the cratered plains. They were thought to be connected with endogenic activity, but the high-resolution Galileo images showed that the bright, smooth plains correlate with heavily fractured and knobby terrain and do not show any signs of resurfacing. The Galileo images also revealed small, dark, smooth areas with overall coverage less than 10,000 km2, which appear to embay the surrounding terrain. They are possible cryovolcanic deposits. Both the light and the various smooth plains are somewhat younger and less cratered than the background cratered plains.

Impact crater diameters seen range from 0.1 km—a limit defined by the imaging resolution—to over 100 km, not counting the multi-ring structures. Small craters, with diameters less than 5 km, have simple bowl or flat-floored shapes. Those 5–40 km across usually have a central peak. Larger impact features, with diameters in the range 25–100 km, have central pits instead of peaks, such as Tindr crater. The largest craters with diameters over 60 km can have central domes, which are thought to result from central tectonic uplift after an impact; examples include Doh and Hár craters. A small number of very large—more than 100 km in diameter—and bright impact craters show anomalous dome geometry. These are unusually shallow and may be a transitional landform to the multi-ring structures, as with the Lofn impact feature. Callisto's craters are generally shallower than those on the Moon.

The largest impact features on Callisto's surface are multi-ring basins. Two are enormous. Valhalla is the largest, with a bright central region 600 km in diameter, and rings extending as far as 1,800 km from the center (see figure). The second largest is Asgard, measuring about 1,600 km in diameter. Multi-ring structures probably originated as a result of a post-impact concentric fracturing of the lithosphere lying on a layer of soft or liquid material, possibly an ocean. The catenae—for example Gomul Catena—are long chains of impact craters lined up in straight lines across the surface. They were probably created by objects that were tidally disrupted as they passed close to Jupiter prior to the impact on Callisto, or by very oblique impacts. A historical example of a disruption was Comet Shoemaker–Levy 9.

As mentioned above, small patches of pure water ice with an albedo as high as 80% are found on the surface of Callisto, surrounded by much darker material. High-resolution Galileo images showed the bright patches to be predominately located on elevated surface features: crater rims, scarps, ridges and knobs. They are likely to be thin water frost deposits. Dark material usually lies in the lowlands surrounding and mantling bright features and appears to be smooth. It often forms patches up to 5 km across within the crater floors and in the intercrater depressions.

On a sub-kilometer scale the surface of Callisto is more degraded than the surfaces of other icy Galilean moons. Typically there is a deficit of small impact craters with diameters less than 1 km as compared with, for instance, the dark plains on Ganymede. Instead of small craters, the almost ubiquitous surface features are small knobs and pits. The knobs are thought to represent remnants of crater rims degraded by an as-yet uncertain process. The most likely candidate process is the slow sublimation of ice, which is enabled by a temperature of up to 165 K, reached at a subsolar point. Such sublimation of water or other volatiles from the dirty ice that is the bedrock causes its decomposition. The non-ice remnants form debris avalanches descending from the slopes of the crater walls. Such avalanches are often observed near and inside impact craters and termed "debris aprons". Sometimes crater walls are cut by sinuous valley-like incisions called "gullies", which resemble certain Martian surface features. In the ice sublimation hypothesis, the low-lying dark material is interpreted as a blanket of primarily non-ice debris, which originated from the degraded rims of craters and has covered a predominantly icy bedrock.

The relative ages of the different surface units on Callisto can be determined from the density of impact craters on them. The older the surface, the denser the crater population. Absolute dating has not been carried out, but based on theoretical considerations, the cratered plains are thought to be ~4.5 billion years old, dating back almost to the formation of the Solar System. The ages of multi-ring structures and impact craters depend on chosen background cratering rates and are estimated by different authors to vary between 1 and 4 billion years.

Atmosphere and ionosphere
Callisto has a very tenuous atmosphere composed of carbon dioxide and probably oxygen. It was detected by the Galileo Near Infrared Mapping Spectrometer (NIMS) from its absorption feature near the wavelength 4.2 micrometers. The surface pressure is estimated to be 7.5 picobar (0.75 μPa) and particle density 4$8.204 km/s$ cm−3. Because such a thin atmosphere would be lost in only about four years (see atmospheric escape), it must be constantly replenished, possibly by slow sublimation of carbon dioxide ice from Callisto's icy crust, which would be compatible with the sublimation–degradation hypothesis for the formation of the surface knobs.

Callisto's ionosphere was first detected during Galileo flybys; its high electron density of 7–17$2,410.3 km$ cm−3 cannot be explained by the photoionization of the atmospheric carbon dioxide alone. Hence, it is suspected that the atmosphere of Callisto is actually dominated by molecular oxygen (in amounts 10–100 times greater than ). However, oxygen has not yet been directly detected in the atmosphere of Callisto. Observations with the Hubble Space Telescope (HST) placed an upper limit on its possible concentration in the atmosphere, based on lack of detection, which is still compatible with the ionospheric measurements. At the same time, HST was able to detect condensed oxygen trapped on the surface of Callisto.

Atomic hydrogen has also been detected in Callisto's atmosphere via recent analysis of 2001 Hubble Space Telescope data. Spectral images taken on 15 and 24 December 2001 were re-examined, revealing a faint signal of scattered light that indicates a hydrogen corona. The observed brightness from the scattered sunlight in Callisto's hydrogen corona is approximately two times larger when the leading hemisphere is observed. This asymmetry may originate from a different hydrogen abundance in both the leading and trailing hemispheres. However, this hemispheric difference in Callisto's hydrogen corona brightness is likely to originate from the extinction of the signal in Earth's geocorona, which is greater when the trailing hemisphere is observed.

Callisto's atmosphere have been modelled to gain better understanding of impact of collisional molecular interactions. Researchers used a kinetic method to model collisions between the constituent elements of Callisto's atmosphere (carbon dioxide, molecular oxygen and molecular hydrogen). The modeling took into account the thermal desorption of these compounds due to solar exposure and the resulting variations in temperature on the surface. The simulation showed that the density of Callisto's atmosphere could be explained by the trapping of hydrogen by the heavier gases, carbon dioxide and oxygen. The model shows how kinetic interactions between molecules affect the atmosphere, although it has limitations in terms of variables considered. The simulated densities correlate with expected thresholds for experimental detection.

Origin and evolution
The partial differentiation of Callisto (inferred e.g. from moment of inertia measurements) means that it has never been heated enough to melt its ice component. Therefore, the most favorable model of its formation is a slow accretion in the low-density Jovian subnebula—a disk of the gas and dust that existed around Jupiter after its formation. Such a prolonged accretion stage would allow cooling to largely keep up with the heat accumulation caused by impacts, radioactive decay and contraction, thereby preventing melting and fast differentiation. The allowable timescale for the formation of Callisto lies then in the range 0.1 million–10 million years.

The further evolution of Callisto after accretion was determined by the balance of the radioactive heating, cooling through thermal conduction near the surface, and solid state or subsolidus convection in the interior. Details of the subsolidus convection in the ice is the main source of uncertainty in the models of all icy moons. It is known to develop when the temperature is sufficiently close to the melting point, due to the temperature dependence of ice viscosity. Subsolidus convection in icy bodies is a slow process with ice motions of the order of 1 centimeter per year, but is, in fact, a very effective cooling mechanism on long timescales. It is thought to proceed in the so-called stagnant lid regime, where a stiff, cold outer layer of Callisto conducts heat without convection, whereas the ice beneath it convects in the subsolidus regime. For Callisto, the outer conductive layer corresponds to the cold and rigid lithosphere with a thickness of about 100 km. Its presence would explain the lack of any signs of the endogenic activity on the Callistoan surface. The convection in the interior parts of Callisto may be layered, because under the high pressures found there, water ice exists in different crystalline phases beginning from the ice I on the surface to ice VII in the center. The early onset of subsolidus convection in the Callistoan interior could have prevented large-scale ice melting and any resulting differentiation that would have otherwise formed a large rocky core and icy mantle. Due to the convection process, however, very slow and partial separation and differentiation of rocks and ices inside Callisto has been proceeding on timescales of billions of years and may be continuing to this day.

The current understanding of the evolution of Callisto allows for the existence of a layer or "ocean" of liquid water in its interior. This is connected with the anomalous behavior of ice I phase's melting temperature, which decreases with pressure, achieving temperatures as low as 251 K at 2,070 bar (207 MPa). In all realistic models of Callisto the temperature in the layer between 100 and 200 km in depth is very close to, or exceeds slightly, this anomalous melting temperature. The presence of even small amounts of ammonia—about 1–2% by weight—almost guarantees the liquid's existence because ammonia would lower the melting temperature even further.

Although Callisto is very similar in bulk properties to Ganymede, it apparently had a much simpler geological history. The surface appears to have been shaped mainly by impacts and other exogenic forces. Unlike neighboring Ganymede with its grooved terrain, there is little evidence of tectonic activity. Explanations that have been proposed for the contrasts in internal heating and consequent differentiation and geologic activity between Callisto and Ganymede include differences in formation conditions, the greater tidal heating experienced by Ganymede, and the more numerous and energetic impacts that would have been suffered by Ganymede during the Late Heavy Bombardment. The relatively simple geological history of Callisto provides planetary scientists with a reference point for comparison with other more active and complex worlds.

Habitability
It is speculated that there could be life in Callisto's subsurface ocean. Like Europa and Ganymede, as well as Saturn's moons Enceladus, Dione and Titan and Neptune's moon Triton, a possible subsurface ocean might be composed of salt water.

It is possible that halophiles could thrive in the ocean. As with Europa and Ganymede, the idea has been raised that habitable conditions and even extraterrestrial microbial life may exist in the salty ocean under the Callistoan surface. However, the environmental conditions necessary for life appear to be less favorable on Callisto than on Europa. The principal reasons are the lack of contact with rocky material and the lower heat flux from the interior of Callisto. Callisto's ocean is heated only by radioactive decay, while Europa's is also heated by tidal energy, as it is much closer to Jupiter. It is thought that of all of Jupiter's moons, Europa has the greatest chance of supporting microbial life.

Past
The Pioneer 10 and Pioneer 11 Jupiter encounters in the early 1970s contributed little new information about Callisto in comparison with what was already known from Earth-based observations. The real breakthrough happened later with the Voyager 1 and Voyager 2 flybys in 1979. They imaged more than half of the Callistoan surface with a resolution of 1–2 km, and precisely measured its temperature, mass and shape. A second round of exploration lasted from 1994 to 2003, when the Galileo spacecraft had eight close encounters with Callisto, the last flyby during the C30 orbit in 2001 came as close as 138 km to the surface. The Galileo orbiter completed the global imaging of the surface and delivered a number of pictures with a resolution as high as 15 meters of selected areas of Callisto. In 2000, the Cassini spacecraft en route to Saturn acquired high-quality infrared spectra of the Galilean satellites including Callisto. In February–March 2007, the New Horizons probe on its way to Pluto obtained new images and spectra of Callisto.

Future exploration
Callisto will be visited by three spacecraft in the near future.

The European Space Agency's Jupiter Icy Moons Explorer (JUICE), which launched on April 14, 2023, will perform 21 close flybys of Callisto between 2031 and 2034.

NASA's Europa Clipper, which is scheduled to launch in October 2024, will conduct nine close flybys of Callisto beginning in 2030.

China's CNSA Tianwen-4 is planned to launch to Jupiter around 2030 before entering orbit around Callisto.

Old proposals
Formerly proposed for a launch in 2020, the Europa Jupiter System Mission (EJSM) was a joint NASA/ESA proposal for exploration of Jupiter's moons. In February 2009 it was announced that ESA/NASA had given this mission priority ahead of the Titan Saturn System Mission. At the time ESA's contribution still faced funding competition from other ESA projects. EJSM consisted of the NASA-led Jupiter Europa Orbiter, the ESA-led Jupiter Ganymede Orbiter and possibly a JAXA-led Jupiter Magnetospheric Orbiter.

Potential crewed exploration and habitation
In 2003 NASA conducted a conceptual study called Human Outer Planets Exploration (HOPE) regarding the future human exploration of the outer Solar System. The target chosen to consider in detail was Callisto.

The study proposed a possible surface base on Callisto that would produce rocket propellant for further exploration of the Solar System. Advantages of a base on Callisto include low radiation (due to its distance from Jupiter) and geological stability. Such a base could facilitate remote exploration of Europa, or be an ideal location for a Jovian system waystation servicing spacecraft heading farther into the outer Solar System, using a gravity assist from a close flyby of Jupiter after departing Callisto.

In December 2003, NASA reported that a crewed mission to Callisto might be possible in the 2040s.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Callisto (moon)


text:
Europa (moon)

Europa, or Jupiter II, is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet of all the 95 known moons of Jupiter. It is also the sixth-largest moon in the Solar System. Europa was discovered independently by Simon Marius and Galileo Galilei and was named (by Marius) after Europa, the Phoenician mother of King Minos of Crete and lover of Zeus (the Greek equivalent of the Roman god Jupiter).

Slightly smaller than Earth's Moon, Europa is made of silicate rock and has a water-ice crust and probably an iron–nickel core. It has a very thin atmosphere, composed primarily of oxygen. Its geologically young white-beige surface is striated by light tan cracks and streaks, with very few impact craters. In addition to Earth-bound telescope observations, Europa has been examined by a succession of space-probe flybys, the first occurring in the early 1970s. In September 2022, the Juno spacecraft flew within about 320 km (200 miles) of Europa for a more recent close-up view.

Europa has the smoothest surface of any known solid object in the Solar System. The apparent youth and smoothness of the surface is due to a water ocean beneath the surface, which could conceivably harbor extraterrestrial life, although it would most likely be that of single celled organisms and bacteria-like creatures. The predominant model suggests that heat from tidal flexing causes the ocean to remain liquid and drives ice movement similar to plate tectonics, absorbing chemicals from the surface into the ocean below. Sea salt from a subsurface ocean may be coating some geological features on Europa, suggesting that the ocean is interacting with the sea floor. This may be important in determining whether Europa could be habitable. In addition, the Hubble Space Telescope detected water vapor plumes similar to those observed on Saturn's moon Enceladus, which are thought to be caused by erupting cryogeysers. In May 2018, astronomers provided supporting evidence of water plume activity on Europa, based on an updated analysis of data obtained from the Galileo space probe, which orbited Jupiter from 1995 to 2003. Such plume activity could help researchers in a search for life from the subsurface Europan ocean without having to land on the moon. In March 2024, astronomers reported that the surface of Europa may have much less oxygen than previously inferred.

The Galileo mission, launched in 1989, provides the bulk of current data on Europa. No spacecraft has yet landed on Europa, although there have been several proposed exploration missions. The European Space Agency's Jupiter Icy Moon Explorer (JUICE) is a mission to Ganymede launched on April 14, 2023, that will include two flybys of Europa. NASA's Europa Clipper is expected to be launched in October 2024, with a complementary lander possible based on its findings.

Discovery and naming
Europa, along with Jupiter's three other large moons, Io, Ganymede, and Callisto, was discovered by Galileo Galilei on 8 January 1610, and possibly independently by Simon Marius. On 7 January, Galileo had observed Io and Europa together using a 20×-magnification refracting telescope at the University of Padua, but the low resolution could not separate the two objects. The following night, he saw Io and Europa for the first time as separate bodies.

The moon is the namesake of Europa, in Greek mythology the daughter of the Phoenician king of Tyre. Like all the Galilean satellites, Europa is named after a lover of Zeus, the Greek counterpart of Jupiter. Europa was courted by Zeus and became the queen of Crete. The naming scheme was suggested by Simon Marius, who attributed the proposal to Johannes Kepler:

"Jupiter is much blamed by the poets on account of his irregular loves. Three maidens are especially mentioned as having been clandestinely courted by Jupiter with success. Io, daughter of the River Inachus, Callisto of Lycaon, Europa of Agenor. Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell... I think, therefore, that I shall not have done amiss if the First is called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth Callisto..."

The names fell out of favor for a considerable time and were not revived in general use until the mid-20th century. In much of the earlier astronomical literature, Europa is simply referred to by its Roman numeral designation as Jupiter II (a system also introduced by Galileo) or as the "second satellite of Jupiter". In 1892, the discovery of Amalthea, whose orbit lay closer to Jupiter than those of the Galilean moons, pushed Europa to the third position. The Voyager probes discovered three more inner satellites in 1979, so Europa is now counted as Jupiter's sixth satellite, though it is still referred to as Jupiter II. The adjectival form has stabilized as Europan.

Orbit and rotation

Europa orbits Jupiter in just over three and a half days, with an orbital radius of about 670,900 km. With an orbital eccentricity of only 0.009, the orbit itself is nearly circular, and the orbital inclination relative to Jupiter's equatorial plane is small, at 0.470°. Like its fellow Galilean satellites, Europa is tidally locked to Jupiter, with one hemisphere of Europa constantly facing Jupiter. Because of this, there is a sub-Jovian point on Europa's surface, from which Jupiter would appear to hang directly overhead. Europa's prime meridian is a line passing through this point. Research suggests that tidal locking may not be full, as a non-synchronous rotation has been proposed: Europa spins faster than it orbits, or at least did so in the past. This suggests an asymmetry in internal mass distribution and that a layer of subsurface liquid separates the icy crust from the rocky interior.

The slight eccentricity of Europa's orbit, maintained by gravitational disturbances from the other Galileans, causes Europa's sub-Jovian point to oscillate around a mean position. As Europa comes slightly nearer to Jupiter, Jupiter's gravitational attraction increases, causing Europa to elongate towards and away from it. As Europa moves slightly away from Jupiter, Jupiter's gravitational force decreases, causing Europa to relax back into a more spherical shape, and creating tides in its ocean. The orbital eccentricity of Europa is continuously pumped by its mean-motion resonance with Io. Thus, the tidal flexing kneads Europa's interior and gives it a source of heat, possibly allowing its ocean to stay liquid while driving subsurface geological processes. The ultimate source of this energy is Jupiter's rotation, which is tapped by Io through the tides it raises on Jupiter and is transferred to Europa and Ganymede by the orbital resonance.

Analysis of the unique cracks lining Europa yielded evidence that it likely spun around a tilted axis at some point in time. If correct, this would explain many of Europa's features. Europa's immense network of crisscrossing cracks serves as a record of the stresses caused by massive tides in its global ocean. Europa's tilt could influence calculations of how much of its history is recorded in its frozen shell, how much heat is generated by tides in its ocean, and even how long the ocean has been liquid. Its ice layer must stretch to accommodate these changes. When there is too much stress, it cracks. A tilt in Europa's axis could suggest that its cracks may be much more recent than previously thought. The reason for this is that the direction of the spin pole may change by as much as a few degrees per day, completing one precession period over several months. A tilt could also affect estimates of the age of Europa's ocean. Tidal forces are thought to generate the heat that keeps Europa's ocean liquid, and a tilt in the spin axis would cause more heat to be generated by tidal forces. Such additional heat would have allowed the ocean to remain liquid for a longer time. However, it has not yet been determined when this hypothesized shift in the spin axis might have occurred.

Physical characteristics
Europa is slightly smaller than the Earth's moon. At just over 3100 km in diameter, it is the sixth-largest moon and fifteenth-largest object in the Solar System. Though by a wide margin the least massive of the Galilean satellites, it is nonetheless more massive than all known moons in the Solar System smaller than itself combined. Its bulk density suggests that it is similar in composition to terrestrial planets, being primarily composed of silicate rock.

Internal structure
It is estimated that Europa has an outer layer of water around 100 km thick – a part frozen as its crust and a part as a liquid ocean underneath the ice. Recent magnetic-field data from the Galileo orbiter showed that Europa has an induced magnetic field through interaction with Jupiter's, which suggests the presence of a subsurface conductive layer. This layer is likely to be a salty liquid-water ocean. Portions of the crust are estimated to have undergone a rotation of nearly 80°, nearly flipping over (see true polar wander), which would be unlikely if the ice were solidly attached to the mantle. Europa probably contains a metallic iron core.

Surface features
Europa is the smoothest known object in the Solar System, lacking large-scale features such as mountains and craters. It has been postulated Europa's equator may be covered in icy spikes called penitentes, which may be up to 15 meters high, due to direct overhead sunlight on the equator, causing the ice to sublime, forming vertical cracks. Although the imaging available from the Galileo orbiter does not have the resolution for confirmation, radar and thermal data are consistent with this speculation. The prominent markings crisscrossing Europa appear to be mainly albedo features that emphasize low topography. There are few craters on Europa, because its surface is tectonically too active and therefore young. Its icy crust has an albedo (light reflectivity) of 0.64, one of the highest of any moon. This indicates a young and active surface: based on estimates of the frequency of cometary bombardment that Europa experiences, the surface is about 20 to 180 million years old. There is no scientific consensus about the explanation for Europa's surface features.

The ionizing radiation level at Europa's surface is equivalent to a daily dose of about 5.4 Sv (540 rem), an amount that would cause severe illness or death in human beings exposed for a single Earth day (24 hours). A Europan day is about 3.5 times as long as an Earth day.

Lineae
Europa's most striking surface features are a series of dark streaks crisscrossing the entire globe, called lineae (lines). Close examination shows that the edges of Europa's crust on either side of the cracks have moved relative to each other. The larger bands are more than 20 km across, often with dark, diffuse outer edges, regular striations, and a central band of lighter material.

The most likely hypothesis is that the lineae on Europa were produced by a series of eruptions of warm ice as Europa's crust slowly spreads open to expose warmer layers beneath. The effect would have been similar to that seen on Earth's oceanic ridges. These various fractures are thought to have been caused in large part by the tidal flexing exerted by Jupiter. Because Europa is tidally locked to Jupiter, and therefore always maintains approximately the same orientation towards Jupiter, the stress patterns should form a distinctive and predictable pattern. However, only the youngest of Europa's fractures conform to the predicted pattern; other fractures appear to occur at increasingly different orientations the older they are. This could be explained if Europa's surface rotates slightly faster than its interior, an effect that is possible due to the subsurface ocean mechanically decoupling Europa's surface from its rocky mantle and the effects of Jupiter's gravity tugging on Europa's outer ice crust. Comparisons of Voyager and Galileo spacecraft photos serve to put an upper limit on this hypothetical slippage. A full revolution of the outer rigid shell relative to the interior of Europa takes at least 12,000 years. Studies of Voyager and Galileo images have revealed evidence of subduction on Europa's surface, suggesting that, just as the cracks are analogous to ocean ridges, so plates of icy crust analogous to tectonic plates on Earth are recycled into the molten interior. This evidence of both crustal spreading at bands and convergence at other sites suggests that Europa may have active plate tectonics, similar to Earth. However, the physics driving these plate tectonics are not likely to resemble those driving terrestrial plate tectonics, as the forces resisting potential Earth-like plate motions in Europa's crust are significantly stronger than the forces that could drive them.

Chaos and lenticulae
Other features present on Europa are circular and elliptical lenticulae (Latin for "freckles"). Many are domes, some are pits and some are smooth, dark spots. Others have a jumbled or rough texture. The dome tops look like pieces of the older plains around them, suggesting that the domes formed when the plains were pushed up from below.

One hypothesis states that these lenticulae were formed by diapirs of warm ice rising up through the colder ice of the outer crust, much like magma chambers in Earth's crust. The smooth, dark spots could be formed by meltwater released when the warm ice breaks through the surface. The rough, jumbled lenticulae (called regions of "chaos"; for example, Conamara Chaos) would then be formed from many small fragments of crust, embedded in hummocky, dark material, appearing like icebergs in a frozen sea.

An alternative hypothesis suggests that lenticulae are actually small areas of chaos and that the claimed pits, spots and domes are artefacts resulting from the over-interpretation of early, low-resolution Galileo images. The implication is that the ice is too thin to support the convective diapir model of feature formation.

In November 2011, a team of researchers from the University of Texas at Austin and elsewhere presented evidence in the journal Nature suggesting that many "chaos terrain" features on Europa sit atop vast lakes of liquid water. These lakes would be entirely encased in Europa's icy outer shell and distinct from a liquid ocean thought to exist farther down beneath the ice shell. Full confirmation of the lakes' existence will require a space mission designed to probe the ice shell either physically or indirectly, for example, using radar.

Work published by researchers from Williams College suggests that chaos terrain may represent sites where impacting comets penetrated through the ice crust and into an underlying ocean.

Subsurface ocean
The scientific consensus is that a layer of liquid water exists beneath Europa's surface, and that heat from tidal flexing allows the subsurface ocean to remain liquid. Europa's surface temperature averages about 110 K at the equator and only 50 K at the poles, keeping Europa's icy crust as hard as granite. The first hints of a subsurface ocean came from theoretical considerations of tidal heating (a consequence of Europa's slightly eccentric orbit and orbital resonance with the other Galilean moons). Galileo imaging team members argue for the existence of a subsurface ocean from analysis of Voyager and Galileo images. The most dramatic example is "chaos terrain", a common feature on Europa's surface that some interpret as a region where the subsurface ocean has melted through the icy crust. This interpretation is controversial. Most geologists who have studied Europa favor what is commonly called the "thick ice" model, in which the ocean has rarely, if ever, directly interacted with the present surface. The best evidence for the thick-ice model is a study of Europa's large craters. The largest impact structures are surrounded by concentric rings and appear to be filled with relatively flat, fresh ice; based on this and on the calculated amount of heat generated by Europan tides, it is estimated that the outer crust of solid ice is approximately 10 to 30 km thick, including a ductile "warm ice" layer, which could mean that the liquid ocean underneath may be about 100 km deep. This leads to a volume of Europa's oceans of 3×1018m3, between two or three times the volume of Earth's oceans.

The thin-ice model suggests that Europa's ice shell may be only a few kilometers thick. However, most planetary scientists conclude that this model considers only those topmost layers of Europa's crust that behave elastically when affected by Jupiter's tides. One example is flexure analysis, in which Europa's crust is modeled as a plane or sphere weighted and flexed by a heavy load. Models such as this suggest the outer elastic portion of the ice crust could be as thin as 200 m. If the ice shell of Europa is really only a few kilometers thick, this "thin ice" model would mean that regular contact of the liquid interior with the surface could occur through open ridges, causing the formation of areas of chaotic terrain. Large impacts going fully through the ice crust would also be a way that the subsurface ocean could be exposed.

Composition
The Galileo orbiter found that Europa has a weak magnetic moment, which is induced by the varying part of the Jovian magnetic field. The field strength at the magnetic equator (about 120 nT) created by this magnetic moment is about one-sixth the strength of Ganymede's field and six times the value of Callisto's. The existence of the induced moment requires a layer of a highly electrically conductive material in Europa's interior. The most plausible candidate for this role is a large subsurface ocean of liquid saltwater.

Since the Voyager spacecraft flew past Europa in 1979, scientists have worked to understand the composition of the reddish-brown material that coats fractures and other geologically youthful features on Europa's surface. Spectrographic evidence suggests that the darker, reddish streaks and features on Europa's surface may be rich in salts such as magnesium sulfate, deposited by evaporating water that emerged from within. Sulfuric acid hydrate is another possible explanation for the contaminant observed spectroscopically. In either case, because these materials are colorless or white when pure, some other material must also be present to account for the reddish color, and sulfur compounds are suspected.

Another hypothesis for the colored regions is that they are composed of abiotic organic compounds collectively called tholins. The morphology of Europa's impact craters and ridges is suggestive of fluidized material welling up from the fractures where pyrolysis and radiolysis take place. In order to generate colored tholins on Europa, there must be a source of materials (carbon, nitrogen, and water) and a source of energy to make the reactions occur. Impurities in the water ice crust of Europa are presumed both to emerge from the interior as cryovolcanic events that resurface the body, and to accumulate from space as interplanetary dust. Tholins bring important astrobiological implications, as they may play a role in prebiotic chemistry and abiogenesis.

The presence of sodium chloride in the internal ocean has been suggested by a 450 nm absorption feature, characteristic of irradiated NaCl crystals, that has been spotted in HST observations of the chaos regions, presumed to be areas of recent subsurface upwelling. The subterranean ocean of Europa contains carbon and was observed on the surface ice as a concentration of carbon dioxide within Tara Regio, a geologically recently resurfaced terrain.

Sources of heat
Europa receives thermal energy from tidal heating, which occurs through the tidal friction and tidal flexing processes caused by tidal acceleration: orbital and rotational energy are dissipated as heat in the core of the moon, the internal ocean, and the ice crust.

Tidal friction
Ocean tides are converted to heat by frictional losses in the oceans and their interaction with the solid bottom and with the top ice crust. In late 2008, it was suggested Jupiter may keep Europa's oceans warm by generating large planetary tidal waves on Europa because of its small but non-zero obliquity. This generates so-called Rossby waves that travel quite slowly, at just a few kilometers per day, but can generate significant kinetic energy. For the current axial tilt estimate of 0.1 degree, the resonance from Rossby waves would contain 7.3 J of kinetic energy, which is two thousand times larger than that of the flow excited by the dominant tidal forces. Dissipation of this energy could be the principal heat source of Europa's ocean.

Tidal flexing
Tidal flexing kneads Europa's interior and ice shell, which becomes a source of heat. Depending on the amount of tilt, the heat generated by the ocean flow could be 100 to thousands of times greater than the heat generated by the flexing of Europa's rocky core in response to the gravitational pull from Jupiter and the other moons circling that planet. Europa's seafloor could be heated by the moon's constant flexing, driving hydrothermal activity similar to undersea volcanoes in Earth's oceans.

Experiments and ice modeling published in 2016, indicate that tidal flexing dissipation can generate one order of magnitude more heat in Europa's ice than scientists had previously assumed. Their results indicate that most of the heat generated by the ice actually comes from the ice's crystalline structure (lattice) as a result of deformation, and not friction between the ice grains. The greater the deformation of the ice sheet, the more heat is generated.

Radioactive decay
In addition to tidal heating, the interior of Europa could also be heated by the decay of radioactive material (radiogenic heating) within the rocky mantle. But the models and values observed are one hundred times higher than those that could be produced by radiogenic heating alone, thus implying that tidal heating has a leading role in Europa.

Plumes
The Hubble Space Telescope acquired an image of Europa in 2012 that was interpreted to be a plume of water vapour erupting from near its south pole. The image suggests the plume may be 200 km high, or more than 20 times the height of Mt. Everest., though recent observations and modeling suggest that typical Europan plumes may be much smaller. It has been suggested that if plumes exist, they are episodic and likely to appear when Europa is at its farthest point from Jupiter, in agreement with tidal force modeling predictions. Additional imaging evidence from the Hubble Space Telescope was presented in September 2016.

In May 2018, astronomers provided supporting evidence of water plume activity on Europa, based on an updated critical analysis of data obtained from the Galileo space probe, which orbited Jupiter between 1995 and 2003. Galileo flew by Europa in 1997 within 206 km of the moon's surface and the researchers suggest it may have flown through a water plume. Such plume activity could help researchers in a search for life from the subsurface Europan ocean without having to land on the moon.

The tidal forces are about 1,000 times stronger than the Moon's effect on Earth. The only other moon in the Solar System exhibiting water vapor plumes is Enceladus. The estimated eruption rate at Europa is about 7000 kg/s compared to about 200 kg/s for the plumes of Enceladus. If confirmed, it would open the possibility of a flyby through the plume and obtain a sample to analyze in situ without having to use a lander and drill through kilometres of ice.

In November 2020, a study was published in the peer-reviewed scientific journal Geophysical Research Letters suggesting that the plumes may originate from water within the crust of Europa as opposed to its subsurface ocean. The study's model, using images from the Galileo space probe, proposed that a combination of freezing and pressurization may result in at least some of the cryovolcanic activity. The pressure generated by migrating briny water pockets would thus, eventually, burst through the crust, thereby creating these plumes. The hypothesis that cryovolcanism on Europa could be triggered by freezing and pressurization of liquid pockets in the icy crust was first proposed by Sarah Fagents at the University of Hawai'i at Mānoa, who in 2003, was the first to model and publish work on this process. A press release from NASA's Jet Propulsion Laboratory referencing the November 2020 study suggested that plumes sourced from migrating liquid pockets could potentially be less hospitable to life. This is due to a lack of substantial energy for organisms to thrive off, unlike proposed hydrothermal vents on the subsurface ocean floor.

Atmosphere
The atmosphere of Europa can be categorized as thin and tenuous (often called an exosphere), primarily composed of oxygen and trace amounts of water vapor. However, this quantity of oxygen is produced in a non-biological manner. Given that Europa's surface is icy, and subsequently very cold; as solar ultraviolet radiation and charged particles (ions and electrons) from the Jovian magnetospheric environment collide with Europa's surface, water vapor is created and instantaneously separated into oxygen and hydrogen constituents. As it continues to move, the hydrogen is light enough to pass through the surface gravity of the atmosphere leaving behind only oxygen. The surface-bounded atmosphere forms through radiolysis, the dissociation of molecules through radiation. This accumulated oxygen atmosphere can get to a height of 190 km above the surface of Europa. Molecular oxygen is the densest component of the atmosphere because it has a long lifetime; after returning to the surface, it does not stick (freeze) like a water or hydrogen peroxide molecule but rather desorbs from the surface and starts another ballistic arc. Molecular hydrogen never reaches the surface, as it is light enough to escape Europa's surface gravity. Europa is one of the few moons in our solar system with a quantifiable atmosphere, along with Titan, Io, Triton, Ganymede and Callisto. Europa is also one of several moons in our solar system with very large quantities of ice (volatiles), otherwise known as "icy moons." Europa is also considered to be geologically active due to the constant release of hydrogen-oxygen mixtures into space. As a result of the moon's particle venting, the atmosphere requires continuous replenishment. Europa also contains a small magnetosphere (approximately 25% of Ganymede's). However, this magnetosphere varies in size as Europa orbits through Jupiter's magnetic field. This confirms that a conductive element, such as a large ocean, likely lies below its icy surface. As multiple studies have been conducted over Europa's atmosphere, several findings conclude that not all oxygen molecules are released into the atmosphere. This unknown percentage of oxygen may be absorbed into the surface and sink into the subsurface. Because the surface may interact with the subsurface ocean (considering the geological discussion above), this molecular oxygen may make its way to the ocean, where it could aid in biological processes. One estimate suggests that, given the turnover rate inferred from the apparent ~0.5 Gyr maximum age of Europa's surface ice, subduction of radiolytically generated oxidizing species might well lead to oceanic free oxygen concentrations that are comparable to those in terrestrial deep oceans.

Through the slow release of oxygen and hydrogen, a neutral torus around Europa's orbital plane is formed. This "neutral cloud" has been detected by both the Cassini and Galileo spacecraft, and has a greater content (number of atoms and molecules) than the neutral cloud surrounding Jupiter's inner moon Io. This torus was officially confirmed using Energetic Neutral Atom (ENA) imaging. Europa's torus ionizes through the process of neutral particles exchanging electrons with its charged particles. Since Europa's magnetic field rotates faster than its orbital velocity, these ions are left in the path of its magnetic field trajectory, forming a plasma. It has been hypothesized that these ions are responsible for the plasma within Jupiter's magnetosphere.

On 4 March 2024, astronomers reported that the surface of Europa may have much less oxygen than previously inferred.

Discovery of atmosphere
The atmosphere of Europa was first discovered in 1995 by astronomers D. T. Hall and collaborators using the Goddard High Resolution Spectrograph instrument of the Hubble Space Telescope. This observation was further supported in 1997 by the Galileo orbiter during its mission within the Jovian system. The Galileo orbiter performed three radio occultation events of Europa, where the probe's radio contact with Earth was temporarily blocked by passing behind Europa. By analyzing the effects Europa's sparse atmosphere had on the radio signal just before and after the occultation, for a total of six events, a team of astronomers led by A. J. Kliore established the presence of an ionized layer in Europa's atmosphere.

Climate and weather
Despite the presence of a gas torus, Europa has no weather producing clouds. As a whole, Europa has no wind, precipitation, or presence of sky color as its gravity is too low to hold an atmosphere substantial enough for those features. Europa's gravity is approximately 13% of Earth's. The temperature on Europa varies from −160 °C at the equator, to −220 °C at either of its poles. Europa's subsurface ocean is thought to be subsequently warm however. It is hypothesized that because of radioactive and tidal heating (as mentioned in the sections above), there are points in the depths of Europa's ocean that may be only slightly cooler than Earth's oceans. Studies have also concluded that Europa's ocean would have been rather acidic at first, with large concentrations of sulfate, calcium, and carbon dioxide. But over the course of 4.5 billion years, it became full of chloride, thus resembling our 1.94% chloride oceans on Earth.

Exploration
Exploration of Europa began with the Jupiter flybys of Pioneer 10 and 11 in 1973 and 1974, respectively. The first closeup photos were of low resolution compared to later missions. The two Voyager probes traveled through the Jovian system in 1979, providing more-detailed images of Europa's icy surface. The images caused many scientists to speculate about the possibility of a liquid ocean underneath. Starting in 1995, the Galileo space probe orbited Jupiter for eight years, until 2003, and provided the most detailed examination of the Galilean moons to date. It included the "Galileo Europa Mission" and "Galileo Millennium Mission", with numerous close flybys of Europa. In 2007, New Horizons imaged Europa, as it flew by the Jovian system while on its way to Pluto. In 2022, the Juno orbiter flew by Europa at a distance of 352 km (219 mi).

In 2012, Jupiter Icy Moons Explorer (JUICE) was selected by the European Space Agency (ESA) as a planned mission. That mission includes two flybys of Europa, but is more focused on Ganymede. It was launched in 2023, and is expected to reach Jupiter in July 2031 after four gravity assists and eight years of travel.

Future missions
Conjectures regarding extraterrestrial life have ensured a high profile for Europa and have led to steady lobbying for future missions. The aims of these missions have ranged from examining Europa's chemical composition to searching for extraterrestrial life in its hypothesized subsurface oceans. Robotic missions to Europa need to endure the high-radiation environment around Jupiter. Because it is deeply embedded within Jupiter's magnetosphere, Europa receives about 5.40 Sv of radiation per day.

In 2011, a Europa mission was recommended by the U.S. Planetary Science Decadal Survey. In response, NASA commissioned Europa lander concept studies in 2011, along with concepts for a Europa flyby (Europa Clipper), and a Europa orbiter. The orbiter element option concentrates on the "ocean" science, while the multiple-flyby element (Clipper) concentrates on the chemistry and energy science. On 13 January 2014, the House Appropriations Committee announced a new bipartisan bill that includes $80 million in funding to continue the Europa mission concept studies.
 * Europa Clipper – In July 2013 an updated concept for a flyby Europa mission called Europa Clipper was presented by the Jet Propulsion Laboratory (JPL) and the Applied Physics Laboratory (APL). In May 2015, NASA announced that it had accepted development of the Europa Clipper mission, and revealed the instruments it would use. The aim of Europa Clipper is to explore Europa in order to investigate its habitability, and to aid in selecting sites for a future lander. The Europa Clipper would not orbit Europa, but instead orbit Jupiter and conduct 45 low-altitude flybys of Europa during its envisioned mission. The probe would carry an ice-penetrating radar, short-wave infrared spectrometer, topographical imager, and an ion- and neutral-mass spectrometer. The mission is scheduled to launch in October 2024 aboard a Falcon Heavy.
 * Europa Lander is a recent NASA concept mission under study. 2018 research suggests Europa may be covered in tall, jagged ice spikes, presenting a problem for any potential landing on its surface.

Old proposals

In the early 2000s, Jupiter Europa Orbiter led by NASA and the Jupiter Ganymede Orbiter led by the ESA were proposed together as an Outer Planet Flagship Mission to Jupiter's icy moons called Europa Jupiter System Mission, with a planned launch in 2020. In 2009 it was given priority over Titan Saturn System Mission. At that time, there was competition from other proposals. Japan proposed Jupiter Magnetospheric Orbiter.

Jovian Europa Orbiter was an ESA Cosmic Vision concept study from 2007. Another concept was Ice Clipper, which would have used an impactor similar to the Deep Impact mission—it would make a controlled crash into the surface of Europa, generating a plume of debris that would then be collected by a small spacecraft flying through the plume.

Jupiter Icy Moons Orbiter (JIMO) was a partially developed fission-powered spacecraft with ion thrusters that was cancelled in 2006. It was part of Project Prometheus. The Europa Lander Mission proposed a small nuclear-powered Europa lander for JIMO. It would travel with the orbiter, which would also function as a communication relay to Earth.

Europa Orbiter – Its objective would be to characterize the extent of the ocean and its relation to the deeper interior. Instrument payload could include a radio subsystem, laser altimeter, magnetometer, Langmuir probe, and a mapping camera. The Europa Orbiter received the go-ahead in 1999 but was canceled in 2002. This orbiter featured a special ice-penetrating radar that would allow it to scan below the surface.

More ambitious ideas have been put forward including an impactor in combination with a thermal drill to search for biosignatures that might be frozen in the shallow subsurface.

Another proposal put forward in 2001 calls for a large nuclear-powered "melt probe" (cryobot) that would melt through the ice until it reached an ocean below. Once it reached the water, it would deploy an autonomous underwater vehicle (hydrobot) that would gather information and send it back to Earth. Both the cryobot and the hydrobot would have to undergo some form of extreme sterilization to prevent detection of Earth organisms instead of native life and to prevent contamination of the subsurface ocean. This suggested approach has not yet reached a formal conceptual planning stage.

Habitability

So far, there is no evidence that life exists on Europa, but the moon has emerged as one of the most likely locations in the Solar System for potential habitability. Life could exist in its under-ice ocean, perhaps in an environment similar to Earth's deep-ocean hydrothermal vents. Even if Europa lacks volcanic hydrothermal activity, a 2016 NASA study found that Earth-like levels of hydrogen and oxygen could be produced through processes related to serpentinization and ice-derived oxidants, which do not directly involve volcanism. In 2015, scientists announced that salt from a subsurface ocean may likely be coating some geological features on Europa, suggesting that the ocean is interacting with the seafloor. This may be important in determining if Europa could be habitable. The likely presence of liquid water in contact with Europa's rocky mantle has spurred calls to send a probe there.

The energy provided by tidal forces drives active geological processes within Europa's interior, just as they do to a far more obvious degree on its sister moon Io. Although Europa, like the Earth, may possess an internal energy source from radioactive decay, the energy generated by tidal flexing would be several orders of magnitude greater than any radiological source. Life on Europa could exist clustered around hydrothermal vents on the ocean floor, or below the ocean floor, where endoliths are known to inhabit on Earth. Alternatively, it could exist clinging to the lower surface of Europa's ice layer, much like algae and bacteria in Earth's polar regions, or float freely in Europa's ocean. Should Europa's oceans be too cold, biological processes similar to those known on Earth could not occur; too salty, only extreme halophiles could survive in that environment. In 2010, a model proposed by Richard Greenberg of the University of Arizona proposed that irradiation of ice on Europa's surface could saturate its crust with oxygen and peroxide, which could then be transported by tectonic processes into the interior ocean. Such a process could render Europa's ocean as oxygenated as our own within just 12 million years, allowing the existence of complex, multicellular lifeforms.

Evidence suggests the existence of lakes of liquid water entirely encased in Europa's icy outer shell and distinct from a liquid ocean thought to exist farther down beneath the ice shell, as well as pockets of water that form M-shaped ice ridges when the water freezes on the surface – as in Greenland. If confirmed, the lakes and pockets of water could be yet another potential habitat for life. Evidence suggests that hydrogen peroxide is abundant across much of the surface of Europa. Because hydrogen peroxide decays into oxygen and water when combined with liquid water, the authors argue that it could be an important energy supply for simple life forms. Nonetheless, on 4 March 2024, astronomers reported that the surface of Europa may have much less oxygen than previously inferred.

Clay-like minerals (specifically, phyllosilicates), often associated with organic matter on Earth, have been detected on the icy crust of Europa. The presence of the minerals may have been the result of a collision with an asteroid or comet. Some scientists have speculated that life on Earth could have been blasted into space by asteroid collisions and arrived on the moons of Jupiter in a process called lithopanspermia.

title: Europa (moon)


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User:Howardzzh

I am a Computer Science student at Caltech in Pasadena

title: User:Howardzzh


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User talk:Howardzzh

Well hello there new fella. Welcome to the 'pedia! You're a Caltechi, eh? Well I hope you like the place and decide to stay. Cheers! --maveric149

title: User talk:Howardzzh


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Talk:Metazoa

Whoops; sorry about covering up the history link again by "fixing" that redirect. Here's a direct link: http://www.wikipedia.com/wiki/Metazoa&action=history Bryan Derksen

Thats OK. The article is a huge mess and needs to be integrated into about a half of a dozen other articles - especially animalia. Metazoa is the old terminology for the Animal Kingdom. Much of the info in the article is outdated. I really think that the addition of scientific articles from very old sources is next to useless -- many of the names have been changed and much of the information is out of date and in fact wrong. That doesn't forgive the person who simply deleted ALL the content in this article and then provided a REDIRECT. --maveric149

I apologize for that. I was hoping to sort out the article but for whatever reason couldn't get the history to work. In retrospect, of course, what I should have done is moved the content out first and then did the redirect. If I help stub the phyla now, will that make it up to you? --User:Josh Grosse

That's cool. Sorry I lost my temper - but if I didn't catch that we would have effectively lost the info contained in the Metazoa article. Right now I am spinning off articles from a bunch of the info that is in the metazoa article. --maveric149

Yep. I think almost everything that is of much use is covered now. I'll let you finish up with what you want to export, and then we can set it back to a redirect. -- JG

Actually it's a Subkingdom and should have it's own article. It's used extensively in the taxoboxes and one shouldn't confuse it with animalia because it's a subset. Williamb 09:53, 1 Aug 2004 (UTC)

Sometimes it's considered a subset, ranked variously from subkingdom to superphylum, but other times not. It's not a standard group, and while it may deserve its own article, the article should reflect that. It definitely shouldn't be used in taxoboxes; its unhelpful and simply makes them more system-dependent (see WikiProject Tree of Life). Josh 07:41, 8 Aug 2004 (UTC)

Removed redirect
Metazoa is a more complex term than a synonym of animal, and is scientifically quite distinct. i kan reed 00:44, 16 January 2007 (UTC)

Synonym for Eumetazoa
According to most of the article, especially the taxobox, this is a synonym for what most articles refer to as Eumetazoa.--Crustaceanguy 00:46, 11 February 2007 (UTC)
 * That appears to be true, but I'm not an expert. Does anyone know if a redirect is valid? i kan reed 18:01, 11 February 2007 (UTC)

The above poster is correct. I'm redirecting this to Animal. I'll post the information on this page on the talk page of Eumetazoa, to see if any of it can be used there. Werothegreat 17:39, 17 March 2007 (UTC)
 * uh... I'd like to point out that animal is a really poor redirect, because Metezoans do not include Sponges, or some Gniderians. The terms are hardly synonymous.  i kan reed 19:34, 17 March 2007 (UTC)

 * Sorry, I probably confused you... What I meant is Metazoa is an outdated name for Animalia, and Eumetazoa is a subkingdom of Animalia which includes all animals except the sponges (phylum Porifera). In this article, however, everything is jumbled up and disordered, that is, the fact that Metazoa is a synonym for Animalia is correct, but the fact that it excludes sponges is not.--Crustaceanguy 23:44, 20 March 2007 (UTC)

title: Talk:Metazoa