이오 (위성)

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이오
이오
갈릴레오 탐사선이 촬영한 이오의 실제 색 사진
발견
최초 발견자 갈릴레오 갈릴레이
발견 일자 1610년 1월 8일
명칭
다른 이름 목성 I
궤도 성질
근일점 420,000 km
원일점 423,400 km
반장축 421,700 km
이심률 0.0041
공전 주기 1.769 137 786 일
평균 공전 속력 17.334 km
궤도 경사 0.05°
(목성의 적도 기준)
모행성 목성
물리적 성질
평균 지름 1821.6 km
(지구의 0.286배)
표면적 4191만 km2
(지구의 0.082배)
부피 2.53 X 10×10^10 km3
(지구의 0.023배)
질량 8.931938 X 10×10^22 km3
(지구의 0.015배)
평균 밀도 3.528 g/cm3
표면 중력 1.796 m/s2
(0.183 g)
탈출 속도 2.558 km/s
자전 주기 조석 고정
반사율 0.63±0.02
표면 온도
최저* 평균 최고
90 K 110 K 130 K
대기 특징
대기압 미량
구성 성분 이산화황

이오(Io, /ˈ./, eye-oh 또는 그리스어: Ἰώ)는 목성의 위성 중 하나로 갈릴레이 위성에 속하는 위성이다. 지름은 3,642km으로 태양계에서 네 번째로 큰 위성이다. 이 위성의 이름은 그리스 신화에서 제우스의 연인 중 한명이자 헤라의 여사제인 이오를 따서 지어졌다.

400개 이상의 활화산을 가진 이오는 태양계에서 지질학적으로 가장 활성화된 위성 중 하나다.[1][2] 이오의 극단적인 지질 활동은 목성과 다른 갈릴레이 위성—에우로파, 가니메데, 칼리스토-이 밀고 당겨 생기는 조석 가열 때문이다. 여러 화산들은 표면 위 500km까지 이산화황의 연기를 뿜어내고 있다. 이오의 표면은 규산염 지각에서 벌어지는 압축에 의해 생긴 100개 이상의 산이 덮고 있다. 그 중 몇몇 개는 에베레스트 산보다 더 크다.[3] 이오의 구성은 외태양계에 있는 다른 위성들과 다르다. 외태양계의 위성들은 주로 얼음으로 구성되어 있는 반면, 이오는 용융 상태의 철과 철 핵을 둘러싼 규산암 바위로 이루어져 있고, 표면은 황과 이산화황의 서리로 덮여 있다.

이오의 화산들은 독특한 '기능'을 각각 담당하고 있다. 화산 폭발로 파편을 날리고 용암을 흐르게 하여 표면을 노란색, 빨간색, 흰색, 검은색, 초록색 황 화합물로 덮는다. 광범위한 용암 분출은 500 km 범위까지 퍼져나가며, 표면에 자국을 남긴다. 이 화산 활동으로 생성된 물질들은 이오의 표면을 얇게 덮고, 얕은 대기를 형성하며, 일부는 목성의 광범위한 자기권에 들어가기도 한다. 이오의 화산 분출물들은 목성에 엄청난 크기의 플라즈마 고리를 형성한다.

이오는 17~18세기 천문학의 발전에 중요한 역할을 했다.이오는 갈릴레오 갈릴레이에 의해 발견되었고, 다른 위성들과 같이 갈릴레이 위성으로 불린다. 이오의 발견은 코페르니쿠스의 태양 중심설을 채택하게 하는 계기가 되었고, 요하네스 케플러의 운동 법칙을 개발하는 계기가 되었으며, 최초의 빛 속도 측정 대상이 되었다. 지구에서 이오는 19세기 후반부터 20세기 초까지 극은 붉고 적도 쪽은 밝다는 것까지 알려졌으며, 그 후 표면의 대규모 용암 형상을 관측할 수 있게 되었다. 1979년, 두 대의 보이저 탐사선은 이오가 지질학적으로 활발한 위성임을 밝혀 냈고, 거대한 화산들과 충돌구들이 비정상적으로 젊다는 것도 관측하였다. 갈릴레오 탐사선은 1990년도와 2000년도에 이오를 지나치며 관측했고, 이오의 내부 구성과 표면 조성에 대한 정보를 얻어냈다. 또한 탐사선들은 이오와 목성 자기권의 연관관계와 이오 궤도 주변의 방사선 띠의 유무도 밝혀 내었다. 이오에는 하루에 3600 Rem의 방사선이 들이친다.[4]

또한 2000년에 카시니-하위헌스호와 2007년 뉴 허라이즌스 호가 목성을 지나치며 이오를 관측했고, 지구허블 우주 망원경도 계속해서 이오를 관측하고 있다.


작명[편집]

이오, 달과 지구의 크기 비교

시몬 마리우스가 갈릴레오 위성을 발견한 유일한 사람이라고 여겨지지는 않지만, 위성에 대한 제안을 내놓았다. In his 1614 publication Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici, he proposed several alternative names for the innermost of the large moons of Jupiter, including "The Mercury of Jupiter" and "The First of the Jovian Planets".[5] Based on a suggestion from Johannes Kepler in October 1613, he also devised a naming scheme whereby each moon was named for a lover of the Greek mythological Zeus or his Roman equivalent, Jupiter. He named the innermost large moon of Jupiter after the Greek mythological figure Io.[5][6] Marius' names were not widely adopted until centuries later, and in much of the earlier astronomical literature, Io was generally referred to by its Roman numeral designation (a system introduced by Galileo) as "Jupiter I",[7] or as "the first satellite of Jupiter".[8][9]

Features on Io are named after characters and places from the Io myth, as well as deities of fire, volcanoes, the Sun, and thunder from various myths, and characters and places from Dante's Inferno: names appropriate to the volcanic nature of the surface.[10] Since the surface was first seen up close by Voyager 1, the International Astronomical Union has approved 225 names for Io's volcanoes, mountains, plateaus, and large albedo features. The approved feature categories used for Io for different types of volcanic features include patera ("saucer"; volcanic depression), fluctus ("flow"; lava flow), vallis ("valley"; lava channel), and active eruptive center (location where plume activity was the first sign of volcanic activity at a particular volcano). Named mountains, plateaus, layered terrain, and shield volcanoes include the terms mons, mensa ("table"), planum, and tholus ("rotunda"), respectively.[10] Named, bright albedo regions use the term regio. Examples of named features are Prometheus, Pan Mensa, Tvashtar Paterae, and Tsũi Goab Fluctus.[11]

관측 역사[편집]

Galileo Galilei, the discoverer of Io

The first reported observation of Io was made by Galileo Galilei on 7 January 1610 using a 20x-power, refracting telescope at the University of Padua. However, in that observation, Galileo could not separate Io and Europa due to the low power of his telescope, so the two were recorded as a single point of light. Io and Europa were seen for the first time as separate bodies during Galileo's observations of the Jupiter system the following day, 8 January 1610 (used as the discovery date for Io by the IAU).[12] The discovery of Io and the other Galilean satellites of Jupiter was published in Galileo's Sidereus Nuncius in March 1610.[13] In his Mundus Jovialis, published in 1614, Simon Marius claimed to have discovered Io and the other moons of Jupiter in 1609, one week before Galileo's discovery. Galileo doubted this claim and dismissed the work of Marius as plagiarism. Regardless, Marius' first recorded observation came from 29 December 1609 in the Julian calendar, which equates to 8 January 1610 in the Gregorian calendar, which Galileo used.[14] Given that Galileo published his work before Marius, Galileo is credited with the discovery.[15]

For the next two and a half centuries, Io remained an unresolved, 5th-magnitude point of light in astronomers' telescopes. During the 17th century, Io and the other Galilean satellites served a variety of purposes, including early methods to determine longitude,[16] validating Kepler's Third Law of planetary motion, and determining the time required for light to travel between Jupiter and Earth.[13] Based on ephemerides produced by astronomer Giovanni Cassini and others, Pierre-Simon Laplace created a mathematical theory to explain the resonant orbits of Io, Europa, and Ganymede.[13] This resonance was later found to have a profound effect on the geologies of the three moons.

Improved telescope technology in the late 19th and 20th centuries allowed astronomers to resolve (that is, see as distinct objects) large-scale surface features on Io. In the 1890s, Edward E. Barnard was the first to observe variations in Io's brightness between its equatorial and polar regions, correctly determining that this was due to differences in color and albedo between the two regions and not due to Io being egg-shaped, as proposed at the time by fellow astronomer William Pickering, or two separate objects, as initially proposed by Barnard.[8][9][17] Later telescopic observations confirmed Io's distinct reddish-brown polar regions and yellow-white equatorial band.[18]

Telescopic observations in the mid-20th century began to hint at Io's unusual nature. Spectroscopic observations suggested that Io's surface was devoid of water ice (a substance found to be plentiful on the other Galilean satellites).[19] The same observations suggested a surface dominated by evaporates composed of sodium salts and sulfur.[20] Radio telescopic observations revealed Io's influence on the Jovian magnetosphere, as demonstrated by decametric wavelength bursts tied to the orbital period of Io.[21]

파이어니어[편집]

파이어니어 11호가 촬영한 이오의 모습.

첫 번째로 가까이서 이오를 지나친 탐사선은 파이어니어 10호파이어니어 11호인데, 각각 1973년 12월과 1974년 12월에 목성을 지나치며 이오를 관측했다.[22] 전파 추적을 통해 이오의 질량과 정확한 크기가 측정되었고, 이오가 네 갈릴레이 위성 중 가장 밀도가 높다는 것이 측정되고, 얼음보다는 주로 규산암으로 구성되어 있다는 것이 밝혀졌다.[23] 파이어니어 탐사선은 이오의 궤도 주변에서 강력한 방사능과 이오의 얇은 대기의 존재를 밝혀냈다. 파이어니어 11호가 찍은 이오는 탐사선에 의해 얻어진 최초의 이오 사진이었고, 북극 지역을 보여주었다.[24] 확대 이미지를 찍는 것은 파이어니어 10호에서도 계획되었으나, 높은 방사능 때문에 관측 기기가 작동하지 않았다.[22]

보이저[편집]

보이저 1호가 촬영한 남극 부분의 합성 영상이다. 사진에서 이오에서 가장 높은 2개의 산들이 보이는데, 왼쪽 위에 유보이아 몬테스가 있고 중앙 쪽에 헤이무스 몬스가 있다.

보이저 1호보이저 2호는 1979년에 이오를 통과했고, 가지고 있던 고급 사진 장비는 더 선명한 사진을 얻어 냈다. 보이저 1호는 1979년 5월 5일 이오에서 20,600 km 떨어진 곳까지 접근했다.[25] 접근할 때 전송된 사진은 이상하였는데, 충돌구들의 모습이 잘 보이지 않았다.[26][27] 높은 해상도의 사진은 구멍들이 잘 보이지 않음을 통해 표면이 상대적으로 젊음을 밝혀 냈고, 산들은 에베레스트 산보다 더 컸으며, 화산이 용암을 분출하는 모습과 닮아 있었다.

이오와의 만남 후, 보이저호의 항법장치 엔지니어인 '린다 A 모라비토'는 표면에서 가스 기둥이 분출되는 모양이 찍혀 있는 사진을 발견했다.[28] 보이저 1호의 다른 사진에는, 표면에서 가스 기둥이 구 모양으로 분출되는 사진이 있었다. 이 사진은 이오가 활발하다는 것을 증명해 주었다.[29] 이 현상은 보이저 1호가 이오에 도착하기 전 논문에서 예측되었던 결과였다. 논문의 저자는 이오가 에우로파와 가니메데의 중력에 의해서 조석 가열 되어야 한다고 생각했다(자세한 과정에 대해서는 조석 가열 문서를 참조하십시오.).[30] 자료에 의하면 이오의 표면은 황과 이산화 황의 서리로 덮여 있다. These compounds also dominate its thin atmosphere and the torus of plasma centered on Io's orbit (also discovered by Voyager).[31][32][33]

Voyager 2 passed Io on 9 July 1979 at a distance of 1,130,000 km (702,000 mi). Though it did not approach nearly as close as Voyager 1, comparisons between images taken by the two spacecraft showed several surface changes that had occurred in the four months between the encounters. In addition, observations of Io as a crescent as Voyager 2 departed the Jovian system revealed that seven of the nine plumes observed in March were still active in July 1979, with only the volcano Pele shutting down between flybys.[34]

갈릴레오[편집]

Galileo image showing a dark spot (interrupting the red ring of short-chain sulfur allotropes deposited by Pele) produced by a major eruption at Pillan Patera in 1997

The Galileo spacecraft arrived at Jupiter in 1995 after a six-year journey from Earth to follow up on the discoveries of the two Voyager probes and ground-based observations taken in the intervening years. Io's location within one of Jupiter's most intense radiation belts precluded a prolonged close flyby, but Galileo did pass close by shortly before entering orbit for its two-year, primary mission studying the Jovian system. Although no images were taken during the close flyby on 7 December 1995, the encounter did yield significant results, such as the discovery of a large iron core, similar to that found in the rocky planets of the inner Solar System.[35]

Despite the lack of close-up imaging and mechanical problems that greatly restricted the amount of data returned, several significant discoveries were made during Galileo's primary mission. Galileo observed the effects of a major eruption at Pillan Patera and confirmed that volcanic eruptions are composed of silicate magmas with magnesium-rich mafic and ultramafic compositions.[36] Distant imaging of Io was acquired for almost every orbit during the primary mission, revealing large numbers of active volcanoes (both thermal emission from cooling magma on the surface and volcanic plumes), numerous mountains with widely varying morphologies, and several surface changes that had taken place both between the Voyager and Galileo eras and between Galileo orbits.[37]

The Galileo mission was twice extended, in 1997 and 2000. During these extended missions, the probe flew by Io three times in late 1999 and early 2000 and three times in late 2001 and early 2002. Observations during these encounters revealed the geologic processes occurring at Io's volcanoes and mountains, excluded the presence of a magnetic field, and demonstrated the extent of volcanic activity.[37] In December 2000, the Cassini spacecraft had a distant and brief encounter with the Jupiter system en route to Saturn, allowing for joint observations with Galileo. These observations revealed a new plume at Tvashtar Paterae and provided insights into Io's aurorae.[38]

Subsequent observations[편집]

Changes in surface features in the eight years between Galileo and New Horizons observations

Following Galileo's planned destruction in Jupiter's atmosphere in September 2003, new observations of Io's volcanism came from Earth-based telescopes. In particular, adaptive optics imaging from the Keck telescope in Hawaii and imaging from the Hubble telescope have allowed astronomers to monitor Io's active volcanoes.[39][40] This imaging has allowed scientists to monitor volcanic activity on Io, even without a spacecraft in the Jupiter system.

The New Horizons spacecraft, en route to Pluto and the Kuiper belt, flew by the Jupiter system and Io on 28 February 2007. During the encounter, numerous distant observations of Io were obtained. These included images of a large plume at Tvashtar, providing the first detailed observations of the largest class of Ionian volcanic plume since observations of Pele's plume in 1979.[41] New Horizons also captured images of a volcano near Girru Patera in the early stages of an eruption, and several volcanic eruptions that have occurred since Galileo.[41]

There are currently two forthcoming missions planned for the Jupiter system. Juno, launched on 5 August 2011, has limited imaging capabilities, but it could monitor Io's volcanic activity using its near-infrared spectrometer, JIRAM. The Jupiter Icy Moon Explorer (JUICE) is a planned European Space Agency mission to the Jupiter system that is intended to end up in Ganymede orbit.[42] JUICE has a launch scheduled for 2022, with arrival at Jupiter planned for January 2030.[43] JUICE will not fly by Io, but it will use its instruments, such as a narrow-angle camera, to monitor Io's volcanic activity and measure its surface composition during the two-year Jupiter-tour phase of the mission prior to Ganymede orbit insertion. The Io Volcano Observer was a proposal for a Discovery-class mission that would launch in 2015. It involved multiple flybys of Io while in orbit around Jupiter; however this mission was not selected for Phase A study by NASA, and remains just a concept.[44]

공전과 자전[편집]

이오, 에우로파, 가니메데의 라플라스 공명

Io orbits Jupiter at a distance of 421,700 km (262,000 mi) from Jupiter's center and 350,000 km (217,000 mi) from its cloudtops. It is the innermost of the Galilean satellites of Jupiter, its orbit lying between those of Thebe and Europa. Including Jupiter's inner satellites, Io is the fifth moon out from Jupiter. It takes 42.5 hours to complete one orbit (fast enough for its motion to be observed over a single night of observation). Io is in a 2:1 mean-motion orbital resonance with Europa and a 4:1 mean-motion orbital resonance with Ganymede, completing two orbits of Jupiter for every one orbit completed by Europa, and four orbits for every one completed by Ganymede. This resonance helps maintain Io's orbital eccentricity (0.0041), which in turn provides the primary heating source for its geologic activity (see the "Tidal heating" section for a more detailed explanation of the process).[30] Without this forced eccentricity, Io's orbit would circularize through tidal dissipation, leading to a geologically less active world.

Like the other Galilean satellites and the Moon, Io rotates synchronously with its orbital period, keeping one face nearly pointed toward Jupiter. This synchronicity provides the definition for Io's longitude system. Io's prime meridian intersects the equator at the sub-Jovian point. The side of Io that always faces Jupiter is known as the subjovian hemisphere, whereas the side that always faces away is known as the antijovian hemisphere. The side of Io that always faces in the direction that Io travels in its orbit is known as the leading hemisphere, whereas the side that always faces in the opposite direction is known as the trailing hemisphere.[45]

목성 자기권과의 상호작용[편집]

목성 자기권의 모습과 이오가 주는 영향(중앙 부근): 플라즈마 고리(빨간색), 중성 구름(노란색), 자속관(녹색), 자기력선(파란색)[46]

Io plays a significant role in shaping the Jovian magnetic field. The magnetosphere of Jupiter sweeps up gases and dust from Io's thin atmosphere at a rate of 1 tonne per second.[47] This material is mostly composed of ionized and atomic sulfur, oxygen and chlorine; atomic sodium and potassium; molecular sulfur dioxide and sulfur; and sodium chloride dust.[47][48] These materials originate from Io's volcanic activity, but the material that escapes to Jupiter's magnetic field and into interplanetary space comes directly from Io's atmosphere. These materials, depending on their ionized state and composition, end up in various neutral (non-ionized) clouds and radiation belts in Jupiter's magnetosphere and, in some cases, are eventually ejected from the Jovian system.

Surrounding Io (at a distance of up to six Io radii from its surface) is a cloud of neutral sulfur, oxygen, sodium, and potassium atoms. These particles originate in Io's upper atmosphere and are excited by collisions with ions in the plasma torus (discussed below) and by other processes into filling Io's Hill sphere, which is the region where Io's gravity is dominant over Jupiter's. Some of this material escapes Io's gravitational pull and goes into orbit around Jupiter. Over a 20-hour period, these particles spread out from Io to form a banana-shaped, neutral cloud that can reach as far as six Jovian radii from Io, either inside Io's orbit and ahead of it or outside Io's orbit and behind it.[47] The collision process that excites these particles also occasionally provides sodium ions in the plasma torus with an electron, removing those new "fast" neutrals from the torus. These particles retain their velocity (70 km/s, compared to the 17 km/s orbital velocity at Io), and are thus ejected in jets leading away from Io.[49]

Io orbits within a belt of intense radiation known as the Io plasma torus. The plasma in this doughnut-shaped ring of ionized sulfur, oxygen, sodium, and chlorine originates when neutral atoms in the "cloud" surrounding Io are ionized and carried along by the Jovian magnetosphere.[47] Unlike the particles in the neutral cloud, these particles co-rotate with Jupiter's magnetosphere, revolving around Jupiter at 74 km/s. Like the rest of Jupiter's magnetic field, the plasma torus is tilted with respect to Jupiter's equator (and Io's orbital plane), so that Io is at times below and at other times above the core of the plasma torus. As noted above, these ions' higher velocity and energy levels are partly responsible for the removal of neutral atoms and molecules from Io's atmosphere and more extended neutral cloud. The torus is composed of three sections: an outer, "warm" torus that resides just outside Io's orbit; a vertically extended region known as the "ribbon", composed of the neutral source region and cooling plasma, located at around Io's distance from Jupiter; and an inner, "cold" torus, composed of particles that are slowly spiraling in toward Jupiter.[47] After residing an average of 40 days in the torus, particles in the "warm" torus escape and are partially responsible for Jupiter's unusually large magnetosphere, their outward pressure inflating it from within.[50] Particles from Io, detected as variations in magnetospheric plasma, have been detected far into the long magnetotail by New Horizons. To study similar variations within the plasma torus, researchers measure the ultraviolet light it emits. Although such variations have not been definitively linked to variations in Io's volcanic activity (the ultimate source for material in the plasma torus), this link has been established in the neutral sodium cloud.[51]

During an encounter with Jupiter in 1992, the Ulysses spacecraft detected a stream of dust-sized particles being ejected from the Jupiter system.[52] The dust in these discrete streams travels away from Jupiter at speeds upwards of several hundred kilometres per second, has an average particle size of 10 μm, and consists primarily of sodium chloride.[48][53] Dust measurements by Galileo showed that these dust streams originate from Io, but exactly how these form, whether from Io's volcanic activity or material removed from the surface, is unknown.[54]

Jupiter's magnetic field lines, which Io crosses, couple Io's atmosphere and neutral cloud to Jupiter's polar upper atmosphere by generating an electric current known as the Io flux tube.[47] This current produces an auroral glow in Jupiter's polar regions known as the Io footprint, as well as aurorae in Io's atmosphere. Particles from this auroral interaction darken the Jovian polar regions at visible wavelengths. The location of Io and its auroral footprint with respect to the Earth and Jupiter has a strong influence on Jovian radio emissions from our vantage point: when Io is visible, radio signals from Jupiter increase considerably.[21][47] The Juno mission, planned for the next decade, should help to shed light on these processes. The Jovian magnetic field lines that do get past Io's ionosphere also induce an electric current, which in turn creates an induced magnetic field within Io's interior. Io's induced magnetic field is thought to be generated within a partially molten, silicate magma ocean 50 kilometers beneath Io's surface.[55] Similar induced fields were found at the other Galilean satellites by Galileo, generated within liquid water oceans in the interiors of those moons.

Structure[편집]

Io is slightly larger than the Moon. It has a mean radius of 1,821.3 km (1,131.7 mi) (about 5% greater than the Moon's) and a mass of 8.9319×10^22 kg (about 21% greater than the Moon's). It is a slight ellipsoid in shape, with its longest axis directed toward Jupiter. Among the Galilean satellites, in both mass and volume, Io ranks behind Ganymede and Callisto but ahead of Europa.

Interior[편집]

Model of the possible interior composition of Io with an inner iron or iron sulfide core (in gray), an outer silicate crust (in brown), and a partially molten silicate mantle in between (in orange)

Composed primarily of silicate rock and iron, Io is closer in bulk composition to the terrestrial planets than to other satellites in the outer Solar System, which are mostly composed of a mix of water ice and silicates. Io has a density of 3.5275 g/cm3, the highest of any moon in the Solar System; significantly higher than the other Galilean satellites and higher than the Moon.[56] Models based on the Voyager and Galileo measurements of Io's mass, radius, and quadrupole gravitational coefficients (numerical values related to how mass is distributed within an object) suggest that its interior is differentiated between a silicate-rich crust and mantle and an iron- or iron-sulfide-rich core.[35] Io's metallic core makes up approximately 20% of its mass.[57] Depending on the amount of sulfur in the core, the core has a radius between 틀:단위 변환/and(-) if it is composed almost entirely of iron, or between 틀:단위 변환/and(-) for a core consisting of a mix of iron and sulfur. Galileo's magnetometer failed to detect an internal, intrinsic magnetic field at Io, suggesting that the core is not convecting.[58]

Modeling of Io's interior composition suggests that the mantle is composed of at least 75% of the magnesium-rich mineral forsterite, and has a bulk composition similar to that of L-chondrite and LL-chondrite meteorites, with higher iron content (compared to silicon) than the Moon or Earth, but lower than Mars.[59][60] To support the heat flow observed on Io, 10–20% of Io's mantle may be molten, though regions where high-temperature volcanism has been observed may have higher melt fractions.[61] However, re-analysis of Galileo magnetometer data in 2009 revealed the presence of an induced magnetic field at Io, requiring a magma ocean 50 km (31 mi) below the surface.[55] Further analysis published in 2011 provided direct evidence of such an ocean.[62] This layer is estimated to be 50 km thick and to make up about 10% of Io's mantle. It is estimated that the temperature in the magma ocean reaches 1,200 °C. It is not known if the 10–20% partial melting percentage for Io's mantle is consistent with the requirement for a significant amount of molten silicates in this possible magma ocean.[63] The lithosphere of Io, composed of basalt and sulfur deposited by Io's extensive volcanism, is at least 12 km (7 mi) thick, but is likely to be less than 40 km (25 mi) thick.[57][64]

Tidal heating[편집]

Unlike Earth and the Moon, Io's main source of internal heat comes from tidal dissipation rather than radioactive isotope decay, the result of Io's orbital resonance with Europa and Ganymede.[30] Such heating is dependent on Io's distance from Jupiter, its orbital eccentricity, the composition of its interior, and its physical state.[61] Its Laplace resonance with Europa and Ganymede maintains Io's eccentricity and prevents tidal dissipation within Io from circularizing its orbit. The resonant orbit also helps to maintain Io's distance from Jupiter; otherwise tides raised on Jupiter would cause Io to slowly spiral outward from its parent planet.[65] The vertical differences in Io's tidal bulge, between the times Io is at periapsis and apoapsis in its orbit, could be as much as 100 m (330 ft). The friction or tidal dissipation produced in Io's interior due to this varying tidal pull, which, without the resonant orbit, would have gone into circularizing Io's orbit instead, creates significant tidal heating within Io's interior, melting a significant amount of Io's mantle and core. The amount of energy produced is up to 200 times greater than that produced solely from radioactive decay.[1] This heat is released in the form of volcanic activity, generating its observed high heat flow (global total: 0.6 to 1.6×1014 W).[61] Models of its orbit suggest that the amount of tidal heating within Io changes with time; however, the current amount of tidal dissipation is consistent with the observed heat flow.[61][66] Models of tidal heating and convection have not found consistent planetary viscosity profiles that simultaneously match tidal energy dissipation and mantle convection of heat to the surface.[66][67]

Surface[편집]

Io's surface map
Rotating image of Io's surface; the large red ring is around the volcano Pele.

Based on their experience with the ancient surfaces of the Moon, Mars, and Mercury, scientists expected to see numerous impact craters in Voyager 1's first images of Io. The density of impact craters across Io's surface would have given clues to Io's age. However, they were surprised to discover that the surface was almost completely lacking in impact craters, but was instead covered in smooth plains dotted with tall mountains, pits of various shapes and sizes, and volcanic lava flows.[26] Compared to most worlds observed to that point, Io's surface was covered in a variety of colorful materials (leading Io to be compared to a rotten orange or to pizza) from various sulfurous compounds.[68] The lack of impact craters indicated that Io's surface is geologically young, like the terrestrial surface; volcanic materials continuously bury craters as they are produced. This result was spectacularly confirmed as at least nine active volcanoes were observed by Voyager 1.[29]

Surface composition[편집]

Io's colorful appearance is the result of various materials produced by its extensive volcanism. These materials include silicates (such as orthopyroxene), sulfur, and sulfur dioxide.[69] Sulfur dioxide frost is ubiquitous across the surface of Io, forming large regions covered in white or grey materials. Sulfur is also seen in many places across Io, forming yellow to yellow-green regions. Sulfur deposited in the mid-latitude and polar regions is often radiation damaged, breaking up normally stable cyclic 8-chain sulfur. This radiation damage produces Io's red-brown polar regions.[8]

Geological map of Io

Explosive volcanism, often taking the form of umbrella-shaped plumes, paints the surface with sulfurous and silicate materials. Plume deposits on Io are often colored red or white depending on the amount of sulfur and sulfur dioxide in the plume. Generally, plumes formed at volcanic vents from degassing lava contain a greater amount of S2, producing a red "fan" deposit, or in extreme cases, large (often reaching beyond 틀:단위 변환/LoffAonDorSoff from the central vent) red rings.[70] A prominent example of a red-ring plume deposit is located at Pele. These red deposits consist primarily of sulfur (generally 3- and 4-chain molecular sulfur), sulfur dioxide, and perhaps Cl2SO2.[69] Plumes formed at the margins of silicate lava flows (through the interaction of lava and pre-existing deposits of sulfur and sulfur dioxide) produce white or gray deposits.

Compositional mapping and Io's high density suggest that Io contains little to no water, though small pockets of water ice or hydrated minerals have been tentatively identified, most notably on the northwest flank of the mountain Gish Bar Mons.[71] This lack of water is likely due to Jupiter being hot enough early in the evolution of the Solar System to drive off volatile materials like water in the vicinity of Io, but not hot enough to do so farther out.

화산[편집]

Active lava flows in volcanic region Tvashtar Paterae (blank region represents saturated areas in the original data). Images taken by Galileo in November 1999 and February 2000.

The tidal heating produced by Io's forced orbital eccentricity has led it to become one of the most volcanically active worlds in the Solar System, with hundreds of volcanic centres and extensive lava flows. During a major eruption, lava flows tens or even hundreds of kilometres long can be produced, consisting mostly of basalt silicate lavas with either mafic or ultramafic (magnesium-rich) compositions. As a by-product of this activity, sulfur, sulfur dioxide gas and silicate pyroclastic material (like ash) are blown up to 200 km (120 mi) into space, producing large, umbrella-shaped plumes, painting the surrounding terrain in red, black, and white, and providing material for Io's patchy atmosphere and Jupiter's extensive magnetosphere.

Io's surface is dotted with volcanic depressions known as paterae.[72] Paterae generally have flat floors bounded by steep walls. These features resemble terrestrial calderas, but it is unknown if they are produced through collapse over an emptied lava chamber like their terrestrial cousins. One hypothesis suggests that these features are produced through the exhumation of volcanic sills, and the overlying material is either blasted out or integrated into the sill.[73] Unlike similar features on Earth and Mars, these depressions generally do not lie at the peak of shield volcanoes and are normally larger, with an average diameter of 41 km (25 mi), the largest being Loki Patera at 202 km (126 mi).[72] Whatever the formation mechanism, the morphology and distribution of many paterae suggest that these features are structurally controlled, with at least half bounded by faults or mountains.[72] These features are often the site of volcanic eruptions, either from lava flows spreading across the floors of the paterae, as at an eruption at Gish Bar Patera in 2001, or in the form of a lava lake.[2][74] Lava lakes on Io either have a continuously overturning lava crust, such as at Pele, or an episodically overturning crust, such as at Loki.[75][76]

Five-image sequence of New Horizons images showing Io's volcano Tvashtar spewing material 330 km above its surface.

Lava flows represent another major volcanic terrain on Io. Magma erupts onto the surface from vents on the floor of paterae or on the plains from fissures, producing inflated, compound lava flows similar to those seen at Kilauea in Hawaii. Images from the Galileo spacecraft revealed that many of Io's major lava flows, like those at Prometheus and Amirani, are produced by the build-up of small breakouts of lava flows on top of older flows.[77] Larger outbreaks of lava have also been observed on Io. For example, the leading edge of the Prometheus flow moved 75 to 95 km (틀:단위 변환/Dual/srnd) between Voyager in 1979 and the first Galileo observations in 1996. A major eruption in 1997 produced more than 3,500 km2 (1,400 sq mi) of fresh lava and flooded the floor of the adjacent Pillan Patera.[36]

Analysis of the Voyager images led scientists to believe that these flows were composed mostly of various compounds of molten sulfur. However, subsequent Earth-based infrared studies and measurements from the Galileo spacecraft indicate that these flows are composed of basaltic lava with mafic to ultramafic compositions. This hypothesis is based on temperature measurements of Io's "hotspots", or thermal-emission locations, which suggest temperatures of at least 1300 K and some as high as 1600 K.[78] Initial estimates suggesting eruption temperatures approaching 2000 K[36] have since proven to be overestimates because the wrong thermal models were used to model the temperatures.[78]

The discovery of plumes at the volcanoes Pele and Loki were the first sign that Io is geologically active.[28] Generally, these plumes are formed when volatiles like sulfur and sulfur dioxide are ejected skyward from Io's volcanoes at speeds reaching 1 km/s (0.6 mps), creating umbrella-shaped clouds of gas and dust. Additional material that might be found in these volcanic plumes include sodium, potassium, and chlorine.[79][80] These plumes appear to be formed in one of two ways.[81] Io's largest plumes are created when dissolved sulfur and sulfur dioxide gas are released from erupting magma at volcanic vents or lava lakes, often dragging silicate pyroclastic material with them. These plumes form red (from the short-chain sulfur) and black (from the silicate pyroclastics) deposits on the surface. Plumes formed in this manner are among the largest observed at Io, forming red rings more than 1,000 km (620 mi) in diameter. Examples of this plume type include Pele, Tvashtar, and Dazhbog. Another type of plume is produced when encroaching lava flows vaporize underlying sulfur dioxide frost, sending the sulfur skyward. This type of plume often forms bright circular deposits consisting of sulfur dioxide. These plumes are often less than 100 km (62 mi) tall, and are among the most long-lived plumes on Io. Examples include Prometheus, Amirani, and Masubi.

Mountains[편집]

Galileo greyscale image of Tohil Mons, a 5.4 km tall mountain

Io has 100 to 150 mountains. These structures average 6 km (4 mi) in height and reach a maximum of 17.5 ± 1.5 km (193–199 mph) at South Boösaule Montes.[3] Mountains often appear as large (the average mountain is 틀:단위 변환/LoffAonDorSoff long), isolated structures with no apparent global tectonic patterns outlined, as is the case on Earth.[3] To support the tremendous topography observed at these mountains requires compositions consisting mostly of silicate rock, as opposed to sulfur.[82]

Despite the extensive volcanism that gives Io its distinctive appearance, nearly all its mountains are tectonic structures, and are not produced by volcanoes. Instead, most Ionian mountains form as the result of compressive stresses on the base of the lithosphere, which uplift and often tilt chunks of Io's crust through thrust faulting.[83] The compressive stresses leading to mountain formation are the result of subsidence from the continuous burial of volcanic materials.[83] The global distribution of mountains appears to be opposite that of volcanic structures; mountains dominate areas with fewer volcanoes and vice versa.[84] This suggests large-scale regions in Io's lithosphere where compression (supportive of mountain formation) and extension (supportive of patera formation) dominate.[85] Locally, however, mountains and paterae often abut one another, suggesting that magma often exploits faults formed during mountain formation to reach the surface.[72]

Mountains on Io (generally, structures rising above the surrounding plains) have a variety of morphologies. Plateaus are most common.[3] These structures resemble large, flat-topped mesas with rugged surfaces. Other mountains appear to be tilted crustal blocks, with a shallow slope from the formerly flat surface and a steep slope consisting of formerly sub-surface materials uplifted by compressive stresses. Both types of mountains often have steep scarps along one or more margins. Only a handful of mountains on Io appear to have a volcanic origin. These mountains resemble small shield volcanoes, with steep slopes (6–7°) near a small, central caldera and shallow slopes along their margins.[86] These volcanic mountains are often smaller than the average mountain on Io, averaging only 1 to 2 km (193–199 mph) in height and 40 to 60 km (틀:단위 변환/Dual/srnd) wide. Other shield volcanoes with much shallower slopes are inferred from the morphology of several of Io's volcanoes, where thin flows radiate out from a central patera, such as at Ra Patera.[86]

Nearly all mountains appear to be in some stage of degradation. Large landslide deposits are common at the base of Ionian mountains, suggesting that mass wasting is the primary form of degradation. Scalloped margins are common among Io's mesas and plateaus, the result of sulfur dioxide sapping from Io's crust, producing zones of weakness along mountain margins.[87]

대기[편집]

이오의 상부 대기권에서 극광이 빛나는 모습이다. 색이 다른 것은 대기가 서로 다른 물질로 이루어져 있기 때문이다(녹색은 나트륨에서 나는 빛이고, 붉은 색은 산소에서 나오고, 파란색은 화산 활동으로 인한 이산화 황으로 인해 나오는 빛이다). 이오가 일식 상태에 있는 동안 사진을 촬영했다.

Io has an extremely thin atmosphere consisting mainly of sulfur dioxide (SO2), with minor constituents including sulfur monoxide (SO), sodium chloride (NaCl), and atomic sulfur and oxygen.[88] The atmosphere has significant variations in density and temperature with time of day, latitude, volcanic activity, and surface frost abundance. The maximum atmospheric pressure on Io ranges from 3.3틀:Esp to 3틀:Esp pascals (Pa) or 0.3 to 3 nbar, spatially seen on Io's anti-Jupiter hemisphere and along the equator, and temporally in the early afternoon when the temperature of surface frost peaks.[88][89][90] Localized peaks at volcanic plumes have also been seen, with pressures of 5틀:Esp to 40 틀:Esp Pa (5 to 40 nbar).[32] Io's atmospheric pressure is lowest on Io's night side, where the pressure dips to 0.1틀:Esp to 1틀:Esp Pa (0.0001 to 0.001 nbar).[88][89] Io's atmospheric temperature ranges from the temperature of the surface at low altitudes, where sulfur dioxide is in vapor pressure equilibrium with frost on the surface, to 1800 K at higher altitudes where the thinner atmospheric density permits heating from plasma in the Io plasma torus and from Joule heating from the Io flux tube.[88][89] The low pressure limits the atmosphere's effect on the surface, except for temporarily redistributing sulfur dioxide from frost-rich to frost-poor areas, and to expand the size of plume deposit rings when plume material re-enters the thicker dayside atmosphere.[88][89] The thin Ionian atmosphere also means any future landing probes sent to investigate Io will not need to be encased in an aeroshell-style heatshield, but instead will require retrorockets for a soft landing. The thin atmosphere also necessitates a rugged lander capable of enduring the strong Jovian radiation, which a thicker atmosphere would attenuate.

Gas in Io's atmosphere is stripped by Jupiter's magnetosphere, escaping to either the neutral cloud that surrounds Io, or the Io plasma torus, a ring of ionized particles that shares Io's orbit but co-rotates with the magnetosphere of Jupiter. Approximately one ton of material is removed from the atmosphere every second through this process so that it must be constantly replenished.[47] The most dramatic source of SO2 are volcanic plumes, which pump 104 kg of sulfur dioxide per second into Io's atmosphere on average, though most of this condenses back onto the surface.[91] Much of the sulfur dioxide in Io's atmosphere sustained by sunlight-driven sublimation of SO2 frozen on the surface.[92] The day-side atmosphere is largely confined to within 40° of the equator, where the surface is warmest and most active volcanic plumes reside.[93] A sublimation-driven atmosphere is also consistent with observations that Io's atmosphere is densest over the anti-Jupiter hemisphere, where SO2 frost is most abundant, and is densest when Io is closer to the Sun.[88][92][94] However, some contributions from volcanic plumes are required as the highest observed densities have been seen near volcanic vents.[88] Because the density of sulfur dioxide in the atmosphere is tied directly to surface temperature, Io's atmosphere partially collapses at night or when Io is in the shadow of Jupiter. The collapse during eclipse is limited somewhat by the formation of a diffusion layer of sulfur monoxide in the lowest portion of the atmosphere, but the atmosphere pressure of Io's nightside atmosphere is two to four orders of magnitude less than at its peak just past noon.[89][95] The minor constituents of Io's atmosphere, such as NaCl, SO, O, and S derive either from: direct volcanic outgassing; photodissociation, or chemical breakdown caused by solar ultraviolet radiation, from SO2; or the sputtering of surface deposits by charged particles from Jupiter's magnetosphere.[92]

High-resolution images of Io acquired when Io is experiencing an eclipse reveal an aurora-like glow.[80] As on Earth, this is due to particle radiation hitting the atmosphere, though in this case the charged particles come from Jupiter's magnetic field rather than the solar wind. Aurorae usually occur near the magnetic poles of planets, but Io's are brightest near its equator. Io lacks an intrinsic magnetic field of its own; therefore, electrons traveling along Jupiter's magnetic field near Io directly impact Io's atmosphere. More electrons collide with its atmosphere, producing the brightest aurora, where the field lines are tangent to Io (i.e. near the equator), because the column of gas they pass through is longest there. Aurorae associated with these tangent points on Io are observed to rock with the changing orientation of Jupiter's tilted magnetic dipole.[96] Fainter aurora from oxygen atoms along the limb of Io (the red glows in the image at right), and sodium atoms on Io's night-side (the green glows in the same image) have also been observed.[80]

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