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Portal:Solar System/Selected article/2
Picture of the Sun in extreme ultraviolet showing its turbulent surface.
The
Sun is the
star at the center of the
Solar System. It is a massive, nearly perfect
sphere of hot
plasma, heated to
incandescence by
nuclear fusion reactions in its core, radiating the energy from its
surface mainly as
visible light and
infrared radiation with 10% at
ultraviolet energies. It is by far the most important source of energy for
life on
Earth. The Sun has been an
object of veneration in many cultures. It has been a central subject for astronomical research since
antiquity.
The Sun orbits the Galactic Center at a distance of 24,000 to 28,000 light-years. From Earth, it is 1 astronomical unit (1.496×108 km) or about 8 light-minutes away. Its diameter is about 1,391,400 km (864,600 mi), 109 times that of Earth. Its mass is about 330,000 times that of Earth, making up about 99.86% of the total mass of the Solar System. Roughly three-quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron. (Full article...)
Portal:Solar System/Selected article/3
Mercury in color, taken by MESSENGER.
Mercury is the first
planet from the
Sun and the smallest in the
Solar System. In English, it is named after the
ancient Roman god
Mercurius (
Mercury), god of commerce and communication, and the messenger of the gods. Mercury is classified as a
terrestrial planet, with roughly the same surface
gravity as
Mars. The surface of Mercury is heavily
cratered, as a result of countless
impact events that have accumulated over billions of years. Its largest crater,
Caloris Planitia, has a diameter of 1,550 km (960 mi), which is about one-third the diameter of the planet (4,880 km or 3,030 mi). Similarly to the
Earth's
Moon, Mercury's surface displays an expansive
rupes system generated from
thrust faults and bright
ray systems formed by
impact event remnants.
Mercury's sidereal year (88.0 Earth days) and sidereal day (58.65 Earth days) are in a 3:2 ratio. This relationship is called spin–orbit resonance, and sidereal here means "relative to the stars". Consequently, one solar day (sunrise to sunrise) on Mercury lasts for around 176 Earth days: twice the planet's sidereal year. This means that one side of Mercury will remain in sunlight for one Mercurian year of 88 Earth days; while during the next orbit, that side will be in darkness all the time until the next sunrise after another 88 Earth days.
Combined with its high orbital eccentricity, the planet's surface has widely varying sunlight intensity and temperature, with the equatorial regions ranging from −170 °C (−270 °F) at night to 420 °C (790 °F) during sunlight. Due to the very small axial tilt, the planet's poles are permanently shadowed. This strongly suggests that water ice could be present in the craters. Above the planet's surface is an extremely tenuous exosphere and a faint magnetic field that is strong enough to deflect solar winds. Mercury has no natural satellite. (Full article...)
Portal:Solar System/Selected article/4
Venus
Venus is the second
planet from the
Sun. It is a
terrestrial planet and is the closest in
mass and size to its orbital neighbour
Earth. Venus has
by far the densest atmosphere of the terrestrial planets, composed mostly of
carbon dioxide with a thick, global
sulfuric acid cloud cover. At the surface it has a mean temperature of 737 K (464 °C; 867 °F) and a
pressure of 92 times that of Earth's at sea level. These extreme conditions compress carbon dioxide into a
supercritical state at Venus's surface.
Internally, Venus has a core, mantle, and crust. Venus lacks an internal dynamo, and its weakly induced magnetosphere is caused by atmospheric interactions with the solar wind. Internal heat escapes through active volcanism, resulting in resurfacing instead of plate tectonics. Venus is one of two planets in the Solar System, the other being Mercury, that have no moons. Conditions perhaps favourable for life on Venus have been identified at its cloud layers. Venus may have had liquid surface water early in its history with a habitable environment, before a runaway greenhouse effect evaporated any water and turned Venus into its present state. (Full article...)
Portal:Solar System/Selected article/5
The Earth seen from Apollo 17.
Earth is the third
planet from the
Sun and the only
astronomical object known to
harbor life. This is enabled by Earth being an
ocean world, the only one in the
Solar System sustaining liquid
surface water. Almost all of Earth's water is contained in its global ocean, covering
70.8% of
Earth's crust. The remaining 29.2% of Earth's crust is land, most of which is located in the form of
continental landmasses within Earth's
land hemisphere. Most of Earth's land is somewhat
humid and covered by
vegetation, while large
sheets of ice at
Earth's polar deserts retain more water than Earth's
groundwater, lakes, rivers and
atmospheric water combined. Earth's crust consists of slowly moving
tectonic plates, which interact to produce mountain ranges,
volcanoes, and earthquakes.
Earth has a liquid outer core that generates a
magnetosphere capable of deflecting most of the destructive
solar winds and
cosmic radiation.
Earth has a dynamic atmosphere, which sustains Earth's surface conditions and protects it from most meteoroids and UV-light at entry. It has a composition of primarily nitrogen and oxygen. Water vapor is widely present in the atmosphere, forming clouds that cover most of the planet. The water vapor acts as a greenhouse gas and, together with other greenhouse gases in the atmosphere, particularly carbon dioxide (CO2), creates the conditions for both liquid surface water and water vapor to persist via the capturing of energy from the Sun's light. This process maintains the current average surface temperature of 14.76 °C (58.57 °F), at which water is liquid under normal atmospheric pressure. Differences in the amount of captured energy between geographic regions (as with the equatorial region receiving more sunlight than the polar regions) drive atmospheric and ocean currents, producing a global climate system with different climate regions, and a range of weather phenomena such as precipitation, allowing components such as nitrogen to cycle. (Full article...)
Portal:Solar System/Selected article/6
Full moon as seen from Earth's northern hemisphere.
The
Moon is
Earth's only
natural satellite. It
orbits at an
average distance of 384,400 km (238,900 mi), about 30 times the diameter of Earth.
Tidal forces between Earth and the Moon have
synchronized the Moon's orbital period (
lunar month) with its rotation period (
lunar day) at 29.5 Earth days, causing the
same side of the Moon to always face Earth. The Moon's gravitational pull—and, to a lesser extent, the
Sun's—are the main drivers of Earth's
tides.
In geophysical terms, the Moon is a planetary-mass object or satellite planet. Its mass is 1.2% that of the Earth, and its diameter is 3,474 km (2,159 mi), roughly one-quarter of Earth's (about as wide as the United States from coast to coast). Within the Solar System, it is the largest and most massive satellite in relation to its parent planet, the fifth largest and most massive moon overall, and larger and more massive than all known dwarf planets. Its surface gravity is about one sixth of Earth's, about half of that of Mars, and the second highest among all Solar System moons, after Jupiter's moon Io. The body of the Moon is differentiated and terrestrial, with no significant hydrosphere, atmosphere, or magnetic field. It formed 4.51 billion years ago, not long after Earth's formation, out of the debris from a giant impact between Earth and a hypothesized Mars-sized body called Theia. (Full article...)
Portal:Solar System/Selected article/7
Mars imaged by the Hubble Space Telescope in 2003.
Mars is the fourth
planet from the
Sun. The
surface of Mars is orange-red because it is covered in
iron(III) oxide dust, giving it the nickname "
the Red Planet". Mars is among the
brightest objects in Earth's sky, and its high-contrast
albedo features have made it a common subject for
telescope viewing. It is classified as a
terrestrial planet and is the second smallest of the
Solar System's planets with a diameter of 6,779 km (4,212 mi). In terms of orbital motion, a Martian solar day (
sol) is equal to 24.5 hours, and a
Martian solar year is equal to 1.88 Earth years (687 Earth days). Mars has
two natural satellites that are small and irregular in shape:
Phobos and
Deimos.
The relatively flat plains in northern parts of Mars strongly contrast with the cratered terrain in southern highlands – this terrain observation is known as the Martian dichotomy. Mars hosts many enormous extinct volcanoes (the tallest is Olympus Mons, 21.9 km or 13.6 mi tall) and one of the largest canyons in the Solar System (Valles Marineris, 4,000 km or 2,500 mi long). Geologically, the planet is fairly active with marsquakes trembling underneath the ground, dust devils sweeping across the landscape, and cirrus clouds. Carbon dioxide is substantially present in Mars's polar ice caps and thin atmosphere. During a year, there are large surface temperature swings on the surface between −78.5 °C (−109.3 °F) to 5.7 °C (42.3 °F) similar to Earth's seasons, as both planets have significant axial tilt. (Full article...)
Portal:Solar System/Selected article/10
Jupiter as seen by the New Horizons spacecraft during its gravity assist in 2007.
Jupiter is the fifth
planet from the
Sun and the
largest in the Solar System. It is a
gas giant with a
mass more than 2.5 times that of all the other planets in the
Solar System combined and slightly less than one-thousandth the mass of the Sun. Its diameter is eleven times that of
Earth, and a tenth that of the Sun. Jupiter orbits the Sun at a distance of 5.20
AU (778.5
Gm), with an
orbital period of
11.86 years. It is the
third brightest natural object in the Earth's
night sky, after the
Moon and
Venus, and has been observed since
prehistoric times. Its name derives from that of
Jupiter, the chief deity of
ancient Roman religion.
Jupiter was the first of the Sun's planets to form, and its inward migration during the primordial phase of the Solar System affected much of the formation history of the other planets. Hydrogen constitutes 90% of Jupiter's volume, followed by helium, which forms 25% of its mass and 10% of its volume. The ongoing contraction of Jupiter's interior generates more heat than the planet receives from the Sun. Its internal structure is believed to consist of an outer mantle of fluid metallic hydrogen and a diffuse inner core of denser material. Because of its rapid rate of rotation, one turn in ten hours, Jupiter is an oblate spheroid; it has a slight but noticeable bulge around the equator. The outer atmosphere is divided into a series of latitudinal bands, with turbulence and storms along their interacting boundaries; the most obvious result of this is the Great Red Spot, a giant storm that has been recorded since 1831. (Full article...)
Portal:Solar System/Selected article/11
A photo of Uranus taken by Voyager 2.
Uranus is the seventh
planet from the
Sun. It is a gaseous
cyan-coloured
ice giant. Most of the planet is made of
water,
ammonia, and
methane in a
supercritical phase of matter, which astronomy calls "ice" or
volatiles.
The planet's atmosphere has a complex layered
cloud structure and has the lowest minimum temperature (49 K (−224 °C; −371 °F)) of all the
Solar System's planets. It has a marked
axial tilt of 82.23° with a
retrograde rotation period of 17 hours and 14 minutes. This means that in an 84-Earth-year
orbital period around the Sun, its poles get around 42 years of continuous sunlight, followed by 42 years of continuous darkness.
Uranus has the third-largest diameter and fourth-largest mass among the Solar System's planets. Based on current models, inside its volatile mantle layer is a rocky core, and surrounding it is a thick hydrogen and helium atmosphere. Trace amounts of hydrocarbons (thought to be produced via hydrolysis) and carbon monoxide along with carbon dioxide (thought to have been originated from comets) have been detected in the upper atmosphere. There are many unexplained climate phenomena in Uranus's atmosphere, such as its peak wind speed of 900 km/h (560 mph), variations in its polar cap, and its erratic cloud formation. The planet also has very low internal heat compared to other giant planets, the cause of which remains unclear. (Full article...)
Portal:Solar System/Selected article/12
Rings of Jupiter.
The
rings of Jupiter are a system of faint
planetary rings. The
Jovian rings were the third ring system to be discovered in the Solar System, after those of
Saturn and
Uranus. The main ring was discovered in 1979 by the
Voyager 1 space probe and the system was more thoroughly investigated in the 1990s by the
Galileo orbiter. The main ring has also been observed by the
Hubble Space Telescope and from Earth for several years. Ground-based observation of the rings requires the largest available telescopes.
The Jovian ring system is faint and consists mainly of dust. It has four main components: a thick inner torus of particles known as the "halo ring"; a relatively bright, exceptionally thin "main ring"; and two wide, thick and faint outer "gossamer rings", named for the moons of whose material they are composed: Amalthea and Thebe. (Full article...)
Portal:Solar System/Selected article/13
Io (moon).
Io (pronounced /ˈaɪoʊ/ eye'-oe, or as Greek Ῑώ) is the innermost of the four
Galilean moons of
Jupiter and, with a diameter of 3,642 kilometers, the fourth largest moon in the
Solar System. It was discovered in 1610 by
Galileo Galilei, along with the other Galilean satellites. This discovery furthered the adoption of the
Copernican model of the Solar System and the development of Kepler's laws of motion. Unlike most satellites in the outer Solar System (which have a thick coating of ice), Io is primarily composed of silicate rock surrounding a molten iron or
iron sulfide core. Io has one of the most geologically active surfaces in the solar system, with over 400 active volcanoes. This extreme geologic activity is the result of
tidal heating from friction generated within Io's interior by Jupiter's varying pull. Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km (310 mi). Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of the moon's silicate crust. Some of these peaks are taller than Earth's
Mount Everest. Most of Io's surface is characterized by extensive plains coated with
sulfur and
sulfur dioxide frost. (
Full article...)
Portal:Solar System/Selected article/14
Europa (moon).
Europa is the sixth-nearest and fourth-largest
natural satellite of the
planet Jupiter. Europa was discovered in 1610 by
Galileo Galilei (and independently by
Simon Marius), and named for a mythical
Phoenician noblewoman,
Europa, who was courted by
Zeus. It is the smallest of the four
Galilean moons - slightly smaller than
Earth's
Moon and is the
sixth-largest moon in the
Solar System. Europa has a tenuous atmosphere composed primarily of
molecular oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This young surface is striated by cracks and streaks, while craters are relatively infrequent. The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably serve as an abode for
extraterrestrial life. Although by 2007 only flyby missions have visited the moon, the intriguing character of Europa has led to several ambitious exploration proposals. The
Galileo mission provided the bulk of current data on Europa, while the
Jupiter Icy Moons Orbiter, canceled in 2005, would have targeted Europa,
Ganymede and
Callisto. Conjecture on extraterrestrial life has ensured a high profile for the moon and has led to steady lobbying for future missions. (
Full article...)
Portal:Solar System/Selected article/15
Formation and evolution of the Solar System.
The
formation and evolution of the Solar System began 4.6 billion years ago with the
gravitational collapse of a small part of a giant
molecular cloud. Most of the collapsing mass collected in the centre, forming the
Sun, while the rest flattened into a
protoplanetary disc out of which the
planets,
moons,
asteroids, and other
small Solar System bodies formed. This widely accepted model, known as the
nebular hypothesis, was first developed in the 18th century by
Emanuel Swedenborg,
Immanuel Kant, and
Pierre-Simon Laplace. Beginning with the initial formation, the Solar System has evolved considerably. Many moons formed from circling discs of gas and dust around their parent planets, while many other moons are believed to have been captured or (in the case of the
Earth's
Moon) to have resulted from a
giant collision. Collisions between bodies have occurred continuously up to the present day and are central to the evolution of the system. The planets' positions often shifted outward or inward, and planets have switched places. This
planetary migration is now believed to be responsible for much of the Solar System's early evolution. Just as the Sun and planets were born, they will eventually die. In roughly 5 billion years, the Sun will cool and bloat outward to many times its current diameter (becoming a
red giant) before casting off its outer layers as a
planetary nebula and leaving behind a stellar corpse known as a
white dwarf. (
Full article...)
Portal:Solar System/Selected article/16
Makemake as seen by the Hubble Space Telescope.
Makemake is the third-largest known
dwarf planet in the
Solar System and one of the two largest
Kuiper belt objects (KBO) in the
classical KBO population. Its diameter is roughly three-quarters that of
Pluto. Makemake has no known satellites, which makes it unique among the largest KBOs. Its extremely low average temperature (about 30
K) means its surface is covered with
methane,
ethane and possibly nitrogen ices. Initially known as 2005 FY
9 (and later given the
minor planet number 136472), it was discovered on March 31, 2005, by a team led by
Michael Brown, and announced on July 29, 2005. On June 11, 2008, the
IAU included Makemake in its list of potential candidates to be given "
plutoid" status, a term for dwarf planets beyond the orbit of
Neptune that would place the object alongside Pluto and
Eris. Makemake was formally classified as a plutoid in July 2008. (
Full article...)
Portal:Solar System/Selected article/17
Neptune, as photographed by Voyager 2.
Neptune is the eighth and farthest
planet from the
Sun in the
Solar System. It is the fourth largest planet by diameter, and the third largest by mass. The planet is named after the
Roman god of the sea. Discovered on September 23, 1846, Neptune was the first planet found by mathematical prediction rather than regular observation. Unexpected changes in the orbit of Uranus led astronomers to deduce the
gravitational perturbation of an unknown planet. Neptune was found within a degree of the predicted position. The moon
Triton was found shortly thereafter, but none of the planet's other 12
moons were discovered before the 20th century. Neptune has been visited by only one spacecraft,
Voyager 2, which flew by the planet on August 25, 1989. Neptune is similar in composition to
Uranus, and both have different compositions from those of the larger
gas giants Jupiter and
Saturn. Traces of methane in the
atmosphere, in part, account for the planet's
blue appearance. At the time of the 1989
Voyager 2 flyby, its southern hemisphere possessed a
Great Dark Spot comparable to the
Great Red Spot on
Jupiter. Neptune has a faint and fragmented
ring system, which may have been detected during the 1960s but was only indisputably confirmed by
Voyager 2. (
Full article...)
Portal:Solar System/Selected article/18
Voyager 2 picture of Uranus' rings
The
rings of Uranus were discovered on March 10, 1977, by
James L. Elliot, Edward W. Dunham, and
Douglas J. Mink. Two additional rings were discovered in 1986 by the
Voyager 2 spacecraft, and two outer rings were found in 2003–2005 by the
Hubble Space Telescope. A number of faint dust bands and incomplete arcs may exist between the main rings. The rings are extremely dark—the
Bond albedo of the rings' particles does not exceed 2%. They are likely composed of water ice with the addition of some dark radiation-processed
organics. The majority of Uranus's rings are opaque and only a few kilometres wide. The ring system contains little dust overall; it consists mostly of large bodies 0.2–20 m in diameter. The relative lack of dust in the ring system is due to
aerodynamic drag from the extended Uranian
exosphere—
corona. The rings of Uranus are thought to be relatively young, at not more than 600 million years. The mechanism that confines the narrow rings is not well understood. The Uranian ring system probably originated from the collisional fragmentation of a number of moons that once existed around the planet. After colliding, the moons broke up into numerous particles, which survived as narrow and optically dense rings only in strictly confined zones of maximum stability. (
Full article...)
Portal:Solar System/Selected article/19
Io, with two plumes erupting from its surface.
Volcanism on Io, a moon of
Jupiter, produces
lava flows,
volcanic pits, and plumes of
sulfur and
sulfur dioxide hundreds of kilometres high. This volcanic activity was discovered in 1979 by
Voyager 1 imaging scientists. Observations of
Io by passing spacecraft and Earth-based astronomers have revealed more than 150 active volcanoes. Io's volcanism makes the satellite one of only four known volcanically active worlds in the solar system. First predicted shortly before the
Voyager 1 flyby, the heat source for Io's volcanism comes from
tidal heating produced by Io's forced
orbital eccentricity. Io's volcanism has led to the formation of hundreds of volcanic centres and extensive lava formations, making the moon the most volcanically active body in the solar system. Three different types of volcanic eruptions have been identified, differing in duration, intensity,
lava effusion rate, and whether the eruption occurs within a volcanic pit. Lava flows on Io, tens or hundreds of kilometres long, have primarily
basaltic composition, similar to lavas seen on Earth at
shield volcanoes such as
Kīlauea in
Hawaii. As a result of the presence of significant quantities of sulfurous materials in Io's crust and on its surface, during some eruptions, sulfur, sulfur dioxide gas, and
pyroclastic material are blown up to 500 kilometres (310 mi) into space, producing large, umbrella-shaped volcanic plumes. (
Full article...)
Portal:Solar System/Selected article/20
Galileo image of 243 Ida. The tiny dot to the right is its moon, Dactyl.
243 Ida is an
asteroid in the
Koronis family of the
main belt. It was discovered on 29 September 1884 by
Johann Palisa and named after a
nymph from Greek mythology. Later telescopic observations categorized Ida as an
S-type asteroid, the most numerous type in the inner asteroid belt. On 28 August 1993, Ida was visited by the spacecraft
Galileo, bound for
Jupiter. It was the second asteroid to be visited by a spacecraft and the first found to possess a
satellite. Like all main-belt asteroids, Ida's orbit lies between the planets
Mars and Jupiter. Its
orbital period is 4.84 years, and its
rotation period is 4.63 hours. Ida has an average diameter of 31.4 km (19.5 mi). It is irregularly shaped and elongated, and apparently composed of two large objects connected together in a shape reminiscent of a
croissant. Its surface is one of the most heavily
cratered in the Solar System, featuring a wide variety of crater sizes and ages. Ida's moon, Dactyl, was discovered by mission member Ann Harch in images returned from
Galileo. It was named after
creatures which inhabited Mount Ida in Greek mythology. Data returned from the flyby pointed to S-type asteroids as the source for the
ordinary chondrite meteorites, the most common type found on the Earth's surface. (
Full article...)
Portal:Solar System/Selected article/21
A map of Jupiter produced by the Cassini probe
The
atmosphere of Jupiter is the largest
planetary atmosphere in the
Solar System. It is primarily made of
molecular hydrogen and
helium in roughly solar proportions; other chemical compounds are present only in small amounts, and include
methane,
ammonia,
hydrogen sulfide and
water. The latter one is thought to reside deep in the atmosphere—its directly measured concentration is very low. The
oxygen,
nitrogen,
sulfur and
noble gas abundances in
Jupiter's atmosphere exceed solar values by a factor of about three. The atmosphere of Jupiter lacks a clear lower boundary and gradually transitions into the fluid interior of the planet. From lowest to highest, the atmospheric layers are the
troposphere,
stratosphere,
thermosphere and
exosphere. Each layer has characteristic
temperature gradients. The lowest layer, the troposphere, has a complicated system of clouds and hazes, comprising layers of ammonia,
ammonium hydrosulfide and water. The upper ammonia clouds visible at Jupiter's surface are organized in a dozen
zonal bands parallel to the
equator and are bounded by powerful zonal atmospheric flows (winds) known as
jets. The bands alternate in color: the dark bands are called
belts, while light ones are called
zones. Zones, which are colder than belts, correspond to upwellings, while belts mark descending air. The zones' lighter color is believed to result from ammonia ice; what gives the belts their darker colors is not known with certainty. The Jovian atmosphere shows a wide range of active phenomena, including band instabilities, vortices (
cyclones and
anticyclones), storms (lightning). (
Full article...)
Portal:Solar System/Selected article/22
Eris, the largest known scattered disc object
The
scattered disc is a distant region of the
Solar System that is sparsely populated by icy
minor planets, a subset of the broader family of
trans-Neptunian objects. The scattered disc objects have
orbital eccentricities ranging as high as 0.8,
inclinations as high as 40° and
perihelia greater than 30
astronomical units. These extreme orbits are believed to be the result of gravitational "
scattering" by the
gas giants,
and the objects continue to be subject to perturbation by the planet Neptune. While the nearest distance to the Sun approached by scattered objects is about 30–35 AU, their orbits can extend well beyond 100 AU. This makes scattered objects "among the most distant and cold objects in the Solar System". The innermost portion of the scattered disc overlaps with a torus-shaped region of orbiting objects known as the Kuiper belt, but its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the belt proper. Due to its unstable nature, astronomers now consider the scattered disc to be the place of origin for most periodic comets observed in the Solar System, with the centaurs, a population of icy bodies between Jupiter and Neptune, being the intermediate stage in an object's migration from the disc to the inner Solar System. (Full article...)
Portal:Solar System/Selected article/23
Callisto, the third largest moon in the Solar System
Callisto is a
moon of the planet Jupiter, discovered in 1610 by
Galileo Galilei. It is the
third-largest moon in the
Solar System and the second largest in the Jovian system, after
Ganymede. It is not a part of the
orbital resonance that affects three inner Galilean satellites—
Io,
Europa and Ganymede—and thus does not experience appreciable
tidal heating. Callisto
rotates synchronously with its orbital period, so the same hemisphere is always turned toward Jupiter. It is composed of approximately equal amounts of
rock and
ices, with a mean
density of about 1.83 g/cm
3. 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 only partially differentiated interior covered by a thick icy crust and possibly a subsurface ocean of liquid water at depths greater than 100 km. Prominent surface features include multi-ring structures, variously shaped
impact craters, and chains of craters and associated
scarps, ridges and deposits. Callisto is surrounded by an extremely thin
atmosphere composed of
carbon dioxide and probably
molecular oxygen, as well as by a rather dense
ionosphere. (
Full article...)
Portal:Solar System/Selected article/24
Ganymede, the largest moon in the Solar System
Ganymede is a
moon of Jupiter and the
largest moon in the
Solar System. Completing an orbit in roughly seven days, it is the seventh moon and third
Galilean moon from
Jupiter. Ganymede participates in a 1:2:4
orbital resonance with the moons
Europa and
Io, respectively. It is larger in diameter than the planet
Mercury but has only about half its mass. It has the highest mass of all planetary satellites with 2.01 times the mass of the Earth's moon. It is composed primarily of
silicate rock and water ice, and a saltwater ocean is believed to exist nearly 200 km below Ganymede's surface. Ganymede is the only satellite in the Solar System known to possess a
magnetosphere, likely created through
convection within the liquid iron core. The satellite has a thin
oxygen atmosphere that includes O, O
2, and possibly
O3. Ganymede's discovery is credited to
Galileo Galilei, who observed it in 1610. The satellite's name was soon suggested by astronomer
Simon Marius, for the mythological
Ganymede, cupbearer of the
Greek gods and
Zeus's beloved. (
Full article...)
Portal:Solar System/Selected article/25
Comet P/Halley as taken March 8, 1986, by W. Liller
Halley's Comet is the best-known of the
short-period comets, and is visible from Earth every 75 to 76 years. Halley is the only short-period comet that is clearly visible to the naked eye, and thus, the only naked-eye comet that might appear twice in a human lifetime. Other naked-eye comets may be brighter and more spectacular, but will appear only once in thousands of years. Halley's returns to the
inner Solar System have been observed by astronomers since at least 240 BC, and recorded by Chinese, Babylonian, and mediaeval European chroniclers, but were not recognised as reappearances of the same object. The comet's periodicity was first determined in 1705 by English astronomer
Edmond Halley, after whom it is now named. It last appeared in the inner
Solar System in 1986 and will next appear in mid-2061. During its 1986 apparition, Halley's Comet became the first to be observed in detail by spacecraft, providing the first observational data on the structure of the
comet nucleus and the mechanism of coma and tail formation. These observations supported a number of longstanding hypotheses about comet construction, particularly
Fred Whipple's "
dirty snowball" model, which correctly surmised that Halley would be composed of a mixture of volatile
ices, such as
water,
carbon dioxide and
ammonia, and dust. However, the missions also provided data which substantially reformed and reconfigured these ideas. (
Full article...)
Portal:Solar System/Selected article/26
The dwarf planet Ceres from the
Dawn spacecraft
A
dwarf planet is a
celestial body orbiting the
Sun that is massive enough to be spherical as a result of its own
gravity but has not
cleared its neighbouring region of
planetesimals and is not a
satellite. They are smaller than
planets, but more massive than
small solar system bodies. The term was adopted in 2006 by the
International Astronomical Union (IAU) as a result of the increase in discoveries of
trans-Neptunian objects that rivaled
Pluto in size, and finally precipitated by the discovery of an even more massive object,
Eris. The IAU currently recognizes five dwarf planets—
Ceres (pictured), Pluto,
Haumea,
Makemake, and Eris. It is suspected that at least another 40 known objects in the
Solar System are dwarf planets, but the number might be as high as 2,000. The 2006 definition has been both praised and criticized, and has been disputed by some scientists. (
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Portal:Solar System/Selected article/27
90377 Sedna is a
trans-Neptunian object currently about three times as far from the Sun as
Neptune. For the majority of its orbit it is the most distant known object in the
Solar System other than long-period
comets. Roughly two-thirds the size of
Pluto, Sedna is hypothetically large enough to be rounded by its own gravity, and thus would qualify as a
dwarf planet under current definitions. However, its distance makes determining its shape difficult.
Spectroscopy has revealed that Sedna's surface composition is similar to that of some other trans-Neptunian objects, being largely a mixture of water,
methane, and
nitrogen ices with
tholins. Its surface is one of the reddest in the Solar System. Its exceptionally long and elongated orbit, taking approximately 12,000 years to complete, and distant point of closest approach to the Sun have led to much speculation as to its origin. Astronomer
Mike Brown, who co-discovered Sedna in 2003, believes it to be the most scientifically important trans-Neptunian object found to date, as understanding its peculiar orbit is likely to yield valuable information about the origin and early evolution of the Solar System. (
Full article...)
Portal:Solar System/Selected article/28
Oberon, as photographed by Voyager 2 in 1986
Oberon is the outermost major
moon of the planet
Uranus. It is the second largest and second most massive of Uranian moons, and the ninth most massive moon in the Solar System. Discovered by
William Herschel in 1787, Oberon is named after a character in Shakespeare's
A Midsummer Night's Dream. Its orbit lies partially outside Uranus's
magnetosphere. Oberon consists of approximately equal amounts of ice and
rock, and is likely differentiated into a rocky
core and an icy
mantle. A layer of liquid water may be present at the core/mantle boundary. The surface of Oberon, which is dark and slightly red in color, appears to have been primarily shaped by asteroid and comet impacts. It is covered by numerous
impact craters reaching 210 km in diameter. Oberon possesses a system of
canyons (
scarps) formed as a result of the expansion of its interior during its early evolution. This moon probably formed from the
accretion disk that surrounded Uranus just after the planet's formation. As of 2010, the Uranian system has been studied up close only once: by the spacecraft
Voyager 2 in January 1986. It took several images of Oberon, which allowed mapping of about 40% of the moon’s surface. (
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Portal:Solar System/Selected article/29
Voyager 2 showing Neptune's full ring system with the highest sensitivity
The
rings of Neptune were first detected in 1980, but only identified in 1989 by the
Voyager 2 spacecraft. The rings are tenuous, faint and
dusty, and resemble the
rings of Jupiter more closely than those of
Saturn or
Uranus. Neptune possesses five known rings, each named for an astronomer who contributed important work on the planet: the Galle, Le Verrier, Lassell, Arago and Adams rings. Neptune also has a faint unnamed ring coincident with the orbit of Neptunian moon
Galatea. The rings of Neptune are made of extremely dark material, likely
organic compounds processed by
radiation similar to that found in the rings of Uranus. The proportion of dust in the rings (between 20 and 70%) is high, while their
optical depth is low, at less than 0.1. Uniquely, the Adams ring is divided into five discrete arcs, named Fraternité, Égalité 1 and 2, Liberté, and Courage. The arcs occupy a narrow range of
orbital longitudes and are remarkably stable, having changed only slightly since their initial detection in 1980. How the arcs maintain stability is still under debate. However, their stability is probably related to the resonant interaction between the Adams ring and its inner
shepherd moon, Galatea. (
Full article...)
Portal:Solar System/Selected article/30
Image of the Trojan asteroids in front of and behind Jupiter
The
Jupiter Trojans are a large group of objects that share the orbit of the planet
Jupiter around the
Sun. Relative to Jupiter, each Trojan
librates around one of the planet's two
Lagrangian points of stability,
L4 and
L5, that respectively lie 60° ahead of and behind the planet in its orbit. Trojan asteroids are distributed in two elongated, curved regions around these Lagrangian points with an average
semi-major axis of about 5.2 AU. The first Trojan,
588 Achilles, was discovered in 1906 by the German astronomer
Max Wolf. A total of 2,909 Jupiter Trojans have been found as of January 2009
[update]. The name "Trojans" derives from the fact that, by convention, they each are named after a mythological figure from the
Trojan War. The total number of Jupiter Trojans larger than 1 km is believed to be about
1 million, approximately equal to the number of asteroids larger than 1 km in the
main asteroid belt. Like main belt asteroids, Trojans form
families. Jupiter Trojans are dark bodies with reddish, featureless
spectra. No firm evidence of the presence of
water,
organic matter or other chemical compounds has been obtained. The Trojans' densities (as measured by studying
binaries or rotational lightcurves) vary from 0.8 to 2.5 g·cm
−3. Trojans are thought to have been captured into their orbits during the early stages of the
formation and evolution of the Solar System or slightly later, during the
migration of giant planets. (
Full article...)
Nominations
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