Jupiter

Jupiter's upper atmosphere is about 90% hydrogen and 10% helium by volume of gas molecules. A helium atom has about four times the mass of a hydrogen atom, so the composition is different when described as the proportion of mass contributed by different atoms. Thus, Jupiter's atmosphere is approximately 75% hydrogen and 24% helium by mass, with the remaining one percent of the mass consisting of other elements. The atmosphere contains trace amounts of methane, water vapour, ammonia, and silicon-based compounds. There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. The outermost layer of the atmosphere contains crystals of frozen ammonia. Through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found.[35] The interior contains denser materials—by mass it is roughly 71% hydrogen, 24% helium, and 5% other elements.[36][37]

The atmospheric proportions of hydrogen and helium are close to the theoretical composition of the primordial solar nebula. Neon in the upper atmosphere only consists of 20 parts per million by mass, which is about a tenth as abundant as in the Sun.[38] Helium is also depleted to about 80% of the Sun's helium composition. This depletion is a result of precipitation of these elements into the interior of the planet.[39]

Based on spectroscopy, Saturn is thought to be similar in composition to Jupiter, but the other giant planets Uranus and Neptune have relatively less hydrogen and helium and relatively more ices and are thus now termed ice giants.[40]

Jupiter's diameter is one order of magnitude smaller (×0.10045) than that of the Sun, and one order of magnitude larger (×10.9733) than that of Earth. The Great Red Spot is roughly the same size as Earth.

Jupiter's mass is 2.5 times that of all the other planets in the Solar System combined—this is so massive that its barycentre with the Sun lies above the Sun's surface at 1.068 solar radii from the Sun's centre.[41] Jupiter is much larger than Earth and considerably less dense: its volume is that of about 1,321 Earths, but it is only 318 times as massive.[2][42] Jupiter's radius is about one tenth the radius of the Sun,[43] and its mass is one thousandth the mass of the Sun, so the densities of the two bodies are similar.[44] A "Jupiter mass" (M_J or M_Jup) is often used as a unit to describe masses of other objects, particularly extrasolar planets and brown dwarfs. For example, the extrasolar planet HD 209458 b has a mass of 0.69 M_J, while Kappa Andromedae b has a mass of 12.8 M_J.[45]

Theoretical models indicate that if Jupiter had much more mass than it does at present, it would shrink.[46] For small changes in mass, the radius would not change appreciably, and above about 500 M_⊕ (1.6 Jupiter masses)[46] the interior would become so much more compressed under the increased pressure that its volume would decrease despite the increasing amount of matter. As a result, Jupiter is thought to have about as large a diameter as a planet of its composition and evolutionary history can achieve.[47] The process of further shrinkage with increasing mass would continue until appreciable stellar ignition was achieved, as in high-mass brown dwarfs having around 50 Jupiter masses.[48]

Although Jupiter would need to be about 75 times more massive to fuse hydrogen and become a star, the smallest red dwarf is only about 30 percent larger in radius than Jupiter.[49][50] Despite this, Jupiter still radiates more heat than it receives from the Sun; the amount of heat produced inside it is similar to the total solar radiation it receives.[51] This additional heat is generated by the Kelvin–Helmholtz mechanism through contraction. This process causes Jupiter to shrink by about 1 mm/yr.[52][53] When formed, Jupiter was hotter and was about twice its current diameter.[54]

Before the early 21st century, most scientists expected Jupiter to either consist of a dense core, a surrounding layer of liquid metallic hydrogen (with some helium) extending outward to about 78% of the radius of the planet,[51] and an outer atmosphere consisting predominantly of molecular hydrogen,[53] or perhaps to have no core at all, consisting instead of denser and denser fluid (predominantly molecular and metallic hydrogen) all the way to the centre, depending on whether the planet accreted first as a solid body or collapsed directly from the gaseous protoplanetary disk. But when the Juno mission arrived in July 2016,[24] it found that Jupiter has a very diffuse core that mixes into its mantle.[55] A possible cause is an impact from a planet of about ten Earth masses a few million years after Jupiter's formation, which would have disrupted an originally solid Jovian core.[56][57] It is estimated that the core is 0.3–0.5 of the planetary radius, and contains heavy elements 7–25 times the mass of Earth.[58]

Above the layer of metallic hydrogen lies a transparent interior atmosphere of hydrogen. At this depth, the pressure and temperature are above hydrogen's critical pressure of 1.2858 MPa and critical temperature of only 32.938 K.[59] In this state, there are no distinct liquid and gas phases—hydrogen is said to be in a supercritical fluid state. It is convenient to treat hydrogen as gas extending downward from the cloud layer to a depth of about 1,000 km,[51] and as liquid in deeper layers. Physically, there is no clear boundary—the gas smoothly becomes hotter and denser as depth increases.[60][61] Rain-like droplets of helium and neon precipitate downward through the lower atmosphere, depleting the abundance of these elements in the upper atmosphere.[39][62] Rainfalls of diamonds have been suggested to occur, as well as on Saturn[63] and the ice giants Uranus and Neptune.[64]

The temperature and pressure inside Jupiter increase steadily inward, this is observed in microwave emission and required because the heat of formation can only escape by convection. At the pressure level of 10 bars (1 MPa), the temperature is around 340 K (67 °C; 152 °F). The hydrogen is always supercritical (that is, it never encounters a first-order phase transition) even as it changes gradually from a molecular fluid to a metallic fluid at around 100–200 GPa, where the temperature is perhaps 5,000 K (4,730 °C; 8,540 °F). The temperature of Jupiter's diluted core is estimated at around 20,000 K (19,700 °C; 35,500 °F) or more with an estimated pressure of around 4,500 GPa.[65]

Jupiter has the deepest planetary atmosphere in the Solar System, spanning over 5,000 km (3,000 mi) in altitude.[66][67]

South polar view of Jupiter

Enhanced color view of Jupiter's southern storms

Jupiter is perpetually covered with clouds composed of ammonia crystals, and possibly ammonium hydrosulfide. The clouds are in the tropopause and are in bands of different latitudes, known as tropical regions. These are subdivided into lighter-hued zones and darker belts. The interactions of these conflicting circulation patterns cause storms and turbulence. Wind speeds of 100 metres per second (360 km/h; 220 mph) are common in zonal jet streams.[68] The zones have been observed to vary in width, colour and intensity from year to year, but they have remained sufficiently stable for scientists to name them.[42]

The cloud layer is about 50 km (31 mi) deep, and consists of at least two decks of clouds: a thick lower deck and a thin clearer region. There may also be a thin layer of water clouds underlying the ammonia layer. Supporting the presence of water clouds are the flashes of lightning detected in the atmosphere of Jupiter. These electrical discharges can be up to a thousand times as powerful as lightning on Earth.[69] The water clouds are assumed to generate thunderstorms in the same way as terrestrial thunderstorms, driven by the heat rising from the interior.[70] The Juno mission revealed the presence of "shallow lightning" which originates from ammonia-water clouds relatively high in the atmosphere.[71] These discharges carry "mushballs" of water-ammonia slushes covered in ice, which fall deep into the atmosphere.[72] Upper-atmospheric lightning has been observed in Jupiter's upper atmosphere, bright flashes of light that last around 1.4 milliseconds. These are known as "elves" or "sprites" and appear blue or pink due to the hydrogen.[73][74]

The orange and brown colours in the clouds of Jupiter are caused by upwelling compounds that change colour when they are exposed to ultraviolet light from the Sun. The exact makeup remains uncertain, but the substances are thought to be phosphorus, sulfur or possibly hydrocarbons.[51][75] These colourful compounds, known as chromophores, mix with the warmer lower deck of clouds. The zones are formed when rising convection cells form crystallising ammonia that masks out these lower clouds from view.[76]

Jupiter's low axial tilt means that the poles always receive less solar radiation than the planet's equatorial region. Convection within the interior of the planet transports energy to the poles, balancing out the temperatures at the cloud layer.[42]

Time-lapse sequence from the approach of Voyager 1, showing the motion of atmospheric bands and circulation of the Great Red Spot. Recorded over 32 days with one photograph taken every 10 hours (once per Jovian day). See full size video.

The best known feature of Jupiter is the Great Red Spot,[77] a persistent anticyclonic storm located 22° south of the equator. It is known to have existed since at least 1831,[78] and possibly since 1665.[79][80] Images by the Hubble Space Telescope have shown as many as two "red spots" adjacent to the Great Red Spot.[81][82] The storm is visible through Earth-based telescopes with an aperture of 12 cm or larger.[83] The oval object rotates counterclockwise, with a period of about six days.[84] The maximum altitude of this storm is about 8 km (5 mi) above the surrounding cloudtops.[85] The Spot's composition and the source of its red color are still unknown.

The Great Red Spot is larger than the Earth.[86] Mathematical models suggest that the storm is stable and may be a permanent feature of the planet.[87] However, it has significantly decreased in size since its discovery. Initial observations in the late 1800s showed it to be approximately 41,000 km (25,500 mi) across. By the time of the Voyager flybys in 1979, the storm had a length of 23,300 km (14,500 mi) and a width of approximately 13,000 km (8,000 mi).[88] Hubble observations in 1995 showed it had decreased in size to 20,950 km (13,020 mi), and observations in 2009 showed the size to be 17,910 km (11,130 mi). As of 2015, the storm was measured at approximately 16,500 by 10,940 km (10,250 by 6,800 mi),[88] and was decreasing in length by about 930 km (580 mi) per year.[86][89]

In 2000, an atmospheric feature formed in the southern hemisphere that is similar in appearance to the Great Red Spot, but smaller. This was created when smaller, white oval-shaped storms merged to form a single feature—these three smaller white ovals were first observed in 1938. The merged feature was named Oval BA and has been nicknamed "Red Spot Junior". It has since increased in intensity and changed from white to red.[90][91][92]

The Great Red Spot is decreasing in size (May 15, 2014)[93]

In April 2017, scientists reported the discovery of a "Great Cold Spot" in Jupiter's thermosphere at its north pole that is 24,000 km (15,000 mi) across, 12,000 km (7,500 mi) wide, and 200 °C (360 °F) cooler than surrounding material. The feature was discovered by researchers at the Very Large Telescope in Chile, who then searched archived data from the NASA Infrared Telescope Facility between 1995 and 2000. They found that, while the Spot changes size, shape, and intensity over the short term, it has maintained its general position in the atmosphere for more than 15 years. Scientists believe the Spot is a giant vortex similar to the Great Red Spot, and also appears to be quasi-stable like the vortices in Earth's thermosphere. Interactions between charged particles generated from Io and the planet's strong magnetic field likely resulted in redistribution of heat flow, forming the Spot.[94]

Aurorae

Aurorae on the north and south poles
(animation)

Aurorae on the north pole
(Hubble)

Infrared view of southern lights
(Jovian IR Mapper)

Jupiter's magnetic field is fourteen times stronger than Earth's, ranging from 4.2 gauss (0.42 mT) at the equator to 10–14 gauss (1.0–1.4 mT) at the poles, making it the strongest in the Solar System (except for sunspots).[76] This field is thought to be generated by eddy currents—swirling movements of conducting materials—within the liquid metallic hydrogen core. The volcanoes on the moon Io emit large amounts of sulfur dioxide, forming a gas torus along the moon's orbit. The gas is ionised in the magnetosphere, producing sulfur and oxygen ions. They, together with hydrogen ions originating from the atmosphere of Jupiter, form a plasma sheet in Jupiter's equatorial plane. The plasma in the sheet co-rotates with the planet, causing deformation of the dipole magnetic field into that of a magnetodisk. Electrons within the plasma sheet generate a strong radio signature that produces bursts in the range of 0.6–30 MHz which are detectable from Earth with consumer-grade shortwave radio receivers.[95][96]

At about 75 Jupiter radii from the planet, the interaction of the magnetosphere with the solar wind generates a bow shock. Surrounding Jupiter's magnetosphere is a magnetopause, located at the inner edge of a magnetosheath—a region between it and the bow shock. The solar wind interacts with these regions, elongating the magnetosphere on Jupiter's lee side and extending it outward until it nearly reaches the orbit of Saturn. The four largest moons of Jupiter all orbit within the magnetosphere, which protects them from the solar wind.[51]

The magnetosphere of Jupiter is responsible for intense episodes of radio emission from the planet's polar regions. Volcanic activity on Jupiter's moon Io injects gas into Jupiter's magnetosphere, producing a torus of particles about the planet. As Io moves through this torus, the interaction generates Alfvén waves that carry ionised matter into the polar regions of Jupiter. As a result, radio waves are generated through a cyclotron maser mechanism, and the energy is transmitted out along a cone-shaped surface. When Earth intersects this cone, the radio emissions from Jupiter can exceed the solar radio output.[97]

Model in the Almagest of the longitudinal motion of Jupiter (☉) relative to Earth (⊕)

Observation of Jupiter dates back to at least the Babylonian astronomers of the 7th or 8th century BC.[106] The ancient Chinese knew Jupiter as the "Suì Star" (Suìxīng 歲星) and established their cycle of 12 earthly branches based on its approximate number of years; the Chinese language still uses its name (simplified as 岁) when referring to years of age. By the 4th century BC, these observations had developed into the Chinese zodiac,[107] with each year associated with a Tai Sui star and god controlling the region of the heavens opposite Jupiter's position in the night sky; these beliefs survive in some Taoist religious practices and in the East Asian zodiac's twelve animals, now often popularly assumed to be related to the arrival of the animals before Buddha. The Chinese historian Xi Zezong has claimed that Gan De, an ancient Chinese astronomer, discovered one of Jupiter's moons in 362 BC with the unaided eye. If true, this would predate Galileo's discovery by nearly two millennia.[108][109] In his 2nd century work the Almagest, the Hellenistic astronomer Claudius Ptolemaeus constructed a geocentric planetary model based on deferents and epicycles to explain Jupiter's motion relative to Earth, giving its orbital period around Earth as 4332.38 days, or 11.86 years.[110]

Galileo Galilei, discoverer of the four largest moons of Jupiter, now known as Galilean moons

In 1610, Italian polymath Galileo Galilei discovered the four largest moons of Jupiter (now known as the Galilean moons) using a telescope; thought to be the first telescopic observation of moons other than Earth's. One day after Galileo, Simon Marius independently discovered moons around Jupiter, though he did not publish his discovery in a book until 1614.[111] It was Marius's names for the major moons, however, that stuck: Io, Europa, Ganymede, and Callisto. These findings were the first discovery of celestial motion not apparently centred on Earth. The discovery was a major point in favor of Copernicus' heliocentric theory of the motions of the planets; Galileo's outspoken support of the Copernican theory placed him under the threat of the Inquisition.[112]

During the 1660s, Giovanni Cassini used a new telescope to discover spots and colourful bands, observe that the planet appeared oblate, and estimate the planet's rotation period.[113] In 1690 Cassini noticed that the atmosphere undergoes differential rotation.[51]

The Great Red Spot may have been observed as early as 1664 by Robert Hooke and in 1665 by Cassini, although this is disputed. The pharmacist Heinrich Schwabe produced the earliest known drawing to show details of the Great Red Spot in 1831.[114]

The Red Spot was reportedly lost from sight on several occasions between 1665 and 1708 before becoming quite conspicuous in 1878. It was recorded as fading again in 1883 and at the start of the 20th century.[115]

Both Giovanni Borelli and Cassini made careful tables of the motions of Jupiter's moons, allowing predictions of when the moons would pass before or behind the planet. By the 1670s, it was observed that when Jupiter was on the opposite side of the Sun from Earth, these events would occur about 17 minutes later than expected. Ole Rømer deduced that light does not travel instantaneously (a conclusion that Cassini had earlier rejected),[37] and this timing discrepancy was used to estimate the speed of light.[116]

In 1892, E. E. Barnard observed a fifth satellite of Jupiter with the 36-inch (910 mm) refractor at Lick Observatory in California. This moon was later named Amalthea.[117] It was the last planetary moon to be discovered directly by visual observation.[118] An additional eight satellites were discovered before the flyby of the Voyager 1 probe in 1979.[lower-alpha 4]

Infrared image of Jupiter taken by ESO's Very Large Telescope

In 1932, Rupert Wildt identified absorption bands of ammonia and methane in the spectra of Jupiter.[119]

Three long-lived anticyclonic features termed white ovals were observed in 1938. For several decades they remained as separate features in the atmosphere, sometimes approaching each other but never merging. Finally, two of the ovals merged in 1998, then absorbed the third in 2000, becoming Oval BA.[120]

In 1955, Bernard Burke and Kenneth Franklin detected bursts of radio signals coming from Jupiter at 22.2 MHz.[51] The period of these bursts matched the rotation of the planet, and they used this information to refine the rotation rate. Radio bursts from Jupiter were found to come in two forms: long bursts (or L-bursts) lasting up to several seconds, and short bursts (or S-bursts) lasting less than a hundredth of a second.[121]

Scientists discovered that there are three forms of radio signals transmitted from Jupiter:

• Decametric radio bursts (with a wavelength of tens of metres) vary with the rotation of Jupiter, and are influenced by the interaction of Io with Jupiter's magnetic field.[122]
• Decimetric radio emission (with wavelengths measured in centimetres) was first observed by Frank Drake and Hein Hvatum in 1959.[51] The origin of this signal was a torus-shaped belt around Jupiter's equator. This signal is caused by cyclotron radiation from electrons that are accelerated in Jupiter's magnetic field.[123]
• Thermal radiation is produced by heat in the atmosphere of Jupiter.[51]

Since 1973, a number of automated spacecraft have visited Jupiter, most notably the Pioneer 10 space probe, the first spacecraft to get close enough to Jupiter to send back revelations about its properties and phenomena.[124][125] Flights to planets within the Solar System are accomplished at a cost in energy, which is described by the net change in velocity of the spacecraft, or delta-v. Entering a Hohmann transfer orbit from Earth to Jupiter from low Earth orbit requires a delta-v of 6.3 km/s,[126] which is comparable to the 9.7 km/s delta-v needed to reach low Earth orbit.[127] Gravity assists through planetary flybys can be used to reduce the energy required to reach Jupiter, albeit at the cost of a significantly longer flight duration.[128]

Beginning in 1973, several spacecraft have performed planetary flyby maneuvers that brought them within observation range of Jupiter. The Pioneer missions obtained the first close-up images of Jupiter's atmosphere and several of its moons. They discovered that the radiation fields near the planet were much stronger than expected, but both spacecraft managed to survive in that environment. The trajectories of these spacecraft were used to refine the mass estimates of the Jovian system. Radio occultations by the planet resulted in better measurements of Jupiter's diameter and the amount of polar flattening.[42][130]

Six years later, the Voyager missions vastly improved the understanding of the Galilean moons and discovered Jupiter's rings. They also confirmed that the Great Red Spot was anticyclonic. Comparison of images showed that the Red Spot had changed hue since the Pioneer missions, turning from orange to dark brown. A torus of ionised atoms was discovered along Io's orbital path, and volcanoes were found on the moon's surface, some in the process of erupting. As the spacecraft passed behind the planet, it observed flashes of lightning in the night side atmosphere.[42][131]

The next mission to encounter Jupiter was the Ulysses solar probe. It performed a flyby maneuver to attain a polar orbit around the Sun. During this pass, the spacecraft studied Jupiter's magnetosphere. Ulysses has no cameras so no images were taken. A second flyby six years later was at a much greater distance.[129]

In 2000, the Cassini probe flew by Jupiter on its way to Saturn, and provided higher-resolution images.[132]

The New Horizons probe flew by Jupiter in 2007 for a gravity assist en route to Pluto.[133] The probe's cameras measured plasma output from volcanoes on Io and studied all four Galilean moons in detail, as well as making long-distance observations of the outer moons Himalia and Elara.[134]

Jupiter as seen by the space probe Cassini

The first spacecraft to orbit Jupiter was the Galileo probe, which entered orbit on December 7, 1995.[47] It orbited the planet for over seven years, conducting multiple flybys of all the Galilean moons and Amalthea. The spacecraft also witnessed the impact of Comet Shoemaker–Levy 9 as it approached Jupiter in 1994, giving a unique vantage point for the event. Its originally designed capacity was limited by the failed deployment of its high-gain radio antenna, although extensive information was still gained about the Jovian system from Galileo.[135]

A 340-kilogram titanium atmospheric probe was released from the spacecraft in July 1995, entering Jupiter's atmosphere on December 7.[47] It parachuted through 150 km (93 mi) of the atmosphere at a speed of about 2,575 km/h (1600 mph)[47] and collected data for 57.6 minutes before the signal was lost at a pressure of about 23 atmospheres and a temperature of 153 °C.[136] It melted thereafter, and possibly vapourised. The Galileo orbiter itself experienced a more rapid version of the same fate when it was deliberately steered into the planet on September 21, 2003, at a speed of over 50 km/s to avoid any possibility of it crashing into and possibly contaminating the moon Europa, which may harbor life.[135]

Data from this mission revealed that hydrogen composes up to 90% of Jupiter's atmosphere.[47] The recorded temperature was more than 300 °C (570 °F) and the windspeed measured more than 644 km/h (>400 mph) before the probes vapourised.[47]

Jupiter viewed by the Juno spacecraft
(February 12, 2019)

NASA's Juno mission arrived at Jupiter on July 4, 2016, and was expected to complete thirty-seven orbits over the next twenty months.[24] The mission plan called for Juno to study the planet in detail from a polar orbit.[137] On August 27, 2016, the spacecraft completed its first fly-by of Jupiter and sent back the first ever images of Jupiter's north pole.[138] Juno would complete 12 science orbits before the end of its budgeted mission plan, ending July 2018.[139] In June of that year, NASA extended the mission operations plan to July 2021.[140] When Juno reaches the end of the mission, it will perform a controlled deorbit and disintegrate into Jupiter's atmosphere. During the mission, the spacecraft will be exposed to high levels of radiation from Jupiter's magnetosphere, which may cause future failure of certain instruments and risk collision with Jupiter's moons.[141][142]

The next planned mission to the Jovian system will be the European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2022,[143] followed by NASA's Europa Clipper mission, scheduled for 2024.[144]

There has been great interest in studying Jupiter's icy moons in detail because of the possibility of subsurface liquid oceans on Europa, Ganymede, and Callisto. Funding difficulties have delayed progress. NASA's JIMO (Jupiter Icy Moons Orbiter) was cancelled in 2005.[145] A subsequent proposal was developed for a joint NASA/ESA mission called EJSM/Laplace, with a provisional launch date around 2020. EJSM/Laplace would have consisted of the NASA-led Jupiter Europa Orbiter and the ESA-led Jupiter Ganymede Orbiter.[146] However, ESA had formally ended the partnership by April 2011, citing budget issues at NASA and the consequences on the mission timetable. Instead, ESA planned to go ahead with a European-only mission to compete in its L1 Cosmic Vision selection.[147]

The moons discovered by Galileo—Io, Europa, Ganymede, and Callisto—are among the largest in the Solar System. The orbits of three of them (Io, Europa, and Ganymede) form a pattern known as a Laplace resonance; for every four orbits that Io makes around Jupiter, Europa makes exactly two orbits and Ganymede makes exactly one. This resonance causes the gravitational effects of the three large moons to distort their orbits into elliptical shapes, because each moon receives an extra tug from its neighbors at the same point in every orbit it makes. The tidal force from Jupiter, on the other hand, works to circularise their orbits.[151]

The eccentricity of their orbits causes regular flexing of the three moons' shapes, with Jupiter's gravity stretching them out as they approach it and allowing them to spring back to more spherical shapes as they swing away. This tidal flexing heats the moons' interiors by friction.[152] This is seen most dramatically in the volcanic activity of Io (which is subject to the strongest tidal forces),[152] and to a lesser degree in the geological youth of Europa's surface, which indicates recent resurfacing of the moon's exterior.[153]

The Galilean moons, compared to Earth's Moon Name IPA Diameter Mass Orbital radius Orbital period km % kg % km % days % Io /ˈaɪ.oʊ/ 3,643 105 8.9×1022 120 421,700 110 1.77 7 Europa /jʊˈroʊpə/ 3,122 90 4.8×1022 65 671,034 175 3.55 13 Ganymede /ˈɡænimiːd/ 5,262 150 14.8×1022 200 1,070,412 280 7.15 26 Callisto /kəˈlɪstoʊ/ 4,821 140 10.8×1022 150 1,882,709 490 16.69 61

The Galilean moons. From left to right, in order of increasing distance from Jupiter: Io, Europa, Ganymede, Callisto.

The Galilean moons Io, Europa, Ganymede, and Callisto (in order of increasing distance from Jupiter)

Jupiter's moons were traditionally classified into four groups of four, based on commonality of their orbital elements.[154] This picture has been complicated by the discovery of numerous small outer moons by Voyager in 1979. Jupiter's moons are currently divided into several different groups, although there are several moons which are not part of any group.[155]

The eight innermost regular moons, which have nearly circular orbits near the plane of Jupiter's equator, are thought to have formed alongside Jupiter, whilst the remainder are irregular moons and are thought to be captured asteroids or fragments of captured asteroids. Irregular moons that belong to a group share similar orbital elements and thus may have a common origin, perhaps as a larger moon or captured body that broke up.[156][157]

The rings of Jupiter

Jupiter has a faint planetary ring system composed of three main segments: an inner torus of particles known as the halo, a relatively bright main ring, and an outer gossamer ring.[161] These rings appear to be made of dust, rather than ice as with Saturn's rings.[51] The main ring is probably made of material ejected from the satellites Adrastea and Metis. Material that would normally fall back to the moon is pulled into Jupiter because of its strong gravitational influence. The orbit of the material veers towards Jupiter and new material is added by additional impacts.[162] In a similar way, the moons Thebe and Amalthea probably produce the two distinct components of the dusty gossamer ring.[162] There is also evidence of a rocky ring strung along Amalthea's orbit which may consist of collisional debris from that moon.[163]

Hubble image taken on July 23, 2009, showing a blemish about 8,000 km (5,000 mi) long left by the 2009 Jupiter impact event.[169]

Jupiter has been called the Solar System's vacuum cleaner:[170] because of its immense gravity well and location near the inner Solar System there are more impacts on Jupiter, such as comets, than on the Solar System's other planets.[171] It was thought that Jupiter partially shielded the inner system from cometary bombardment.[47] However, recent computer simulations suggest that Jupiter does not cause a net decrease in the number of comets that pass through the inner Solar System, as its gravity perturbs their orbits inward roughly as often as it accretes or ejects them.[172] This topic remains controversial among scientists, as some think it draws comets towards Earth from the Kuiper belt while others think that Jupiter protects Earth from the Oort cloud.[173] Jupiter experiences about 200 times more asteroid and comet impacts than Earth.[47]

A 1997 survey of early astronomical records and drawings suggested that a certain dark surface feature discovered by astronomer Giovanni Cassini in 1690 may have been an impact scar. The survey initially produced eight more candidate sites as potential impact observations that he and others had recorded between 1664 and 1839. It was later determined, however, that these candidate sites had little or no possibility of being the results of the proposed impacts.[174]