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More about Jupiter from Wikipedia, The Planets.org and Space.com. See my main Astronomy page for other information about many aspects of Astronomy.
Jupiter and its “big four” moons (Io, Europa, Ganymede and Callisto) are described here, as are some of the other Jovian moons.
This page also describes the spacecraft that have visited the planet.
A composite image of Jupiter taken by the Cassini spacecraft.
The dark spot is the shadow of the moon Europa, and the Great Red Spot, believed to be a gigantic storm that has been raging for centuries, is on the right.
Jupiter, the fifth planet from the Sun in the Solar System is also the largest. It has 67 known satellites and a feeble ring system.
It orbits the Sun once in 4,332.59 Earth days (11.8618 years or 10,475.8 Jupiter solar days). Its temperature at the 1-bar pressure level is about 165 K, and at the 0.1-bar level is 112 K (see in the diagram that the atmosphere is extremely thick, perhaps 1,000 km, and as the planet has no identified ‘surface’, this may be the best way of indicating its atmospheric temperature).
Jupiter’s average distance from the Sun is 778,547,200 km (5.204267 AU) varying between 740,573,600 km (4.950429 AU) and 816,520,800 km (5.458104 AU).
Its mean radius is 69,911±6 km, its equatorial radius is 71,492±4 km (11.209 Earth’s), and its polar radius is 66,854±10 km (10.517 Earth’s); these numbers indicate a significant flattening of 0.06487±0.00015. Jupiter’s surface area is 6.1419×1010 km2 (121.9 times Earth’s), its volume is 1.4313×1015 km3 (1321.3 times Earth’s). Its mass is 1.8986×1027 kg (317.8 times Earth’s or 1/1047 of the Sun’s). Its mean density is 1.326 g/cm3.
Topics covered in this page include more about the planet, the famous Great Red Spot and Jupiter’s magnetosphere; there are also descriptions of its moons [Io, Europa, Ganymede and Callisto are described elsewhere], and animations of the orbits of its moons; also Jupiter has many associated asteroids (“Trojans”, etc) and a ring system.
Uniquely in the solar system, the barycentre (common centre of gravity) of the Sun–Jupiter system is outside the Sun. This raises the interesting possibility that Jupiter is wrongly classified as a “planet” but is one (very minor) component of a binary system. That’s one for the IAU to chew over for some time!
Jupiter, the fifth major planet from the Sun, is also the largest in the Solar System. Its existence as a “wandering star” has been known since antiquity and was associated with the mythology and religious beliefs of many cultures. The Romans named the planet after their god “Jupiter”. But it was only with the invention of the telescope that Galileo Galilei in 1610 recognized that it was something special – it had moons of its own!
Jupiter is a gas giant, along with Saturn, Uranus and Neptune. It has a mass of one-thousandth that of the Sun but is two-and-a-half times the mass of all the other planets in the Solar System combined. Together, the four gas giant planets are sometimes referred to as the Jovian or outer planets. When viewed from Earth, Jupiter can reach an apparent magnitude of −2.94, making it on average the third-brightest object in the night sky after the Moon and Venus. (Mars can briefly match Jupiter’s brightness at certain points in its orbit.)
Jupiter is primarily composed of hydrogen; a quarter of its mass is helium, although hydrogen molecules outnumber helium by ten to one. It may also have a rocky core of heavier elements, but like the other gas giants, it lacks a well-defined solid surface. Because of its rapid rotation, the planet’s shape is that of an oblate spheroid (it possesses a slight but noticeable bulge around the equator). The outer atmosphere is visibly segregated into several bands at different latitudes, with turbulence and storms along their interacting boundaries. A prominent feature is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope.
Surrounding Jupiter is a faint planetary ring system and a powerful magnetosphere.
[Left] This true-colour composite frame, made from narrow angle images taken by the Cassini spacecraft in 2000, captures Io and its shadow in transit against the disk of Jupiter with its Great Red Spot. The distance of the spacecraft from Jupiter was 19.5 million km. The scale is 117 km per pixel.
[Right] A detail of Jupiter’s atmosphere taken by the LEISA infrared camera on the New Horizons spacecraft, re-mapped to visible colours and contrast-enhanced.
Jupiter has been explored on several occasions by spacecraft, most notably during the early Pioneer and Voyager fly-by missions and later by the Galileo orbiter. A recent probe that visited Jupiter was the Pluto-bound New Horizons spacecraft in late February 2007, which used the gravity from Jupiter to increase its speed in a sling-shot maneouvre. The Juno spacecraft is currently orbiting the planet. Future targets for exploration in the Jovian system include the possible ice-covered liquid ocean on the moon Europa.
In almost every photograph of Jupiter, the Great Red Spot appears. It is a persistent anticyclonic storm. White atmospheric bands, termed zones, represent areas of upwelling; reddish bands, called belts, represent areas of downwelling. They display high-altitude ammonia ice clouds and lower clouds of unknown composition, respectively. The Great Red Spot is 22° south of Jupiter’s equator; Earth observations establish a minimum storm lifetime of, variously, 183 years to possibly 348 years. The storm is large enough to be visible through Earth-based telescopes, first being observed by Samuel Heinrich Schwabe in 1831 as a drawing of the gap formed around it, and possibly even earlier, as a “permanent spot” was described by Gian Domenico Cassini between 1665 and 1713.
The Great Red Spot rotates counterclockwise, with a period of about six Earth days or 14 Jovian days. Its dimensions are 24 to 40,000 km west-to-east and 12 to 14,000 km south-to-north. The spot is large enough to contain two or three planets the size of Earth. At the start of the 21st century, the Great Red Spot had approximately half the longitudinal extent it had a century ago, when it was 40,000 km in diameter. At the present rate of reduction it could potentially become circular by 2040, although this is unlikely because of the distortion effect of the neighboring jet streams. It is not known how long the spot will last, or whether the change is a result of normal fluctuations; however, the Hubble Space Telescope has seen that it appears to be decreasing in size.
There’s a much fuller description here.
As well as the famous Great Red Spot, the Cassini spacecraft discovered an equally impressive Dark Spot near the planet’s north pole (which is why it hadn’t been observed from the Earth).
“I was totally blown away when I saw it – a dark cloud twice as big as Earth swirling around Jupiter’s north pole,” says Bob West, a planetary scientist at the Jet Propulsion Laboratory.
This composite of Cassini ultra-violet (UV) images reveals the “Great Dark Spot” swirling near Jupiter’s north pole. Jupiter’s auroral zone is denoted by the blue curve
West has been chasing this cloud for some time. He first saw it – “just a glimpse,“ says West – in an ultraviolet (UV) picture of Jupiter taken by the Hubble Space Telescope in 1997. But it only appeared in one image out of many spanning a period of years. “I didn’t know what to make of it,” he recalls. Now he knows. “The Cassini spacecraft was en route to Saturn in 2000 when it passed by Jupiter and had a good view of the planet’s north pole,” says West. “At first there was nothing unusual – just ordinary polar clouds. Then the Dark Spot emerged.” For weeks Cassini’s UV-sensitive cameras watched as the cloud grew into an oval the size of the Great Red Spot itself. It swirled, darkened and changed shape until, as Cassini was departing, it began to fade again.
“The Dark Spot is ephemeral,” says West. That’s probably why Hubble saw it only once. And if Cassini had arrived a month or two later, it might not have seen the Dark Spot at all. Instead, Cassini’s cameras monitored the cloud for 11 straight weeks, and those data have allowed West to draw some conclusions: “The Great Dark Spot and the Great Red Spot are entirely different,” he says. The Great Red Spot is deep. “It’s a high-pressure storm system rooted in Jupiter’s troposphere far below the cloud-tops. The Great Dark Spot is apparently shallow and confined to Jupiter’s high stratosphere.”
Everyone knows of the rings of Saturn, but other planets have them too. Uranus was the second planet to be shown to possess a ring system, and then Jupiter.
Jupiter’s ring system (like Saturn’s but very much smaller) was discovered by Voyager 1 in a single image that was targeted specifically to search for a faint ring system. Subsequently, Voyager 2 was reprogrammed to take a more complete set of images. It has since been thoroughly investigated in the 1990s by the Galileo orbiter. It has also been observed by the Hubble Space Telescope and from ground-based telescopes on the Earth for the past 23 years.
The ring is now known to be composed of three major components. The main ring is about 7000 km wide and has an abrupt outer boundary 128,940 km from the centre of the planet. The main ring encompasses the orbits of two small moons, Adrastea and Metis, which may act as the source for the dust that makes up most of the ring. At its inner edge the main ring merges gradually into the halo. The halo is a broad, faint torus of material about 10,000 km thick and extending halfway from the main ring down to the planet s cloud-tops. Just outside the main ring is a pair of broad and exceedingly faint “gossamer rings”, one bounded by the orbit of the moon Amalthea and the other by the orbit of Thebe.
The 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”.
A ring could possibly exist around the moon Himalia’s orbit. One possible explanation is that a small moon had crashed into Himalia and the force of the impact caused material to blast off Himalia. There’s a much fuller description here of the main and other jovian ring systems.
A magnetosphere is the area of space near an astronomical object in which charged particles are controlled by that object’s magnetic field. Near the surface of the object, the magnetic field lines resemble those of an ideal magnetic dipole. Farther away from the surface, the field lines are significantly distorted by external currents, such as the solar wind.
The magnetosphere of Jupiter is the largest in the Solar System, extending up to 7,000,000 km on the day side and almost to the orbit of Saturn on the night side. Jupiter’s magnetosphere is stronger than the Earth’s by an order of magnitude (around a factor of ten), and its magnetic moment (next paragraph) is approximately 18,000 times larger.
The magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it. Both the magnetic moment and magnetic field have a magnitude and a direction. The direction of the magnetic moment points from the south to north pole of a magnet. The magnetic field produced by a magnet is proportional to its magnetic moment. The dipole component of an object’s magnetic field is symmetric about the direction of its magnetic dipole moment, and decreases as the inverse cube of the distance from the object.
Jupiter has 67 moons (and counting), including its four large moons, the “Galilean” moons, Io, Europa, Ganymede and Callisto.
For convenience, and to explain some of their similarities, the jovian moons are grouped into a number of classes (full details of their sizes, orbits, etc. are in Wikipedia):
Regular satellites. These have prograde (they orbit in the same direction as Jupiter) and nearly circular orbits of low inclination.
Irregular satellites. These are substantially smaller objects with more distant and eccentric orbits. They form families with shared similarities in orbit (semi-major axis, inclination, eccentricity) and composition; it is believed that these are at least partially collisional families that were created when larger (but still small) parent bodies were shattered by impacts from asteroids captured by Jupiter’s gravitational field. The families bear the names of their largest members.
Just for the record, the satellites so far discovered are, in order of their average distance from Jupiter:
Metis, Adrastea, Amalthea, Thebe, Io, Europa, Ganymede, Callisto, Themisto, Leda, Himalia, Lysithea, Elara, S/2000 J 11, Carpo, S/2003 J 12, Euporie, S/2003 J 3, S/2003 J 18, S/2011 J 1, S/2010 J 2, Thelxinoe, Euanthe, Helike, Orthosie, Iocaste, S/2003 J 16, Praxidike, Harpalyke, Mneme, Hermippe, Thyone, Ananke, Herse, Aitne, Kale, Taygete, S/2003 J 19, Chaldene, S/2003 J 15, S/2003 J 10, S/2003 J 23, Erinome, Aoede, Kallichore, Kalyke, Carme, Callirrhoe, Eurydome, S/2011 J 2, Pasithee, S/2010 J 1, Kore, Cyllene, Eukelade, S/2003 J 4, Pasiphaë, Hegemone, Arche, Isonoe, S/2003 J 9, S/2003 J 5, Sinope, Sponde, Autonoe, Megaclite, S/2003 J 2
The naming convention of newly discovered moons of Jupiter is after lovers and favourites of the god Jupiter (Zeus), and since 2004, after their descendants also. All of Jupiter’s satellites from Euporie in 2003 are named after daughters of Jupiter or Zeus.
Some asteroids share the same names as moons of Jupiter: 9 Metis, 38 Leda, 52 Europa, 85 Io, 113 Amalthea and 239 Adrastea. Two more asteroids previously shared the names of Jovian moons until spelling differences were made permanent by the IAU: Ganymede and asteroid 1036 Ganymed; and Callisto and asteroid 204 Kallisto.
Cloud bands along the edge of Jupiter
Start of polar gravity assist
Depiction of Pioneer 11 in deep space
Galileo (1989-084B) was launched on 18th October 1989 at 16:53:00 UTC from the Space Shuttle Atlantis (STS-34) after a long and troubled development process, and it suffered a crippling malfunction early in its mission when its high-gain antenna failed to open. It studied the planet Jupiter and its moons, as well as several other solar system bodies. It consisted of an orbiter and entry probe. It arrived at Jupiter on 8th December 1995 at 01:20:00 UTC, after gravitational assist fly-bys of Venus (on 10th February 1990) and Earth (on 8th December 1992 and 28th August 1993), and became the first spacecraft to orbit Jupiter. It launched the first probe into Jupiter, directly measuring its atmosphere.
Calvin J. Hamilton’s website View of the Solar System describes this image as follows:
“This picture of Gaspra is a combination of the highest-resolution images and colour information obtained by the Galileo spacecraft. The Sun is shining from the right. The subtle colour variations on Gaspra’s surface have been exaggerated. Albedo and colour variations are associated with surface topography. The bluish areas are regions of slightly higher albedo and tend to be associated with some of the crisper craters and with ridges. The slightly reddish areas, apparently concentrated in low areas, represent regions of somewhat lower albedo. In general, such patterns can be explained in terms of greater exposure of fresher rock in the brighter bluish areas and the accumulation of some regolith materials in the darker reddish areas.” [USGS/NASA/JPL]
Galileo achieved the first asteroid fly-by, of 951 Gaspra on 29th October 1991 within 1,600 km, and discovered the first asteroid moon, Dactyl, around 243 Ida on 28th August 1993, passing within 2,400 km.
It witnessed the comet Shoemaker–Levy 9 (D/1993 F2) crash into Jupiter in July 1994; Galileo was the only observatory that had a direct view of the impact, which happened on Jupiter’s night side; Earth-based telescopes had to wait until Jupiter’s rotation brought the impact zone into view hours later. On 21st September 2003 at 18:57:00 UTC, after 14 years in space and 8 years in the Jovian system, Galileo’s mission was terminated by sending the orbiter into Jupiter’s atmosphere at a speed of over 48 km/sec, to avoid any possibility of the spacecraft contaminating Europa’s salty ocean with material brought from Earth.
Galileo was the first spacecraft to dwell in a giant planet’s magnetosphere long enough to identify its global structure and investigate the dynamics of Jupiter’s magnetic field. It revealed that Jupiter’s ring system is formed by dust kicked up as interplanetary meteoroids smash into the planet’s four small inner moons and that the planet’s outermost ring is actually two rings, one embedded within the other. The spacecraft’s mission was extended three times in order to study the Galilean satellites Io, Europa, Ganymede, and Callisto. Galileo made many discoveries about these moons: Io’s extensive volcanic activity is 100 times greater than that found on Earth; Europa has a salty ocean up to 100 km underneath its frozen surface, containing about twice as much water as all the Earth’s oceans; Callisto and Ganymede may also feature a liquid salt-water layer; and Ganymede has an iron core, like Earth, and a magnetic field, making this moon the first satellite known to possess a magnetic field. In order to avoid any possibility of the spacecraft contaminating Europa’s salty ocean with material brought from Earth, the spacecraft was deliberately destroyed by sending it onto a collision course with Jupiter.
This animation shows all the moons in orbit around Jupiter, which is in the centre.
You can see the inner moons in regular orbits and how much farther away from Jupiter the rest are.
This distribution emphasizes how most of the moons are captured into orbits where they are only loosely held.
(From Tony Dunn, Gravity Simulator, http://www.orbitsimulator.com/gravity/articles/joviansystem.html)
Some of the moons of Jupiter and their orbits, seen from a slightly inclinated angle.
The red line is Jupiter’s orbit. The animation is intended to give an idea of depth.
These four moons of Jupiter were discovered by Galileo Galilei; by March 1610, he had sighted these massive Galilean moons with his 30× magnification telescope:
With radii that are larger than any of the dwarf planets, Io, Europa, Ganymede and Callisto are some of the largest objects in the solar system. They contain almost 99.999% of the total mass in orbit around Jupiter; Jupiter is almost 5,000 times more massive than the Galilean moons. Io, Europa and Ganymede have a 1:3:2 orbital resonance.
No additional satellites were discovered until E. E. Barnard observed Amalthea in 1892.
Now the total number has reached 67.
Io, one of the four famously found by Galileo when he first pointed his newly-invented telescope at Jupiter, was probably also discovered independently of him in 1610 by the German astronomer Simon Marius, who named it after “Io” of Greek mythology.
The brilliant red-yellow colour of the moon is due to deposits from its sulphurous volcanoes. New Horizons photographed an active volcano on Io as it flew by towards Pluto.
Io rotates at the same rate that it revolves around Jupiter (1.769 Earth days) and so it always presents the same face to Jupiter, much as the Moon does towards the Earth. Its nearly circular orbit has an inclination of only 0.04° to Jupiter’s equatorial plane and a radius of 1820 km.
[Right] Detail of the volcanic crater Tupan Patera on Io, seen from the Galileo spacecraft. This image shows the results of lava interacting with sulphur-rich materials, and is only slightly enhanced with infrared light compared to what the human eye would actually see. This image of Tupan Patera (named after a Brazilian thunder god), was taken at a resolution of 135 m per pixel.
The entire body of Io, about the size of Earth’s Moon, is periodically flexed as it speeds around Jupiter and feels, as a result of its non-circular orbit, the periodically changing gravitational pull of the planet. The heat arising in Io’s interior from this continual flexure makes it the most volcanically active body in the solar system, with more than a hundred active volcanoes. The white and reddish colors on its surface are due to the presence of different sulphurous materials. The black areas are silicate rocks.
Europa is about 3,160 km in diameter, or about the size of Earth’s moon.
Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of water ice and is one of the smoothest in the Solar System. This surface is striated by cracks and streaks, while cratering is 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. This hypothesis proposes that heat energy from tidal flexing causes the ocean to remain liquid and drives geological activity similar to plate tectonics. As a preliminary experiment, scientists are, according to The Guardian, search[ing] for life... in [a] lake entombed under [the] Antarctic ice.
The Galileo mission, launched in 1989, provided the bulk of current data on Europa. Although only fly-by missions have visited the moon, the intriguing characteristics of Europa have led to several ambitious exploration proposals. The next mission to Europa will be the European Space Agency’s Jupiter Icy Moon Explorer (JUICE), due to be launched in 2022.
The photograph on the left was taken by the solid state imaging television camera onboard the Galileo spacecraft in 1996; at a range of 677,000 km during its second orbit around Jupiter, and shows a view of the trailing hemisphere of Jupiter’s ice-covered satellite, Europa. It shows the approximate natural colour appearance of Europa. Long, dark lines are fractures in the crust, some of which are more than 3,000 km long. The bright feature containing a central dark spot in the lower third of the image is a young impact crater some 50 km in diameter. This crater has been provisionally named “Pwyll” after the Celtic god of the underworld.
The dark spots, or “lenticulae”, are about 10 km across, and appear to be places where ice from below erupts onto the surface. The warmer ice below rises upwards, displacing the colder surface ice that flows downwards. NASA have now suggested that Europa’s ocean may have an Earth-like chemical balance.
Callisto is the second largest moon orbiting Jupiter. It is the third largest moon in the Solar System after Ganymede, and Saturn’s moon Titan. It is 4,820 km across and orbits 1,880,000 km from the planet. It orbits Jupiter every 16.7 Earth days.
Callisto has an average density of 1.83 g/cm3;, which leads scientists to believe that it is composed of equal parts silicate rock and various ices. Spectroscopic examination of the moon has shown water ice, carbon dioxide, silicates, and organic compounds. The Galileo spacecraft returned data that seems to indicate the presence of a rocky core and an ocean of liquid water that is at least 100 km below the surface ice. Callisto has an extremely tenuous atmosphere mainly composed of carbon dioxide with a small amount of molecular oxygen mixed in. The outer edges of the atmosphere are an intense ionosphere.
Callisto’s surface is heavily cratered and does not show any signs of recent restructuring from tectonic action, volcanic eruptions, or tidal flexing from Jupiter’s gravity. The entire moon is thought to have evolved from frequent impacts with other bodies. The surface is dominated by features that normally result from impacts, like multi-ring structures, intact impact craters, chains of craters (“catenae”), scarps, ridges and deposits. Many of the features have been worn by time and are only remnants. The surface of Callisto is thought to be nearly 4 billion years old. It is believed to have formed by slow accretion from the disk of the gas and dust that surrounded Jupiter after its formation. The slow pace of the accretion combined with a lack of tidal heating prevented the moon from becoming completely differentiated. Callisto did have some interior convection, allowing it to partially differentiate, which may have allowed the formation of the suspected subsurface ocean.
The likely presence of an ocean under the surface of Callisto has led to speculation that the moon could be harbouring life on some scale. Many scientists agree that the possibility of life is much higher on Europa than it is on Callisto. Callisto has been studied several times because of its low radiation which make the moon a possible location for a human base for exploring the remainder of the Jovian system. So Callisto is of great interest to scientists and will remain so for decades to come.
Callisto is named after one of Zeus’s many lovers in Greek mythology. Callisto was a nymph (or, according to some sources, the daughter of Lycaon) who was associated with the goddess of the hunt, Artemis. The name was suggested by Simon Marius soon after the moon’s discovery; Marius attributed the suggestion to Johannes Kepler.
Ganymede is the largest satellite in our solar system with a mean radius of 2634.1±0.3 km. It is bigger than Mercury (mean radius 2,439.7±1.0 km) and Pluto (mean radius 1,153±10 km), and three-quarters the size of Mars (equatorial radius 3,396.2±0.1 km, polar radius 3,376.2±0.1 km). If Ganymede orbited the Sun instead of orbiting Jupiter, it would easily be classified as a planet.
Ganymede has three main layers. A sphere of metallic iron at the centre (the core, which generates a magnetic field), a spherical shell of rock (its mantle) surrounding the core, and a spherical shell mostly of ice surrounding the rock shell and the core. The ice shell on the outside is very thick, maybe 800 km thick. The surface is the very top of the ice shell. Though it is mostly ice, the ice shell might contain some rock mixed in. Scientists believe there must be a fair amount of rock in the ice near the surface. The moon’s magnetic field is embedded inside Jupiter’s massive magnetosphere.
Astronomers using the Hubble Space Telescope found evidence of a thin oxygen atmosphere on Ganymede in 1996.
In 2004, scientists discovered irregular lumps beneath the icy surface of Ganymede; these may be rock formations, supported by Ganymede’s icy shell for billions of years. This suggests that the ice is probably strong enough, at least near the surface, to support these rock masses from sinking to the bottom of the ice. However, this anomaly could also be caused by piles of rock at the bottom of the ice.
Spacecraft images of Ganymede show the moon has a complex geological history. Ganymede’s surface is a mixture of two types of terrain. 40% of the surface is covered by highly cratered dark regions, and the remaining 60% is covered by a light grooved terrain, which forms intricate patterns across the moon. The largest identified area on Ganymede is Galileo Regio. The term “sulcus” (plural sulci) describes a long parallel groove on a planet or a moon; Uruk Sulcus is a bright region of grooved terrain adjacent to Galileo Regio. This terrain is probably formed by tensional faulting or the release of water from beneath the surface. Groove ridges as high as 700 m have been observed and the grooves run for thousands of kilometers across Ganymede’s surface. The grooves have relatively few craters and probably developed at the expense of the darker crust. The dark regions on Ganymede are old and rough, and the dark cratered terrain is believed to be the original crust of the satellite. Lighter regions are young and smooth (unlike those of the Earth’s Moon).
The large craters on Ganymede have almost no vertical relief and are quite flat; they lack central depressions common to craters often seen on the rocky surface of the Moon. This is probably due to slow and gradual adjustment to the soft icy surface. These large phantom craters are called “palimpsests” (a term originally applied to reused ancient writing materials on which older writing was still visible underneath newer writing). A typical example is Memphis Facula, a 340 km wide palimpsest. Palimpsests range from 50 to 400 km in diameter. Both bright and dark rays of ejecta exist around Ganymede’s craters – rays tend to be bright from craters in the grooved terrain and dark from the dark-cratered terrain.
Beneath its icy shell, Jupiter’s largest moon harbours a massive, salty ocean of water – one of the key ingredients for the emergence of life as we know it – based on the latest evidence from the Hubble Space Telescope. “A deep ocean under the icy crust of Ganymede opens up further exciting possibilities for life beyond Earth”, John Grunsfeld, associate administrator for NASA’s Science Mission Directorate, said in a news release.
Using data from the telescope, researchers said they found evidence for the existence of Ganymede’s hidden sea by measuring the motions of auroras caused by the moon’s magnetic field. The ocean sits beneath a 95-mile-thick sheet of ice and could have a depth of 60 miles, the scientists said.
It’s not the first time someone has raised the possibility that Ganymede’s ice might conceal a deep ocean. Earlier studies have suggested the moon might have alternating layers of ice and water that one NASA researcher compared to “a Dagwood sandwich”. Two other of Jupiter’s moons, Europa and Callisto, are also thought to have water beneath their ice-frosted crusts.
In mythology, “Ganymede” was a beautiful youth, a son of Tros, first king of the classical land known as Troy. He tended sheep there on the slopes of Mount Ida. One day Zeus (the Greek equivalent of the Roman god Jupiter) caught a glimpse of the young boy and was overwhelmed with a desire to bring Ganymede to Olympus to serve as the cup bearer of the gods. Jupiter then changed his own shape into that of an eagle, swooped down and carried the boy off to the home of the gods.
However the position of cup bearer of the gods was already filled by Hebe, the daughter of Jupiter and his wife Juno. Once Ganymede arrived at the royal court a competition began between Hebe and Ganymede for the honour of serving the gods. Eventually Ganymede won, and stayed on also as the favoured companion to Jupiter. To honour the events surrounding the elevation of Ganymede to “cup bearer and servant of the gods”, Jupiter placed the eagle (the shape he had assumed when abducting Ganymede to Olympus) into the heavens as the constellation Aquila (The Eagle), and immortalized Ganymede as the constellation Aquarius (The Water Bearer).
There are a large number of asteroids that are associated with Jupiter. A Jupiter-crosser is a minor planet whose orbit crosses that of Jupiter. Jupiter’s Trojans are classified as inner grazers (105), outer grazers (52), co-orbitals (183), and crossers (537). See this List of Jupiter-crossing Minor Planets.
Juno (2011-040A) is a Jupiter polar orbiter. It was launched on 5th August 2011 at 1625:00 UTC by an Atlas V rocket, and arrived at Jupiter on 4th July 2016. Juno is the second mission in NASA’s New Frontiers program.
Following a lengthy cruise and October 2013 Earth flyby, Juno will survey Jupiter from a polar orbit, carrying a suite of instruments designed to study the planet’s interior. It will investigate the existence of an ice-rock core; determine the amount of global water and ammonia present in the atmosphere; study convection and deep wind profiles in the atmosphere; investigate the origin of the jovian magnetic field; and explore the polar magnetosphere. Its science mission does not require a camera, but it does carry one, specifically designed to capture unusual and beautiful views of Jupiter from its polar perspective for public pleasure. Juno is the first solar-powered spacecraft to have gone into the outer reaches of the solar system.
Latest (24th June 2016): Juno enters Jupiter’s Magnetic Field; (4th July 2016): Juno captured by Jupiter’s gravity; more to follow.
Juno’s journey to Jupiter
Jupiter’s radiation belts
Juno’s orbits round Jupiter