Jupiter

Jupiter has only about 1/1,000th of Sol's mass, but it would have become a star if it was perhaps only 75 times more massive than it is. However, Jupiter is even too small to meet one of today's definition of a substellar brown dwarf because it can't even fuse deuterium (hydrogen nuclei composed one neutron as well as one proton) like young brown dwarfs that have accumulated at least 13 times the mass of Jupiter can.

Cassini-Huygens, NASA
Planetary Photojournal

Larger polar images.

Jupiter's atmosphere
has fast winds blowing
in opposite directions
in adjacent wide bands
of latitude (more on north
and south polar images).

Jupiter radiates more energy into space than it receives from the Sun. Traditionally, this heat was thought to be produced from the slow gravitational compression of the planet so that its core may be as hot as 20,000 °C (or F). This great interior heat would be generating convective movements deep within the planet that may also be leading to the motions of the clouds in its atmosphere. According to astronomer Geoffrey W. Marcy (Astronomy, October 2006), the planet appears to have a rocky core of around three Earth-masses. According to another interesting hypothesis, Jupiter (and possibly all of the planets and many of their larger moons) may formed around a rocky core with an inner core of fully crystallized nickel silicide around an inner sub-shell of nuclear decay and fission products surrounding a sub-core of uranium and plutonium. Acting like a natural, fast-breeder nuclear fission reactor, the sub-core generates heat that propels charged particles to ultimately produce the planet's magnetic field in a non-linear process that periodically weakens and strengthens, and may even reverse polarity. Lighter than the uranium-plutonium sub-core, fission by-products absorb neutrons and gradually slow core fission until they slowly float out to join the sub-shell, and fission resumes and strengthens. (See: J. Marvin Herndon, 1998; Brad Lemley, "Nuclear Planet," Discover, August 2002; and www.NuclearPlanet.com.)

The planet has the most circular orbit of all the planets. However, because of Jupiter great distance from Sol, its orbit takes almost 12 years to complete. In contrast, its very fast rotation results in a "day" that last only just over four tenths of an Earth day -- 10 hours.

Jupiter's composition is very similar to that of the primordial Solar nebula from which the Sun is formed. The planet is made almost exclusively of gaseous elements (about nine-tenths hydrogen and one-tenth helium by numbers of atoms, but closer to three-quarters and one-quarter by mass) with traces of methane, water, ammonia, and rockier elements. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium. Like the other gas giants, Jupiter does not have a true solid surface. Its gaseous substance simply gets denser with depth.

NASA, JPL, and the University of Arizona

This color "movie" depicts changes in some of the bands in Jupiter's atmosphere
-- a complete circumference from 60° north to 60° south -- over 24 rotations between
October 31 and November 9, 2000. The Great Red Spot is visible at lower left of center.

What we see of Jupiter is just the outer shell of its thick atmosphere which is composed of molecular hydrogen and helium. The atmosphere also has traces of water, carbon dioxide, methane, and other simple molecules that are gaseous or liquid when present on Earth's surface. Astronomers believe that there are three types of clouds made of ammonia ice, ammonium hydrosulfide, or ice and water that float at different levels in Jupiter's atmosphere. As a result, colors related to each type of cloud are visible from different altitudes with red being highest, browns and whites in the middle, and blue farther down (where the lower clouds are seen through through holes in the upper layers). In addition, there are very fast winds (exceeding 640 km, or 400 miles, per hour) blowing in opposite directions in adjacent, wide bands of latitude, whose vivid colors result from slight differences in chemical make-up and temperature.

NASA (Jupiter's Great Red Spot)

The Great Red Spot is a huge storm measuring 12,000 by 25,000 km (7,500 by 15,500 miles), which is big enough to hold two Earths side by side. While Jupiter's cloud patterns can change within hours or days like on Earth, the Spot has lasted for over 300 years. Modern infrared observations and the counter-clockwise direction of its rotation suggest that the Spot is a high-pressure region whose cloud tops are significantly higher and colder than surrounding regions. There are other, if smaller, storm spots on Jupiter, and similar storms have been seen on Saturn and Neptune. Some planetary scientists speculate that the Great Red Spot was created by the merger of smaller storms. On March 3, 2006, NASA announced that a large white spot ("Oval BA") -- about half the size of the Great Red Spot that had formed from smaller storms between 1999 and 2000 -- had changed color to brown then red (more information and images from NASA and Astronomy Picture of the Day).

NASA


Larger and pre-merger
color images.


In March 1999, two giant storms (each about half the size of the Earth) on Jupiter were observed to collide and merge to form an even bigger storm. The resulting storm began as three smaller, white oval storms about 60 years ago, in band of Jupiter's atmosphere south of the Great Red Spot. In early 1998, two of the ovals merged as Jupiter went behind the Sun, out of sight from Earth. In 1999, the resulting larger storm approached the third remaining oval where winds were swirling counter-clockwise at about 470 km (290 miles) per hour. One storm was about 9,000 km (5,600 miles) across, the other slightly smaller. A third, darker oval that was swirling clockwise formed temporarily between the two white ovals, but this new middle storm was pushed farther south and torn apart, as all three storms passed near the Great Red Spot in December 1999. Then, in March 2000, the two remaining white storms collided and merged over about three weeks, after circling each other in a counter-clockwise direction. The resulting, single storm is about one-third wider than either of the pre-merger storms. (March 3, 2006 update.)

Amy A. Simon-Miller,
Nancy J. Chanover,
Glenn Orton, GSFC,
JPL, ESA, NASA



Larger and jumbo images.



The newest red-spotted
storm has been shredded
by a collision with the
two larger storms (more).

On May 9 and 10, 2008, astronomers observing with the Hubble Space Telescope and the 10-meter Keck II telescope found a third red (storm) spot near the other two. The second red spot formed in 2006, initially as smaller whitish storms that merged and later reddened. This latest and smallest red spot, however, was moving along the same band of clouds as the Great Red Spot and is drifting towards it.

Michael Wong, Imke de Pater,
Christopher Go, U.C. Berkeley,
ESA, NASA




Larger and jumbo images.




A third red-spotted storm
along Jupiter's equator
(as well as more whitish
storms) has formed as
the region warms (more).

The tops of the three red storms actually extend above their surrounding clouds around some five miles (eight kilometers), and their red coloration is throught to be due to substances dredged up from below the clouds. While the Great Red Spot has almost twice Earth's diameter, the newer spots are less than one Earth-diameter in width. Jupiter's recent outbreak of other persistent, whitish high-pressure storms as well as red spots may be to large scale climate change as the gas giant planet appears to be getting warmer around its equator (U.C. Berkeley press release; hubblesite.org; and Astronomy Picture of the Day). On July 8, 2008, astronomers following the movements of the third red spot found that it appears to been greatly diminished by a collision beginning July 3rd with the two larger and older red-hued, planet-sized storms on Jupiter and is now expected to be drawn in and absorbed by the larger storm (Great Spot) (Rachel Courtland, New Scientist, July 9, 2008; and Astronomy Picture of the Day).

Infrared spectra of Jupiter's outer gas envelope suggests that the planet has less helium than the Sun in its upper atmosphere. This suggests that Jupiter's primordial helium (which is heavier than hydrogen) has been "raining" down through its hydrogen-rich atmosphere towards its center, although there must be planetary processes over time that move some helium back up. Moreover, the concentration of neon (which can dissolves in helium under certain conditions) is only one-tenth of Sol's and so may be lost below with the helium. Given its apparent depletion on Saturn, helium may also be precipitating out of Saturn's outer layers as well.

NASA, JPL, JHUAPL -- larger image


Jupiter has a huge magnetosphere that,
if it glowed at night, would appear to be
two to three times the size of the Sun or
Moon to viewers on Earth (more from
NASA and Chandra).

Even hydrogen, the bulk of the substance of Jupiter other than some helium and traces of various ices, becomes liquid in the interior of the gas planet when the pressure exerted by the weight of the upper layers exceeds four million bars. (By comparison, one bar is the atmospheric pressure found on Earth's surface.) This liquid hydrogen occurs when the individual hydrogen atoms are broken up and the electrons are freed from the now bare nuclei. The liquid hydrogen is metallic in behavior and functions as an electrical conductor that generates a huge magnetic field that encompasses its moons and extends in a windsock shape all the way past the orbit of Saturn. Indeed, the particles trapped in its equivalent of Earth's Van Allen radiation belts would be immediately fatal to unprotected humans.

According to astronomer Richard Crowe, since Jupiter's polar diameter is about six percent less than its equatorial diameter, the planet may have a rocky core of about 1.5 times Earth's diameter and around 10 times more massive than Earth -- three percent of Jupiter's mass -- that is resisting rotational flattening at the poles ("Ask Astro", Astronomy, October 2004). On November 20, 2008, The Astrophysical Journal Letters published the results of a new computer simulation indicating that Jupiter may have a central core of 14 to 18 Earth-masses of elements heavier than hydrogen and helium (including metals, non-metallic rocks, and ices), which is roughly double previously estimates. The gas giant may also have a few more Earth-masses of ices in the mostly hydrogen and helium mantle, which is consistent with the findings of the 1995 Galileo Entry Probe mission. According to one of the researchers Burkhard Militzer, their results provide additional support for the theory that Jupiter developed through core-accretion of planetesimals, where a large planetary core forms before a massive outer layer of gas is attracted by the core's gravity (Militzer et al, 2008). These model results have not been accepted by all astronomers, however.

Paul Kalas,
Michael Fitzgerald,
Franck Marchis,
James Graham,
Keck Observatory






Larger composite
image.






The latest asteroidal
or cometary impact
on Jupiter is large,
relative to the size
of the Earth (more).

On July 19, 2009, Anthony Wesley, an amateur astronomy, observed a large dark spot on Jupiter's south polar region (more). The new spot was subsequently confirmed in images by astronomers using the Keck II telescope (Keck news release; and Lisa Grossman, New Scientist, July 20, 2009). Like the 1994 impacts of more than 20 fragments of Comet Shoemaker-Levy 9, the "Wesley impact" was apparently the result of a recent collision with an asteroid or comet, which may have been a few hundred meters wide (more discussion and a Hubble close-up image of the atmosphere's "impact scar" from Science@NASA).

NASA, Chandra X-Ray Observatory -- full planet animation

An x-ray source blinks every 45 minutes near Jupiter's north
magnetic pole, that may be caused by the crash of Solar
oxygen and sulfur ions that are trapped in its magnetic field
and bounce back and forth to and from its south pole (more
from NASA and Chandra).

Contributing to Jupiter's radiation belts are particles from its moons Io and Europa. Volcanoes on Io spew out sulfur and oxygen atoms that collect in a donut-shaped cloud (torus) along the moons' orbit. A similar toroidal cloud of mostly hydrogen and oxygen gas also originates from Europa. Stretching millions of miles around Jupiter, the cloud may result from the bombardment of Europa's icy surface by ionized radiation from Jupiter's magnetosphere that kicks up and breaks apart water-ice molecules into its constituent atoms. The resulting atoms form neutral gases that are dispersed along Europa’s orbit and is estimated to mass around 60,000 tons. The cloud is hypothesized to act as both a source and a sink of charged particles.

APL/JHU, JPL/Cassini, NASA



Larger image.



Jupiter is surrounded
by donut-shaped clouds
of atoms from its moons
Io and Europa (more).

NASA -- larger and planetary images and fact sheet


Jupiter has fainter, smaller, and darker rings than Saturn. Its rings are probably composed of very small grains of rocky material, but no ice unlike Saturn's rings. These ring particles are probably lost to Jupiter's atmospheric and magnetic drag comparatively quickly. However, they are probably also being continuously resupplied by dust driven into space from energic, interplanetary micrometeors that smash into Jupiter's four small, innermost moons: Metis, Adrastea, Thebe, and Amalthea -- more. (See an Astronomy Picture of the Day of the rings and Jupiter in eclipse and an animation of the rings' orbits, known "shepherd moons", and other satellites around Jupiter, with a table of basic orbital and physical characteristics.)

More on Jupiter's ring system is available from NASA's Planetary Rings Node.

NASA (Jupiter's four largest moons, from left: Io, Europa, Ganymede, and Callisto -- see comparison with Earth's moon and fact table)

At latest count in May 2005, Jupiter has been found to have at least 63 satellites orbiting outside of its rings. (See an animation of the orbits of many of these satellites around Jupiter, with a table of basic orbital and physical characteristics, or a NASA fact sheet.) In addition to the four innermost moons, the next four are also the largest, called "Galilean" moons because they were first seen in 1610 by the astronomer Galileo Galilei, who was subsequently persecuted by the Church for suggesting that at least some celestial bodies do not revolve around the Earth. By order of their increasing distance from the giant planet, the four moons are Io, Europa, Ganymede, and Callisto. These Galilean moons are probably the last survivors of at least five generations of large moons that formed from the debris disk which once circled the gas giant during its formation (Canup and Ward, 2008; and Marcus Chown, New Scientist, March 7, 2009). As with Earth and its Moon, Jupiter's rotation is gradually slowing down from the tidal drag of the large Galilean moons, and these tidal forces are also gradually causing the moons' orbits to move farther away from the planet.

The three closest Galilean moons are locked into orbital manuevers called Laplace resonance. For each time that Ganymede orbits Jupiter once (in 7.2 days), Europa orbits twice (3.6 days), while Io orbits four times (1.8 days). These orbital motions cause gravitational variations that pull and pushes the orbits of the moons, leading to greater orbital eccentricity. The moons move farther then closer to Jupiter during each orbit, causing tidal flexing in each.

Galileo Project, JPL, NASA


Larger image.


Volcano on Io with molten red sulfur, surrounded by
black lava rock and yellowish sulfur-rich terrain (more).

These tidal forces significantly heat the two innermost Galilean moons. The gravitational pull of the mother planet and sister moons constantly yank and bend Io, heating it up in the same way repeated twisting heats up a metal wire. Two percent larger than Earth's Moon, Io has a molten interior and occasionally tremendous volcanic eruptions. As a result, lava lakes sizzle between 1430 to 1730 degrees Celsius (2,600 to 3,140 degrees Farenheit), although the temperature at other places on Io can be as cold as minus -160° C (250° F). This the intense heat probably caused any water present to evaporate billions of years ago, but it has generated a thin atmosphere of sulfur dioxide.

NASA

Cassini-Huygens Mission
to Saturn and Titan


Larger image.


Jupiter with Europa
and its shadow.

In contrast, Europa (which is 12 percent smaller than Earth's Moon) appears to have a sparsely cratered shell of water ice that may be only 10 to 250 million years old. Estimates of the thickness range from less than 12 miles (20 km) to more than 90 miles (150 km) thick. Most important, some of the ice below may have been melted by radioactive decay and tidal heating from gravitational interactions with Jupiter to create a deep ocean of electrically conductive liquid such as salty water that generates fluctuations in the moon's magnetic pole. Some scientists estimated in 1999 that this possible ocean may be at least 4.5 miles (7 km) deep, average about 100 km (62 miles) in depth, but range mostly between 80 and 170 km (50 and 110 miles). Biologists now speculate that, if liquid water does exist beneath Europa's shell of ice, then Earth-type microbial-sized or larger lifeforms may have developed as well (more discussion about the possibility of life on Europa can be found here). Near-infrared mapping spectrometer on NASA's Galileo spacecraft has also found evidence of the presence of molecules made of oxygen, carbon, sulfur, hydrogen, and nitrogen on Europa, and a suggestion of the presence of a class of complex organic compounds called tholins (see NASA news brief).

Courtesy Jet Propulsion Laboratory. Copyright (c) California Institute of Technology, Pasadena, CA. All rights reserved.

Callisto and Ganymede are slightly larger in diameter (two and eight percent, respectively) than the planet Mercury. However, Callisto is about one quarter water ice. While Ganymede is denser, its bright surface is made of about 90 percent ice or frost. Recent evidence from the magnetometer aboard the Galileo spacecraft indicates that both Ganymede and Callisto may also have liquid oceans deep beneath their icy surfaces (see NASA news brief). The largest "moon" in the Solar System with a diameter near 5,270 km (3,270 miles), Ganymede (like Europa) has a very thin atmosphere of oxygen, while Callisto appears to have its oxygen locked up in ice and rocks as its atmosphere is composed of mostly of carbon dioxide instead.

Ganymede is the only known planetary satellite to have its own dipolar magnetic field. It appears to have a dense core about 3,000 km (1,900 miles) in diameter of iron or iron sulfide, enveloped in rock. This core is surrounded by an icy mantle of about 700 km (440 miles). Because the orbits of the Galilean moons may have shifted over time, Ganymede may still be cooling off from past tidal heating, and so its core may still have sufficient heat to generate convection in its mantle to produce its magnetic field.

NASA (Clockwise from top left: Io, Europa, Callisto, and Ganymede -- larger image)

Except for Io, the other three large Galilean moons all have a layer of ice and possibly
liquid water surrounding their rocky cores. Ganymede, like Europa, has a very thin
atmosphere of oxygen, while Callisto appears to have its oxygen locked up in ice and
rocks as its atmosphere is composed of mostly of carbon dioxide instead.

Farthest out of the Galileans, Callisto orbits outside the main radiation belts of Jupiter and so is not subject to the intense tidal forces that contort its neighboring sisters. It appears to have the oldest and most cratered landscape in the Solar System although its surface seems to show evidence of geologic activity. While it was long thought to be a dead world, Callisto appears to have weathered some type of erosion, perhaps from the evaporation of carbon dioxide ice.

Some of Jupiter's outer moons are and clustered in two distinct groups. The inner group are all inclined between 24 and 29 degrees from the planet's equator, while the outer group is even more greatly inclined between 147 and 164 degrees -- which means that the outer moons are basically in retrograde orbits. Not only that, some of the outer moons are black from the presence of carbonaceous minerals like those found in certain dark asteroids and meteorites. Hence, it is possible that some of these outer moons are cometary or asteroidal bodies that were captured Jupiter's gravity and perhaps fragmented by collision or drag forces.

In recent years, astronomers have found numerous small moons, including satellites discovered up through May 2005, all of these moons are very small, with estimated diameters ranging from one to eight kilometers (about 0.6 to five miles) (more). Like the other outer moons suspected to be captured cometary or asteroidal bodies, these moons have large and highly eccentric orbits, and virtually all move in retrograde orbits. Jupiter could have captured the moons during the first million years of the Solar System's development by growing so fast that it captured nearby objects. Another theory suggests that the planet's atmosphere slowed passing asteroids enough to trap them in orbit.

© Petr Scheirich
(Used with permission)



Larger image.



The eccentric orbits of the
Hilda asteroids suggest that
Jupiter may have migrated
inward towards Sol by about
0.45 AUs (see orbit animation
and Franklin et al, 2004).

According to computer simulations inspired by the finding of "hot" Jupiter-class planets found in inner orbits around nearby stars, the Solar System's own Jupiter may have formed 10 percent farther from the Sun than it is now, and then spiralled in by about 0.45 AUs (70 million kilometers or 44 million miles) over at least 100,000 years as it lost angular momentum to drag within the thick dust disk that surrounds young stars. Supporting evidence of this in-migration from an unusual group of 700 or so rocky bodies known as the Hilda asteroids, which orbit the Sun three times for every two made by Jupiter, and of which vast majority have slightly elongated elliptical orbits, whereas many other asteroids have near-circular orbits. Computer simulations (led by Fred Franklin’s team at the Harvard-Smithsonian Center for Astrophysics) indicate that Jupiter's early in-migration would have ejected any proto-Hilda asteroids with circular orbits from the Solar System and would have further elongated the orbits of those that remained (Franklin et al, 2004). Luckily, Sol's dust disk was probably thin compared with those stars that dragged their outer gas giants into inner orbits closer than Mercury or into the stars themselves, perturbing the orbits of any developing, inner terrestrial planets. Indeed, astronomer Phil Armitage speculates that the in-migration of Jupiter could also have disturbed the proto-planetary bodies of the inner Solar System so that they collided more frequently, to spur the formation and growth of Earth itself.