Jupiter (planet)

Jupiter is the fifth planet from the Sun and, by far, the largest within our solar system; some have described the solar system as consisting of the Sun, Jupiter, and assorted debris. It and the other gas giants Saturn, Uranus, and Neptune are sometimes referred to as "Jovian planets." It was named after the Roman god Jupiter.

Overview

Jupiter is 2.5 times more massive than all the other planets combined, so massive that its barycenter with the Sun actually lies above the Sun's surface (1.068 solar radii from the Sun's center). It is 318 times more massive than Earth, with a diameter 11 times that of Earth, and with a volume 1300 times that of Earth. It has been termed by many a "failed star." As impressive as it is, extrasolar planets have been discovered with much greater masses. However, it is thought to have about as large a diameter as a planet of its composition can, as adding extra mass would only result in further gravitational compression. There is no clear-cut definition of what distinguishes a large and massive planet such as Jupiter from a brown dwarf but in any case it would need to be about seventy times as massive as it is to become a star.

Jupiter also has the fastest rotation rate of any planet within the solar system resulting in a flattening easily seen through a telescope. Its best known feature is probably the Great Red Spot, a storm larger than Earth. The planet is perpetually covered with a layer of clouds.

Jupiter is usually the fourth brightest object in the sky (after the Sun, the Moon and Venus; however at times Mars appears brighter than Jupiter, while at others Jupiter appears brighter than Venus). It has been known since ancient times. Galileo Galilei's discovery, in 1610, of Jupiter's four large moons Io, Europa, Ganymede and Callisto (now known as the Galilean moons) was the first discovery of a celestial motion not apparently centered on the Earth. It was a major point in favor of Copernicus's heliocentric theory of the motions of the planets; Galileo's outspoken support of the Copernican theory got him in trouble with the Inquisition.

Physical characteristics

Planetary composition

Jupiter is composed of a relatively small rocky core, surrounded by metallic hydrogen, surrounded by liquid hydrogen, which is surrounded by gaseous hydrogen. There is no clear boundary or surface between these different phases of hydrogen; the conditions blend smoothly from gas to liquid as one descends.

Atmosphere

Jupiter's atmosphere is composed of ~86% hydrogen and ~14% helium (by number of atoms, the atmosphere is ~75%/24% by mass; with ~1% of the mass accounted for by other substances - the interior contains denser materials such that the distribution is ~71%/24%/5%). The atmosphere contains trace amounts of methane, water vapour, ammonia, and "rock". There are also negligible amounts of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. This atmospheric composition is very close to the composition of the solar nebula. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium.

Jupiter's upper atmosphere undergoes differential rotation, an effect first noticed by Cassini (1690). The rotation of Jupiter's polar atmosphere is ~5 minutes longer than that of the equatorial atmosphere. In addition, bands of clouds of different latitudes flow in opposing directions on the prevailing winds. The interactions of these conflicting circulation patterns cause storms and turbulence. Wind speeds of 600 km/h are not uncommon.

The outermost layer of the atmosphere contains crystals of frozen ammonia.

Planetary rings

Jupiter has a faint planetary ring system composed of smoke-like dust particles knocked off of its moons by meteor impacts. The main ring is made of dust from the satellites Adrastea and Metis. Two wide gossamer rings encircle the main ring, originating from Thebe and Amalthea. There is also an extremely tenuous and distant outer ring that circles Jupiter backwards. Its origin is uncertain, but this outer ring might be made of captured interplanetary dust.

Magnetosphere

Jupiter has a very large and powerful magnetosphere. In fact, if you could see Jupiter's magnetic field from Earth, it would appear five times as large as the full moon in the sky despite being so much farther away. This magnetic field collects a large flux of particle radiation in Jupiter's radiation belts, as well as producing a dramatic gas torus and flux tube associated with Io.

Exploration of Jupiter

Jupiter has been known of since ancient times and is visible to the naked eye in the night sky. In 1610, Galileo Galilei discovered the four largest moons of Jupiter using a telescope, the first observation of moons other than Earth's.

A number of probes have visited Jupiter, all of them American in origin. Pioneer 10 flew past Jupiter in December of 1973, followed by Pioneer 11 exactly one year later. Voyager 1 flew by in March 1979 followed by Voyager 2 in July of the same year. The Galileo probe went into orbit around Jupiter in 1995, dropping a smaller subprobe into Jupiter's atmosphere and conducting multiple flybys of all of the Galilean moons. The Galileo probe also witnessed the impact of Comet Shoemaker-Levy 9 into Jupiter as it approached the planet in 1994, giving a unique vantage point for this spectacular event.

After the discovery of a liquid ocean on Jupiter's moon Europa and the end of the Galileo probe, which was deorbited in September 2003, NASA is planning a mission dedicated to the icy moons. The JIMO, or Jupiter Icy Moons Orbiter, is expected to be launched sometime after 2012.

Jupiter's moons

For a complete listing of these moons, please see Jupiter's natural satellites.

The orbits of 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 moons to distort their orbits into elliptical shapes, since 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 circularize their orbits. This constant tug of war causes regular flexing of the three moons' shapes, Jupiter's gravity stretching the moons more strongly during the portion of their orbits that are closest to it and allowing them to spring back to more spherical shapes when they're farther away. This flexing causes tidal heating of the three moons' cores. This is seen most dramatically in Io's extraordinary volcanic activity, and to a somewhat less dramatic extent in the geologically young surface of Europa indicating recent resurfacing.

Jupiter's moons fall into four major groups:

1. The inner group were all discovered during the Voyager program except for Amalthea, all have diameters of less than 200 km and orbit at radii less than 200,000 km, and have orbital inclinations of less than half a degree.
2. The Galilean moons were all discovered by Galileo Galilei, orbit between 400,000 and 2,000,000 km, and include the largest moons in the solar system.
3. The third group were all discovered in the 20th century but before Voyager, have diameters less than 200 km, and orbit between 11,000,000 and 12,000,000 km with an orbital inclination between 26° and 29°.
4. The outer moons were also discovered in the 20th century before Voyager, but have diameters under 50 km and orbit between 21,000,000 and 24,000,000 km. They are particularly notable for having retrograde orbits with inclinations between 145° and 165°.

It is thought that the three groups of smaller moons may each have a common origin, perhaps as a larger moon or captured body that broke up into the existing moons of each group.

On April 4, 2003, the official moon count for Jupiter jumped to 58. The latest discoveries were made by a team led by Scott Sheppard and David Jewitt at the University of Hawaii, along with Jan Kleyna of Cambridge University. The discoveries were made using the world's two largest digital cameras at the Subaru and Canada-France-Hawaii telescopes atop Mauna Kea in Hawaii. All six newfound satellites are estimated to be about 2 kilometers wide. The same team earlier this year found the smallest known moons, a pair of 1-kilometer satellites orbiting the giant planet.

On May 15, 2003, Scott Sheppard published in the journal Nature his discovery of 23 new moons around the giant planet. This brought the total number of known moons to at least 61. Now there are 63 known moons.

References: [1] [2]

Cometary impact

During the period July 16 to July 22, 1994, over twenty fragments from the comet Shoemaker-Levy 9 hit Jupiter's southern hemisphere, providing the first direct observation of a collision between two solar system objects. It is thought that due to Jupiter's large mass and location near the inner solar system it receives the most frequent comet impacts of the solar system's planets.

Jupiter in fiction

In Sir Arthur C. Clarke's novel 2010: Odyssey Two, fictional technology converts Jupiter into a star by increasing the density of its core. Jupiter is also home to Jupiter Station, a fictional space station in the Star Trek universe. Numerous science fiction novels, such as Green Mars and Blue Mars in Kim Stanley Robinson's Mars trilogy, are set on the moons of Jupiter--in fact, its moons are a more common setting than the planet itself.