DEFINITION OF JUPITER

The fifth planet from the Sun, Jupiter is the gaseous

giant of the solar system, named after the most powerful

Roman god of the Pantheon. It has more than twice the mass

of all the other planets combined, estimated at 1.9 × 1027

kilograms, or 318 Earth masses, and a radius of 71,400 kilometers,

or 11.2 Earth radii. Volumetrically, it would take

1,400 Earths to fill the space occupied by Jupiter. Jupiter is

the third brightest object in the night sky, following the

Moon and Venus. Four of its many moons are visible from

the Earth. It orbits the Sun at a distance of 483 million miles

(778 million km, at its semi-major axis) and takes 11.9 Earth

years to complete each orbit.

Visual observations of the surface of Jupiter indicate that

the gaseous surface has a rapid differential rotation rate, with

the equatorial zones rotating with a period of 9 hours and 50

minutes, and higher latitudes rotating with a period of 9

hours and 56 minutes. The interior of the planet is thought to

be rotating with a period of 9 hours 56 minutes, since the

magnetic field rotates at this rapid rate. The rapid rotation

has distorted the planet so that the equatorial radius (44,365

miles, or 71,400 km) is 6.5 percent greater than the polar

radius (41,500 miles, or 66,800 km).

The outer layer of Jupiter is made of gases, with temperatures

at the top of the cloud layers estimated to be 185°F)

and a pressure of 10 bars. This is underlain by a layer of

molecular hydrogen extending to 12,425 miles (20,000 km)

below the surface. The temperature at this depth is estimated

to be 19,340°F (11,000°K), with 4 Megabars pressure. Below

this a layer of metallic hydrogen extends to 37,280 miles

(60,000 km), with basal temperatures of 44,540°F

(25,000°K), and 12 Megabars pressure. An internal rocky

core extends another 6,215 miles (10,000 km).

Jupiter’s surface and atmosphere are visibly dominated

by constantly changing colorful bands extending parallel to

the equator, and a great red spot, which is a huge hurricanelike

storm. The bands include yellows, blues, browns, tans,

and reds, thought to be caused by chemical compounds at

different levels of the atmosphere. The most abundant gas in

the atmosphere is molecular hydrogen (86.1 percent), followed

by helium (13.8 percent). Other chemical elements

such as carbon, nitrogen, and oxygen are chemically mixed

with helium. Hydrogen is so abundant on Jupiter because the

gravitational attraction of the planet is so large that it can

retain hydrogen, and most of the planet’s original atmosphere

has been retained.

Since Jupiter has no solid surface layer, the top of the

troposphere is conventionally designated as the surface. A

haze layer lies above the troposphere, then grades up into the

stratosphere. The colorful bands on Jupiter are thought to

reflect views deep into different layers of the atmosphere.

Several to tens of kilometers of white wispy clouds of ammonia

ice are underlain by a layer of red ammonium hydrosulfide

ice, then blue water ice extending to about 62 miles (100

km) below the troposphere. The cloud layers are constantly

changing, reflecting different weather and convective systems

in the atmosphere, creating the bands, and the Great Red

Spot. The leading hypothesis about the origin of the bands is

that the light or bright colored bands represent regions where

the atmosphere is warm and upwelling, whereas the darker

bands represent places where the atmosphere is downwelling

back to deeper levels. The rapid rotation of Jupiter causes

these convective bands to be wrapped around the planet in

elongate bands, unlike on Earth where they tend to form isolated

convective cells. The rotation of the planet causes a

strong zonal flow, with most wind belts moving the atmosphere

to the east at tens to several hundreds of kilometers

per hour. Several belts are moving westward however, with

the largest and fastest being the 124-mile-per-hour (200

km/hr) westward-moving belt associated with the northern

edge of the Great Red Spot. The southern edge of the Great

Red Spot is in an eastward flowing zone (also about 200

km/hr), and the Great Red Spot rotates with the planet,

caught between these two powerful belts. Many smaller ovalshaped

vortices spin off the edges of the Great Red Spot and

are thought to be smaller storms that may persist for several

or several tens of years. Many similar features are found elsewhere

on the planet.

Jupiter has many moons, with the Galilean satellites

resembling a miniature solar system. The four largest moons

include Io (1.22 Earth/Moon masses), Europa (0.65 Earth/

Moon masses), Ganymede (2.02 Earth/Moon masses), and

Callisto (1.47 Earth/Moon masses). Each moon is distinct and

fascinating, showing different effects of the gravitational

attraction of nearby Jupiter. Io and Europa are rocky planetlike

bodies, with Io exhibiting active sulfur-rich volcanism and

very young surface material. The energy for the volcanism is

thought to be the gravitational attraction of Jupiter, and the

sulfur particles emitted from the volcanoes get entrained as

charged ions in Jupiter’s magnetosphere, forming a plasma

torus ring around the planet. Europa has an icy surface with a

rocky interior, crisscrossed by cracks on the surface that may

be analogous to pressure ridges on terrestrial ice flows. The

surface is not heavily cratered and must be relatively young.

Ganymede and Callisto are both thought to be icy planets

with low densities, and Ganymede is heavily cratered, reflecting

that it has an old surface. Callisto also has many craters,

including two huge ones with multiple rings, reflecting cataclysmic

impacts in its history.

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