JUPITER

Key Concepts

Jupiter, the most massive planet in the solar system, consists mainly of hydrogen and helium.
Jupiter's atmosphere contains belts and zones of clouds - and the Great Red Spot.
Jupiter has a strong magnetic field created in a layer of metallic hydrogen in the planet's interior.

The Jovian planets, of which Jupiter is the largest, systematically differ from the terrestrial planets in their properties. Some differences:

Terrestrial planets (Mercury, Venus, Earth, Mars):
close to Sun,
small,
high density,
few moons,
no rings,
slow rotation,
solid surface.
Jovian planets (Jupiter, Saturn, Uranus, Neptune):
far from Sun,
large,
low density,
many moons,
circled by rings,
rapid rotation,
no solid surface.

Much of what we know about the Jovian planets comes from the Voyager 1 and Voyager 2 spacecraft, launched in 1977. In addition, the Galileo spacecraft, launched in 1989 and currently orbiting Jupiter, has taught us much about Jupiter and its moons. The Cassini spacecraft, launched in 1997 and scheduled to go into orbit around Saturn in June 2004, should tell us much about Saturn and its giant moon Titan.

(1) Jupiter, the most massive planet in the solar system, consists mainly of hydrogen and helium.

Jupiter's orbit has a semimajor axis of about 5 A.U. The orbital period of Jupiter is just under 12 years. Jupiter is spinning rapidly on its axis; the rotation period of Jupiter is only 10 hours. The speed of rotation at Jupiter's equator is 27 times the speed of rotation at the Earth's equator. Thanks to its rapid rotation, Jupiter is flattened at its poles and bulges outward at its equator. The equatorial diameter of Jupiter is 6.5% greater than its polar diameter. (The equivalent difference in diameters for the slowly rotating Earth is only 0.3%; the Earth really is close to spherical.)

[Image credit: Hubble Space Telescope]

Jupiter, unlike the terrestrial planets, has been able to retain the hydrogen and helium in its atmosphere.

Jupiter's escape velocity = 60 kilometers/second
Jupiter's air temperature = 170 Kelvin

(The above numbers are measured at the top of Jupiter's clouds.) Compare the values for Jupiter with the equivalent numbers for Earth:

Earth's escape velocity = 11.2 kilometers/second
Earth's air temperature = 300 Kelvin

Hydrogen and helium remain gravitationally bound to Jupiter; however, they are able to escape from Earth, with its higher temperature and lower escape velocity. As a consequence, Jupiter consists almost entirely of hydrogen and helium (which together make up 95% of its mass) and is much more massive than the Earth (roughly 318 times more massive).

The temperature of the clouds of Jupiter turned out to be about 15 degrees Kelvin warmer than expected. In fact, if you add up the amount of energy that Jupiter receives from the Sun in the form of light and compare it to the amount that Jupiter radiates away in the form of infrared light and radio waves, the books don't balance. Jupiter radiates away nearly twice as much energy as it receives from the Sun. Where does the extra energy come from? Energy can't be created out of nothing. Jupiter must have an internal energy source which generates the extra energy radiated away into space. Most of the internal energy is simply residual heat, left over from when Jupiter, like all the other planets, was much hotter than it is today. Since Jupiter is the largest, most massive planet, it is the one that has taken the longest to cool off. (The Earth's surface is heated from above by sunlight at an average rate of 350 watts per square meter; it is heated from below by the Earth's internal heat at an average rate of only 0.06 watts per square meter. Jupiter's cloud layer is heated from above by sunlight at an average rate of 13 watts per square meter; it is heated from below by Jupiter's internal heat at an average rate of 10 watts per square meter.)

(2) Jupiter's atmosphere contains belts and zones of clouds - and the Great Red Spot.

The atmosphere of Jupiter is heated from above by the Sun and from below by the internal heat of Jupiter. The additional energy pumped into the atmosphere from below means that Jupiter has stronger winds and larger storms than you would expect for a planet so far from the Sun.

When we look at Jupiter (as in the Hubble Space Telescope image below)

we are looking at its outer layer of clouds, which consist of crystals of frozen ammonia (NH₃). Frozen ammonia is white; the colors of the clouds result from the presence of trace amounts of complex chemical compounds (probably based on sulfur and/or phosphorus).

A look at Jupiter's atmosphere tells you that it is striped horizontally; that is, the clouds are drawn out in long bands that circle the planet east to west. The cloud bands consist of alternate zones and belts.

Zones (lighter in color) are high-pressure, high-temperature regions.
Belts (darker in color) are low-pressure, low-temperature regions.

The boundaries between zones and belts are where the winds of Jupiter blow most swiftly. At the boundaries, strong winds, comparable to the jet streams of Earth, blow at speeds up to 400 kilometers per hour (a hurricane on Earth blows at 120 km/hr).

Winds on Jupiter blow predominantly east to west, or west to east. There are, however, numerous circular storms:

[Image credit: NASA, Galileo spacecraft]

The largest and longest-lived circular storm on Jupiter is the famous Great Red Spot, shown in the Voyager image below. (Click on the small image to get a better view, including the intricate details of the swirling clouds.)

The Great Red Spot is an enormous circular storm in the southern hemisphere of Jupiter. It varies a bit in size, but at maximum it is 40,000 kilometers across (about three times the diameter of the Earth!) The Great Red Spot is a high pressure system; air flows outward from the center of the Spot, shoved by the high pressure air behind it. The air is deflected to the left (remember, flowing air is deflected to the left in the southern hemisphere by the Coriolis effect). As a result, the Great Red Spot whirls around counterclockwise. It takes about 6 days to complete one rotation. The Great Red Spot was first seen by Earthly astronomers in 1664; thus, it has existed for over three centuries. Not only is it a BIG storm, it is a long-lasting one.

(3) Jupiter has a strong magnetic field created in a layer of liquid metallic hydrogen in the planet's interior.

Jupiter is differentiated. Models for the interior of Jupiter indicate that it has four distinct layers. Working from the outside in:

Ordinary molecular hydrogen (and helium): mostly molecular hydrogen (H₂) and helium at relatively low pressure.
Liquid metallic hydrogen (and helium): higher pressures squeeze electrons out of the hydrogen molecules, converting the hydrogen from a non-conducting liquid to an electrically conducting metal. (Hydrogen doesn't become a liquid metal until it is subjected to a pressure equal to 1.4 million times the air pressure at sea level on Earth.)
Liquid water, methane, and ammonia: The high temperature keeps these ``ices'' in a liquid state.
Rocky core: The mass of the central, solid, rocky core, is thought to be about 8 times that of the Earth. The immense pressures in the central core (about 70 million atmospheres) squeeze it down to a density roughly twice that of lead.

The inner rocky & `icy' core of Jupiter is hidden away beneath a layer of hydrogen and helium tens of thousands of kilometers thick.

Note that Jupiter has a layer of liquid metal in its interior, just as the Earth does. As on Earth, a magnetic field is generated by circulation of the liquid metal. Since Jupiter has rapid rotation and a very thick layer of metallic hydrogen, it is no surprise that the magnetic field of Jupiter is very strong. At the level of Jupiter's clouds, the magnetic field is 14 times stronger than the magnetic field at the surface of the Earth. Jupiter's huge magnetosphere traps a large number of charged particles.

See Jupiter's Moon's.

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