Jupiter

Moons - Orbital and Rotational Detail - Atmosphere - Interior - Magnetic Field - The Star that Never Was - Dust Rings

Jupiter is the giant of the planets, appropriately named after the Roman god who was the ruler of gods and men. Jove is another name for Jupiter and that is why the four largest planets are called the Jovian planets, or gas giants, because of their size and massive atmospheres.

Along with Saturn, Jupiter is the most beautiful planet to see through a telescope because of its colored pattern of belts and zones, and its red spot. We now know that these patterns result from the transport of heat from lower to upper layers in the atmosphere, which get wrapped around the planet because of its fast rotation period of 10 hours. The coloring comes from the different chemicals that make up the atmosphere.

With a telescope you can also see the four Galilean moons orbiting the planet. These are the largest moons of Jupiter and were first observed by Galileo in 1610 when he pointed his telescope to the sky. Jupiter contains 71% of the total mass of the planets, but it is only 0.1% the mass of the Sun. Its diameter is 11 times that of Earth's and one tenth that of the Sun's. In recent times we have learned a great deal about Jupiter from the various satellite missions that have gone to study this planet, in particular the Pioneer 10, Pioneer 11, Voyager 1, Voyager 2 and Galileo spacecraft.

Jupiter has at least 63 moons that vary enormously in shape, size, and orbit. The most famous of these moons are the four known as the Galilean moons -- Io, Europa, Ganymede, and Callisto -- and were discovered in 1610 by Galileo Galilei. They are also the largest moons in the Jupiter system and can be seen from the Earth with the aid of a small telescope.

Like our Moon, all four Galilean moons rotate synchronously, meaning that their period of revolution is equal to their period of rotation about their axes, so they always show the same face to Jupiter. This is due to the exertion of tidal forces from Jupiter over billions of years.

Io

Io
Io is the innermost of the Galilean moons and the third in size (its diameter is 3630 km). It takes 1.8 Earth days to orbit Jupiter. Io is composed mostly of rock and is a volcanically active moon; photographs of it show bright colors of white, orange, yellow and black, which result from sulfur deposits on the surface that come from volcanic eruptions. These eruptions result from the heating of Io's interior, caused by friction from a gravitational tug-of-war between the other Galilean moons and Jupiter.

The moon Europa

Europa
Europa is the second Galilean moon in distance from Jupiter and the smallest in size, with a diameter of 3138 km. Its rotation and revolution periods are 3.55 days. Europa is covered by a smooth surface layer of ice, it has some craters and no mountains. Streaks and cracks crisscross its surface, these are formed by tidal flexing of the moon, much like the volcanoes are formed in Io. It is speculated that under the icy surface, Europa may have an ocean of liquid water, and wherever there is liquid water in the universe, we like to think about the possibility of life existing there (but certainly not big ETs, maybe only microscopic life forms). Photographs of Europa taken with satellites show that tectonic activity is still occurring, and that Europa has a thin atmosphere of molecular oxygen that probably results from solar ultraviolet radiation braking up the water molecules on the moon's surface.

Ganymede

Ganymede
Ganymede is the largest moon in the solar system, its diameter is 5262 km—larger than Mercury! Its period of revolution is 7.15 days. It is believed to have a thick mantle of ice, as evidenced by jagged islands of dark crust separated by bands of heavily grooved ice. One side of Ganymede is heavily cratered and the other side is not, implying that the former one is older terrain than the latter because the latter has already erased all the craters that were formed by meteorites falling on the surface more than 3.9 billion years ago. Mountain ridges are present on the surface, pointing to past tectonic activity on this moon.

Callisto
Callisto is the outermost of the Galilean moons and the second in size (diameter = 4800 km). Its rotation and revolution periods are 16.69 days. The impact craters seen on its surface reveal an ancient icy crust. The absence of grooved terrain implies that it never had tectonic activity, probably because it froze too fast as it was forming. In fact, a huge impact created a basin surrounded by ripples that froze into place.

Since Jupiter's rotation axis is only inclined 3° with respect to the perpendicular of its orbital plane, it practically has no seasons. Jupiter rotates differentially, meaning that different regions of the planet rotate at different speeds. Regions near the equator rotate faster than regions near the poles. The rotation periods of these regions are measured by looking at features in the clouds as the planet rotates. Since Jupiter does not have a solid surface, the internal rotation period, i.e., the true rotation period, is the period of rotation of the magnetic field that is measured from radio emissions.

The rapid rotation of Jupiter makes it oblate. Oblateness = 0 means the object is spherical (like Venus), and oblateness = 1 means it is a straight line.

Astronomers can infer the chemical make up of a planet simply by knowing its density and comparing it to the known density of compounds here on Earth. For example, the density of water is 1 g/cm³, and the density of rock is about 3 g/cm³; therefore Jupiter is more liquid than solid or gaseous.

The table gives a heat ratio of 1.9. This is the ratio of energy that Jupiter emits to the energy absorbed from the Sun. Why does Jupiter emit almost twice the energy that it absorbs from the Sun? Partly because Jupiter is still shrinking and the contraction due to gravity produces heat, and partly because it is still cooling down from the time when it was formed. The planets and the Sun all formed at the same time, 4.6 billion years ago. All the planets were a lot hotter when they formed, and Jupiter is so big that it is taking a long time to cool down.

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