Jupiter

Jupiter is the largest planet in the solar system, and by far the most massive. It contains over twice as much matter as all of the other planets combined. Jupiter is a very different planet from the Earth, or any of the other inner planets. It is composed mostly of liquid and gaseous hydrogen, and has no solid surface. For this reason, Jupiter is the first of the "gas giant" planets.

Orbit and Observation

Jupiter is the fifth planet from the Sun, and orbits at an average distance of 480 million miles (780 million km) from it - that is, 5.20 times as far as the Earth. Jupiter takes 11.86 years to complete one orbit around the Sun. With an equatorial diameter of about 89,000 miles (143,000 km), Jupiter is over 11 times the diameter of the Earth. In spite of its size, Jupiter is the fastest-rotating planet, taking only 9 hours and 56 minutes to complete one rotation. Because of its rapid rotation, Jupiter is noticeably flattened at the poles.

Jupiter is large enough, and close enough to Earth, to show features that are easily visible in a small telescope. The most prominent are the equatorial bands, which look like brownish stripes. Higher magnification will reveal many more features, and show the complexity of Jupiter's atmosphere. The most prominent feature visible to small telescopes is the Great Red Spot (GRS). This is a long-lived cyclone in Jupiter's southern hemisphere; it has lasted for at least 300 years, and is larger than the entire Earth. It rotates counter-clockwise and, interestingly, moves in the opposite direction as the cloud belts around it.

Jupiter has four large moons, first discovered by Galileo in 1610: Io, Europa, Ganymede, and Callisto. These "Galilean" moons are easily seen in a small telescope or even binoculars, lined up along Jupiter's equatorial plane. They move quickly in their orbits around Jupiter; one can see their motion relative to the planet over a period of a few hours. The shadows of the Galilean moons are often easily seen as well, transiting across the face of Jupiter.

Composition and Structure

Most of Jupiter's mass is composed of hydrogen (90%) and helium (9%). The colorful patterns in its clouds are produced by trace amounts of methane, ammonia, water ice crystals, and other elements. Beneath the clouds, the atmospheric pressure rises quickly, compressing the hydrogen and helium into a liquid state. Further down, the pressure is so great that electrons are stripped from hydrogen atoms, and wander freely through the planet's interior like those in a metal. This liquid metallic hydrogen comprises the bulk of Jupiter's mass. At its center, Jupiter may possess a core of rocky material about the size of the Earth.

Jupiter's electrically-conductive interior and rapid rotation produce a very large and strong magnetic field, some 20,000 times as powerful as the Earth's. In addition, Jupiter possesses a strong internal heat source, radiating nearly 2.5x as much energy into space as it receives from the Sun. This heat source is believed to be residual energy left over from the gravitational contraction that formed Jupiter. It is what powers much of the turbulent weather that we see at the cloud tops. Nevertheless, in spite of Jupiter's internal energy source, its surface is still very cold: -234°F (-148°C) at the cloud tops.

If Jupiter contained even more matter, the pressure and temperature at its center would be so great that nuclear reactions would take place. Hydrogen would fuse into helium, and Jupiter would be a star, like our Sun. For that reason, Jupiter has sometimes been described as a "failed star". However, Jupiter would have to be much larger - about ten times more massive than it actually is - in order for nuclear fusion reactions to take place inside its core.

Exploration and Ring System

Jupiter has been studied extensively from Earth, and visited by several spacecraft, starting with Pioneer 10 in 1973. Our knowledge of Jupiter and its moons increased dramatically with the Voyager 1 and 2 flybys in 1979, and again with the Galileo mission, which orbited Jupiter from 1995 to 2003, and dropped an instrumented probe into Jupiter's atmosphere. The Galileo orbiter itself was destroyed by being deliberately flown into Jupiter in 2003, to avoid any possibility of later impacting and contaminating the surface of Jupiter's moon, Europa.

Voyager 1 discovered that Jupiter has a faint ring system, like Saturn. But unlike Saturn's, Jupiter's rings are dark and faint, and nearly invisible from Earth.

The Moons of Jupiter

After the Sun, Jupiter is the most influential body in the solar system. It has at least 63 moons, including the four planet-sized objects discovered by Galileo. Jupiter's inner three Galilean satellites (Io, Europa, and Ganymede) have interesting orbital characteristics. All four have nearly circular orbits, with very low inclinations to Jupiter's equatorial plane. They are in a three-way, 1:2:4 orbital resonance. In other words, for each orbit of Ganymede, Europa orbits Jupiter twice, and Io orbits Jupiter four times. All four Galilean moons are tidally locked with Jupiter, forever showing the same face toward the giant planet.

Before the Voyager spacecraft, little was known about any moons in the solar system other than Earth's. The expectation was that the other planets' moons would be barren, cratered, and rocky, like our own. Instead, the first close-up views of Jupiter's Galilean moons showed that they are an amazing variety worlds in their own right. Io is the most volcanically active body in the solar system. Europa has the smoothest surface of any moon in the solar system; its icy surface floats on an ocean of liquid water that makes it one of the most likely candidates in the solar system for life. Ganymede is the largest moon in our solar system; its surface is mottled by light and dark regions, long grooves, mountain ranges, and "lava" flows of water ice, implying a complex geological history. Callisto is the most heavily cratered object in the solar system, and with a nearly-complete absence of any geological activity on its surface, it is thought to be a world long dead.

All the rest of Jupiter's 63+ moons are small, less than 200 km in diameter. They are also dark, with albedos of about 0.04. They form a complex system, and are classified by their orbital properties into "families", in the same manner as asteroids. Most have retrograde orbits, and are in fact believed to be captured asteroids.

Another object captured into orbit around Jupiter was the ill-fated comet Shoemaker-Levy 9, which broke into more than a dozen fragments that collided with Jupiter in July, 1994. This was the first comet impact to have been observed on another planet, and it left Earth-sized blemishes that lasted for weeks before being dissipated by Jupiter's turbulent atmosphere.

Almost exactly fifteen years later, in July, 2009, a similar atmospheric "scar" appeared near Jupiter's south pole. It was believed to have been caused by a lone comet or asteroid strike, though the impact itself was not observed.