Jupiter

INTRODUCTION

The nearest of the outer planets to the Sun, Jupiter was named after the most powerful god in Roman mythology, and aptly so, since it is the largest planet in the solar system.

BASIC FACTS

Jupiter is a gas giant. It is the 4th brightest object in the sky, after the Sun, Moon and Venus, and as such makes it easily identifiable. It has a sidereal orbit of 11.9 Earth years and an orbital semi-major axis of 5.20 A.U. (778 million km). Jupiter has an eccentricity of about 0.05. As such, its brightness varies throughout its orbit. At opposition, Jupiter can come close to a distance of 3.95 A.U., revealing much of the planet's surface.

Jupiter has an equatorial radius of 71400 km and a polar radius of 66800 km, indicating that Jupiter bulges at its equator. This relatively pronounced bulge is due to Jupiter's rapid rotation rate of 9 hrs 56 mins, and the fact that Jupiter is made up of gas. Imagine a ball of loosely-held material being spun rapidly. The high-speed rotation would cause the ball to bulge at its midsection. This is exactly what happens to Jupiter.

In the case of Jupiter, observing surface features to calculate rotation rate would be impossible due to differential rotation of Jupiter's atmosphere. Astronomers had to resort to observing the period of radio emissions from Jupiter's magnetosphere.

Jupiter's mass of 1.9 x 10^27 kg puts it 318 times heavier than the Earth. Its high gravity allows it to retain its gaseous atmosphere. If we take the masses of all the other planets combined, it would still be less than half of Jupiter's mass. It is theorized that Jupiter has a small dense rocky core 10 to 20 times the mass of the Earth.

There are 16 known moons orbiting Jupiter, with varying sizes and properties. The 4 largest moons, known as the Galileo moons, are visible from Earth through a telescope, or even on a clear night sky, with your naked eye.

ATMOSPHERE

By making spectroscopic studies of sunlight reflected from Jupiter, scientists are able to determine the composition of Jupiter's atmosphere. Molecular hydrogen (86%) is the most abundant gas, followed by helium (13.8%). The remaining 1% is made up of methane, ammonia and water vapour.

Jupiter is well-known for the Great Red Spot, an oval-shaped Earth-size hurricane that has lasted for hundreds of years. Yet another atmospheric feature, perhaps not as well known but still as distinctive, is theseries of ever-changing multi-coloured atmospheric bands that lie parallel to the equator.

Complex and continuous chemical reactions occur in Jupiter's atmosphere, producing clouds that lie at different altitudes. The atmosphere is in constant motion, evident from the various atmospheric bands and the Great Red Spot. It is believed that the energy required for these processes come from the planet's own heat, solar UV radiation from the Sun, high-energy particles escaped from the planet's magnetosphere and lightning discharges within the atmosphere. However we are still unable to fully comprehend the reason for Jupiter's turbulent atmosphere.

The coloured atmospheric bands we see are made up of bright zones and dark belts that vary both in position and intensity throughout the year. However, there is no fixed pattern by which they vary. The bands are the result of convective currents in Jupiter's atmosphere. Convection produces pressure differences between the zones and the belts that remind one of the high and low pressure systems that cause weather on Earth. As a result, the zones lie slightly above the belts. There is also a stable zonal flow from east to west below the bands.

British scientist Robert Hooke first discovered the Great Red Spot in the mid-seventeenth century. It is actually a huge and persistent atmospheric storm that has been around since its discovery. Its size varies, but it is about twice the diameter of Earth. Gas rotates in a counter-clockwise direction around the spot. The spot has a period of 6 days. No one knows for sure where the energy for the spot comes from. In fact, there are many such storms present in Jupiter's atmosphere, but most are of a much smaller scale and do not last as long, disappearing after many years or decades.

SHOEMAKER-LEVY 9'S ENCOUNTER WITH JUPITER

Shoemaker-Levy 9 was a comet with a somewhat elliptical orbit that partially intersected that of Jupiter's. On approaching Jupiter in July 1992, it got too close and broke up under Jupiter's gravitational effect. The remnants then went on to collide with Jupiter's atmosphere in July 1994. Read more about this at our Comets section.

INTERNAL STRUCTURE

Jupiter emits about twice as much energy as it receives from the Sun. As such, scientists theorize that Jupiter must have its own heat source. One such theory states that the heat source is the slow emission of gravitational energy released and subsequently trapped during the planet's formation. Jupiter's clouds are less than 200 km thick. The atmosphere becomes denser and denser until at a depth of a few thousand kilometres, the gaseous hydrogen in the atmosphere gradually turns into liquid state. The hydrogen becomes a solid at a depth of about 20000 km. Metallic hydrogen is an excellent conductor of electricity, which explains for why Jupiter's magnetic field is so strong. At its centre is a core about 10 to 20 times the mass of Earth and is probably made up of heavier, "rocky" material.

MAGNETOSPHERE

We were only able to realize the full extent of Jupiter's magnetic field in the mid-1970s, when the Mariner and Voyager spacecraft flew by the planet. Jupiter's magnetosphere is made up of mostly electrons and protons that resemble Earth's Van Allen belts but is much larger. Its magnetic field is so strong that it is able to accelerate the electrons and protons close to the speed of light.

Jupiter's magnetosphere has been measured to be almost 30 million km across, even larger than the Sun's! Another feature of Jupiter's magnetosphere is that it has a tail that extends to Saturn's orbit. Jupiter's outer magnetosphere is also highly unstable and is easily deflected by solar winds. In the inner magnetosphere, the planet's rapid rotation has forced the charged particle into a flat sheet, lying on the planet's magnetic equator. So large an extent is Jupiter's magnetosphere that it is no wonder it is 20000 times stronger than the Earth's.

MOONS

Jupiter has 16 moons that are known to Man. Its 4 largest moons – the Galilean satellites – are each comparable in size to the Moon. Named after the mythical attendants of Jupiter, they are Io, Europa, Ganymede and Callisto.

Within the orbit of Io lies 4 small moons. The largest of the 4 is Amalthea, an irregularly shaped rock less than 300 km across.

Beyond the Galileo moons lie 8 more small satellites. These 8 moons can be split into 2 main groups. Scientists theorize that these 8 moons must have been part of 2 bodies that were captured during Jupiter's formation. They have since broken up , resulting in the 2 groups of 4 moons orbiting around Jupiter. These 2 groups of moons orbit around Jupiter in 2 different orbits. One group orbits in the prograde sense while the other orbits in the retrograde sense, which further serves to enhance the fact that they were originally 2 different bodies.

RINGS

When Voyager flew by Jupiter in 1979, it discovered a faint thin ring of matter encircling Jupiter in the plane of the equator. However, the ring is only a few tens of kilometres thick, very much less distinct than those of Saturn's.