JUPITER

At 5.2 Astronomical units (AU), Jupiter is the 5th planet from the sun and the most massive of the Giant Planets, which extend in the outer solar system up to heliocentric distances of 30 AU. The planet has been rightly named so after the supreme Roman God, Jupiter. Its mass amounts to 318 earth masses, and its diameter is 11 times the terrestrial one. Like the other giant planets, Jupiter has a comparatively low density which reflects its chemical composition, mostly dominated by hydrogen and helium.

As one of the brightest objects in the sky, Jupiter has been known from antiquity. Its astronomical observation started almost four centuries ago with the appearance of the first refractors and telescopes. Galileo is believed to be the first astronomer to have discovered, in 1610, the four large satellites orbiting the planet, known since that time as the Galilean satellites. Our knowledge of Jupiter started with the beginning of telescopic observations, more than three centuries ago. Over the last three decades, considerable progress has come from space exploration, but also from the on-going technological developments of ground-based astronomy.

The giant planet is primarily composed of hydrogen with about quarter of its mass being helium; it may also have a rocky core of heavier elements. Due of its rapid rotation, Jupiter is shaped like an oblate spheroid (it possesses a slight but noticeable bulge around the equator). The outer atmosphere is visibly segmented into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. A prominent result of this is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope.

Jupiter has a ring system and many satellites. Its ring is very tenuous; the planet is surrounded by 63 satellites (discovered to date), including the four main ones known as the Galilean satellites. Another prominent feature of Jupiter is its magnetosphere, more massive than those of the other giant planets, and comparable in some regards to the terrestrial magnetosphere. The first step in the space exploration of the Jovian system was its encounter by the Pioneer 10 and 11 spacecraft, in 1973 and 1974 respectively.

The Pioneer missions provided the first high-resolution images of the atmospheric structure, and an in situ investigation of its magnetosphere. The space era really started in 1979 when the two Voyager spacecraft encountered the planet. Not only did the cameras reveal an unexpected variety and complexity in the cloud structures and their dynamics, but they also discovered the ring system of Jupiter, active volcanoes on Io (providing the first evidence for active volcanism outside the Earth), and a great diversity in the surfaces of the other Galilean satellites. The Galileo orbiter mission of 1989 has also provided considerable amount of information about the Jovian atmosphere and also discovered the possible existence of water in its moon, Europa.

Questions to Ponder:

• Why doesn't the ammonia and methane in Jupiter's atmosphere sink into the core owing to its high gravitational pull?