The planet ‘JUPITER’
Jupiter is the largest planet in the Solar System, weighing more than twice as much as all the other planets put together. It spins on its axis faster than any other planet – one rotation takes less than 100 hours. The visible surface of Jupiter consists of swirling, distinctively banded clouds. Beneath them, the planet is composed mostly of liquid hydrogen and helium.
Jupiter is one of the most interesting planets for the amateur observer. It is usually the brightest planet after Venus (although at times Mars can become slightly brighter), and it is so large that even binoculars show it as a rounded disc. A small telescope reveals the most prominent cloud bands, which are usually the dark North and South Equatorial Belts, either side of the bright Equatorial Zone. Either side of the bright Equatorial Zone. An aperture of 75 mm (3 in) reveals some of the larger features within the clouds. Over a period of about 10 minutes, these features can be seen to be moving. A small telescope also shows Jupiter’s disc to be elliptical – the planet’s equatorial diameter is over 9,000 km (5,600 miles) greater than its polar diameter, due to its rapid rotation. Jupiter looks biggest and brightest when it is at opposition (see p.15), which occurs every 13 months. At that time, at a magnification of only 40 times it will appear as large as the full moon looks to the naked eye.
OBSERVING THE GALINEN MOONS
Jupiter’s four largest moon – lo, Europa, Ganymede, and Callisto – were first seen in 1610 by the Italian scientist Galileo Galilei, and are now known as the Galilean moons. Through a small telescope or binoculars, they appear like faint stars lined up on either side of the plants equator, and change position as they orbit the planet. Sometimes one or more may be missing, either passing in front of or behind Jupiter, or lost in its shadow.
STRUCTURE AND ATMOSPHERE
Jupiter is enveloped in a cloudy atmosphere about 1,000 km (600 miles) deep. Clouds of different composition form at various levels depending on the temperature and pressure, which both increase with depth. Beneath the clouds there is no solid surface. Instead, hydrogen and helium are compressed into liquids by Jupiter’s great gravitational pull. Deep in the interior, where liquid hydrogen acts like molten metal, convection produces a strong magnetic field that extends for millions of kilometers into space. At Jupiter’s center there is thought to be a rocky core.
CLOUD BELTS AND STORMS
Jupiter’s rapid rotation draws its clouds into light and dark bands. In the lighter areas, termed zones, gas rises from the warm interior and condenses to form high-altitude clouds. The dark colures of the belts range from red an brown to blue, depending on the compounds that they contain. Individual cloud features seldom last more than a few weeks, but some white oval clouds have lasted for over half a century. The most prominent feature is the Great Red Spot, on the southern edge of the South Equatorial Belt. This swirling, high-altitude storm cloud rotates anti-clockwise once a week or so. It has been tracked since 1831 although a similar feature was seen in the 17th century.
RINGS AND MOONS
Jupiter’s 28 known satellites (including 12 very small ones recently discovered) fall into three groups: the inner eight, including the four Galilean moon, which have circular orbits in the planet’s equatorial plane: the middle six, which have elliptical orbits inclined between 25 and 46 degrees to Jupiter’s equator; and the outer 14, which have elliptical an retrograde (east-to-west) orbits. The last two groups are probably captured asteroids. Apart from Galilean moons, the satellites are small and faint. Jupiter has a faint ring of dust, over 60,000 miles (100,000 km) wide.
THE GALLILEAN MOONS
Lo, the innermost Galilean satellite is similar in size to our Moon, having a diameter of 2,260 miles (3,630 km). it is covered with yellow-orange sulfur, erupted by volcanoes that are still active. Its interior is kept molten by Jupiter’s gravitational forces. Europa, 1,950 miles (3,140 km) in diameter, is the smallest of the four. Its smooth, icy surface resembles a cracked eggshell. Ganymede-diameter 3,270 miles (5,260 km) – is the largest moon in the Solar System, bigger than Mercury. Its compiles, partly cratered surface has dark patches and lighter grooves. Outer-most of the four moons is Callisto- diameter 2,980 miles (4,800 km) – the dark surface of which is covered with impact craters.