Jupiter's Ring

Unlike Saturn's intricate and complex ring patterns, Jupiter has a simple ring system that is composed of an inner halo, a main ring and a Gossamer ring. To the Voyager spacecraft, the Gossamer ring appeared to be a single ring, but Galileo imagery provided the unexpected discovery that Gossamer is really two rings. One ring is embedded within the other. The rings are very tenuous and are composed of dust particles kicked up as interplanetary meteoroids smash into Jupiter's four small inner moons Metis, Adrastea, Thebe, and Amalthea. Many of the particles are microscopic in size.

The innermost halo ring is toroidal in shape and extends radially from about 92,000 kilometers (57,000 miles) to about 122,500 kilometers (76,000 miles) from Jupiter's center. It is formed as fine particles of dust from the main ring's inner boundary 'bloom' outward as they fall toward the planet. The main and brightest ring extends from the halo boundary out to about 128,940 kilometers (80,000 miles) or just inside the orbit of Adrastea. Close to the orbit of Metis, the main ring's brightness decreases.

The two faint Gossamer rings are fairly uniform in nature. The innermost Amalthea Gossamer ring extends from the orbit of Adrastea out to the orbit of Amalthea at 181,000 kilometers (112,000 miles) from Jupiter's center. The fainter Thebe Gossamer ring extends from Amalthea's orbit out to about Thebe's orbit at 221,000 kilometers (136,000 miles).

Jupiter's rings and moons exist within an intense radiation belt of electrons and ions trapped in the planet's magnetic field. These particles and fields comprise the jovian magnetosphere or magnetic environment, which extends 3 to 7 million kilometers (1.9 to 4.3 million miles) toward the Sun, and stretches in a windsock shape at least as far as Saturn's orbit - a distance of 750 million kilometers (466 million miles).

Jupiter's moons.Nearly four centuries ago Galileo Galilei turned his homemade telescope towards the heavens and discovered three points of light, which at first he thought to be stars, hugging the planet Jupiter. These stars were arranged in a straight line with Jupiter. Sparking his interest, Galileo observed the stars and found that they moved the wrong way. Four days later another star appeared. After observing the stars over the next few weeks, Galileo concluded that they were not stars but planetary bodies in orbit around Jupiter. These four stars have come to be know as the Galilean satellites.

Over the course of the following centuries another 12 moons were discovered bringing the total to 16. Another 12 satellites have been recently discovered and given provisional designators until they are officially confirmed and named. Finally in 1979, the strangeness of these frozen new worlds was brought to light by the Voyager spacecrafts as they swept past the Jovian system. Again in 1996, the exploration of these worlds took a large step forward as the Galileo spacecraft began its long term mission of observing Jupiter and its moons.

Twelve of Jupiter's moons are relatively small and seem to have been more likely captured than to have been formed in orbit around Jupiter. The four large Galilean moons, Io, Europa, Ganymede and Callisto, are believed to have accreted as part of the process by which Jupiter itself formed.

TEXT 6

MARS

Mars is the fourth planet from the Sun and is commonly referred to as the Red Planet. The rocks, soil and sky have a red or pink hue. The distinct red color was observed by stargazers throughout history. It was given its name by the Romans in honor of their god of war. Other civilizations have had similar names. The ancient Egyptians named the planet Her Descher meaning the red one.

Before space exploration, Mars was considered the best candidate for harboring extraterrestrial life. Astronomers thought they saw straight lines crisscrossing its surface. This led to the popular belief that irrigation canals on the planet had been constructed by intelligent beings. In 1938, when Orson Welles broadcasted a radio drama based on the science fiction classic War of the Worlds by H.G. Wells, enough people believed in the tale of invading Martians to cause a near panic.

Another reason for scientists to expect life on Mars had to do with the apparent seasonal color changes on the planet's surface. This phenomenon led to speculation that conditions might support a bloom of Martian vegetation during the warmer months and cause plant life to become dormant during colder periods.

In July of 1965, Mariner 4, transmitted 22 close-up pictures of Mars. All that was revealed was a surface containing many craters and naturally occurring channels but no evidence of artificial canals or flowing water. Finally, in July and September 1976, Viking Landers 1 and 2 touched down on the surface of Mars. The three biology experiments aboard the landers discovered unexpected and enigmatic chemical activity in the Martian soil, but provided no clear evidence for the presence of living microorganisms in the soil near the landing sites. According to mission biologists, Mars is self-sterilizing. They believe the combination of solar ultraviolet radiation that saturates the surface, the extreme dryness of the soil and the oxidizing nature of the soil chemistry prevent the formation of living organisms in the Martian soil. The question of life on Mars at some time in the distant past remains open.

Other instruments found no sign of organic chemistry at either landing site, but they did provide a precise and definitive analysis of the composition of the Martian atmosphere and found previously undetected trace elements.

AtmosphereThe atmosphere of Mars is quite different from that of Earth. It is composed primarily of carbon dioxide with small amounts of other gases. The six most common components of the atmosphere are:

Carbon Dioxide (CO2): 95.32%
Nitrogen (N2): 2.7%
Argon (Ar): 1.6%
Oxygen (O2): 0.13%
Water (H2O): 0.03%
Neon (Ne): 0.00025 %

Martian air contains only about 1/1,000 as much water as our air, but even this small amount can condense out, forming clouds that ride high in the atmosphere or swirl around the slopes of towering volcanoes. Local patches of early morning fog can form in valleys. At the Viking Lander 2 site, a thin layer of water frost covered the ground each winter.

There is evidence that in the past a denser martian atmosphere may have allowed water to flow on the planet. Physical features closely resembling shorelines, gorges, riverbeds and islands suggest that great rivers once marked the planet.

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