Jupiter

The giant planet Jupiter, like Venus, is shrouded in clouds. The clouds occur in bands of changing color – yellow, red, brown, blue, purple, gray – and there are some semipermanent markings, such as the Red Sport some tens of thousands of kilometers across. The latter make possible a determination of the planet’s period of rotation. This turns out to be less than 10 h, which means that points on Jupiter’s equator travel at the enormous speed of 45,000 km/h; the earth’s equatorial speed is only 1,670 km/h. Because of its rapid rotation, Jupiter bulges much more at the equator than the earth does.

The four satellites of Jupiter that Galileo discovered over 3 centuries ago are conspicuous objects in a small telescope. The largest is as big as Mercury, and the smallest is about the size of the moon. The other eight satellites are very small (25 to 250 km in diameter), and one of them escaped detection until 1951.

Jupiter’s volume is about 1,300 times that of the earth, but its mass is only 300 times as great. The resulting low density – only a third more than that of water – means that Jupiter cannot be composed of a mixture of rock, iron, and nickel as is the earth. Like the other giant planets (Saturn, Uranus, and Neptune), Jupiter must consist chiefly of hydrogen and helium, the two lightest elements. Probably Jupiter does not have an actual surface; instead, its atmosphere gradually becomes thicker and thicker with increasing depth until it becomes a liquid. A terrestrial analogy might be the slushy surface of a snowbank on a warm winter day.

Jupiter’s interior is believed to be very hot, about 500,000⁰ C according to some estimates, but not hot enough for nuclear reactions to occur in its hydrogen content whose release of energy would turn Jupiter into a star. But if Jupiter’s mass were 30 times greater, the increased internal pressure would push the temperature to 20 million⁰ C, and the result would be a miniature star.

Jupiter’s atmosphere apparently contains such gases as ammonia, methane, and water vapor as well as hydrogen and helium. As mentioned earlier, laboratory experiments show that when a mixture of these gases is exposed to energy sources such as are usually present in a planetary atmosphere (for instance lightning, ultraviolet light, streams of fast ions), the various organic compounds characteristic of life are formed. It seems entirely possible – some biologists think probable – that some form of life has evolved in the dense lower atmosphere of Jupiter. It is interesting that simple microorganisms such as bacteria and yeasts are able to survive when exposed to gas mixtures that simulate the Jovian atmosphere at temperatures and pressures comparable to those on Jupiter.

The American spacecraft Pioneer 10 passed close to Jupiter late in 1973 after a journey that lasted 20 months and covered over a billion kilometers. Of the wealth of information radioed back, a few items are especially notable. For example, Jupiter has a complex magnetic field about 8 times stronger than the earth’s, and this field traps high-energy protons and electrons from the sun in belts that extend many Jovian radii outward, (The Van Allen belts around the earth are similar, but 10,000 times weaker). Another important finding confirmed that Jupiter radiates over twice as much energy as it receives from the sun, which means that it has powerful internal sources of energy; by contrast, the atmospheres of Venus, Earth, and Mars are in balance, and radiate only as much energy as they get from the sun. It has been suggested that Jupiter is still contracting gravitationally, and in this contraction potential energy is turned into heat just as compressing air in a tire pump warms up the air.

Saturn

In its setting of brilliant rings, Saturn is the most beautiful of the earth’s kindred. The planet itself is much like Jupiter: similarly flattened at the poles by rapid rotation, similarly possessing a dense atmosphere, its surface similarly hidden by banded clouds. Farther from the sun than Jupiter, Saturn is considerably colder; ammonia is largely frozen out of its atmosphere, and its clouds consist mostly of methane.

The famous rings, two bright ones and a fainter inner one, surround the planet in the plane of its equator. This plane is somewhat inclined to Saturn’s orbit. Hence, as Saturn moves in its leisurely 29-year journey around the sun, we see the rings from different angles. Twice in the 29-year period the rings are edgewise to the earth; in this position they are practically invisible, which suggests that their thickness is small, perhaps 20 km as compared with the 270,000-km diameter of the outer ring.

The rings are not the solid sheets they appear to be but instead consist of myriad small bodies ranging in size from boulders a meter or more across to dust particles, each of which revolves about Saturn like a miniature satellite. No satellite of substantial size can exist close to its parent planet because of the disruptive effect of tide-producing forces, which are proportionately less the farther distant the satellite. The Roche limit is the minimum radius that a satellite orbit must have if the satellite is to remain intact; the limit is named in honor of E.A.Roche, who investigated the origin of Saturn’s rings a century ago. For Saturn the Roche limit is calculated to be 2,4 times the planet’s radius, and in fact the outer rim of the outer ring is 2,3 radii from the center of Saturn and the closest satellite never approaches closer that 3,1 radii from the center. Saturn has 10 ordinary satellites outside the rings; the innermost of these was discovered in 1966.