The Origin and Evolution of the Solar System

12 The structure of the Solar Systemcratered uplands on one hemisphere and smoother ‘filled’ terrain on the other. OnMars the division is approximately north–south with the volcanoes in the north—in contrast to the Moon whose smooth hemisphere faces the Earth. Unlike theMoon the Martian surface has channel features which have almost certainly beencaused by running water (Pollack et al 1990). The polar caps contain substantialpermanent deposits of ice with the addition of solid CO ¾ which comes and goeswith the seasons. Since the orbit of Mars has an eccentricity which varies withtime and may rise to 0.14 it is possible that Mars has had wet episodes in its existence.The present surface pressure is about 6 millibar (mb) and its atmosphere is95% CO ¾ .1.3.2 The major planetsThe four major planets differ markedly in both structure and appearance fromthe terrestrials. Even a small telescope shows Jupiter as the most colourful anddynamic planet in the system. The banded appearance of its upper atmosphere,composed mainly of molecular hydrogen and helium, is due to the rapid rotationof the planet and has been studied for over three centuries. There is no visiblesolid surface and so no evidence of any collision history. However, the fact thatJupiter probably has absorbed many smaller bodies was well illustrated by the collisionsof the broken-up Comet Shoemaker–Levy 9 in 1994. These collisions, bythrowing up material from deep inside the planet, acted as probes for its internalcomposition.The atmospheric bands parallel to the equator contain spots or ovals of variouscolours whose longevity seem to be size-dependent. The largest of these isthe Great Red Spot (GRS) that has persisted for more than 300 years. This hugefeature is roughly elliptical with axes some 25 000 by 13 000 km. Its colour is notconstant but it is a notable feature even when its red colour fades. The ovals andspots are thought to be eddies formed between neighbouring bands moving withrelative speeds of up to 150 m s ½ . This theory is a plausible one for applicationto small ovals with a lifetime of a few days but it seems not too successful in thecase of the GRS (Ingersoll 1990).In most respects Saturn is similar to Jupiter. The atmosphere has the samecomposition and the body of the planet has a banded appearance, although the differentiationof zones is far less prominent. With only about one-third of the massof Jupiter, Saturn is less compressed and its rapid rotation makes it more oblate.Wind speeds in the upper atmosphere are greater even than those of Jupiter, reaching500 m s ½ . The most remarkable feature of Saturn is, of course, its extensivering system (figure 1.5). It is now known that all the major planets have one ormore orbiting rings, but those of Jupiter, Uranus and Neptune are much less substantialthan those of Saturn and more difficult to detect and observe. Uranus andNeptune also have hydrogen–helium atmospheres but have a much more uniformappearance than the two larger gas giants. Neptune does have a Great Dark Spot,a storm system similar to the GRS on Jupiter.