THE PLANETS, ASTEROIDS AND COMETS

 THE PLANETS, ASTEROIDS AND COMETS

Planets

Mercury-the Elusive' Planet

Mercury, the planet nearest to the Sun, has long remained a mysterious body. Because .of the Sun's glare, it is difficult to see it clearly all the mare so because it is very small. Mercury appears for a brief time as a morning object in the eastern sky, rising just Solar System before sunrise. Then it can be briefly seen as an evening object in the western sky, setting just after sunset. Even through the largest telescopes in use today, Mercury appears less distinct than the Moon does to the naked eye.

Mercury

Photographs of the surface of Mercury show craters and wrinkles on it, separated by  substantially large smooth areas . Mercury possesses no atmosphere. It has a temperature of almost  + 427°C on the side facing the Sun to a low of almost -270°C on its dark side. It has no satellite.

Venus-the Clouded Planet

Venus, our nearest planetary neighbour, only 40 million km away, has its surface hidden from view by a dense yellowish-white cloud, which extends to 80 km above the surface. Venus appears to be the third brightest object in the sky after the Sun and the Moon, because of its short distance from us and because the white cloud reflects almost 76% of the sunlight that falls on it. Venus appears so bright at times that, under ideal atmospheric conditions, it may be seen with the unaided eye in daytime. Venus can be best seen about three hours after sunset when it is a night object or about three hours before sunrise when it is a morning obiect. Venus appears to go through phases, quite like those oi the Moon. It requires almost 20 months for an observer to see Venus in all its phases.

As revealed from the space probes, Venus has turned out to be a broiling hot planet. The surface temperatures of Venus are around 480°C. The atmosphere of Venus is made up of 96%  carbon dioxide gas and clouds of sulphuric acid with small quantities of hydrochloric and hydrofluoric acid. There we small traces of water vapour, nitrogen, argon, sulphur dioxide and carbon monoxide gases. The atmospheric pressure is 90 times the pressure we feel from the Earth's atmosphere. With its searing heat, crushing pressures and poisonous gases, Venus seems less the goddess of love of mythology and more an incarnation of hell! Life cannot survive on Venus.

Venus 

The high surface temperature of Venus comes about through what is known as the greenhouse effect. Sunlight passes through the clouds and atmosphere of Venus, and reaches its surface. The surface on being heated, gives out infrared radiations. The carbon dioxide in the atmosphere of Venus does not let the infrared radiation escape. Thus, the heat of the Sun is efficiently trapped with only very little being able to escape. As a result, the surface temperature rises.

Now that human beings have unveiled the mysteries of the clonded planet, many imaginative ideas have been put forward to turn Venus into a habitable plane,. One idea is to introduce small living organisms, which can consume carbon dioxide and give off oxygen, into the planet's atmosphere. So, as time passes, the greenhouse effect will  become less as carbon dioxide gas decreases in the atmosphere. This will'cool the planet's surface, and water vapour will condense. As rain falls, the heat-retaining clouds will begin to clear. This will create a planet with an oxygen-rich atmosphere and a cool temperature to sustain a variety of life forms. It may even become suitable for human beings to live! This may seem like a fantasy today. But, there are plausible elements in this idea and it might become a reality in the not so distant future!

Mars-the Red Planet

After Venus, Mars is our closest neighbour. It seems very earth-like. There are ice-caps on its poles, drifting white clouds and raging dust storms in its atmosphere. Seasonally changing patterns occur on its red surface. There are large dark areas on its surface called maria (meaning 'seas'). It even has a twenty-four-hour day. Mars experiences summer and winter seasons, each of which lasts for almost six earth months. However, the planet's distance from the Sun causes these seasonal changes to be more extreme.

It is tempting to think of Mars as an inhabited world. In 1877, the Italian astronomer, Schiaparelli, observed an intricate network of single and double lines criss-crossing the bright arps of the planet Mars. He gave them the name of canali. Canali in Italian means channels or grooves. But it was promptly translated into English as 'canals'. Since then, people have wondered who had made these canals and how. Were there living beings on the planets?

Mars

Many satellites have now been sent into orbit around Mars. Two automated laboratori,es have been landed on its surface. The entire planet has been mapped. Martian surface has craters of sizes, ranging from 5 km to 121 km in diameter, created by meteorite impact . It also has enormous volcanoes. The largest volcano on Mars, Olympus Mons or Mount Olympus is nearly three times as high as Mount Everest. It is not active any more. The Martian surface has deep ridges and valleys. Pictures alsa show islands made where water once flowed around the existing craters, and river beds, dry for hundreds of millions of years. The surface features indicate that Mars may have had both atmosphere and ocean in the past. Viking space probes, send by U.S.A., did find evidence that 'liquid water once flowed on the planet and the atmosphere was also more dense than what it is now. Martian soil is mostly like the Earth's soil made up mainly of silicates. However, about 16 % of the soil is made up of iron oxide, giving it its red colour.

The atmospheric pressure on Mars is very low. It is comparable to what it is on the Earth at a height of 32 km from sea level. Thus, Mars has a very thin atmosphere. It is made up of 95 per cent carbon dioxide. The rest is nitrogen, argon and a small amount of water vapour. Tiny amounts of hydrogen, oxygen and ozone have also been detected. Though Martian atmosphere has clouds of frozen water, carbon dioxide and of reddish dust, it does not contain enough gases to trap the Sun's heat. This makes Mars a very cold planet. The surface temperature may rise to 21°C or 27°C near its   Equator, at noon. But, during the night, it becomes as low as-84°C. The present conditions on Mars-cold, extreme dryness, intense ultraviolet light and little oxygen-are hostile to the familiar forms of life. The Viking experiments on Mars showed no signs of life.

Mars possesses two natural satellites. They are called Phobos (F&) and Deimos (Terror). Phobos, about 27 km in diameter, is about 9,300 km away from Mars. Deimos is even smaller, about 14 km in diameter. The latter is 24,000 km from the surface of Mars. More information about Mars and its satellites is expected in the near future from the Soviet Union's space missions to the planet.

The Asteroid Belt-Rubble of the Solar System

There is a gap of 547 million kilometers, between the orbits of Mars and Jupiter. In this gap are thousands of bodies made of rock and metal, ranging in size from mere specks of dust to hundreds of kilometers. These are called asteroids, and are believed to represent original solar material which failed to bind together to form a planet. The first asteroid was discovered in 1801 by Giuseppi Piazzi, who named it Ceres. It is 960 km across. The number of asteroids known now runs into thousands.

Asteroid Belt

Most asteroids are irregular in shape. They reflect varying amounts of light as they travel through space. Their shapes suggest that they may be the product of a collision  or an explosion. Thousands of fragments of asteroids enter the Earth's atmosphere every year, producing a streak of light across the sky. This streak of light shooting across the sky is called a meteor. Of the objects that enter the Earth's atmosphere, mgny are of the size of a grain of sand or a pebble and they are destroyed in flight. The objects that are large enough to survive the flight and hit the Earth's surface are called meteorites. A meteorite is like a piece of the Solar System right in our laboratory. The chemical analysis of meteorites provides us vital information about the Solar System.

Jupiter-A Strange Veiled Giant

Jupiter is the largest planet of the Solar System. If Earth were placed on the face of Jupiter, it would look like a 50 paise coin on a dinner plate. It weighs more than twice as much as all the other eight planets put together. It has sixteen known satellites. Jupiter is not only the biggest planet bur also the liveliest. It is full of mysteries and surprises. Its highest clouds are mainly crystals of frozen ammonia gas at a temperature of about -140"C. It is veiled in a turbulent, gaseous atmosphere made up of hydrogen and helium with significant amounts of ammonia and methane. The atmosphere. reaches thousands of kilometers deep to the surface. It gradually thickens into a churning liquid due to the immense pressures scientists assume that it finally turns into a liquid metallic core. At its centre, Jupiter has a small rocky core where temperatures probably reach 20,000"C, about three times the temperature of the Sun's surface. There may also be iron, silicon and other heavy elements in the rocky core. Jupiter seems to be more like the Sun in its composition than the other planets.

The most outstanding feature on the surface of Jupiter is the Great Red Spot. It is a long oval area which is so huge that two earths. side by side, could be  dropped through it. Sometimes it becomes pale pink in colour and at other times a fiery orange red. For long, the Great Red Spot puzzled the astronomers. The Pioneer and Voyager missions to Jupiter revealed that the Red Spot is a huge cyclonic disturbance in the atmosphere.

Jupiter

Jupiter is mainly a quick-spinning ball of gas and liquid with no solid surface. It also emits radio waves. Its composition, size and the number of moons gave rise to the idea that Jupiter is not a planet but rather a star with a "sdar system" of its own, that did1 not start 'burning'. It is estimated that if Jupiter,were just ten times heavier, it could have started to produce its own energy like the Sun.

Jupiter's four largest satellites, 10, Europa, Ganymede and Callisto are called the Galilean satellites after Galileo, their discoverer. They are fascinating worlds in themselves. Io,,of diamw 3240 km, is almost like a fireball'with frequent-volcanic eruptions. Beyond 10 are Europa, diameter 3120 Km, Ganymede, diameter 4900 km and Callisto, diameter 4U0 lan. Europa is thought to'& made-up of frozen ice. - Ganymede and Callisto have a thick icy covering on them. The remaining satellites of Jupiter have not been so well studied.


With ammonia clouds, intense emission of radio waves, enormous storms and satellites of fire and ice, Jupiter seems to be a most unlikely place to find life. Yet, in some ways, conditions on the present day Jupiter are not very different from those that did produce life on Earth about four billion years ago. Water, ammonia and methane, considered essential to the formation of life, are all present in the atmosphere of Jupiter. Moreover, lightning bolts continuously flash through Jupiter's clouds. Even if only a few organic compounds aie found to exist on Jupiter, it will strengthen the idea that life is a normal phenomenon throughout the universe.

Saturn-the Ringed Planet

Saturn, the sixth planet, is the last one visible to the naked eye. Its rings, visible only through a telescope, make it the most striking and beautiful sight in the Solar System . On seeing Saturn through his telescope, Galileo described the planet as having ears! These were actually three rings that girdle Saturn's equator. It is the second largest planet, exceeded in size and mass only by Jupiter. Saturais colder than Jupiter. The temperature at the top of its clouds is -180"C. Like Jupiter, Saturn consists mainly of hydrogen and helium, with traces of ammonia, methane and other compounds. Its average density is less than that of water. Thus, if we could put Saturn in a large enough ocean, it would float!

Saturn

The rings of Saturn are its most distinctive feature, giving it a matchless grace. The rings are in fact a thousand tightly packed individual ringlets, like the grooves on a gramophone record. Even the supposed gaps in the rings have been found to contain small particles. Most astounding of all, a new outermost ring contains two strands twisted around each other like the threads of rope. The rings are made up of large chunks of solid matter-probably ice coated rocks.

Saturn, too, has at least 16 satellites orbiting it at the edge of the rings. The close-ups Solar System of the known satellites reveal that they are icy and heavily cratered by the impact of meteorites. Saturn's main satellite is Titan, 5800 km in diameter. It is large enough to have been a planet by itself. It is the only satellite in our Solar System which has an atmosphere nearly as dense as our .own. Its atmosphere is made up of 90 per cent nitrogen gas and organic compounds such as hydrogen cyanide. But it is a very cold world, with a surface temperature of - 184°C. It is far too cold for any life as we know it, apart from having the highly poisonous hydrogen cyanide.

Planets Beyond the Reach of the Eye

The remaining three planets Uranus, Neptune and Pluto, invisible to the unaided eye, were discovered later.

Non Planets of the Solar System scale

Uranus

Uranus  appears as a green disc with vague markings, even through the largest telescopes. Its colour is produced by the large amounts of methane and ammonia clouds in its outer atmosphere. The temperature of the ammonia clouds is about -217°C. Uranus is made up of gases such as hydrogen, helium and methane just like,Jupiter and Saturn. Uranus is unique in the Solar System because its axis of rotation is tilted at an angle of 980 to the perpendicular and lies almost in the plane of its orbit around the Sun, with one pole sometimes pointing directly towards the Sun. Uranus. would seem that the planet has toppled over its side, rolling along the orbit like a wheel. Thus, the Sun shines directly on its poles. Uranus has fifteen satellites. In 1977, .-nine faint rings of rocky debris were also discovered around Uranus.

Uranus

Neptune

The eight planet, Neptune, was discovered in 1846 by astronomer Johann Gottfried Galle.  . It is so far away that, seen from Neptune, the Sun must appear just as a bright point. Neptune's atmosphere has methane hut no detectable ammonia. Its cloud temperature is about -237°C. Neptune is orbited by Triton, one of the biggest satellites in the Solar System. It orbits Neptune in a clockwise direction, i.e. opposite to the planet's own rotation. It has an atmosphere of nitrogen and methane. It may even have an ocean of liquid nitrogen. 'Triton is accompanied by a smaller satellite, ere id. In 1989, Voyager 2- passed within 35,000 km of Neptune and within 40,000 km of Triton resulting in a significant increase in our knowledge about Neptune and its satellites.

Pluto

Pluto's existence was also proposed to account for deviations in the orbit of Uranus.  Even after the influence of Neptune~had been accounted for, an American astronomer Percival Lowell, detected that the orbit of Uranus was still disturbed. Neptune's orbit too showed a similar disturbances. Lowell and Pickering did some calculations to predict the mass and radius of the orbit of Planet X which was supposed to cause these disturbances. In 1916, Pluto was discovered in about the right place in the sky. However, its mass turned out to be much smaller, about that of our Moon. Small, cold and dark, Pluto is about one-fifth of the size of the Earth. Its surface is coated with ,frozen methane. In 1938, a satellite of Pluto was discovered and named Charon. Not much is really known about Pluto.

Pluto

Pluto's orbit crosses that of Neptune's. No other planetary orbits cross in this way, and it is possible that Pluto is an escaped satellite of Neptune. Pluto's discovery had led astronomers to believe that it was the Planet X. But now calculations show that ,the mass of Pluto is too low to cause irregularities in. Uranus' orbit. Thus, the search for the elusive Planet X goes on.

A Cloud Made of Comets

The cold outer areas beyond Pluto are the regions of comets, those visitors that dash, around the Sun, seldom to be seen again. Comets are of great interest because they are the relics of the early history of the Solar System. When solar matter was churning and the Sun had just been ignited, its heat drove ,the lighter elements into the outer reaches of the Solar System. Hydrogen, oxygen, nitrogen and carbon collected into something like snowy cotton balls and they still float as a "cloud", at a distance of 100.000 times the distance of the Earth from the sun! This thin cloud of comets reaches out to a distance halfway to the nearest star, i.e. almost a few billion kilometers.

Comets are made mainly of 'ices', that is ordinary water ice mixed with frozen gases such as methane, carbon dioxide and ammonia. The ices are mixed with specks of dust that makes them look like dirty snowballs. Billions of comets stay in their great cloud, moving slowly in enormous orbits around the Sun. While still in this cloud, comets do not shine. Once in a while the gravity of a passing star discribes this cloud. A few comets then move into interstellar space and are lost to the Solar System. Others move towards the Sun.

Comet

Seen from the Earth, the comets shine more brightly than anything in the sky, except the Sun and the Moon. You may wonder how dirty looking snowballs turn into bright long-tailed comets. As a comet moves towards the Sun, its surface is warmed by the strengthening sunlight. Some of its frozen material turns into gas, forming a rapidly growing cloud called its head or coma, around its centre. On getting nearer to the Sun, more gas evaporates and its head becomes bigger and brighter. Also, a brilliant tail, made of dust and gas, is pushed out of the head by the pressure of the sunlight and the solar wind. This tail extends in a direction away from the Sun.

If comets pass very close to the Sun, they acquire enormous speeds, more than a million kilometers per hour, and move off into space with their tails pointing ahead. Most comets depart on long elliptical orbits, billions of kilometres into deep space, ana remain there for thousands of years. But a few do not escape from the Sun so easily. If they happen to pass near a large planet, particularly Jupiter, its gravitation pushes them into short-period orbits around the Sun. One of the most famous comets is Halley's comet which returns once every 74 to 79 years . It was last seen in 1986. Sometimes fragments from the comets fall on the Earth producing meteors. Comets returning repeatedly lose their gases each time. When all their ices melt, comets disintegrate, leaving a stream of small particles that spreads out thinly and loses its identity.