- Mercury: The second smallest of the planets ( c. one twentieth of the Earth's mass ) and is the closest to the sun, some 58 million km ( 36 million miles ) distant from it. It appears to resemble the Earth in it's interior and has the same density, but its surface resembles the moon and is heavily cratered. Mercury was visited by Mariner 10 which passed within a few hundred kilometres in March 1974 and which relayed pictures on the two following passes in September 1974 and March 1975 before control of its altitude was lost. It nevertheless continues to revisit Mercury every 176 days ( two Mercurian years ). Mariner 10 was guided to Mercury by the gravitational field of Venus before it became a satelite of the smaller planet. The relayed pictures showed that the planet has an approximately 59 day rotation period, exactly two-thirds of the orbital period of 88 days. This relationship means that unlike on earth the seasonal variations are with longitude not latitude. Surface tempuratures vary from 400°C to -200°C and there is virtually no atmosphere except a thin envelope of helium gas. Mariner 10 measured a small magnetic field at the surface about 1% of the Earth's but much stronger than for Venus or Mars. Mercury is 4900km ( 3000 miles ) in diameter and has no moons.
- Venus: The brightest of the planets, whose orbit lies between the orbits of Mercury and Earth, and is second in order from the Sun and is 108 million kilometres ( 67 million miles ) away from the Sun. It approaches the Earth to within 40 million kliometres ( miles ). It moves around the Sun in the same direction as Earth and the other planets, but rotates in the opposite sense. It has been visited by a number of American and Russian probes, some flybys, some soft landings, others atmospheric probes. Venus has 100 times the atmospheric pressure of the Earth and temperatures of 500°C. It is covered in a very thick cloud mainly of carbon dioxide which some scientists believe acts like a greenhouse trapping the heat from the Sun. Complex reactions occur in the atmosphere and the rain consists mainly of sulphuric and hydrochloric acids. The cloud rotates about six times faster than Venus rotates and swirls into the polar regions like giant whirlpools. The terrain has been studied by radar, much of it ground based, but a vast amount of information has come from both the Pioneer Venus orbiter and the recent successful Magellan probe, which between them have mapped nearly the entire Venusian surface. There are two major continents, Aphrodite Terra, is about the same size as Africa, and the smaller Ishtar Terra, about the size of Australia, which consists of a very high plateau ( c 3,300 m ) and a mountain range dominated by Maxwell Montes ( at 10,600 m higher than Everest ). Aphrodite Terra has a deep trench 2,900 m below 'sea level'. Most of the planet ( 60 per cent ) is relatively flat. The Magellan probe ( 1990 ) revealed a carbon dioxide atmosphere, 100 times thicker than Earth's, with sulphur escaped from volcanoes as well as the longest channel ( 4,200 m ) in the solar system. It also found that there are no asteroid impact craters bigger than 5 miles across. Venus is 12,100km ( 7,500 miles ) in diameter and, like Mercury, has no moon, but on Earth is often seen and referred to as either the 'morning' or 'evening' star .
- Earth: Our habitable globe, and is the third of the planets of the solar system in order from the sun and on an average throughout the year takes 24 hours to turn completely round relative to the Sun, the whole Earth rounds the Sun in a slightly elliptical orbit once in a year of 365.2564 days. The mean distance of the Earth from the Sun is 149,597,000km ( ?miles ). Recent Earth satellite studies have shown that small variations of the surface gravity field ( or geoid ) occur which are believed to be related more to the structures deep within the Earth's mantle than to the location and dimension of the crustal features, such as oceans, continents and mountain ranges. The crust consists of a skin, 30km thick under the continents, but only about 6 - 8km thick under the ocean bed, comprised of rocks and sediments or soil. At the base of the crust is a sharp discontinuity ( the Mohorovicic Discontinuity ) to denser rocks of the mantle. This region, nearly 3,000 kilometres thick, is in a process of slow but inexorable change, one of convection responding to heat sources ( due mainly to radioactivity of the materials within the Earth ). This slow convection is responsible for many changes in the surface of the Earth - continental drift, earthquakes and volcanic activity. The core is a region of very high density and temperature, comprised of heavy elements such as iron and nickel. The crustal rocks are mainly comprised of oxygen, silicon, aluminium, sodium, potassium, iron, calcium and magnesium, with traces of many other elements. The mass of the Earth is about 6,000 million million million tonnes, and it was formed about 4,600 million years ago. The Earth is 12,756km ( 7,926 miles ) in diameter and has one natural satellite, the Moon.
- Mars: The fourth nearest planet to the Sun. The enormous amount of new information from pioneer visits to Mars, primarily by the US Mariner and Viking spacecraft, in 1971 and 1976 respectively, has revolutionised our understanding of the planet. These missions showed that the planet's surface displayed many contrasts - the ancient cratered terrains glimpsed previously, enormous volcanoes, far larger than any on Earth, vast wastelands, particularly near the polar regions, scoured by wind-blown dust over many aeons, and the most tantalising, in many regions water had set unmistakable marks. We see the plains which have been sublect to torrential deluges of rain in, probably, a number of different episodes over hundreds of millions of years. Other regions, extending over hundreds of kilometres, show great gorges and canyons cut by catastrophic floods, when some natural dam has burst, releasing the pent-up waters of a gigantic lake or sea. This pictorial evidence, and more detailed information of the atmosphere and surface obtained from the Viking spacecraft tell us that there must have been episodes in the planet's history when its climate was quite different from that observed today. The atmosphere, if only for brief interludes, must have been much denser, and warmer, and rain must have been able to fall. Mars is today too cold and dry for water but there are polar ice caps, and the nature of the meteoric craters suggests that the surface is rock over ice and solid carbon dioxide. The search for life carried out by Viking produced much new information, but no traces of organic compounds as evidence, or precursors, of life. Findings from the Mars Patfinder mission ( which landed on Mars in 1997 ) suggests there was abundant water in the past and also that Mars may be much more like Earth than previously thought. Further information will be added when I get round to reading up on the Mars Polar Lander ( launched in January 1999 ). Mars is 6,800km ( 4,200 miles ) in diameter and has 2 moons called Phobos and Deimos.
- Jupiter: It is the largest of the planets, 11 times the diameter of Earth, 318 times its mass but only one fourth of its density. It is the fifth farthest from the Sun and is the second brightest. Our knowledge of Jupiter, its moons, its rings and its magnetosphere was enormously increased in 1979 by data from the Voyager 1 and 2 space-probes which passed the planet at distances of 349,000 km and 722,000 km on March 5 and July 9 respectively. Jupiter is a gaseous planet composed mainly of hydrogen and helium like the Sun, possibly with a small molten core of silicates. The centre is presumed to be at a very high temperature ( 30,000°C ) and enormous pressure - a hundred million Earth atmospheres - which make hydrogen not only liquid but metallic. This metallic flux leads to intense magnetic fields. Jupiter's large magnetosphere is distorted by the solar wind and contains trapped high energy particles in radiation belts which would be lethal to man. Its existence was first inferred from its radio emissions and was confirmed by Pioneer 11 in 1974.
Jupiters outer gaseous surface is very cold ( -120°C ) and consists of three layers of crystal clouds ( ice, ammonium-hydrogensulphide and ammonia ) interspersed with gaseous hydrogen and helium. It shows massive persistent features like the swirling high pressure Red Spot ( larger when first seen in 1664 but which could hold two Earths ) and the neighbouring white oval formations. All these features are in relative motion, some moving east and others west which account for Jupiter's banded appearence. The persistence of the features arises because Jupiter has no terrain to break them up and because the surface temperature is low. Lightning continually flashes over the Juvian surface and at the poles there are large auroras caused by an electric current of five million amperes which flows from the poles out to Io, one of the moons discovered in 1610 by Galileo. The others were Europa, Ganymede and Calisto. The fourteenth moon was discovered in 1979 by Voyager 2 at the outer edge of the thin ring of particles discovered by Voyager 1, which also discovered high plumes due to volcanic activity on Io, the first seen beyond Earth ( although a recent suggestion is that the plumes are caused by the electric current ). Io's sulphurous surface shows no meteoric craters and Europa's smooth ice surface shows only a few impacts and is probably young, unlike the older Ganymede and Calisto whose ice-crust surfaces have many. Ganymede's crust has been reworked, however, by extensive tectonic movements. Ganymede is now known to be the largest moon in the solar system, larger than the planet Mercury. In 1994, Jupiter was struck by the Shoemaker-Levy-9 comet whose impact sent seismic waves across the planet
In 1995, the Hubble telescope found oxygen on Europa, one of Jupiter's moons. More recently salt has been discovered. Late in 1995 the Galileo spacecraft successfully entered the atmosphere of Jupiter. First data from Galileo suggested Jupiter's atmosphere was hotter and drier than expected with hurricane winds of 330 m.p.h. Images from Galileo in 1997 appeared to show iceberg-like structures on Europa. The origin of Jupiter's swirling rings was discovered in 1998 to be the detritus thrown up by meteoroids crashing into the moons Almathea, Thebes, etc. Colour images of Io have shown great volcanic activity. Jupiter is 143,000 km ( 89,400 miles ) in diameter and has 16 known moons. Check out Moons of Jupiter.
- Saturn: The sixth planet in order from the sun, from which it is distant 1427 x 106 km and around which it makes a revolution in 29.46 years. It rotates on its axis in 10 hours 14 minutes and is 9.42 times larger than Earth. Saturn is noted for its magnificent ring system which is composed of myriads of ice covered particles. Among the planets moons are Mimas, Enceladus, Tethys, Dione, Titan, Rhea, Hyperion, Iapetus and Phoebe. Titan is the largest ( 5150 km diameter ), being the second largest satellite in the solar system, about the size of the planet Mercury. Galileo was the first to observe that Saturn was ringed; Christian Huygens the first to study the nature of the rings; and Jacques Cassini the first to suggest that they were composed of myriads of particles. Our knowledge of the planet and its moons and rings has been greatly increased by the camera observations of the spacecrafts Pioneer 11 ( flyby Sept 1979 ) and Voyager 2 ( flyby August 1981 ). The Cassini space probe, launched back in 1997, is planned to reach Titan ( where water vapour is believed to exist ) in 2004. Saturn is 120,00 km ( 75,000 miles ) in diameter and has 18 known moons. Check out Moons of Saturn.
- Uranus: This planet was discovered by Herschel in March 1781, and is seventh planet in order from the Sun. Its diameter is 51,118 km ( 32,250 miles ) and its mean distance from the Sun is 2,871 x 106 km. When Voyager 2 visited Uranus in January 1986, it raised the number of known moons from five to 15 . The number of rings surrounding Uranus were also increased to 11. Uranus has a large axial tilt which means it rotates on its side, seeemingly 'rolling' round its orbit. Each pole faces the Sun every half revolution around the Sun, i.e. every 42 years. Uranus now has 17 moons as two further moons, Caliban and Sycorax, were discovered in 1998.
- Neptune: Apart from Pluto this is the most distant of the planets ( 8th ), estimated to be 4,497 x 106 km from the Sun, and taking about 165 years to revolve round it and rotates once every 10 - 20 hours. Discovered by the German astronomers Galle and D'Arrest in Sept 1846, after its existence had been predicted by Leverrier and Adams. The planet was visited by Voyager 2 in 1989, its last planetary stop. This revealed Neptune had eight moons ( not just Triton and Nereid as previously thought) and a ring system. A huge Earth-sized storm has been observed, which has been named the 'Great Dark Spot'. Winds blow at nearly 900 m.p.h. round Neptune's equator, making it the windiest corner of the solar system. Neptune is for some periods outside the orbit of Pluto due primarily to the shape of Pluto's orbit. It's diameter is 49,528 km ( 30,400 miles ). Check out Moons of Neptune.
- Pluto: The last planet to be discovered was searched for following the 1914 predictions of P. Lowell and discovered by C. W. Tombaugh at the Flagstaff Observatory in Arizona in January 1930. Generally the most distant of the planets ( 9th ) its orbit is much more elliptical than the other planetary orbits and so it sometimes approaches the Sun closer than Neptune. Pluto takes 248 years to orbit the Sun, and spins on its axis once in 6.4 days. It was thought to be different from the other outer Jovian planets which are gaseous and to consist of rock covered in frozen methane gas. Recently the size of Pluto has been revised downwards by interferromagnetic measure of its diameter. The diameter of Pluto is currently ( 1999 ) estimated to be 2,250 km. It is the only planet in the solar system not yet visited by spacecraft, but N.A.S.A. plans a probe in 2003. Pluto's own moon was also detected by Earth based telescopes using special photometric measurements. It is composed almost entirely of ice. Images of the surface of Pluto reveal icy Polar caps and clusters of bright and dark features. Some astronomers do not think Pluto should be classed as a planet.
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