- By Muneeb Jamal
The special features of the planets are as follows:
- Mercury is the planet nearest to the Sun.
- Venus is the hottest planet of the solar system.
- The Earth is the only planet where water is present in all its three forms, ice, liquid and vapors. It is unique to support conditions necessary for life. There is movement of crustal plates on the Earth.
- Mars: Mars is the red planet with change of weathers.
- Jupiter is the largest planet of the solar system.
- Saturn is the lightest planet of the solar system. It is also the planet with largest number of moons.
- Uranus is the dark planet with black rings.
- Neptune is the planet farthest away from the sun.
The orbits of the planets are elliptical (not circular) and lie roughly in the same plane. The reference for the plane of the orbits or the planets is the orbit of the earth. The orbit of Uranus almost matches that of earth, with an inclination of only one degree. The inclination of Mercury's orbit is 7° (degrees) and Pluto has the most oblique planetary orbit in the solar system 17°2. The planets have been categorized into two group according to their constituent materials that is rocky planets and gaseous planets. However the outermost planets are also different in nature.
1. Rocky Planets:
The four innermost members of our solar system Mercury, Venus, Earth and Mars are all solid rocky bodies. For that reason they are called rocky planets or the terrestrial planets. Earth bears evidence of surface erosion processes, such as volcanism or meteorite bombardment, which have played a significant role in shaping the surface we see today. In 1974 and 1975 Mariner 10 space probe revealed the surface of Mercury with mountainous regions, valleys and plains.The surface was like that of moon.
In 1978 American Pioneer Venus mission's radar mapping demonstrated a surface that consisted of upland regions with two dominant highland areas — Ishtar Terra and Aphrodite Terra. The outermost of the terrestrial planets is Mars. Mars is geologically very active. The effects of wind and water erosion are very much in evidence on the planet's surface. Mars is also considered by some astronomers to be still volcanically active.
2. The Gaseous Planets:
Jupiter, Saturn, Uranus and Neptune are gaseous planets. Jupiter and Saturn are very large planets. The main body of these two planets is composed of liquid hydrogen and helium. Jupiter and Saturn are known to have solid rocky cores and magnetic field. Uranus and Neptune are smaller than Jupiter or Saturn. They have similar atmospheres with hydrogen, helium and methane, ethane is also found on Neptune.
Mercury is the planet nearest to the sun. Mercury is visible with naked eye shortly after the sunset or shortly before the sunset. The radius of the planet is 2,433 kilometer or its diameter is 4,878 kilometers. The mass of the planet is 0.055 times the mass of the earth. Mercury is the smallest planet of the solar system. The surface of Mercury is composed of silicate rocks and has craters due to bombardment of meteorite debris. There is no atmosphere around the planet.
It is like the Moon. The average distance of Mercury from the Sun is 57.9 x ,106 (m kilometers. It completes its period of one revolution around the Sun in 87.969 days (of the earth). The period of rotation on its axis is equal to 58.646 days. This greater length of Mercury's day is due to greater pull of the nearby Sun. As a result of high temperature and smaller size (low gravity) of the planet gases left the planet. US spacecraft Mariner 10 flew thrice close to the planet in 1973, 1974 and 1975, which gave a greater understanding of the planet.
The basaltic (of dark colored volcanic rocks) crust of the planet is apparently like the crust of the earth, the moon and Mars and its composition indicates the geological history, past melting and geochemical differentiate. The surface is covered with numerous craters occupying nearly the whole surface area. These craters are formed by the falling meteorite debris and are called impact craters. The largest impact on the observed part of Mercury is named Caloris Basin.
It is about 1,300 Km in diameter. The crater has raised mountain ridges (the Montes Caloris) and a lava flooded interior marked by an intricate system of cracks, fissures and ridges. The basin is surrounded by relatively flat plains also flooded with lava. About 20 other basins are also observed. - Although Mercury is closest to the sun but it is not the hottest planet. On Mercury, the maximum surface temperature varies with the distance from the sun.
It ranges from 800 °F (when closer to sun) to -260 °F at night. This is the greatest diurnal variation of temperature in the solar system. The crust of the planet is composed of porous soil or rock powder. It is basaltic which means that it is composed of dark colored rocks of volcanic, origin. Mercury is the only planet which has tectonic features like the earth. These features are due to similar cores of molten iron. Mercury has an abundance to iron and other metals. Iron is now deposited at the core, and the surface contains silicates.
Venus is the nearest neighbor of the Earth among the solar family. Its size is nearly the same as that of the earth. In spite of its nearness, the planet remained shrouded in mystery due to its the layers of clouds. In October 1990 NASA's spaceship Magellan sent most detailed image of the landscape of the planet. The topography or the surface has been revealed to have fault like cracks arranged in a regular pattern, and craters as big as of a diameter, 50 Km across.
The craters show typical features of impact craters formed by the encounters of meteorites with the surface. There is an evidence of recent volcanic activity on the planet. Thus Venus is the fourth body in the solar system, after the Earth, Jupiter's moon Io and Neptune's moon Triton where volcanism is observed. Venus is the hottest planet of the solar system. The atmosphere of the planet is extremely thick and the atmospheric pressure of gases is 90 times more as compared with that of the Earth. These gases trap heat during greenhouse effect which keeps the temperature of the planet at 470 °C. Liquid water cannot exist at this temperature, rather metals like lead would melt. Venus rotates from east to west, opposite to most other planets and completes one trip in 243 days.
The diameter of Venus is 12,100 kilometers and its average distance from the Sun is 108.2 x 106 kilometers. Its density is 5.2 times that of water and mass is 0.8 times that of the earth. Its surface temperature is 480 °C. The similarity of densities of the Earth and Venus shows that Venus is made up of same rocks as formed the Earth. However, due to proximity of Venus to the Sun and the resulting hot environment, it may have contained a smaller amount of sulfur and water containing compounds. The interior of both the planets is having a central iron core, a middle mantle of rocks rich in silicon, oxygen, iron and magnesium and a thin outer crust having rocks with higher proportion of silicon compared with those of mantle.
The cloud layer surrounding Venus is not penetrable by visible light but it is only transparent to long wavelength radio waves. Carbon dioxide is dominant in the atmosphere of Venus. Water vapors are also present in the atmosphere of the planet. Rare gases such as helium, neon and organ are found on Venus. Atmosphere of Venus is moving rapidly. Near the cloud top it is moving with a jet-stream-like velocity of about 200 miles/hour from east to west in the direction of Venus's rotation. The wind speed decreases gradually at most parts and sharply at a few parts with the decrease in altitude.
Mars is usually visible to naked eye as a bright red spot the sky. The diameter of the planet is 6787 kilometers, mass ten times less than that of the Earth, average surface temperature is -50°C and the distance from the sun is 227.9 I 106 kilometers. The period of its rotation is 24.6 hours while period of revolution around the Sun is 687 days. After landing on the Moon, the next target for man is Mars. A manned mission to Mars is planned in the early decades of 21st century. A spacecraft was sent to Mars in December 1998 and it landed in North pole of Mars while the second (Mars Polar Lander) launched on January 3, 1999 landed in the Southern hemisphere.
Another mission the Mars Observer was launched on September 26, 1992 which was lost on the way. However, in 1976 two American spacecrafts the Viking Landers successfully landed on the surface of Mars. Lander 1 dropped in a region known as Chryse Planitia on July 20, and Lander 2 arrived at Utopia Planting on September 3. The landers sent a wealth of information about the nature, composition and organic contents of the surface material of Mars. They also sent information's about the climatic pattern of the lower atmosphere. There is change of weathers on the planet. Mars usually appears as a bright reddish disc with semi permanent dark areas and caps on the polar region called the polar caps.
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