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Universe and the Solar system - Part 2
5.0 Planets of the solar system
A celestial body moving in an elliptical orbit round a star is known as a planet. Planets are generally divided into : (i) the Inner Planets (Mercury, Venus, Earth and Mars), and (ii) the Outer Planets (Jupiter, Saturn, Uranus, Neptune, and Pluto - dwarf planet). Of course, icy giants is also a category within (ii).
The four inner or terrestrial planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of refractory minerals, such as the silicates, which form their crusts and mantles, and metals such as iron and nickel, which form their cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets that are closer to the Sun than Earth is (i.e. Mercury and Venus).
The four outer planets, or gas giants (sometimes called Jovian planets), collectively make up 99% of the mass known to orbit the Sun. Jupiter and Saturn are each many tens of times the mass of the Earth and consist overwhelmingly of hydrogen and helium; Uranus and Neptune are far less massive (<20 Earth masses) and possess more ices in their makeup. For these reasons, some astronomers suggest they belong in their own category, "ice giants". All four gas giants have rings, although only Saturn's ring system is easily observed from Earth. The term superior planet designates planets outside Earth's orbit and thus includes both the outer planets and Mars.
6.0 Earth
Diameter: 12,756 km (7,926 miles)
Mass: 5,976 million, million, million tons
Temperature: –88° to 58°C
Distance from the Sun: 150 million km (93 million miles)
Length of day: 23.92 hours
Length of year: 365.25 earth days
Surface gravity: 1 kg = 1 kg
The Earth is shaped like a ball, but it is not perfectly round. The force of the Earth's rotation makes the world bulge very slightly at the equator and go a little flat at the North and the South poles. So the Earth is actually a flattened sphere, or a 'geoid'.
The Earth is unique among the terrestrial planets (Mercury, Venus, Earth, and Mars) because of its size and distance from the Sun. The Earth is unique in the solar system in that it supports life: its size, gravitational pull and distance from the Sun have all created the optimum conditions for the evolution of life. It is large enough to develop and retain an atmosphere and a hydrosphere. Temperature ranges on the Earth are such that water can exist on its surface as liquid, solid, and gas. Water, more than anything else, makes the planet Earth unique. The Earth is a delicate ball wrapped in filmy white clouds. The blue water and swirling patterns of clouds that dominate the scene underline the importance of water in the Earth's system. Thus, life on Earth can be considered an accidental outcome of a very lucky combination of several factors, all beyond, of course, the control of man!
6.1 The Earth's atmosphere
During the early stages of the Earth's formation, ash, lava, carbon and water vapour were discharged onto the surface of the planet by constant volcanic eruptions. The water formed the oceans, while carbon dioxide entered the atmosphere or was dissolved in the oceans. Clouds, formed of water droplets, reflected some of the Sun's radiation back into space. The Earth's temperature stabilised and early life forms began to emerge, converting carbon dioxide into life-giving oxygen.
The Earth's oceans and seas cover more than 367 million sq km, i.e. twice the surface of Mars and nine times the surface of the Moon.
The Pacific Ocean contains the deepest places on the Earth's surface - the ocean trenches. The very deepest is the Challenger Deep in the Mariana Trench which plunges 11022 m into the Earth's crust. If Mount Everest, the highest point on land at 8,848 m was dropped into the trench, its peak would not even reach the surface of the Pacific.
7.0 ASTEROIDS & OTHER BODIES
Asteroids are small Solar System bodies composed mainly of refractory rocky and metallic minerals, with some ice.
The asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.
Asteroids range in size from hundreds of kilometres across to microscopic. All asteroids except the largest, Ceres, are classified as small Solar System bodies.
The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter. Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of the Earth. The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident. Asteroids with diameters upto 1 metre are called meteoroids. A meteoroid enters the earth’s atmosphere, becomes a meteor, and if it reaches the surface, is called a meteorite.
7.1 Comets
Comets are among the most spectacular and unpredictable bodies in the solar system. They have been compared with large, dirty snowballs, since they are made of frozen gases (water, ammonia, methane and carbon dioxide) which hold together small pieces of rocky and metallic minerals. Many comets travel along very elongated orbits that carry them beyond the Pluto. On their return, the comets are visible only after they have moved within the orbit of Saturn. One of the larger comets is the Halley's Comet. The orbit of Halley's Comet brings it close to the Earth every 76 years. It last visited in 1986.
7.2 Meteoroids
Meteor is a body of matter travelling at a great speed through space which becomes luminous when enters into the atmosphere (ionosphere) at about 200 km above the Earth's surface, because it is heated by friction. Generally, this latter process dissipates the material into meteoric dust. A meteor is popularly termed a 'shooting star' or 'falling star'.
7.3 Meteorite
Any particle of solid matter that has fallen to the Earth, the Moon, or another planet from the space. It is composed of various proportions of a nickel-iron alloy (typically about 10% nickel and about 90% iron) and silicate minerals.
A meteor crater in Arizona (USA) is 4,200 ft (1,300 m) deep is the largest meteor crater in the world. It was formed over 10,000 years ago.
8.0 The Moon
With the development of space programme, the Moon has become one of the best understood planetary bodies in the solar system. The Moon is the natural satellite of the Earth, with a diameter of 3,470 km. The Moon is pock-marked with billions of craters, which range in size from microscopic pits on the surface of rock specimens to huge circular basins of hundreds of kilometers in diameter. Each of these must have been formed due to crashing bits of rocks onto the Moon’s surface.
The Moon rotates more slowly than the Earth and takes a little over 27 days to rotate once. Since it takes about the same time to revolve around the Earth, it always presents the same face or hemisphere to the Earth. The interval between one Full Moon and the next is 29.5 days. The Moon seems to have different shapes at different times of the month because of its changing position in relation to the Earth. The different shapes are known as the phases of the Moon.
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