EARTH:-
Sunday, September 21, 2008
VENUS
VENUS:-Venus is the second-closest planet to the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love. It is the brightest natural object in the night sky, except for the Moon, reaching an apparent magnitude of −4.6. Because Venus is an inferior planet from Earth, it never appears to venture far from the Sun: its elongation reaches a maximum of 47.8°. Venus reaches its maximum brightness shortly before sunrise or shortly after sunset, for which reason it is often called the Morning Star or the Evening Star.Classified as a terrestrial planet, it is sometimes called Earth's "sister planet," because the two are similar in size, gravity, and bulk composition. Venus is covered with an opaque layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light; this was a subject of great speculation until some of its secrets were revealed by planetary science in the twentieth century. Venus has the densest atmosphere of all the terrestrial planets, consisting mostly of carbon dioxide, as it has no carbon cycle to lock carbon back into rocks and surface features, nor organic life to absorb it in biomass. It has become so hot that the earth-like oceans that the young Venus is believed to have possessed have totally evaporated, leaving a dusty dry desertscape with many slab-like rocks. The best hypothesis is that the evaporated water vapor has dissociated, and with the lack of a planetary magnetic field, the hydrogen has been swept into interplanetary space by the solar wind.[6] The atmospheric pressure at the planet's surface is 92 times that of the Earth.
Physical characteristics:-Venus is one of the four solar terrestrial planets, meaning that, like the Earth, it is a rocky body. In size and mass, it is very similar to the Earth, and is often described as its 'sister'. The diameter of Venus is only 650 km less than the Earth's, and its mass is 81.5% of the Earth's. However, conditions on the Venusian surface differ radically from those on Earth, due to its dense carbon dioxide atmosphere. The mass of the atmosphere of Venus is 96.5% carbon dioxide, with most of the remaining 3.5% composed of nitrogen.[7]
Internal structure:-
Though there is little direct information about its internal structure, the similarity in size and density between Venus and Earth suggests that it has a similar internal structure: a core, mantle, and crust. Like that of Earth, the Venusian core is at least partially liquid. The slightly smaller size of Venus suggests that pressures are significantly lower in its deep interior than Earth. The principal difference between the two planets is the lack of plate tectonics on Venus, likely due to the dry surface and mantle. This results in reduced heat loss from the planet, preventing it from cooling and providing a likely explanation for its lack of an internally generated magnetic field.[8]
Geography:-
About 80% of Venus's surface consists of smooth volcanic plains. Two highland 'continents' make up the rest of its surface area, one lying in the planet's northern hemisphere and the other just south of the equator. The northern continent is called Ishtar Terra, after Ishtar, the Babylonian goddess of love, and is about the size of Australia. Maxwell Montes, the highest mountain on Venus, lies on Ishtar Terra. Its peak is 11 km above Venus's average surface elevation. The southern continent is called Aphrodite Terra, after the Greek goddess of love, and is the larger of the two highland regions at roughly the size of South America. Much of this continent is covered by a network of fractures and faults.[9]
As well as the impact craters, mountains, and valleys commonly found on rocky planets, Venus has a number of unique surface features. Among these are flat-topped volcanic features called farra, which look somewhat like pancakes and range in size from 20–50 km across, and 100–1000 m high; radial, star-like fracture systems called novae; features with both radial and concentric fractures resembling spiders' webs, known as arachnoids; and coronae, circular rings of fractures sometimes surrounded by a depression. All of these features are volcanic in origin.[10]
About 80% of Venus's surface consists of smooth volcanic plains. Two highland 'continents' make up the rest of its surface area, one lying in the planet's northern hemisphere and the other just south of the equator. The northern continent is called Ishtar Terra, after Ishtar, the Babylonian goddess of love, and is about the size of Australia. Maxwell Montes, the highest mountain on Venus, lies on Ishtar Terra. Its peak is 11 km above Venus's average surface elevation. The southern continent is called Aphrodite Terra, after the Greek goddess of love, and is the larger of the two highland regions at roughly the size of South America. Much of this continent is covered by a network of fractures and faults.[9]
As well as the impact craters, mountains, and valleys commonly found on rocky planets, Venus has a number of unique surface features. Among these are flat-topped volcanic features called farra, which look somewhat like pancakes and range in size from 20–50 km across, and 100–1000 m high; radial, star-like fracture systems called novae; features with both radial and concentric fractures resembling spiders' webs, known as arachnoids; and coronae, circular rings of fractures sometimes surrounded by a depression. All of these features are volcanic in origin.[10]
MERCURY
MERCURY:-Mercury is the innermost and smallest planet in the solar systemorbiting the Sun once every 88 days. Mercury is bright when viewed from Earth, ranging from −2.0 to 5.5 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun is only 28.3°. It can only be seen in morning or evening twilight. Comparatively little is known about it; the first of two spacecraft to visit Mercury was Mariner 10, which mapped only about 45% of the planet’s surface from 1974 to 1975.[8] The second is the MESSENGER spacecraft, which mapped another 30% during its flyby of January 14, 2008.[8] MESSENGER will make two more passes by Mercury, followed by orbital insertion in 2011, and will then survey and map the entire planet.Mercury is similar in appearance to the Moon: It is heavily cratered, has no natural satellites and no substantial atmosphere. However, unlike the moon, it has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth.[9] It is an exceptionally dense planet due to the large relative size of its core. Surface temperatures range from about 90 to 700 K (-183 °C to 427 °C),[10] with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.
Internal structure:-Mercury is one of four terrestrial planets, and is a rocky body like the Earth. It is the smallest planet in the solar system, with an equatorial radius of 2439.7 km.[3] Mercury is even smaller—albeit more massive—than the largest natural satellites in the solar system, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material.[13] Mercury's density is the second highest in the Solar System at 5.427 g/cm³, only slightly less than Earth’s density of 5.515 g/cm³.[3] If the effect of gravitational compression were to be factored out, the materials of which Mercury is made would be denser, with an uncompressed density of 5.3 g/cm³ versus Earth’s 4.4 g/cm³.[14
Impact basins and craters:-
Mercury’s Caloris Basin is one of the largest impact features in the Solar System.
Craters on Mercury range in diameter from small bowl-shaped cavities to multi-ringed impact basins hundreds of kilometers across. They appear in all states of degradation, from relatively fresh rayed craters to highly degraded crater remnants. Mercurian craters differ subtly from lunar craters in that the area blanketed by their ejecta is much smaller, a consequence of Mercury's stronger surface gravity.[32]
The largest known craters are Caloris Basin, with a diameter of 1550 km,[33] and the Skinakas Basin with an outer-ring diameter of 2300 km.[34] The impact that created the Caloris Basin was so powerful that it caused lava eruptions and left a concentric ring over 2 km tall surrounding the impact crater. At the antipode of the Caloris Basin is a large region of unusual, hilly terrain known as the "Weird Terrain". One hypothesis for its origin is that shock waves generated during the Caloris impact traveled around the planet, converging at the basin’s antipode (180 degrees away). The resulting high stresses fractured the surface.[35] Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin’s antipode.[36]
Thursday, September 11, 2008
SUN
SUN:-
The Sun is the star at the center of the Solar System. The Earth and other matter (including other planets, asteroids, meteoroids, comets, and dust) orbit the Sun,[9] which by itself accounts for about 99.8% of the Solar System's mass. Energy from the Sun, in the form of sunlight and heat, supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather.
The surface of the Sun consists of hydrogen (about 74% of its mass, or 92% of its volume), helium (about 24% of mass, 7% of volume), and trace quantities of other elements, including iron, nickel, oxygen, silicon, sulfur, magnesium, carbon, neon, calcium, and chromium.[10] The Sun has a spectral class of G2V. G2 means that it has a surface temperature of approximately 5,780 K, giving it a white color that often, because of atmospheric scattering, appears yellow when seen from the surface of the Earth. This is a subtractive effect, as the preferential scattering of shorter wavelength light removes enough violet and blue light, leaving a range of frequencies that is perceived by the human eye as yellow. It is this scattering of light at the blue end of the spectrum that gives the surrounding sky its color. When the Sun is low in the sky, even more light is scattered so that the Sun appears orange or even red.[11]
Overview:-
The Sun is a Population I, or heavy element-rich,[16] star.[17] The formation of the Sun may have been triggered by shockwaves from one or more nearby supernovae.[18] This is suggested by a high abundance of heavy elements such as gold and uranium in the Solar System relative to the abundances of these elements in so-called Population II (heavy element-poor) stars. These elements could most plausibly have been produced by endergonic nuclear reactions during a supernova, or by transmutation via neutron absorption inside a massive second-generation star.
STRUCTURE:-
An illustration of the structure of the Sun
The Sun is a yellow main sequence star comprising about 99% of the total mass of the Solar System. It is a near-perfect sphere, with an oblateness estimated at about 9 millionths,[35] which means that its polar diameter differs from its equatorial diameter by only 10 km (6 mi). As the Sun exists in a plasmatic state and is not solid, it rotates faster at its equator than at its poles. This behaviour is known as differential rotation. The period of this actual rotation is approximately 25 days at the equator and 35 days at the poles. However, due to our constantly changing vantage point from the Earth as it orbits the Sun, the apparent rotation of the star at its equator is about 28 days. The centrifugal effect of this slow rotation is 18 million times weaker than the surface gravity at the Sun's equator. The tidal effect of the planets is even weaker, and does not significantly affect the shape of the Sun.
The Sun is the star at the center of the Solar System. The Earth and other matter (including other planets, asteroids, meteoroids, comets, and dust) orbit the Sun,[9] which by itself accounts for about 99.8% of the Solar System's mass. Energy from the Sun, in the form of sunlight and heat, supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather.The surface of the Sun consists of hydrogen (about 74% of its mass, or 92% of its volume), helium (about 24% of mass, 7% of volume), and trace quantities of other elements, including iron, nickel, oxygen, silicon, sulfur, magnesium, carbon, neon, calcium, and chromium.[10] The Sun has a spectral class of G2V. G2 means that it has a surface temperature of approximately 5,780 K, giving it a white color that often, because of atmospheric scattering, appears yellow when seen from the surface of the Earth. This is a subtractive effect, as the preferential scattering of shorter wavelength light removes enough violet and blue light, leaving a range of frequencies that is perceived by the human eye as yellow. It is this scattering of light at the blue end of the spectrum that gives the surrounding sky its color. When the Sun is low in the sky, even more light is scattered so that the Sun appears orange or even red.[11]
Overview:-The Sun is a Population I, or heavy element-rich,[16] star.[17] The formation of the Sun may have been triggered by shockwaves from one or more nearby supernovae.[18] This is suggested by a high abundance of heavy elements such as gold and uranium in the Solar System relative to the abundances of these elements in so-called Population II (heavy element-poor) stars. These elements could most plausibly have been produced by endergonic nuclear reactions during a supernova, or by transmutation via neutron absorption inside a massive second-generation star.
STRUCTURE:-
An illustration of the structure of the Sun
The Sun is a yellow main sequence star comprising about 99% of the total mass of the Solar System. It is a near-perfect sphere, with an oblateness estimated at about 9 millionths,[35] which means that its polar diameter differs from its equatorial diameter by only 10 km (6 mi). As the Sun exists in a plasmatic state and is not solid, it rotates faster at its equator than at its poles. This behaviour is known as differential rotation. The period of this actual rotation is approximately 25 days at the equator and 35 days at the poles. However, due to our constantly changing vantage point from the Earth as it orbits the Sun, the apparent rotation of the star at its equator is about 28 days. The centrifugal effect of this slow rotation is 18 million times weaker than the surface gravity at the Sun's equator. The tidal effect of the planets is even weaker, and does not significantly affect the shape of the Sun.
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