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An Introduction to World History. "Any scientist, I think, will agree that in a hundred years time, many of the details of the story we tell now may look kind of cute and naive, because well have moved on beyond them. But if you happen to live now, " he concludes, "this is the best story going. " said David Christians and it is really true. We lived in a beautiful world but dont know a lot about cosmology, geology, archeology, and population and environmental studies. Today with the help of David Christian book Maps of Time well try to discuss one of the main questions of human begins. Well speak about Solar System. Hadean time is not a geological period as such.
No rocks on the Earth are this old - except for meteorites. During Hadean time, the Solar System was forming, probably within a large cloud of gas and dust around the sun, called an accretion disc. The relative abundance of heavier elements in the Solar System suggests that this gas and dust was derived from a supernova, or supernovas - the explosion of an old, massive star. Heavier elements are generated within stars by nuclear fusion of hydrogen, and are otherwise uncommon. We can see similar processes taking place today in so-called diffuse nebulae in this and other galaxies - such as the nebula M 16, shown above left. The sun formed within such a cloud of gas and dust, shrinking in on itself by gravitational compaction until it began to undergo nuclear fusion and give off light and heat.
Surrounding particles began to coalesce by gravity into larger lumps, or planetesimals, which continued to aggregate into planets. "Left-over" material formed asteroids and comets - like asteroid Ida, on the upper right. Because collisions between large planetesimals release a lot of heat, the Earth and other planets would have been molten at the beginning of their histories. Solidification of the molten material into rocks happened as the Earth cooled. The oldest meteorites and lunar rocks are about 4. 5 billion years old, but the oldest Earth rocks currently known are 3. 8 billion years old. Sometime during the first 800 million or so years of its history, the surface of the Earth changed from liquid to solid. Once solid rock formed on the Earth, its geological history began.
This most likely happened prior to 3. 8 billion years, but hard evidence for this is lacking. Erosion and plate tectonics has probably destroyed all of the solid rocks that were older than 3. 8 billion years. The beginning of the rock record that is currently present on the Earth is the inception of a time known as the Archaean. Pluto is the farthest planet from the Sun (usually) and by far the smallest. Pluto is smaller than seven of the solar system's moons (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton). In Roman mythology, Pluto (Greek: Hades) is the god of the underworld.
The planet received this name (after many other suggestions) perhaps because it's so far from the Sun that it is in perpetual darkness and perhaps because "PL" are the initials of Percival Lowell. Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing of the error, Clyde W.
Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway. Pluto is the only planet that has not been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface (left and above). There is a planned mission called New Horizons that will launch in 2006 if it gets funded.
Pluto is the second most contrasty body in the Solar System (after Iapetus). Pluto is locked in a 3: 2 resonance with Neptune; i. e. Pluto's orbital period is exactly 1. 5 times longer than Neptune's. Its orbital inclination is also much higher than the other planets'. Thus though it appears that Pluto's orbit crosses Neptune's, it really doesn't and they will never collide.
Like Uranus, the plane of Pluto's equator is at almost right angles to the plane of its orbit. The surface temperature on Pluto varies between about - 235 and - 210 C (38 to 63 K). The "warmer" regions roughly correspond to the regions that appear darker in optical wavelengths. Pluto's composition is unknown, but its density (about 2 gm / cm 3) indicates that it is probably a mixture of 70 % rock and 30 % water ice much like Triton.
The bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of (solid) methane, ethane and carbon monoxide. The composition of the darker areas of Pluto's surface is unknown but may be due to primordial organic material or photochemical reactions driven by cosmic rays. The unusual nature of the orbits of Pluto and of Triton and the similarity of bulk properties between Pluto and Triton suggest some historical connection between them. It was once thought that Pluto may have once been a satellite of Neptune's, but this now seems unlikely. A more popular idea is that Triton, like Pluto, once moved in an independent orbit around the Sun and was later captured by Neptune. Perhaps Triton, Pluto and Charon are the only remaining members of a large class of similar objects the rest of which were ejected into the Oort cloud.
Like the Earth's Moon, Charon may be the result of a collision between Pluto and another body. I dont thins Pluto is a really problem, cause nowadays we have a lot of scientific's who take some researches inches sphere. Maybe, in some years well know a new page in the World History. 1. web 2.
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