雅思阅读真题文章：Venus

　　Venus and Earth Compared

　　Venus, the second planet from the Sun, lies, on average, 108 million km from the Sun, about 30% closer than the Earth. Venus is often referred to as our sister planet because of similarities in size, mass, density and volume. It is believed that both planets share a common origin forming at the same time out of a condensing nebulosity around 4.5 billion years ago.

　　There the similarities end. Venus has no surface water, a toxic, heavy atmosphere made up almost entirely of carbon dioxide with clouds of sulphuric acid and at the surface the atmospheric pressure is over 90 times that of the Earth at sea-level.

　　The surface of Venus is the hottest in the solar system at a searing 750 K . This high temperature has been caused by a catastrophic greenhouse effect due to the carbon dioxide rich atmosphere. Incident sunlight is trapped by the atmosphere and cannot radiate out into space with a resulting boost to the surface temperature by over 475 K.

　　The final anomaly between the two worlds is the rotation of Venus. Firstly its axis of rotation is inclined at 177.36 degrees . This means that Venus rotates in a retrograde direction from east to west, making the Sun rise in the west and set in the east. Further to this the rotation is very slow: a sidereal day on Venus lasts 243 Earth days. This is even longer than a Venusian year which is only 224.7 Earth days.

　　Despite several missions to Venus there is still much that is unknown about our celestial neighbour. The Russian Venera landers touched down in the 1980s and survived the hostile environment for about 60 minutes during which time images were taken revealing a barren world. Other missions to Venus, both Russian Venera orbiters, and NASAs Magellan spacecraft have concentrated on radar mapping the surface.

　　From the data returned by these missions scientist believe that the surface of Venus is relatively young - it appears to have undergone resurfacing around 400 million years ago. The topography consists of vast plains and mountainous regions.

　　Impact craters cover the surface. Unlike other worlds, however, there are virtually no craters less than 2 km in size. The impactors that would cause them simply burn up in the thick atmosphere. Those that do exist are believed to be caused by the fragmentation of a large meteorite just before impact with the surface.

　　Venus Transits

　　When Venus passes directly between the Earth and the Sun, this event is called a transit of Venus. Venus orbits the Sun at a smaller distance than does the Earth, so one might expect that Venus transits would occur often. However, the orbital plane of Venus is not parallel to that of the Earth, so transits occur very rarely - only four times every 243 years.

　　The last Venus transit occurred in 2004; the next will occur in June 2012, although unlike the 2004 transit this one will not be visible in its entirety from Europe. After that, the following transit will not occur until 2117.

　　Venus transits are of great historical significance, as they provided the first quantitative measure of the distance scales of the Solar System: the Astronomical Unit, where 1 AU is defined as the distancebetween the Earth and the Sun). Astronomers had started to measure the distances to nearby stars in units of AU but the value of the AU was still unknown. Therefore, many expeditions were sent out from European nations to observe the Venus transits of 1761 and 1769, resulting in the first measurement of the AU. Further refinement in the measurement was then obtained when the next two transits occurred in 1874 and 1882.

　　Venus transits have also provided the first evidence of an atmosphere existing on a planet other than the Earth.