An icy body, orbiting in the solar system They appear to be composed of frozen water, carbon dioxide, methane and ammonia, in which dust and rocky material is embedded. As a comet approaches the Sun, solar heating starts to vaporize the ices, releasing gas that forms a diffuse luminous sphere, called the coma, around the nucleus. The coma may be up to a million kilometres across. The nucleus itself is too small to be observed directly. Observations in the ultraviolet region of the spectrum made from spacecraft have shown comets to be surrounded by huge clouds of hydrogen, many millions of kilometers in size. The hydrogen comes from the breakdown of water molecules by solar radiation
A small rocky object in the solar system. The largest, Ceres, is nearly 1,000 kilometres (600 miles) across and they range in size down to dust particles. Many thousands have been individually identified and it is believed that there could be half a million with diameters larger than 1.6 kilometres (1 mile). Most asteroid orbits are concentrated in the asteroid belt between Mars and Jupiter.
Earth from moon
The Earth's only natural satellite. It is a barren, heavily cratered world, lacking water or an atmosphere. The same side of the Moon now always faces the Earth, it was heavily cratered by the impact of large numbers of meteorites, the largest of which created the mare basins
nearest major planet to the Sun and the smallest of the terrestrial
Telescopic observation of Mercury from the Earth is very difficult, partly because of its small size and partly because it can never be more than 28° from the Sun since its orbit lies well inside the Earth's The planet's rotation period is such that a "day" on Mercury lasts two "years". This leads to immense temperature contrasts: the subsolar point reaches 430°C; the night-time temperature plunges to -170°C. The high daytime temperatures and the small mass of the planet make it impossible for an atmosphere to be retained.
The fourth major planet from the Sun, often known as the Red
Planet because of its distinctive colour, noticeable even to the naked
Mars is one of the terrestrial planets with a diameter just over half that of the Earth. It had long been regarded as the planet (other than Earth) most likely to have life, a view encouraged by the presence of polar ice caps and observations of seasonal changes. Nineteenth-century observers, notably Percival Lowell, convinced themselves that they could make out systems of straight channels, canals, that might be artificially constructed. Exploration of the planet by spacecraft has virtually eliminated the possibility that life exists currently on Mars. However, studies of meteorites believed to be of martian origin have fuelled speculation that microscopic life at least may have existed on Mars in the remote past when the climate was wetter and warmer
Jupiter is ten times the size of the Earth and one-tenth of the Sun's diameter. Its mass is 0.1 per cent that of the Sun and its composition (by number of molecules) is very similar to the Sun's: 90 per cent hydrogen (in its molecular form in Jupiter) and 10 per cent helium. Of trace gases, the most significant are water vapour, methane and ammonia. There is no solid surface beneath the cloud layer. Instead, a gradual transition from gas to liquid takes place as the pressure increases with depth below the outermost layers, followed by an abrupt change to a metallic liquid, in which the atoms are stripped of their electrons. At the very centre there may be a small core of rock and perhaps ice
Saturn is one of the four "gas giants", second in size only to Jupiter. Its equatorial diameter is 9.4 times the Earth's and its mass 95 times greater. However, its average density is only 0.7 times that of water. Hydrogen and helium make up the bulk of the mass. There is a rocky central core, ten or fifteen times the mass of the Earth, which is surrounded by a thick mantle of liquid hydrogen and helium. In the high-pressure region surrounding the core, the hydrogen takes on the form of a metal. The outermost layers of the planet are gaseous; the visible features of the planet are cloud bands at the top of this atmosphere. The most striking feature is the spectacular ring system. The rings lie in the planet's equatorial plane, which is tilted at an angle of 27° to its orbit round the Sun they consist of many thousands of narrow concentric ringlets, resulting in a grooved appearance. They are only one kilometre thick and are made up of a huge number of separate rocks and particles, perhaps ranging in size from a hundred metres down to a micrometre
Venus can be viewed either in the western sky in the evening, or in the eastern sky in the morning. It is sometimes called the "morning star" or the "evening star". The surface is perpetually covered by dense, highly reflecting clouds which reveal a banded structure The atmosphere is almost entirely of carbon dioxide, and the surface pressure is more than 90 times that at the surface of the Earth. The exceptionally high surface temperature of 730 K (450°C) is a result of the greenhouse effect.
Uranus is one of the four "gas giant" planets of the solar system, with a diameter four times the Earth's and a mass fifteen times greater. It is composed almost entirely of hydrogen and helium. It is generally believed that there is a small rocky core at the centre of the planet, which is surrounded by a thick icy mantle of frozen water, methane and ammonia which merges into the outermost layer, an atmosphere of hydrogen and helium, with small quantities of some molecular compounds.
Neptune It is one of the four "gas giant" planets, having a small rocky core surrounded by an icy mantle of frozen water, methane and ammonia. Its diameter is almost four times the Earth's. The outer atmosphere is mainly molecular hydrogen with 15-20 per cent helium (by mass) and some methane. Not visible to the naked eye. With high magnification and larger telescopes, it is seen as a faintly bluish disc, the colour coming from methane in the upper atmosphere
Mass of Pluto is less than one-fifth that of the Moon. The diameter is 2,300 ± 40 kilometres. Pluto's overall density is approximately twice that of water and it is thought likely to consist of a thick layer of water ice overlying a core of partially hydrated rock Pluto's surface is reddish in colour. Methane ice was detected on Pluto Nitrogen and carbon monoxide ices were discovered on the surface in 1992. The surface temperature is about 40 K
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