The Solar System

Mercury is the planet closest to the sun and is approximately the same size as our moon. It revolves around the sun once every 88 days and has a very elliptical orbit. It doesn't have an atmosphere and its high density suggests that it is composed of a relatively large metallic core, surrounded by a silicate mantle.

A large part of Mercury is covered by craters and it looks very similar to our moon. The craters range in size from very small pits to large basinal structures (such as the Caloris Basin which is 1300 km across). The craters appear to have formed by meteorite impacts.

Mercury also has extensive smooth plains that may represent either material ejected by impacts or by lava flows. Mercury has large fault scarps which appear to have formed early when the molten interior of the planet cooled, shrank and caused the outer solid brittle crust to contract around the shrinking core.

Venus among the Pleiades, April 6. Photo: © Akira Fujii/DMI. Venus is very similar to the Earth in both size and overall density but has a very dense atmosphere (90 times that of the Earth). It orbits around the sun every 224.7 days. The surface appears to be composed of mainly basalt. It has flat extensive plains though there are two elevated areas, similar in size to Australia. There is a large circular structure rising some 5 km above the surrounding plains and 700 km in diameter which may be a shield volcano. There is little evidence for any craters on Venus. This may be because

• the surface rocks are young (e.g. sediments and volcanics)
• wind erosion may have destroyed any craters
• the dense atmosphere may have caused any meteorites to break up before reaching the surface
• the early crust may have been too thin to support any relief

The atmosphere of Venus largely consists of carbon dioxide (95%), along with minor traces of nitrogen, oxygen, sulfur dioxide, sulfuric acid and water vapour.

Mars revolves around the Sun once every 687 days. It does not have continents or oceans. The southern hemisphere of Mars is heavily cratered and there are large impact basins. In contrast, the northern hemisphere of Mars consists of vast expanses of relatively crater-free plains which are covered by basaltic lava flows. Mars also contains very large volcanoes with calderas at their summits. One of these, Olympus Mons, is probably the largest volcano in the Solar System as it has a height of more than 20 km and is over 700 km across.

Near the equator of Mars there is an extensive system of canyons known as Valles Marineris (Mariner Valley). This large rift system is believed to have formed by the tension, stretching and collapse of the crust. The soils of Mars are rich in iron oxides which explains the planet's red colour. The atmosphere on Mars is very thin (approximately one hundredth of Earth's) and consists mainly of carbon dioxide with traces of argon, oxygen and water vapour.

Between Mars and Jupiter, some 400 million kilometres from the sun, there is a zone known as the Asteroid Belt that contains a multitude of small bodies known as asteroids. There may be up to 100 000 asteroids in this belt, however the largest is only some 1000 km across. They are too small to retain atmospheres as any gases would be lost to space because of the weak gravitational field. Although they were all once thought to represent exploded fragments of planets, many asteroids are now believed to represent planetesimals, the building blocks from which planets are formed. Most evidence suggests that meteorites are derived from the Asteroid Belt, although some come from the Moon, Mars or comets.

Jupiter makes one revolution around the Sun in about 11.86 Earth years (the orbital period). The Great Red Spot on Jupiter is a great cyclonic storm system that has persisted for at least the last 300 years. The atmosphere of Jupiter is composed largely of hydrogen (90%) and helium (10%), along with trace amounts of water, methane and ammonia. Jupiter has an inner rocky core and an inner mantle of metallic hydrogen.

Saturn is smaller and less dense than Jupiter. Both the atmosphere and internal structure of Saturn resemble those of Jupiter. Saturn is best known because of its ring system that is over 300 000 km in diameter but less than 2 km thick. The ring system consists of hundreds of individual rings that are composed of many small particles ranging in size from centimetres to metres. The particles are mainly rocky fragments coated in ice.

Uranus and Neptune are of similar size and consist largely of hydrogen and methane. They have rocky cores surrounded by solid mantles of crystalline methane, ammonia and water (in the form of ice) grading into atmospheres of hydrogen, helium and methane.

Pluto was once considered to be the smallest of the planets, as it is even smaller than Earth's moon. It takes 247 years to orbit the sun. Because it is so far from Earth, we don't really know much about either its composition or surface features.

Comets are small bodies from the outer reaches of the Solar System which only become visible when they move into the inner part of the Solar System. They consist largely of ices, along with silicate dust particles and frozen compounds of carbon, nitrogen, oxygen and hydrogen. On approaching the sun, the ice melts to become gas and the solar energy ionises these to form the comet's tail. The head of a comet is only about 10 km across.

Meteorites are extraterrestrial objects that enter the Earth's atmosphere with velocities of 11 km - 74 km per second. They undergo intense friction when entering the Earth's atmosphere during which the surface of the meteorite is melted and blown away (ablation) leaving a fiery trail in the sky.

Small objects (less than a gram in size) which enter the Earth's atmosphere are usually totally burnt up. These are known as meteors or, more commonly, shooting stars. Larger bodies (few grams to hundreds of tons) lose a large part of their mass by ablation, but are large enough to survive descent and fall to the Earth's surface. These are known as meteorites. Falls are meteorites that were actually observed to fall from the sky. Finds are meteorites whose fall from the sky was not observed.

Meteorites can be classified into three main groups:

• Stony meteorites: those composed largely of silicate minerals.
• Iron meteorites: those composed largely of iron-nickel alloy.
• Stony-iron meteorites: those composed of equal amounts of silicate minerals and iron-nickel metal.

Stony meteorites can be subdivided into a number of different types:

• Ordinary stony meteorites are called chondrites, and contain the same proportions of the heavier elements as observed in the Sun. The silicate minerals occur as subrounded to rounded grains called chondrules.
  Carbonaceous chondrites contain chondrules, and complex organic compounds, along with calcium and aluminium rich minerals.
• Those meteorites composed almost entirely of silicate minerals but lacking chondrules are called achondrites.
  Iron meteorites are massive chunks of metallic iron, usually with 4% - 12% nickel. When their polished surface is chemically etched, many show a regular pattern of platy crystals made of two different iron-nickel minerals. This is called Widmanstatten structure.
• Stony-iron meteorites include Pallasites and mesosiderites. Pallasites are composed of coarse olivine crystals and nickel-iron metal, while mesosiderites are stony-iron meteorites in which the silicate phase consists of pyroxenes.

Most meteorites are 4.6 billion years old, though a small proportion are only 1.3 billion years old and may have been derived from the surface of Mars.

Tektites are of common occurrence in Australia - Australites, South-East Asia - Indochinites, Central Europe - Moldavites, Ivory Coast (Africa), Texas and Georgia (USA). They are rounded pieces of glass formed by meteorite impact. They have been splashed up into the Earth's atmosphere and then fallen back to the Earth's surface.

The Moon has a diameter of 3476 km, about a quarter of that of the Earth. It has a gravitational force on its surface one sixth of that of the Earth. It has no atmosphere as its gravitational force is too low. The side facing the sun has a surface temperature of 132° C while the opposite dark side has a surface temperature of -185° C.

The surface of the Moon is completely covered by a thin veneer of rock fragments a few metres thick called the regolith. This has been produced by the impact of millions of meteorites on the Moon's surface. The regolith is composed of rock fragments, finely-ground rock, and glass formed by melting during meteorite impact. Occasionally, the meteorite impact is large enough to melt the regolith together. This is called soil breccia. The light coloured mountainous areas on the Moon are called lunar highlands and are saturated by impact craters. The dark areas are lava plains called Mare.

• Lunar highlands: are the oldest parts of the Moon and are largely composed of the mineral plagioclase (hence the light colour). They are rich in calcium and aluminium and most are breccias produced by meteorite impact. These are very similar to some plagioclase-rich rocks on Earth called anorthosites.
• Mare: Mostly composed of basaltic lavas derived from the interior of the Moon. They are low in silica and high in iron.

The crust of the Moon is not of uniform thickness, varying from 60 km - 100 km. The mantle begins below this and is solid down to 800 km - 1000 km. Below this zone the Moon appears to be partially molten. The Moon may also have a small metallic core, up to 350 km in diameter.

The Moon formed approximately 4.6 billion years ago, most likely in direct association with the Earth. They accreted from different mixtures of metallic iron and silicates with the smaller Moon growing from a debris ring around the Earth.