The Planets

by Chris Lawton

There are nine planets within in the solar system. They fall into two main categories - the gas giants and the rocky minors. The gas giants Jupiter, Saturn, Uranus and Neptune represent most of the mass in the solar system (after that in the sun of course !). The rocky minor planets Mercury, Venus, Earth, Mars and Pluto make up very little of the mass but apart from the last one represent our closest neighbours in space.

In the data table at the start of each planet's description the mass, radius and gravity are relative to the earth's values. The values for the earth are 5.976x1024 kg, 6.4x103 km and 9.78 m s-2 respectively. The escape velocity is how fast one must travel to break free from the gravitational pull of the planet.

Mercury

Observations :

Being the closest planet it is one of the hardest to see. It is visible to the naked eye but always within 2 hours of the sun. This problem means that viewing must either take place just after sunset or just before sunrise. Once located telescopic observations are difficult to perform due to the solar glare. In the 1970s the American Mariner 10 satellite was sent to Mercury. It produced maps of around a third of the planetary surface.

Surface & Composition :

The images from Mariner revealed a lunar type landscape. However the composition is more earthlike than lunar. There is evidence for volcanic type activity suggesting a once molten core. Mercury is a very savage place with temperatures ranging from -170 C at night to over 400 C at day.

Images :

Click here for Mariner 10 images of Mercury.

Venus

Observations :

Venus is the brightest planet. When observable it is bright enough to cast faint shadows in favourable conditions. Telescopic observations are much easier than Mercury with Venus visible for several hours after sunset or before sunrise. Nearly all our information on Venus has come from satellite missions. Both Soviet and American orbiters, particularly Magellan, have now mapped most of the surface via radar. Radar mapping is essentially due to the thick toxic clouds which fill the atmosphere. Probes have also been deployed to the surface sampling soil types and atmospheric conditions.

Surface & Composition :

The Venetian surface is vast and rolling with signs of recent volcanic activity. Perhaps of more interest is the atmosphere. It is predominantly carbon dioxide with sulphuric acid clouds. Clearly one of the most inhospitable places in the solar system. The planet is very similar to the earth in terms of geophysical composition. Due to the atmosphere the surface temperature is over 400 C and atmospheric pressure 90 times that on earth.

Images :

Click here for images. These images are from the NASA Magellan probe which has extensively mapped the surface via microwave radar. The level of detail is such that features of less than 1 km can be seen.

Mars

Observations :

Mars, even to the naked eye, appears red. When at closest it shines brightly and is easily identified. Surface detail is difficult to obtain with small telescopes and most of the information we have has again come from missions. In the 1960s the Mariner 4 probe orbited the planet. In the next few years various Mariner missions flew by the planet obtaining more detailed images. In 1976 the Viking spacecraft landed on the surface providing images and information on the soil type and atmospheric content. Mars also has two moons, Phobos and Demios which are probably captured asteroids.

Surface & Composition :

Mars is famous for its red dusty surface. The Martian landscape contains some of the most dramatic features known. Its surface is split into two distinctive regions. The southern hemisphere is heavily cratered and stands around 2 km proud of the 'sea-level'. The northern hemisphere is very sparse containing vast seas or plantia. There is an exception to this simple morphology - the Tharsis region. This vast area is 10 km high and shield volcanos rise out of this up. The highest feature is Olympus Mons which towers some 27 km high.

Images :

Click here for images of Mars obtained from various missions as stated.

Jupiter

Observations :

Jupiter, when visible, shines brightly in the night sky. It is second in brightness only to Venus. Observations form earth reveal little of its complex detail. The banded nature of the planet is discernible but the turbulence within it is unobservable. The great red spot, Jupiter's most famous feature, is visible in moderate sized telescopes. Also visible in telescopes are points of light around the planet which represent the four Galilean satellites - Io, Europa, Callisto and Ganymede. Detail of these and the complexity of Jupiter's atmosphere came from the NASA deep space missions. These missions involved the Pioneer 10 and Voyager missions to the outer planets. On their vast trips they gathered a wealth of information on the planets and their moons. Recently the Hubble Space Telescope (HST) has also produced dynamic pictures. Last year observations of Jupiter increased as the dramatic comet Shoemaker-Levy IX impact occurred.

Surface & Composition :

Jupiter is the first of the giant gas planets. It is difficult to say what the interiors of these planets hold but the existence of a solid surface is open to question. Many observations have been made especially in the Pioneer and Voyager flybys and the comet impact also helped in our understanding of the structure. The atmosphere of Jupiter is mainly made up from methane and amonia with many other gases.

Images :

Click here for an image of Jupiter's moons.

Saturn

Observations:

As we go ever outwards and the planets now ever smaller it becomes increasingly difficult to make good observations form earth. Saturn is famous for its rings and their true complexity was revealed by the Voyager spacecraft. With a suitably sized telescopes there are, however, two features to look for. Firstly a shadow on the Saturnian atmosphere cast by the rings. Secondly the Cassini division - a gap within the ring structure. Even under low power, however, the rings can be seen as an elongation of the planet's main body.

Surface & Composition :

Very similar to Jupiter but fewer of the minor gases have been detected due to the relative coolness of the planet.

Images :

Click here for a recent image of Saturn from HST showing Saturn's rings edge on.

Uranus

Observations :

Uranus was the first planet to be discovered in recent times by Herschel in 1781. Voyager 2 has provided us with the only detailed images of Uranus. Again a thin ring system was system confirmed although this had been detected some years earlier from earth. Little was seen in the upper atmosphere and images were on the whole rather diappointing. Several of the moons around the planet were also seen in detail for the first time and revealed icy, cratered and scared worlds.

Surface & Compositon :

Uranus's atmosphere is principally made up from methane hence its blue colour. Other than that little is known about the atmosphere of this planet.

Neptune

Observations :

Neptune was discovered in 1846 independently by Leverrier and Adams. Due to excentricities in the orbit it is currently the furthest planet from the sun and will remain so until 1999. Detailed observations had to wait until Voyager 2 reached there in 1989 although some earth based observations had been made. Neptune was in complete contrast to Uranus. It's deep blue colour was brought to life by storms and clouds blowing around its upper atmosphere. It has several moons the beautifully pink coloured Triton being one of them. This distinct colour comes from methane ice. The surface of this icy world -240 C is the coldest place in the solar system. It also happens to be my favourite planet.

Surface & Composition :

Despite these excellent images very little is known about the atmosphere and deeper compositon of Neptune. All we can conclude, agan from the blue colour, is that large amounts of methane must be present.

Images :

Click here and here for some more images of Neptune.

Pluto

General :

Very little is known about this distant body. Discovered in 1930 by Clive Tombaugh it is an enigma. Its size is small and the origins uncertain. What is even stranger is that it owns a moon - Charon which is almost identical in size. Recently the HST has made observation of this planet system and has been able to resolve both planet and moon (see above). A cold lonely place, often overlooked, yet part of our solar system.
Back to Astronomy and Astrology home page

On to The Moon

More pages on Science and Christianity

Copyright © Chris Lawton 1995, with thanks to NASA/ESA for some of the astronomy images