The Solar System
• The solar system is located 26,100 light years from the centre of the Milky Way and was formed about 4540 million years ago from a globe of gas and dust.
• Most of the mass became concentrated in the central core to form the Sun, but part of the surrounding matter formed a flattened disc, which evolved into the eight planets (now there are only 8 planets with Pluto being relegated to the status of Dwarf planet) and other objects of the solar system.
The boundary between the inner and outer solar system is taken to be the orbit of Jupiter. The inner solar system consists of the four terrestrial planets, Mercury, Venus, Earth and Mars, as well as the asteroid belt. The outer solar system comprises the four next planets - Jupiter, Saturn, Uranus and Neptune. These outer planets are all giant ‘gas’ planets; although they are all expected
to have rock-iron cores, it is the composition and behaviour of the outer gas layers which gives each its unique characteristics.
Mercury’s surface is dominated by large basins representing different epochs in the formation of the crust. There are a large number of impact craters. The high density of the planet is due to its iron-rich core, which is 3600 km (2200 mi) in diameter and accounts for 65% of Mercury’s mass.
The atmospheric pressure is only one-trillionth (10-12) that of Earth and, without the protection of an atmosphere, there is a wide variation in surface temperature, from up to 420°C (790°F) in the day down to - 180°C (-290°F) at night.
Although similar in size to Earth, Venus is an extremely hostile planet with an atmosphere rich in carbon dioxide. It has a surface pressure 94 times that of the Earth at an overall temperature of 464°C (867°F). A thick planet-wide cloud cover, 50-75 km above the surface, contains a high concentration of droplets of sulphuric acid, which may be due to emissions from active volcanoes. Although the surface cannot be seen from Earth, Venus is essentially flat (with 80% being within 1 km of the planet’s average radius). Venus does not have the same type of tectonic geological structure as the Earth, but appears to periodically undergo extensive resurfacing, so that the current surface is relatively young – only 500 million years old. Although the rotation period is longer than its year, a Venusian ‘day’ (sunrise to sunrise) is equivalent to 116 Earth days.
The Earth is the largest and densest of the inner planets, with an atmosphere consisting mainly of 78% nitrogen and 21% oxygen at an average temperature of 15°C (59°F). Twothirds of the surface is covered by oceans and the mean (average) sea level can be used as reference from which the maximum deviations are a depth of 11,022 m (36,160 ft) for the Mariana Trench in the
Pacific ocean and a maximum height of 8863 m (29,078 ft) for Mount Everest in the Himalaya, a difference of 19.9 km - only 0.3% of the Earth’s radius.
With an equatorial diameter of 3476.3 km, a polar diameter of 3471.9 km and a mass of 7.348 x 1019 tonnes or 0.0123 Earth masses, the Moon is the fifth largest and fifth most massive satellite in our solar system.
Extensive space probe photography has now recorded the whole of the lunar surface showing the presence of craters, mountain ranges and broad plains of frozen lava known as ‘seas’ or ‘maria’. Because the Moon has no atmosphere, there is a wide variation in surface temperature, from 117°C (243°F) at the Equator at mid-day to -163°C (-261 °F) after nightfall.
The current theory of the Moon’s formation is the ‘giant impact’ theory, suggesting that in the violent early history of the solar system, the newly formed Earth was struck by at least one and possibly several very large planetesimals.
Like Venus, the atmosphere of Mars is mainly carbon dioxide, but only at a surface pressure about 116th that of Earth at an average temperature of - 53°C (-63°F), which is 68°C (122°F) lower than that of Earth. The surface is highly complex, consisting of flat plains, craters, dormant volcanoes and pole caps. There are also a number of large channels such as ‘Valles Marineris’, which is at least 4000 km long, up to 600 km wide and 7 km deep. It now appears that life on Mars could only have developed to a microbiological level. Although there is evidence that many channels must have been fashioned by large quantities of water in the past, there is now no evidence of water in the outermost surface layers or in the atmosphere,although water ice is still present
under the frozen carbon dioxide polar caps.
A model of Jupiter suggests that it may have a central rockiron core about 15,000 km in diameter, weigh about 15 Earth masses, and is surrounded by a shell of metallic hydrogen extending to a radius of 55,000km from its centre. The ‘Great Red Spot’, which was first seen in 1664, is a long-lived swirling storm which rises up to 8 km above the surrounding cloud deck. The planet has a very strong magnetic field, about five to ten times stronger than that of the Earth. This results in the formation of extremely lethal radiation belts about 10,000 times more powerful than those of Earth.
The ring system on Jupiter was discovered in 1979 and has three components.
Saturn’s internal structure is generally considered to be similar to that of Jupiter, but with a much smaller metallic hydrogen
layer extending to only 26,000 km from the centre of the planet. The true nature of Saturn’s rings, initially vaguely observed by Galileo in 1610, was deduced by Christiaan Huygens (Netherlands) in 1659. Composed mainly of water ice, the main ring system is 273,550 km in diameter, but only about 10 m (33 ft) thick.
Although officially Saturn has eighteen satellites, many more candidates have been observed from re-examination of photographs from the Voyager 2 encounter and from Hubble Space Telescope observations. The largest of the moons, Titan, is the only satellite in the solar system with an extensive atmosphere.
Uranus can just be seen with the naked eye, this much smaller gas planet is believed to have a rock-iron core surrounded by
a ‘sea’ of water, methane and ammonia. The outer atmosphere is composed mainly of hydrogen, but with about 26% of helium and a small amount of methane, which is responsible for the greenish colour of the atmosphere.
The large tilt of Uranus’s axis (98°) means that day and night on some parts of the planet may last up to 21 years, but the sunlit ‘south’ pole and dark ‘north’ pole differ very little in temperature.
The planet Neptune is invisible to the naked eye. Neptune is an extremely dynamic world with many discernible cloud
features and extremely high wind speeds. The planet radiates 161% more heat back into space than it receives from the Sun. The bluish colour of the planet is due to the presence of methane in the atmosphere.
The Voyager 2 encounter proved that there were three distinct rings and a diffuse ring of material between 38,000 to 59,000 km from Neptune’s centre.In 1989, the Voyager imaging team discovered six new satellites in addition to the two already known. The largest of the satellites, Triton, has a large orbital inclination (157°) and a retrograde orbit (opposite to the direction of Neptune’s rotation).
THE SUN At the centre of the solar system is the Sun, a mass composed of 73% hydrogen, 25% helium and 2% other elements.
The internal structure of the Sun consists of a helium-rich core, in which hydrogen undergoes fusion to helium. The atmosphere of the Sun consists of a ‘chromosphere’, which extends to about 10,000 km above the photosphere. It has a sufficiently high temperature. The chromosphere only becomes visible during total eclipses, when the photosphere is blocked out. The outer atmosphere or ‘corona’ is an extremely thin gas at a high temperature which appears as a white halo. It allows the continuous dispersal into space of the Sun’s matter in the form of a plasma of charged particles. This ‘solar wind’ permeates the whole of the solar system. Solar flares are huge jets of gas flung many thousands of kms from the chromosphere.