Category Science & Technology

JUPITER

Jupiter is the largest planet in the Solar System. Large enough to contain more than 1300 Earths inside it, Jupiter is more massive than all the other planets combined. Along with Saturn, Uranus and Neptune, Jupiter is known as a “gas giant”, because it is mostly made of gas with no solid surface at all.

The colourful patterns of red, brown, yellow and white on Jupiter’s surface are produced by the chemicals sulphur and phosphorus in the swirling atmosphere. Jupiter’s extremely quick rotation is probably responsible both for separating the clouds into different colour “zones” (the lighter bands) and “belts” (the darker bands), and for the continual storms. The Great Red Spot, its most famous feature, is such a storm. The quick rotation also causes Jupiter to bulge at its equator, so that it measures 7500 kilometres less from pole to pole.

Jupiter has a system of rings consisting of dark grains of dust. The four largest of its moons are bigger than the planet Pluto. The beautiful, ever-changing patterns on Jupiter’s globe are violent winds.

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VENUS

About the same size as Earth, Venus is shrouded in thick, unbroken clouds made of droplets of deadly sulphuric acid. Because its cloud cover reflects the light of the Sun from its surface, Venus is a very bright object in the night sky.

Some 25 kilometres thick, the clouds prevent most sunlight from reaching the surface. But another kind of radiation from the Sun, called infrared, does get though and Venus’s dense atmosphere stops it from escaping. The result is a constant surface temperature hotter than the melting point of lead and the hottest in the Solar System. If any space explorer landed on Venus, he or she would be simultaneously incinerated, suffocated by the unbreathable carbon dioxide air, dissolved by acid and crushed by air pressure about 90 times that on Earth.

Venus spins slowly on its axis, actually taking longer to complete one rotation than to orbit the Sun. Relative to all the other planets except Pluto, it spins backwards.

            Venus is covered by thick clouds. They race round in the planet in just four days. The interior of Venus is similar to that of Earth, although its metallic core is much larger than Earth’s.

            Beneath the clouds, Venus’s barren surface features tens of thousands of volcanoes (some possibly still active) surrounded by vast lava plains. Lava flows have cut channels in the ground that look as if they may have been carved by rivers. Odd, dome-shaped volcanoes, or “pancakes”, as they have been described, have formed where lava has oozed to the surface, and then cooled as it spread out in all directions.

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MERCURY

          Mercury, the closest planet to the Sun, is the second smallest planet in the Solar System. Because it is so near the Sun, it can be seen from Earth only with difficulty – low in the dawn or twilight sky close to the Sun.

          Mercury’s surface looks quite similar to that of our Moon. Bare and rocky, it is covered with craters, the result of continual bombardment by meteorites during the first billion years of its existence. Originally molten, Mercury’s surface shrank as it cooled after the bombardment eased, resulting in “wrinkles” – long mountain chains. With no winds or water to erode the rocks, Mercury’s landscape has remained the same ever since.

           Mercury’s orbit has an unusual shape All the other planets, except Pluto, have nearly circular orbits, but Mercury’s is elliptical – more like an oval. At its closest, Mercury is 46 million kilometres from the Sun, 70 million kilometres away at its most distant.

            Mercury has great extremes of temperature. Where it faces the Sun, it can exceed 400°C, but during the long nights (lasting about 59 Earth days) and with no atmosphere to keep the heat in, temperatures can plummet to – 170°C.

            Mercury’s surface is made up of thousands of craters, as well as mountains and lava plains.

            Mercury, the densest planet apart from Earth, has a large metal core made of iron and nickel, surrounded by a thin rocky shell.

            The landscape of Mercury is dominated by thousands of craters. The huge Sun burns with a fierce heat – turning to severe cold when this face of the planet is turned away from it. Large boulders falling from space have produced craters in Mercury’s surface measuring many kilometres across, some with smaller craters inside. Because there is hardly any atmosphere, Mercury’s skies remain black even during the day.

            When a meteorite strikes the surface of Mercury, it punches a saucer-shaped crater in the ground. Debris is blasted out in all directions, creating long streaks.

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THE PLANETS

A planet is a large object in orbit around a star. It can be made of rock, metal, liquid, gas, or a combination of these. Planets do not produce light, but reflect the light of their parent star.

In our own Solar System, there are nine planets, including Earth, orbiting the Sun, our parent star. Observations of other stars made by astronomers using powerful telescopes indicate that they, too, have planets. There could therefore be billions of other planets in the Universe.

The Earth is the largest of the four inner, or “terrestrial”, planets: Mercury, Venus, Earth and Mars. They are, as the scale illustration demonstrates, dwarfed by the four “gas giants”, Jupiter, Saturn, Uranus and Neptune, so called because they have comparatively small rocky cores surrounded by thick layers of liquid and gas. Pluto fits into neither category, being a small, outer planet made of ice and rock.

The diagram shows the relative distances of the planets from the Sun. Pacing out their positions would give an even better idea of the huge distances between them. If the Sun were a football, Mercury would be pinhead 10 paces away from it. Earth (the size of a peppercorn) is a further 16 paces on from Mercury, with the Moon a thumb’s length away from Earth. Another 209 paces would bring you to Jupiter (a large marble), while Pluto lies 884 more paces distant. To reach the nearest star, Proxima Centauri, you must walk another 6700 kilometres!

EXPLORING THE PLANETS

Because the giant planets lie so far from Earth, it would take too long for people to travel to them. So space probes have been launched to “fly by” every planet except Pluto and send back pictures. Voyager 2 made the greatest journey. Space probe Cassini visits Saturn in 2004.

THE PLANETS FORM

The Solar System began life as a cloud of gas and dust drifting across the Milky Way Ga1axy. It is thought that a supernova may have sent shock waves racing across space, striking the cloud and somehow causing it to collapse under its own gravity.

Within 100,000 years, the collapsed cloud became a swirling disc, called a solar nebula. Under pressure from gas and dust spiralling inwards, the centre became hotter and denser and began to bulge. It would soon evolve into the infant Sun.

Away from this central furnace, particles of dust began to clump together like snowflakes, first into small fragments of rock, then becoming large boulders. Over millions of years, some grew into blocks several kilometres across, called planetesimals. These eventually started to collide with one another, building up like snowballs to become the four rocky inner planets, Mercury, Venus, Earth and Mars, and the cores of the four gas giants, Jupiter, Saturn, Uranus and Neptune.

The solar wind stripped away any remaining dust and gas, including the atmospheres around the four inner planets. The giant planets lay beyond the solar wind’s fiercest blast, so they were able to hold on to their thick blankets of gas.

Jupiter’s gravitational pull caused nearby planetesimals to destroy one another rather than build up into another planet, leaving a belt of rock fragments, known as asteroids, still orbiting the Sun, as they do today.

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SOLAR SYSTEM

The solar system consists of the Sun and an array of objects that orbit it. These objects include the nine known planets, their 64 known moons, asteroids, comets, meteoroids and huge amounts of gas and dust. The Sun’s great size relative to the other objects in the Solar System gives it the gravitational pull to keep them permanently in orbit around it.

The planets orbit the Sun in the same direction (anticlockwise in this illustration) and in elliptical (oval-shaped) paths. Pluto’s orbit is the most elliptical of all the planets. For part of its journey around the Sun, its orbit actually lies inside that of Neptune. All the planets, and most of their moons, travel on approximately the same plane, with the exception of Mercury and, once again, Pluto, both of which have tilted orbits.

Constantly streaming away from the Sun in all directions is the solar wind, made up of electrically-charged particles (parts of atoms).Travelling at more than 400 kilometres per second, it produces electric currents inside a giant magnetic “bubble” called the heliosphere. The heliosphere protects the Solar System from cosmic rays arriving from space. Its edge, some 18 billion kilometres from the Sun, marks the true boundary of the Solar System.

EARLY ASTRONOMERS

Thousands of years ago, in the time of the ancient civilizations of Egypt and China, people thought that the Sun and Moon were gods, the Earth was flat and the sky was a great dome suspended above it.

In later years, astronomers from ancient Greece proved that the Earth was round. Many believed that the stars were fixed to a great sphere that rotated around the Earth each day. One Greek astronomer, Aristarchus, proposed that the planets, including Earth, orbited the Sun, a star, but most astronomers of this time thought that the Sun, Moon and planets all travelled in circular paths around Earth, the centre of the Universe. Ptolemy, who lived in the 2nd century AD, observed that, while the stars moved across the night sky along regular paths, the planets appeared to “wander” from theirs. He proposed that they each moved in their own small circles, called epicycles, as they orbited Earth.

The Polish priest and astronomer, Nicolaus Copernicus, challenged Ptolemy’s view of the Solar System, declaring that the Sun lay at the centre of a system of orbiting planets. Only the Moon orbited the Earth. Copernicus wrongly believed that the planets’ orbits were perfect circles and that they moved in epicycles. It was left to the German astronomer Johannes Kepler (1571-1630), who showed that the planets moved in elliptical, rather than perfectly circular, orbits. The shapes of their orbits also explained the “wandering” that so perplexed earlier observers, thus disproving the idea that the planets moved in epicycles.

The Italian astronomer Galileo (1564-1642) was the first to use a telescope. From his observations of the moons of Jupiter in orbit around that planet, and the changing shape of Venus as it orbited the Sun, he concluded that Copernicus had been correct: the planets do orbit the Sun.

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