The solar system Earth Sun CClip l i p SScience en FFile a ... · The Shoemaker-Levy 9 comet that struck Jupiter in 1994 was actually a series of 21 comets! The explosions caused

SunEarth

1 AU1 1 AUAU

SunEarth

1 AU

Sun

Mercury

Venus

Earth

Mars

Jupiter Saturn Uranus Neptune

Fig 8.4.2 The distances in this diagram are to scale, but the sizes of the planets are not.

Hey you, AU!The average distance from the Earth to the Sun is called an astronomical unit (AU) and equals 149 600 000 kilometres. If you travelled at 100 kilometres per hour, then it would take about 170 years to get from the Earth to the Sun!

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Mercury

Mass 0.056 times that of Earth

Moons None

Diameter 4878 km ( = 0.38 × Earth’s diameter)

SurfaceSimilar to Earth’s Moon, with craters, lava-flooded plains and smooth mountains

AtmosphereMainly helium, which blows past Mercury from the Sun

Gravity 0.38 times that on Earth

Surface temperature

Drops to –170°C at night and rises to 430°C in the day

Period of rotation (day)

59 Earth days

Tilt of axis 0°

Distance from Sun 0.39 AU (58 million km)

Time to orbit Sun (year)

88 Earth days

Scale model (Sun = 300 mm)

Diameter 1 mm

Distance from Sun 12.5 m

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Fig 8.4.3 An astronomical unit (AU) is the distance from Earth to the Sun.

The terrestrial planetsMercury, Venus, Earth and Mars all can be seen without a telescope. For this reason they were known to most ancient civilisations. The terrestrial planets could be easily referred to as the ‘ancient’ planets.

Each planet is given a symbol by modern astronomers. These symbols were invented by the Greeks who imagined each planet to be a god.

MercuryMercury was known in ancient Sumer (now Iraq) some 5000 years ago. This planet moves very quickly across the sky. It was named after the Roman god, Mercury, who was the swift messenger of the gods.Fig 8.4.4 Mercury shows its heavily cratered surface.

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would take aboutrs to get from thethe Sun!

Di

Su

At

Gr

Su te

Pe(d

Diameter measurements are made at the equator for each planet. For comparison, the diameter of the Sun is 1 392 000 km.

Venus


Moons None

Diameter12 103 km ( = 0.95 × Earth’s diameter)

SurfaceExtensive cratering, volcanic activity, mountain ranges, a 1500 km trench.

Atmosphere

80 km thick layer of carbon dioxide with some water vapour. Clouds contain concentrated sulfuric acid droplets.

Atmospheric pressure90 times that on Earth (enough to crush early Space probes)


Surface temperature 460°C

Period of rotation (day) 243 Earth days

Tilt of axis 30°


Time to orbit Sun (year) 225 Earth days


Diameter 2.6 mm



Fig 8.4.5 A radar image of Venus sent from the Magellan mission.

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Mercury is the closest planet to the Sun, so close that it is often difficult to observe. One time that it is very visible is in the dark of the morning or evening. For this reason it appears as a morning and evening star. Despite its proximity to the Sun, Mercury is not the hottest planet in the solar system.

Mercury is the smallest planet of the solar system, being about the size of Earth’s Moon. Like the Moon, it has a very old surface containing craters and plains. Much of our knowledge of Mercury comes from the American spacecraft Mariner 1. In the 1970s it flew past Mercury three times, photographing and mapping roughly half the planet’s surface. In 2004, the Messenger spacecraft was launched to investigate the geology, atmosphere and magnetic field of the planet Mercury. It took three and a half years for Messenger to get to Mercury, flying within 200 kilometres of its surface. In 2008, it pulled itself onto a path that will lead it to back to orbit Mercury in 2011. It takes ten minutes for Messenger’s radio signals to reach Earth and its flight controllers at NASA.

VenusVenus was recorded by the Babylonians in approximately 3000 BCE and it is also mentioned in the astronomical records of the ancient civilisations of China, Central America, Egypt and Greece.

Venus is the hottest planet in the solar system and is the planet closest in size to Earth. Venus has an acidic and crushing atmosphere that would make it impossible for life to exist on it. When visible, Venus is the third brightest object in the sky after the Sun and Moon. It is so bright that it is known as the morning and evening star.

Venus spins from east to west, a direction opposite to the spin of Earth and the other planets. This opposite spin is called retrograde movement. Venus also revolves very slowly. A day is longer than its year. Scientists are curious about why Venus developed so differently from Earth and the other planets. One thought is that a comet or asteroid may have crashed into Venus in the distant past and that this collision slowed its rotation and reversed its spin.

The solar system

Earth

Mass1.0 times that of Earth(5 980 000 000 000 000 000 000 000 kg)

Moons One (known as ‘the Moon’)

Diameter 12 756 km

Surface Two-thirds water, one-third land

Atmosphere78% nitrogen, 21% oxygen, 1% carbon dioxide, argon and water vapour and other gases


Surface temperature

Average 22°C


1 day

Tilt of axis 23.5°

Distance from Sun 1 AU (150 million kilometres)


365.25 days


Diameter 2.7 mm


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Fig 8.4.6 Earth showing Australia and snow-covered Antarctica.

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The surface of Venus cannot be observed directly using telescopes because of its thick, acidic cloud layer. Most of our knowledge of Venus comes from the Magellan mapping probe that left Earth in 1989 and fell into orbit around Venus in 1990. The images it sent back showed sharp-edged craters and massive volcanoes, and were used to map 98 per cent of the planet’s surface. The Venus Express, which was launched by the European Space Agency in 2005, continues to send back scientific data about the planet.

EarthEarth is the third planet from the Sun. Seventy per cent of Earth’s surface is covered by water and so the planet is also known as the blue or water planet.

Earth is the only planet currently known to support life. It has a molten core, upon which float the massive rocky tectonic plates that make up its surface. Earth is orbited by its Moon and many artificial communication satellites.

MarsMars is named after the Roman god of war. Mars is known as the ‘red planet’ and has been the subject of many science fiction movies and books. Mars would be accurately called ‘the rusty planet’, as its red appearance is due to rust (iron oxide) in its surface soil and rocks. There are some similarities between Earth and Mars—a Martian day is only 30 minutes longer than an Earth day and its 25.2° tilt causes seasons similar to Earth’s (only twice as long).

Many space probes have been sent to Mars to collect data on its rocks and weather, and to see if there is any evidence of past or microscopic life. Some of the most important missions were Viking 1 and 2, Pathfinder, the Mars Global Surveyor (which found that water once existed on Mars) and Odyssey (which discovered ice under its surface). These were space probes that collected data about the rocks, weather and any evidence of microscopic life. In 2007, the Phoenix Mars Mission was launched as a ‘budget’ Mars Exploration Program. This mission was designed to determine the existence of water and the habitability and potential biology on the planet.

Mars


MoonsTwo (Phobos—diameter 23 km, Deimos—diameter 10 km)

Diameter6794 km ( = 0.53 × Earth’s diameter)

Surface

Soft red soil containing iron oxide (rust), giving the planet its red appearance. Cratered regions, large volcanoes, a large canyon and possible dried-up water channels. Polar caps of frozen carbon dioxide and water.

Atmosphere Very thin, mainly carbon dioxide


Surface temperature

–120°C to 25°C


1.03 Earth days

Tilt of axis 25.2°

Distance from Sun

1.52 AU (228 million km)


687 Earth days


Diameter 1.4 mm

Distance from Sun

49.1 m


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The asteroid beltThe asteroid belt is made up of thousands of small rocky metallic bodies and dust in orbit around the Sun. The largest asteroid is Ceres, having a diameter of about 1000 kilometres. Researchers have found several near-Earth asteroids, but none are predicted to crash into Earth in the near or distant future.

The solar system

Fig 8.4.7 Mars showing red earth and polar caps.

Fig 8.4.8 The Mars Phoenix mission. The landing system on Phoenix allows the spacecraft to touch down within 10 kilometres of its targeted landing area.

Fig 8.4.9 Thousands of asteroids lie in a belt between Mars and Jupiter. One is Ida, an asteroid big enough to have a gravitational field that has trapped its own orbiting moon, Dactyl.

Hooked on astronomyRobert Hooke (1635–1703) was the English scientist who discovered cells. He was also one of the first men to build a reflecting telescope. He used this telescope to discover a previously unknown star in the constellation of Orion. His observations with his telescope led him to suggest in 1664 that Jupiter rotated on its axis just like Earth. His detailed sketches of Mars were used two hundred years later to determine how fast it spun.


Shoemaker puts the boot inThe Shoemaker-Levy 9 comet that struck Jupiter in 1994 was actually a series of 21 comets! The explosions caused by the impacts had the equivalent power of an atomic bomb going off every second for five or six years. Some of the resulting dust clouds were bigger than Earth.


The Great Red Spot is a giant hurricane about three times the size of the Earth.

Fig 8.4.10 Jupiter showing alternating east and west wind belts.

Jupiter

Mass 318 times that of Earth

Moons

Over 60 moons and four rings, including the four largest moons: Io, Ganymede, Europa and Callisto. These are known as the ‘Galilean’ moons.


Surface Liquid hydrogen

Atmosphere

Hydrogen (84%) and helium (15%). Upper layer contains white clouds, probably composed of solid ammonia.


Surface temperature Cloud top –150°C


9 h 55 min

Tilt of axis 3.1°

Distance from Sun5.2 AU (778 million kilometres)


11.8 Earth years


Diameter 30 mm

Distance from Sun 168 m


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The gas giantsJupiterJupiter is the largest planet in the solar system with a diameter more than 11 times that of Earth. Ancient astronomers named the planet Jupiter, after the ruler of the gods in the Roman state.

In March 1979, Voyager 1 flew by Jupiter and detected a faint series of rings around the planet, measuring 29 kilometres thick and 6400 kilometres wide. The first active volcano outside Earth was also observed on Io, one of Jupiter’s sixty or so moons. In July 1994, the Hubble space telescope photo-graphed the collision of the comet Shoemaker-Levy 9 with Jupiter.

Unit Unit 8.48.4

Ancient and modern planetsAlthough the other planets of the solar system were known to ancient civilisations, Uranus and Neptune were discovered only relatively recently. This is because they can be seen only by using a telescope. Neither of these planets appear in any of the textbooks printed before Captain Cook landed at Botany Bay in 1770. These books show only six planets—the ancient planets.

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SaturnSaturn is the second-largest planet in the solar system. It was named after the Roman god of agriculture and is most easily recognised by its impressive ring system. This system was discovered by Galileo in 1610. The space probe Voyager 2 detected over 100 000 rings when it flew by Saturn in 1981. The rings are only tens of metres thick, spread out to a diameter of 270 000 kilometres and are thought to be composed of particles of ice and ice-covered rock, ranging from tiny particles to large rocks. Like Jupiter, Saturn is a world of gas—a planet so light that it would float on water.

UranusThe English astronomer William Herschel accidentally discovered Uranus in 1781. Uranus was named after the Greek god of the heavens. Uranus is a strange planet because its axis is tilted at an angle of 98°—an angle that makes it virtually lie on its side as it orbits the Sun.

The solar system

Fig 8.4.12 Uranus, showing the vertical rings and moons (white spots) orbiting the planet.

Saturn


MoonsAt least 30 moons and rings in seven bands

Diameter 120 536 km (= 9.45 × Earth’s diameter)

Surface Liquid hydrogen. Winds up to 1800 km/h

Atmosphere Very thick layer of hydrogen and helium


Surface temperature –180°C

Period of rotation (day) 10 h 39 min

Tilt of axis 26.7°


Time to orbit Sun (year) 29.5 Earth years


Diameter 25 mm



Fig 8.4.11 Saturn showing the cloudy atmosphere and the separation between the two bright rings (called the Cassini Division).

Worksheet 8.1 Saturn’s rings

Zombie bumsAuthor Andy Griffiths’ description of Uranus in his novel Zombie Bums from Uranus is partly accurate. It does have an explosive methane atmosphere (with added hydrogen for extra bang!), but its surface is definitely not sticky and poo-like!


Planet GeorgeWhen William Herschel discovered Uranus in 1781, he first thought it was a comet. When he then found it was a planet, he wanted to name it George, after the British King George III.


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Uranus


Moons At least 21 moons and 11 rings

Diameter51 200 km (= 4.01 × Earth’s diameter)

SurfaceLikely to be frozen hydrogen and helium

AtmosphereHydrogen, helium and very turbulent, with winds of over 600 km/h




Tilt of axis 98°


Time to orbit Sun (year) 84 Earth years


Diameter 10.1 mm



The Great Dark Spot is a huge cyclonic storm with winds up to 2400 km/h.

Extrasolar planetsSince July 2003, about 117 planets have been located revolving around other stars outside our solar system. Several of these are in what are called ‘habitable zones’ around their parent stars, where the temperature would permit water to remain in a liquid state. Perhaps life exists on one of these planets.


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This tilt gives Uranus the strangest seasons of all the planets, with each season lasting 21 years! Like Saturn, Uranus has a large number of moons, and a ring system that is quite faint in comparison. Voyager 2 discovered additional moons and rings when it flew by in 1986.

NeptuneNeptune was identified by German astronomer Johann Galle in 1846 after it was noticed that Uranus had strayed from its orbit. The cause was the gravitational attraction of ‘nearby’ Neptune. Neptune is sometimes referred to as the twin of Uranus and is named after the Roman god of the sea.

Voyager 2 flew past Neptune in 1989, examining its rings. These are the least known and understood of all the ring systems.

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Fig 8.4.13 Neptune, showing its blue-green atmosphere. The Great Dark Spot seen at the centre is about 13 000 kilometres by 6600 kilometres in size.

Neptune


Moons Eight moons and five rings


Surface Frozen hydrogen and helium

AtmosphereMainly hydrogen, helium. Very high winds of over 600 km/h




Tilt of axis 29.3°


Time to orbit Sun (year) 165 Earth years


Diameter 9.7 mm



Planet XFor several years, many scientists believed there was an undiscovered planet, ‘Planet X’, beyond Pluto that was responsible for unexplained deviations in the orbits of Uranus and Neptune. When Voyager 2 provided more accurate information about the masses of Uranus and Neptune, it was discovered that both planets were heavier than first thought. New calculations using these more accurate masses were then able to explain the deviations in orbit, ending any support for the existence of Planet X.

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The solar system

Remembering 1 List the planets in order from:

a largest to smallestb closest to most distant from the Sunc lowest to highest gravity.

2 Name the planets known as the gas giants. 3 Name three spacecraft, and at least one planet visited by

each.

Understanding 4 a Name the body in the solar system that lost its

status as a planet in 2006.b Explain why it is now regarded as a dwarf planet.c Name two other dwarf planets.

5 Clarify what is meant by a terrestrial planet. 6 Explain why there is less information available about the outer

planets than the inner ones.

Applying 7 Identify the following planets by unscrambling their names:

a PETENUNb SUNEVc ARMSd RATUNSe ITUPREJf SUNRAUg TEHARh RECYRUM

8 Identify which planets could be called:a ancient planetsb modern planets.

9 Identify which planet:a is the hottestb is the coldestc has a giant hurricane raging that is larger than the Earth

8.4 QUESTIONS

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The solar system Earth Sun CClip l i p SScience en FFile a ... · The Shoemaker-Levy 9 comet that struck Jupiter in 1994 was actually a series of 21 comets! The explosions caused