P1: The Earth in the Universe

The solar system was formed over a very long period of time, about 5000 million years ago.

• The solar system started as clouds of dust and gas, which were pulled together by the force of gravity.
• This created intense heat.Eventually,nuclear fusion began to take place and the Sun appeared
• The remaining dust and gas formed smaller masses, which were attracted to the Sun.

The smaller masses in our solar system are:

• planets - eight large masses that orbit the Sun
• moons - small masses that orbit the planets
• asteriods - small, rocky masses that orbit the Sun
• comets - small, icy masses that orbit the Sun
• dwarf planets - small spherical objects that have not cleared their orbit of other objects.

Planets, moons and asteroids all move in elliptical orbits. Comets move in highly elliptical orbits. It takes Earth one year to make a complete orbit around the Sun.

• The Sun's energy comes from nuclear fusion.
• Hydrogen nuclei fuse together to produce a nucleus with a larger mass, i.e. a new chemical element.
• During fusion, some of the energy trapped inside the hydrogen nuclei is released.
• All the chemical elements with a larger mass than helium were formed by nuclear fusion in earlier stars.

• At 5000 million years old, the Sun is only 500 million years older than the Earth.
• The Universe is much older than this; approx 14000 million years old.
• The Sun is one of thousands of millions of stars in the Milky Way.

• Earth has 12800km diameter.
• Our star (the sun) is 100 times wider than Earth.
• The solar system is approx 10 billion km across.
• The Milky Way is 100000 light years across, containing at least 100 billion stars.
• The Universe contains billions of galaxies, with vast distances between them.

• Light travels at very high but finite (limited) speeds. This means that if the distance to an object is great enough, the time taken for light to get there can be measured.
• The speed of light is 300000 km/s in a vacuum (around one million times faster than sound), So, light from Earth takes just over one second to reach the Moon (approx 384400 km away).
• Light from the Sun takes eight minutes to reach the Earth. This means that when we look at the Sun, we are actually seeing what it looked like eight minutes ago.
• Vast distances in space are measured in light-years. One light-year is the distance light travels in one year (approx 9500 billion km). The nearest galaxy to the Milky Way is 2.2 million light-years away. This means that light from this galaxy has taken 2.2 million years to reach the Earth and so we are seeing the galaxy as it was in the past.

Astronomers work out the distances to different stars using two different methods:

• Relative brightness - In general, the dimmer the star is, the further away it is. However, stars can vary in brightness so we can never be 100% certain.
• Parallax - As the Earth orbits the Sun, stars in the near distance appear to move against the background of very distant stars. The closer they ar, the more they appear to move.
• The position of a star is measured at six-monthly intervals. These measurements can be used to calculate its distance from Earth. However, the further away the star is, the more difficult and less accurate the measurement is.

• Because the stars are so far away, everything we know about them is worked out from the radiation they produce: visible light and other types of radiation, including ultraviolet and infrared.
• All our electric lights on Earth illuminate the night sky, so it is very difficult to see the stars sometimes. This is called light pollution. In 1990 the Hubble Space Telescope was launched. It orbits the Earth at a height of 600km so it is not affected by light pollution or atmospheric conditions.

• If a source of light is moving away from us, the wavelengths of the light are longer than they would be if the source was stationary.
• The wavelengths of light from almost all galaxies are longer than scientists would expect. This means the galaxies are moving away from us.
• In 1929 Edwin Hubble discovered that light from distant galaxies had even longer wavelengths. Therefore, they must be moving away from us faster. As a result, he developed Hubble's Law.
• The speed at which galaxies are moving away from us is proportional to their distance from us (e.g. the faster a galaxy is moving, the further away it is).
• If all the galaxies are moving away from one another, this must mean that space is expanding (getting bigger).

If a wave source is moving away from or towards an observer, there will be a change in the observed wavelength and frequency.

If a source of light moves away from an observer, the wavelengths of the light in its spectrum are longer than if it was not moving. This is known as redshift because the wavelengths 'shift' towards the red end of the spectrum.

All distant galaxies appear to be 'redshifted', which means they are moving away from us.

When scientists trace the paths of galaxies, they all appear to be moving away from the same point.

There have been many theories about how the Universe began. The one that best explains this evidence is the Big Bang theory, which says that the Universe started with a huge explosion 14000 million years ago.

It is difficult to predict the fate of the Universe because it is very hard to measure the very large distances involved. It is also very difficult to study the motion of very distant objects.

The future depends on the amount of mass in the Universe. If there is not enough mass, the Universe will keep expanding. If there is too much mass, gravity will be strong enough to pull everything back together and the Universe will collapse with a big crunch. Measuring the amount of mass in the Universe is very difficult, so its ultimate fate is hard to predict.

Galaxies are further apart now than they were in the past - the Universe is expanding.

People once thought that the Earth was only 6000 years old. There was no way of testing this theory, so people believed it for a long time. We now know that rocks provide evidence of how the Earth has changed and clues as to its age.

Erosion - the Earth's surface is made up of layers of rock, one on top of the other, with the oldest at the bottom. The layers are made of compacted sediment, which is produced by weathering and erosion. Erosion changes the surface of the planet over long periods of time.

Craters - the surface of the Moon is covered with impact craters from collisions with meteors. However, the Earth, which is much larger, has had fewer meteor collisions (due to Earth's atmosphere), but craters have also been erased by erosion.

Mountain formation - if new mountains were not being formed, the whole Earth would have been worn down to sea level by erosion.

Fossils - plants and animals trapped in layers of sedimentary rock have formed fossils, providing evidence of how life on Earth has changed over millions of years.

Folding - some rocks look as if they have been folded like plasticine. This would require a big force to be applied over a long period of time - further evidence that the Earth is very old.

Radioactive dating - all rocks are radioactive, but the amount of radiation they emit decreases over time. Radioactive dating measures radiation levels to find out how old they are.

Scientists estimate that the Earth is around 4500 million years old - it has to be older than its oldest rocks - and when it was first formed it was completely molten (hot liquid) and would have taken a very long time to cool down.

The oldest rocks that have been found on Earth are about 4000 million years old.

Thin rocky crust:

• Its thickness varies between 10km and 100km.
• Oceanic crust lies beneath the oceans.
• Continental crust forms continents.

The mantle:

• Extends almost halfway to the centre.
• Has a higher density, and a different composition, than rock in the crust.
• Very hot, but under pressure.

The core:

• Made of nickel and iron.
• Over half of the Earth's radius; has a liquid outer part and a solid inner part,
• The decay of radioactive elements inside the Earth releases energy, which keeps the interior of the Earth hot.

Alfred Wegener was a meteorologist who put forward a theory called continental drift. He saw that the continents all fitted together like a jigsaw, with the mountain ranges and sedimentary rock patterns matching up perfectly. There were also fossils of the same land animals on different continents. Wegener proposed that the different continents were once joined together, but had become separated and drifted apart. Wegener also claimed that when two continents collided, they forced each other upwards to make mountains. Geologists at the time did not accept Wegener's theory because:

• he was not a geologist and was considered an outsider.
• it was a big idea but he wasn't able to provide much evidence.
• the evidence could be explained more simply by a land bridge connecting the continents that has now sunk or been eroded.
• the movement of the continents was not detectable.

Wegener's evidence for continental drift could be summarised as:

• similar patterns of rocks, which contain fossils of the same plants and animals.
• closely matching coastlines.

We now know that the Earth's crust is cracked into several large pieces (tectonic plates). The plates float on the Earth's mantle because they are less dense. They can move apart, move towards each other or slide past each other. The lines where the plates meet are called plate boundaries. These are where volcanoes, earthquakes and mountain building usually occur. Earthquakes that occur near coastlines or at sea can often result in a tsunami. The movement of the tectonic plates can happen suddenly due to a build up in pressure and can sometimes have disastrous consequences, e.g. tsunamis and earthquakes. Tectonic plates can move in three ways:

• Slide past each other - When plates slide, huge stresses and strains build up in the crust which eventually have to be released in order for movement to occur. This 'release' of energy results in an earthquake. A classic example of this is in California.
• Move away from each other - Constructive Plate Boundaries - When plates move away from each other at an oceanic ridge, fractures occur. Molten rock rises to the surface, where it solidifies to form new ocean floor (seafloor spreading). New rock being formed.
• Move towards each other - Destructive Plate Boundaries - As plates are moving away from each other in some places, it follows that they must be moving towards each other in other places. When plates collide, one is forced under the other. Earthquakes and volcanoes.

Just below the Earth's crust the mantle is fairly solid. Further down it is liquid and able to move.

Convection currents in the mantle cause magma (molten rock) to rise to the surface. The force is strong enough to move the solid part of the mantle and the tectonic plates. When the magma reaches the surface, it hardens to form new areas of oceanic crust (seafloor), pushing the existing floor outwards.

New oceanic crust is continuously forming at the crest of an oceanic ridge and old rock is gradually pushed further outwards. The Earth has a magnetic field which changes polarity every million years or so. Combined with the spreading of the seafloor, this produces stripes of rock of alternating polarity. Geologists can work out how quickly new crust is forming frm the widths of the stripes. This occurs at constructive plate boundaries, where the plates are moving apart.
When an oceanic plate and a continental plate collide, the denser oceanic plate is forced under the continental plate. This is called subduction. The oceanic plate then melts and the molten rock can rise upwards to form volcanoes. This occurs at destructive plate boundaries. Mountain ranges form along plate boundaries as sedimentary rock is forced upwards by the pressure created in a collision.
Earthquakes occur most frequently at plate boundaries when plates slide past each other or collide. Pressure builds up over many years due to force of plates pushing against each other. The stored energy's released in a sudden upheaval of crust and spreads outwards in waves.
Plate movement plays a crucial role in the rock cycle:

• Old rock is destroyed through subduction.
• Igneous rock is formed when magma reaches the surface.
• Plate collisions produce high temperatures/pressures causing rock to fold.

Evidence for the layered strucure of the Earth has been gained through the study of earthquakes. These are due to the fracture of large masses of rock inside the Earth. The energy that is released travels through the Earth as a series of shock waves-seismic waves-directed using seismographs.

There are two types of shock waves: P-waves and S-waves. Differences in the speed of P- and S- waves can be used to give evidence for the structure of the Earth:

• P-waves - Longitudinal waves where the ground is made to vibrate in the same direction as the shock wave is travelling.
• Pass through solids and liquids.
• Faster than S- waves.
• Speed increases in denser material.
• S-waves - Transverse waves where the ground is made to vibrate at right angles to the direction the shock wave is travelling.
• Pass through solids only.
• Slower than P- waves.
• Speed increases in denser material.

Waves are regular patterns of disturbance that transfer energy in the direction the wave travels without transferring matter. There are two types of wave - longitudinal and transverse.

All waves transfer energy from one point to another without transferring particles of matter. If we consider that each coil of the slinky spring.

The distance travelled by a wave can be worked out using the formula:

distance (metres, m) = wave speed (metres per second, m/s) X time (seconds, s)

All waves have several important features:

• Amplitude - the maximum disturbance caused by a wave. It is measured by the distance from a crest or trough of the wave to the undisturbed position.
• Wavelength - the distance between corresponding points on two adjacent disturbances.
• Frequency - the number of waves produced, in one second. Frequency is measured in hertz (Hz).

Each particle moves backwards and forwards in the same plane as the direction of wave movement. Each particle simple vibrates to and fro about its normal position.

Sound travels as longitudinal waves.

Each particle moves up and down at right angles to the direction of wave movement. Each particle simply vibrates up and down about its normal position. Light and water ripples travel as transverse.

If a wave travels at a constant speed, then:

• increasing its frequency will decrease its wavelength
• decreasing its frequency will increase its wavelength.

If a wave has a constant frequency, then:

• decreasing its wave speed will decrease its wavelength
• increasing its wave speed will increase its wavelength.

Wave speed, frequency and wavelength are related by the following formula:

wave speed (metres per second, m/s) = frequency (hertz, Hz) X wavelength (metres, m)