The Earth

3.5.3 The Earth

Mass

24 kg

Radius (mean)

6371 km

Escape velocity

11.18 km s

Day length

1 day

Rotation period 23 h 56 min 04 s (sidereal day) Axial tilt

Semi-major axis of orbit

1 AU Average Orbital Speed

149 597 887.5 km

29.78 km s

Orbital inclination

0° (Our orbit provides the reference plane.)

The Earth–Moon system was formed when an object, thought to have had a mass of about 10% that of the Earth, impacted with it soon after the formation of the Solar System (Figure 3.8). A portion of the combined mass was thrown off into space and formed the Moon, initially far closer to the Earth than now. As the

Our Solar System 2 – The Planets

Figure 3.8 Earthrise as seen by the Apollo 8 crew as they rounded the Moon’s far side in December 1968. Image: NASA Apollo 8 Crew.

Moon’s relative mass and size is comparable with that of the Earth the two are sometimes thought of as a ‘double planet’.

The name Earth derives from the Anglo-Saxon word ‘erda’ meaning ground or soil. This became ‘eorthe’ in Old English and then ‘erthe’ in Middle English. Finally, from around 1400, the name Earth was used – the only planetary name not derived from Greek and Roman mythology.

Out-gassing from volcanoes produced its secondary atmosphere and water vapour condensed to form the oceans, with additional water provided by the impact of comets. About 4 billion years ago a self-replicating molecular system – life – arose. Photosynthesis allowed the trapping of solar energy and the by-product, oxygen, accumulated in the atmosphere. A resulting layer of ozone reduced the fl ux of ultraviolet radiation on the surface which allowed life forms to survive on land.

The surface of the Earth has been shaped by plate tectonics – the movement of sections of the crust across the underlying magma – which at times formed vast continents such as Pangaea which allowed species to colonize much of the surface.

In the Cambrian era, which followed a period of extreme cold when much of the Earth was covered in ice, multicellular life forms began to fl ourish. In the ∼535 million years since then, there have been fi ve ‘mass extinctions’ when many species died out. The last of these was 65 million years ago when a large asteroid or comet of at least 10 km in diameter formed the Chicxulub crater on (and offshore of) the Yucatan Peninsular in Mexico. The dust produced would have reduced the amount of sunlight reaching the ground and hence the growth of vegetation. This may have been one of the causes of the demise of the dinosaurs at about this time along with ∼70% of all species then living on the Earth. Shrew-like small mammals were spared and their evolution fi nally gave rise to human beings.

94 Introduction to Astronomy and Cosmology

Our Earth is a rocky body with a core, mantle and crust. Its rotation causes a bulge at the equator whose diameter is 43 km greater than the polar diameter. As

a result, the peak of Mount Chimborazo in Equador lies furthest from the Earth’s centre. With an extreme variation from 8.8 km above (Mount Everest) to 10.9 km below (the Mariana Trench), the surface of the Earth is actually smoother than a billiard ball!

The central core has a temperature of ∼7000 K as a result of the decay of radio- active isotopes of potassium, uranium and thorium, all of which have half-lives of over 1 billion years. Convection currents within the molten rock brought this heat up towards the crust giving rise to thermal hotspots and produced the volcanic activity which gave the Earth its secondary atmosphere. The atmosphere is now largely composed of nitrogen (78%) and oxygen (21%) with the remaining 1% made up of water vapour, carbon dioxide, ozone, methane and other trace gases. It is worth pointing out again that without the warming due to the greenhouse gases, carbon dioxide, water vapour and methane, the average surface tempera-

Ancient peoples put the Earth at the centre of the universe and regarded the human race as special. As it was found that our Sun is one of billions of stars in our Galaxy, a principle of mediocrity arose. Planets like Earth were thought to have been very common and so advanced life like ours was thought to be widespread – we were not special. As we have learnt more about the history of our Earth, how plate tectonics have helped recycle carbon dioxide back into the

atmosphere, how Jupiter prevents too many comets from impacting the Earth and how our large moon has stabilized the Earth’s rotation axis, some scientists now believe that the conditions that have allowed intelligent life to arise here may well

be very uncommon. It really could be that we are special, and it is not impossible that we are the only advanced life form now within our Milky Way Galaxy.