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Earth - A sanctuary for life
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When humans gaze out across the universe we see galaxies 
colliding, supernova explosions, and a huge diversity in values 
such temperature, distance, and time. Events happen on such a 
grand scale that it is hard not to feel a little insignificant 
in comparison. In reality, one of the most remarkable features 
of our Solar System is the near-constant stability of factors 
such as temperature, which have nurtured the development of life 
on Earth. A difference of few degrees was sufficient to turn 
Venus into a suffocating inferno, and yet Earth has retained the 
delicate balance required for millions of years.

While no single factor is responsible for the regulation, plants 
have an important role to play in the recycling of oxygen. If no 
plants existed on Earth the humans and animals would use up all 
the oxygen in less than 300 years. The future of the Earth's 
ecosystem is by no means assured. The use of phrases such as the 
greenhouse effect and global warming are now commonplace; in a 
geological blink of an eye we threaten to destroy the very 
balance that we depend on for life.

Water is a very unusual material, with lots of perculiar 
properties, and many scientists have now concluded that it is 
an essential component for the development of life. The search 
for life in our solar system has now turned into the search for 
water. Where ever water existed in a liquid form, life may have 
developed and thrived. The most likely candidate at present is 
Europa, one of Jupiter's icey moons. It is thought that 
geological activity could result in a warm water ocean beneath 
its frozen surface. The rarity of liquid water is another 
indication of how lucky we are here on Earth. Our tiny planet 
contains so much water that if the Earth was a perfectly smooth 
sphere it would be covered by a single ocean 2.8 kilometres 
deep.

One of our planets less obvious features is its magnetic field. 
Charged particles from the Sun, called the Solar Wind, pass the 
Earth at speeds of around a million miles per hour. As they slam 
into the magnetic field, a protective cocoon is created that 
surrounds the Earth. When the Solar Wind is particularly strong, 
some of the particles slip through and collide with the Earth's 
atmosphere, creating the spectacular aurora around the north and 
south poles.

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Images
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1.  The planet Earth from space
(http://www.solarsystem.f2s.com/earth/img1a.jpg)

The crew of Apollo 17 took this photograph of Earth in December 
1972 while the spacecraft was traveling between the Earth and 
the Moon.

Public domain image courtesy of NASA.  

2. Side on view of the Earth's atmosphere
(http://www.solarsystem.f2s.com/earth/img2a.jpg)

This false-colored image was acquired during the Clementine mission. 
It shows airglow of the upper atmosphere as a thin blue line. The 
bright spot toward the right of the image is an urban area. 

Public domain image courtesy of NASA. 

4.  The spectacular aurora in northern Scotland
(http://www.solarsystem.f2s.com/earth/img4a.jpg)

The spectacular aurora can be seen close to the north and south 
poles of the Earth during times of strong solar activity. 
High-energy particles collide with the molecules in the 
atmosphere, causing them to glow like a neon light.