Scientific Background
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  For many people, it goes without saying that our sun has an enormous impact on Earth.  The sun is the major source of energy for phenomena on the earth's surface, such as growth of plants, winds, ocean currents, and the water cycle. Sun drives Earth's ocean currents and weather patterns.

However, the amount of the Sun's energy available to and retained by Earth depends on several factors. Interestingly, distance from the Sun isn't the only, or even the most important, variable.  For example, Mercury, the closest planet to the Sun, has an average temperature around 600 F.  However Venus, more than twice the distance away from the Sun as Mercury, has an average temperature of around 900 F.  Clearly, a planet's distance from the Sun may not be the most important influence on a planet's temperature. 

Earth and Moon as seen by Galileo Spacecraft A planet's temperature may be much more dependent on the thickness and characteristics of its atmosphere.  Although Earth and Moon are essentially the same distance from the Sun, Earth's temperature varies from -80 F to +140 F, whereas the moon's temperature varies between -200 F and +200 F.  The primary differences are the rate of rotation and the presence of an atmosphere.  The point here is that the variables affecting a planet's temperature, in order of importance, are 

    (1) atmosphere greenhouse strength; 
    (2) atmosphere albedo; and 
    (3) distance from Sun.
Earth's changing polar ice caps.A planet's atmosphere acts like a blanket.  First, it shields living things from the harmful rays of the Sun.  In fact, the atmosphere reflects most of the energy incident on Earth; this reflectivity is called a planet's albedo.  The second thing a planet's atmosphere does is hold in heat.  Without the atmosphere, Earth's night time temperature would plunge to - 100 F every night.  Conversely, an atmosphere can work too well.  Venus has a very thick atmosphere that keeps its temperature around + 900 F all day and all night.  The effectiveness of the atmosphere is called the Greenhouse Strength (a number that varies from 0 to 1000); it gets its name from the Greenhouse Effect which allows people to keep plants growing in the winter by placing them in glass buildings - the same effect that makes the inside of a car heat up enormously in the summer when parked outside. 

It is important to understand the variables that impact the living conditions on a planet.  As astronomers begin to discover planets around other stars (current listings), there are only certain conditions and temperature ranges suitable for water to exist in liquid form--and subsequently, be hospitable for extra-terrestrial life. 

Mars image courtesy of NASA HSTOn-line Resources