<p><hr><p>
<table cell padding=10 border=10 bgcolor=purple><tr><td>
<img src="http://zebu.uoregon.edu/~imamura/122/images/faint_young_sun.jpg">
</td><td><h3><font color=gold><center>Faint Young Sun Paradox</center><p>
The luminosity of the Sun 
has increased as it has aged; 3.8 billion years ago the Sun
was ~25 % fainter than today. 
This is a conundrum because there was liquid water on the
Earth at least 3.7 billion years ago and a 
simple argument leads to a prediction 
for what is referred to as the <i>Equilibrium Temperature</i>,
T<sub>e</sub> for the Earth which at that time, would be below the 
freezing point of water, T<sub>e</sub> = -40 C!. Note that
T<sub>e</sub> is determined by simply finding the temperature for the Earth
where it radiates exactly the same amount of energy per second as it receives
from the Sun in the absence of clouds and an atmosphere. Further, if we were 
to include an atmosphere with the composition of our current atmosphere, the
temperature would rise but would still be less than the freezing point of 
water.
</td></td></table>
<p>
The answer to the question of then, why do we have liquid oceans? 
requires that our atmosphere in the past had a much 
different chemical composition than today
so that the Greenhouse Effect could maintain liquid oceans or, perhaps, the Sun 
was much brighter in the past than we now believe.
<p><hr><p>