<< The surface of Mars today gets more UV than earth.
Back before earth got its ozone layer, the surface
of Mars still got more UV than does the average
surface location on earth - if you remember that
those locations are mostly under water.

For the most part I agree with this post.
But the above is a bit of a catch 22.
IF Mars had no water, it may have had more UV than Earth, but how can life
start with no water.

IF it indeed had water, then its UV would be less than Earth with water too.

By the way, what is your opinion on dimers as being a part of selection
pressure?



  And even
if you limit yourself to dry land, the UV photons
on Mars are just as strong as on earth - only they
are about 2.5 times less abundant.  That is still
a lot of selection against pyrimidines.

 >>
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