Earth's Fragile Shield Myth

Lovelock talks of the Earth's Fragile Shield Myth in
Ages of Gaia. Specifically that the fear of UV is out of
all reasonable proportion in origin scenarios.
It is perhaps too long, but the point is important.
And I think that instead of trying to avoid UV in origin scenarios, the
tide is turning and we see it as a major factor in setting up the origin.
Also note that stromatolites, some of our earliest life colonies, rose
out of the water!

"The false image, that lingers like a mirages, is the shibboleth,
"Earth's fragile shield." In a way the atmospheric scientists
L.V. Berkner and L.C. Marshall started it. Some thirty years ago,
they introduced their famous theory on the evolution of
atmospheric oxygen. Crucial to this was the assumption that
there was a flux of lethal untraviolet radiation before oxygen
was present in the air, and that this prevented life from
colonizing the land surfaces. Indeed, it was further held that
life before oxygen must have been obliged to exist deep in the
sea at levels where the UV could not penetrate. It was only after
oxygen appeared in the air that ozone could form and act as a
shield to prevent the UV from reaching the surface. Once this
happened, the way was open for an abundance of life to colonize
the land and for the growth of oxygen concentration by increased
photosynthesis to its present level of 21% Some details of
their theory we now suspect are wrong...

The scientific establishment accepted their ideas enthusiastically.
Among those ideas was the minor postulate that the presence
of a stratospheric ozone layer is an essential requirement
for surface life. Almost every scientist now accepts it as
if it were a proven fact of science...

The intensity of UV in the absence of ozone would have
been 30 times higher than it now incident upon the Earth's
surface. Such an irradiation, it is said, would have sterilized
the land surfaces. The more committed believers in the
potency of UV hold that 10 to 30 meters of ocean water
are needed to filter out the deadly radiation. Life, they say,
could not have existed in shallower depths of the sea,
let alone on the surface.

Much more probably, "Earth's fragile shield" is a myth...

The memory of these experiments left me disinclined
to accept that the much weaker irradiation of the land
surfaces in the Archean by natural UV could have
prevented their colonization.... It is also wrong to assume
that ozone alone among atmospheric gases can filter out UV light....

But even if the full unfiltered solar UV shone on the
surface, it still would not have much hindered life...

This belief that UV radiation is unconditionally lethal
to life on Earth has sustained a distored view of the
Archean and of other periods in the evolution of Gaia.
And it is a view still deeply entrenched in scientific thinking

I could not help wondering how they could think that
there was life on the intensely irradiated surface of
Mars and at the same time believe that the land beneath
the thick and murky Archean atmosphere of Earth was
sterile. How could they fit into their minds two such
contrary ideas."

Tom Hendricks

.

Relevant Pages