Past climate change - Climate Change: Biological and Human Aspects - page 64

those of Venus (Genda and Abe, 2005). However, the notion has not become firmly
established with any definitive link as to biosphere formation.
As for what was effectively the 'climate' immediately following the Thea-Earth
collision, a proportion of the Earth (and presumably Thea) was vaporised and the
Earth would have been surrounded by a tenuous photosphere with a temperature of
around 2300 K. Subsequent cooling to allow liquid rock to form on the surface took
the order of 1000 years and another 10 million years or so were needed for a thin, solid
crust to form. After that there would have been some tectonic cycling of rocks, which
would have drawn down some of the great excess of carbon dioxide thought to exist
at that time. This too would have taken millions of years, possibly a hundred million.
Following the creation of the Moon and cooling on Earth, the oceans formed
from water arising from the proto-Earth's rocks and from the comets, asteroids
and planetesimals still bombarding it. At the time the Earth's atmosphere was very
different from today, being largely oxygen-free and dominated not by nitrogen but
by carbon dioxide and possibly methane. There is also some debate as to whether
there was a significant amount of hydrogen in the very early mix. Either way, the
early Earth's atmosphere must have conferred a powerful greenhouse effect. This was
needed at that time as the early Sun (as a typical main-sequence star) was thought to
be only 30% as bright as it is today. Studies of other main-sequence stars have made it
clear that young main-sequence stars do not generate as much energy as when they are
older. Given, from the previous chapter, that there is currently a natural greenhouse
effect, the early Earth under a fainter Sun would have needed a far more powerful
natural greenhouse effect if liquid water (which would greatly facilitate life) was
common on the planet: this is known as the faint young Sun paradox. So, as we shall
see, greenhouse gases were vitally important to ensure that conditions on the Earth's
surface were warm enough for life. Furthermore, as the Earth's atmosphere evolved
so a different mix of greenhouse gases were required to maintain temperatures. Life
was fundamental to this process.
3.1.2 TheearlybioticEarth(3.8-2.3bya)
Life arose fairly quickly in planetary geological terms, if suggestive isotopic evidence
(possible microfossils containing a high proportion of 12 C) is to be believed. It is
thought to have begun somewhere between 3.5 and 4.2 bya, which is about when the
heavy bombardment of the Earth ceased (somewhere around, or shortly after, 4 bya).
However, both the Earth and the Moon were still subject to hits by large planetesimals,
as the 1000 km-wide impact basins visible on the Moon testify, although the frequency
of such hits gradually waned.
There is reasonable evidence that water existed on the Earth at this time and there is
a growing body of literature on the pre-biotic chemistry that led to the building blocks
for life. Work by Oleg Abramov and Stephen Mojzsis in 2009 indicated that early
microbial life, had it arisen before the late heavy bombardment, could have survived
this period. This period would have seen the Earth's crust fractured, possibly allowing
more hydrothermals, but irrespectively the energy of such impacts would have added
heat to the environment (albeit locally/regionally). At around the same time, in
2008, work by Eric Gaucher, Sridhar Govindarajan and Omjoy Ganesh elucidated