Geology Reference
In-Depth Information
until eventually all the matter had been swept into a planet, which was left orbiting the
trajectory where each band of dust had once been. All the planets in our solar system
came together in the same way, but the first four, Mercury, Venus, Earth and Mars are
rocky; the next four (Jupiter, Saturn, Uranus and Neptune) are gaseous. Pluto, the last,
is also mostly rock.
It was fortunate that the nebular cloud that eventually formed the Earth was poor in
carbon and water, for too much carbon would have meant so much carbon dioxide in the
atmosphere that the surface temperature of the Earth would have been very high right
from the outset. The high temperatures would have evaporated all of the abundant wa-
ter into the atmosphere, driving the temperature still higher because water vapour is a
powerful greenhouse gas in its own right. With all its water in the atmosphere, Earth
would have faced the inevitability of total desiccation. High-energy sunlight at the top
of the atmosphere would sunder the bonds between hydrogen and oxygen like so many
swords cutting through butter, and the newly liberated hydrogen atoms would have then
eagerly shot off into space because hydrogen (a being with basically just one proton and
one electron) is so light that Earth's gravity cannot prevent its escape into the surround-
ing void. The remaining oxygen, bereft of hydrogen, would have satisfied its hunger for
electrons by reacting with sulphur gases from volcanoes to give sulphuric acid. With no
hydrogen at all and no free oxygen, the water could never have been reconstituted. This
was the fate suffered by Venus, our nearest sunward neighbour, where the surface tem-
perature is high enough to melt lead and the acidity is sufficient to corrode even the most
resistant of materials. But things on our planet were different: its initially low levels of
carbon and water were supplemented to just the right extent by impacts with meteorites
and comets from the outer reaches of the solar system where these elements were much
more abundant.
Our infant planet was special in many other ways. Its orbit was just the right distance
from the sun to allow liquid water to remain on its surface, and its mass provided just the
right amount of gravitational attraction for holding the atmosphere and ocean in a pro-
tective embrace around the Earth. The sun itself provided a relatively steady output of
energy, without too much sterilising ultraviolet radiation. The configuration and masses
of the other planets in the solar system were also well tuned, so that mutual gravitation-
al influences on each other and on the Earth produced the enduring emergent property
of stability in the Earth's orbit. Had any of the masses of the planets been even slightly
different, our own planet's orbit may well have been chaotic, making the evolution of a
Gaia with complex multicellular life impossible. The moon was created when a Mars-
sized piece of interplanetary shrapnel struck the Earth some 4,500 million years ago.
The impact released so much energy that both planets melted to their cores, sending a
great mass of molten debris into space which later condensed into our nearest planet-
