Geoscience Reference
In-Depth Information
have broken that down quickly to give an atmosphere of carbon dioxide and nitrogen. No
one yet knows for certain how life began. There are even claims that it may have had an
extra-terrestrial origin, arriving on Earth in meteorites from Mars or beyond. But laborat-
ory studies are beginning to show how some chemical systems can begin to self-organize
and catalyse their own reproduction. Analysis of present-day life forms suggests that the
most primitive are not the sort of bacteria that scavenge organic carbon or that use sunlight
to help them photosynthesize but those that use chemical energy of the sort that is found
today in deep-sea hydrothermal vents.
By 3,500 million years ago, there were almost certainly microscopic cyanobacteria and ar-
guably primitive algae - the sort of thing we see today in pond scum. These began to have a
dramatic effect. Using sunlight to power photosynthesis, they took in carbon dioxide from
the atmosphere, effectively eating the blanket that, by the greenhouse effect, kept the Earth
warm when the power of the Sun was weak. This may be what led ultimately to the late
Pre-Cambrian glaciation. But long before that, it resulted in the worst pollution incident the
world has known. Photosynthesis released a gas that had not existed on Earth before and
which was probably toxic to many life forms: oxygen. At first, it did not last long in the
atmosphere but quickly reacted with dissolved iron in sea water, resulting in thick deposits
of banded iron oxide. Almost literally, the world went rusty. But photosynthesis continued,
and free oxygen began to build up in the atmosphere from about 2,400 million years ago,
paving the way for animal life that could breathe the oxygen and eat the plants.
Birth of Earth
About 4,500 million years ago there was a great cloud of gas and dust, the product of sev-
eral previous generations of stars. It began to contract under gravity, perhaps boosted by
the shock waves from a nearby exploding star or supernova. A slight rotation in the cloud
accelerated as it contracted and spread the dust out into a flattened disc around the proto-
star. Eventually, the central mass, mostly of hydrogen and helium, contracted sufficiently
to trigger nuclear fusion reactions at its core and the Sun began to shine. A wind of charged
particles began to blow outwards, clearing some of the surrounding dust. In the inner part
of the nebula, or disc, only refectory silicates remained. Further out, the hydrogen and heli-
um accumulated to form the giant gas planets Saturn and Jupiter. Volatile ices such as wa-
ter, methane, and nitrogen were driven still further out and formed the outer planets, Kuiper
belt objects, and comets.
The inner planets - Mercury, Venus, the Earth, and Mars - formed by a process known as
accretion which began as particles bumped into one another, sometimes splitting, occasion-
ally joining together. Eventually, the larger lumps developed sufficient gravitational attrac-
tion to pull others to them. As the mass increased, so did the energy of the impacts, melting
