Introduction
Seen from space, the Earth would strike even a casual observer as unique.
Though it is similar in size to the other inner planets
(Mercury, Venus, and
Mars), and, like those planets, composed mainly of
silicates, the Earth is set apart at first glance by its liquid water
oceans, which cover almost 75% of the planet at average depths of a few
kilometers. The Earth's atmosphere, comprised largely of nitrogen
(N2; 78%) and oxygen (O2; 21%) also set it apart. Mercury has
no real atmosphere at all, and those of Mars and Venus are predominantly
carbon dioxide. The presence of oxygen in the Earth's atmosphere is the
product of the biological process of plant photosynthesis. Thus, the
very makeup of Earth's atmosphere gives clues that the planet harbors life
forms.
Another characteristic unique to Earth, which is less obvious and has only
become clear to scientists in the last few years, is that the Earth is the
only planet in our solar system that has an active plate tectonics.
Whereas the surfaces of the other rocky inner planets remain fundamentally
the same through time, the Earth largely resurfaces itself every few
hundred million years by slowly creating new crust while melting older
crust in very particular regions of the planet. Not coincidentally, these
regions host the highest concentrations of volcanic and seismic
activity.
The factors underlying the Earth's unique mechanism of plate tectonics are
still elusive. Scientists have thus far been unable to explain why the
Earth has such an active geological system while a planet such as Venus,
which is comparable in size and composition, has none. In this respect, as
well as in many others concerning its biosphere, the Earth is probably
the most mysterious body in the solar system.
