Earth

 Though comprising only 0.2% of the total mass
of all the planets in the Solar System, Earth nonetheless stands
out as unique among the terrestrial planetary bodies in having
a substantial atmosphere. Earth's atmosphere is mostly nitrogen
and oxygen-its nearest neighbors' atmospheres are predominantly
carbon dioxide, with little or no nitrogen or oxygen. Though quite
thin (75 percent of our atmosphere's mass is within 11 km of the
surface), if not for this very thin film of gas, Earth's average
surface temperature would be 35 oC colder than it is, or -20 oC.
Therefore, without this atmosphere, the Earth's erosion cycle
and its surface would be totally different, and life would have
evolved along a very different path.

  Though our oceans cover 70 percent of the surface,
they are also very thin, and if spread evenly over the entire
planet, they would be only 1.5 km thick. Yet the extremely thin
(by Solar System standards) atmosphere and hydrosphere have made
Earth unique. Our oceans also hide the most significant geologic
features on the planet-the globe-circling mountain ranges that
are the center of constantly forming new crust. It is this ongoing
formation of new crust, and the destruction of old crust elsewhere-in
concert with the hydrologic cycle-that constantly change the face
of the Earth and redistribute and concentrate minerals and rocky
materials into minable accumulations that we use as resources.
Where tectonic plates converge, geologic forces have, over the
eons, built up continents that provided an exotic new environment
for ocean life to exploit. Continued alteration of the continents
by plate motions affect climates that in turn challenge life to
constantly adapt. To put in perspective how important the geologic
forces of Earth are, consider that if not for geologic activity
and the oceans, Earth might look like the impact-battered surface
of the Moon. Instead, it has high-standing continents, low ocean
basins, millions of life forms, and is brilliantly colored.