The Solar System and the Planet Earth 315
named oceans and a number of smaller seas such as the Mediterranean,
Black, North and Baltic Seas. There are also some saline land-locked-
enclosed seas, which do not connect to the world Ocean such as the
Caspian Sea, the Dead Sea and the Great Salt Lake.
Atmosphere
The Earth’s atmosphere consists primarily of nitrogen (78%), oxygen
(21%), Argon (1%), traces of other gases and up to 1% water depending
on the humidity. The atmosphere adheres to Earth because of gravity
becoming less and less dense with altitude such that 75% of the
atmosphere is within 11 kilometers of the Earth’s surface. The boundary
between the atmosphere and outer space is not well defined but
atmospheric effects on reentering space vehicles begins to be noticeable
at about an altitude of 120 km. The total mass of the atmosphere is
5 × 1018 kilograms.
One of the components of the atmosphere is the ionosphere, which
stretches from 50 to 1,000 km above the Earth and consists of ionized
molecules. It has practical importance because it reflects radio waves and
permits radio communication across the globe. The ionosphere also gives
rise to the northern and southern lights, i.e. the aurora borealis and the
aurora australis.
Another component of the atmosphere that is essential for the
protection of living organisms from the harmful ultraviolet (UV) rays of
the Sun is the ozone layer. Located 10 to 50 kilometers above the
Earth this high concentration of ozone (O 3 ) absorbs 95 ± 2% of the
Sun’s UV rays. Unfortunately a form of air pollution due to chlorine and
bromine fluorocarbons have been breaking down ozone creating holes in
the ozone layer over the North and South poles.
Another form of atmospheric pollution is due to the emissions of
sulfur dioxide and nitrogen oxides, which give rise to acid rain. Acid rain
has a deleterious effect on freshwater and terrestrial ecosystems as well
as architectural buildings and monuments such as the Great Sphinx in
Egypt.
Greenhouse Effect
The most serious form of atmospheric pollution, however, is the
emission of greenhouse gases, primarily carbon dioxide and to some
extent methane. These gases trap sunlight reflected from the Earth’s