a. The sun's temperature is right
to provide the range of light wavelengths suitable for life. Higher temperatures would
result in too much ultraviolet radiation, lower temperatures in too much infrared.
b. The sun is at the correct
distance from the earth and has the proper size and temperature to provide the total
amount of radiation required to maintain surface temperatures on earth suitable for life.
Even small changes in any of these factors would probably destroy all life.
c. The nearly circular orbit of
the earth limits temperature variations.
d. The water vapor and carbon
dioxide in the atmosphere produce a so-called "greenhouse effect" which
moderates the temperature extremes.
e. A high altitude ozone layer
effectively absorbs the lethal fraction of solar ultraviolet rays which would destroy life
on the earth's surface were the ozone layer removed.
f. The 23 degree inclination of
the earth's axis of rotation from the perpendicular to the plane of its orbit provides for
the seasons. It probably also considerably increases the land area in the northern
hemisphere suitable for intensive summer agriculture.
g. The earth's magnetic field,
extending tens of thousands of miles into space, shields the earth's surface from much of
the cosmic radiation which probably would prove deleterious to life.
h. The lunar gravitation produces
important tidal circulation effects in the oceans, which make conditions much more
suitable for sea life in the shallow zones along shores and in estuaries.
i. The mass and size of the earth
are adjusted to provide gravitational force and atmospheric pressure suitable for life,
and also to prevent the escape of the earth's atmosphere.
k. The earth's surface is blessed
with a concentration unknown any-where else in the universe, of liquid water, the only
possible solvent and medium for living cells. The physical properties of water are unique
and absolutely essential to all life processes. These properties of water include (1) the
highest heat of fusion(melting), (2) a liquid temperature range which includes the
temperatures at which enzymes and other life molecules can exist and function, (3) the
highest heat capacity(heat required to increase its temperature), (4) the highest heat of
vaporization, (5) the highest dielectric constant of all of the common liquids, (6) the
greatest solvent powers, (7) the greatest power to form a special chemical bond called a
hydrogen bond, (8) the greatest ionizing power of all the common liquids, (9) the property
(unique except for bismuth) of expanding when it crystallizes, and (10) a strong power to
absorb infrared radiation.
Liquid ammonia is sometimes proposed as a possible liquid for some
imagined kind of life system. It has slightly higher values in several of the above
properties, but it is distinctly inferior to water in the others. The idea of life in any
solvent other than water is mere fantasy.
l. The elements carbon, hydrogen,
oxygen, nitrogen, and phosphorus, together with liquid water, are the basis for the only
remotely possible chemical framework for life. The surface of the earth is the only known
place in the entire universe where these substances are found in suitable quantities and
in the proper forms for life to exist.
