Conditions for life on any planet
conditions must hold for plant life and animal life to exist on any planet.
The planet must have a suitable living temperature range.
The planet must have a sufficient and right kind of atmosphere.
The planet must have considerable amount of water on its surface.
The ratio of the amount of solar
energy reflected by the planet to that incident on it is known as albedo. From the knowledge of albedo, we
get information about the existence of atmosphere in the planets. The albedo of
Venus is 0.85. It reflects 85% of the incident light, the highest among the
nine planets. It is supposed to be covered with thick layer of atmosphere. The
planets Earth, Jupiter, Saturn, Uranus and Neptune have high albedoes, which
indicate that they possess atmosphere. The planet Mercury and the moon reflect
only 6% of the sunlight. It indicates that they have no atmosphere, which is
also confirmed by recent space probes.
There are two factors which
determine whether the planets have atmosphere or not. They are (i) acceleration
due to gravity on its surface and (ii) the surface temperature of the planet.
The value of g for moon is very small (¼th of the Earth). Consequently the
escape speed for moon is very small. As the average velocity of the atmospheric
air molecules at the surface temperature of the moon is greater than the escape
speed, the air molecules escape.
Mercury has a larger value of g than moon. Yet there is no atmosphere
on it. It is because, Mercury is very close to the Sun and hence its
temperature is high. So the mean velocity of the gas molecules is very high.
Hence the molecules overcome the gravitational attraction and escape.
Surface temperatures of the planets
The planets do not emit light of
their own. They reflect the Sun's light that falls on them. Only a fraction of
the solar radiation is absorbed and it heats up the surface of the planet. Then
it radiates energy. We can determine the surface temperature T of the planet
using Stefan's law of radiation E = σ T4 where σ is the Stefan's constant and E
is the radiant energy emitted by unit area in unit time.
In general, the temperature of
the planets decreases as we go away from the Sun, since the planets receive
less and less solar energy according to inverse square law. Hence, the planets
farther away from the Sun will be colder than those closer to it. Day
temperature of Mercury is maximum (340oC) since it is a planet closest to the
Sun and that of Pluto is minimum (−240oC). However Venus is an exception as it
has very thick atmosphere of carbon−di−oxide. This acts as a blanket and keeps
its surface hot. Thus the temperature of Venus is comparatively large of the
order of 480oC.