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 (Vsth 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.
