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| Atmosphere of Earth has negligible hydrogen gas. ( Image Credit: NASA) |
The atmosphere of Earth has 78% Nitrogen and 21% oxygen but
no Hydrogen gas. Mercury does not have atmosphere whereas Venus and Mars has
abundance of Carbon Dioxide. This indicates absence of Hydrogen gas in the
atmosphere of inner rocky planets of our solar system.
If we look at the outer gaseous planets, hydrogen gas is the
main component of atmosphere. For example, Jupiter has 82% hydrogen and 18%
Helium similarly Saturn contains 96.3% Hydrogen and 3.25% Helium. Uranus and
Neptune also have 82.5% and 80% Hydrogen respectively.
From the above data, it is clear that Hydrogen is present
only in the atmosphere of outer gaseous planets. What is the reason behind this
or is it a coincidence?
There are two main factors which can account for this type
of distribution of Hydrogen gas in atmosphere of planets of our solar system.
First one is the gravity and another is distance from Sun.
This problem can be explained by the thermal escape
mechanism. Temperature is the average Kinetic Energy of molecules of gas. In a
gas, the average velocity of molecules of gas is determined by the temperature
of gas, although the velocity of individual molecules varies continuously due
to collision with other molecules but the average velocity remains nearly
constant. The variation in Kinetic Energy among the molecules is described by
the Maxwell distribution. The Kinetic Energy and velocity is related by
following relation:
K.E.=mv2/2
Now velocity is directly proportional to the temperature
which is average Kinetic Energy of molecules of gas. The individual molecules
may acquire a velocity in the higher end of velocity distribution and reach the
escape velocity, at a level in the atmosphere where the mean free path is
comparable to the scale height, and leave the atmosphere of the planet.
For a given temperature of any planet, the more massive the
molecule of a gas is, the lower the average velocity of molecules of that gas
and the less likely it is that any of them reach escape velocity.
Hydrogen molecules have very less mass as compared to molecules of Carbon dioxide. This is why Hydrogen can easily acquire velocity greater than escape velocity in comparison to Carbon dioxide and escapes from the atmosphere of planet easily. Also if the planet has a higher mass, the escape velocity is greater and fewer particles will escape. This is why the gas giant planets still retain
significant amount of Hydrogen and Helium, which
have largely escaped from Earth’s atmosphere.
The other factor is the distance of a planet from the Sun. A
close planet has a hotter atmosphere which corresponds to higher Kinetic Energy
of molecule of gas and provides velocity greater then escape velocity easily.
Opposite to it, a distant planet has a cooler atmosphere which provides lower
velocity and prevents escape from atmosphere. This helps, Titan in retaining
its atmosphere although Titan is smaller as compared to Earth but distance from
the Sun helps it.
Both the higher mass and distance from Sun provide
favourable conditions for retaining Hydrogen in the atmosphere of outer planets
of our solar system. Opposite to this, both factor provide drastic conditions
for Mercury which unable to hold even its atmosphere.