Why do inner planets of our solar system have negligible hydrogen in the atmosphere?

The Planet Earth
The 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 have an abundance of Carbon Dioxide. This indicates an 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 the 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 the atmosphere of planets of our solar system. First one is the gravity and another is the 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 are related by the following relation:

K.E.=mv2/2,

where, m is the mass of the particle and v is the velocity of the particle.

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 the 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 the molecule of gas and provides velocity greater than escape velocity easily. Opposite to it, a distant planet has a cooler atmosphere which provides lower velocity and prevents escape from the atmosphere. This helps Titan in retaining its atmosphere although Titan is smaller as compared to Earth but the 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 the factor provide drastic conditions for Mercury which unable to hold even its atmosphere.

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