Why the Earth has atmosphere? The question is quite obvious. One of the vital factors why life exists on Earth is its Atmosphere.
This little post tells us why the Earth holds the atmosphere and why the Moon doesn't.
We must understand two concepts -
1) Escape Velocity
In physics, escape velocity is the minimum speed needed for an object to escape from the gravitational influence of a massive body, such as the Earth or the Moon.
For example - if you throw a stone in the air, it falls back down, because the Earth's gravitational force attracts it. However, if you throw a stone with an initial velocity equals or more than the escape velocity of the Earth, the stone will escape into the space, and won't come back !
The escape velocity of the Earth is 11.2 km/s
The escape velocity of the Moon is 2.38 km/s
2) Root Mean Square (RMS) velocities of gases
When you provide hit to the gas, the pressure increases. This is because each individual gas molecule gains kinetic energy, and starts moving more rapidly.
In simple terms, RMS velocity is the resultant velocity calculated by considering the velocity of each individual particle of the gas. This is dependent upon the total kinetic energy of the gas.
When the solar system war forming, or the time of formation of the Moon, the temperatures were very high in thousands of degree Celsius. Obviously, gases that hot must have had the tremendous velocities !
Earth holds the atmosphere because - Earth's escape velocity is simply too much !
RMS velocity of gases in Earth’s atmosphere "was and is" much smaller than the escape velocity of the Earth, which is 11.2 km/s. However, even today, Earth loses several tons of Hydrogen and Helium each year !
On the other hand, Moon's escape velocity is much smaller and can be easily achieved by the hot gases. Therefore the Moon lost almost all of its atmosphere in the past.
Another reason is, Moon is geologically dead. There are not many active volcanoes or tectonic plate movements. Earth has many active volcanoes that erupt many gases in to the atmosphere.
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Amateur Astronomers Group