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The Space Environment and Earth's Atmosphere

Some planets have an atmosphere and others do not

Our planet is enveloped by layers of gases that enable us to breathe and protect us from the elements of space. Without our atmosphere, there would not be life on Earth. Why don’t all planets have an atmosphere?

Gravity

Gravity is responsible for the presence of our atmosphere. Without it, the gases surrounding Earth would vanish into space over time.

The Law of Universal Gravity, formulated by Sir Isaac Newton, states that all objects have a force of attraction that act on other bodies. The force of attraction is proportional to their respective mass. The greater the mass of an object, the greater the force of attraction it has on its neighbours.

The Earth is so massive that it attracts the envelope of gases surrounding it (and on any other object that is not anchored to the ground, including us).

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Certain huge planets, such as Saturn or Jupiter, are clearly more massive than Earth and have atmospheres that measure several thousands of kilometres in thickness.

For comparison purposes, our planetary atmosphere is approximately 500 km thick whereas Mars, for example, has a mass that is approximately ten times thinner than the Earth’s having, over time, lost part of its original atmosphere which its gravitational force could not maintain.

As for our satellite, the Moon, its small mass and proximity to the Earth make it incapable of maintaining an atmosphere.

Solar wind

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Solar wind is made of a large quantity of gas emitted by the Sun. The source of these gases is located in the solar corona, where the temperature is so hot that even the gravitational force of our star cannot hold them back.

These gas jets (solar flares) are scattered into space at speeds varying between 300 and 800 km/s, and can sometimes travel toward planets. In the case of Earth, its magnetic field effectively deviates most of these gas attacks.

Mars, however, is not as well protected, and we believe that these bursts of gas may have contributed to the thinning of its atmosphere.

• Composition and structure of the atmosphere