The Gain or Loss of Atmospheric Gas

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  • 0:02 Gain or Loss of…
  • 0:54 Primary & Secondary Atmosphere
  • 1:54 Temperature, Mass & Atmosphere
  • 5:39 Atmospheric Gas Shifts
  • 6:51 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will discuss how a planet's atmosphere is formed, how it is shaped by the planet's characteristics, and how it evolves with time. Afterwards, you can take a short quiz to test your knowledge of planetary atmospheres.

The Gain or Loss of Atmospheric Gas

When you put gas into your car, you're actually putting in a liquid: gasoline. That liquid is then used by your engine as an energy source to power your vehicle forward. The byproducts of this process leave your car as an actual gas or mixture of gases. Surely, you have seen this before as black smoke coming from semis, or white plumes coming out of a car in front of you on a really cold day.

In a way, your car's combustible processes lose gas from the car but add gas into the Earth's atmosphere at the same time: gas, like carbon dioxide, which we all know contributes to global warming. But this discussion won't be about global warming. Instead, it's about how an atmosphere, especially right here on Earth, forms, loses gas molecules, or gains them.

The Primary and Secondary Atmosphere

Every planet's original atmosphere is gained from the solar nebula from which the sun and planets formed. The solar nebula is a gaseous cloud of interstellar matter from which the solar system, including the planets and the sun, formed.

A planet's original atmosphere, made during its formation, is called its primary atmosphere. Such an atmosphere would be largely composed of light primordial gases, like hydrogen and helium. The primary atmosphere changes radically in planets closer to the sun for quite a few reasons, and it later evolves into a secondary atmosphere.

The secondary atmosphere is, therefore, quite different from the primary atmosphere. The secondary atmosphere is the atmosphere formed later in a planet's history through outgassing, and outgassing is the release of gas from the interior of the planet, during things like volcanic activity.

Temperature, Mass, and Atmosphere

But the solar nebula is only one component of how a planet gains its atmosphere as a whole or a particular gas comprising part of the atmosphere. You see, it is hypothesized that planets closer to our sun, the terrestrial planets, may not have derived too much benefit from the solar nebula for very long when compared to the outer planets of our solar system. This is because by the time these inner planets formed, the solar wind of our sun would've blown quite a bit of this solar nebula into the far reaches of our solar system.

That's not all, though. The inner planets are quite small, have weak gravity, and are hot compared to the outer gas giants, like Jupiter. This size, gravity, and temperature difference plays an important role in how atmospheric gases are gained or lost in a planet's atmosphere.

Let's take a look at an example using what we've learned so far to explain how and why. A planet's atmosphere can be made up of several different gases, like the light helium and hydrogen I discussed before, and heavier molecules, like carbon dioxide, methane, and ammonia.

For a given temperature, lighter molecules have higher velocities than molecules that are heavier. You can think of the temperature as an engine of a car. If the same exact engine is powering a little sedan versus a huge monster truck, which one do you think will move faster? Obviously, the little car, since the engine is the same one.

Furthermore, in hotter atmospheres, gas molecules will move a lot faster than in colder ones. This one is also easy to understand. Think about how much more agile and quick you are when your muscles are warmed up or when the outside temperature is warmer. Keep these two latter points in mind: hotter temperatures cause gas molecules to move faster and lighter molecules have higher velocities than heavier ones.

If a gas molecule is in the upper part of a planet's atmosphere and is moving upwards at a high velocity, it could actually exceed the planet's escape velocity, the minimum velocity an object requires to break free from a celestial body's gravitational field.

The escape velocity depends on the mass of the planet. The larger the mass, the stronger the force of gravity. The stronger the force of gravity, the higher velocity the gas molecules need to escape the gravitational field.

The gravitational field is like a rubber band pulling the molecules trying to escape back down to the planet. The larger the planet, the larger, stronger, and thicker this rubber band is, and the faster the molecules need to move to escape the inevitable snap-back gravity induces.

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