Atmosphere :: The Sphere of Gas Around Planets and Stars

An atmosphere is the distinct layers of gas surrounding a celestial body, which will typically be a planet, though stars also possess an atmosphere called a stellar atmosphere.

The term atmosphere comes from the Greek words $\alpha \tau \mu \omicron \sigma$ (atmos) meaning vapour, and $\sigma \phi \alpha \iota \rho \alpha$ (sphaira) meaning sphere, referring to the sphere of gas around a planet. It is composed of a collection gases – various molecules, compounds, and chemical elements in their gaseous state – which are bound in a sphere around the planet due to gravity.

The planet rotates as the local star heats the atmosphere with its radiation, the region of direct alignment with the star heating most efficiently. As nature fundamentally seeks balance, the temperature of the atmosphere moves towards equilibrium by sending great convective atmospheric currents of hot gas heated by the star to cold regions, while cool atmospheric gases are sent towards the day side in order to maintain equilibrium. These processes result in wind and weather patterns.

A planetary atmosphere is matter, of atoms, which makes it a fluid like water, though less dense. A planetary atmosphere differs in density, pressure, and temperature with distance from the planetary surface. The closer you are to the surface of the planet (or to sea level) atmospheric pressure and density are both going to reach their peak.

A given region of the Earth will literally have the weight of all the air vertically above that region pressing down upon it. The further you get from the surface the thinner the atmosphere is, becoming less dense, and colder too. There are less particles per unit of volume surrounding you (lower density) and far less atmosphere above you to push down upon you.

The Earth’s Atmosphere has been officially designated as extending 100 km from the surface. However, some 3/4 of it’s entire mass are condensed within the first 7 – 17 km, within the first 20% of the altitude of the atmosphere.

Composition of an Atmosphere

The exact chemical composition of an atmosphere depends on the chemical composition of the planets stellar neighborhood. The atmospheric composition of a planet is the result of the chemical elements available at the time of its formation. Planets are formed during the process of star formation, when a stellar nebula (space cloud) collapses under the gravitational forces from its own mass into a proto-star surrounded by great rings of matter which coalesce into planets.

The stellar nebula passes on its chemical composition to both planet and star, the ratios of elements of the periodic table relative to one another as unique to it as a fingerprint, which ultimately becomes every living creature, plant, environment, ecosystem and substance on the surface.

Younger generation stars form out of nebula imbued with a rich concentration of heavier elements, which have been manufactured over hundreds of millions of years in stellar cores. Older stars of an earlier generation possess smaller concentrations of heavier elements which had yet to be forged. This greater chemical complexity seems to result in an exponential increase in the potential environmental, geological, and biological complexity of the planet, which in turn has an impact on atmospheric composition.

For example, when the Earth was young it possessed a great deal of $CO_2$ in the atmosphere and very little oxygen. It wasn’t until small living organisms evolved which consumed that carbon dioxide and released oxygen into the environment through photosynthesis, resulting in a gradual millennial increase in the amount of oxygen in the atmosphere.

In modern times human beings are increasing the amount of $CO_2$ in the environment through pollution, which has already resulted in measurable changes.

Structure & Dynamics of an Atmosphere

The structure and dynamics of an atmosphere are complex. Modern scientists have made several distinctions in atmospheric density and composition which define the atmospheric structure of the Earth.

The main layers from surface to space of the Earth’s atmosphere are the troposphere, stratosphere, mesosphere, ionosphere, thermosphere, and exosphere. The exosphere marks the beginning of space at the Karman Line at an altitude of 100 km.

The composition of each layer changes slightly, as well as many of the properties of that layer. Lighter elements and compounds rise, while heavier elements sink. The Stratosphere contains the Earth’s ozone layer which is responsible for blocking the majority of harmful UV radiation from the Sun, while the Mesosphere is the layer of the atmosphere where the majority of meteors (space ice and dust) burn up in ephemeral fiery streaks across the stars.

Atmospheric Pressure

The gases of an atmosphere are composed of molecules and particles, just like liquids are. The deeper that you go in water – a fluid like the atmosphere – you can feel the weight of the water pressing upon you and your eardrums. Even though all the air above us in a cylinder around our bodies all the way to space is literally resting upon us, we cannot feel it because it is such as small force, and because we have known nothing else.

This is called atmospheric pressure. Atmospheric pressure is the amount of force exerted on a unit of area by the weight of all the gas above that unit of area, enclosed within the borders of the perimeter of that area extending vertically into space.

Atmospheric pressure falls with distance from the surface of a body. The top of Mount Everest or the altitude of an airplane will have a significantly lower atmospheric pressure than a location at sea level on a beach of Fiji or Mexico.

This is partially because the atmosphere is at its most dense near to the surface, the layer of the Earth’s atmosphere that we call the troposphere, where 3/4 of the atmosphere’s mass resides and where weather develops. However, the main reason is that there is far less atmosphere above between a high altitude position and space, at a lower density, which means less material above you to exert a downward force.

Atmospheric Escape :: Gradually Leaking of Atmosphere Into Space

The gravitational force around each planet varies according to the mass and density of each planet. The greater the mass of the planet, the greater its surface gravity, gravitational pull, and the amount of gravitational forces it can exert at greater distances.

Every cosmic body has an escape velocity related to its mass. The escape velocity is the velocity an object must surpass to escape the gravitational pull (and atmosphere) of the planet or star to reach space.

The more massive the planet is, the higher the escape velocity will be. The escape velocity of the Earth is only about 11 km/s, while the escape velocity around far more massive Saturn is about 36 km/s. However, the Sun contains around 99.86% of the mass of our entire solar system resulting in a relatively enormous escape velocity of 618 km/s.

Escape velocity is the speed a spacecraft needs to surpass in order to enter space. The least massive particles such as hydrogen, helium, and lithium have a very small gravitational force acting upon them, especially when they are high in the atmosphere. When they are heated by the Sun they move more rapidly with a higher propensity for collision.

When the angles and velocities are just right, particles are ejected into space by natural heating processes, meaning the atmosphere is constantly leaking into space at a small rate. Planets of greater mass (such as the gas giants) can retain lighter elements much better than smaller planets. The solar wind blowing off of the Sun blows away the atmosphere of planets that are too close to it.

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