Atmosphere

An atmosphere is a layer of gases that surrounds an astronomical object, held in place by the object's gravity. The word comes from Ancient Greek words meaning "vapour" and "sphere." Objects usually get their atmospheres when they are very young, either by pulling in nearby matter or by releasing gases from inside themselves. The mix of chemicals in an atmosphere can change because of reactions with the planet's surface or with energy from the Sun. Planets with stronger gravity and cooler temperatures can hold onto their atmospheres for much longer. The stream of particles from the Sun, called the solar wind, can slowly remove parts of a planet's atmosphere, but a protective magnetic field called a magnetosphere can help shield it.

Besides Mercury, all the planets in our Solar System have important atmospheres, including the dwarf planet Pluto and the moon Titan. Big planets like Jupiter and the other gas giant planets have very strong gravity and cold temperatures, so they keep huge atmospheres made mostly of hydrogen and helium. Smaller rocky planets, like Earth, orbit closer to the Sun and keep denser atmospheres made of carbon, nitrogen, and oxygen, with small amounts of other gases. Scientists have also found atmospheres around planets outside our Solar System, called exoplanets, such as HD 209458 b and Kepler-7b.

Stars also have atmospheres, which are the outer layers above the bright surface called the photosphere. Cooler stars may have atmospheres containing molecules. Other objects with atmospheres include special types of stars called brown dwarfs and active comets.

Occurrence and compositions

In the nebular hypothesis, stars form when clouds of gas and dust come together due to gravity. This forms a spinning disk, with a hot young star in the middle and planets forming from the material around it. Planets begin with a basic atmosphere as they gather gas from the disk surrounding their star.

The air around planets like Venus and Mars is mostly made of carbon dioxide, with some nitrogen and argon. Venus has a very thick atmosphere because it has no rain to wash away the carbon dioxide. Mars has a thin atmosphere because it is small and cold.

Earth’s air is mostly nitrogen and oxygen, with smaller amounts of other gases. This mix helps support life and is protected by Earth’s magnetic field. The large planets like Jupiter, Saturn, Uranus, and Neptune have thick atmospheres made mostly of hydrogen and helium. Some moons, like Titan and Triton, also have their own thin layers of air.

Conditions

Main articles: Atmospheric pressure and Atmospheric temperature

An atmosphere stays in balance because the air pressure from moving molecules is held back by gravity, stopping them from floating away. The pressure gets lower as you go higher up, creating a difference in pressure from one place to another. This pressure is the force pushing on a surface from the weight of the air above it.

The temperature of the atmosphere depends on how much energy comes from the Sun and how much heat the planet sends back into space. The distance from the Sun and how much sunlight the planet reflects affect this balance. When a planet has the same amount of energy coming in and going out, it reaches a certain temperature, but the actual temperature we feel can be different because of how the atmosphere traps heat.

Structure

Planetary atmospheres are made of layers that have different features, like what gases are there, how hot or cold it is, and how much air pressure there is.

For Earth, Mars, and Venus, the lowest part of the atmosphere is called the troposphere. This is where most clouds and weather happen. It reaches up from the ground to about 65 kilometers on Venus, 40 kilometers on Mars, and 17 kilometers on Earth. Most of the air is found here. The temperature changes depending on how high you go because heat from the ground moves upward. The next layer is the stratosphere, where the temperature gets warmer as you go up. This layer has something called the ozone layer, which helps protect the planet from harmful sun rays.

Above this is the mesosphere, where it gets colder as you go higher. For Venus and Mars, there are areas here where the temperature stays about the same. Higher up is the thermosphere, where the temperature rises because it takes in energy from the sun. The very top layer is the exosphere, where the air is so thin that particles can sometimes float off into space.

Gas giant planets, like Jupiter and Saturn, are mostly made of hydrogen and helium. They have layers of clouds made from different substances, like ammonia and methane. These planets also give off more heat than they get from the sun. They formed far from the sun where it was cold enough for certain materials to clump together.

Circulation

Main article: Atmospheric circulation

The atmosphere moves because of temperature differences. When the sun heats one part of a planet more than another, the warm air rises and cooler air sinks. This movement helps balance the heat across the planet.

On planets like Earth, this creates patterns of moving air called cells. For example, near the equator, warm air rises and then moves toward cooler areas. This helps create steady winds that blow in certain directions, like the trade winds near the equator. Larger planets like Jupiter and Saturn show bands of clouds moving in different directions because of their strong winds.

Escape

Main article: Atmospheric escape

The gravity of a planet or other space object helps keep its atmosphere close. Big planets like Jupiter can hold onto light gases such as hydrogen and helium, while smaller objects cannot. The distance from the Sun also matters, as it affects how much energy heats the atmosphere. Faraway objects like Titan, Triton, and Pluto keep their atmospheres even though their gravity is not very strong.

Over time, some gases can escape into space. Lighter gases move faster and are lost more quickly. For example, Venus and Mars may have lost much of their water this way. Earth's magnetic field helps protect our atmosphere, but some gases can still escape slowly. Other forces, like the stream of particles from the Sun and impacts from space, can also wear away an atmosphere. Small objects like comets can only hold their atmospheres briefly as they get closer to the Sun.

Terrain

The part of a planet's atmosphere that touches the ground is called the planetary boundary layer. Atmospheres shape the land by moving things around with wind. This wind can slowly change the shape of mountains and erase marks left by old space crashes or volcanoes. Because atmospheres create pressure, they allow liquids like water to exist on a planet's surface, forming lakes, rivers, and oceans. We see this on Earth and Titan, and Mars may have had water in the past.

Planets without atmospheres, or with only very thin layers of gas, are covered in craters. Without protection from an atmosphere, space rocks called meteoroids crash directly into the surface, creating these craters. But planets with thick atmospheres usually protect their surfaces — most meteoroids burn up as bright streaks of light called meteors before they can reach the ground. If they do hit the surface, wind and weather often erase the marks over time.

Fields of study

From the view of a planetary geologist, the atmosphere helps shape a planet's surface. Wind can pick up dust and other small pieces, which hit the land and change its shape. Frost and precipitations also affect the land, depending on what the air is made of. Changes in climate can even change how a planet changes over time. By studying Earth's surface, we can learn about the air and weather on other planets.

For a meteorologist, what the air is made of helps decide the climate and how it changes. For a biologist or paleontologist, the air around Earth is linked to how life appeared and grew. In astrobiology, the air of planets far away can tell us if there might be extraterrestrial life.