A secondary atmosphere is an

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

of a

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

that did not form by accretion during the formation of the planet's

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

. A secondary atmosphere instead forms from internal

volcanic A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often fo ...

activity, or by accumulation of material from

comet A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...

impacts. It is characteristic of

terrestrial planet A terrestrial planet, tellurian planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate, rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to ...

s, which includes the other terrestrial planets in the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

: Mercury,

Venus Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...

, and

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

. Secondary atmospheres are relatively thin compared to

primary atmosphere A primary atmosphere, often called a primordial or proto-atmosphere, is an atmosphere of a protoplanet that forms by Accretion (astrophysics), accretion of Gas, gaseous matter from the protoplanetary disk. Gas giant planets including Jupiter, Satu ...

s like

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

's. Further processing of a secondary atmosphere, for example by the processes of

biological life Life, also known as biota, refers to matter that has biological processes, such as signaling and self-sustaining processes. It is defined descriptively by the capacity for homeostasis, organisation, metabolism, growth, adaptation, res ...

, can produce a tertiary atmosphere, such as that of

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

Atmosphere evolution

Starting point

During planet formation, gas and dust from the Sun's protoplanetary disc accrete onto all forming planets. Depending on the final size of the planet, it may or may not have enough gravity to retain this first,

or the star's solar winds strip the atmosphere off of the planet. The

giant planets A giant planet, sometimes referred to as a jovian planet (''Jove'' being another name for the Roman god Jupiter), is a diverse type of planet much larger than Earth. Giant planets are usually primarily composed of low-boiling point materials ( vo ...

, such as

and

Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...

, became large enough where they were able to hold onto their primary atmosphere that they gained during formation while

terrestrial planets A terrestrial planet, tellurian planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate, Rock (geology), rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner plane ...

, such as

and

, do not have enough gravity to hold onto the original atmosphere. By being made of rock, they are able to go through geologic processes that will produce gas into the atmosphere.

Post-primary atmosphere

If the planet is too small, then its gravity isn't strong enough to hold onto all of the gas it gained during formation. This causes the primary atmosphere, which is mostly made of

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

(H₂) from the nebula the solar system formed in, to run away and leave the planet entirely. Hydrogen, being the lightest element, will naturally escape the atmosphere due to it being the most buoyant. If a planet is to gain a new atmosphere, it must create one with materials found within the planet itself.

Volcanism Volcanism, vulcanism, volcanicity, or volcanic activity is the phenomenon where solids, liquids, gases, and their mixtures erupt to the surface of a solid-surface astronomical body such as a planet or a moon. It is caused by the presence of a he ...

is an example of a geologic process that will pump out volcanic gasses, such as

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

(CO₂) and

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...

(SO₂), that come out from a variety of sources. During the hot protoplanet phase, the molten planet or moon will cool off which will cause the surface to solidify. Material still want to outgas which causes openings in the crust which will spew out gas that is often trapped in the cavities of rocks which include the

asteroids An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

meteors A meteor, known colloquially as a shooting star, is a glowing streak of a small body (usually meteoroid) going through Earth's atmosphere, after being heated to incandescence by collisions with air molecules in the upper atmosphere, creating a ...

, and

comets A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or coma surrounding the nucleus, an ...

that bombard the surface of a planet during, and after, its formation. The molten

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

is able to retain gasses that are dissolved or bonded with the magma or lava itself and release at the opening of the volcano where the pressure becomes low enough to sublimate.

Water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

(H₂O) is a very common molecule throughout the universe, so asteroids and comets during the solar system's formation were likely what brought water around the solar system. When the water is first delivered, it mixes in with the surrounding lava and stays trapped until it cools off enough. The volcanoes, along with the other gasses mentioned above, will also spew out water vapor that was trapped in the magma. In the case of Earth, its earliest secondary atmosphere was almost entirely made up of water vapor and carbon dioxide.

Planetary conditions

The result of a planet gaining an atmosphere once the first is lost is heavily dependent on the planet's individual conditions as well as the conditions of its placement in the solar system. Different types of stars have varying amounts of solar wind and harmful radiation. These are the components that tend to strip a planet of its atmosphere in the first place. For example, Mercury does not have an atmosphere because it is so close to the sun that the solar winds have stripped it from the planet. Earth has an atmosphere largely because of the magnetic field which deflects most of the harmful radiation and solar winds away from the surface. Not only does Earth have an atmosphere due to outgassing but its favorable conditions has given rise to life that produces gas as a byproduct such as photosynthesis creating oxygen, which is the characteristic needed to create a tertiary atmosphere. Venus, on the other hand, does not have a magnetic field which means it is bombarded by solar wind and radiation, but the planet's volcanic activity continues to pump out more carbon dioxide into its atmosphere.

References

Atmosphere Atmospheric sciences Planetary atmospheres {{Climate-stub