A secondary atmosphere is the gaseous envelope a planet develops after losing its initial, or primordial, atmosphere. This atmosphere is not a remnant of the solar nebula, but is built up from materials originating within the planet itself or delivered from space. The existence of a secondary atmosphere signifies a dynamic geological and chemical history, representing a major step in the evolution of a rocky world. This gaseous layer must be continually replenished and maintained against forces that seek to strip it away.
Distinguishing Secondary from Primary Atmospheres
Planets first form with a primary atmosphere, captured directly from the solar nebula. This initial envelope consists almost entirely of the lightest and most abundant elements, primarily hydrogen and helium. Massive gas giants like Jupiter and Saturn, formed in the cold outer solar system, were able to retain this primary atmosphere.
Terrestrial planets, which are smaller and orbit closer to the sun, cannot hold onto this initial gaseous blanket for long. Higher temperatures, lower planetary gravity, and the intense solar wind provide light hydrogen and helium atoms enough energy to reach escape velocity. Once the primary atmosphere is lost, the planet must build a new gaseous layer fundamentally different in composition. This secondary atmosphere is dominated by heavier, chemically reactive molecules, such as water vapor (\(\text{H}_2\text{O}\)), carbon dioxide (\(\text{CO}_2\)), and nitrogen (\(\text{N}_2\)).
Key Processes for Building a Secondary Atmosphere
The most significant process for constructing a secondary atmosphere is volcanic outgassing, where gases trapped deep within the planet’s interior are released to the surface. As a terrestrial planet cools and solidifies, volatile compounds are expelled through volcanic eruptions and fissures in the crust. This continuous release provides the bulk of the initial components, including water vapor, carbon dioxide, and sulfur-containing gases.
Accretion and impact delivery also play a substantial role, especially in the early stages of planetary formation. Volatiles are delivered by comets and icy asteroids that collide with the planet, a process known as late accretion. These impacts vaporize the incoming material and release the gases directly into the growing atmosphere.
Impact Degassing
The intense heat generated by large impacts can also cause impact degassing, vaporizing volatile-rich material already present in the planet’s crust and mantle.
Mantle Geochemistry
The composition of the outgassed material is determined by the geochemistry of the planet’s mantle, particularly its oxidation state. For instance, a more reduced mantle will outgas gases like methane (\(\text{CH}_4\)) and hydrogen (\(\text{H}_2\)). A more oxidized mantle will release carbon dioxide (\(\text{CO}_2\)) and sulfur dioxide (\(\text{SO}_2\)). The slow, sustained release of these heavier molecules allows a secondary atmosphere to accumulate, provided the planet’s gravity is strong enough to hold them.
Atmospheric Loss and Planetary Examples
The fate of a secondary atmosphere is determined by a continuous balance between the processes that build it up and the mechanisms that strip it away. Atmospheric gases can be lost to space through various methods.
Mechanisms of Loss
These methods include thermal escape, where light molecules like hydrogen reach escape velocity due to high thermal energy. Non-thermal processes, such as solar wind stripping, are also effective, particularly on planets lacking a global magnetic field to deflect the star’s charged particles.
The three terrestrial planets—Earth, Mars, and Venus—demonstrate the varying outcomes for secondary atmospheres.
Earth
Earth, being sufficiently massive and having a strong magnetic field, retained its outgassed atmosphere. This atmosphere evolved into its current nitrogen and oxygen-rich state through chemical and biological processes. Water vapor condensed to form oceans, which sequestered vast amounts of carbon dioxide into surface rocks.
Venus
Venus, despite its similar size to Earth, is closer to the sun, preventing the condensation of water into oceans. This led to a runaway greenhouse effect. Its massive secondary atmosphere is now 96.5% carbon dioxide and is nearly 100 times denser than Earth’s. The outgassed water likely escaped to space through photodissociation and subsequent thermal escape of hydrogen.
Mars
Mars, being much smaller, had a weaker gravitational pull and lost its protective magnetic field early in its history. Its secondary atmosphere, once thick enough to allow liquid water, was largely stripped away by the solar wind and impact erosion. Today, Mars has a very thin, cold atmosphere, primarily composed of carbon dioxide, with a surface pressure less than one percent of Earth’s.