Outgassing
Outgassing is the volcanic or magmatic release of volatiles (CO, CO₂, H₂, SO₂, H₂O, and others) from a planet’s interior into its atmosphere. On rocky planets, outgassing is the primary builder of a secondary-atmosphere after the primordial H/He envelope is lost.
Role in atmospheric composition
The composition of an outgassed atmosphere is governed by the interior oxygen fugacity (redox state):
- Low oxygen fugacity → CO- and H₂-rich atmosphere (e.g., 55-cancri-e)
- High oxygen fugacity → CO₂- and SO₂-rich atmosphere (e.g., l-98-59-d)
Time-variable signals
Outgassing can be episodic or transient, producing variable atmospheric signals detectable as changing eclipse depths across multiple JWST observations (demonstrated in 55-cancri-e, where five eclipses showed inconsistent depths — src-55-cancri-e-hydrogen-atmosphere-2026-06).
Icy-moon outgassing
On icy moons, outgassing refers to the release of ocean volatiles through surface vents or geysers — relevant to biosignature detection on Enceladus, Europa, and Triton (src-active-moons-review-2026-04).