Saturn, the solar system’s second-largest planet, is classified as a gas giant, dominated by hydrogen and helium. The question of whether oxygen exists in the Saturnian system depends entirely on the specific location being examined. While molecular oxygen gas (\(\text{O}_2\)), the kind we breathe, is virtually nonexistent within the planet’s main atmosphere, oxygen is abundant in other forms, locked away in compounds across the planet’s vast surrounding systems.
Oxygen in Saturn’s Primary Atmosphere
The main atmosphere of Saturn is composed of roughly 96% molecular hydrogen and 3% helium. Molecular oxygen gas, which is highly reactive, cannot exist for long in this hydrogen-rich environment. Trace amounts of oxygen are present only in the form of oxygen-bearing molecules found deep within the planet’s atmosphere.
Water vapor, which contains oxygen, is theorized to be concentrated in a cloud deck far below the visible cloud tops. This water cloud layer is predicted to exist where the temperature is high enough for its formation and the atmospheric pressure approaches that of Earth’s sea level. Carbon monoxide (CO) is another oxygen-containing compound detected in trace amounts. The upper atmosphere also receives a continuous influx of water vapor from the plumes erupting from the moon Enceladus.
The Vast Reservoir: Oxygen as Water Ice
The vast majority of oxygen in the Saturnian system is chemically bound in the form of water ice (\(\text{H}_2\text{O}\)). This icy composition is where the element resides on a massive scale. The most visible manifestation is the planet’s spectacular ring system, which is composed of billions of particles that are nearly pure water ice.
These ring particles range dramatically in size, from microscopic dust grains to chunks the size of small mountains. Many of Saturn’s icy moons are also giant reservoirs of frozen water. For instance, Enceladus has a global subsurface ocean of liquid saltwater beneath its icy crust. This ocean vents plumes of water vapor and ice particles into space, which acts as the source material for the diffuse E-ring. The surfaces of other moons, such as Tethys and Rhea, are also dominated by water ice.
Molecular Oxygen in Exospheres and Its Source
Molecular oxygen (\(\text{O}_2\)) has been scientifically detected in the extremely thin atmospheres, or exospheres, of some icy moons. The Cassini spacecraft directly measured trace amounts of this gas around moons like Rhea and Dione. This detection confirms that oxygen gas can form in the outer solar system without the presence of biological processes.
The molecular oxygen is created through a process known as radiolysis. This occurs when high-energy charged particles, primarily trapped within Saturn’s powerful magnetosphere, constantly bombard the water ice on the moons’ surfaces. This energetic radiation breaks the chemical bonds in the \(\text{H}_2\text{O}\) molecules, freeing the oxygen atoms. These liberated oxygen atoms then recombine to form a sparse cloud of molecular oxygen gas in the surrounding exosphere. The density of this oxygen is extremely low, estimated to be five trillion times less dense than the air at Earth’s surface.