One Saturn Moon Blows Oxygen to Another
The atmosphere of Titan can be seen on the Saturn moon's limb in this stunning view from NASA's Cassini spacecraft.
Credit: NASA/JPL/SSI

A comprehensive model of Saturn's magnetosphere and moons has revealed that the ringed planet's largest moon Titan gets its oxygen from the icy geysers on another.

Until now, the method for oxygen making its way into Titan's surface chemistry has remained a mystery, the scientists said in a NASA statement.

Icy geysers on Saturn's moon Enceladus shoot water molecules into the planet's magnetosphere, where they break down into oxygen and hydrogen as they travel to Titan, according to a model developed by scientists at NASA's Goddard Space Flight Center in Greenbelt, Md.

"Titan and Enceladus, another icy moon of Saturn, are chemically connected by the flow of material through the Saturn system," said John Cooper, a plasma spectrometer team scientist with NASA's Cassini mission to Saturn, in a recent statement.

Cooper and fellow Cassini scientist Edward Sittler led two different studies into the transportation of oxygen between Enceladus and Titan. NASA announced the research this month.

Icy moon calls the shots   

Scientists have known that Titan's atmosphere receives an infusion of oxygen ions since 2004, when it was detected by the Cassini spacecraft currently orbiting Saturn.

At the time, the very tiny amounts of the gas and their high energy surprised scientists accustomed to a Titan atmosphere composed mostly of nitrogen, but they wrote it off as the breakup of icy interplanetary grains of dust.

It was not until Cassini's observations revealed the ice jets on Enceladus that researchers understood "that Enceladus was dominating the whole system," Sittler said. [Photos: Moons of Saturn]

Though roughly 10 times smaller than Titan, Enceladus was observed dumping 661 pounds (300 kg) of frozen water every second into Saturn's magnetosphere, researchers said. Gas beneath a cavity of water on the moon's south pole continuously builds up, causing water to erupt from cracks on the icy surface.

The icy jets escape Enceladus in plumes of vapor similar to the Old Faithful Geyser at Yellowstone Park in the U.S., reaching orbit easily because the moon's escape velocity is only a few hundred meters per second, Sittler said.    

Titan's hydrocarbons are key

In their studies, the two researchers found that oxygen particles can attach to soccer ball-shaped carbon molecules called fullerenes on Titan after they migrate to the cloud-covered moon from Enceladus.

These fullerene cages protect the oxygen reacting with the methane in Titan's atmosphere and could ultimately reach the moon's surface attached to dust particles, the researchers said.

It's a unique setup, and one that arises only because of Saturn and its interactions with its moons.

"That's what's unique about this whole story: you need Titan, you need Enceladus, and you need the magnetosphere," Sittler told SPACE.com.

Furthermore, the fullerene hydrocarbons form some of the building blocks of more complex compounds, Sittler said. When radiated by highly energetic cosmic rays, the oxygen immersed in these molecules can produce amino acids, the foundation of pre-biological chemistry.

The fullerene link

At 10 hours and 29 minutes, the rotation of Saturn subjects materials in its magnetosphere to velocities similar to those found in a particle accelerator, more than enough to break down water molecules from Enceladus into their component parts.

By the time molecules are ripped apart and headed for the upper atmospheres of nearby moons, they are energetic enough to pierce the cages of fullerene, researchers said.

"You need energetic reactions to get the molecules in the fullerenes," Sittler explained, likening the action to shooting a particle through a solid ball.

Once oxygen atoms are trapped in the hollow fullerene balls, the molecules can condense into aerosols and act as protection and crucible for chemical reactions.

Aerosols are dust particles that can cause the reddish glow seen on Earth during sunset. They are large enough to gradually fall with time, making them a viable path for oxygen to be transported to the Titan surface, researchers said. Without the protection of the fullerenes in Titan's hydrocarbon-rich atmosphere, oxygen atoms would react with the moon's methane to produce carbon dioxide or carbon monoxide, they added.