Oxygen in Saturn’s E Ring

Cassini Detects Oxygen Buildup in Saturn’s E Ring

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Saturn rings A-G. Cassini passed through the F and G ring gaps. The E-ring is beyond the outer G ring shown. Image Credit: JPL/NASA

A massive oxygen buildup was seen by Cassini’s Ultraviolet Imaging Spectrograph (UVIS) instrument earlier this year, while the spacecraft was en route to Saturn, mission scientists said Friday. Saturn’s rings are composed, for the most part, of pure water ice, good ol’ H2O. As this icy material is bombarded by charged particles from Saturn’s magnetosphere, it breaks down into its constituents, hydrogen and oxygen. So there is always some oxygen floating around in the ring system. But what UVIS detected in Saturn’s E ring wasn’t just "some" oxygen; it was a tremendous burst of the stuff, that seemed suddenly to come out of nowhere.

The E ring is Saturn’s outermost and widest ring. Its inner edge lies 181,000 kilometers (112,000 miles) from the planet and the ring extends outward for more than 300,000 kilometers (188,000 miles). In observations of Saturn’s rings made between late December 2003 and mid-January 2004, UVIS measured normal oxygen levels in the E ring. Then, for about a month, UVIS stopped taking measurements. Bad timing, it turns out. Because when UVIS checked back again in mid-February, it found that while it was napping something had caused a massive buildup of oxygen in the E ring. The total amount of oxygen detected, said UVIS Co-investigator Donald Shemansky, was 4 times the mass of all of the known material in the E ring.

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Two moons near ring plane Image Credit: JPL/NASA

There is an explanation – or at least a theory. To date, the only E-ring particles that have been detected are tiny micron-sized grains. But scientists speculate that the E ring also contains much larger chunks of ice. These larger ice balls, they believe, vary in size between roughly 1 and 10 kilometers (0.6 and 6.2 miles) in diameter. Similar-sized objects – they’re referred to as "parent bodies" – have been found in the F ring, but to date not in the E ring. Nevertheless, scientists suspect that a collision between a pair of these parent bodies provided the material that led to the oxygen buildup. In fact, they cite the oxygen buildup as evidence supporting their theory. The oxygen "has to come out of the ice in the rings, somehow," said Shemansky. There is no other source for it.

The theory is bolstered by the fact that when UVIS first detected the oxygen increase, it was concentrated heavily on one side of the E ring. One month later, it had spread out more evenly. This is consistent with the occurrence of a single event, such as a collision, followed by the normal dissipation and distribution of the icy detritus throughout the ring. Scientists believe that through an ongoing process of such collisions and the subsequent dissipation of the liberated material, the E ring is being slowy eaten away. They predict that within 100 million years, the ring will disappear completely.

In the mean time, they are hopeful that Cassini will be able to detect parent bodies. Cameras aboard previous spacecraft that visited Saturn did not have high enough resolution to find them. If they’re there, Cassini’s cameras should be able to see them. Otherwise, someone’s going to have come up with another way to explain all that oxygen.