UM scientist helps discover
leaky Saturn moon

Enceladus, the icy sixth moon of Saturn, has sprung a leak.

Scientists analyzing data from NASA’s Cassini spacecraft — including researcher Dan Reisenfeld at The University of Montana — have discovered a plume of gas venting from the moon’s south pole. This plume evidently explains the mysteriously high levels of water vapor found in Saturn’s magnetosphere.

The team’s work was published in the March 10 issue of Science, one of the world’s leading research journals.

“I love being part of this,” Reisenfeld said. “This is pure discovery. You are learning something that nobody on the entire planet ever knew before.”

Named for a Titan from Greek mythology, Enceladus [en-SELL-ah-dus] was long considered an average Joe among Saturn’s nearly 50 known moons. Only 310 miles in diameter, the white moon reflects most of the sunlight striking it, keeping the surface a chilly minus 330 degrees Fahrenheit.

But upon closer examination the satellite becomes more interesting. The surface is scoured by fissures, plains, corrugated terrain and other crustal formations that suggest it has been resurfaced in the recent geologic past.

Reisenfeld said Cassini spacecraft scientists noticed an odd deflection of the magnetic field of Saturn around Enceladus last year, so they reorientated the probe’s trajectory for a close flyby of the innocuous moon. On July 14, 2005, the craft zoomed within about 109 miles of the surface.

In the process, NASA had a few “Eureka!” moments. First, Enceladus has a slight atmosphere, making it only the third moon in the solar system (along with Jupiter’s Io and Saturn’s Titan) to have one. Secondly, Enceladus has volcanic activity.

“The plume of ionized water plasma coming off the south pole enshrouds the moon,” Reisenfeld said. “However, this gas isn’t gravitationally bound, so it would quickly dissipate if it weren’t being continually replenished.”

Like the Earth, Saturn has a magnetosphere — an invisible shroud of ionized gas trapped by a world’s magnetic field. (The Earth’s magnetosphere protects us from harmful space radiation.) Cassini found that water ions dominate composition of the magnetosphere closer to Saturn.

Reisenfeld said scientists previously theorized that the ionized water around Saturn came from its rings, which are mostly made of water. But now they know Enceladus is the source.

Why is a frigid moon volcanically active? Researchers don’t know for sure, Reisenfeld said, but they speculate the gravity of Saturn and some of its other moons tear and elongate Enceladus, creating tidal warming on the small world in which the south pole is warmer than at the equator.

“It would be like if we discovered Antarctica was warmer than the Sahara,” he said. “The source must be internal heat. It’s a reasonable theory.”

Before becoming a UM astronomy and physics professor, Reisenfeld worked on NASA projects at Los Alamos National Laboratory in New Mexico from 1998 to 2004. He remains an active player with several space probe missions, and with Cassini his group interprets data from the ion mass spectrometer, an instrument that detects ionized particles.

At his Missoula office, Reisenfeld takes raw Cassini data and graphs it. “All the code that does the analysis is here at UM,” he said. The ion mass spectrometer takes periodic snapshots of the atoms and molecules around Saturn and its moons, and he can then determine the proportions of ionized helium, atomic oxygen, hydronium or whatever is in a given area.

“NASA built the instrument [Cassini], and then the data comes down to Earth and is streamed to all these different scientists around the country that do different things with it,” Reisenfeld said. “So if the data shows me something, I’m the first person on the planet to know this. It’s a great feeling.”

He said his group’s Enceladus findings were published in Science under the title “The Interaction of the Atmosphere of Enceladus With Saturn’s Plasma.” His team leader and the article’s lead author is Bob Tokar of Los Alamos National Laboratory.

Reisenfeld’s upcoming plans call for outfitting a Montana Space Flight Prototype Facility at UM, which would test designs for future spacecraft.