Comet Impact Triggered Saturn Moon's Huge Jets?

Richard A. Lovett in San Francisco
for National Geographic News
December 16, 2008
A comet impact millions of years ago may have helped set in motion the events that created the huge geyser-like jets on Saturn's icy moon Enceladus, scientists said yesterday.

The finding came from photos taken on recent flybys of the moon by NASA's Cassini spacecraft.

"The goal of two of these flybys, in August and October, was to target specific places in the source regions of the jets," said Carolyn Porco of the Space Science Institute in Boulder, Colorado, and head of the Cassini imaging team.

The close-range flybys gave scientists the most detailed looks yet at the moon's "tiger stripes," long cracks known to be sources of the jets.

(Related: "Cassini to Go Through Saturn Moon Plumes" [March 12, 2008].)

Spreading Centers

The new photos reveal a fractured terrain that looks remarkably similar to Earth's mid-ocean ridges, said Paul Helfenstein of Cornell University, at a meeting of the American Geophysical Union in San Francisco, California.

On Earth seafloor ridges represent "spreading centers," where the crust is rifting apart, allowing magma to rise from below.

Unlike Earth's lonely mid-ocean ridges, the tiger stripes lie together in roughly parallel lines. They are even intersected by features that look like the fault lines that separate mid-ocean ridges into separate blocks.

The discovery of these crisscrossing faults "was the first thing that clued us in that something like spreading might be going on on Enceladus," Helfenstein said.

To test this idea, he created a computer model that "slid" the stripes back together along the fault lines. Some of the crustal blocks, he added, appear to have slid as far as 45 miles (73 kilometers) from their birthplaces.

When he did this, he found that many seemingly random terrain features matched up. Some curved ridges, for example, unexpectedly fit together to form a mound that looks a lot like the bulging surface trace of a diapir, an upward-moving mass of soft material that is piercing through more brittle layers above it.

Porco notes that the "spreading" discovery is yet another sign that Enceladus has a level of geologic activity that seems to demand the presence of liquid water below the south polar terrain.

"We're growing more and more confident that it is virtually impossible to explain what we see on Enceladus without liquid water somewhere in its interior," she said.

Flowing Ice

The strange dome straddling what appears to be the oldest of the tiger stripes may be an important clue to what eventually set the jets in motion, Helfenstein said.

If it is the top of an ancient diapir, the dome may mark a place where warm ice or even liquid water flowed upward from deeper beneath Enceladus's surface.

Perhaps, he added, that process might have been initiated by an impact on the surface. A comet impact, for example, may have sent shock waves rebounding from the moon's core, creating surface fractures and a warm region below.

Once heated, more energy could have been generated as tides driven by Saturn's pull flexed the 300-mile-wide (480-kilometer-wide) moon and caused the fractured blocks of crust to rub against each other.

Such a process could have powered not only the jets but may have produced enough heat to cause the crust to begin spreading.

The new announcement, however, is far from the final word on Enceladus and its epic jets.

"It's an hypothesis," Helfenstein said. "We have to think very carefully about how to test it, or if it's even testable."

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