Stellar occultation offers new insights on Enceladus’ geysers

Cassini was able to measure the amount of water vapor erupting from the geysers, offering new insights on geologic activity beneath the moon’s surface.
By | Published: May 9, 2016
Enceladus' icy geysers
Narrow jets of gas and icy particles erupt from the south polar region of Enceladus, contributing to the moon’s giant plume. A cycle of activity in these small-scale jets may be periodically lofting extra particles into space, causing the overall plume to brighten dramatically.

NASA/JPL/Space Science Institute
The Cassini spacecraft viewed a bright star passing behind a plume of gas and dust spewing from Saturn’s moon Enceladus. The Saturn-orbiting Cassini probe, using its Ultraviolet Imaging Spectrograph (UVIS), was able to measure the amount of water vapor erupting from Enceladus, offering new insights on geologic activity beneath the moon’s surface.

“This new data is very important to our understanding of the ‘plumbing’ of the water conduits below the surface of Enceladus that connect the water vapor eruption to the reservoir of water below the surface,” said Candice Hansen from the Planetary Science Institute in Tucson, Arizona.

Earlier data from Cassini showed that Enceladus continuously spews out a broad geyser of gas and dust-sized grains originating from the moon’s subsurface ocean of salty liquid water. The plume, originating from the region around the moon’s south pole and extending hundreds of miles into space, is more than 90 percent water vapor.

Other instruments on the Cassini spacecraft have observed that the number of water ice grains being ejected from the small moon was three times greater when Enceladus was farthest from Saturn compared to when it was closest in its elliptical orbit. This was Cassini’s first opportunity to see if the amount of gas that propels the particles from fissures — called tiger stripes — across Enceladus’ south pole also changes with its orbital position.

Saturn's gravitational pull
The gravitational pull of Saturn changes the amount of particles spraying from the south pole of Saturn’s active moon Enceladus at different points in its orbit. More particles make the plume appear much brighter in the infrared image at left.

NASA/JPL-Caltech/University of Arizona/Cornell/SSI
“The UVIS team did not find the total number of water molecules to be the predicted two times higher; rather, it is only approximately 20 percent greater,” said Hansen, a UVIS team member. “We went after the most obvious explanation first, but the data told us we needed to look deeper at what was happening closer to the moon’s surface.”

Hansen and her team focused their attention on one supersonic jet known informally as “Baghdad I.” The researchers found while the amount of gas in the overall plume did not change much, this particular jet was four times more active than previously observed during other times in Enceladus’ orbit. Instead of providing just 2 percent of the plume’s total water vapor, as Cassini had previously observed, it was at this time supplying 8 percent of the plume’s gas.

The increase in the jet’s activity is what causes more icy dust grains to be lofted into space, with the jets that lift the observed ice grains much stronger when Enceladus is farthest from Saturn.

“How do the tiger stripe fissures respond to the push and pull of tidal forces as Enceladus goes around its orbit to explain this difference? We now have new clues!” Hansen said. “It may be that the individual jet sources along the tiger stripes have a particular shape or width that responds most strongly to the tidal forcing each orbit to boost more ice grains at this orbital longitude.”