Author: Nancy Atkinson

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A cold volcano seems like an oxymoron, but active “cryovolcanoes” may actually be spewing a super-chilled liquid into the atmosphere of Saturn’s moon Titan. The beauty of Cassini’s long and now extended mission is the numerous flybys the spacecraft is able to take of several of Saturn’s most interesting moons. We reported yesterday how scientists have been able to see how Enceladus’ surface and its geysers are changing over time, and now data collected during several recent flybys of Titan show alternations in that moon’s surface as well. “Cassini data have raised the possibility that Titan’s surface is active,” said Jonathan Lunine, a Cassini interdisciplinary scientist from the Lunar and Planetary Laboratory, at the University of Arizona, Tucson . “This is based on evidence that changes have occurred on the surface of Titan, between flybys of Cassini, in regions where radar images suggest a kind of volcanism has taken place.”

Rather than erupting hot, molten rock, it is theorized that the cryovolcanoes of Titan would erupt volatiles such as water, ammonia and methane. Scientists have suspected cryovolcanoes might populate Titan, and the Cassini mission has collected data on several previous passes of the moon that suggest their existence. Imagery of the moon has included a suspect haze hovering over flow-like surface formations. Scientists point to these as signs of cryovolcanism there.

What led some Cassini scientists to believe that things are happening now were changes in brightness and reflectance detected at two separate and distinct regions of Titan. Reflectance is the ratio of light that radiates onto a surface to the amount reflected back. These changes were documented by Visible and Infrared Mapping Spectrometer data collected on Titan flybys from July 2004 to March 2006. In one of the two regions, the reflectance of the surface surged upward and remained higher than expected. In the other region, the reflectance shot up but then trended downward. There is also evidence that ammonia frost is present at one of the two changing sites. The ammonia was evident only at times when the region was inferred to be active. Watch a video of the changes.

“Ammonia is widely believed to be present only beneath the surface of Titan,” said Robert M. Nelson of JPL, a scientist for Cassini’s Visual and Infrared Mapping Spectrometer team. “The fact that we found it appearing at times when the surface brightened strongly suggests that material was being transported from Titan’s interior to its surface.”

But other scientists aren’t certain that cryovolcanoes are responsible for the changes seen on Titan. Instead the changes might result from the transient appearances of ground “fogs” of ethane droplets very near Titan’s surface, driven by atmospheric rather than geophysical processes. Nelson has considered the ground fog option, stating, “There remains the possibility that the effect is caused by a local fog, but if so, we would expect it to change in size over time due to wind activity, which is not what we see.”

An alternative hypothesis to an active Titan suggests the Saturnian moon could be taking its landform evolution cues from a moon of Jupiter.

“Like Callisto, Titan may have formed as a relatively cold body, and may have never undergone enough tidal heating for volcanism to occur,” said Jeffrey Moore, a planetary geologist at the NASA Ames Research Center, Moffett Field, Calif. “The flow-like features we see on the surface may just be icy debris that has been lubricated by methane rain and transported downslope into sinuous piles like mudflows.”

But scientists will continue to analyze and collect more data in attempt to pinpoint exactly what is happening on Titan. Cassini’s next Titan flyby is scheduled for Dec. 21, when the spacecraft will come within 970 kilometers (603 miles) of its cloud-shrouded surface.

Source: JPL

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Saturn’s moon Enceladus not only has jets of water vapor spewing from vents on the southern hemisphere, but the moon’s surface in the same region shows evidence of changes over time, providing surprising indications of Earth-like tectonics. New high resolution images from the Cassini spacecraft’s recent flybys of Enceladus show close views of the moon’s distinctive “tiger stripe” fractures, yielding new insight into what may be happening inside the fractures. “Of all the geologic provinces in the Saturn system that Cassini has explored, none has been more thrilling or carries greater implications than the region at the southernmost portion of Enceladus,” said Carolyn Porco, Cassini imaging team leader.

A special spacecraft maneuver dubbed “the skeet shoot” was employed to make smear-free imaging at close range possible. The ground track of the camera’s pointing was selected to cut swaths across three tiger stripes, or sulci, the prominent rifts through which jets of water vapor and ice particles are actively jetting. The full-resolution images are absolutely astounding. Take a look at the large images and movies here.

Cassini’s flybys on Aug. 11 and Oct. 31 of this year targeted Enceladus’ fractured southern region, and an Oct. 9 flyby took the spacecraft deep into the plume of water vapor and ice shooting out of the moon’s vents. Interestingly, the plume is not constant varies over time. Scientists think that condensation from the jets erupting from the surface may create ice plugs that close off old vents and force new vents to open. The opening and clogging of vents also corresponds with measurements indicating the plume varies from month to month and year to year. This movie shows the locations of the vents on a “spinning” Enceladus.

“We see no obvious distinguishing markings on the surface in the immediate vicinity of each jet source, which suggests that the vents may open and close and thus migrate up and down the fractures over time,” Porco said. “Over time, the particles that rain down onto the surface from the jets may form a continuous blanket of snow along a fracture.”

The varying cloud of vapor and particles extends into space and has a far-reaching effect on the entire Saturn system by supplying the ring system with fresh material and loading ionized gas from water vapor into Saturn’s magnetosphere.

“Enceladus has Earth-like spreading of the icy crust, but with an exotic difference — the spreading is almost all in one direction, like a conveyor belt,” said Paul Helfenstein, Cassini imaging associate at Cornell University in Ithaca , N.Y. “Asymmetric spreading like this is unusual on Earth and not well understood.”

“Enceladus has asymmetric spreading on steroids,” Helfenstein added. “We are not certain about the geological mechanisms that control the spreading, but we see patterns of divergence and mountain-building similar to what we see on Earth, which suggests that subsurface heat and convection are involved.” This video demonstrates the observed tectonic spreading along tiger stripes in the South Polar Terrain of Enceladus.

The tiger stripes are analogous to the mid-ocean ridges on Earth’s seafloor where volcanic material wells up and creates new crust. Using Cassini-based digital maps of the south polar region of Enceladus, Helfenstein reconstructed a possible history of the tiger stripes by working backward in time and progressively snipping away older and older sections of the map. Each time he found that the remaining sections fit together like puzzle pieces.

With water vapor, organic compounds and excess heat emerging from Enceladus’ south polar terrain, scientists are intrigued by the possibility of a liquid-water-rich habitable zone beneath the moon’s south pole.

Cassini’s next flyby of Enceladus will be in November 2009.

The Cassini team presented their findings and recent images at the American Geophysical Union’s fall meeting in San Francisco.

Source: NASA, CICLOPS