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What came before the
Big Bang?

...a question that has been pondered by scientists and philosophers alike could soon be answered, thanks to a mathematical model that explains an anomaly in the early Universe...

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Water found 11 billion light years away

...the most distant occurrence of water yet seen in the Universe is in a galaxy more than 11 billion light years from Earth...

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Icy volcanism likely on Titan

...cyrovolcanoes could be spewing a super-chilled liquid into Titan’s atmosphere...

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Video archive

STS-120 day 2 highlights

Flight Day 2 of Discovery's mission focused on heat shield inspections. This movie shows the day's highlights.


STS-120 day 1 highlights

The highlights from shuttle Discovery's launch day are packaged into this movie.


STS-118: Highlights

The STS-118 crew, including Barbara Morgan, narrates its mission highlights film and answers questions in this post-flight presentation.

 Full presentation
 Mission film

STS-120: Rollout to pad

Space shuttle Discovery rolls out of the Vehicle Assembly Building and travels to launch pad 39A for its STS-120 mission.


Dawn leaves Earth

NASA's Dawn space probe launches aboard a Delta 2-Heavy rocket from Cape Canaveral to explore two worlds in the asteroid belt.

 Full coverage

Dawn: Launch preview

These briefings preview the launch and science objectives of NASA's Dawn asteroid orbiter.

 Launch | Science

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Cassini reveals Enceladus’ shifting terrain



Posted: 19 December, 2008

The latest fly-bys of Saturn’s dynamic moon Enceladus have revealed new signs that the south polar surface changes over time in an eerily Earthlike fashion.

The south polar region of Enceladus has long been a source of fascination to planetary scientists since jets of water vapour and icy particles were found to spew from vents along the moon’s distinctive ‘tiger stripe’ fractures, and now evidence has been presented to suggest that Earthlike plate tectonic activity is in action. "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," says Carolyn Porco, Cassini imaging team leader.

These two images compare a characteristic sea-floor spreading feature on Earth, known as a spreading ridge transform, to a very similar looking arrangement of “tiger stripe” rift segments in the south polar terrain region of Saturn’s moon Enceladus. Image: NASA/JPL/Space Science Institute.

The science team liken the spreading to Earth’s mid-ocean ridges where volcanic material wells up and creates new crust, but with one major difference: the spreading on Enceladus is almost all in one direction. "Asymmetric spreading like this is unusual on Earth and not well understood," says Paul Helfenstein, Cassini imaging associate at Cornell University. "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."

Using digital maps derived from Cassini fly-bys, 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 piece together like a jigsaw puzzle, strong evidence that the icy crust is indeed spreading apart over time.

These two images compare a "twisted" sea-floor spreading feature on Earth, known as an Offset Spreading Center (OSC), to a very similar looking twisted break, or axial discontinuity, in the Damascus Sulcus “tiger stripe” on Saturn’s moon Enceladus. Image: NASA/JPL/Space Science Institute.

The latest images also support the idea that condensation from the jets erupting from the surface may create ice plugs that close off old vents and force new vents to open. This idea also corresponds to measurements that imply that plume activity varies from month to month and year to year. "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," says Porco. "Over time, the particles that rain down onto the surface from the jets may form a continuous blanket of snow along a fracture."

With water vapour, 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, and with every fly-by they take a step closer to uncovering Enceladus’ secrets, but will have a year to wait until the next close up look.