Cassini unveils the wintry world of Enceladus


The Cassini-Huygens mission has just returned to Saturn's icy moon Enceladus for a flyby of its previously unseen northern region.  Bill Condie charts the mission's latest discoveries.


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On Oct. 9, 2008, just after coming within 25 kilometers (15.6 miles) of the surface of Enceladus, NASA's Cassini captured this stunning mosaic as the spacecraft sped away from this geologically active moon of Saturn.
NASA / JPL / Space Science Institute
This view from NASA's Cassini spacecraft shows battered terrain around the north pole of Saturn's icy moon Enceladus. Craters crowd and overlap each other, each one recording an impact in the moon's distant past. – NASA / JPL-Caltech / Space Science Institute

The last time NASA's Cassini spacecraft flew by Saturn's moon Enceladus its northern regions were shrouded in darkness during the depths of winter. Now, in the final days of its mission, Cassini is back and delivering images of the moon's north pole in unprecedented detail, taken from just 1,839 kilometres above the surface.

Before this flyby, scientists expected the north polar region to be heavily cratered, based on low-resolution images from the Voyager mission, and they weren't disappointed. The craters were there, but a more complex picture began to emerge. The surface also displays an unexpected and circuitous pattern of cracks and fractures, as can be seen in the images below.

"The northern regions are crisscrossed by a spidery network of gossamer-thin cracks that slice through the craters," said Paul Helfenstein, a member of the Cassini imaging team at Cornell University. "These thin cracks are ubiquitous on Enceladus, and now we see that they extend across the northern terrains as well."

NASA's Cassini spacecraft zoomed by Saturn's icy moon Enceladus on Oct. 14, 2015, capturing this stunning image of the moon's north pole. – NASA / JPL-Caltech / Space Science Institute
NASA's Cassini spacecraft spied this tight trio of craters as it approached Saturn's icy moon Enceladus for a close flyby on Oct. 14, 2015. The craters, located at high northern latitudes, are sliced through by thin fractures -- part of a network of similar cracks that wrap around the snow-white moon. – NASA / JPL-Caltech / Space Science Institute

That is an exciting discovery as scientists believe the fractures may be the result of surface movement and more evidence of a vast ocean beneath the frozen surface of the moon.

Earlier observations by Cassini revealed a fine spray of water vapour, icy particles and simple organic molecules from fractures near Enceladus' south pole, as can be seen in the images below. They believe that is being fed by a vast liquid water reservoir beneath the surface.

That supported earlier evidence of the ocean when researchers found the magnitude of the moon's very slight wobble, as it orbits Saturn, could only be accounted for if its outer ice shell is not frozen solid to its interior.

Cassini imaging scientists used views like this one to help them identify the source locations for individual jets spurting ice particles, water vapor and trace organic compounds from the surface of Saturn's moon Enceladus. – NASA / JPL / Space Science Institute

The basin of fractures near Enceladus' south pole. – NASA / JPL-Caltech / Space Science Institute

Cassini's next encounter with Enceladus is planned for later this week when the spacecraft will come within about 50 kilometres of the moon's south polar region. The spacecraft will dive deeply through the plumes of icy spray in a bid to understand the chemistry of the ocean beneath the ice.

Scientists hope the data will provide evidence of how much hydrothermal activity is occurring in the moon's ocean. This in turn may give insights into the possibility that Enceladus could harbour life.

Cassini's final close Enceladus flyby will take place on 19 December at an altitude of 5,000 kilometres when it will measure the amount of heat coming from the moon's interior. It will give the fullest picture yet of this beautiful, ice-bound moon.

A masterpiece of deep time and wrenching gravity, the tortured surface of Saturn's moon Enceladus and its fascinating ongoing geologic activity tell the story of the ancient and present struggles of one tiny world. This is a story that is recounted by imaging scientists in a paper published in the journal Science on March 10, 2006. – NASA / JPL / Space Science Institute
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