NASA Zeroing in on Saturn’s Enceladus in Search for Life

502721main_pia13620-466 New images and data from NASA's Cassini spacecraft have given scientists a unique view of active fissures through the south polar region of Saturn's moon Enceladus, revealing for the first time a more complicated web of warm fractures. The images and maps come from the Aug. 13, 2010, Enceladus flyby, Cassini's last remote sensing flyby of the moon until 2015.


Scientists working jointly with Cassini's composite infrared spectrometer and its high-resolution imaging camera have constructed the highest-resolution heat intensity maps yet of the hottest part of a region of long fissures spraying water vapor and icy particles from Enceladus. These fissures have been nicknamed "tiger stripes."

Additional high-resolution spectrometer maps of one end of the tiger stripes Alexandria Sulcus and Cairo Sulcus reveal never-before-seen warm fractures that branch off like split ends from the main tiger stripe trenches. They also show an intriguing warm spot isolated from other active surface fissures.

"The ends of the tiger stripes may be the places where the activity is just getting started, or is winding down, so the complex patterns of heat we see there may give us clues to the life cycle of tiger stripes," said John Spencer, a Cassini team scientist based at Southwest Research Institute in Boulder, Colo.

The geometry of the many flybys between now and 2015 will not allow Cassini to do thermal scans like these, because the spacecraft will be too close to scan the surface and will not view the south pole. This Enceladus flyby, the 11th of Cassini's tour, also gave Cassini its last look at any part of the active south polar region in sunlight.

Spencer said he isn't sure if this tiger stripe is just more active than it was the last time Cassini's spectrometer scanned it, in 2008, or if the hottest part of the tiger stripe is so narrow that previous scans averaged its temperature out over a larger area. In any case, the new scan had such good resolution, showing details as small as 800 meters (2,600 feet), that scientists could see for the first time warm material flanking the central trench of Damascus, cooling off quickly away from the trench.

The Damascus thermal scan also shows large variations in heat output within a few kilometers along the length of the fracture. This unprecedented resolution will help scientists understand how the tiger stripes deliver heat to the surface of Enceladus.

Cassini acquired the thermal map of Damascus simultaneously with a visible-light image where the tiger stripe is lit by sunlight reflecting off Saturn. The visible-light and thermal data were merged to help scientists understand the relationships between physical heat processes and surface geology.

A potential life-harboring mother lode was discovered at the Enceladus' South Pole, said Carolyn Porco, director of flight operations and imaging team leader for the Cassini spacecraft in orbit around Saturn, describing Cassini’s findings of elevated temperatures in the moon’s polar region, as well as an enormous plume of icy particles shooting tens of thousands of kilometers into space.

Analysis of the icy trail, which includes water vapor and trace amounts of organic materials such as methane, carbon dioxide, and propane, suggests it is fueled by geysers erupting from a pocket of salt water within the moon.

The findings, noted Porco, point to the possibility of “an environment where life itself might be stirring. Should we ever discover that a second genesis had occurred in our solar system, independently outside the Earth,” she added, “then I think at that point the spell is broken. The existence theorem has been proven, and we could safely infer from it that life was not a bug but a feature of the universe in which we live, that it’s commonplace and has occurred a staggering number of times.”

On the surface, Saturn's icy moon Enceladus is one of the oddest places in our solar system to look for extraterrestrial life.  Located in the frigid outer solar system, it should have frozen solid billions of years ago.

Unlike Mars or Jupiter's moon Europa, which give hints that they might harbor liquid water beneath their surfaces. With a diameter only slightly more than 500 miles, Enceladus just doesn't have the mass needed for its interior to stay warm enough to maintain liquid water underground.

Although its surface temperatures hover around 324 degrees below zero Fahrenheit, in 2005 NASA's Cassini spacecraft discovered a giant plume of water shooting up from cracks in the surface over the moon's south pole, indicating that there was a perhaps a global ocean of water beneath the ice.

Analysis of the plume by Cassini revealed that the water is salty, scientists estimate from the Cassini data that the south polar heating is equivalent to a continuous release of about 13 billion watts of energy generated by radiation coupled with tidal heating. During the formation of the solar system, if Enceladus was able to gather greater amounts of rock, which contained radioactive elements, enough heat could have been temporarily generated by the decay of the radioactive elements in its interior to melt the body.

But to  keep Enceladus warm enough for liquid water to remain under its surface it's theorized that Enceladus' slightly oval-shaped orbit generates heat from friction deep within Enceladus, called gravitational tidal forcing.

The gravitational tides also produce stress that cracks open the surface ice  at the south pole, opening and closing the cracks by shearing them back and forth generating friction, which releases heat.

To test the tidal heating theory, scientists with the Cassini team overlaid a map of the gravitational tidal stress on the moon's icy crust to a map of the warm zones created using Cassini's composite infrared spectrometer instrument (CIRS).

"However, they don't exactly match," says Dr. Terry Hurford of NASA's Goddard Space Flight Center, Greenbelt, Md. "For example, in the fissure called the Damascus Sulcus, the area experiencing the greatest amount of shearing is about 50 kilometers (about 31 miles) from the zone of greatest heat."

Hurford and his team believe that this discrepancy is caused by Enceladus' rotation rate, which wobbles slightly as it rotates.  "Cassini observations have ruled out a wobble greater than about 2 degrees with respect to Enceladus' uniform rotation rate," says Hurford.

The team created a computer simulation that made maps of the surface stress on Enceladus for various wobbles, and found a range where the areas of greatest stress line up better with the observed warmest zones.

"Depending on whether the wobble moves with or against the movement of Saturn in Enceladus' sky, a wobble ranging from 2 degrees down to 0.75 degrees produces the best fit to the observed warmest zones," said Hurford.

The wobble also generates about five times more heat in Enceladus' interior than tidal stress alone, and the extra heat makes it likely that Enceladus' ocean could be long-lived, according to Hurford. This is significant in the search for life, because life requires a stable environment to develop.

"Enceladus is not completely spherical, so as it moves in its orbit, the pull of Saturn's gravity generates a net torque that forces the moon to wobble," said Hurford. Also, Enceladus' orbit is kept oval-shaped, maintaining the tidal stress, because of the gravitational tug from a neighboring larger moon Dione. Dione is farther away from Saturn than Enceladus, so it takes longer to complete its orbit. For every orbit Dione completes, Enceladus finishes two orbits, producing a regular alignment that pulls Enceladus' orbit into an oval shape.

Enceladus-Geysers

Casey Kazan via NASA/JPL/Cassini

The new images are available at: http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.
More details are also available at the imaging team's website http://ciclops.org and the composite infrared spectrometer team's website http://cirs.gsfc.nasa.gov .

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