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Planetary News: Cassini-Huygens (2006)

Liquid Water at Enceladus? The Case Grows Stronger

By Emily Lakdawalla
March 9, 2006
Fountains of Enceladus
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Fountains of Enceladus
On November 27, 2005, Cassini captured a series of images of Enceladus from its night side. The back-lit view lights up fine particles streaming from Enceladus' south-polar geysers. Credit: NASA / JPL / Space Science Institute

The Cassini science team announced today the possibility -- but not proof -- of liquid water very close to the surface of Saturn's moon Enceladus.  The announcement occurred just prior to the March 10 release of a raft of articles about this tiny, dynamic moon in the journal Science.  "We realize that this is a radical conclusion -- that we may have evidence for liquid water within a body so small and so cold," said Carolyn Porco, leader of the Cassini imaging team. "However, if we are right, we have significantly broadened the diversity of solar system environments where we might possibly have conditions suitable for living organisms."  The conclusion is also radical because, if true, Enceladus would possess liquid water closer to its surface than any other body in the solar system except Earth.

That Enceladus is a dynamic place has been known since last July, when Cassini completed the third of a series of very close flybys of the moon.  Nearly all of Cassini's science instruments contributed to the discovery of a thin atmosphere only at Enceladus' south pole, followed by the thrilling discovery of the source of that atmosphere: geysers spewing forth from south polar features dubbed "tiger stripes" by the scientists.

"We actually flew through the geysers that are producing this gas, and imaged these geysers emanating from the south polar region of Enceladus, spraying ice particles and water vapor, escaping from Enceladus and into the magnetosphere around Saturn, filling it up with water vapor and water products, effectively producing a south polar jet from this little moon," says Torrence Johnson, a member of the Cassini imaging team and a veteran of the Voyager mission.  "This is the type of thing that scientists who study the solar system really live for."

Now, 8 months after the last of the three close encounters, the science teams have had time to explore the data they gathered, and there have been further surprises.  For instance, one of the key observations that indicated that something very interesting was happening in the interior of Enceladus was the detection of unusually high temperatures near those "tiger stripe" features.  A closer look at the data, says Composite Infrared Spectrometer team member John Spencer, has yielded even higher temperatures than they saw in their first look at the data.  "In July we were talking 125 kelvins, now the highest we've actually seen in our entire data set is 145 kelvins," Spencer said.

These are still extremely cold temperatures; the Kelvin scale begins at absolute zero, and the freezing temperature of water is at 273 kelvins.  But most of Enceladus is at a temperature of around 70 kelvins (-200 Celsius or -330 Fahrenheit), making a measured temperature of 145 kelvins a true hot spot.  Spencer hopes to see hotter temperatures on one final flyby, planned for an altitude of only 25 kilometers (15 miles) in 2008.  "When we fly past Enceladus again, we'd like to look down the throat of these cracks and see 273-kelvin temperatures down there."

Temperatures across an Enceladan 'tiger stripe'
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Temperatures across an Enceladan 'tiger stripe'
One of the types of measurements that the Cassini Composite Infrared Spectrometer can make is to measure the temperatures of the cold, icy surfaces of Saturn's moons and rings. Two of its three components consist of linear arrays of ten detectors sensitive to thermal infrared radiation that is emitted by these cold surfaces. On the July 14, 2005 flyby of Enceladus, CIRS captured this measurement of temperatures across one of the little moon's south polar "tiger stripe" features. The temperatures are in Kelvins, and have measurement uncertainties indicated. (CIRS has difficulty measuring temperatures lower than 75 Kelvins, so the uncertainties for the lowest temperatures are large). The CIRS detector that sampled a tiger stripe (center, orange) found a temperature of 90 Kelvins (-183 Celsius, -297 Fahrenheit). This temperature is an average across the whole area underneath the square, which is 6 kilometers (3.7 miles) across; the temperature in the center few hundred meters of the fissure might be as high as 145 Kelvins (-128 Celsius, -199 Fahrenheit), according to the CIRS team. Credit: NASA / JPL / GSFC / Space Science Institute

This is powerful thermal activity for a body that, at 504 kilometers (313 miles) in diameter, is only the sixth largest moon of Saturn and smaller even than the three largest asteroids (Ceres, Pallas, and Vesta).  In fact, Johnson said, "The thermal power output from the south polar region is sufficient to make us believe that there may be liquid water just tens of meters, or a kilometer, beneath the surface of that region."  But an explanation of just how it could be so hot is still eluding scientists.  "Enceladus is so tiny that it's unlikely that radiogenic heating could do much of anything, so tidal heating is the only plausible heat source," Spencer said.  "But then you must explain why Mimas, which should have a lot more tidal heating, has absolutely none.  It's closer in [to Saturn], and its orbit is way more eccentric."  Mimas is only slightly smaller than Enceladus, at 418 kilometers (260 miles) diameter.

Further information about the Cassini science team's new take on Enceladus will come out in the Science articles, and will be followed on March 15, by five presentations by members of the science team at the annual Lunar and Planetary Science Conference in Houston, Texas.  The Planetary Society will be there to find out what the science team is concluding from the wealth of data returned by Cassini, and will follow up with a more detailed report here at planetary.org.  One thing is clear: Cassini has proved that the Saturn system, containing youthful and active worlds like Enceladus and Titan, is just as exciting a destination as Jupiter and its moons.