Enceladus: Saturn’s sprightly moon looks young
NASA / JPL / Space Science Institute
Geologically active ‘tiger stripes’ at the south pole of Enceladus
NASA’s flybys of Saturn’s moon Enceladus by their Cassini spacecraft reveal more evidence of an active world that continues to confound scientists.
William Herschel (1738–1822) discovered this moon in 1789 on his first night of viewing with the large new telescope he had built. Little was known about Enceladus until the 1980s, when the Voyager 1, Voyager 2 and Pioneer 11 spacecraft flew by Saturn.1
The images they returned to Earth were astounding. They showed the moon was only about 500 km (300 miles) in diameter and that it reflects almost 100% of the light it receives from the sun.
Most scientists expected Saturn’s moons to be cold and dead, and their surfaces to be covered with impact craters from exposure to meteors for billions of years. That’s because they believed that the sun and all the planets of our solar system formed when a hypothetical solar nebula condensed some 4.5 billion years ago.
Instead, the images from Voyager 2 revealed that Enceladus’ surface was mostly smooth. Scientists still asserted that the moon was billions of years old, but said it must have acquired a new surface in the recent past. NASA scheduled more space visits to investigate.2
The Cassini spacecraft was launched in 1997 but did not arrive at Saturn until 2004. It was 2005 before the cameras turned toward Enceladus. Since then there have been numerous flybys with spectacular images and more surprises.
How can such a small moon have enough heat to power the geysers?
The first was that powerful geysers were blasting water vapour and tiny ice particles hundreds of kilometres into space from vents near the South Pole. Enceladus should have been dead but it is still geologically active.
Scooting over the southern polar region, the Cassini spacecraft captured close-up images of distinctive fractures described as “tiger stripes”, V-shaped cracks up to 300 m (1,000 ft) deep.
This is the area with the powerful geysers. Such is the immense pressure behind them that the jets of water vapour and icy particles are spewed into space as fast as a supersonic jet. Infrared measurements estimated the energy produced by the geysers at 5.8 gigawatts—enough to power a city with 6 million people.
Subsequent flybys in 2008 captured even more detail, and a few daring manoeuvres took Cassini close enough to pass through the plume, collect samples and analyze them.
This tiny moon seems to have an enormous influence on the whole ring system of Saturn.
NASA / JPL / Space Science Institute
Enhanced colour image of geysers spraying water vapour far into space
First, the jets produce the material for the outermost E ring—a 300,000-km-wide (200,000 miles) donut-shaped mist of micron-sized particles. The gas from the jets is also ionized and tends to drag the magnetosphere so strongly that it distorts our measurements of Saturn’s rotation. And even more surprisingly, the plasma cloud around Saturn is being drawn into the brighter A ring, much closer to the planet.
But how can such a small moon have enough heat to power the geysers?
One suggestion was that radioactive substances inside the moon are heating it. But even though Enceladus is the rockiest of Saturn’s major moons, it could not contain enough radioactive material to produce the observed heat.3
Another idea is tidal heating. Just as our moon distorts the shape of the earth and creates ocean tides, the gravitational pull of Saturn changes the shape of Enceladus as it orbits every 1.37 days. It heats the moon like a metal wire heats when you bend it backwards and forwards.
But why is Enceladus still flexible? After 4.5 billion years the moon should be frozen solid and tidal friction should not work. When James Roberts and Francis Nimmo of the University of California modelled the moon’s interior, they found it would have frozen after only 30 million years—less than 1% of its supposed age.4
What about the effects of Enceladus’ out-of-round orbit imposed on it by Dione (a larger nearby moon of Saturn)? Again, no. Still not enough heat.5
Is it possible that Enceladus contained the heat when Saturn and its moons formed, and it has not cooled down yet? That is a simple idea and would explain the observations without any complicated mental gymnastics. But that would mean that Saturn and its moons are not billions of years old.
Such ideas are off limits. Most space scientists won’t entertain these options because they have a “sacred” secular belief that the solar system is billions of years old.
But the enigmatic moon of Saturn, with its deep, icy chasms and spectacular, shimmering fountains, fits beautifully with a solar system created just 6,000 years ago as biblical history records.
- Vittorio, S., Cassini visits Enceladus—new light on a bright world, <csa.com/discovery guides/enceladus/review.pdf>, July 2006. Return to text.
- Porco, C., The restless world of Enceladus, Scientific American 299(6):26–35, p. 26, December 2008. Return to text.
- Ref. 2, p. 31. Return to text.
- Schirber, M., Frigid future for ocean in Saturn’s moon, Astrobiology Magazine, <space.com/scienceastronomy/080619-am-enceladus-ocean.html> 19 June 2008. Return to text.
- Ref. 2, p. 32. Return to text.
Thanks for your prompt and convincing response to my posting. It is interesting to know that NASA acknowledges that such erruptions could not have been going on continuously for aeons and as you say, it is surely "special pleading" to suggest that Cassini was lucky to pick them up on this occasion.
Sirs would you care to comment on the extract from a NASA blog below.
Tidal forces acting on fault lines in the moon's icy shell cause the sides of the faults to rub back and forth against each other, producing enough heat to transform some of the ice into plumes of water vapor and ice crystals, according to a new study published in the May 17 issue of the journal Nature.
Francis Nimmo, assistant professor of Earth and planetary sciences at the University of California, Santa Cruz, and his co-authors calculated the amount of heat that could be generated by this mechanism and concluded that it is the most likely explanation for the plumes and other features observed in the south polar region of Enceladus. This region is warmer than the rest of the frozen surface of Enceladus and has features called "tiger stripes" that look like tectonic fault lines.
"We think the tiger stripes are the source of the plumes, and we made predictions of where the tiger stripes should be hottest that can be tested by future measurements," Nimmo said.
Driving the whole process is the moon's eccentric orbit, which brings it close to Saturn and then farther away, so that the gravitational attraction it feels changes over time.
"It's getting squeezed and stretched as it goes around Saturn, and those tidal forces cause the faults to move back and forth," Nimmo said.
Unlike some other proposals for the origin of the plumes, this mechanism does not require the presence of liquid water near the surface of Enceladus, noted co-author Robert Pappalardo of NASA's Jet Propulsion Laboratory in Pasadena, Calif.
"The heat is sufficient to cause ice to sublimate, like in a comet -- the ice evaporates into vapor, and the escaping vapor drags particles off into space," Pappalardo said.
The essential point being that this explanation does not need the whole planet to be flexible to produce heating, just the "Tiger stripe" fault lines.
The above seems to be?? a convincing explanation for this remarkable discovery.
That may be a plausible explanation. Of course it involves many assumptions about features and structures that are wholly unobserved and other information may cause it to be abandoned down the track.
However, this explanation does not change the fact that the plumes could not have been erupting for billions of years or the volatiles would have long been gone. That is why scientists expected the moon to be cold, dead and uninteresting.
In order to preserve the long ages Nimmo has proposed that these eruptive periods are rare, and for most of the time the moon is inactive (See Icy Saturn Moon Burps Up Heat and Ice). Supposedly, it was just amazingly lucky that Cassini happened to fly past when the eruptions were in full flow. This is clearly a case of special pleading just to keep the time-frame in tact.