Analog SFF, June 2007

by Dell Magazine Authors

  A lot appears to be going on there, but the most recent discovery is that it's snowing.

  The finding, announced in the March 10 issue of the journal Science by Robert Brown of the University of Arizona's Lunar and Planetary Laboratory, is one of many recent discoveries that have converted this icy chunk of outer Solar System real estate into one of the most exciting places ever studied.

  To begin with, Enceladus's snow appears to be creating one of Saturn's rings. And, as if that's not enough, scientists now think that the moisture originates from pools of water that may be the Solar System's best prospect for extraterrestrial life.

  The story began with a close flyby of Enceladus, in early 2005, in which Cassini's instruments detected oddities in the way in which the moon interacts with Saturn's magnetic field. The only plausible explanation was that Enceladus had a tenuous atmosphere containing ionized water. Other instruments found water vapor extending 180 miles into space—but only above the south pole. Presence of a vapor plume was further confirmed by watching changes in the light of a star that passed behind it.

  But seeing is believing. On November 27, the Cassini team took a long-range photo of Enceladus, backlit by the Sun. The angle of light was perfect to highlight a spreading cloud of ice particles, condensed from water vapor leaving Enceladus's surface. Not only that, but there were distinct jets that appeared to emanate from the tiger stripes.

  But why would the tiger stripes be spewing dust and vapor into space? The answer appears to lie in one of the most surprising finds from the probe's closest flyby, on July 14, 2005. As Cassini swept across Enceladus's south pole, it trained infrared cameras on the surface zipping by, barely 100 miles beneath it. What they found was a hot spot, precisely at the location of the stripe.

  That makes the pole, which should have been cold, the hottest place on Enceladus, says Torrence Johnson of NASA's Jet Propulsion Laboratory. The stripes are cracks, offering glimpses of “hot” ice underneath.

  That far from the Sun, of course, “hot” is a relative term. Most of Enceladus's southern reaches are about—315 degrees F. John Spencer of the Southwest Research Institute in Boulder, Colorado, estimates that in order to produce the infrared signatures seen from space, the hot spots have to be at least—260 degrees F—surprisingly warm for an airless worldlet, nearly ten times farther than we are from the Sun.

  The source of the heat is anybody's guess. Spencer has estimated that the total power output from the south polar region is between four and twelve gigawatts: a subterranean energy source equivalent to several large electrical power plants.

  There are two possible sources for all of that energy. One is decay of radioactive elements in subsurface rocks. But to produce enough heat, that would require an unusually radioactive ore body beneath the south pole, and nobody knows why that might be the case. Alternatively, interactions with other moons might generate frictional heat by repeatedly flexing Enceladus, like a child squeezing a rubber ball. Normally, such heat would be dispersed throughout the planet's interior, and there wouldn't be enough of it to produce the plume. Scientists therefore speculate that something in Enceladus's internal structure may cause much of that energy to be concentrated at the south pole.

  "We're looking at some kind of focusing,” Porco says. “We can't say why it would be at the south pole."

  The same factors may explain why the young, uncratered terrain is in the south, while the older, heavily cratered surfaces are in the north. “Perhaps that has something to do with the south being warmer and squishier,” Porco says. “Global symmetry is not what Enceladus is about."

  Below ground, of course, it's going to be warmer than—260 degrees . How much depends on what's producing the jets.

  Initially, there were two theories.

  One allowed it still to be quite cold beneath the ice. At temperatures above about—100 degrees F, ice undergoes a process called sublimation, in which it evaporates, without ever melting. This process is well known in cold, Earthly climes. North of the Arctic Circle in Greenland, villagers take advantage of it by hanging out wet laundry on dry winter days. First the laundry freezes. Then it dries, just as it would if hung on a clothesline in midsummer. But while sublimation can produce water vapor, it's a slow process: too slow to produce the density of ice particles seen in the escaping plume.

  That means the plume must be fed by pools of liquid water, boiling into space. Thus, the jets are rapidly freezing steam from a geyser-like process that Susan Kieffer, a geologist at the University of Illinois, has dubbed “Cold Faithful."

  JPL's Candice Hansen has calculated the rate at which Enceladus is venting water vapor, based on the amount of water Cassini's instruments have measured in the plume and the speed at which it appears to be moving. Her conclusion: Enceladus is blasting out 360 kilograms of water vapor per second—enough to fill a suburban swimming pool every couple of minutes. And that doesn't count the ice crystals, which can't be measured by her instruments.

  All of this is exciting for two reasons. One is that Enceladus lies in the heart of the mysterious E ring, which is so faint it wasn't discovered until 1979. Not only is the E ring tenuous, but it appears to be comprised almost entirely of extremely fine, dust-sized motes of ice. From the moment it was discovered, scientists suspected a connection between the ring and Enceladus, but nobody knew what it might be.

  Now they know. Some of the ice crystals and water vapor venting from the south polar jets is falling back to the surface, forming the fresh snow seen by Brown. But at least as much appears to be escaping from this tiny worldlet whose gravity is only 1.2 percent of Earth's.

  The E ring is losing water at the rate of about a kilogram per second due to chemical reactions with sunlight and collisions between particles. Enceladus appears to be pumping out more than enough water to keep the ring supplied, indefinitely. One of Saturn's many mysteries has been solved.

  Even more exciting, though, are the implications for astrobiology. That's because the water pools that feed the geysers probably lie only a few dozen meters below the surface. “That's really close,” Porco says.

  "Once you have liquid water, you have the potential for living organisms,” she adds. “That's why this has been so exciting. On this cold little moon we have an environment that is potentially suitable for living organisms."

  In fact, Porco says, all of the building blocks of life seem to be present. Not only is there liquid water and heat, but signs of organic chemicals potentially useful to life have been seen in the vicinity of the tiger stripes. And that makes Enceladus the most likely place in the solar system to have life—not something anyone would ever have predicted.

  * * * *

  Iapetus: A Two-Faced Walnut

  If there were a prize for Saturn-system mysteries, Iapetus would be the odds-on favorite. An ice/rock worldlet about 1,470 kilometers in diameter,[6] it has long been known to be weird, with one side ten times brighter than the other. Close views show that the dark material appears to lie atop the light material, as though sprinkled there from somewhere. But what it is and how it got there remain a mystery.[7]

  [FOOTNOTE 6: Internet searching reveals numerous, slightly differing figures for the diameters of Saturn's moons. This article uses the figures stated at the 2005 GSA meeting, presumably the most current.]

  [FOOTNOTE 7: Several of Saturn's icy moons appear to have thin coverings of dark material. Is it the same substance on all of them? If so, does it have a common origin? And what kind of process might sprinkle it across several moons? From a science-fictional perspective, the fun answer is “something blew up,” but that's wildly speculative.]

  But that's not Iapetus's greatest mystery. Not only is that worldlet divided east/west into light and dark hemispheres: it's also divided north and south by a vast seam, like nothing else in the Solar System.

  When I was a child, my family owned “Toas-Tite” irons: clamshell-shaped pieces of cast iron mounted on long handles, used to make hot sandwiches. You put the s
andwich inside, sealed it shut (crimping off the corners of the bread in the process) and heated it in a campfire to produce a remarkably tasty treat. The resulting sandwiches looked like a pair of mini-Frisbees cemented together, with a rim around the edge, where two halves of the clamshell met.

  Iapetus looks a bit the same, but rounder.

  Most folks think it looks like a walnut. What makes it unique is that it has a ridge, 10-20 kilometers high and at least as many wide, running nearly halfway around its equator. And like so many planetary features, the closer you get to it, the more complex it looks. Iapetus's equatorial ridge turns out to have multiple crests—in places, as many as three—running in parallel, with deep valleys between.

  At the December 2005 meeting of the American Geophysical Union, W. Ip of Taiwan's National Central University argued that the ridge is the result of a “collapsed” ring, which somehow fell onto the planet's equator. Maybe. The Saturn system is weird enough that it's unwise to discount any semi-feasible theory. But what the structure looks like is a pressure ridge. (The multi-ridge structure is particularly common in pressure ridges.)

  The leading hypothesis is that it was created by centrifugal forces during a slowdown in the planet's spin, early in its existence. (The fact that the ridge is right on the equator is a red flag, suggesting that whatever created it must have had something to do with the moon's spin.) Once upon a time, the theory goes, Iapetus probably had a fairly average spin. Now it's tide-locked to Saturn, rotating once every 90 days, so it always keeps the same face toward its primary, just like Earth's moon. The idea is that the forces that slowed it down somehow caused it to squirt up that big ridge.

  Unfortunately, nobody's been able to produce a decent mathematical model of how this could happen. It's possible to design models that create ridges, but they require the underlying material to be soft and fluid enough that the ridge should have subsided under its own weight, once the spin had slowed.

  Another prospect is that the ridge is a tectonic feature, caused by shrinkage of Iapetus's surface as the planet cooled. This would allow lava to erupt from below, creating a range of massive volcanoes. Alternatively, shifts in plate segments might have caused the northern and southern halves of the planet's crust to press against each other. On Earth, the Himalayas are produced by such a collision, and despite Earth's much higher gravity, they have reached impressive heights.[8]

  [FOOTNOTE 8: We still need to explain why this feature lies so precisely on the equator. I'm not a geologist, but I've been around enough geologists to be willing to offer my own speculation: perhaps the ridge was created by a two-step process. First, the spin-slowdown created a weakness at the equator. Then subsequent mountain-building tectonics occurred along the same line of weakness. It's probably wrong, but if it does turn out to be right, you read it first, here!]

  * * * *

  Cyclops and the Death Star

  Sometime early in their histories, two of Saturn's moons really got clobbered. One was Mimas (diameter 398 kilometers), which bears an enormous crater, one-quarter its diameter, that caused it to be dubbed “the Death Star World” because of its remarkable similarity to the spaceship of Star Wars fame. As of this writing, not much else about it is known.

  The other cyclops world is Tethys, diameter 1,072 kilometers. Its impact crater is called Odysseus and it, too, produces a world that looks like a giant eyeball, staring off into space, although the effect is not as dramatic because Tethys's bigger size produces enough gravity that the planet has slowly “relaxed” back to a more spherical shape, smoothing out the crater's topography.

  Big impact craters are spectacular, but planetary scientists are more interested in mountains and valleys because these are indicative of other types of processes at work. Tethys has a fascinating one: a huge valley several kilometers deep and 100 kilometers wide that runs three-quarters of the way around the planet.

  The valley has nothing to do with Odysseus. Rather, it appears to be a very old feature: much like the rim that circles Iapetus, but sunken rather than raised. Geologically, it looks like a graben, which is a deep valley created when a chunk of a planet's crust collapses along parallel faults. You can find such features in America's Great Basin and Africa's Rift Valley, where tectonic forces have attempted to rip continents apart. But nothing on Earth comes remotely close to matching the graben on Tethys.

  If you're looking for explanations, the simple one is that something caused Tethys's surface to contract and tear apart. But at this point, it's anyone's guess.

  * * * *

  Wispy Dione

  As long as we're talking about planet-girdling tectonic features, we should also pay a brief visit to Dione, diameter 1,206 kilometers.

  * * * *

  Hyperion, as viewed by Cassini.

  * * * *

  On first glance, it looks like an icy version of Earth's moon. But it isn't uniformly cratered, indicating that portions have been active sometime in the relatively recent past. Its most interesting trait is terrain that looked “wispy” in the Voyager pictures. Higher resolution photos now indicate that these gauzy bands aren't rays from big impact craters or deposits from geyser-like volcanoes. Rather, they appear to be belts of crevasses or fractures, running long distances across the surface. You can even see bright, clean-looking material spilling down from the tops of these scarps like rocks scaling off earthly cliffs. In places, the wispy terrain's fractures cut across craters, indicating that the planet was tectonically active more recently than those particular craters were formed.[9]

  [FOOTNOTE 9: The highest-resolution photos also indicate that in places there is a finer fabric of smaller fractures angling across the big ones. The small ones appear to have come first, indicating that Dione has gone through at least two phases of tectonic activity.]

  The tectonics of the Saturn system won't be fully understood until we have a theory that explains why you get an enormous ridge on Iapetus, a huge rift valley on Tethys, and “wisps” on Dione.[10]

  [FOOTNOTE 10: A moon we haven't discussed is Rhea, diameter 1,500 kilometers. As of this writing, Cassini has yet to make a close visit to it, but from a distance, it shows no sign of major tectonic features. If that proves to be the case on closer inspection, then in its case, it will be the absence of such features that will have to be explained.]

  * * * *

  Floating Rocks

  On my bookshelf, I have a potato-sized chunk of a rock called pumice. It has the unique property that if you put it in a bucket of water, it floats.

  If you could find a big enough bucket, several of Saturn's smaller moons might do the same. These “under-dense” worlds have densities as low as half a gram per cubic centimeter, which is half the density of liquid water, still a lot less dense than ice.[11]

  [FOOTNOTE 11: You can calculate a planet's density by knowing its volume (easily measurable) and its mass (determined by how its gravity affects other objects, such as your spaceship.)]

  What a setting these worlds would make for an adventure story! Their low densities indicate that they must have the consistency of Swiss cheese. It's possible, of course, that the bubbles simply come from a frothy rock, like pumice. But they could also be caves. And if they're big enough, they might make great hideouts for bandits or serve as ready-made prospecting tunnels running deep into the subsurface. Only small moons can have these features because larger ones would crush them beneath the weight of the overlying rock and ice. But explorers would need to be careful because even in microgravity, having an entire world collapse onto you would be a bad thing!

  The best studied of the under-dense worlds is Hyperion, diameter 282 kilometers. It's also one of the strangest objects in the entire Saturn system.

  Hyperion made headlines when Cassini made a flyby ... and released photos of an object whose surface looks like a honeycomb, or perhaps a big chunk of coral.

  Within weeks, scientists were tentatively suggesting that these bizarre features might be suncups.

 
Suncups can be found on earthly snowfields, where they are the result of uneven solar heating. The process typically starts when a dark, sun-warmed rock begins melting into the snow. As a depression forms, it acts as a reflecting oven, capturing more and more sunlight and melting ever deeper. By late summer, suncups can be hip deep on the upper slopes of mountains such as Washington's High Cascades. Hikers hate them.

  Close views of Hyperion show that its surface is highly cratered, with the crater walls including outcrops that spill dark talus onto the light-colored material of the crater floor. The hypothesis is that the dark material heats the underlying ice, gradually converting a small crater into an enormous suncup, many kilometers deep and wide.

  For this process to work at this scale, you need two things: an ice that vaporizes at the right temperatures, and a small moon. If the moon is too large, you wouldn't get the deep, honeycomb-shaped craters because gravity would cause it to relax into a more spherical shape.

  * * * *

  Titan: Cryovolcanic Badlands

  As the largest moon in the Solar System, and the only Saturnian satellite with a dense atmosphere, Titan will always be an object of special interest. Partly that's because of its atmosphere, which blocks visibility like a bad day in Los Angeles: not being able to see what's down there makes it all the more intriguing. But it's also because Titan is big: 5,150 kilometers in diameter—slightly bigger than Mercury. It was discovered in 1655 by Dutch astronomer Christiaan Huygens, and is big enough to be a planet in its own right.

  Titan is also interesting because of the methane in its atmosphere. Chemically, it shouldn't be there because, in Titan's upper atmosphere, ultraviolet light from the Sun should long ago have destroyed it. The fact that there is methane means it's being replenished from somewhere, probably via cryovolcanism, about which we'll say more in a moment.

  The methane is also interesting because, at Titan temperatures, it pays a role similar to water in the Earth's atmosphere: forming clouds (which can be seen on Cassini flybys) and precipitating as rain or snow. Methane rain and melting methane snow should scour the landscape like flowing water, before evaporating back into the atmosphere.