Analog Science Fiction and Fact - September 2014

by Penny Publications

  "Oh no," Luis said, giving Marjan a stricken look. "Have you checked your Proctor, Marjan? You can't be serious."

  "My Proctor's green," Marjan said. "15 percent chance of irrationality, which is the best I've done all day. You see it, don't you? There's an obvious compromise, which gives both sides what they want."

  "Just so we're clear," Luis said, pushing his tray away from him, "you're suggesting we offer the Gammans a blood-f iltering Proctor which they can... what? Use to trigger shornoth at controlled intervals?"

  "Right," Marjan said triumphantly, and watched her proctor's assessment of her irrationality spike to 20 percent based on her exuberance. "No more hydroponics rampages, or broken memory sticks, or containment breaches. Which you know they've had, if they can flip out over an uncomfortable chair."

  "No more dismembered junior diplomats," Luis said, his gaze unfocused. "Oh, Pua'a will like that. You know he thinks putting a price on sapient life is barbaric."

  "You think we might be able to get rid of the interclan database altogether?" Marjan asked.

  Luis made a face. "Dunno. They'll still be smashing furniture, won't they?"

  Marjan thought about that for a moment. "Maybe we can sell them some that's meant to get broken up and reassembled."

  Pua'a beamed benevolently over the proceedings as Koorsor and Tsak held up the symbolic first Proctors for the Gamman species, baring broad teeth for the recording devices. It was an historic moment, and the two human junior diplomats were in the background. At the crucial still-image moment, Luis even had his head turned away to talk to one of the xenomedical staff.

  But the Gammans were more appreciative of their part in the arrangements than the verdict of history was likely to be. Tsak cornered them after all the recording was done and the real Proctors—long since implanted, and far less shiny than the symbolic ones—had been switched on.

  "It was you who thought of these, wasn't it?" she said. "You hoomans."

  "How did you know?" asked Luis. "It was mostly Marjan," he added hastily. "I only helped a little."

  Tsak's broad teeth flashed again. "The Sophonts know a lot. Koorsor is very impressed with them. Of course, I am too. But they do without the Proctors. I don't think they really understand what it means to have one. I think it takes another—another supposedly semisapient race to understand that."

  Marjan smiled back at her. "I hope it won't offend you if I tell you that part of my motivation was keeping you from ripping my head off."

  "Not at all," said Tsak. "That's the basis of all diplomacy."

  * * *

  SATURN'S "JET-PROPELLED" MOON AND THE SEARCH FOR EXTRATERRESTRIAL LIFE

  Richard A. Lovett | 5081 words

  Science Fact

  It was a shame, Aimee Miller thought, that the first alien life form discovered by humanity had to be killed in the process of looking at it. But such was the way with electron microscopy: in order to see the alien cell in enough detail, it was necessary to hit it with radiation guaranteed to kill it. Just as, afterward, it was necessary to chemically pick it apart to prove that this tiny bit of one-time life melted out of an ice grain wasn't something that had somehow hitchhiked from Earth.

  But if there was one microbe, hopefully there were more. Aimee wasn't sure exactly where in its elaborate gravitational dance with Saturn's moons the Saturn orbiter currently was, but one thing was clear. The tiny grain of ice it had already plucked from the giant planet's tenuous E ring would forever change humanity's view of its place in the Universe. And as the mission progressed, the orbiter would be making repeated flybys of the mother lode of enigmatic ice grains:the tiny moon Enceladus, where for millions of years ice geysers had spewed microscopic snowflakes from underground cauldrons now proven to be cradles of life beyond Earth.

  Simply stated, this small, frozen worldlet, a moon the size of England, had just become the hottest place in the Solar System.

  Far fetched? Not as much as you might think. The most exotic part of this scenario might be Aimee's fancy bio-lab, which can do complex analyses no space mission has yet dreamed of. As for seeking life at Saturn? Many scientists think that might actually be the best (and easiest), place to look.

  Sure, NASA's Curiosity rover has repeatedly confirmed what many have suspected: that once upon a time, Mars was habitable. But that was billions of years ago. If you switch the question to "Where, right now, might we find conditions suitable for life?" Mars becomes less interesting. In fact, the best options might be in places that barely crossed astrobiologists' radar as little as a few decades ago.

  In Larry Niven's classic Known Space series, the first interstellar colonies were scouted by robotic probes designed to search out habitable worlds. Due to a programming error, however, what the probes reported as habitable planets turned out simply to be habitable "places"—small niches on otherwise bleak planets.

  In Niven's stories, this was something of a running joke, as unsuspecting colonists were forced to adapt to worlds a lot less Earthlike than anticipated. But Niven's little gag may have been unexpectedly visionary. When it comes to seeking potential abodes for extraterrestrial life, the approach we've been taking, with its focus on Mars-roving and exoplanet-hunting, might be unnecessarily terra-centric (at a minimum, planet-centric). There might, in fact, be many exotic places where life could exist. One of the first to be recognized was Jupiter's moon Europa, which appears to have a planet-girdling sea beneath its icy surface, but it's possible to spin an argument for many such seas, even including one deep beneath the surface of Pluto. 1

  But for the moment, the astrobiological spotlight is on a much more accessible world: Enceladus—a three-hundred-mile-wide moon of Saturn that many think might be the Solar System's best place to look for extraterrestrial life. "It has liquid water, organic carbon, nitrogen [in the form of ammonia], and an energy source," says Chris McKay, an astrobiologist at NASA's Ames Research Center, Moffett Field, California. "There is no other environment in the Solar System [except Earth] where we can make all those claims."

  Superficially, Enceladus seems a singularly uninteresting place to visit. White, round, and icy, it looks from a distance like nothing more than an overgrown cue ball. Named for a giant from Greek mythology, it isn't even all that big; Saturn alone has five larger moons. Enceladus could snuggle nicely between Baltimore and Boston with room to spare.

  But sometimes, nomenclature is irony. In Greek lore, the giant Enceladus was buried under Italy's Mt. Etna, one of the world's most active volcanoes. In some versions of the story, in fact, it's his sleeping breath that fuels Etna's fires. And while nobody could have foreseen it when British astronomer William Herschel first spotted Enceladus in 1789, it turns out that the not-so-giant moon behaves in some ways like an enormous volcano.

  "It's the only unambiguously cryovolcanically active moon in the Solar System," Dennis Matson, a planetary scientist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, said in May 2011 at a gathering of Enceladus experts at the SETI Institute in Mountain View, California. 2

  As the term indicates, cryovolcanoes don't spew lava—at least not lava as we know it. Rather, they extrude a mix of water and ammonia that's only hot compared to everything else nearby. And in Enceladus's case, it's not really lava, but instead geyser-like jets of ice spicules and chilly vapor—not so much an extraterrestrial Etna as a deep-frozen Yellowstone.

  Although some people will argue that this isn't really a volcano, what makes Enceladus's cryovolcanoes, cryogeysers, cryofumeroles, or whatever you want to call them interesting is that their icy vapor most likely originated as water. If so, somewhere beneath the surface there may be an environment warm enough for life as we know it.

  Better yet, in Enceladus's low surface gravity (slightly more than 1.1% that of Earth's) many of those ice grains are blasted all the way into space, where a properly equipped spacecraft might be able to swoop close and collect samples, including, perhaps, a few stray microbes caught up by the jets.
<
br />   To look for life signs on Mars, we have to land rovers and drill into rocks for samples. On Enceladus, we might simply have to fly by and let the samples come to us.

  Tiger Stripes

  The first inkling Enceladus might be more than simply an iceball came in 1980 and 1981 when NASA's two Voyager spacecraft sped through the Saturn system, passing within 120,000 miles of Enceladus en route to Uranus, Neptune, and the edge of the Solar System.

  Even from that close, the cameras of the era weren't good enough to reveal more than tantalizing hints that something about this moon was weird. But one of those hints was that the surface, while not truly cue-ball smooth, was unusually free of big craters: a sign that something had "resurfaced" the planet relatively recently, erasing traces of old impacts. Also, the Voyagers found that Enceladus orbits in the heart of Saturn's tenuous E ring (so faint it wasn't even seen until 1967), raising the possibility that somehow, the ring and moon were linked—most likely with the ring being created by material somehow being ejected from Enceladus's surface.

  But that was pretty much all we knew until NASA's Cassini spacecraft arrived on the scene in 2004, more than two decades later.

  Initially, Cassini's prime target was Saturn's truly giant moon, Titan, which has revealed a remarkably complex surface topography and a weather cycle in which methane plays the role of water, complete with lakes, seas, streambeds, thunderstorms, and seasons. But it didn't take the scientists long to take a look at Enceladus as well, on the first of what are now scheduled to be some two dozen flybys by the time the mission ends in 2017.

  What they found became more exciting with each flyby.

  One of the first discoveries was of strange, bluish bands near the south pole, quickly dubbed "tiger stripes." There were four major ones, now named for cities in The Arabian Nights —Alexandria, Cairo, Baghdad, and Damascus—lying in parallel, roughly 130 kilo-meters long and 35 kilometers apart. Clearly fault lines of some sort, they confirmed the Voyager-era discovery that Enceladus wasn't simply frozen in time, but bears unmistakable scars of recent geological activity.

  But tectonic features are a dime a dozen in the outer Solar System. The tiger stripes were interesting, but not game changing.

  The next surprise came a couple of months later, when a subsequent flyby revealed that the stripes weren't just cracks in Enceladus's icy façade—they were hot, at least by the standards of an airless moon, 870 million miles from the Sun, where the average temperature is somewhere around –330°F. In places, the tiger stripes have now been measured with temperatures as high as –139°F—still far colder than anything ever measured on the surface of Earth, but a strong sign of an internal heat source. It was a bit like going to Antarctica and measuring hot spots with temperatures higher than anything ever recorded in the Sahara. Clearly, something interesting was going on.

  But the biggest find came later that year, when Cassini scientists released a photo showing a cloud of backlit particles jetting from Enceladus's south polar regions, right above the tiger stripes.

  It would have made a great cover for a science fiction novel, making it look almost as though the moon was rocket propelled, blasting northward, out of its orbit. That's not actually the case, of course. (Nor is there enough impulse from the plume to overcome other forces holding Enceladus in its orbit.) Rather, the plume is Matson's cryovolcano: the jets from what are now known to be dozens of geysers spewing a total of 440 pounds of water vapor and ice particles into space each second—enough to fill an Olympic-sized swimming pool every few hours.

  It was evidence that Enceladus is indeed the source of the E ring, but the photo was so startling that the scientists took months before releasing it, double-checking their instruments to make sure the plume was real and not some odd type of lens flare or other instrumental artifact.

  Since 2005, Cassini has made nearly twenty more flybys, one dipping as close as 13 miles to the surface, sampling the plume by flying right through its heart. Other flybys have photographed the surrounding terrain, made increasingly detailed thermal maps, and used spectroscopes to analyze the composition, speed, and density of the plume's dust grains and gas. One thing that's now known, for example, is that the stripes are valleys, each about 1,600 feet deep and 1.2 miles wide.

  Flanking them on each side are low ridges, about three hundred feet high, giving the whole thing a vaguely "M"-shaped profile. (Although in this "M" the dip in the middle would extend well into the next line of type.) Even on Earth, these would be dramatic features, roughly comparable to Colorado's Black Canyon of the Gunnison. On much smaller Enceladus, they are enormous.

  Not surprisingly, the geyser-like jets feeding the plume originate from vents in the tiger stripes, corresponding nicely to the hot spots. But this doesn't mean the tiger stripes are jetting vapor everywhere. Rather, the individual vents appear to be quite small, by some calculations (based on exhaust velocities of the particles) as narrow as a few tens of centimeters —roughly a foot.

  Between the jets and the heat, Enceladus appears to be emitting an enormous amount of energy into space—about 16 gigawatts, Enceladus scientists have calculated: enough to power Washington D.C. and several of its suburbs. And with all of that energy comes not only water vapor and ice crystals, but a stew of intriguing chemicals: ammonia, methane, carbon dioxide, hydrogen cyanide, formaldehyde, acetylene, and other hydrocarbons.

  None of this means that Enceladus has life. But with each successive find, the possibility of a habitable zone not far beneath its surface becomes ever more likely.

  Water, carbon, nitrogen, and an energy source... Cassini wasn't sent to Saturn looking for life. But what it's found with instruments designed for other purposes definitely makes one wonder what a future mission, with a more specialized instrument package, might someday uncover.

  The big question, of course, is what is producing the plumes.

  Most of the energy needed to drive them probably comes from gravitational flexing, as Enceladus moves though Saturn's enormous gravity field. That causes the moon's inner materials to rub against each other, generating heat. But even if you also throw in heat from radioactive decay, there doesn't seem to be 16 gigawatts of energy, which means that there's either a missing energy source the scientists have yet to figure out, or that the plume isn't active all the time.

  In fact, says Frank Postberg, a physicist at Heidelberg University, Germany, "the energy output suggests that [the plume] cannot be active much more than 10% of the time, at least at the current scale." Many Enceladus scientists therefore believe that instead of blasting away continuously, the plumes probably follow what engineers call a duty cycle, in which they turn on and off at intervals, accumulating energy for future eruptions during the inactive periods, much as earthly geysers do, but on a longer time scale.

  Nobody knows how long the "on" cycles might last. All we know for sure is that the geysers have been at full blast ever since Cassini first saw them, a decade ago. And, since they do feed the E ring, it's also a pretty good guess that they've been going at least since the E ring was first spotted, 47 years ago. In fact, they've presumably been active long enough for the E ring as we know it to have formed— a process Postberg estimates would take somewhere between a few hundred and "maybe" a thousand years.

  But maybe there's another marker for how long they've been active, says Paul Schenk, a planetary scientist at the Lunar and Planetary Institute, Houston, Texas.

  Snow.

  Talcum Powder

  When the plumes are operating, Enceladus isn't just a cold, airless world. It has weather. Cold and sunny, with a chance of snow flurries.

  That's because many of the ice grains ejected from the geysers don't get far into the E-ring before Enceladus reclaims them. "The vast majority, we think, fall back to the surface," says Carolyn Porco, head of the Cassini imaging team.

  But the snowfall isn't evenly distributed. In a 2010 study in the journal Icarus, a team led by Sascha Kempf, then of the Max Planck Institute i
n Germany, calculated that the dual effects of Enceladus's and Saturn's gravity would concentrate it into two narrow bands, stretching hundreds of kilometers northward from the tiger-stripe zone. 3

  Based on this, Schenk went looking for snowdrifts. Using the highest-resolution photos available for the fallout zones, he found a 12x12 mile region where successive flybys had taken photos from different angles, allowing him to construct 3D "stereo" images showing an area whose topography was muted into rounded contours indicative of deep snow, with the ghostly outlines of underlying features peaking through. "It's different from what you see in other areas that have been photographed at high resolution," he says. "Those had incredible detail everywhere you look. This is much smoother. It really does look as if it's been mantled by a deposit."

  The superfine ice crystals (finer-grained even than talcum powder, Schenk says) might make for the Solar System's best powder skiing, but the goal wasn't to find an exotic science-fictional vacationland. Rather, it was to measure the depth of the snow—something Schenk was able to do by using stereo images of snow-draped canyons, up to 1,600 feet deep and a mile across. From these photos, Schenk was able to distinguish the overlying snowpack from the underlying topography. "Slope breaks near ridge crests can be interpreted as a weak layer [snow] over a solid layer," he says. "I get a thickness of 125 meters (410 feet)—give or take fifty meters. It varies from site to site." 4

  That's a lot of snow, anywhere. But on Enceladus, it's enormous, because Kempf's model doesn't show it coming down at blizzard rates. Rather, his team calculated that even in the highest snowfall zones it was accumulating at only a rate of one millimeter every 1,500 years. At that rate, it would take nearly two hundred million years to pile up 125 meters.

  That said, there's a bit of apples and oranges going on in these calculations, because Kempf's calculations were based on highly compressed snow or ice, and Schenk's drifts might be quite fluffy—meaning they could pile up a lot deeper, faster. But even if there's a factor of ten difference, Schenk's finding still suggests that the plume must have been going for nearly twenty million years. And realistically, even in Enceladus's low gravity, snow at the bottom of that giant pile would be substantially compacted, which means that twenty million years is a lower-bound estimate.