But the point isn't to debate whether the plume has been going for twenty million years or two hundred million. Rather, it's that the jets aren't a brand-new feature. "It's a big step from a thousand to ten million years," says Postberg. "This is a whole new order of constraints for how long the plumes have been active."
Furthermore, it's possible that Postberg's duty cycle doesn't require the plumes to shut off entirely. Rather, they may ramp up and ramp down, without ever totally subsiding. If so, the average rate of snowfall on the rim of Schenk's canyon might be much lower than at present, and Schenk's snowfall record might extend not merely over tens of millions of years, but over hundreds of millions.
And that's important because the longer the geysers have been active, the greater the chances that the processes producing them might have given life enough time to evolve.
Urine Dumps
Life as we know it, of course, needs liquid water, not snow, so the mere fact the tiger stripes are jetting out moisture doesn't necessarily mean there's liquid beneath. Instead, the jets could be fueled by "dry" processes in which ice grains and vapor are spewed out without melting.
One such process could involve sublimation (in which ice evaporates directly into vapor, without ever getting warm enough to melt). Another might involve the decomposition of subsurface layers of clathrate deposits (a type of ice composed of a cage-like lattice, tightly woven enough to trap gas molecules).
Both processes occur on Earth. Fishermen often dredge up methane-containing clathrates (also called methane hydrates) from the Arctic waters, watching them decompose into water and methane. And villagers in Greenland use sublimation to air-dry laundry on cold, sunny days when temperatures never get close to freezing.
But these processes produce mostly water vapor, not the fifty/fifty mix of water vapor and ice crystals observed by Cassini's instruments, says Andrew Ingersoll, a planetary scientist from California Institute of Technology in Pasadena. "It's hard to get solid-to-gas ratios of more than 1% if you think all the particles are forming from vapor," he says.
Water droplets, on the other hand, easily form clouds of tiny ice particles. "If you throw a blob of liquid into vacuum," Ingersoll says, citing astronaut "urine dumps" as an example, "it's not going to freeze into a big chunk. It'll break up and explode into a cloud of smaller particles."
An even worse blow to the "dry" theories comes from what happens when ice grains hit Cassini's cosmic dust analyzer as the spacecraft f lies through the heart of the plume. These collisions are energetic enough to produce a quick flash of light as the impact not only vaporizes the ice grain but ionizes its constituents, allowing scientists to determine the grain's composition from its spectra. And in a 2001 paper in Nature, Postberg's team saw clear indications of sodium chloride: salt. 5
Salty ice particles, Postberg says, are hard to explain with "dry" plume models, because even if salt is present underground, sublimation or clathrate degeneration should either leave it behind or bring up sodium vapor in the plume, something that has not been seen. For appreciable quantities of salt to be in the ice grains, he says, they must have begun as frothy bubbles fizzing out of a salty, subsurface ocean, rich in dissolved gases: bubbles that then collect into a wind that blows up through narrow fissures, emerging at the surface as the geyser-like jets seen from space.
Matson calls this a "Perrier ocean" and compares it to being on a beach on a blustery day. "When the surf is up," he says, "there's a lot of spray. You can smell the salt in the air." The amount of salt, Postberg adds, is about half that in Earth's oceans—comparable to what you'd get if you put liquid water in contact with primordial water for millions of years.
So far, we've been talking as though the existence of an underground ocean is all but proven. But nothing on the frontiers of science is that cut and dry, and not all the evidence points to liquid water.
One problem is the energy problem, which doesn't give enough heat to maintain a layer of water beneath the ice, everywhere, says Francis Nimmo, a planetary scientist at the University of California, Santa Cruz.
But that's just a minor difficulty, because if all of the gravitational energy is focused in one zone—such beneath the tiger stripes—there is plenty to produce a large, regional sea. And once such a sea forms, Nimmo says, it would concentrate gravitational flexion (and therefore heating) into its own region, thereby becoming self-perpetuating. In fact, he says, it's possible that such a regional sea might be a remnant of a once-larger ocean that long ago mostly froze. Another prospect is that a south-polar underground sea was created by a long-ago asteroid impact (a theory that draws support from the fact that the southern region containing the tiger stripes is separated from the rest of Enceladus by a "dichotomy boundary" marked by features that might—or might not—be remnants of exactly such a crater rim.)
On the other hand, complex fracture patterns in the ice around the tiger stripes region (including ones that appear to be remnants of several generations of prior tiger stripes) indicate that the entire ice shell of that part of Enceladus has slowly rotated, presumably due to asymmetric torques from Enceladus's notperfectly-circular orbit.
"There's no way to do that without a global ocean," says Simon Kattenhorn, a planetary geologist at the University of Idaho. "You can't have just the south polar terrain rotating above a local sea."
Another problem comes from the zoo of compounds found within the plume. Some of them, says Hunter Waite, a space physicist at the Southwest Research Institute, San Antonio, Texas, are inconsistent with a liquid source because they are ones like hydrogen cyanide, which, if they had ever met liquid water should have reacted to produce other compounds not yet found. Others, like methane, simply aren't soluble in water.
In discussing this, however, Waite is careful not to argue that there's no liquid water: he merely notes that some of these chemicals are consistent with liquid water... and some aren't. "We don't know how to sort that out," he says. One possibility, he adds, is simply that chemicals in the plumes might come from multiple sources, some involving water, some not.
It's not a debate that's likely to be fully resolved in the near future. In the latest pro-ocean salvo, a 2013 paper in Nature spearheaded by Matt Hedman, a planetary scientist at Cornell University, Ithaca, N.Y., reported that the intensity of the jets feeding Enceladus's plume varies with Enceladus's distance from Saturn. 6 "The jets of Enceladus apparently work like adjustable garden hose nozzles," Hedman said in a NASA press release. 7 "The nozzles are almost closed when Enceladus is closer to Saturn and are most open when the moon is farthest away. We think this has to do with how Saturn squeezes and releases the moon with its gravity."
That itself might simply be an intriguing factoid. But Hedman's coauthor, JPL planetary scientist Christophe Sotin, argues that it wouldn't work that way under "dry" models of plume formation. "The way the jets react so responsively to changing stresses on Enceladus suggests they have their origins in a large body of liquid water," he said in the same press release.
12C, 13 C, and Acetylene
Even if Enceladus has liquid water, that's not proof of life. Just to begin with, there has to be some way for microbes to turn the available energy into something it can use.
One option that would not be available in Enceladus's dark underground would be photosynthesis, the process by which earthly plants (and some microorganisms) harvest energy from sunlight. But one possibility that might be available, says McKay, is a "methanogen system" in which microorganisms live by obtaining both energy and biological building blocks by synthesizing methane from carbon dioxide. McKay is particularly fond of this idea because it might be sustainable over very long time periods if geological processes carry some of the methane produced by the bacteria back into zones hotter than 500°C. Those temperatures would decompose it back into building blocks usable by methanogen bacteria, meaning that overall Enceladus could have an ecosystem in which organisms subsist on chemicals recycled by geochemical heat. "The fact we see m
ethane in the plume makes it interesting," McKay adds.
Ronald Oremland, a microbial biogeochemist with the U.S. Geological Survey's off ice in Menlo Park, California, believes an even better food source would be acetylene.
On Earth, this chemical doesn't occur naturally, although humans synthesize it as fuel for welding torches. But it exists in comets and there are hints it might be present in Enceladus's jets. And acetylene-eating organisms do exist on Earth. If there's enough acetylene on Enceladus, Oremland says, it could be "fast food" for microbes—a primordial food source on which Enceladus bugs might still be nibbling away. If so, possible biomarkers (chemicals indicating the existence of biological processes) for such organisms might be byproducts of acetylene metabolism, such as acetate and acetaldehyde.
Other possible biomarkers are amino acids, especially if they can be tested for "chirality," which is the degrees to which they mix mirror-imaged shapes known as D and L isomers.
Abiotic processes tend to produce an even mix of the two isomers. Biological ones favor one or the other. On Earth that's the L versions, though there's no known reason extraterrestrial life couldn't do the reverse. 8 But even if Enceladus organisms had the same L preference as earthly ones, any imbalance between the two isomers is a biomarker. "If we find amino acids and there's a strong chiral preference, that's persuasive evidence for a biological origin," McKay says.
But the best biomarkers, McKay and Oremland agree, might be found in the ratios of carbon's two stable isotopes, 12 C and 13 C. That's because biological processes produce compounds slightly enriched in 12 C, compared to nonbiological ones. (Testing 12 C/ 13 C ratios, in fact, is one way sports authorities can catch drug-cheating athletes, because synthetic hormones, produced in a lab, have different ratios from those produced by the athlete's own bodies.)
Thus, if the methane coming from Enceladus is unexpectedly rich in 12 C, it's a strong indication it's being made by bacteria. Similarly, if the plume is emitting acetylene byproducts that are richer in light carbon, Enceladus's primordial acetylene, it suggests that acetylene-eating bacteria may have been at work.
Unlike the electron microscope on Aimee's hypothetical spacecraft, the equipment to conduct carbon-isotope tests isn't actually all that elaborate. It just happens not to have been included in Cassini's payload, which means that carrying out such tests would require a return to Enceladus.
Several such missions are possible, said Nathan Strange, a mission architect at NASA's Jet Propulsion Agency in Pasadena, California, at a 2010 discussion group. 9 The simplest would merely be to go back to Saturn with a dedicated Enceladus mission. But it might be more efficient to use a multi-purpose mission that piggybacked a small Enceladus orbiter onto another dream project: a Titan rover.
A fancier mission might be an Enceladus lander, or possibly a "hopper," capable of launching itself from one landing zone to another. "The low gravity makes this possible," Strange said. "You could even hop across a jet."
A sample-return mission is also possible in which a probe flies through Enceladus's plume, collects some ice-grains and hurls them back to Earth for analysis. "This is really the low-hanging fruit" of sample-return missions, Peter Tsou of Sample Exploration Systems in La Canada, California, told space.com in conjunction with a December 5, 2012 presentation at a meeting of the American Geophysical Union in San Francisco, California. "It would be a shame not to pick it." 10
Not that any of this can occur instantly. Every couple of decades it's possible to use Jupiter as a gravity-boost en route to Saturn, but the nearest window for that is most likely lost. "We're probably looking at 9 to 10 year flight times," Strange said.
Still, Enceladus presents a unique opportunity, both for scientists seeking answers to the ultimate question, and for science fiction fans who enjoy playing mental games of "what if?" "What we're being handed at Enceladus is a potential gift of looking at life in the outer Solar System," says Oremland. "What's appealing about Enceladus is that you have some of the conditions for life. There's liquid water under the ice. It seems to have been around a long time. How long, nobody knows. One hundred million years? A billion? That's a long time for life to get going, provided there's something to eat."
And for science fiction fans, Enceladus provides a world unlike anything the best minds in the field had ever, on their own, imagined.
From Dune to Dyson spheres, from Game of Thrones to "Nightfall," one of the mainstays of science fiction is "world-building"—the imagining of scientifically plausible planets sufficiently unlike our present existence to be worth a story, but not so implausible as to fall into the realm of fantasy. Sometimes, those are extrapolated from contemporary Earth.
Sometimes, they involve places like Arrakis, Trantor, or the Ringworld, with no known real-life analogs.
But sometimes, they are inspired by worlds like Enceladus, where reality outstrips imagination and science and wonder unexpectedly combine.
Footnotes:
1 Lisa Grossman, "Pluto's icy exterior may conceal an ocean," New Scientist, 16 September 2011.
2 Much of the background for this article comes from that meeting, of the Enceladus Focus Group, May 23–24, 2011.
3 Sascha Kempf, Uwe Beckmann, and Jürgen Schmidt, "How the Enceladus dust plume feeds Saturn's E ring," Icarus, Volume 206, Issue 2, April 2010, pp. 446–457.
4 I got this from the Enceladus Focus Group meeting, ibid, but Schenk later presented it at a joint meeting of U.S. and European planetary scientists in Nantes, France, October 3, 2011.
5 F. Postberg, J. Schmidt, J. Hillier, S. Kempf & R. Srama, "A salt-water reservoir as the source of a compositionally stratified plume on Enceladus," Nature 474, 620–622 (30 June 2011) doi:10.1038/nature10175.
6 "An observed correlation between plume activity and tidal stresses on Enceladus," M. M. Hedman, C. M. Gosmeyer, P. D. Nicholson, C. Sotin, R. H. Brown, R. N. Clark, K. H. Baines, B. J. Buratti & M. R. Showalter, Nature 500, 182–184 (8 August 2013) doi:10.1038/nature12371.
7 http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20130731/.
8 One SF story to examine this is "Technical Error," also known as "The Reversed Man," first published in 1950 by Arthur C. Clarke. It's not about alien life, per se, but a man who is mirror-imaged at the molecular level.
9 Enceladus Focus Group meeting, ibid.
10 Whether we want to bring possible alien microorganisms back to Earth is a different question (and one of the reasons why other Enceladus experts tend to view an orbiter as a more likely proposal).
* * *
HaiKu
Kate Gladstone | 9 words
Who rejects Darwin
show up for new flu shots and
antibiotics
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EDITORIAL
THESE ARE NOT THE DRONES YOU'RE LOOKING FOR Trevor Quachri | 1424 words
Like the much-maligned "bailout," "celebutante," 1 or "hipster," the word "drone" has become an increasingly relevant, yet unloved, part of our lexicon over the past several years.
As drones have occupied more and more of the 24-hour news cycle and our awareness of them has increased, so too have concerns over them, and, accordingly, rallies and protests specifically calling for the end of their use as weapons of war.
With the word carrying enough weighted connotation to inspire dedicated activists, we should ask: just what is a drone?
Also commonly called a UAV (unmanned aerial vehicle), a remotely piloted aircraft (RPA, per the USAF), or unmanned aircraft system (UAS, per the FAA), a "drone" is simply "a remotely controlled pilotless aircraft or missile." They can range in size from eighty feet long to the size of a pen, and in price from the $17 million dollar Reaper to $300 for a used quadrotor on eBay. The line between "pilot-less vehicle" and "remote-controlled toy" is a slim one at the smaller end of the spectrum, and like remote-controlled toys, they're not hard for hobbyists to make on their own: in fact, the prototype of the Albatross, the non-combat predecessor of the Predator and Reap
er, was literally built in a garage, using a go-kart engine, in the early '80s.
Militarily, they have two primary purposes: reconnaissance and combat, with the vast majority performing the former. The single most common drone is a carried glider that a soldier throws, by hand, to see what's beyond the visible terrain.
In a combat role, the two most common U.S. combat drones, the Predator and the Reaper, launch Hellfire missiles—the same airto-ground anti-tank weapon used by many helicopters, fixed-wing aircraft, boats, and, experimentally, a Humvee.
So if they're not especially advanced, why are they becoming so prolific?
For one thing, they're cheap: that $17 million for a Reaper doesn't seem so bad compared to $150 million for an F-22 fighter jet or $200 million for an F-35. It's also easier to train a drone operator than a pilot. (They train in less than half the time; under one year for an operator, instead of two for a pilot.) More practically, while most are short range, some drones can be operated from half a world away—operators in the U.S. can control drones in Afghanistan—so the lives of the operators aren't at risk. They can also stay aloft past human endurance, swapping out operators in shifts, while the drone is limited only by fuel capacity. The U.S.'s Global Hawk can stay in the air for 35 hours; typical fighter planes only fly for two. In general, according to Mary Cummings of the MIT Humans and Automation Lab: "Planes can fly longer, they can pull more Gs, they can be more precise when they bomb, if a human is not in the cockpit." 2
Analog Science Fiction and Fact - September 2014 Page 19
