The first one to gaze on Enceladus was the British-German astronomer and musician Frederick William Herschel, who in 1789 focused what was then the largest telescope in the world (1.2 meters) on the ringed planet Saturn. The name of the moon was derived from the giant Enkelados (Latin: Enceladus) in Greek mythology. This was actually a mistake, because Enkelados, as one of the giants, never joined with the Titans in their war against the gods (whose leader was Kronos, called Saturn by the Romans). The giants, including Enkelados, rebelled later, after Zeus had locked away the Titans (the sons of the ancestral mother Gaia) in the underworld.
For almost 200 years Enceladus remained an unremarkable if unusually bright spot in the sky. Due to its closeness to Saturn and Saturn’s rings, which were so much brighter, the moon was hard to observe. Voyager 1 was the first object created by humans to pay it a visit. On November 12, 1980, this probe flew past it at a distance of 202,000 kilometers. The pictures taken then already showed that Enceladus had a very young face that showed no deep craters. On August 26, 1981, Voyager 2 came even closer, as close as 87,010 kilometers, and thus provided images with considerably better resolution. These pictures excited scientists. How could such a cold, small moon have such differently formed areas, some of which could only be a few million years old?
The answer finally was provided in 2004 by the Cassini probe, which had been sent by NASA and ESA, and which had already placed a lander on the larger moon Titan. Afterward, Cassini followed an orbit around Saturn that often swung very close to Enceladus. Initially, fly-bys at distances down to 1,500 kilometers had been planned, but after it was discovered that water vapor shot into space from the region around the South Pole, planners decided to take an even closer look at this moon. Cassini sometimes came as close as 25 kilometers from the surface of Enceladus, and it was able to send spectacular photos. On October 28, 2015, it even managed to fly through a geyser eruption. During this, the probe analyzed the composition of the ejected material, which was so dense that it measurably slowed Cassini’s rate of travel.
Currently, no space agency on Earth has specific plans for another visit to Enceladus. At the beginning of the 21st century, ESA discussed the concept of a Titan-Enceladus mission named TandEM. This was merged in 2009 with a similar concept at NASA and became the Titan Saturn System Mission (TSSM). For budgetary reasons, TSSM was canceled in favor of EJSM (Europa Jupiter System Mission, now often called Europa Clipper), which was proposed to explore the ice moon Europa orbiting Jupiter, scheduled to launch in the 2020s.
Time will tell whether that was a smart decision—Europa orbits within the radiation belt of Jupiter. While Cassini was able to provide data from Saturn and Enceladus for a decade, the Europa Clipper probably won’t even last a year.
Parallel to that, ESA is considering the mission JUICE (JUpiter ICy moons Explorer). The plan is to have a probe arrive near Jupiter in 2030 and explore the ice moons Ganymede, Callisto, and Europa for three years. They, at minimum, expect to discover an ocean on Europa similar to that on Enceladus, though the ice layer is considerably thicker there.
So far, ELF (Enceladus Life Finder) only exists as a proposal. It should be launched no sooner than 2021 and the goal is to hunt for traces of life, such as amino acids or fatty acids, in the water vapor of the geysers. For this purpose, the probe would make several flights through geyser eruptions. Parallel to that, NASA and JAXA are jointly developing LIFE (Life Investigation For Enceladus), a probe that could take samples from the geysers and bring them back to Earth. In 2015, both projects applied for one of five spots in NASA’s Discovery Program (in which each mission cannot cost more than 450 million dollars). Yet when the decision was made in the summer of 2016, neither of the two missions was selected.
Yet there is some hope for the future. The lower-priced Discovery missions focus on using only proven technologies. ELF and LIFE would have used solar energy, which is rather optimistic at that distance from the sun. The Europa mission (which also uses solar energy) could now demonstrate that this technology has matured.
Furthermore, NASA announced in early 2016 that it would also accept proposals explicitly for expeditions to Titan and Enceladus for its New Frontiers program. This program (of which the New Horizons probe to Pluto was a part) accepts innovative technologies—and such proposed missions can be more expensive, in the two to three billion dollar range. In 2017, NASA selected a proposed mission to Enceladus as the recipient of technology funds in preparation for future mission competitions. Therefore, it is not improbable that ELF might be revived.
Flying to Enceladus with a Crew?
Currently, humanity lacks the technological capability of sending a manned spaceship to Saturn and its moons—at least if one looks at it realistically. A robot probe can take as much time as it needs and can do without many things humans absolutely need—life support (oxygen, water, food,) gravity, protection against radiation, a return ticket. While it might be possible in principle to build a spaceship for a Saturn mission with today’s technology, it would be very, very expensive.
The Juno probe, which weighed almost four tons and entered an orbit around Jupiter in the fall of 2016, cost about a billion dollars. A manned spaceship would be at least ten times heavier and more complex. After all, it would have to accelerate for the return flight and then decelerate again. Therefore, it does not just need twice as much fuel, but several times as much. Considering all these requirements, it might cost 100 billion dollars or more. Juno, the fastest probe so far, took five years to reach Jupiter—and Saturn is twice as far from us. Therefore, the crew would spend 20 years on this mission.
However, there are already propulsion concepts that might offer alternatives ten or twenty years from now.
A magnetoplasmadynamic drive (MPD) uses very strong magnetic fields to accelerate a reaction mass (like the noble gas argon or the metal lithium) out of the thruster. This makes high exit-velocities of up to 40 km/s possible. A spaceship thus equipped could reach Saturn in one to two years. A current commercial variant is called VASIMR. Its manufacturer, Ad Astra, built a 100 kw prototype that, in a mid-2017 NASA test, worked for 100 continuous hours. One drawback of the MPD, though, is that it only works with an energy input of electricity. As solar energy is rather sparse in the environs of Saturn, one would need a small nuclear reactor, or Radioisotope Thermoelectric Generator (RTG). The latter creates electricity from the heat released by the decay of plutonium.
Another alternative would be a nuclear drive. The concept was already developed in the 1950s, ‘Project Orion.’ Back then, the plan was to ignite a nuclear bomb behind a spaceship (which was shielded by a large mirror) to propel it this way. Using about 1,000 such bomb ignitions in sequence, Saturn could be reached in one or two years. However, these days it would hardly be acceptable to send real atom bombs into space by rocket. The concept was further developed into Mini-Mag Orion. In that project, a small piece of fissile material would be compressed in a magnetic field until it ignited in a miniature explosion. Even that would not be possible without releasing damaging radiation, so the drive would have to be carefully shielded.
The most promising concept these days seems to be the Direct Fusion Drive (DFD). The idea of using nuclear fusion for powering a spaceship has been discussed since the 1990s. In contrast to nuclear fission, nuclear fusion does not create very much radioactive garbage. The only problem might be released neutrons, which then might be captured by stable atoms, turning them into unstable nuclides. Currently, the company Princeton Satellite Systems (PSS) is predominantly active in this field of research, and it also advised me on this book. In the DFD conceptualized by PSS, deuterium (heavy hydrogen) reacts with helium-3 (an isotope of helium) to form helium-4. It achieves an exit velocity of 70 km/s. Under these conditions, a spaceship would reach Mars in a month and Saturn in a year. The fusion reactor generates relatively few neutrons, so the spaceship would not require cumbersome shielding. At the same time, the DFD also generates electricity.
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p; Neither deuterium nor helium-3 is radioactive. However, helium-3 is quite expensive, as it is very rare. There are only about 3,000 tons present in the entire atmosphere of Earth. The annual consumption on Earth is approximately eight kilograms, and one kilogram of the gas costs about 16 million dollars. PSS estimates a round-trip flight to Saturn would require about 20 kilograms of helium-3. Earth’s moon might be a good source. In its upper rock formations, the content of helium-3 is up to 1,000 times higher than on Earth. A DFD could also serve as a source of energy for everything a human crew would want to do on Enceladus.
The final proof of the existence of life on Enceladus could, in all probability, only be found by an expedition directly exploring its ocean. The conditions on Enceladus are actually quite good for this, as at least parts of the ice crust are thinner than on other moons.
Humans have experience drilling through layers of ice. Usually, conventional drill rigs are used for this, but it would be impractical to take them along to Enceladus.
The alternative would be a cryobot, an ice-drilling robot already developed by the German physicist Karl Philberth in the 1960s. In 1968, his ‘Philberth Probe’ reached a depth of 1,000 meters in the Greenland ice sheet.
The current leader in this area is the U.S. company Stone Aerospace with its ‘Valkyrie.’ This ‘Very deep Autonomous Laser-powered Kilowatt-class Yo-yoing Robotic Ice Explorer’ is partially financed by NASA. It does not carry an energy source on board, but is supplied by a laser via a fiber-optic cable. Currently, Stone Aerospace is testing their Valkyrie with a power level of 5-kilowatts on Earth. The version for Enceladus would have to be considerably larger, but it would work according to the same principle. It would need from 250 kilowatts upwards to 1 megawatt of power. The more power, the faster the cryobot can drill, although “drill” is not really the correct term. The laser heats up the water, and the hot water is aimed at the ice, which then melts like butter. This works more quickly and requires less maintenance (which is very important) than a metal drill that would wear out. In addition, the hot water can also be used for generating energy. The fiber-optic cable, in turn, can be used for transmitting information.
However, the Valkyrie concept only works if you bring along a source of energy. One cannot simply generate 5 megawatts with an emergency generator. One would need a small nuclear power plant. Or one could use the dual function of the DFDs, each of which provides 10 megawatts of power. As Enceladus does not have an atmosphere, the energy could be beamed with negligible loss via laser from the orbiting vessel to the lander module, which then would feed it to Valkyrie through the fiber-optic cable.
Life on Enceladus?
If there is life on Enceladus, it would be located at the bottom of its ocean. Here, as already described, serpentinization reactions continually occur. These generate heat, hydrogen, and methane—each important for life. At the same time, other minerals that are important as nutrients for life might be dissolved in the water, along with the various salts it contains. The ocean probably has existed longer than there have been conditions on Earth suitable for life as we know it. Thus, there really was plenty of time.
Of course, these organisms would have to survive without photosynthesis, as no light reaches the bottom of the ocean. They also would have to go without oxygen. Yet even here on Earth, researchers have already identified three habitats with similar conditions. One of them was found in the depths of a South African mine. It is based on radioactive decay energy and consists of bacteria-reducing sulfur. However, there is hardly any sulfur on Enceladus. The other two were found by scientists in volcanic rocks near hot springs deep below the ground. They are dominated by archaea. These organisms consume the hydrogen emerging here due to plate movements, and they burn it with carbon dioxide and generate the energy they need for living, as well as traceable byproduct amounts of methane and water. Together with bacteria and eukaryota, the archaea form the three domains of life, and they are the most ancient. Archaea are single-celled, and the DNA containing their genetic information is circular. They possess simple organs of movement (flagella) and sometimes build a kind of skeleton to stabilize their shape. They differ from bacteria in the structure of the ribosomal RNA, which is responsible for translating genetic information into proteins.
On Earth, archaea are often found under extreme conditions. Some varieties only flourish at temperatures above 80 degrees Celsius, while others prefer living in highly concentrated saline solutions, or very acidic or alkaline environments (pH value below 0 or above 10). Even the pressure at the bottom of the ocean, which measures between 2.8 and 4.5 MPa, should pose no problem. After all, there are microorganisms in the Mariana Trench which can withstand a pressure of 50 MPa. Even multi-celled organisms like Pseudoliparis amblystomopsis, a species of snailfish, can survive under these conditions.
Archaea can also perform amazing feats—they are among the fastest creatures on Earth, for instance. In the category ‘body lengths per second’ they achieve a value of 400 to 500. A cheetah only reaches 20, a human 11, and a horse 7. A sports car would have to drive at 6,000 km/h to rival the archaea. The reason archaea are so much faster than bacteria is that they have more flagella (50 versus 5 to 7), and they can also rotate these faster, as they possess a more efficient ‘motor.’ Humans use archaea, among other things, in biogas systems to generate methane.
On Earth, the archaea, which on average measure one micrometer, are much more common and more important for chemical cycles than was long suspected. After all, they don’t absolutely need extreme environmental conditions. They are as common in fresh water as in the sea, where in some areas they represent up to 90 percent of living beings, and in the soil. They also exist as symbionts in the intestinal tract of animals and humans. Archaea have even been found in the human navel. The total number of archaea in the oceans is estimated to be more than 10 to the power of 28, that is, a 1 followed by 28 zeroes. The estimated number of all cells in a human multiplied by the number of human beings is about 10 to the power of 22 (the number 1 followed by 22 zeroes), which is six magnitudes smaller.
How Life Might Have Started
We do not know yet whether Enceladus harbors life—but if it does, it must have started at some point in time, with either the chicken or the egg. We are not sure about what happened afterward, even in the case of Earth. There are two theories about the origin of life that might also apply to Enceladus.
Theory 1: Origin in the Primordial Soup
The theory that inorganic molecules in a water-based ‘primordial soup’ randomly combine to form the first organic compounds, and then assemble the most primitive forms of cells, was proposed by Charles Darwin. In 1953, Stanley Miller and Harold Urey showed in a spectacular experiment how this might have happened on Earth. They had simulated lightning striking a mixture of methane, ammonia, water vapor, and hydrogen. Within two weeks, various amino acids had formed in the solution. These complex molecules are the basic building blocks of life. Miller and Urey were wrong concerning the possible composition of the primordial soup, and they did not create life, but the experiment proved that under the right circumstances complex molecules can be formed from simple compounds. These basic materials and the necessary energy supply in the form of heat are also present on Enceladus, and they have been for billions of years.
Theory 2: Origin from Hydrothermal Vents
At the bottom of Earth’s oceans, hot water, in which various chemicals are dissolved, emerges from the crust of the planet. Some vents eject water with a temperature of almost 500 degrees. These might have been the places where life developed on Earth. In such places, chemical energy provided by the vents in the form of dissolved reduced gases meets with suitable reaction partners. If such vents exist at the bottom of the Enceladus Ocean, they could also form the starting point for life. After life developed at such hotspots, it could have gradually adapted to cooler environments and spread across the entire ocean.
Thanks for visiting Enceladus with me! I hope you h
ad a pleasurable ride. If you’d like to see all these places in their colorful glory, you can get the PDF version of the Guided Tour of Enceladus for free by leaving me your e-mail address at: hard-sf.com/subscribe
Glossary of Acronyms
AI – Artificial Intelligence
API –Application Program Interface; Acoustic Properties Instrument
ASCAN – AStronaut CANdidate
AU – Astronomical Unit (the distance from the Earth to the sun)
BIOS – Basic Input/Output System
C&DH – Command & Data Handling
CapCom – Capsule Communicator
Cas – CRISPR-associated system
CELSS – Closed Ecological Life Support System
CIA – (U.S.) Central Intelligence Agency
COAS – Crewman Optical Alignment Site
Comms – Communiques
CRISPR – Clustered Regularly Interspaced Short Palindromic Repeats
DEC PDP-11 – Digital Equipment Corporation Programmable Data Processor-11
DFD – Direct Fusion Drive
DISR – Descent Imager / Spectral Radiometer
DNA – DeoxriboNeucleic Acid
DoD – (U.S.) Department of Defense
DPS – Data Processing Systems specialist (known as Dipsy)
DSN – Deep Space Network
ECDA – Enhanced Cosmic Dust Analyzer
EECOM – Electrical, Environmental, COnsumables, and Mechanical
EGIL – Electrical, General Instrumentation, and Lighting
EJSM – Europa Jupiter System Mission
ELF – Enceladus Life Finder
EMU – Extravehicular Mobility Unit
Ice Moon 1 The Enceladus Mission Page 30