Habitability: what does it take for a planet or moon to host life? Eric Hedin has elucidated the conditions on Mars, which for a long time has been one of the principal targets for NASA astrobiology missions. Due to uncomfortable facts gathered after 48 missions to Mars, he unfortunately had to play the spoilsport about its prospects for life. In my previous article, I gave a similar assessment of the prospects for astrobiology at Titan. But there’s another fascinating body orbiting Saturn, a little moon named Enceladus. It’s about as wide as Iowa or Arizona and hosts an array of geysers that astrobiologists envision as fountains of life. With a likely warm interior, an ocean under the icy crust, and some minerals available, could it be a lively place? 

The geysers provide a convenient take-home experiment for spacecraft: collect the particles in the spray and learn about what’s happening down below. The first-ever trial of that experiment succeeded during the Cassini mission, on which I worked between 1997 and 2011. Cassini scientists had eyes for Enceladus almost as big as for any other part of the Saturn family. In the early 1980s, Voyagers 1 and 2 had shown Enceladus to be the brightest object in the solar system, reflecting almost all the sunlight it received. This suggested a constant re-coating of the surface with fresh ice. Were “ice volcanoes” erupting there? And why did the densest part of Saturn’s E-ring coincide with the orbit of Enceladus? The reality would turn out to astonish everyone beyond their wildest imaginations.

Excitement of Discovery

On July 15, 2005, I was in the room with the leading Cassini scientists to see the first images arriving from Enceladus on Cassini’s third flyby (labeled E0, E1, E2, etc.). During the first distant pass (E0) at 1,260 km on Feb 17, Cassini’s magnetometer noticed a disturbance in the magnetic field lines around the little moon. Something was going on there. What was it? One image showed a possible eruption, but scientists could not rule out an imaging artifact. The closer images from 497 km during E1 on March 9 were not definitive, although we could see in much finer detail some of the ice flows cutting through impact craters that Voyager 2 had detected in 1981 from a long distance of 112,000 km. But there was no smoking gun… yet. Then during E2, on July 14, 2005, from only 166 km, geyser plumes were detected! Image processing showed clear evidence of vapor plumes issuing rapidly out of Enceladus. Stranger still, the material was all emerging from one side — the south pole! 

The discovery set off a frenzy of reporting back on Earth. To learn more, mission planners immediately set out to modify the flight plan to include more flybys of Enceladus. They even dared to send the spacecraft through the geyser plumes, so that its Cosmic Dust Analyzer (CDA), looking oddly like a shiny garbage can but much more sophisticated, could obtain samples to analyze with its mass spectrometer. When the Cassini mission ended in 2017, JPL had obtained voluminous and unprecedented data from all the science instruments on the spacecraft during 23 encounters with Enceladus, some as close as 50 km (31 miles) above the surface. We owe the mission navigators applause for being able to fly at speeds up to 32,000 mph so close to a moving target from 800 million miles away! They were able to do it using laws of celestial mechanics first elucidated by Johannes Kepler and Isaac Newton.

The Upshot and the Output

Cassini found about 100 geysers shooting water vapor into space at speeds up to 1,360 mph (mach 5 to 10) into space. Many of the entrained ice particles in the plumes exceeded escape velocity, proving that this little moon produces the broad, diffuse E-ring around Saturn. Some particles follow ballistic paths back onto Enceladus, coating it (and some neighboring moons) with fresh ice “paint” resulting in the high albedo. And very unexpectedly, all the geysers were erupting from parallel cracks at the south pole. Before they knew what was happening there, mission scientists called these cracks “tiger stripes” by their appearance. They are deep cracks or canyons about 80 miles long and a mile wide and were found to be the warmest parts of the moon. So much material has erupted from the cracks that it has built up piles of ice 300 feet high along the flanks. Most planetary scientists believe that these cracks reach down into a subsurface ocean where energy from tidal heating forces the particles out of the cracks and into space.

The output of this moon is astonishing. Enceladus ejects 79 gallons of water per second, enough to fill an Olympic swimming pool in just a couple of hours. The eruptions are nearly continuous, having been inferred from the two Voyager spacecraft in 1980-1981 and known to be still going on 53 years later. As Cassini was approaching Saturn in 2004, scientists detected large quantities of oxygen coming from the E-ring which later proved to be the result of a major outburst from Enceladus. And just last year, on May 30, 2023, the James Webb Space Telescope detected a “surprisingly large plume” at Enceladus, 6,000 miles in length according to Space.com. If eruptions have been continuing at the average rate of 300 kg/s throughout the lifetime of the moon, it would have erupted over 150 times its current mass by now. Cassini measured 15.8 gigawatts of heat power in the tiger stripes, 2.6 times the power output of all the hot springs in Yellowstone. How such a tiny moon could sustain so much activity when nearby Mimas orbits quietly remains a puzzle. I’ve seen papers claiming or denying that tidal flexing is a sufficient heat source.

Astrobiology and Habitability

Motivated by their “follow the water” motto, NASA astrobiologists tend to focus on the presence of water and a possible liquid ocean as reasons to look for life at Europa and Enceladus. It’s hard to find a news story about Enceladus unaccompanied by the L-word, life. But as I discussed in the Titan article, and as Michael Denton recently explained at Evolution News, life requires much more than water and a few common molecules.

After four flights through the plumes, the Cassini instruments were able to characterize the particles: 91 percent water and 9 percent other materials, including carbon dioxide, salt, ammonia, and methane, with a few trace minerals like silicates and possibly phosphorus. This leads into our discussion of habitability for Enceladus. I’ll share the same factors I listed for Titan.

Magnetic field. Enceladus is well within Saturn’s magnetosphere, reducing its exposure to damaging solar radiation to benign levels, although some galactic cosmic rays and energetic charged particles within the magnetosphere could impinge on the surface.

Temperature. The surface is -330°F (-201°C), but a subsurface ocean implies suitable temperatures for life 30-40 km below the icy crust.

Atmosphere. Without an atmosphere to speak of, any putative biological activity would be confined to its ocean. For the same reason, weather at Enceladus is not an issue, and the surface is not considered a habitat for life by astrobiologists.

Interior. Based on its measured density, Enceladus has a core of silicates and ices. Most likely it would be lacking iron and other heavy elements required for life as we know it.

Water. The moon has plenty of H₂O in solid and probably liquid form, but it is likely salty, which could inhibit the formation of biopolymers.

Carbon. CO₂ was detected in the geyser plumes, making up about 3.2 percent. Additionally, 1.6 percent methane (CH₄) was detected, along with other hydrocarbons like acetylene and propane. Some carbon-nitrogen compounds like HCN and possible amines (R-NH₂ compounds) were detected, as well as ammonia (NH₃), in quantities less than 1 percent (source).

Elemental composition. Characterizing the non-water molecular cupboard at Enceladus was tricky, but the Ultraviolet Imaging Subsystem (UVIS), Ion and Neutral Mass Spectrometer (INMS), and CDA instruments detected salts, implying sodium chloride (NaCl) and maybe potassium (K). Claims of phosphorus detection in the plume data were announced in 2023, making astrobiologists excited because phosphorus is an essential element for nucleic acids and metabolic proteins in all Earth organisms, and can be a limiting factor for biology. Other elements might be delivered by meteorites but would be highly unlikely to reach 30 km down to the subsurface ocean where needed.

Astrobiologists boast of the possibility of amino acids forming at Enceladus, but the lack of elements beyond those detected (C, H, O, N, Na, and P) severely limits the plausibility of life. The simplest microbes on Earth require S, Fe, Mn, Ni, Zn, Cu, and Co for metabolic enzymes and cofactors beyond the common CHON elements and P. As Catherine Neish said as quoted in my previous article, “Titan is the most organic-rich icy moon in the Solar System, so if its subsurface ocean is not habitable, it does not bode well for the habitability of other known icy worlds.” Unless one can envision life made of water, salt, methane, ammonia, and cyanide, the excitement of possible life at Enceladus seems vastly overblown. Except for water and salt, the other required molecules would likely be at prohibitively low concentrations in a cool brine.

Well, Get Excited Anyway

This hasn’t quenched the enthusiasm of astrobiologists like those at the University of Stuttgart who built an apparatus to see whether existing chains of amino acids (on Earth) could survive launch through the pressures found in the Enceladus plumes.

A research team publishing in Science Advances assured the astrobiology community that bacterial counterparts could be detected within a single ice grain issuing from the geyser vents, “even if an ice grain contains much less than one cell.” 

If astrobiology keeps the funding coming for missions to the outer planets, so be it. The probability is exceedingly low that they will find life at Enceladus. Most likely new data will further constrain the extremely narrow conditions for life that we have in abundance on Earth. That would be useful knowledge to have for intelligent design advocates. But until astrobiologists have evidence, I prefer to rearrange the letters to call it bio-astrology.