New models trying to infer the geology of potentially habitable moons orbiting Jupiter and Saturn hint at surprisingly cool, geologically inactive worlds, the opposite of what a diverse alien ecosystem would need
Imagine a spaceship finally landing on Europa and slowly drilling into the ice. After weeks of very careful progress, it pierces the moon’s frozen shell and releases a small semi-autonomous submarine connected to the probe with an umbilical to ensure constant communication and a human taking over in case of an emergency. Much of the time, it will chart a course of its own since piloting it with an hour long delay between command and response would be less than ideal. It navigates through the salty ocean, shining its light on structures never before seen by a human eye, making its way deeper and further into the alien environment to find absolutely… nothing at all.
That’s the sad scenario proposed by a team of geologists who crunched the numbers on the four leading contenders to host alien life in our outer solar system: Europa, Ganymede, Titan, and Enceladus. According to their models, looking at gravity, the weight of water and ice on the rocks underneath, and the hardness of the rocks themselves, these moons would be more or less geologically dead. Without volcanoes or sulfur vents, there would be very little in terms of nutrient exchange and therefore, very little food and fuel for an alien ecosystem more complex than microbe colonies.
Of course, these results are a pretty serious departure from the hypotheses commonly held by planetary scientists that the gravity of gas giants cause tidal kneading inside their moons, citing Io as an example. According to the researchers’ model, only Enceladus would be a promising world to look for life, as evidenced by the plumes breaking through its icy crust, spraying organic material into space. The reason why the numbers are different, they say, is because its core is likely to be porous, meaning its ocean would be heated deep inside the moon, fueling geysers and churning organic matter while effectively making the little world a ball of soggy slush.
Since these findings are so different from what’s implied by observations, the researchers aren’t in a rush to publish them are are soliciting other scientists’ opinions to make sure they have a complete picture, and lead investigator Paul Byrne grumbled about his disappointment with what the models indicate. That said, while he’s hoping to be proven wrong, we shouldn’t forget that these are alien worlds and while we’ve spent decades studying them, our knowledge came in bursts. Simply put, we might know a fair bit but far from everything and disappointing surprises may lurk under their icy surfaces and subterranean oceans.
The day before Cassini plunged into Saturn’s atmosphere, dramatically ending 13 years of Saturn exploration (and nearly two decades in space), I was sitting on a bench outside the Von Karman Visitor Center on the NASA Jet Propulsion Laboratory campus in La Cañada Flintridge with Linda Spilker, who served as the mission’s project scientist since before Cassini was launched.
“I feel very fortunate to be involved with Cassini since the very beginning … and just to be there, to be one of the first to see SOI [Saturn Orbital Insertion] with those first incredible ring pictures,” she told me. “I love being an explorer. I worked on the Voyager mission during the flybys of Jupiter, Saturn, Uranus and Neptune; that sort of whet my appetite and made me want more, to become an explorer to go to the Saturn system.”
Spilker especially loved studying Saturn’s rings, not only from a scientific perspective, but also because they are so beautiful, she continued. “It’s been a heartwarming experience,” she said.
But Cassini’s “legacy discovery,” said Spilker, was the revelation that the tiny icy moon of Enceladus is active, venting water vapor into space from powerful geysers emerging from the moon’s “tiger stripes” — four long fissures in the moon’s south pole. After multiple observations of these geysers and direct sampling of the water particles during flybys, Cassini deduced that the icy space marble hides a warm, salty ocean.
“What Cassini will be remembered for — its legacy discovery — will be the geysers coming from Enceladus with the ocean with the potential for life. It’s a paradigm shift.” — Linda J. Spilker, Cassini project scientist, NASA Jet Propulsion Laboratory (JPL), Sept. 14, 2017.
Alongside Jupiter’s moon Europa, Enceladus has become a prime destination for future explorations of life beyond Earth. Its subsurface ocean contains all the ingredients for life as we know it and Cassini was the mission that inadvertently discovered its biological potential. So now we know about this potential, Spilker is keen to see a dedicated life-hunting mission that could go to Enceladus, perhaps even landing on the surface to return samples to Earth.
As Enceladus is much smaller and less massive than Europa, its gravity is lower, meaning that landing on the surface is an easier task. Also, the radiation surrounding Saturn is much less aggressive than Jupiter’s radiation belts, meaning less radiation shielding is needed for spacecraft going to Saturn’s moons.
But if we ever send a surface mission to Enceladus (or any of the icy moons in the outer solar system), the planetary protection requirements will be extreme.
“If any life were found on these moons, it would be microbial,” said Larry Soderblom, an interdisciplinary scientist on the Cassini mission. “Some [terrestrial] bacteria are very resilient and can survive in hot acid-reducing environments. They can be tenacious. We have to make sure we don’t leave any of these kinds of Earthly bacteria on these promising moons.”
Soderblom has a unique perspective on solar system exploration. His career spans a huge number of NASA missions since the 1960’s, including Mariner 6, 7, 9, Viking, Voyager, Galileo, Magellan, Mars Pathfinder, the Mars Exploration Rovers, Deep Space 1, to name a few. While chatting to me under the shade of a tree on the JPL campus, he pointed out that the outer solar system was seen as a very different place over half a century ago.
“When I started to explore the solar system as a young guy just out of graduate school, our minds-eye view of the outer solar system was pretty bleak,” he remembered. “We expected lifeless, dead, battered moons with no geologic activity.”
After being involved with many outer solar system missions, this view has radically changed. Not only have we discovered entire oceans on Enceladus and Europa, there’s active volcanoes on Jupiter’s tortured moon Io, an atmosphere on Titan sporting its own methane cycle and surface lakes of methane and ethane. Other moons show hints of extensive subsurface oceans too, including distant Triton, a moon of Neptune. When NASA’s New Horizons flew past Pluto in 2015, the robotic spacecraft didn’t see a barren, dull rock as all the artistic impressions that came before seemed to suggest. The dwarf planet is a surprisingly dynamic place with a rich geologic history.
Sending our robotic emissaries to these distant and unforgiving places has revolutionized our understanding of the solar system and our place in it. Rather than the gas and ice giant moons being dull, barren and static, our exploration has revealed a rich bounty of geologic variety. Not only that, we’re almost spoilt for choices for our next giant leap of scientific discovery.
Missions like Cassini are essential for science. Before that spacecraft entered Saturn orbit 13 years ago, we had a very limited understanding of what the Saturnian system was all about. Now we can confidently say that there’s a tiny moon there with incredible biological potential — Enceladus truly is Cassini’s legacy discovery that will keep our imaginations alive until we land on the ice to explore its alien ocean.
For more on my trip to JPL, read my two HowStuffWorks articles:
A little frozen Saturn moon, with a diameter that could easily fit inside the state of New Mexico, holds some big promises for the possibility of finding basic alien life in our solar system.
Enceladus is often overshadowed by its larger distant cousin, Europa, which orbits Jupiter and the Jovian moon’s awesome potential has been widely publicized. But Enceladus has one thing Europa doesn’t — it has been visited very closely by a robotic space probe that could take a sniff of its famous water vapor plumes. And this week, there was much excitement about another facet of the moon’s complex subsurface chemistry, thanks to analysis carried out on data gathered by NASA’s Cassini mission.
But before we get into why this new discovery is so cool, let’s take a very quick look at the other signs of Enceladus’ life-giving potential.
The Cocktail Of Life
Being living, breathing creatures on a habitable planet, it may not come as a surprise to you that for biology to evolve, it needs a few basic ingredients. Liquid water is a definite requirement, of course. Heat also helps. Throw some organic chemistry into the mix and we have a party.
Enceladus, however, is a tiny icy globe, there’s no sign of liquid water on its surface. But when Cassini arrived at Saturn in 2004, Enceladus revealed some of its best-kept secrets. Firstly, it may be a smooth ice ball, but the moon has a large quantity of water under its surface. This water even escapes as geysers, through fissures in its icy crust, producing stunning plumes that eject material hundreds of miles high and into Saturn’s rings.
Before Cassini was launched to Saturn, we had little clue about Enceladus’ watery potential — though this finding explained why Enceladus appeared so bright and how it contributes material to Saturn’s E-ring. Fortunately, the spacecraft has an instrument on board — a mass spectrometer — that could be used to “taste” the watery goodness of these plumes. During its Enceladus flybys, Cassini was able to fly through the plumes, revealing a surprisingly rich chemical cocktail — including a high concentration of organic chemistry.
It’s as if all the building blocks of life have been thrown into a small icy cocoon, shaken up and gently heated from within.
Now, another fascinating discovery has been made. Further analysis of Cassini data from its last 2015 plume fly-through, molecular hydrogen has been detected and planetary scientists are more than a little excited to add this to Enceladus’ habitable repertoire.
Deep In The Enceladus Abyss
“Hydrogen is a source of chemical energy for microbes that live in the Earth’s oceans near hydrothermal vents,” said Hunter Waite, principal investigator of Cassini’s Ion Neutral Mass Spectrometer (INMS) at the Southwest Research Institute (SwRI), in a statement on Thursday (April 13). “Our results indicate the same chemical energy source is present in the ocean of Enceladus.”
This hydrogen could be a byproduct of chemical reactions going on between the moon’s rocky core and the warm water surrounding it. And there’s a lot of hydrogen gas being vented, probably enough to sustain basic lifeforms deep in the Enceladus abyss.
“The amount of molecular hydrogen we detected is high enough to support microbes similar to those that live near hydrothermal vents on Earth,” added co-author Christopher Glein, who specializes in extraterrestrial chemical oceanography, also of SwRI. “If similar organisms are present in Enceladus, they could ‘burn’ the hydrogen to obtain energy for chemosynthesis, which could conceivably serve as a foundation for a larger ecosystem.”
Yes, we’re talking alien microbes. (Also, “extraterrestrial chemical oceanography” — oceans on other worlds! — is one hell of a mind-blowing topic to specialize in, just sayin’.) And did he mention “larger ecosystem”? Why yes! Yes he did.
So, in short, we know Enceladus has a liquid water ocean. We know that it has an internal heat source (hence the liquid oceans). We also know there’s organic chemistry. And now there’s solid hints that there’s water-rock interactions going on that terrestrial microbes living at Earth’s ocean vents like to munch on. If that’s not a huge, blinking neon sign pointing at Enceladus, saying: “We need a surface mission here!” I don’t know what is.
Although the researchers are keen to emphasize that alien microbes have not been found (because Cassini isn’t capable of looking for life), the universe has given us a moon-sized Petri dish where an “ecosystem” may have taken hold. All the ingredients are there, wouldn’t it be cool to find out if Enceladus could be another place in the solar system where life may be hanging out?
In October 2008, Cassini flew very close to the surface of Saturn’s icy moon Enceladus. From a distance of only 50 km from the moon, the spacecraft was able to collect samples of a plume of ice. In an earlier “skeet shot”, Cassini captured detailed images of the cracked surface, revealing the source of geysers blasting the water into space. At the time, scientist were able to detect that it was in fact water ice, but little else would be known until the molecular weight of chemicals in the plume could be measured and analysed.
At the European Geophysical Union meeting in Vienna this week, new results from the October Enceladus flyby were presented. Frank Postberg and colleagues from the Max Planck Institute for Nuclear Physics have discovered traces of sodium salts and sodium bicarbonate in the plume for the first time.
It would appear that these chemicals originated in the rocky core of the moon and were leached from the core via liquid water. The water was then transported to the surface where it was ejected, under pressure, into space. Although scientists are aware that the chemical composition in the plume may have originated from an ancient, now frozen, sub-surface ocean, the freezing process would have isolated the salt far from the surface, preventing it from being released.
“It is easier to imagine that the salts are present in a liquid ocean below the surface,” said Julie Castillo of NASA’s Jet Propulsion Laboratory in Pasadena, California. “That’s why this detection, if confirmed, is very important.”
This is the best evidence yet that Enceladus does have a liquid ocean, bound to cause a stir amongst planetary scientists and re-ignite excitement for the search for life living in a salty sub-surface ocean.