Chapter 1: What makes Enceladus a significant target for astrobiology?
This is Enceladus, Saturn's sixth largest moon. A tiny world orbiting a giant planet. But don't let its seemingly unimpressive size fool you. When the Cassini space probe found huge cryovolcanic plumes erupting from its south pole, this little world brought the entire scientific community to a standstill. What was going on? Flying through these icy jets got us closer to an answer.
The probe discovered water, organic molecules, and hints of a hidden ocean filled with hydrothermal vents. But in 2017, Cassini ran out of fuel. As it dived into Saturn, it took with it our only means of probing Enceladus, leaving us with countless unanswered questions. Does it have a global ocean? What exactly is happening below the surface? And most importantly, could it be home to life?
A decade on, we've not yet returned to Enceladus, but the science hasn't stopped. The James Webb Space Telescope has turned its attention to the Moon, and researchers are still mining the data that Cassini sent back. And luckily, we found more answers than you might think.
Chapter 2: What did the Cassini mission reveal about Enceladus' ocean?
I'm Alex McColgan, and you're watching Astrum. Join me today as we return to this surprise in the Saturn system, explore the incredible chemical complexity hiding below its surface, and reveal how 10-year-old data is still shocking us, all in the search for life beyond Earth. At just 500 km across, Enceladus is a tiny world. It's similar in size to the state of Arizona.
It orbits 238,000 km from Saturn, where the Sun is a mere distant spark, and surface temperatures average a bone-chilling minus 201 degrees Celsius. All these factors make Enceladus an incredibly difficult target to track. It was finally discovered by Sir William Herschel in 1789, and to spot it required the largest telescope the world had ever seen.
The instrument, built by Herschel and his sister Caroline, was so pioneering that its half-ton mirror required more than a year of painstaking polishing. Yet, even through this behemoth scope, Enceladus looked like nothing more than an unremarkable icy rock. Herschel's telescope was so far ahead of its time that this remained our only perspective of the Saturnian satellite for nearly 200 years.
It wasn't until the 1980s, when Voyager 1 and 2 performed their swift lie-bys, that our perspective shifted forever. The images sent back didn't show the featureless world we expected, but instead revealed regions that were remarkably smooth and crater-free, suggesting the Moon had been recently seismically active.
Most curiously, Enceladus was found sitting directly in the densest part of Saturn's E-ring, leading scientists to suspect the Moon was somehow feeding the ring with material.
Was Enceladus alive? The mysteries raised by the Voyager flybys made Enceladus a high priority target for a follow-up, and directly fuelled the planning of the Cassini-Huygens mission.
The Titan IV-B rocket launched Cassini in 1997. and in late 2004, six years after its double slingshot maneuver around Venus, it reached Saturn to carry on Voyager's legacy. It was a monumental effort that has since generated more than 4,000 scientific papers, but no discovery caused more hype than the events of early 2005.
During its first flybys, Cassini's cameras caught sight of massive geysers erupting from Enceladus into the vacuum of space. These vents were ejecting 250kg of water and ice every second at speeds of over 1,000 km per hour. This confirmed it. Enceladus had an atmosphere and active geology.
NASA immediately shifted the mission's course, ordering Cassini to perform a maverick maneuver, passing just 50 kilometers above the surface to taste the plumes. What was worth such a risk, you might ask? Well, NASA were looking for signs of extraterrestrial life. And I for one can't wait to find out for sure if alien life exists elsewhere in the universe.
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Chapter 3: How did the James Webb Space Telescope contribute to our understanding of Enceladus?
This result tells us that Enceladus is generating a lot more internal heat than we thought, and suggests the ocean may have been there for a while. But, this far from the sun, it is bitterly cold. As I mentioned, the surface temperature of Enceladus is minus 201 degrees Celsius. There is no way water should be liquid. So, next on the list for scientists to check out was energy.
Where was the heat coming from? If Encella does generated heat by radioactive decay in its core, like Earth does, the energy produced would only be 1% of what Cassini was witnessing. So in 2017, a team led by Gail Schoble proposed a solution. Enceladus is tidally locked to Saturn, meaning the Moon only ever shows one face to the planet.
This distorts Enceladus, which creates heat inside the Moon through internal friction, and powers geological activity. But it's not enough to just generate and radiate the heat, it needs to be physically transported from the core to the ocean above. How? Well, incredibly.
It seems that water perfuses the silicate mineral core where it is heated, causing it to rise in focused plumes that pepper the seafloor around the South Pole. In 2022, a study led by Wan-Ying Han found further proof of these events. Based on Cassini's data, they created models of Enceladus' icy surface to try and work out the salinity of the ocean.
They found that it's just a little less salty than we find here on Earth, and that means that there must be, or have been, water-rock reactions, or in other words, vents, at some point. Hydrothermal vents are credited by scientists as powering the origin of life on Earth.
That these vents may also exist on a small moon around a gas giant is an electrifying discovery, because they give us the third requirement for life, chemicals. Cassini was equipped with a mass spectrometer, an instrument that can identify individual molecules by the mass of their ions. On its flybys, it detected hydrogen, believed to be a fuel source for early life.
and complicated hydrocarbons containing oxygen, carbon and nitrogen. But scientists racing to process the huge volume and richness of data couldn't keep up. Before they could get through it all, the Cassini mission came to an end, and on the 15th of September 2017, it was sent hurtling towards Saturn.
and was vaporised by the planet's atmosphere, in a self-destructive manoeuvre designed to protect potential habitable life on Enceladus and Titan from contamination by Earth-born microbes. Though Cassini's mission had been completed, ours had not.
An analysis in 2018 had confirmed the existence of ring-shaped organic molecules and simple oxygen-containing molecules, indicating some limited chemistry was occurring. Another huge boost to the search for life came in 2022, when the last element crucial to life was observed in the data – phosphorus.
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Chapter 4: What evidence suggests that Enceladus has a global ocean?
By understanding the typical pattern of how an object breaks under certain conditions, we can be more confident of what it used to be. Improved analytical tools helped a team led by Nozea Kavajar from the Institute of Space Systems in Stuttgart do exactly that.
After more than a decade of studying the plume flybys, Kavajar's team published their results last year, using samples that were collected when Cassini passed just 20km from the Moon's surface.
Unlike others the probe collected, they were just minutes old. and therefore unchanged by ionising radiation from the sun like other particles in the E-ring. These samples were a pristine example of what's in the oceans of Enceladus.
In the most extreme examples, they found molecules more closely related to life than ever before – carbon dioxide, carbon-rich alkanes and alkenes. cyclic esters and ethers, ethyls, and other nitrogen and oxygen-bearing compounds. In other words, complex chemistry, potentially powered by hydrothermal vents. These molecules are the Lego blocks of chemistry.
More functional groups allow for more complex reactions. In a lab, these can become so complex that they even demonstrate Darwinian evolution. And once you have evolution, There's just one last requirement. Time. What we need to know is, how stable and frequent is this geothermal activity? Would life have had enough time to emerge on Enceladus?
Although Cassini was at Saturn for 13 years, that was, of course, too short a time to be conclusive. Enter the James Webb Space Telescope. A team led by Dr. Jeronimo Villanueva used 10 hours of the James Webb's time to check in on Enceladus, and what they saw was shocking.
The moon had an even more enormous plume erupting from its south pole, expanding to 20 times the size of the moon itself, that is significantly larger than what Cassini saw. These observations widened the known window of Enceladus' activity, and they solved another massive mystery. Where does Saturn's atmospheric water come from?
The images show that Enceladus feeds a large torus of ice particles around Saturn, making it the only moon in the solar system that affects the atmospheric chemistry of its host planet. But these water-ice particles also have another impact on the planet. As they travel through the Saturn system, they're stripped of electrons, becoming ionized.
This then creates an electrically charged plasma, which scientists have recently discovered interacts with the planet's magnetic field. Plasma waves 504,000 km long, that's 1,000 times the moon's diameter, known as the Alphen wings, stream from the moon. The main ones go from Enceladus to the planet, but others are reflected back, creating an intriguing lattice of plasma around Saturn.
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