Enceladus hovers above this side view of Saturn's rings. The Cassini mission is exploring this icy moon through a series of flybys.
Updated Wednesday, October 28: Cassini made its closest approach to Enceladus at 11:22 a.m. ET.
The Cassini spacecraft takes its deepest plunge yet into an alien ocean on Wednesday.
Or, more specifically, the ocean comes to Cassini, as the probe flies through a huge icy plume near the south pole of Saturn’s moon Enceladus. Stretching thousands of kilometers into space, the plume is the creation of at least 101 geysers of water vapor and ice, erupting from a formation of fissures known as “tiger stripes.”
“Enceladus is not just an ocean world, it's a world that might provide a habitable environment for life as we know it,” said Cassini program scientist Curt Niebur at a NASA press conference held on Monday. “And we will collect the best samples ever from an ocean beyond Earth.”
Skimming just 30 miles (49 kilometers) above the moon's surface, the spacecraft will have unprecedented access to heavier particles that were out of reach during previous, higher-altitude sweeps through the plume, raising the possibility that it might find organic materials that were previously undetected.
With the probe whizzing by at 19,000 miles per hour, the flyby will be over in just a few tens of seconds. But, analysis of the data sent back to Earth over the ensuing weeks will have lasting implications.
Thar She Blows
Although Cassini, which left Earth in 1997, isn’t equipped with scientific instruments designed to detect life directly, the spacecraft’s 20-year-old technology has managed to provide crucial clues about whether Enceladus has the right stuff to sustain extraterrestrial organisms.
“Enceladus has surprised us,” Jonathan Lunine, a planetary scientist at Cornell University recently said at a hearing held by the House Science, Space, and Technology Committee. “Make a list of the requirements for terrestrial-type life—liquid water, organics, minerals, energy, chemical gradients—and Cassini has found evidence for all of them in the plume of Enceladus.”
Cassini first spotted that plume in 2005. Was it ice vaporizing from the surface? Or was it evidence of subsurface water—a regional sea, or perhaps even a vast, planet-wide ocean, like Jupiter’s moon, Europa?
Over the next decade, Cassini gathered data that offered potential answers to those questions—and revealed Enceladus to be more complex than planetary scientists had dared to imagine. A 2008 flyby through the plume sniffed out organic molecules—the fundamental starting point of life's evolution—that included methane, propane, acetylene, and formaldehyde. “Astrobiologically speaking, this moon is one of the most interesting places in the solar system,” said Cassini scientist Hunter Waite.
A year later, salt was found in the plume’s icy particles. The scientists who made the discovery concluded that there had to be liquid water beneath Enceladus, since that would be the only way to dissolve enough minerals to account for the levels of salt they detected.
Earlier this year, researchers produced further evidence of an ocean environment through a curious discovery. Using Cassini’s instruments, the scientists found tiny grains of silica dust in Saturn’s E-ring. Silica is a material found in sand or mineral quartz, so precisely how grains ended up in orbit around Saturn was a bit of a mystery.
Eventually, the researchers concluded that hydrothermal activity—the circulation of warm or hot water through rock—within Enceladus must have produced the grains, which were then ejected in the moon's plume and added to Saturn's E-ring.
That discovery made it increasingly apparent that Enceladus has a dynamic ocean encased within its icy outer rind. But, how deep is it? Estimates were made public in September, after a team of researchers scrutinized seven years of images taken by Cassini. Over time, they detected a slight wobble as the moon orbited Saturn.
If the interior of Enceladus were solid dead weight, then this wobble, known as a libration, would have been much smaller than what the scientists had found. The libration could be explained only if there was a global layer of liquid—16 to 19 miles (26 to 31 kilometers) in depth—separating the icy surface of the moon from its core.
NASA has high hopes that Wednesday’s flyby will shore up the mounting evidence that Enceladus is capable of supporting life.
“We know we’ve seen organics,” said Cassini project scientist Linda Spilker at NASA’s press conference. “We’ve seen methane, carbon dioxide, a number of key ingredients and, in this case, with our much deeper dive through the plume, we’ll have a chance to sample potentially larger particles, and a greater density of both the gas and the particles. We may find new organics that we haven't seen previously or are just at the limits of our detection.”
Another item high on NASA’s wish list is that Cassini will detect molecular hydrogen, which could help reveal how much hydrothermal activity is occurring on the seafloor of Enceladus. What’s more, molecular hydrogen is an excellent source of energy for organisms—especially those that would be cut off from other energy sources, such as sunlight.
Still, evidence that a world is habitable is not proof that it is inhabited. If we really want to know if there’s life on Saturn’s moon, we’ll eventually need to send another spacecraft designed explicitly to search for it.
Lunine believes it can be done. He’s the principal investigator for a proposed mission he calls the Enceladus Life Finder (ELF). Essentially, ELF would do much of what Cassini is currently doing, except with state-of-the-art scientific instruments.
One objective would be to further assess the habitability of the ocean. “We’ll measure the ratios of certain compounds—chlorine, sodium ions, and many other things that will tell us the basic properties,” Lunine says. “What’s the acidity? What’s the ability to generate organic molecules and keep them stable?”
And, of course, ELF will look for signs of life—not actual critters, but indicators of biological processes that can’t be duplicated inorganically, such as the ratio of amino acids. “We know what amino acids look like in a non-biological setting,” Lunine says. “Those are the ones we find in meteorites. They’re full of glycine and alanine. But, among life on Earth, they’re relatively minor. We’ll look to see if they’re the dominant amino acids or under-abundant.”
NASA turned down the initial ELF proposal, but Lunine will continue to make the case for it, especially since, unlike Europa, another target for exploration, we don’t need to penetrate the thick, icy crust to learn about the ocean on Enceladus.
“The plume is a free sample,” he says. “The only thing nature asks is that we get ourselves out there. Once we get ourselves out there, it's free. No drilling, melting, hammering, fracturing, or precision navigation into a crack á la Jules Verne is required.”
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