I’m sure you remember the Cassini mission to Saturn. What I remember most about it is that my daughter, who is about to attend college in the fall, used to love to watch videos about it when she was barely old enough to speak. “Cassini!” she would shout as she pointed at the TV.
On September 15, 2017, after thirteen years exploring Saturn and its moons, Cassini took a final dive into the atmosphere of Saturn, sending us measurements until the very end, fighting to point its transmitter toward Earth until there was no more data to be had:
Oh, but don’t think we’re done learning from Cassini. Far from it.
Why, just a month ago, data from Cassini’s Cosmic Dust Analyzer (CDA) showed that Saturn’s rings cannot be more than 400 million years old, and are likely closer to only 100 million years old, judging by how dirty they’d become from incoming micrometeoroid material, and the known rate at which that happens. We’re lucky to have such spectacular rings in our Solar System, because at the rate they’re falling into Saturn, as also reported just a month ago from yet another Cassini finding, they may only be around for another 100 million years or so.
Another “posthumous” Cassini finding came a couple of years ago, when the spacecraft’s data was used to establish that a significant amount of methane appears in the plumes of Enceladus. It is very tempting to say that this methane is being produced by microorganisms, especially because modeling shows that this is actually the most likely explanation. Not only that, but Cassini also found that there are plenty of other organic molecules in the plumes. All of that doesn’t necessarily mean there are living things on Enceladus, but it sure makes you wonder.
But if life on Enceladus were to be anything at all like life here on Earth, it would require phosphate, something never detected in water on Enceladus or in fact anywhere besides Earth. So many key life processes absolutely depend on phosphate. It’s part of the very structure of DNA and RNA, it’s a terrific way to chemically store energy as adenosine triphosphate (ATP), it’s a charged tag that keeps organic molecules from escaping cellular compartments where they belong, and it’s a modifier that can activate or deactivate proteins in ways that we could not function without.
A short four-base strand of DNA. There’s one phosphate group for every base
It’s true that one very kooky organism has been found on Earth that in a pinch can substitute atomically similar arsenic for phosphorus and still survive somehow (although it still really, really prefers phosphorus), but that just goes to show you that life never ceases to amaze and will stubbornly find a way. (Though Dischorran astutely points out in the comments that while this organism is amazing, it’s amazing in a different way than first assumed.)
But realistically, if you’re going to propose that life exists somewhere, it’s a much tougher sell if there is no phosphate, and there’d been no real evidence of that on Enceladus. But, thanks to data from Cassini’s CDA, gathered as the spacecraft flew through Enceladus’ plume material, and newly reexamined by researchers in Germany, Japan, Finland, and the USA, that has just changed. There is indeed phosphate there, and quite a bit. You can read all about it in the June 14 issue of Nature.
Enceladus not only orbits within the diffuse E ring of Saturn but is responsible for producing it.
The E ring is the wispy outermost one
Enceladus within the E ring that it produces
In fact, an image released by the James Webb Space Telescope team just a couple of weeks ago, along with spectroscopic data, shows that Enceladus’ plumes create a much larger volume of spray — about 20 times the size of the moon itself — than had been appreciated. Busy time for Enceladus right now!
A water vapor plume jets from the southern pole of Saturn’s moon Enceladus, extending out 20 times the size of the moon itself. The inset, an image from the Cassini orbiter, emphasizes how small Enceladus appears in the Webb image compared to the water plume
Back when it was still exploring Saturn, Cassini flew through this plume area and took samples with the CDA. Last fall a subset of our researchers were doing some geochemical modeling based in part on that data, considering what the pH of the ocean is, how long it’s been there, what else is in it, and what the rock beneath it is likely made of. They found that not only should phosphorus be present in significant amounts in Enceladus’ ocean, but it should be predominantly in the form of phosphate, the form that life can use most effectively.
But no one had ever gone back through the Cassini plume data to actually look for phosphate! Modeling isn’t always right, after all, so these researchers decided to do just that.
They gathered together dust data from Cassini’s passes through the E ring (which you’ll recall is made out of Enceladus plume material) and found that during those passes, 345 dusty ice particles had hit the CDA with enough energy to trigger the acquisition of a mass spectrum. This is where the dust particle is vaporized, ionized, and shot into a mass spectrometer to find out what sizes the molecules (positive ions only) are. Heavier ions take longer to be pulled into the ion detector by an opposing negative charge, so you can separate the molecules by size.
Nine of these tiny particles gave a pattern the researchers had not seen before, and it looked an awful lot like a pattern you’d see if you ionized sodium salts of phosphate. They added those nine mass spectra together to cut down on the noise (hey, it’s tough to do precise experiments in space!), and that made a pretty decent graph:
“u” is atomic mass units; for example, a sodium ion (Na+) has an atomic mass of 23 (11 protons plus 12 neutrons). Just because something has a mass of 23 doesn’t mean it’s sodium, but when all the masses in the spectrum are uncannily related to each other, it starts making sense
Back on Earth, they could then try and reproduce and confirm this pattern by laser-ionizing some water droplets that contained known species of phosphates, and they arrived at a combination that approximated the spectrum that the CDA mass spectrometer had made. They got much nicer peaks on Earth, of course!
Laboratory mass spectrum of an aqueous solution of 0.420 M Na2HPO4 and 0.038 M Na3PO4. The “M” means “molar”, or moles per liter. One mole is about 602 sextillion molecules, the number of carbon atoms that weigh twelve grams. A mole is just like a dozen but way, way bigger (602 sextillion of something instead of 12 of something)
The formula used here was an 11:1 ratio of disodium phosphate (Na2HPO4) to trisodum phosphate (Na3PO4), so we can say the phosphate found in the E ring looks roughly like that. As far as phosphate concentration in Enceladus’ ocean, they divided the total mass of phosphate detected by the volume of all 345 ice particles to arrive at a fair estimate of phosphate concentration in Enceladus’ ocean of at the very least 100 milligrams per liter (and probably a lot more), which would be substantially higher than the phosphate concentration of Earth’s own oceans.
So it seems Enceladus’ ocean now checks all the big boxes for containing life, but of course we can’t say whether it’s there for sure until we go back and actually find it — or don’t!
A mission to Enceladus was recommended as the 2nd highest priority — only a Uranus mission ranked higher — by the very influential Planetary Science Decadal Survey for 2023-2033, so NASA is busy planning one as we speak! And NASA’s not the only one.
NASA/JPL scientist Linda Spilker, who can be spotted in the Cassini video above but wasn’t involved in the phosphate study, had this to say:
“This latest discovery of phosphorus in Enceladus’ subsurface ocean has set the stage for what the habitability potential might be for the other icy ocean worlds throughout the Solar System. Now that we know so many of the ingredients for life are out there, the question becomes: Is there life beyond Earth, perhaps in our own Solar System? I feel that Cassini’s enduring legacy will inspire future missions that might, eventually, answer that very question.
Even though it’s gone now, we’ll still be learning from Cassini for a long time.
“Cassini!”
⭐️
We learned earlier today that someone many of us adored here at Daily Kos passed away yesterday. I think it’s fitting that my diary happens to be about Cassini today, because we’ll also still appreciate CameronProf — a.k.a. BFSkinner — long, long after he’s gone. When all of this is said and done, and we look back fondly on our years here at Daily Kos, he’ll be a big reason it was what it was. As nancyjones aptly put it earlier today:
Another bright star in the sky.
