Astrobiology Revealed #7: Frank Postberg

On the pivotal discovery of phosphorus in Enceladus’ ocean

by Aubrey Zerkle

This week we had an exciting discussion with Frank Postberg, Professor of Planetary Sciences in the Institute of Geological Sciences at the Freie Universität Berlin. In their recent paper, “Detection of phosphates originating from Enceladus’s ocean,” Frank and coauthors describe the surprising detection of vast amounts of the bioessential nutrient, phosphorus, in the liquid ocean beneath Enceladus’s icy shell. Frank describes how they made this astonishing discovery, and why it could be a harbinger for habitability in the outer solar system. (This interview has been edited for length and clarity.)

Why is Enceladus an interesting target for astrobiology research?

There are two main reasons. First, from all we know it’s currently the most habitable environment outside Earth. And second, it’s easy to sample.

There’s a subsurface ocean, from which we know that the salinity and the pH are not extreme, they are moderate. At the bottom of the ocean there’s a rocky seafloor, with which the water interacts and has dissolved lots of minerals. We also know of a wide variety of organic compounds that are present in the ocean. And we have strong indications for hydrothermal activity at the seafloor as an energy source.

Even before our finding of phosphate, we learned all that from sampling the material that is shot into space from the ocean by its cryovolcanic activity. This is another unique feature of Enceladus that allows us in a simple, straightforward way to sample its ocean. So now we have confirmed the 6 bioessential elements [the CHNOPS elements - carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur]. 

But one word of caution here - sulfur is still a tentative detection. That has been written incorrectly in several articles. The sulfur detection is not water-proof. That was found in the gas phase, as H2S. In 2009 they reported the detection, but then a few years later they said this is not 100% water-proof, there might be other interpretations. It’s still the best fit, but it’s not totally unambiguous. 

So it’s got CHNOP, but not necessarily CHNOPS?

Right, the S may be with some parentheses around it! But phosphorus was seen as the most important one, because it’s the least abundant cosmochemically and it doesn’t like to dissolve in water. Phosphate is poorly soluble. It’s normally bound up with calcium in the rock, and that’s one of the things that makes the bioavailability so hard. And now [we’ve found] it’s there in large abundances in the ocean. 

So it ticks all the boxes! As far as we know, there’s no other environment that is so well constrained that ticks all these habitability boxes. It doesn’t mean it’s inhabited, but it just says it’s a favorable environment. At least for extant life, for current life. So, that’s why I think it’s pretty attractive for astrobiology.

You said it’s easy to sample. Is that because the material that makes up the E-ring of Saturn is thought to be ejected from Enceldaus’s ocean by its cryovolcanism?

Yes, that’s the current idea. At least a substantial fraction of the ice grains [in the E-ring] appear to be frozen ocean droplets. We see the different salts in the same proportions, and this only makes sense if there’s a direct conduit to the ocean. We also see minerals that have to come from the rocky core, like silicates. And the phosphates, there’s no way they’re just somewhere buried in the ice shell. So, this is all very strong indication that we have a direct connection to the subsurface ocean. We also have this material in the plume [of material being ejected from Enceladus] which eventually also ends up in the E-ring.

The fate of the ejected material is two-fold. The slower particles don’t escape Enceladus, and fall back as an umbrella of snow covering the southern hemisphere of the moon. Most of the deposits you find directly near the sources, so you can expect tens of meters of snow there. The faster particles escape the gravitational influence of Enceladus, since the gravity is pretty low, just a little more than 1% of Earth’s gravity, and ejection speeds are high. On its way around Saturn, Enceladus constantly loses ice grains from the plume, and that makes this ring-shaped structure, the E-ring, which can be seen as an extension of the plume. From the E-ring we have much more data, because Cassini [NASA’s mission to Saturn] spent much more time in the E-ring, compared to very short traverses through the plume. And we needed these good statistics for the phosphate detection.

You detected the phosphate in Enceladus’s ocean by looking at mass spectra from the ice grains sampled by Cassini. How does a probe that’s hurtling through space analyze the mass spectra of an ice grain?

It’s not easy! I’m surprised myself that this works really well. It is an instrument that previously was not very common on spacecrafts. The Cosmic Dust Analyzer on Cassini was basically the first of its kind, at least with such a large aperture. It’s an impact ionization mass spectrometer, so you use the kinetic energy of the impact of a single ice grain or dust grain that hits the bottom of the instrument, its metal target. The spacecraft flies so fast around Saturn that the impact speeds are kilometers per second, even 10 kilometers per second. You can translate that in miles per hour yourself, but it’s 10’s of thousands of miles per hour for sure. [In fact it’s more than 22,000 miles per hour!] 

So this ice grain makes a poof, and partially ionizes. The ions that are created by the impact we then analyze with time of flight mass spectrometry, and with that can reverse engineer the make-up of the ice grain that hit us. We get a mass spectrum for each micrometer-sized ice grain. And this is fantastic – we need only a tiny sample to make this compositional assessment!

That’s remarkable! What are the implications of finding phosphorus for the first time in an ocean beyond Earth?

Well, it is probably found for the first time beyond Earth in Enceladus because it’s so easy to sample that ocean. I would predict that if you look at other ocean moons, at Europa, at Titan, at Pluto even, you would probably find phosphate there as well. But Enceladus makes the job so easy, and we had a spacecraft right there on the spot. 

It was also a lucky coincidence that Cassini had such diverse explorative instrumentation. Nowadays you specialize the spacecraft, but this was like a dinosaur of the good ole times. It had a diverse instrumentation that could react immediately on the discovery of the plume, and then later the subsurface ocean. That’s why this is the first subsurface ocean that was really sampled thoroughly.

With Europa Clipper, we will do a similar good job at Europa, but Cassini was the first one at Enceladus. And we found large amounts of phosphate, surprisingly large amounts. We were astonished ourselves, because the geochemical computer models previously predicted much lower [phosphate] abundances.

The previous models predicted scarce phosphate in Enceladus’s ocean, but your models predict enormous amounts - up to hundreds of times the phosphate levels in Earth’s oceans! What’s changed in these models such that they’ve gone from “there’s no phosphorus” to “there’s tons of it”?

The first model that came out in 2018 predicted that Enceladus’s ocean and the ocean moons in general should all be particularly depleted in phosphorus. Like 1000 times less than Earth oceans, that’s what they said in the conclusion of that paper. Of course, there was some ambiguity and an error bar, but that was the sweet spot of their scientific modelling. But they made some wrong assumptions, mostly about the rock composition in the outer solar system. They took rock analogues from Earth.

The ocean composition of Enceladus is somewhat different. It has one critical difference to Earth oceans, which in the end makes the phosphorus dissolve much better than in the Earth oceans. And the rock composition is different. In 2022, I was co-authoring a modelling approach, still without data, that took a better analogue for the peculiarities of the outer solar system. In this modelling paper we already predicted much higher levels. We submitted the paper just a few months before we actually found the phosphorus, so at the time of writing we had no clue [what we would actually find]. What we actually found [with the data from Cassini] is really at the upper end of that more optimistic prediction. 

So we went back to the lab. Instead of doing theoretical modelling, we teamed up with a world-class geochemistry lab in Tokyo, led by Yasuhito Sekine. For more than a year, they went into the lab and did experiments. They used Enceladus ocean simulant water, because from previous Cassini measurements we already had a good idea what the salinity is, the pH is, what the ingredients are - ammonia and carbonates, both of which are at much lower abundances in Earth’s ocean. 

They put [the simulant ocean water] into contact with a simulant for Enceladus’s ocean floor, a carbonaceous chondrite, which is seen as a good analogue material for rocks in the outer solar system. They made several experiments, each of which were several months long, monitoring how the composition of the rock and the water changed. In the end that told us what’s the trick of Enceladus, what’s the geochemistry behind this. That told us why there is so much more phosphate dissolved [in Enceladus’s ocean] compared to Earth’s ocean and other places.

So what is the trick? What’s the special thing about Enceladus’s ocean that keeps phosphorus in solution?

It’s the soda ocean, in a short word. It’s the high carbonate and CO2 content. This is evident in the plume outgassing. The volatiles in the plume are mostly water vapor, but there’s a substantial fraction of CO2. We also see large amounts of sodium carbonates in the ice grains. These have been previously dissolved in the ocean like the phosphates, and when the droplets froze, the salts crystallized in the ice grain. So the CO2-rich ocean water is able to dissolve much more phosphates from the rock than without carbonates and carbon dioxide.

It’s no surprise that there’s phosphates around in the outer solar system. Phosphorus has been found in meteorites, it has been found in comets, so that was no surprise. It’s not very abundant, it’s the least abundant of the 6 bioessential elements, but it’s not super trace. A hundred parts per billion or so is the typical concentration in carbonaceous chondrites. It’s a little bit less than on Earth, but still it’s there. The surprise was that it’s so easily soluble in Enceladus’s ocean, and that it’s readily available for potential formation of life. The carbonate-rich ocean water has the property that it can more easily dissolve the phosphates in this carbonaceous rocky material.

There was another interesting detail I only learned recently when we wrote the paper. There’s no soda ocean on Earth, but there are soda lakes, carbonate-rich lakes with similar pH. And there you observe the same thing, that the phosphate concentration goes up. That has another implication, because CO2 and carbonates should be way more abundant in the outer solar system compared to the inner solar system.

You stated in the paper that “Enceladus’s ocean could be a harbinger of high phosphorus availability in subsurface oceans across most of the outer Solar System.” Which, if so, that would be extraordinary! 

Yes, exactly! That’s why, in the last paragraph of our paper, we extrapolate that most of the oceans in the outer solar system should be carbonate-rich. And if they get in contact with any kind of rock, specifically with carbonaceous chondritic rock, they should also be rich in phosphates. And with that, phosphorus availability should not be a bottleneck [for life] on any of these ocean worlds in the outer solar system. 

If we now have confirmation that Enceladus ticks all the boxes for habitability, when are we going back there?

When? I don’t know!

Are there any missions to Enceladus currently in the works?

There are two big flagship missions from ESA and NASA going to the Jupiter ocean moons. These will be very cool missions, and I think we will learn about Europa and Ganymede as much as we learned about Enceladus with Cassini, or even more. The ESA mission JUICE is on its way to Jupiter already, it launched two months ago. Europa Clipper, the big NASA spacecraft, will launch next year. This is  something new in the outer solar system, its only overarching science goal is to explore the habitability of Europa.

But for Enceladus, there are mission proposals, there are teams developing mission designs, mission scenarios, and suitable mission instrumentation. The next mission to Enceladus is something really new that hasn’t been done since Viking, that you design a mission that should look for life. The habitability question, I think, is not a question anymore. So, if you go back to Enceladus, you need to look for life in the best possible way, so that it’s not a false positive, not a false negative, that it’s unambiguous. And this is tricky. That’s why for a few years several teams are already trying to find the best model payload, the most affordable payload, since you need to spend the taxpayers’ money carefully. You don’t want to repeat the Viking lander. That was a pretty expensive mission, to look for life on Mars in the 70’s, and the outcome was inconclusive. 

Both NASA and ESA have ranked the ocean moons, and specifically Enceladus, very high in their strategic planning. The NASA Decadal Survey, and the ESA Voyage 2050 both clearly say that. So, it’s just a matter of time, probably a few years, until a mission will materialize and be scheduled for the next visit to Enceladus!