There’s no way, as a chemist and telescope owner, that I could let this story go by. A new paper reports mass spec data from ice grains that have been sprayed from Saturn’s moon Enceladeus, and let’s just say that there’s a lot of stuff in them.
Enceladeus and Europa (a broadly similar moon around Jupiter) are two of the most interesting destinations in the solar system. They both orbit gas giant planets fairly closely, to the point that gravitational flexing around those orbits kneads them gently but noticeably. A closer orbit gives you a moon like Io, the most wildly volcanically active planet or moon known. Io is like something out of Dante, with lakes of molten sulfur, giant spewing fountains of sulfur dioxide and other noxious volatiles, and massive constantly erupting volcanoes reshaping the terrain year by year. Further out, though, you get iceballs.
Water is incredibly common in the outer solar system (so much for all those movies, with up-to-date special effects and plots out of 1930s pulp magazines, that have rapacious aliens coming to attack Earth because of our oceans). Europa and Enceladeus each have far more water than Earth does, but a good part of it is in the form of an icy crust that’s kilometers thick. Under that, though, you have massive world oceans overlaying what may very well be geologically active surfaces (somewhere down there). It’s hard to know for sure, of course, but the best guesses are the Europa’s ice crust is 15 to 25 km thick, with a massive ocean 60 to 150 km deep below that, while Enceladeus’ ice crust is maybe 40 km thick with a 5 to 30 km deep ocean under that. And the crusts down there may well be geologically active, considering the heat that’s needed to keep that liquid ocean going.
There are definite signs of activity on the surfaces of each moon, which are geologically young. Shown at right is a famous shot of Europa’s surface from the Galileo mission, which shows clear evidence that the surface of the ice is not only ridged and lined with fault-like structures, but has at some point broken up into rafts and frozen solid again. Meanwhile, Enceladeus, in the shot below, shows more ancient cratered ice terrain along with smoother areas that have clearly been remodeled in more recent times. Overall, it would seem that Europa is a bit warmer and more active, but that both moons have extremely interesting geology and hydrology.
What’s caught even more attention is evidence that both moons are actively spraying out plumes of ice particles. A reanalysis of the Galileo data suggests that’s happening on Europa, and the more recent Cassini mission to Saturn has provided a large amount of unequivocal evidence of them on Enceladeus. Indeed, Cassini flew right through them, and a good part of Saturn’s E ring seems to be made up of icy particles that have been ejected in this way. If you want samples of that interior ocean, this is by far the easiest way to get them.
That leads us to this new paper, which provides a look at mass spec data from the plume material and from the E ring. And similar to the recent mass spec data from Mars, there is evidence of complex organic material. Cassini’s Cosmic Dust Analyzer included a time-of-flight instrument that could handle masses up to about 200 daltons, and as the ice grains hit its target, a whole range of organic fragments showed up. The signal is consistent with them coming from a source of higher-MW material, rather than being a set of small individual species on their own. The mass differences also indicate mostly unsaturated carbon atoms, rather than methylene groups. And there are definitely heteroatoms present (oxygen and nitrogen at least), because many of the mass fragments cannot be obtained from just hydrocarbon formulae.
There’s another interesting point: there are easily identifiable peaks for benzene (77 daltons) and tropylium cationic species (91), but the tropylium cation is much more stable, as organic chemists well know. The fact that relatively little of it is seen must mean that the parent structures don’t allow fragmentation in that fashion. Meanwhile, polycyclic aromatic hydrocarbons don’t form either of those single-ring species very well, so we’re already getting some constraints on what the complex organics might be. Data from another onboard mass spec are completely consistent with these results – and in that case, the small organics are only seen at high flyby speed collections, indicating that they are indeed coming from fragmentation events that don’t happen at lower velocities. Overall, the data are consistent with some sort of polymerized/crosslinked material with numerous isolated aryl rings, short aliphatic chains (up to four carbons), and oxygen/nitrogen functional groups scattered throughout.
But there are several types of ice grains seen in this paper (and in other Cassini analyses). There’s nearly pure water ice, there are salty (high sodium) particles that appear to be direct frozen spray from the subsurface ocean) and then there are these organic-laden ones. The hypothesis is that there is a layer of floating organic material in that subsurface ocean, and that some of the plumes are sampling it, while others are sampling the salt-water layer underneath. Aerosols of organic material from sea spray here on Earth have been extensively studied, and we might be seeing the same sort of thing out by the orbit of Saturn. The organics could be picked up on the surface of bubble rising from deeper in the subsurface ocean – at least, that’s how it often works on Earth.
As with the Mars results, the immediate question is whether these compounds are associated with life. For now, in both cases, we have absolutely no way to tell. But it could be. Or it couldn’t. We’re right on the edge of knowing, and I very much hope that we get that answer in time for those of us reading this to see it. Sample collection is rather a long, expensive, and difficult process in these cases, and you don’t get many shots at it. There’s an answer out there. We just don’t know what it is.