No one ever said Europa Clipper would be able to detect life beneath the ice, but as we look at the first imagery from the spacecraft’s star-tracking cameras, it’s helpful to keep the scope of the mission in mind. We’re after some critical information here, such as the thickness of the ice shell, the interactions between shell and underlying ocean, the composition of that ocean. All of these should give us a better idea of whether this tiny world really can be a home for life.
Image: This mosaic of a star field was made from three images captured Dec. 4, 2024, by star tracker cameras aboard NASA’s Europa Clipper spacecraft. The pair of star trackers (formally known as the stellar reference units) captured and transmitted Europa Clipper’s first imagery of space. The picture, composed of three shots, shows tiny pinpricks of light from stars 150 to 300 light-years away. The starfield represents only about 0.1% of the full sky around the spacecraft, but by mapping the stars in just that small slice of sky, the orbiter is able to determine where it is pointed and orient itself correctly. The starfield includes the four brightest stars – Gienah, Algorab, Kraz, and Alchiba – of the constellation Corvus, which is Latin for “crow,” a bird in Greek mythology that was associated with Apollo. Besides being interesting to stargazers, the photos signal the successful checkout of the star trackers. The spacecraft checkout phase has been going on since Europa Clipper launched on a SpaceX Falcon Heavy rocket on Oct. 14, 2024. Credit: NASA/JPL-Caltech.
Seen in one light, this field of stars is utterly unexceptional. Fold in the understanding that the data are being sent from a spacecraft enroute to Jupiter, and it takes on its own aura. Naturally the images that we’ll be getting at the turn of the decade will far outdo these, but as with New Horizons, early glimpses along the route are a way of taking the mission’s pulse. It’s a long hike out to our biggest gas giant.
I bring this up, though, in relation to new work on Enceladus, that other extremely interesting ice world. You would think Enceladus would pose a much easier problem when it comes to examining an internal ocean. After all, the tiny moon regularly spews material from its ocean out through those helpful cracks around its south pole, the kind of activity that an orbiter or a flyby spacecraft can readily sample, as did Cassini.
Contrast that with Europa, which appears to throw the occasional plume as well, though to my knowledge, these plumes are rare, with evidence for them emerging in Hubble data no later than 2016. It’s possible that Europa Clipper will find more, or that reanalysis of Galileo data may point to older activity. But there’s no question that in terms of easy access to ocean material, Enceladus offers the fastest track.
Enceladus flybys by the Cassini orbiter revealed ice particles, salts, molecular hydrogen and organic compounds. But according to a new paper from Flynn Ames (University of Reading) and colleagues, such snared material isn’t likely to reveal life no matter how many times we sample it. Writing in Communications Earth and Environment, the authors make the case that the ocean inside Enceladus is layered in such a way that microbes or other organic materials would likely break down as they rose to the surface.
In other words, Enceladus might have a robust ecosystem on the seafloor and yet produce jets of material which cannot possibly yield an answer. Says Ames:
“Imagine trying to detect life at the depths of Earth’s oceans by only sampling water from the surface. That’s the challenge we face with Enceladus, except we’re also dealing with an ocean whose physics we do not fully understand. We’ve found that Enceladus’ ocean should behave like oil and water in a jar, with layers that resist vertical mixing. These natural barriers could trap particles and chemical traces of life in the depths below for hundreds to hundreds of thousands of years.”
The study relies on theoretical models that are run through global ocean numerical simulations, plugging in a timescale for transporting material to the surface across a range of salinity and mixing (mostly by tidal effects). Remarkably, there is no choice of variables that offers an ocean that is not stratified from top to bottom. In this environment, given the transport mechanisms at work, hydrothermal materials would take centuries to reach the plumes, with obvious consequences for their survival.
From the paper:
Stable stratification inhibits convection—an efficient mechanism for vertical transport of particulates and dissolved substances. In Earth’s predominantly stably stratified ocean this permits the marine snow phenomena, where organic matter, unable to maintain neutral buoyancy, undergoes ’detrainment’, settling down to the ocean bottom. Meanwhile, the slow ascent of hydrothermally derived, dissolved substances provides time for scavenging processes and usage by life, resulting in surface concentrations far lower than those present nearer source regions at depth.
Although its focus is on Enceladus, the paper offers clear implications for what may be going on at Europa. Have a look at the image below (drawn not from the body of the paper but from the supplementary materials linked after the footnotes) and you’ll see the problem. We’re looking at these findings as applied to what we know of Europa.
Image: From part of Figure S7 in the supplementary materials. Caption: “Tracer age (years) at Europa’s ocean-ice interface, computed using the theoretical model outlined in the main text. Note that age contours are logarithmic.” Credit: Ames et al.
The figure shows the depth of the inversion and age of the ice shell for the same ranges in ocean salinity as inserted for Enceladus. Here we have to be careful about how much we don’t know. The ice thickness, for instance, is assumed as 10 kilometers in these calculations. Given all the factors involved, the transport timescale through the stratified layers of the modeled Europa is, as the figure shows, over 10,000 years. The same stratification layers impede delivery of oxidants from the surface to the ocean.
So there we are. The Ames paper stands as a challenge to the idea that we will be able to find evidence of life in the waters just below the ice, and likewise indicates that even if we do begin to trace more plumes from Europa’s ocean, these would be unlikely to contain any conclusive evidence about biology. Just what we needed – the erasure of evidence due to the length of the journey from the ocean depths to the ice sheet. Icy moons, it seems, are going to remain mysterious even longer than we thought.
The paper is Ames et al., “Ocean stratification impedes particulate transport to the plumes of Enceladus,” Communications Earth & Environment 6 (6 February 2025), 63 (full text).
