Centauri Dreams

Imagining and Planning Interstellar Exploration

Pondering Life in an Alien Ocean

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).

Putting AI to Work on Technosignatures

As a quick follow-up to yesterday’s article on quantifying technosignature data, I want to mention the SETI Institute’s invitation for applicants to the Davie Postdoctoral Fellowship in Artificial Intelligence for Astronomy. The Institute’s Vishal Gajjar and his collaborators both in the US and at IIT Tirupati in India will be working with the chosen candidate to focus on neural networks optimized for processing image data, so-called ‘CNN architectures’ that can uncover unusual signals in massive datasets.

“Machine learning is transforming the way we search for exoplanets, allowing us to uncover hidden patterns in vast datasets,” says Gajjar. “This fellowship will accelerate the development of advanced AI tools to detect not just conventional planets, but also exotic and unconventional transit signatures including potential technosignatures.”

As AI matures, the exploration of datasets is a critical matter as these results from missions like TESS and Kepler are packed with both exoplanet data as well as stellar activity and systematics that can mislead investigators. Frameworks for sifting out anomalies should help us distinguish unusual candidates including disintegrating objects, planets with rings, exocomets and perhaps even megastructures and other technosignatures, all flagged by their deviation from our widely used transit models.

The data continue to accumulate even as our AI tools sharpen to look for anomalies. I can think of several Centauri Dreams readers who should find this work right up their alley. If you’re interested, you can find everything you need to apply for the fellowship here. The deadline for applications is March 15, 2025.

Quantifying the Chances of a Technosignature

It’s one thing to talk about technology as we humans know it, but applying it to hypothetical extraterrestrials is another matter. We have to paint with a broad brush here. Thus Jason Wright’s explanation of technosignatures as conceived by SETI scientists. The Penn State astronomer and astrophysicist defines technology in that context as “the physical manifestations of deliberate engineering.” That’s saying that a technology produces something that is in principle observable. Whether or not our current detection methods are adequate to the task is another matter.

Image: Artist’s concept of an interesting radio signal from galactic center. But the spectrum of possible technosignature detections is broad indeed, extending far beyond radio. Credit: UCLA SETI Group/Yuri Beletsky, Carnegie Las Campanas Observatory.

A technosignature need not be the sign of industrial or scientific activity. Consider: In a new paper in The Astronomical Journal, Sofia Sheikh (SETI Institute) and colleagues including such SETI notables as Wright himself, Ravi Kopparapu and Adam Frank point out that the extinction of ancient megafauna some 12,800 years ago may have contributed to changes in atmospheric methane that fed into a period of cooling known as the Younger Dryas, to be followed by growing human agricultural activity whose effects on carbon dioxide and methane in the atmosphere would be detectable.

As a technosignature, that one has a certain fascination, but it’s not likely to be definitive in ferreting out extraterrestrials, at least not at our stage of detection technology. But Sheikh and team are really after a much less ambiguous question. We know what our own transmission and detection methods are. How far away can our own technosignature be detected? By studying the range of technosignatures we are producing on Earth, the authors produce a scale covering thirteen orders of magnitude in detectability, with radio still at the top of the heap. The work establishes quantitative standards for detectability based on Earth’s current capabilities.

We might, for example, use the James Webb Space Telescope and the upcoming Habitable Worlds Observatory to provide data on atmospheric technosignatures as far out as 5.7 light years away. That takes us interstellar, with that interesting system at Proxima Centauri in range. Let’s tarry a bit longer on this one. While carbon dioxide is implicated in manmade changes to Earth’s atmosphere, the paper points to other sources, zeroing in on one in particular:

…there are other atmospheric technosignatures in Earth’s atmosphere that have very few or even no known nontechnological sources. For example, chlorofluorocarbons (CFCs), a subcategory of halocarbons, are directly produced by human technology (with only very small natural sources), e.g., refrigerants and cleaning agents, and their presence in Earth’s atmosphere constitutes a nearly unambiguous technosignature (J. Haqq-Misra et al. 2022). Nitrogen dioxide (NO2), like CO2, has abiotic, biogenic, and technological sources, but combustion in vehicles and fossil-fueled power plants is a significant contributor to the NO2 in Earth’s atmosphere (R. Kopparapu et al. 2021).

And indeed nitrogen dioxide (NO2) is what the authors plug into this study, drawing on earlier work by some of the paper’s authors. Note the fact that biosignatures and technosignatures overlap here given how much work has proceeded on characterizing exoplanet atmospheres. It turns out that the wavelength bands that the Habitable Worlds Observatory will see best in its search for biosignatures are also those that include the NO2 technosignature, a useful example of piggybacking our searches.

But of course the realm of technosignatures is wide, including everything from the lights of cities to ‘heat islands’ (inferring cities), orbiting satellites, radio transmissions and lasers. I’m aware of no other study that combines the various forms of technosignature in a single analysis. If you start looking at the full range of technosignatures according to distance, you find objects on a planetary surface to be the toughest catch, with heat islands swimming into focus only from a distance as far as outer planetary orbits in the Solar System. The current technology with the most punch is planetary radar, whose pulses should be detectable as much as 12,000 light years away, although such a signal would be a fleeting and non-repeating curiosity.

SETI does find signals like that now and then. But precisely because they are non-repeating, we simply don’t know what to make of them.

Image: The maximum distances that each of Earth’s modern-day technosignatures could be detected at using modern-day receiving technology, in visual form. Also marked are various astronomical objects of interest. Credit: Sheikh et al.

Think back to the early days of SETI and ponder how far we’ve come in trying to understand what other civilizations might do that could get us to notice them. SETI grew directly out of the famous work by Giuseppe Cocconi and Philip Morrison that laid out the case for artificial radio signals in 1959, followed shortly thereafter by Frank Drake’s pioneering work at Green Bank with Project Ozma. Less known is the 1961 paper by Charles Townes and R. N. Schwartz that got us into optical wavelengths.

And while ‘Dysonian SETI’, which explicitly searches for technosignatures, is usually associated with vast engineering projects like Dyson spheres, the point here is that a civilization will produce evidence for an outside observer that will continue to deepen as that observer’s tools increase in sophistication. The search for technosignatures, then, actually grows into a multi-wavelength effort, but one that spans a vast range. Making all this quantitative involves a ‘detectability distance scale.’ The authors choose one known as an ichnoscale. Here’s how the paper describes it:

Using Earth as a mirror in this way, we can employ the concept of the ichnoscale (ι) from H. Socas-Navarro et al. (2021): “the relative size scale of a given technosignature in units of the same technosignature produced by current Earth technology.” An ι value of 1 is defined by Earth’s current technology. This necessarily evolves over time—for this work, we set the ichnoscale to Earth-2024-level technology, including near-future technologies that are already in development.

Considering how fast our detection methods are improving as we build extremely large telescopes (ELTs) and push into ever more sophisticated space observatories, learning the nature of this scale will become increasingly relevant. While we realized in the mid-20th Century that radio was detectable at interstellar distances, we’re now able to detect not just an intentional signal but radio leakage, at least from nearby stellar systems. That’s an extension of the parameter space that involves levels of power we have already demonstrated on Earth. The ichnoscale framework quantifies these signatures that will gradually become possible to detect as our methods evolve.

We see more clearly which methods are most likely to succeed. This is an important scaling because the universe we actually live in may not resemble the one we construct in our imaginations. Let me quote the paper on this important point:

…the focus on planetary-scale technosignatures provides very specific suggestions for which searches to pursue in a Universe where large-scale energy expenditures and/or travel between systems is logistically infeasible. While science fiction is, for example, replete with mechanics for rapid interstellar travel, all current physics implies it would be slow and expensive. We should take that constraint seriously.

And with this in mind we can state key results:

1. Radio remains the way that Earth is most detectable at ι = 1.
2. Investment in atmospheric biosignature searches has opened up the door for atmospheric technosignature searches.
3. Humanity’s remotely detectable impacts on Earth and the solar system span 12 orders of magnitude.
4. Our modern-day planetary-scale impacts are modest compared to what is assumed in many technosignature papers.
5. We have a multiwavelength constellation of technosignatures, with more of the constellation becoming visible the closer the observer becomes.

Let’s pause on item 4. The point here is that most notions of technosignatures assume technologies visible on astronomical scales, and indeed it is usually assumed that our first SETI detections, when and if they come, will involve civilizations vastly older and superior in technology than ourselves. Planets bearing technologies like those we have today are a supremely difficult catch, because the technosignatures we are throwing are tiny and all but trivial compared to the Dyson spheres and starship beaming networks we typically consider. And this point seems overwhelmingly obvious:

We should be careful about extrapolating current technosignatures to scales of ιTS = 10 (or even ιTS = 2) without considering the changing context in which these technologies are being developed, used, and (sometimes) mitigated or phased out (e.g., the recovery of the ozone hole; J. Kuttippurath & P. J. Nair 2017). As another example, we are becoming aware of the negative health effects of the UHI [urban heat index] (as detailed in, e.g., A. Piracha & M. T. Chaudhary 2022); thus, work may be done to mitigate the concentrated regions of high infrared flux discussed in Section 4.3.

Indeed. How many of the technosignatures we are producing are stable? Chlorofluorocarbons in the atmosphere are subject to adjustment on astronomically trivial timeframes. The chances of running into a culture that is about to realize it is polluting itself just before it takes action to mitigate the problem seem remote. So all these factors have to be taken into account as we rank technosignature detection strategies, and it’s clear that in this “multiwavelength constellation of technosignatures” the closer we are, the better we see. All the more reason to continue to pursue not just better telescopes but better ways to get ever improving platforms into the outer Solar System and beyond. Interstellar probes, anyone?

The paper is Sheikh et al., “Earth Detecting Earth: At What Distance Could Earth’s Constellation of Technosignatures be Detected with Present-day Technology?” Astronomical Journal Vol. 169, No. 2 (3 February 2025), 118 (full text). The Cocconi and Morrison paper is “Searching for Interstellar Communications,” Nature 184 (19 September 1959), 844-846 (abstract). The 1961 paper on laser communications is Schwartz and Townes, “Interstellar and Interplanetary Communication by Optical Masers,” Nature 190 (15 April 1961), 205-208 (abstract).

A Fast Radio Burst in a Dead Elliptical Galaxy

Work is healing, so let’s get back to it. I’m enthralled with what we’re discovering as we steadily build our catalog of fast radio bursts (FRB), close to 100 of which have now been associated with a galaxy. These are transient radio pulses of short duration (down to a fraction of a millisecond, though some last several seconds), the first being found in 2007 by Duncan Lorimer, an astronomer at West Virginia University. Sometimes FRBs repeat, although many do not, and one is known to repeat on a regular basis.

What kind of astrophysical processes might be driving such a phenomenon? The leading candidate appears to be supernovae in a state of core collapse, producing vast amounts of energy as stars more massive than the Sun end their lives. Out of such catastrophic events a type of neutron star called a magnetar may be produced, its powerful magnetic field pumping out X-ray and gamma ray radiation. Young, massive stars and regions of active star formation are implicated under this theory. But as we’re learning, magnetars are only one of a possible range of candidates.

For the event known as FRB 20240209A, detected in 2024 by the Canadian Hydrogen Intensity Mapping Experiment (CHIME), has dealt us a wild card. Remember, a single FRB can produce more energy in a quick burst than our Sun emits in an entire year. This one has repeatedly fired up, producing 21 pulses between February and July of last year. And the problem with it is that it has been traced to a galaxy in which star formation has ceased. That finding is verified by data from the Gemini North telescope and the Keck Observatory using its Low Resolution Imaging Spectrometer (LRIS).

Yuxin (Vic) Dong is an NSF Graduate Research Fellow and second author on one of two papers recently published on the event:

“For nearby galaxies, there is often archival data from surveys available that tells you the redshift — or distance —to the galaxy. However, in some cases, these redshift measurements may lack precision, and that’s where Keck Observatory and the LRIS instrument becomes crucial. Using a Keck/LRIS spectrum, we can extract the redshift to a very high accuracy. Spectra are like fingerprints of galaxies, and they contain special features, called spectral lines, that encode tons of information about what’s going on in the galaxy like the stellar population age and star formation activity. What’s really fascinating in this case is that the features we saw from the Keck/LRIS spectrum revealed that this galaxy is quiescent, meaning star formation has shut down in the galaxy. This is strikingly different from most FRB galaxies we know which are still actively making new stars.”

Image: The ellipse shows the location of the FRB and the crosshairs point to its host galaxy, taken with the Gemini North telescope from Maunakea. Credit: Shah et al.

It turns out that FRB 20240209A is coming from a galaxy fully 11.3 billion years old some 2 billion light years from Earth. This is painstaking work and quite productive, for the papers’ authors report that the galaxy is both extremely luminous and the most massive FRB host galaxy yet found. Moreover, while FRBs that have been associated with their host galaxies are usually located deep within the galaxy, this one occurs 130,000 light years from galactic center, in a region with few stars nearby.

When you’re dealing with a new phenomenon, finding similar events can be productive. In this case, there is one other FRB that can be placed in the outskirts of a galaxy, the spiral M81. While FRB 20240209A occurred in an ancient elliptical galaxy, it like the M81 event is far from areas of active star formation, again raising the possibility that FRBs have causes we have yet to pin down. From the Eftekhari et al. paper:

Since the first host associations, investigations into FRB host demographics have offered valuable insights into the origins of FRBs and their possible progenitor systems. Such studies remain in their infancy, however. With the development of interferometric capabilities for various FRB experiments and the promise of hundreds of precisely localized events, the discovery landscape for new and unforeseen hosts and environments presents considerable potential.

And as the paper notes, FRB 20240209A isn’t the first FRB that challenges our assumptions:

Indeed, the connection of a few FRBs with remarkable environments, including dwarf galaxies (S. Chatterjee et al. 2017; C. H. Niu et al. 2022), a globular cluster (F. Kirsten et al. 2022), and the elliptical host of FRB 20240209A, implicate exotic formation channels as well as older stellar populations for some FRBs and demonstrate that novel environments offer significant constraining power for FRB progenitors. A larger sample of host associations will further uncover intriguing diversity in host environments and may identify interesting subpopulations or correlations with FRB repetition, energetics, or other burst characteristics, contributing to a clearer understanding of FRB origins.

Image: CHIME detectors. Credit: CHIME, Andre Renard, Dunlap Institute for Astronomy & Astrophysics, University of Toronto.

Clearly we have a long way to go as the FRB catalog grows. Senior author Wen-fai Fong, who was involved in both papers, likes to talk about the surprises the universe has in store for us, disrupting any possibility of scientific complacency. Instead, we are often confronted with yet another reason to revise our thinking, in what Fong refers to as “a ‘dialogue’ with the universe” as we pursue time-domain astronomy, the analysis of changes in brightness and spectra over time so suited for mysterious FRBs.

The papers are Shah et al., “A repeating fast radio burst source in the outskirts of a quiescent galaxy,” Astrophysical Journal Letters Vol. 979, No. 2 (21 January 2025) (full text) and Eftekhari et al., “The massive and quiescent elliptical host galaxy of the repeating fast radio burst FRB 20240209A,” Astrophysical Journal Letters Vol. 979 No. 2 (21 January 2025). Full text.

Centauri Dreams to Resume Soon

I’d like to thank all of you who wrote comments and emails about the recent pause in Centauri Dreams. My beautiful wife Eloise passed away on January 17. It was as peaceful a death as can be imagined, and I am so pleased to say that she was able to stay at home until the end. As she had battled Alzheimer’s for eleven gallant years, death was simply a bridge that now had to be crossed. As she did with everything else in her life, she did it with class.

This is to let you know that I will be getting Centauri Dreams back into action again in about three weeks. When I began the site in 2004, my primary goal was to teach myself as much as I could about the topics we address by writing about them, which is how I’ve always tended to learn things. I’ve always welcomed comments that informed me, caught my errors and extended the discussion into new realms. No one could work with a better audience than the readers I’ve been privileged to address, and for this I am profoundly grateful.

A Necessary Break

It’s time to write a post I’ve been dreading to write for several years now. Some of my readers already know that my wife has been ill with Alzheimer’s for eleven years, and I’ve kept her at home and have been her caregiver all the way. We are now in the final stages, it appears, and her story is about to end. I will need to give her all my caring and attention through this process, as I’m sure you’ll understand. And while I have no intention of shutting down Centauri Dreams, I do have to pause now to devote everything I have to her. Please bear with me and with a bit of time and healing, I will be active once again.

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).

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