Uranus: Diamond Rain, Bright Rings

Thinking about the ice giants, as I have been doing recently in our look at fast mission concepts, reminds me of the ‘diamond rain’ notion that has grown out of research into experiments with the temperatures and pressures found inside worlds like Uranus and Neptune. The concept isn’t new, but I noted some months ago that scientists at the Department of Energy’s SLAC National Accelerator Laboratory had been studying diamond formation in such worlds in the presence of oxygen. Oxygen, it turns out, makes it more likely that diamonds form that may grow to extreme sizes.

So let me turn back the clock for a moment to last fall, when news emerged about this exotic precipitation indicating that it may be more common than we had thought. Using a material called PET (polyethylene terephthalate), the SLAC researchers created shock waves within the material and analyzed the result with X-ray pulses. The scientists used PET because of its balance between carbon, hydrogen and oxygen, components more closely mimicking the chemical composition of Neptune and Uranus.

While earlier experiments had used a plastic material made from hydrogen and carbon, the addition of oxygen made the formation of diamonds more likely, and apparently allowed them to grow at lower temperatures and pressures than previously thought possible. The team, led by Dominik Kraus (SLAC/University of Rostock), suggests that such diamonds under actual ice giant conditions might reach millions of carats in weight, forming a layer around the planetary core. Silvia Pandolfi, a SLAC scientist involved in this work, was quoted in a SLAC news release last September:

“We know that Earth’s core is predominantly made of iron, but many experiments are still investigating how the presence of lighter elements can change the conditions of melting and phase transitions. Our experiment demonstrates how these elements can change the conditions in which diamonds are forming on ice giants. If we want to accurately model planets, then we need to get as close as we can to the actual composition of the planetary interior.”

Image: Studying a material that even more closely resembles the composition of ice giants, researchers found that oxygen boosts the formation of diamond rain. The team also found evidence that, in combination with the diamonds, a recently discovered phase of water, often described as “hot, black ice” could form. Credit: Greg Stewart/SLAC National Accelerator Laboratory.

Diamond rain is a startling concept, hard to visualize, and given the possibility that ice giants may be one of the most common forms of planet, the phenomenon may be occurring throughout the galaxy. Something to ponder as we look at the new image from Uranus just in from the James Webb Space Telescope, which highlights the planet’s rings as never before, while also revealing features in its atmosphere. The rings themselves have only rarely been imaged, but have been seen through Voyager 2’s perspective and through the adaptive optics capabilities of the Keck Observatory.

The brightness of the rings is striking, the result of the telescope’s Near-Infrared Camera (NIRCam) working through filters at 1.4 and 3.0 microns, shown here in blue and orange. The Voyager 2 imagery was as featureless as Voyager 1’s image of Titan, showing a lovely blue-green orb in visible wavelengths, but the power of working in the infrared is clear with the JWST results. Note the brightening at the northern pole (Uranus famously lies on its side almost 90 degrees from the plane of its orbit). This ‘polar cap’ formation appears when it is summer at the pole and disappears in the fall.

Image: This zoomed-in image of Uranus, captured by Webb’s Near-Infrared Camera (NIRCam) Feb. 6, 2023, reveals stunning views of the planet’s rings. The planet displays a blue hue in this representative-color image, made by combining data from two filters (F140M, F300M) at 1.4 and 3.0 microns, which are shown here as blue and orange, respectively. Credit: NASA, ESA, CSA, STScI. Image processing: J. DePasquale (STScI).

A few other features emerge here beyond the edge of the cap, including a second bright cloud at the left limb of the planet that seems to be related to storm activity. As to Uranus’ 13 known rings, 11 of them appear in the image. The other two, quite faint, are more visible, according to JWST scientists, during ring-plane crossings, which is a time in the planetary orbit when we see the rings edge-on. The Hubble instrument first discovered them during the last crossing, in 2007; the next will occur in 2049.

We’re dealing here with a brief exposure (12 minutes), but even so, a number of the planet’s moons can be found in the wider view shown below. Looking at this oddball system, I have to wonder whether the idea of a giant impact knocking it onto its side holds water. Can we get this result from resonance effects and the gravitational influence of the gas giants through periods of migration? The fact that the question can even be asked highlights how little we know about this particular ice giant. And whatever the cause, imagine a world where the Sun disappears for 42 years, a world of water, methane and ammonia, a rocky core and perhaps a rain of diamonds.

Image: This wider view of the Uranian system with Webb’s NIRCam instrument features the planet Uranus as well as six of its 27 known moons (most of which are too small and faint to be seen in this short exposure). A handful of background objects, including many galaxies, are also seen. Credit: NASA, ESA, CSA, STScI. Image processing: J. DePasquale (STScI).

As I’ve mentioned before, a mission to Uranus including an orbiter has been identified as a priority in the 2023-2033 Planetary Science and Astrobiology decadal survey. A flagship mission of Cassini-class at Uranus would be a great boon to science, but the suspicion grows that before it can fly, we’ll have learned how to reach the ice giants faster, and with mission strategies far different from those used for Cassini.

The paper on diamond rain is Zhiyu He et al., “Diamond formation kinetics in shock-compressed C?H?O samples recorded by small-angle x-ray scattering and x-ray diffraction,” Science Advances Vol. 8, Issue 35 (2 September 2022). Full text.


The Latest from New Horizons

New Horizons is, like the two Voyagers, a gift that keeps on giving, even as it moves through the Kuiper Belt in year 17 of its mission. Thus the presentations that members of the spacecraft team made on March 14 at the 54th Lunar and Planetary Science Conference. Papers will flow out of these observations, including interpretations of the twelve mounds on the larger lobe of Arrokoth, the contact binary that is being intensely studied through stereo imaging to identify how these features formed around a larger center mound. Alan Stern (SwRI) is principal investigator for the New Horizons mission:

“We discovered that the mounds are similar in many respects, including their sizes, reflectivities and colors. We believe the mounds were likely individual components that existed before the assembly of Arrokoth, indicating that like-sized bodies were formed as precursors to Arrokoth itself. This is surprising, and a new piece in the puzzle of how planetesimals – building blocks of the planets, like Arrokoth and other Kuiper Belt objects come together.”

Science team members also discussed the so-called ‘bladed terrain,’ evidently the product of methane ice, that seems to stretch across large areas of Pluto’s ‘far side,’ as observed during the spacecraft’s approach. It was intriguing to learn as well about the spacecraft’s observations of Uranus and Neptune, which will complement Voyager imaging at different geometries and longer wavelengths. And Pluto’s ‘true polar wander’ (the tilt of a planet with respect to its spin axis came into play (and yes, I do realize I’ve just referred to Pluto as a ‘planet’). Co-investigator Oliver White:

“We’re seeing signs of ancient landscapes that formed in places and in ways we can’t really explain in Pluto’s current orientation. We suggest the possibility is that they formed when Pluto was oriented differently in its early history, and were then moved to their current location by true polar wander.”

Image: Pluto’s Sputnik Planitia, the huge impact basin found in Pluto’s ‘heart’ region, seems to have much to do with the world’s axial tilt, while the possibility of a deep ocean pushing against the basin from below has to be taken into account. This image is from the presentation by Oliver White (SETI Institute) at LPSC. Credit: NASA/Johns Hopkins APL/SwRI/James Tuttle Keane.

But let me pause today on the quest for other Kuiper Belt Objects as the search for a second flyby candidate continues. Not that a flyby is essential. Using the Japanese Subaru Telescope in Hawaii and the Victor M. Blanco instrument at Cerro Tololo, the team is now applying a deep learning algorithm (a ‘convolutional neural network’) to analyze imagery. Wes Fraser, a member of the science team, is quoted on the New Horizons site as saying “The software network’s classification performance is extremely good, significantly cutting back on ‘false’ candidate sources. An entire night’s worth of search data requires only a few hours of human vetting. Compare that to the weeks it used to take to do this!”

Image: A “stack” of images from one night of observing with the Subaru Telescope’s Hyper Suprime-Cam, showing myriad stars that illustrate the difficulty of spotting an undiscovered Kuiper Belt object. The animation below shows movement – across the center-right of the frame — of a newly discovered KBO in one of these images. Credit: NASA/Johns Hopkins APL/Southwest Research Institute/Subaru Telescope.

The point is that we’re learning a great deal about KBOs even in the absence of another flyby, discovering a surprising number of objects like that shown (look carefully) in the animation below. The Subaru Telescope produced, with its wide field of view, some 87 new KBOs in 2020 in the direction of the spacecraft’s trajectory. It was heartening to learn that running that same data through the new software enabled a search that was both 100 times faster but also revealed another 67 KBOs. Some of these – and about 20 will be close enough to observe from a distance of millions of miles – should be grist for the mill as New Horizons examines them in the coming two years.

Image: This animation shows movement—across the center-right of the frame—of a newly discovered Kuiper Belt object in one of the Subaru Telescope Hyper Suprime-Cam images. Credit: NASA/Johns Hopkins APL/Southwest Research Institute/Subaru Telescope.

Will JHU/APL’s Interstellar Probe design eventually be approved and join the spacecraft now departing our Solar System? Or will JPL’s Solar Gravity Lens mission to the gravitational focus become our next deep space sojourner? As we ponder mission designs and the likelihood of their approval, keeping an eye on our existing assets in deep space reminds us of the outstanding science return we’ve achieved thus far.


Oceanic Surprise: Pushing Europa’s Ice

Getting Europa Clipper to its target to analyze the surface of Jupiter’s most interesting moon (in terms of possible life, at least) sets up a whole range of comparative studies. We have been mining data for many years from the Galileo mission and will soon be able – at last! – to compare its results to new images pulled in by Europa Clipper’s flybys. Out of this comes an interesting question recently addressed by a new paper in JGR Planets: Is Europa’s ice shell changing in position with time?

An answer here would establish whether we are dealing with a free-floating shell moving at a different rate than the salty ocean beneath. Computer modeling has previously suggested that the ocean’s effects on the shell may affect its movement, but this is evidently the first study that calculates the amount of drag involved in this scenario. Ocean flow may explain surface features Galileo revealed, with ridges and cracks as evidence of the stretching and straining effects of currents below.

Hamish Hay (University of Oxford) is lead author of the paper on this work, which was performed at the Jet Propulsion Laboratory during his postdoctoral tenure there. The study reveals a net torque on the ice shell from ocean currents moving as alternating east-west jets, sometimes spinning up the shell and at other times spinning it down as convection is altered by the evolution of the moon’s interior. Says Hay:

“Before this, it was known through laboratory experiments and modeling that heating and cooling of Europa’s ocean may drive currents. Now our results highlight a coupling between the ocean and the rotation of the icy shell that was never previously considered.”

Thus we are forced to reconsider some old assumptions, one of them being that the primary force acting on Europa’s surface is the gravitational pull of Jupiter. The paper calculates that an average ‘jet speed’ of at least ~1 cm s-1 produces enough ice-ocean torque to be comparable to tidal torque. Calling these results “a huge surprise,” Europa Clipper project scientist Robert Pappalardo (JPL) notes that thinking about ocean circulation as the driver of surface cracks and ridges takes scientists in a new direction: “[G]eologists don’t usually think, ‘Maybe it’s the ocean doing that.’”

Image: This view of Jupiter’s icy moon Europa was captured by JunoCam, the public engagement camera aboard NASA’s Juno spacecraft, during the mission’s close flyby on Sept. 29, 2022. The picture is a composite of JunoCam’s second, third, and fourth images taken during the flyby, as seen from the perspective of the fourth image. North is to the left. The images have a resolution of just over 1 to 4 kilometers per pixel. As with our Moon and Earth, one side of Europa always faces Jupiter, and that is the side of Europa visible here. Europa’s surface is crisscrossed by fractures, ridges, and bands, which have erased terrain older than about 90 million years. Credit: NASA, with image processing by citizen scientist Kevin M. Gill.

It was the introduction of drag into the simulations that demonstrated the effects of ocean currents on the shell’s rotational speed. The under-ice flow depicted in this paper is complex, with supercomputing modeling showing water flow being bent by Europa’s overall rotation into east-west and west-east currents. The results depend upon a model of internal heating from radioactive decay as well as tidal heating to drive warmer water to the top of the ocean. They imply changes to the surface over time as the amount of interior heating varies, a process that presumably would occur on other ocean worlds as well.

The paper notes another aspect of the drag model that is unusual:

We have for the first time estimated the time-mean stress field and resulting torque that must exist between the flowing ocean and solid ice shell of Europa. Perhaps unintuitively, the stress field due to alternating zonal jets does not necessarily cancel out once integrated over the entire surface. This means that it is likely that ocean dynamics that manifest in east-west jets exert a net unidirectional torque on the ice shells of Europa and other ocean worlds.

Moreover, ice-ocean torque is a process whose effects can change dramatically. Notice the reversal process described below. The ‘equatorial jet’ mentioned here is accompanied in the simulations by one to two alternating jets at higher latitudes:

The scaling analysis shows that strengthening of turbulent convection reverses the equatorial jet and resulting torque such that it acts against the direction of rotation. The reversal occurs when the thermal buoyancy forcing becomes large enough to drive highly turbulent convection. If the energetic state of Europa’s interior has changed sufficiently over time, perhaps due to the depletion of radioactive heat producing elements or changes in tidal forcing, it is possible that a reversal has taken place. We speculate that this provides a novel mechanism to stop, start, and even reverse nonsynchronous rotation of the ice shell.

So we see the ice shell’s rotation being speeded up and at other times slowed down by the ocean currents below, sometimes stretching and at other times collapsing, with possible effects on surface topography that Europa Clipper can examine. How interesting that we can learn about the dynamics of the ocean below through the speed of the shell’s rotation, which is something the mission may be able to measure. The craft, now in assembly, test, and launch operations phase at JPL, is on target for a launch in 2024. Orbital operations at Jupiter begin in 2030, with some 50 Europa flybys on the schedule.

The paper is Hay et al., “Turbulent Drag at the Ice-Ocean Interface of Europa in Simulations of Rotating Convection: Implications for Nonsynchronous Rotation of the Ice Shell,” JGR Planets 19 February 2023 (full text).


Uranus Orbiter and Probe: Implications for Icy Moons

What do you get if you shake ice in a container with centimeter-wide stainless steel balls at temperature of –200 ?C? The answer is a kind of ice with implications for the outer Solar System. I just ran across an article in Science (citation below) that describes the resulting powder, a form of ‘amorphous ice,’ meaning ice that lacks the familiar crystalline arrangement of regular ice. There is no regularity here, no ordered structure. The two previously discovered types of amorphous ice – varying by their density – are uncommon on Earth but an apparently standard constituent of comets.

The new medium-density amorphous ice may well be produced on outer system moons, created through the shearing process that the researchers, led by Alexander Rosu-Finsen at University College London, produced in their lab work. There is a good overview of this water ‘frozen in time’ in a recent issue of Nature. The article quotes Christoph Salzmann (UCL), a co-author on the Science paper:

The team used a ball mill, a tool normally used to grind or blend materials in mineral processing, to grind down crystallized ice. Using a container with metal balls inside, they shook a small amount of ice about 20 times per second. The metal balls produced a ‘shear force’ on the ice, says Salzmann, breaking it down into a white powder.

Firing X-rays at the powder and measuring them as they bounced off — a process known as X-ray diffraction — allowed the team to work out its structure. The ice had a molecular density similar to that of liquid water, with no apparent ordered structure to the molecules — meaning that crystallinity was “destroyed”, says Salzmann. “You’re looking at a very disordered material.”

Disruptions in icy surfaces caused by the process would have implications for the interface between ice and liquid water that is presumed to exist on moons like Europa and Enceladus. The surface might be given to disruptions that would expose ocean beneath.

What goes on in the icy moons of the outer system is always of interest, especially given the astrobiological possibilities, and it was probably the thought of an ice giant orbiter at Uranus that triggered my interest in the amorphous ice issue. Kathleen Mandt (Johns Hopkins University Applied Physics Laboratory) just wrote the former up in Science as well, noting how little we’ve learned since the solitary Voyager flyby of the planet in 1986. In addition to planetary structure and atmosphere, we could do with a lot more information about its moons and their possible liquid water oceans.

Image: This 2006 image taken by the Hubble Space Telescope shows bands and a new dark spot in Uranus’ atmosphere. Credit: NASA/Space Telescope Science Institute.

The planetary science decadal survey released in 2022, called Origins, Worlds, and Life, which reviewed over 500 white papers and 300 presentations over the course of its 176 meetings, flagged the need for such a mission in the coming decade, a planetary flagship mission as a next step forward after Europa Clipper. I don’t want to downplay the role of such a mission in deepening our understanding of ice giant formation and migration, not to mention the Uranian atmosphere, but the moon system here has proven an extreme challenge for observers. Its study becomes a major driver for the Uranus Orbiter and Probe (UOP) mission:

The system’s extreme obliquity…limits visibility of the moons to one hemisphere during southern and northern summers. Voyager 2 could only image the moons’ southern hemispheres, but what was seen was unexpected. The five largest moons, predicted to be cold dead worlds, all showed evidence of recent resurfacing, suggesting that geologic activity might be ongoing. One or more of these moons could have potentially habitable liquid water oceans under an ice shell, making them “ocean worlds.” Ariel, the most extensively resurfaced moon, is a strong ocean worlds candidate along with the two largest moons, Titania and Oberon… UOP will image and measure the composition of the full surfaces of the moons to search for ongoing geologic activity, and measure whether magnetic fields vary in their interiors owing to the presence of liquid water.

Miranda, Ariel, Umbriel, Titania and Oberon, the five largest moons of Uranus, could all contain subsurface oceans (and I don’t want to leave out of the UOP story the fact that it would carry a Uranus atmospheric probe designed to reach a depth of at least 1 bar in pressure, a fascinating investigation in itself). The decadal survey has recommended a launch by 2032 to take advantage of a Jupiter gravity assist that would allow arrival before the northern autumn equinox in 2050 for better study of the moon system. “The space science community has waited more than 30 years to explore the ice giants,” writes Mandt, “and missions to them will benefit many generations to come.”

The first of today’s papers is Rosu-Finsen et al., “Medium-density amorphous ice,” Science Vol. 379, No. 6631 (2 February 2023), pp. 474-478 (abstract). The Mandt article is “The first dedicated ice giants mission,” Science Vol. 379, No. 6633 (16 February 2023), pp. 640-642 (full text).


Gathering the Evidence for Life on Enceladus

With a proposal for an Enceladus Orbilander mission in the works at the Johns Hopkins Applied Physics Laboratory, I continue to mull over the prospects for investigating this interesting moon. Something is producing methane in the ocean under the Enceladus ice shell, analyzed in a 2021 paper from Antonin Affholder (now at the University of Arizona) and colleagues, using Cassini data from passages through the plumes erupting from the southern polar regions. The scientists produced mathematical models and used a Bayesian analysis to weigh the probabilities that the methane is being created by life or through abiotic processes.

The result: The plume data are consistent with both possibilities, although it’s interesting, based on what we know about hydrothermal chemistry on earth, that the amount of methane is higher than would be expected through any abiotic explanation. So we can’t rule out the possibility of some kind of microorganisms under the ice on Enceladus, and clearly need data from a future mission to make the call. I won’t go any further into the 2021 paper (citation below) other than to note that the authors believe their methods may be useful for dealing with future chemical data from exoplanets of a wide variety, and not just icy worlds with an ocean beneath a surface shell.

Now a new paper has been published in The Planetary Science Journal, authored by the same team and addressing the potential of such a future mission. A saltwater ocean outgassing methane is an ideal astrobiological target, and one useful result of the new analysis is that it would not take a landing on Enceladus itself to probe whether or not life exists there. Says co-author Régis Ferrière (University of Arizona):

“Clearly, sending a robot crawling through ice cracks and deep-diving down to the seafloor would not be easy. By simulating the data that a more prepared and advanced orbiting spacecraft would gather from just the plumes alone, our team has now shown that this approach would be enough to confidently determine whether or not there is life within Enceladus’ ocean without actually having to probe the depths of the moon. This is a thrilling perspective.”

Image: This graphic depicts how scientists believe water interacts with rock at the bottom of Enceladus’ ocean to create hydrothermal vent systems. These same chimney-like vents are found along tectonic plate borders in Earth’s oceans, approximately 7000 feet below the surface. Credit: NASA/JPL-Caltech/Southwest Research Institute.

Microbes on Earth – methanogens – find ways to thrive around hydrothermal vents deep below the surface of the oceans, in regions deprived of sunlight but rich in the energy stored in chemical compounds. Indeed, life around ‘white smoker’ vents is rich and not limited to microbes, with dihydrogen and carbon dioxide as an energy source in a process that releases methane as a byproduct. The researchers hypothesize that similar processes are at work on Enceladus, calculating the possible total mass of life there, and the likelihood that cells from that life might be ejected by the plumes.

The team’s model produces a small and sparse biosphere, one amounting to no more than the biomass of a single whale in the moon’s ocean. That’s an interesting finding in itself in contrast to some earlier studies, and it contrasts strongly with the size of the biosphere around Earth’s hydrothermal vents. But the quantity is sufficient to produce enough organic molecules that a future spacecraft could detect them by flying through the plumes. The mission would require multiple plume flybys.

Actual cells are unlikely to be found in the plumes, but detected organic molecules including particular amino acids would support the idea of active biology. Even so, we are probably going to be left without a definitive answer, adds Ferrière:

“Considering that according to the calculations, any life present on Enceladus would be extremely sparse, there still is a good chance that we’ll never find enough organic molecules in the plumes to unambiguously conclude that it is there. So, rather than focusing on the question of how much is enough to prove that life is there, we asked, ‘What is the maximum amount of organic material that could be present in the absence of life?'”

An Enceladus orbiter, in other words, would produce strong evidence of life if its measurements were above the threshold identified here. Back to the JHU/APL Enceladus Orbilander, thoroughly described in a concept study available online. The mission includes both orbital operations as well as a landing on the surface, with thirteen science instruments aboard to probe for life in both venues. The mission would measure pH, temperature, salinity and availability of nutrients in the ocean as well as making radar and seismic measurements to probe the structure of the ice crust.

Image: Artist’s impression of the conceptual Enceladus Orbilander spacecraft on Enceladus’ surface. Credit: Johns Hopkins APL.

Here the chances of finding cell material are much higher than in purely orbital operations, where survival through the outgassing process of plume creation seems unlikely. The lander would target a flat space free of boulders at the moon’s south pole with the aim of collecting plume materials that have fallen back to the surface. The team points out that the largest particles would not reach altitudes high enough for sampling from orbit, making the lander our best chance for a definitive answer.

The paper, indeed, points to this conclusion:

…cell-like abiotic structures (abiotic biomorphs) that may form in hydrothermal environments could cause a high risk of a false positive… Assuming that cells can be identified unambiguously…, we find that the volume of plume material that needs to be collected to confidently sample at least one cell might require a large number of fly-throughs in the plume, or using a lander to collect plume particles falling on Enceladus’s surface (e.g., the Enceladus Orbilander; MacKenzie et al. 2021).

The paper is Affholder et al., “Putative Methanogenic Biosphere in Enceladus’s Deep Ocean: Biomass, Productivity, and Implications for Detection,” Planetary Science Journal Vol. 3, No. 12 (13 December 2022), 270 (full text). The paper on methane on Enceladus is Affholder at al., “Bayesian analysis of Enceladus’s plume data to assess methanogenesis,” Nature Astronomy 5 (07 June 2021), 805-814 (abstract).


A Role for Comets in Europa’s Ocean?

The role comets may play in the formation of life seems to be much in the news these days. Following our look at interstellar comets as a possibly deliberate way to spread life in the cosmos, I ran across a paper from Evan Carnahan (University of Texas at Austin) and colleagues (at JPL, Williams College as well as UT-Austin) that studies the surface of Europa with an eye toward explaining how impact features may evolve.

Craters could be cometary in origin and need not necessarily penetrate completely through the ice, for the team’s simulations of ice deformation show drainage into the ocean below from much smaller events. Here comets as well as asteroids come into play as impactors, their role being not as carriers of life per se but as mechanisms for mixing already existing materials from the surface into the ocean.

Image: Tyre, a large impact crater on Europa. Credit: NASA/JPL/DLR.

That, of course, gets the attention, for getting surface oxidants produced by solar irradiation through the ice has been a challenge to the idea of a fecund Europan ocean. The matter has been studied before, with observational evidence for processes like subduction, where an ice surface moves below an adjacent sheet, and features that could be interpreted as brine drainage, where melting occurs near the surface, although this requires an energy source that has not yet been determined. But many craters show features suggestive of frozen meltwater and post-impact movement of meltwater beneath the crater.

The Carnahan paper notes, too, how many previous studies have been done on impacts that penetrate the ice shell and directly reach the ocean, which would move astrobiologically interesting materials into it, but while impacts would have been common in the history of the icy moon, the bulk of these may not have been penetrating. Much depends upon the thickness of the ice, and on that score we await data from future missions like Europa Clipper and JUICE to probe more deeply. Current thinking seems to be coalescing around the idea that the ice is tens of kilometers thick.

The authors believe that impacts need not fully penetrate the ice to have interesting effects. Such impacts should produce melt chambers, some of them of considerable size, allowing heated meltwater to then sink through the ice remaining below them. This meltwater mechanism copes with a thick ice shell and does not require that it actually be penetrated to mix surface ingredients with the water below. The observational evidence can support this, not only on Europa but elsewhere. Implicit in the discussion is the idea that the ice surrounding a melt chamber is not rigid. From the paper:

…impacts that generate melt chambers also significantly warm and soften the surrounding ice making it susceptible to viscous deformation. Furthermore, although not explored here, the impact may generate fractures that allow for transport of melts short distances away from the crater melt pond… Importantly, the crater record of icy moons includes craters of varying complexities (Schenk, 2002; Turtle & Pierazzo, 2001) with anomalous features such as collapsed pits, domes, and central Massifs that imply post-impact modifications (Bray et al., 2012; Elder et al., 2012; Korycansky, 2020; Moore et al., 2017; Silber & Johnson, 2017; Steinbrügge et al., 2020). These observed crater features suggest that both impact structures and the generated melts experience significant post-impact evolution that has so far received little attention.

Image: An artist’s concept of a comet or asteroid impact on Jupiter’s moon Europa. Credit: NASA/JPL-Caltech.

The method here is to deploy mathematical simulations to study the evolution of these melt chambers on Europa. The term is ‘foundering,’ which is the movement of meltwater through the ice as it potentially transports oxidants below. If surface ice can be transferred into the ocean in a sustained way, and thus not just through massive impacts but through a range of smaller ones, the chances of developing interesting biology below only increase. The work also implies that Europa’s so-called ‘chaos’ terrain, which some have explained as the result of meltwater near the surface, may have other origins, for in this model most of the meltwater does not remain near the surface. Says Carnahan: “We’re cautioning against the idea that you could maintain very large volumes of melt in the shallow subsurface without it sinking.”

The researchers modeled comet and asteroid impacts using a shock-physics cratering simulation and massaged the output by factoring in both the sinking of dense meltwater and its refreezing within the ice shell. The modeling required analysis of the energies involved as well as the deformation of the surface ice after impact. UT’s Carnahan developed the ice shell convection model that the authors extended to match the geometry of surface impact simulations and subsequent changes in the ice.

The conclusions are striking:

Our simulations show that impacts that generate significant melt chambers lead to substantial post-impact viscous deformation due to the foundering of the impact melts. If the transient cavity depth of the impact exceeds half the ice shell thickness the impact melt drains into the underlying ocean and forms a continuous surface-to-ocean porous column. Foundering of impact melts leads to mixing within the ice shell and the transfer of melt volumes on the order of tens of cubic kilometers from the surface of Europa to the ocean.

Image: A computer-generated simulation of the post-impact melt chamber of Manannan Crater, an impact crater on Europa. The simulation shows the melt water sinking to the ocean within several hundred years after impact. Credit: Carnahan et al.

So we have a way to get surface materials through to the Europan ocean, a method that because it does not require large impacts, has likely been widespread throughout Europa’s history. It’s interesting to speculate on how this process could leave evidence beyond what we’ve already uncovered in the craters visible on the surface and what corroboration in support of the analysis Europa Clipper and JUICE may be able to provide. Other icy worlds come to mind here as well, with the authors mentioning Titan as a place where even an exceedingly thick ice shell may still be susceptible to exchanging material with the surface.

Given how little we know about abiogenesis, it’s conceivable not only that life might develop under Europan ice, but that icy moons elsewhere in the Solar System may hold far more life in the aggregate than exists in what we view as the habitable zone. If that is the case, then the argument that life is ubiquitous in the universe receives strong support, but it will take a lot of hard exploration to find out, a process of discovery whose next steps via Europa Clipper and JUICE will represent only a beginning.

The paper is Carnahan et al., “Surface-To-Ocean Exchange by the Sinking of Impact Generated Melt Chambers on Europa,” Geophysical Research Letters Vol. 49, Issue 24 (28 December 2022). Full text.