An Ice Giant’s Possible Oceans

Further fueling my interest in reaching the ice giants is a study in the Journal of Geophysical Research: Planets that investigates the possibility of oceans on the major moons of Uranus. Imaged by Voyager 2, Uranus is otherwise unvisited by our spacecraft, but Miranda, Ariel, Titania, Oberon and Umbriel hold considerable interest given what we are learning about oceans beneath the surface of icy moons. Hence the need to examine the Voyager 2 data in light of updated computer modeling.

Julie Castillo-Rogez (JPL) is lead author of the paper:

“When it comes to small bodies – dwarf planets and moons – planetary scientists previously have found evidence of oceans in several unlikely places, including the dwarf planets Ceres and Pluto, and Saturn’s moon Mimas. So there are mechanisms at play that we don’t fully understand. This paper investigates what those could be and how they are relevant to the many bodies in the solar system that could be rich in water but have limited internal heat.”

Image: This is Figure 1 from the paper. Caption: Densities and mean radii of the Uranian moons compared to those of other large moons and dwarf planets. Miranda has a low density similar to Saturn’s moon Mimas, whereas the densities of the other Uranian moons are more similar to Saturn’s moons Dione and Rhea. After Hussmann et al. (2006).

The interest is more than theoretical, for as we’ve recently discussed the Planetary Science and Astrobiology Decadal Survey for 2023–2032 has put a Uranus Orbiter and Probe mission on its short list of priorities. A mission to Uranus would open up the prospect for confirming oceans, or the lack of same, within the five large moons. Recent work explored in the Castillo-Rogez paper has made the case that magnetic fields induced by such oceans should be detectable by a Uranus orbiter’s flybys.

Much has happened to call for new modeling of this system. The paper notes recent advances in surface chemistry and geology, revised models of system dynamics, and the knowledge gained on icy bodies in the size range of the Uranian moons as studies have continued on Enceladus and the moons of Saturn as well as Pluto and Charon, not to mention the availability of data from the Dawn mission at Ceres. The team’s modeling produces likely interior structures that are promising for four of the moons.

These moons are indeed small objects, and while Uranus has 27 moons, it is only when we reach the size of Ariel (1160 kilometers) that we can start talking realistically about interior oceans. Titania is the largest of these at 1580 kilometers. The paper argues that of the five largest moons, we can exclude Miranda (470 kilometers) as being too small to sustain the heat to support an internal ocean. But the other four appear promising, revising and contradicting earlier work that had focused primarily on Titania and Oberon in the belief that Ariel, Umbriel, and Miranda would be frozen at present.

Image: New modeling shows that there likely is an ocean layer in four of Uranus’ major moons: Ariel, Umbriel, Titania, and Oberon. Salty – or briny – oceans lie under the ice and atop layers of water-rich rock and dry rock. Miranda is too small to retain enough heat for an ocean layer. Credit: NASA/JPL-Caltech.

Of the large Uranian moons, Ariel may emerge as the best possibility. From the paper:

Ariel is particularly interesting as a future mission target because of the potential detection of NH3-bearing species on its surface (Cartwright et al., 2021) that could be evidence of recent cryovolcanic activity, considering these species should degrade on a geologically short timescale. Geologic features, visible in Voyager 2 Imaging Science Subsystem images of Ariel, show some evidence for cryovolcanism in the form of double ridges and lobate features that may represent emplaced cryolava (Beddingfield & Cartwright, 2021).

But oceans tens of kilometers deep at Titania and Oberon may yet excite astrobiological interest, depending on what we learn about heat sources here.

Based on current understanding, we conclude that the Uranian moons are more likely to host residual or “relict” oceans than thick oceans. As such, they may be representative of many icy bodies, including Ceres, Callisto, Pluto, and Charon (De Sanctis et al., 2020). The detection and characterization (depth and thickness) of deep oceans inside the Uranian moons… and refined constraint on surface ages would help assess whether these bodies still hold residual heat from a recent resonance crossing event and/or are undergoing tidal heating due to dynamical circumstances that are currently unknown (as was the case at Enceladus before the Cassini mission).

The Uranus Orbiter and Probe mission holds great allure for answering some of these questions. The issue of detection by a spacecraft is still charged, however. The authors note from the outset that an ocean maintained primarily by ammonia would be well below the water freezing point, in which case its electrical conductivity might be too low to register on the UOP’s sensors. In other words, ammonia essentially acts as an antifreeze, with electrical conductivity near zero. Temperatures below ~245 K would mean an ocean would have to be detected by the exposure of deep ocean material, in which case we come back to Ariel as the most likely target for the closest scrutiny.

The paper is Castillo-Rogez et al., “Compositions and Interior Structures of the Large Moons of Uranus and Implications for Future Spacecraft Observations,” JGR Planets Vol. 128, Issue 1 (January 2023). Abstract.


Probing the Shifting Surface of Icy Moons

In celebration of the recent JUICE launch, a few thoughts on what we’re learning about Ganymede, with eight years to go before the spacecraft enters the system and eventually settles into orbit around the icy moon. Specifically, let’s consider a paper just published in Icarus that offers results applicable not just to Ganymede but also Europa and Enceladus, those fascinating and possibly life-bearing worlds. We learn that when we look at the surface of an icy moon, we’re seeing in part the result of quakes within its structure caused by the gravitational pull of the parent planet.

Image: ESA’s latest interplanetary mission, Juice, lifted off on an?Ariane 5 rocket?from?Europe’s Spaceport?in French 09:14 local time/08:14 EDT on 14 April 2023 to begin its eight-year journey to Jupiter, where it will study in detail the gas giant planet’s three large ocean-bearing moons: Ganymede, Callisto and Europa. Credit: ESA.

The Icarus paper homes in on the link between such quakes, long presumed to occur given our understanding of gravitational interactions, and the landslides observable on the surface of icy moons. It’s one thing to tag steep ridges surrounded by flat terrain as the result of ‘ice volcanoes’ spouting liquid, but we also find the same result on moons whose surface temperature makes this explanation unlikely.

Thus the new work, described by lead author Mackenzie Mills (University of Arizona), who analyzed the physical pummeling icy terrain takes during tidally induced moonquakes:

“We found the surface shaking from moonquakes would be enough to cause surface material to rush downhill in landslides. We’ve estimated the size of moonquakes and how big the landslides could be. This helps us understand how landslides might be shaping moon surfaces over time.”

Image: NASA’s Galileo spacecraft captured this image of the surface of Jupiter’s moon Ganymede. On Earth, similar features form when tectonic faulting breaks the crust. Scientists modeled how fault activity could trigger landslides and make relatively smooth areas on the surfaces of icy moons. Credit: NASA/JPL/Brown University.

This is ancient terrain indeed, located within Ganymede’s Nicholson Regio near the border with Harpagia Sulcus. Which leads to a quick digression: One of the pleasures of discovery is the growing familiarity with the names of surface features we are beginning to see close up. We’ve known enough about Ganymede thanks to craft in the system (the above image is from the Galileo probe) to have already named many features, but with New Horizons we were naming as we went, seeing surface detail for the first time. Ponder how familiar we will become with the surface features of Ganymede once JUICE settles into its multi-year orbit around the moon. We’ll be tossing off references to Nicholson Regio with ease.

As to the latter, the terrain is ancient indeed, heavily cratered, and as you can see, riddled with steep slopes and cliffs (scarps) causing crustal fracturing. We’re seeing frozen geological history here, a useful pointer to how moon and planet have interacted over the aeons, and information which may tell us about Ganymede’s interior structure when complemented by the data we can expect from JUICE. Here the scarps form a series of blocks that delineate the boundary between dark and light terrain.

The image in question covers approximately 16 by 15 kilometers, and was taken on May 20, 2000 at a range of just over 2000 kilometers. It’s been some time since I’ve written about Galileo imagery used for anything other than the study of Europa, but of course the craft gave us priceless data about the entire Jovian moon system despite its high-gain antenna problems. Here the resolution is 20 meters per pixel. Below is another Galileo snapshot, this one of Europa and likewise showing scarp formations.

Image: An image of Jupiter’s moon Europa captured in the 1990s by NASA’s Galileo shows possible fault scarps adjacent to smooth areas that may have been produced by landslides. Credit: NASA/JPL-Caltech.

We’re looking at what appear to be fault features, scarps adjacent to much smoother areas. Is this the result of material cascading out into surrounding terrain as the result of a landslide? Co-author Robert Pappalardo (JPL) notes the likelihood, even when we’re talking about much smaller celestial objects than Ganymede or Europa. Much studied Enceladus, in fact, has a mere 3% of the surface area of Europa:

“It was surprising to find out more about how powerful moonquakes could be and that it could be simple for them to move debris downslope. Because of that moon’s small gravity, quakes on tiny Enceladus could be large enough to fling icy debris right off the surface and into space like a wet dog shaking itself off.”

The paper underlines the point:

By measuring scarp dimensions, we aim to better understand the formation of faults and associated mass wasted deposits, given abundant evidence of past and/or recent tectonic and seismic environments on these icy worlds. For studied scarps, we estimate a moonquake moment magnitude range Mw = 4.0–7.9. On Earth, quakes of similar magnitude are the middle and upper end of the log-based magnitude scale and commonly cause significant destruction, including causing mass movements such as landslides. Occurrence of similarly large quakes on icy satellites, which have surface gravities much less than Earth, implies that such quakes could induce significant seismic effects.

You can imagine how much JUICE and Europa Clipper will help in the decoding of such surface features, with sharp improvements in the resolution of our imagery and the prospect of stereo imaging along with subsurface radar sounding deployed for Ganymede and Europa. Thus we build our library of information about the geological processes at work in such exotic venues, and also learn about whether or not their surfaces continue to be active. The nature of the ice shell on Europa is a prime science objective for Europa Clipper, providing further information about the ocean beneath.

The paper is Mills et al., “Moonquake-triggered mass wasting processes on icy satellites,” Icarus Vol. 399 (15 July 2023), 115534 (full text).

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).