by Paul Gilster | Aug 25, 2023 | Outer Solar System |
We’re living in a prime era for studying the Solar System’s movement through the galaxy, with all that implies about stellar evolution, planet formation and the heliosphere’s interactions with the interstellar medium. We don’t often think about movements at this macro-scale, but bear in mind that the Sun and the planets are now moving through the outer edges of what is known as the local interstellar cloud (LIC), having been within the cloud by some estimates for about 60,000 years.
What happens next? I always think about Poul Anderson’s wonderful Brain Wave when contemplating such matters. In the classic 1954 tale, serialized the year before in Space Science Fiction during the great 1950s boom in science fiction magazines, Brain Wave depicts the Earth’s movement out of an energy-damping field it had moved through since the Cretaceous. When the planet moves out of this field at long last, everyone on the planet gets smarter. What will happen when we leave the LIC?
Nothing this dramatic, as far as anyone can tell, but we’re moving into the so-called G-cloud, a somewhat denser region we’ll get to know within a few thousand years (Alpha Centauri is already within the G-cloud). The LIC appears to be about 30 light years across, sporting somewhat higher density hydrogen than the interstellar medium around it and flowing from the direction of Scorpius and Centaurus. We seem to be moving through its outer edge.
Image: That’s a Richard Powers cover on the first edition of Poul Anderson’s Brain Wave, illustrating a novel Anderson considered one of his best.
The word ‘denser’ has to be kept in perspective. Consider that about half of the interstellar gas – hydrogen and helium for the most part – takes up 98 percent of the space between the stars and is thus extraordinarily low in density, The other half of this gas is compressed into 2% of the volume and is found in molecular clouds, mostly molecular hydrogen, that include some carbon monoxide and higher concentrations of dust than the surrounding ISM.
We’d like to know more about the G-cloud, and in particular such factors as its temperatures and density, for just how the heliosphere reacts to this changing environment, presumably through compression, could have effects upon its shape and thus its shielding effects from cosmic rays. Thus the importance of the Interstellar Probe mission (ISP) out of Johns Hopkins Applied Physics Laboratory that we’ve discussed often in these pages (I’ll point you to Mapping Out Interstellar Clouds for more on the subject). One thing I noted in an article last year was that there are alternative models to the Sun’s nearby cloud environment, some challenging the idea that the LIC and the G-cloud are distinct regions. Interstellar Probe would help in the investigation.
Image: Our solar journey through space is carrying us through a cluster of very-low-density interstellar clouds. Right now the Sun is inside of a cloud (Local cloud) that is so tenuous that the interstellar gas detected by the IBEX (Interstellar Boundary Explorer mission) is as sparse as a handful of air stretched over a column that is hundreds of light-years long. These clouds are identified by their motions, indicated in this graphic with blue arrows. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan.
The fate of Interstellar Probe is the hands of the National Academies of Sciences, Engineering, and Medicine, which prioritizes in its decadal studies where we are going in space exploration over ten year periods. We find out next year whether ISP has been selected among several strong candidates for the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Until then, I continue to scout the literature both for Interstellar Probe as well as JPL’s Solar Gravitational Lens (SGL) mission, a candidate for the same study. These are huge decisions in space science.
Be aware of a new paper tackling the issues of heliospheric and dust science that a mission like Interstellar Probe could examine, with a solid backgrounder on the kind of instrumentation that would make this first dedicated interstellar probe a game-changer in our understanding of the local interstellar environment. The paper, by lead author Veerle J. Sterken (ETH Zürich) and colleagues, was originally submitted to the heliophysics Decadal, and is here available in a modified version (citation below) that includes the new instrumentation discussion among other modifications.
I won’t run through the entire paper; suffice it to say that its discussion of interstellar dust in and around the heliosphere is comprehensive and its analysis of how best to study changing environments in the interstellar medium is highly informative. The benefit of a probe explicitly designed to examine the transition between dust and gas states along its trajectory are clear. From the paper, which notes here the particular importance of heliopause passage:
ISP will fly throughout approximately 16 years, more than a solar cycle, while passing through interplanetary space, the termination shock, the heliosheath, up to the heliopause and beyond, making it an optimal mission for studying the heliosphere-dust coupling and using this knowledge for other astrospheres. Beyond the heliopause, the tiny dust with gyro-radii of only a few to 100 AU (for dust radii < 0.1 μm…), will help study the interstellar environment (magnetic field, plasma) and may detect local enhancements of smaller as well as bigger ISD [interstellar dust]. The strength of the mission lies in flying through all of these diverse regions with simultaneous magnetic field, dust, plasma and pickup ion measurements. No mission so far has flown a dedicated dust dynamics and composition suite into the heliosheath and the vast space beyond.
Analyzing the interaction between the heliosphere and interstellar dust acts as a probe into the history of our own Solar System, since we know that some of this material was affected by near-Earth supernovae, still raining down in the form of dust today. Mid-sized dust particles in the range of 0.1 to 0.6 μm in radius can make it through the heliosphere into the Solar System. Some smaller particles (30-100 nm) as well may escape filtering, but the authors note the limitations of our knowledge: “The exact lower cut-off size and time-dependence of particles that can enter the solar system is not yet exactly known, but Ulysses and Cassini already have measured ISD particles with radii between 50 and 100 nm.”
Thus we have markers for stellar and galactic evolution as factors in this study. But I’ll also remind those interested in interstellar flight that assessing dust density and the size distribution of particles will play a major role when we reach the point where we can send craft at a significant fraction of the speed of light. Larger particles in the local interstellar environment could cause catastrophic failure, while Ian Crawford has pointed out that other properties of this region could inform propulsion concepts like the interstellar ramjet.
Something else I learned from Ian Crawford (University of London) and wrote about twelve years ago (see Into the Interstellar Void) is that a spacecraft moving at 0.1c could make daily measurements 17 AU apart, which is roughly half the radius of the Solar System. So as we develop the fastest probe we can manage today in the form of designs like Interstellar Probe, we can see a larger picture in which such studies assist as we develop future technologies capable of actually making a crossing to Alpha Centauri, sampling widely along the way two different interstellar clouds and the boundary between them.
The paper is Sterken et al., “Synergies between interstellar dust and heliospheric science with an Interstellar Probe,” submitted as white paper for the National Academies Decadal Survey for Solar and Space Physics 2024-2033 and available as a preprint here.
by Paul Gilster | Aug 21, 2023 | Outer Solar System |
As a book-dazzled kid growing up in St. Louis, I had the good fortune to be surrounded by books from previous generations, and specifically those belonging both to my father and my half-brother, who had died long before I was born. Among these was a multi-volume encyclopedia from the 1920s I’ve never been able to identify. All I have is the memory of looking through its musty volumes and realizing that Pluto was not listed in it, as the publication date was a few years earlier than Clyde Tombaugh’s epic search for the world.
I do remember thinking that without Pluto, the Solar System only had eight planets, and musing in my teenage boy way about how odd this incomplete view of the Solar System was. Little did I know how much more was in store! As to that eighth planet, Neptune was a puzzler not only to the encyclopedia but to science fiction writers of the Gernsback era. Thus James Morgan Walsh’s “The Vanguard to Neptune,” published in Wonder Stories Quarterly in the Spring, 1932 issue. In the cover by Frank R. Paul, that’s Neptune hanging in the sky, looking for all the world like a terrestrial planet, here seen from Triton. The explorers assume the blue areas are ice until they cross to the planet.
Image: Frank R. Paul’s cover illustration for J. M. Walsh’s “The Vanguard to Neptune.” Walsh (1897-1952) was an interesting figure in his own right for those of us who spent a career living off the printed word. Settling in the UK, the Australian novelist would pen an astounding 94 novels across a wide range of genres and under a variety of pseudonyms. It was possible to do that kind of thing in the pulp era.
Spurring these recollections are images of Neptune revealed in a new study on the planet’s cloud cover and its relation to the solar cycle. They’re so stunning that I wanted to reproduce them here, thinking about how our knowledge of the Solar System has advanced since my first acquaintance with the planet in that encyclopedia as no more than a speck of light amongst countless others. There’s also a bit of the Voyager 2 thrill as the craft approached Neptune back in 1989 deep in the summer night here. To see new worlds open before us. Astonishing.
I suppose one day we’ll get so completely accustomed to imaging exoplanets that such thrills will seem commonplace, or maybe not, given their sheer diversity. But the images below still work for me, the first set from Hubble.
Image: This sequence of Hubble Space Telescope images chronicles the waxing and waning of the amount of cloud cover on Neptune. This nearly-30-year-long set of observations shows that the number of clouds grows increasingly following a peak in the solar cycle – where the Sun’s level of activity rhythmically rises and falls over an 11-year period. The Sun’s level of ultraviolet radiation is plotted in the vertical axis. The 11-year cycle is plotted along the bottom from 1994 to 2022. The Hubble observations along the top, clearly show a correlation between cloud abundance and solar peak of activity. The chemical changes are caused by photochemistry, which happens high in Neptune’s upper atmosphere and takes time to form clouds. Credit: NASA, ESA, LASP, Erandi Chavez (UC Berkeley), Imke de Pater (UC Berkeley).
I don’t mean to neglect the import of the paper that features these observations, which comes from astronomers at UC-Berkeley, or their conclusions, which use the numerous changes in the patterning of Neptune’s clouds to point to the connection with the flip in the Sun’s magnetic field every eleven years. It’s intriguing to learn that when the Sun emits more intense ultraviolet light, and in particular the strong hydrogen Lyman-alpha emission, there is increasing cloud cover on Neptune fully two years later.
Imke de Pater (UC-Berkeley) is senior author on the study:
“These remarkable data give us the strongest evidence yet that Neptune’s cloud cover correlates with the Sun’s cycle. Our findings support the theory that the Sun’s UV rays, when strong enough, may be triggering a photochemical reaction that produces Neptune’s clouds.”
We see 2.5 cycles of cloud activity on Neptune recorded over a 29-year period in observations not only from Hubble but Keck Observatory and Lick Observatory, in which it also becomes clear that there is a relationship between the number of clouds and the planet’s observed brightness. Below is the Keck imagery.
Image: A dramatic change in Neptune’s appearance was observed in late 2019 and has persisted through June 2023. As shown by this compilation of images at 1.63 µm (microns) obtained with the NIRC2 and adaptive optics system on the Keck II Telescope, Neptune had numerous cloud features organized in latitudinal bands from before 2002 through late 2019. Afterwards, clouds appeared almost absent except near the south pole. The images are displayed using a Asinh function which, like a log-scale display, decreases the contrast between the features; if displayed on a linear scale, only the brightest features would be visible. Credit: Imke de Pater, Erandi Chavez, Erin Redwing (UC Berkeley)/W. M. Keck Observatory.
This is tricky analysis, because as the paper points out, clouds not related to photochemical reactions, as for example those produced by storms rising up from the deep atmosphere, would complicate correlations with the solar cycle. More recent imagery from the summer of this year has begun to show more clouds in the northern latitudes and at high altitude, which de Pater says reflects the observed increase in the solar ultraviolet flux in the past two years. It’s chastening to realize that even with 30 years of high resolution data covering almost three solar cycles, we have still covered only 20 percent of Neptune’s orbit. Oh for an ice-giant orbiter to depict up close the chaotic actions on a planet whose winds are the strongest known in the Solar System.
The paper is Chavez et al., “Evolution of Neptune at near-infrared wavelengths from 1994 through 2022,” Icarus Vol. 404 (1 November 2023), 115667 (abstract).
by Paul Gilster | May 9, 2023 | Outer Solar System |
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.
by Paul Gilster | Apr 18, 2023 | Outer Solar System |
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).
by Paul Gilster | Apr 11, 2023 | Outer Solar System |
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.
by Paul Gilster | Mar 15, 2023 | Outer Solar System |
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.
