by Paul Gilster | Dec 21, 2022 | Outer Solar System |
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.
by Paul Gilster | Dec 2, 2022 | Outer Solar System |
I keep an eye on recent findings about Europa because fine-tuning procedures for the science that missions like Europa Clipper and JUICE (Jupiter Icy Moons Explorer) will perform at the Jovian moon is an ongoing process that doesn’t stop at launch. The more we learn now – the more anomalies we uncover or processes we begin to glimpse – the better able we’ll be to adjust spacecraft observing strategies to go after the answers to these phenomena. A new study teaches us a bit more about Europa’s plate tectonics, the only solid evidence of tectonics we know of other than Earth’s. And it will take new high-resolution imagery to confirm the theories put forth within it.
Appearing in the Journal of Geophysical Research: Planets, the paper looks at the processes that evidently govern the evolution of the fractured Europan surface the Galileo mission revealed to us back in the 1990s. What’s intriguing here is the identification of Europan tectonic plates in the context of deep time. If a tectonic plate shows a broad continuity, it is bounded by surface discontinuities. The authors show that a sequence can be established based on how discontinuities cut through surface features, and we can begin to make statements about how the surface changes.
Image: A complex pattern of ridges and bands named Arachne Linea is seen in this false-color image of Europa’s surface taken by the Galileo spacecraft on 26 September 1998. New research shows that this landscape was formed by the jostling of nearby tectonic plates. Credit: NASA/JPL-Caltech/SETI Institute.
The work, led by Geoffrey Collins (Wheaton College, MA), looks at three areas on Europa – spread out in terms of latitude so as to include the high northern and southern latitudes as well as the equatorial areas – to reconstruct what they would have looked like before surface plates moved. It’s fascinating to contrast the findings with what we see on Earth, because on Europa tectonic movement is scattered, with some areas showing no plates at all. That reveals a regional process in which plate travel distance can be less than 100 kilometers. Plate tectonics, in other words, occurs only in limited areas on Europa, covers only a small degree of motion across the surface, and only appears to operate intermittently. It does not appear to be happening now, at least in the areas the paper surveys.
I come back to Europa Clipper, the mission that should benefit from this analysis, for although we’ve studied plate tectonics on Europa before, the work has been hampered by lack of imaging data at high enough resolution to attain a widespread view. The authors argue that the improved imaging from missions like Clipper and JUICE will detect many more areas with plate-like motions to add to their three examples. This should highlight the fact that while Earth’s tectonic plates form a global system, Europa’s are sharply confined in a process that is likely local or regional.
Note the stop-and-start aspect of plate movement, as discussed in the paper (italics mine):
In all of the study areas, young ridges and ridge complexes overprint the plate boundaries. The young ridges do not accommodate offsets like those seen in the plate boundaries. Thus, whatever process was driving the plate motions came to an end, and is not actively driving plate motions today in any of the areas studied. The relationship between the Castalia Macula study area and the older Belus area to its north… demonstrates that the plate tectonic-like behavior on Europa did not occur all at the same time. Combined with the previous conclusion, we develop the view that plate tectonic-like behavior on Europa occurs in regional patches and turns on and off at different times in different places.
Image: This is Figure 19 from the paper, illustrating tectonic change over time. Caption: An area exhibiting plate-like motions north of Belus Linea is outlined by a red dashed line. The purple lines in the south are plate boundaries CM2 and CM4 from the Castalia Macula reconstruction… The blue line shows a ridge that is crosscut by CM2 and CM4, and extends all the way south to be crosscut by Acacallis Linea (off the southern edge of this figure). The Blue Ridge crosscuts the green ridge, which crosscuts all of the candidate plate boundaries in the area north of Belus. This shows that all of the plate-like activity in the area north of Belus is older than the activity in the Castalia Macula study area. Orthographic projection centered at 15°N, 135°E. Credit: Collins et al.
The study of these three areas of Europa implies that something stops plate motion from persisting and spreading, possibly because of the nature of the surface material or else the mechanism that forces plate motions in the first place. Here’s an interesting point that has further ramifications for future observations by our spacecraft: Is convergence – where surface material is lost and pre-existing terrain must be reconstructed – always apparent from the limited data we have available? We’ll need plenty of high resolution imagery from future spacecraft to make the call on that.
There is much for Europa Clipper to examine in terms of how tectonics functions here, not the least of which is the question of what drives the plate motions thus far observed on Europa. If plate tectonics is indeed as sharply limited as this study implies, just what is it that forces a plate movement and then apparently shuts it down?
And in terms of that ever-fascinating ocean below the ice, is surface material drawn in large quantities into the ice shell, and are there times when the lower crust or ocean is exposed while the plate motion occurs? Reconstructing plate motions is an open investigation with serious implications for habitability. Europa Clipper and the JUICE mission should give us data that sharply refine our modeling of surface and ocean.
The paper on Europa’s tectonics is Collins et al, “Episodic Plate Tectonics on Europa: Evidence for Widespread Patches of Mobile?Lid Behavior in the Antijovian Hemisphere,” Journal of Geophysical Research: Planets Vol. 127, Issue 11 (06 November 2022). Full text.
by Paul Gilster | Sep 30, 2022 | Outer Solar System |
Juno’s close pass of Europa on September 29 (1036 UTC) took it within 352 kilometers of the icy moon, marking the third close pass in history below 500 kilometers. The encounter saw the spacecraft come within a single kilometer of Galileo’s 351 kilometers from the surface back in January of 2000, and it provided the opportunity for Juno to use its JunoCam to home in on a region north of Europa’s equator. Note the high relief of terrain along the terminator, with its ridges and troughs starkly evident.
Image: The complex, ice-covered surface of Jupiter’s moon Europa was captured by NASA’s Juno spacecraft during a flyby on Sept. 29, 2022. At closest approach, the spacecraft came within a distance of about 352 kilometers. Credit: NASA/JPL-Caltech/SWRI/MSSS.
This first image from JunoCam captures features at the region called Annwn Regio, and was collected in the two-hour window available to Juno as it moved past Europa at 23.6 kilometers per second. What we hope to gain from analysis of the data should be high resolution images at approximately 1 kilometer per pixel, along with data on the ice shell covering the moon’s ocean, along with a good deal more about its surface composition, its internal structure and tenuous ionosphere. Says Candy Hansen, a Juno co-investigator (Planetary Science Institute, Tucson):
“The science team will be comparing the full set of images obtained by Juno with images from previous missions, looking to see if Europa’s surface features have changed over the past two decades. The JunoCam images will fill in the current geologic map, replacing existing low-resolution coverage of the area.”
In other words, more JunoCam imagery to come, all useful to the upcoming Europa Clipper and JUICE missions. In particular, data from the spacecraft’s Microwave Radiometer should fill in our understanding of variations in Europa’s ice beneath the crust, and possibly point to regions where liquid water may be captured in subsurface pockets.
by Paul Gilster | Sep 26, 2022 | Outer Solar System |
As the Europlanet Science Congress (EPSC) has just wrapped up in Spain’s Palacio de Congresos de Granada, I’m reminded how little time I’ve had recently to keep up with such gatherings. I do hope to have some entries on EPSC-announced findings in the near future. Today I simply note the news of an unexpected ‘heat wave’ (700?) extending 130,000 kilometers just below Jupiter’s northern aurora, one traveling at high speed toward the equator, as announced by James O’Donoghue at the EPSC.
Says JAXA’s O’Donoghue:
“While the auroras continuously deliver heat to the rest of the planet, these heat wave ‘events’ represent an additional, significant energy source. These findings add to our knowledge of Jupiter’s upper-atmospheric weather and climate, and are a great help in trying to solve the ‘energy crisis’ problem that plagues research into the giant planets.”
I mention this work in particular because of my interest in the EPSC results but also because Jupiter has been on my mind thanks to the Juno mission extension. The spacecraft will now remain in operation in Jupiter space through September of 2025, becoming an explorer of the moon system here. Specifically, multiple rendezvous are planned for Ganymede, Europa and Io, even as the spacecraft continues magnetic field studies and radio occultation science. The extended mission will also take multiple passages through the planet’s thin system of rings.
Image: A far more distant encounter with Europa than the one about to happen. This image from the spacecraft’s JunoCam was taken at a distance of about 82,000 kilometers. Color and reflectance variations across Europa’s regions can be seen Although the resolution of the images is just 50 to 60 km per pixel, the data fills in a previously un-imaged area around the north pole near the center of the image. Credit: NASA/JPL-Caltech/SwRI/MSSS. Image processing: Andrea Luck CC BY.
The words ‘previously un-imaged’ have a nice resonance, reminding us how reliant we have been on the Galileo dataset for Europa surface studies. As Juno’s orbit evolves, the spacecraft continues to investigate this system from new angles, with perijove (closest approach to Jupiter) migrating northward over the course of the mission, allowing for example close views of cyclones at Jupiter’s northern poles. Note the beautiful gravitational ballet going on here, as multiple satellite flybys steer our spacecraft through the Jupiter system and reduce its orbital period.
But my attention is drawn primarily to the implications for Europa Clipper and JUICE (Jupiter Icy Moons Explorer), and not just because of the close observations of icy moon surfaces that the Juno orbits will allow. Note this: During the course of these investigations, Juno will fly through the Europa and Io ring tori. These are ring shaped clouds of ions whose characterization will assist planning as Europa Clipper and JUICE controllers anticipate the radiation environment near the large icy moons.
Obviously, the more we can learn about the radiation situation we’ll face near Europa, the better for conducting operations around that intriguing world. Juno’s icy moon encounters have already begun, with a low-altitude flyby of Ganymede in June of 2021. Gravitational interactions there have in turn set up the close flyby of Europa we can expect in just a few days, on September 29, which will be followed by close approaches of Io, one on December 30 of 2023, the next on February 3 of 2024.
The upcoming Europa flyby will reduce Juno’s orbit around Jupiter from 43 to 38 days, and will represent the closest a NASA spacecraft has approached Europa since the days of the Galileo probe. The latter came within 351 kilometers of the moon back in 2000, and we’ve been examining that precious data ever since as we investigate the moon’s surface looking for clues about the ocean underneath. Now we’ll come within 358 kilometers while collecting high-resolution images of portions of the surface.
If passage through the ring tori will be valuable for future missions, so will the additional data on Europa’s ionosphere and its interactions with Jupiter’s magnetosphere help us understand more about this world and its intriguing interior. Juno will throw every science instrument and sensor it has into the effort, from the Jovian Auroral Distributions Experiment (JADE) to its X-band medium-gain radio antenna. The Jupiter Energetic-Particle Detector Instrument (JEDI) and Magnetometer will collect information about the ionosphere and plasma environment.
Moreover, Juno’s Microwave Radiometer (MWR) should return data on the icy crust. And for the visually oriented (which probably means all of us), JunoCam will take four visible light images that should be helpful in finding any changes in surface features since Galileo. Expect a resolution better than 1 kilometer per pixel. We’ll also get a high-resolution black-and-white image from Juno’s star camera, while the Jovian Infrared Auroral Mapper (JIRAM) will take infrared images of the surface.
Data collection is to begin about an hour before closest approach, with the spacecraft still over 80,000 kilometers from Europa. That makes for a swift flyby indeed. John Bordi is Juno deputy mission manager at the Jet Propulsion Laboratory:
“The relative velocity between spacecraft and moon will be 14.7 miles per second (23.6 kilometers per second), so we are screaming by pretty fast. All steps have to go like clockwork to successfully acquire our planned data, because soon after the flyby is complete, the spacecraft needs to be reoriented for our upcoming close approach of Jupiter, which happens only 7½ hours later.”
Image: Juno’s extended mission includes flybys of the moons Ganymede, Europa, and Io. This graphic depicts the spacecraft’s orbits of Jupiter – labeled “PJ” for perijove, or point of closest approach to the planet – from its prime mission in gray to the 42 orbits of its extended mission in shades of blue and purple. Credit: NASA/JPL-Caltech/SwRI.
Nearly 50 Europa flybys are on tap from Europa Clipper, while JUICE will add several more as the spacecraft adjusts its trajectory enroute to extended orbital operations at Ganymede. Oh for faster propulsion! It takes a long time to get to Jupiter with today’s methods, but the early 2030s should be a harvest of information about the small world that seems to be the Solar System’s most likely place for life as we don’t know it.
by Paul Gilster | Aug 26, 2022 | Outer Solar System |
We’re in that earliest phase of interstellar exploration that is all about nudging outward from our system into the local interstellar medium. That has already involved the Voyagers, but my plan is to keep checking in on both the Interstellar Probe concept at the Johns Hopkins Applied Physics Laboratory and the SGL probe study steadily maturing at the Jet Propulsion Laboratory. These are absorbing ventures as scientists figure out ways to do propulsion, in-flight maintenance (and in the case of SGL, in-flight assembly) and data return on timescales the Voyager team wasn’t imagining when those doughty craft were launched in 1977.
Nudging outward. Let’s check in a bit with New Horizons, because here we have a Kuiper Belt explorer that is fully operational, and with instruments specifically designed for the environment it explores, now some 54 AU from the Sun. It’s striking to think that the Juno mission is ten times closer to our star than New Horizons. The Pluto/Charon flyby seems a long time ago, as does that of the KBO Arrokoth. Indeed the spacecraft is now 1.6 billion kilometers further out than Arrokoth, which it visited in 2019.
Meanwhile the pace of analysis has been intense, with more than 65 publications from the New Horizons science team making their way into the literature last year alone. Have a look at Arrokoth as visualized from recent analysis and realize that data from the encounter continue to stream back to Earth even now. In fact, as New Horizons begins its second extended mission on October 1, completing the data transfer is among the priorities, according to principal investigator Alan Stern in his latest PI’s Perspective.
Image: Recently published discoveries from New Horizons have run the gamut across astrophysics, heliophysics and planetary science. This image is one of many geophysical data products resulting from New Horizons’ 2019 flight past Arrokoth, the first and (so far) only Kuiper Belt object explored by spacecraft, It shows surface slopes on Arrokoth derived from New Horizons stereo imagery, and illustrates one important aspect to understanding both the origin and the geological evolution of Arrokoth. Credit: From a paper led by James Tuttle Keane in the June 2022 issue of Journal of Geophysical Research (JGR) Planets (citation below).
The illustration above is from a recent paper in which lead author James Keane (JPL) and colleagues delve into the peanut-shaped Arrokoth, which the authors point out is probably the least evolved object ever explored by a spacecraft. The paper takes on the ambitious challenge of figuring out the object’s gravity field, noting that bright material seems to collect in its lowest locations. New Horizons was not able to measure Arrokoth’s density directly, but the latter can be inferred using methods that have been fine-tuned in the study of asteroids and comets. It turns out to be unusually low.
The authors describe Arrokoth as akin to fluffy snow on Earth, making it one of the lowest density objects ever explored. It’s intriguing to see that there are comparisons between Arrokoth and some of the smallest moons found within Saturn’s rings. From the paper:
The only objects in the Solar System with consistently low, Arrokoth-like, measured densities are Saturn’s ring moons. These small worlds are thought to form from the gentle accretion of icy ring particles—which may not be unlike the formation of planetesimals via streaming instability and other processes in the early Kuiper Belt, although this comparison requires more investigation.
A useful analogue indeed, if it can be shown that a ring system that Cassini has already provided huge amounts of data on can illuminate processes from the earliest days of the Solar System. The paper continues:
Expanded models of ring and planetesimal dynamics may partially support testing this hypothesis, as could continued analysis of Cassini data. New, ultra-high-resolution observations of dense rings around gas giants (like those proposed by the Saturn Ring Skimmer mission concept; Tiscareno et al., 2021) may be particularly illustrative of how these small, low-density worlds form and evolve.
Let’s pause for a moment on Saturn Ring Skimmer, which comes out of an effort led by Matthew S. Tiscareno (SETI Institute) and has been submitted to the 2023 Planetary Science Decadal Survey. The mission is described as a “ballistic tour” that makes repeated low altitude passes over Saturn’s main rings in a span of 162 days without the use of propellant, covering the main ring regions in 13 low-altitude flybys. The authors of the white paper on the idea say that Saturn Ring Skimmer would get 100 times closer to the ring system than Cassini when its best ring images were taken, and would be able to measure material surrounding the rings in situ.
Image: This is Figure 1 from the paper on Ring Skimmer. Caption: Polar plot (left) illustrating 13 passes over Saturn’s rings corresponding to the 162-day long prototype ballistic tour; the altitude (middle) and relative velocity (right) curves represent the passes over the rings. The black solid lines on the left panel represent the region of the rings shadowed by the Sun and, thus, in eclipse. This ring-skimming trajectory is ballistic and exploits four Titan gravity assists. For reference, the ring passes are color coded and grouped by Titan flybys. Figure from Vaquero et al. (2019).
So we have one possibility for augmenting even our Cassini data with measurements that may shed light on the New Horizons findings at Arrokoth. Out of the exhaustive analysis in the Keane paper, we begin to build a picture of Kuiper Belt Objects assembling gently in the outer Solar System, probably within the first few million years of its formation. The New Horizons extended mission should also be able to help if suitable targets can be found. As of now, 24 KBO systems have density constraints, all of these determined by studying multiple object systems (other than Triton, which is most likely a KBO that was captured). Returning to the Keane paper:
Arrokoth is a much smaller object than these other characterized KBOs (it is the smallest KBO with an inferred density). All other characterized KBOs are binary or multiple systems with individual components at least 3× larger than Arrokoth. It is widely recognized that KBO densities decrease with decreasing size, however it is unclear how far that trend goes (and it clearly cannot go to zero density at zero size). Without a more complete sample, it is unclear if Arrokoth’s inferred low density is (a) representative of other small KBOs, (b) an outlier, or (c) simply incorrect due to some flawed assumption(s) used in inferring its density.
So we have a lot to learn. The authors point out how many questions emerge from the Arrokoth flyby. To understand KBO densities, we need further density analysis of comets (only 67P/Churyumov-Gerasimenko has precise density measurements). For that matter, we need more information about KBO binaries and their rotation and orbital dynamics, a thought that Arrokoth emphasizes because there is close alignment between its two lobes. Did two tidally locked objects slowly spiral together before merging? Finding a KBO binary ahead would be pure gold for New Horizons as we try to refine our understanding of the evolution of these objects.
The possibility of another KBO flyby is enhanced by the fact that New Horizons has about 11 kilograms of fuel onboard, though finding an object within range is a daunting task. Both Gemini South (Chile) and the Subaru telescope in Hawaii are looking for a target now, aided by new machine learning tools recently developed. Says Stern:
…by the time New Horizons emerges from hibernation at the beginning of March, we’ll be deep into planning observations of new, much more distant KBOs, as well as a look back at distant Uranus and Neptune to observe how these two “ice giant” planets reflect sunlight – which will tell us more about what drives their internal energy balance. We also plan to make the most extensive and sensitive studies of the cosmological visible light and ultraviolet light backgrounds ever made; such measurements constrain origin theories of the universe while shedding new light on the total number of galaxies in the universe.
Flybys make the news, while much of the essential data gathering proceeds quietly and relatively behind the scenes, which is why I focus on it here. Now in hibernation, New Horizons has until next spring in a relatively quiescent state, though as Stern points out, the Venetia Burney Student Dust Counter (SDC) as well as the PEPSSI and SWAP charged-particle plasma spectrometers remain active. The second priority of the extended mission is collecting and archiving data on the Kuiper Belt environment and also looking outward to the interactions between the heliosphere and the interstellar medium. The plan is to continue these observations while creating the first all-sky ultraviolet maps of the heliosphere, as well as studying clouds in the interstellar medium.
The paper is Keane et al., “The Geophysical Environment of (486958) Arrokoth—A Small Kuiper Belt Object Explored by New Horizons,” JGR Planets 15 May 2022 (full text). The paper on Saturn Ring Skimmer is Tiscareno et al, “The Saturn Ring Skimmer Mission Concept: The next step to explore Saturn’s rings, atmosphere, interior, and inner magnetosphere,” a white paper submitted to the 2023 Planetary Science Decadal Survey (2020). Full text.
by Paul Gilster | Aug 23, 2022 | Outer Solar System |
The new Jupiter photos from JWST’s Near Infrared Camera (NIRCam) are unusual, enough so that I decided to fold one into today’s post. It’s a pretty good fit because I had already put together most of the material I was going to use about Europa. It would have been an additional plus if Europa showed up in the image below, but even without it, note that we can see moons as small as Adrasta here. Imke de Pater (UC-Berkeley), who led the observations, noted that both tiny satellites and distant galaxies show up in the same image. And here’s Thierry Fouchet, a professor at the Paris Observatory, who likewise worked on the observing effort:
“This image illustrates the sensitivity and dynamic range of JWST’s NIRCam instrument. It reveals the bright waves, swirls and vortices in Jupiter’s atmosphere and simultaneously captures the dark ring system, 1 million times fainter than the planet, as well as the moons Amalthea and Adrastea, which are roughly 200 and 20 kilometers across, respectively. This one image sums up the science of our Jupiter system program, which studies the dynamics and chemistry of Jupiter itself, its rings and its satellite system.”
And yes, these images are significantly processed, in this case by citizen scientist Judy Schmidt in California and Ricardo Hueso (University of the Basque Country, Spain). Hurdo is a co-investigator on the Early Release Science program and also leads the NIRCam observations of Jupiter’s Atmosphere. I think Schmidt, who has been working with space observations for a decade, says it best when she describes her goal as “to get it to look natural, even if it’s not anything close to what your eye can see.”
Image: This false-color composite image of Jupiter was obtained July 27 with the NIRCam instrument on board the JWST. Jupiter’s faint rings — a million times dimmer than the planet — and two of its small satellites, Amalthea (left) and Adrastea (dot at edge of ring), are clearly visible against a background of distant galaxies. The diffraction pattern created by the bright auroras and the moon Io (to the left out of the image), form a complex background of scattered light around Jupiter. (Image credit: NASA, European Space Agency, Jupiter Early Release Science team. Image processing: Ricardo Hueso [UPV/EHU] and Judy Schmidt).
I had this image on-screen this morning as I looked into progress on Europa Clipper, which is in the midst of its most significant year so far. By the end of 2022, most flight hardware and all the science instruments are expected to be installed at the Jet Propulsion Laboratory’s Spacecraft Assembly Facility. Engineers and technicians will be assembling the spacecraft’s main body in the installation’s High Bay 1. That includes installation of the craft’s science instruments as well as the aluminum electronics vault that shields the electronics from Jupiter’s radiation. Launch is currently scheduled for October, 2024. We should get nearly 50 close passes of Europa out of all this.
Image: Engineers and technicians use a crane to lift the core of NASA’s Europa Clipper spacecraft in the High Bay 1 clean room of JPL’s Spacecraft Assembly Facility. Credit: NASA/JPL-Caltech.
Watching a spacecraft come together is a fascinating exercise, and we’ll keep an eye on NASA’s updates on the Clipper as the process continues. Just as fascinating, though, is the continual inflow of information about what Europa Clipper’s science instruments will be looking for, a process just as critical if we are to interpret its data correctly.
On that score, what an interesting paper has recently turned up in Astrobiology. In the hands of lead author Natalie Wolfenbarger, it comes out of the University of Texas at Austin, where Europa Clipper’s radar instrument has been developed. A key issue is the composition of the moon’s ice shell, which in turn will feed our conclusions about the ocean lying beneath. Europa’s ocean has been likened to the waters beneath an Antarctic ice shelf on Earth. A good comparison?
To find out, Wolfenbarger and colleagues went to work on how water freezes under ice shelves, which takes us into two unusual terms. ‘Congelation ice’ forms under the ice shelf, while ‘frazil ice’ floats upward in the form of ice flakes in supercooled seawater. These form a kind of snow that coats the bottom of the ice shelf. Interestingly, both ice production mechanisms produce ice with less salinity than seawater itself.
In other words, we may have been assuming that Europa’s ocean is saltier than it actually is, particularly given the paper’s finding that scaling up what happens under Antarctica to an ice shell the size and age of Europa’s produces ice that is less salty still. Frazil ice in particular retains only a small fraction of seawater salt, and the authors make the case that it should be common on Europa. A less saline ice shell is significant because salinity governs its strength and the movement of heat through it.
Image: Mounds of snow-like ice under an ice shelf. Credit: ©Helen Glazer 2015 from the project Walking in Antarctica.
Thus we use our own planet as a research model to understand mechanisms likely at play on a Jovian moon, in ways that help us prepare for Europa Clipper’s look via its ice penetrating instrument, which is called REASON (Radar for Europa Assessment and Sounding: Ocean to Near-surface). This is the only one of the spacecraft’s nine instruments that can look directly into the ice shell, a process that we have experience with on Earth, as REASON principal investigator Don Blankenship notes: “We’ve used ice-penetrating radar for decades. That’s how we know Earth’s ice sheets’ thickness.”
The thickness of the ice is important for everything from getting future probes through the shell into the ocean beneath to creating conditions for an ocean with more likelihood of habitable conditions. For Europa is constantly bathed in particles flung against its surface by Jupiter’s magnetic field, so that compounds emerge that would be useful to life below. A thinner ice sheet would make it more likely that these compounds enter the ocean. No wonder the thickness of the ice has been such a contentious matter among scientists, whose estimates range from a few kilometers to tens of kilometers thick.
Europa Clipper’s REASON instrument uses different wavelengths of radio waves and will be capable of penetrating the ice shell as much as 30 kilometers. This should get interesting.
The paper is Wolfenbarger et al., “Ice Shell Structure and Composition of Ocean Worlds: Insights from Accreted Ice on Earth,” Astrobiology Vol. 22, No. 8 (25 July 2022). Abstract.
