TAOS II is the Transneptunian Automated Occultation Survey, designed to spot comets deep in our Solar System. It may also be able to detect comets of the interstellar variety, of which we thus far have only one incontrovertible example, 2I/Borisov. And TAOS II, as well as the Vera C. Rubin Observatory (both are slated for first light within a year or so) could have a lot to work with, if a new study from Amir Siraj and Avi Loeb (Center for Astrophysics | Harvard & Smithsonian) is correct in its findings.
I cite Borisov as thus far unique in being an interstellar comet because the cometary status of ‘Oumuamua is still in play. On my way to looking at his paper on Borisov, I had an email exchange with Avi Loeb, from which this:
Observations with the Spitzer Space Telescope of `Oumuamua placed very tight limits on carbon-based molecules in its vicinity, implying that it was not made of carbon or oxygen. This led to suggestions that perhaps it is made of pure hydrogen or pure nitrogen, but these would be types of objects we had never seen before. Borisov appeared to be just like a regular comet that we had seen many times before. Clearly, `Oumuamua and Borisov are of very different composition and origin (irrespective of whether `Oumuamua is natural or artificial in origin).
Image: Comet 2I/Borisov. Credit: NASA, ESA and D. Jewitt (UCLA).
The paper refers to Borisov as “the first confirmed interstellar comet with a known composition,” but if this comet is alone in our catalog, it’s unlikely to remain that way long. Siraj and Loeb argue that there exist more interstellar objects in the Oort Cloud than objects born in the Solar System. Indeed, Loeb in his email cited “a hundred trillion Borisov-like interstellar comets” in this vast space, which extends from roughly 2,000 AU perhaps as far out as 50,000 AU, with some sources citing an outer edge as far as 200,000 AU. That should ring a few bells — Alpha Centauri is 268,000 AU from the Sun, meaning our Oort Cloud could mingle with any similar cloud in that system.
The prospect of studying interstellar objects without leaving our own system is enhanced by these results, even if the calculations contain significant uncertainties. There should be many Borisovs, a small number of which should enter the inner system. This is a reversal of earlier thinking that interstellar visitors should be rare, all part of a reevaluation of the subject forced by the detection of ‘Oumuamua and 2I/Borisov in recent years, and the coming upgrades in equipment and surveys mentioned above.
We are only now getting into position to be able to see these objects and identify their true nature. The detection of Borisov in 2019 allowed scientists to calculate a number density for such objects per star based on a statistical analysis of the likelihood of a single object like this being within 3 AU of the Sun. Other researchers had applied this kind of calibration to ‘Oumuamua, with the number density implied by both being approximately the same. Similarly, the population of bound Oort Cloud comets can be inferred through observations of long-period comets. Figure 1 in the paper shows the comparison.
Image: This is Figure 1 from the paper. Caption: Comparison of the relative abundance per star of bound Oort cloud objects, as implied by the observed rate of long-period comets (Brasser & Morbidelli 2013), and interstellar objects, as implied by the detection of Borisov (Jewitt et al. 2020), with a differential size distribution for power-law index, q, values of 2.5, 3, and 3.5, displayed for reference. The error bar indicates the 3? Poisson error bars for the implication of a singular interstellar object detection on the abundance. The shaded band correspond[s] to the plausible range of nucleus radii for Borisov, given the central value for Borisov’s abundance. The error bounds on the abundance of bound Oort cloud objects are not resolvable on this plot. Credit: Siraj and Loeb.
These are calibrations with, as the paper notes, uncertainties of several orders of magnitude, but even adjusting for these, interstellar objects still prevail in the Oort. They also come with limits that can be tested by observation. Interstellar objects experience negligible gravitational focusing because of their speed (on the order of 30 kilometers per second) and the nature of their orbits as related to their distance from the Sun. Bound Oort Cloud objects should have characteristic orbits that can be differentiated from the orbits of objects that have entered the Oort from elsewhere.
Note: ‘Gravitational focusing’ refers not to gravitational lensing but to the likelihood that two particles will collide based on their mutual gravitational attraction. The authors are saying that bound Oort objects are significantly affected by gravitational focusing. We wind up with a wide dispersion in these two populations:
Given that the number density of interstellar objects may be ?103 larger than that of bound Oort cloud objects far from the Sun, the Oort cloud objects may be still a factor of ?10 more abundant than interstellar objects in the inner Solar system, due to the unequal influence of gravitational focusing on the two populations. The fact that interstellar objects outnumber Oort cloud objects per star is consistent with the Oort cloud having lost most of its initial mass. However, the degree to which interstellar objects outnumber Oort cloud objects is still very uncertain. Stellar occultation surveys of the Oort cloud will be capable of confirming the results presented here, by differentiating between the two populations through speed relative to the Sun…
Thus we can look to planned surveys of the sort mentioned above to test the abundances of the two classes of objects, and can expect more visitors of the Borisov kind, even if such comets are far more common in the Oort Cloud than in the inner system. Siraj points out that such an abundance of interstellar objects indicates that planetary formation leaves a great deal more debris than previously thought:
“Our findings show that interstellar objects can place interesting constraints on planetary system formation processes, since their implied abundance requires a significant mass of material to be ejected in the form of planetesimals. Together with observational studies of protoplanetary disks and computational approaches to planet formation, the study of interstellar objects could help us unlock the secrets of how our planetary system — and others — formed.”
The paper is Siraj & Loeb, “Interstellar objects outnumber Solar system objects in the Oort cloud,” Monthly Notices of the Royal Astronomical Society Vol. 507, Issue 1 (October, 2021) L-16-L18 (abstract).
Rotorcraft have certainly been in the news lately, with Ingenuity, the Mars helicopter, commanding our attention. The Dragonfly mission to Titan involves a far more complex rotorcraft capable of visiting numerous destinations on the surface. In fact, Dragonfly makes use of eight rotors and depends upon an atmosphere more helpful than what Ingenuity has to work with on Mars. Titan’s atmosphere is four times denser than what we have on Earth, allowing Dragonfly to move its entire science payload from one location to another as it examines surface landing zones while operating on a world whose gravity is but one-seventh that of Earth.
I want to call your attention to the publication of the science team that just appeared in the Planetary Science Journal, because it lays out the rationale for the various decisions made thus far about operations on and above Titan’s surface. It’s a straightforward, interesting read, and makes clear how much work we have to do here. Yes, we had Cassini for a breathtaking tour that lasted 13 years, with repeated flybys and investigations on Titan using radar, but while we know a lot about structures like lakes and mountains on the surface, we know all too little about their composition.
In fact, as Alex Hayes points out, we didn’t know at the time Cassini launched whether the Huygens probe would find a global ocean at Titan or a solid surface of ice and organics. Because of the uncertainty, the Huygens science experiments were primarily atmospheric, meant to function during the descent phase. Hayes (Cornell University) is a co-investigator for Dragonfly. He adds:
“The science questions we have for Titan are very broad because we don’t know much about what is actually going on at the surface yet. For every question we answered during the Cassini mission’s exploration of Titan from Saturn orbit, we gained 10 new ones.”
Image: What we do know. This is Figure 1 from the paper. Caption: Dragonfly will image from the surface to provide context for sampling and measurements, as well as in flight to identify sites of interest at a variety of locations. (Left) Huygens image of Titan’s surface; cobbles are 10-15 cm across and may be water ice (Tomasko et al. 2005; Keller et al. 2008; Karkoschka & Schröder 2016a). (Right) Huygens aerial view of terrain akin to the diverse equatorial landscapes that Dragonfly will traverse and image at higher resolution. Credit: NASA/ESA/Barnes et al.
Dragonfly’s 2.7 year mission, starting upon arrival at Titan in 2034 during winter in the northern hemisphere, will commence at a landing site that was chosen for its safety factors (broad, relatively flat terrain) as well as its proximity to nearby interesting scientific targets. The goal is to set down at the equatorial dune fields called Shangri-La, which NASA notes are similar to the dunes found in Namibia on Earth. A series of short flights will explore this area before longer flights of up to eight kilometers begin, the beauty of the design being that Dragonfly will be able to sample interesting surface areas along the route to its destination, the Selk impact crater.
As the mission now stands, the lander should log on the order of 175 kilometers across Titan enroute to Selk. The latter is an interesting place because there is evidence here of past liquid water as well as organics, complex molecules containing carbon, along with hydrogen, oxygen and nitrogen. Methane rain and a snow of organics keep Titan’s weather systems complex amidst a landscape containing the building blocks of life.
But let’s get back to that landing site. The paper refers to Shangri-La as an “organic sand sea,” with the touchdown site located 134 kilometers south of Selk Crater, and approximately 175 kilometers north-northwest of the Huygens Landing Site. The image below is Figure 7 from the paper, giving the landing site in context.
Image: Dragonfly landing site. Credit: Barnes et al.
As the paper notes, a ‘sand sea’ is only partially sand. Dunes can be separated by flat sand-free areas called ‘interdunes,’ a feature likewise common to Namibia, where the Namib sand sea is covered only 40 percent by sand. The interdunes that make up the balance are primarily gravel. Cassini was able to resolve Titan’s interdunes to reveal their predominantly icy character, one that matches the spectral properties at the Huygens landing site. The authors find the correlation interesting because it implies the Shangri-La interdunes will include water-ice gravels, “potentially a fine-grained layer damp with condensed methane,” and thus offering a chance for Dragonfly to sample both Titan’s organic sands and materials with a water ice component.
Image: This illustration shows NASA’s Dragonfly rotorcraft-lander approaching a site on Saturn’s exotic moon, Titan. Taking advantage of Titan’s dense atmosphere and low gravity, Dragonfly will explore dozens of locations across the icy world, sampling and measuring the compositions of Titan’s organic surface materials to characterize the habitability of Titan’s environment and investigate the progression of prebiotic chemistry. Credit: NASA/JHU-APL
The path to Selk Crater should take in a variety of terrain with different compositions, which will include the edge of the crater’s ejecta deposits. From Cassini data, the authors believe this material is similar in composition to the Huygens landing site, representing an area likely to feature water ice. Selk itself is 80 kilometers in diameter. Cassini data along with the Dragonfly team’s modeling show the spectral signature of organic sand in the interior and water-ice around the edges of the crater floor.
As you can see, the astrobiological examinations Dragonfly will engage in are both water-based and hydrocarbon-based, meaning a potential biosignature is possible from impact melt deposits or interactions with the interior ocean — this would be life as we know it — or from a form of life we have yet to discover that draws on liquid hydrocarbons within Titan’s lakes, seas and aquifers. The mission is designed around the ability to seek out both, as the paper explains:
We designed the science of Dragonfly around the themes of prebiotic chemistry, habitability, and the search for biosignatures, with an explicit consideration of both water and hydrocarbon solvents. To address prebiotic chemistry, we will determine the inventory of prebiotically relevant organic and inorganic molecules and reactions on Titan. In the realm of habitability, we will determine the role of Titan’s tropical atmosphere and shallow subsurface reservoirs in the global methane cycle, determine the rates of processes modifying Titan’s surface and rates of material transport, and constrain what physical processes mix surface organics with subsurface ocean and/or melted liquid-water reservoirs. Our search for biosignatures will entail a broad-based search for signatures indicative of past or extant biological processes.
The paper is Barnes et al., “Science Goals and Objectives for the Dragonfly Titan Rotorcraft Relocatable Lander,” Planetary Science Journal Vol. 2, No. 4 (full text).
Seeing spacecraft coming together is always exciting, and when it comes to Europa Clipper, what grabs my attention first is the radiation containment hardware. This is a hostile environment even for a craft that will attempt no landing, for flybys take sensitive electronics into the powerful radiation environment of Jupiter’s magnetosphere. 20,000 times stronger than Earth’s, Jupiter’s magnetic field creates a magnetosphere that affects the solar wind fully three million kilometers before it even reaches the planet, trapping charged particles from the Sun as well as Io.
We have to protect Europa Clipper from the intense radiation emerging out of all this, and in the image below you can see what the craft’s engineers have come up with. Now nearing completion at the Jet Propulsion Laboratory, the aluminum radiation vault will ultimately be attached to the top of the spacecraft’s propulsion module, connecting via kilometers of cabling that will allow its power box and computer to communicate with systems throughout the spacecraft. The duplicate vault shown below is used for stress testing before final assembly of the flight hardware.
Image: Engineers and technicians in a clean room at NASA’s Jet Propulsion Laboratory display the thick-walled aluminum vault they helped build for the Europa Clipper spacecraft. The vault will protect the spacecraft’s electronics from Jupiter’s intense radiation. In the background is a duplicate vault. Credit: NASA/JPL-Caltech.
The ATLO phase (Assembly, Test, and Launch Operations) begins in the spring of 2022 at JPL, with the radiation vault being one of the first components in place as Europa Clipper enters its final stage of fabrication. The 3-meter tall propulsion module was recently moved from Goddard Space Flight Center in Greenbelt, Maryland to the Johns Hopkins Applied Physics Laboratory (APL) in preparation for the installation of electronics, radios, antennae and cabling. Science instruments, meanwhile, are being tested at the universities and other institutions contributing to the mission.
Jan Chodas (JPL) is Europa Clipper Project Manager:
“It’s really exciting to see the progression of flight hardware moving forward this year as the various elements are put together bit by bit and tested. The project team is energized and more focused than ever on delivering a spacecraft with an exquisite instrument suite that promises to revolutionize our knowledge of Europa.”
Before the ATLO phase begins, Europa Clipper will also be subject to a System Integration Review later this year, a process in which all instruments are inspected and plans for the assembly and testing of the spacecraft are finalized. The destination for all these components and instruments is the main clean room at JPL in Pasadena, where what NASA describes as the ‘choreography’ of building a flagship mission will draw together components from workshops and laboratories in the US and Europe.
Image: Contamination control engineers in a clean room at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, evaluate a propellant tank before it is installed in NASA’s Europa Clipper spacecraft. The tank is one of two that will be used to hold the spacecraft’s propellant. It will be inserted into the cylinder seen at left in the background, one of two cylinders that make up the propulsion module. Credit: NASA/GSFC Denny Henry.
The travels of the propulsion module emphasize the collaborative nature of any complex spacecraft assembly. The two cylinders making up the module were built at the Applied Physics Laboratory and then shipped to JPL, where thermal tubing carrying coolant to regulate the spacecraft’s temperature in deep space was added. The cylinders then went to Goddard, where the propellant tanks were installed inside them and the craft’s sixteen rocket engines were attached to the outside. It then returned to APL for the installation of electronics and cabling mentioned above.
Image: Engineers and technicians in a clean room at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, integrate the tanks that will contain helium pressurant onto the propulsion module of NASA’s Europa Clipper spacecraft. Credit: NASA/GSFC Barbara Lambert.
Connecting to the thermal tubing will be Europa Clipper’s radiator, which will radiate enough heat into space to keep the spacecraft in its operating temperature range. APL is now integrating the propulsion module and the radios, antennae and cabling for communications, while a company called Applied Aerospace Structures Corporation in Stockton, California is building the 3-meter high-gain antenna. By the spring of next year, the antenna will be in place at JPL for insertion in the ATLO process.
Nine science instruments will fly aboard Europa Clipper, all being assembled and undergoing testing at NASA centers as well as partner institutions and private vendors. The spacecraft is to investigate the depth of the internal ocean as well as its salinity and the thickness of the ice crust. The latter is obviously a huge factor in any future plans to sample the ocean beneath the ice, but so is the question of whether Europa vents subsurface water into space through plumes that may one day be sampled.
Image: NASA’s Jet Propulsion Laboratory in Southern California is building the spectrometer for the agency’s Europa Clipper mission. Called the Mapping Imaging Spectrometer for Europa (MISE), it is seen in the midst of assembly in a clean room at JPL. Pronounced “mize,” the instrument will analyze infrared light reflected from Jupiter’s moon Europa and will map the distribution of organics and salts on the surface to help scientists understand if the moon’s global ocean – which lies beneath a thick layer of ice – is habitable. Credit: NASA/JPL-Caltech.
We’ll finally be able to update those Galileo images that have served scientists so well in the study of Europa’s surface with new, detailed looks at the surface geology. Launch is currently planned for October, 2024 aboard a Falcon Heavy rocket. Europa Clipper isn’t a life detection mission, but we’ll learn a good deal more about Europa’s potential for supporting life. What kind of mission grows out of that is something it would be foolhardy to predict. One step at a time as Europa reveals its mysteries.
Hubble observations from the past two decades have been recently re-examined as a way of investigating what is happening in the tenuous atmosphere of Ganymede, the largest moon in the Solar System. It was in 1998 that the telescope’s Space Telescope Imaging Spectrograph took the first images of Ganymede at ultraviolet wavelengths, showing auroral bands — ribbons of electrified gas — that reinforced earlier evidence that the moon had a weak magnetic field. Now we have news of sublimated water vapor within the atmosphere, an earlier prediction now verified.
Ganymede’s atmosphere, such as it is, is the result of charged particles and solar radiation eroding its icy surface, producing both molecular (02) and atomic oxygen (0) as well as H20, with the molecular oxygen long thought to be the most abundant constituent overall. Surface temperatures are as extreme as you would expect, roughly between 80 K and 150 K (-193 °C to -123 °C).
In 2018, a team led by Lorenz Roth (KTH Royal Institute of Technology in Stockholm) again turned to Hubble, this time in support of the ongoing Juno mission. The goal was to measure Ganymede’s atomic oxygen (0) as a way of resolving differences in the 1998 ultraviolet observations, which were thought to be the result of higher concentrations of atomic oxygen in some parts of the atmosphere. The result was surprising.
Roth’s team used data from Hubble’s Cosmic Origins Spectrograph along with archival data from the Space Telescope Imaging Spectrograph taken in 1998 and 2010. There was little trace of atomic oxygen in Ganymede’s atmosphere, meaning that differences in the auroral images must have some other explanation. The relative distribution of the aurorae allowed the scientists to map them against projected water sources on the surface, released when the moon sublimates water molecules as the equator warms. The phenomenon has no connection with the moon’s subsurface ocean, thought to be buried 150 kilometers beneath the surface.
The fit is strong: The area where water vapor would be expected in Ganymede’s atmosphere, around noon at the equator, correlates with the differences in the ultraviolet images. Roth thinks water vapor produced by sublimation (ice turning directly to vapor with no intervening liquid state) is the explanation:
“So far only the molecular oxygen had been observed,. This is produced when charged particles erode the ice surface. The water vapor that we measured now originates from ice sublimation caused by the thermal escape of water vapor from warm icy regions.”
Image: In 1998, Hubble’s Space Telescope Imaging Spectrograph took these first ultraviolet images of Ganymede, which revealed a particular pattern in the observed emissions from the moon’s atmosphere. The moon displays auroral bands that are somewhat similar to aurora ovals observed on Earth and other planets with magnetic fields. This was illustrative evidence for the fact that Ganymede has a permanent magnetic field. The similarities in the ultraviolet observations were explained by the presence of molecular oxygen. The differences were explained at the time by the presence of atomic oxygen, which produces a signal that affects one UV color more than the other. Credit: NASA, ESA, Lorenz Roth (KTH).
The paper goes on to point out the significance of the observation (italics mine):
The low oxygen emission ratios in the center of Ganymede’s observed hemispheres are consistent with a locally H2O-dominated atmosphere. With phase angles around 10? …the disk centers are close to the sub-solar points…. A viable source for H2O in Ganymede’s atmosphere can be sublimation in the low-latitude sub-solar regions, where an H2O-dominated atmosphere was indeed predicted by atmosphere models. Our derived H2O mixing ratios are in agreement with these predictions. While previously detected tenuous atmospheres around icy moons in the outer solar system were consistent with surface sputtering (or active outgassing) as source for the neutrals our analysis provides the first evidence for a sublimated atmosphere on an icy moon in the outer solar system.
Image: This image of the Jovian moon Ganymede was obtained by the JunoCam imager aboard NASA’s Juno spacecraft during its June 7, 2021, flyby of the icy moon. At the time of closest approach, Juno was within 1,038 kilometers of its surface – closer to Jupiter’s largest moon than any other spacecraft has come in more than two decades. Credit: NASA/JPL-Caltech/SwRI/MSSS.
So we have an atmosphere with what the paper describes as “a pronounced day/night asymmetry.” It’s an important finding for future missions, for atmospheric asymmetries will turn up in data on the magnetosphere and space plasma, meaning numerical simulations and data analysis for future missions have to incorporate them. This points to JUICE, the Jupiter Icy Moons Explorer mission, which will put eleven science instruments past Ganymede in a series of flybys and later orbital operations.
That itself is a stunning thought to someone who grew up thinking about Ganymede as a Poul Anderson novel setting. We’ll have a spacecraft orbiting the moon for a minimum of 280 days, if all goes according to plan, giving scientists abundant data about both the surface and atmosphere. The Roth paper provides significant information about the role of sublimation that will refine the JUICE observing plan.
The paper is Roth et al., “A sublimated water atmosphere on Ganymede detected from Hubble Space Telescope observations,” Nature Astronomy 26 July 2021 (abstract / preprint).
Our recent discussion about Europa (Europa: Below the Impact Zone) has me thinking about those tempting Galilean moons and the problems they present for exploration. With a magnetic field 20,000 times stronger than Earth’s, Jupiter is a radiation generator. Worlds like Europa may well have a sanctuary for life beneath the ice, but exploring the surface will demand powerful radiation shielding for sensitive equipment, not to mention the problem of trying to protect a fragile human in that environment.
Radiation at Europa’s surface is about 5.4 Sv (540 rem), although to be sure it seems to vary, with the highest radiation areas being found near the equator, lessening toward the poles. In human terms, that’s 1800 times the average annual sea-level dose. Europa is clearly a place for robotic exploration rather than astronaut boots on the ground.
Jupiter offers up an environment where the solar wind, hurling electrically charged particles at ever-shifting velocities, interacts with the powerful magnetosphere, stretching it out almost 1000 kilometers away from the Sun. It’s essential that we learn more about the behavior of the magnetic fields generated by gas giants, and on that score new work out of Goddard Space Flight Center offers some insight. GSFC’s Yasmina Martos and team have been using the inner Galilean moon, Io, as a probe, studying what sets off one type of radio emissions known to emanate from Jupiter.
Io’s volcanoes have been observed since the first Voyager flyby, driven by internal heat as the moon experiences the gravitational pull not only of Jupiter but neighboring large moons. Gas and particles released by this activity are ionized and swiftly captured by Jupiter’s magnetic field, being accelerated along the field toward the Jovian poles.
Out of this we get decametric radio emissions (DAM) as electrons spiral in the magnetic field, waves that the Juno spacecraft’s Juno Waves Instrument has been detecting. Jupiter also produces radio waves at centimeter and decimeter wavelengths, caused by atmospheric phenomenon as well as activity in the magnetosphere apart from the Io interactions. The planet is, in fact, the noisiest radio emitter in the Solar System apart from the Sun. Homing in on the Io emissions, the GSFC work deploys a new magnetic field model with higher accuracy near the moon and targets the particular geometric configurations of planet and moon needed for Juno to detect the emissions.
Studying the radio emissions mediated by Io doesn’t help us cope with the radiation problem, but it does offer clues about this particular magnetosphere, a phenomenon we’ll come to know much better as future missions arrive. The researchers, reporting in the Journal of Geophysical Research: Planets, found that the decameter radio waves are controlled by not just the strength but the shape of Jupiter’s magnetic field. They emerge from a cone-like space thus formed, so that the spacecraft can only receive the radio signal when Jupiter’s rotation moves that cone across the instrument. The effect is similar to a lighthouse beacon sweeping out to sea.
Image: The multicolored lines in this conceptual image represent the magnetic field lines that link Io’s orbit with Jupiter’s atmosphere. Radio waves emerge from the source and propagate along the walls of a hollow cone (gray area). Juno, its orbit represented by the white line crossing the cone, receives the signal when Jupiter’s rotation sweeps that cone over the spacecraft. Credit: NASA/GSFC/Jay Friedlander.
I can remember trying to pick up radio emissions from Jupiter with my first shortwave receiving set — they’ve been a known phenomenon since 1955, detectable from Earth at between 10 and 40 MHz. What we get thanks to the Juno observations is a clarification about why decametric radio waves originating in the northern hemisphere seem more abundant than those from the southern. The paper explains for the first time the particular geometric configurations producing these effects. The authors offer up a plain language summary to go along with the paper’s abstract, from which this:
Thanks to Juno, the geometry of the magnetic field has been better constrained as waves and magnetic field data have been continuously collected within the Jovian environment since July 2016. In this study, we estimate where the radio waves generate and the energy of the electrons that generate these waves, which is up to 23 times higher than previously proposed. We ultimately demonstrate that the geometry of Jupiter’s magnetic field is a primary controller for the higher observation likelihood of radio wave groups originating in the northern hemisphere relative to those originating in the southern hemisphere.
Video: The decametric radio emissions triggered by the interaction of Io with Jupiter’s magnetic field. The Waves instrument on Juno detects radio signals whenever Juno’s trajectory crosses into the beam which is a cone-shaped pattern. This beam pattern is similar to a flashlight that is only emitting a ring of light rather than a full beam. Juno scientists then translate the radio emission detected to a frequency within the audible range of the human ear. Credit: University of Iowa/SwRI/NASA.
What a fascinating place the Jovian system is. Learning the precise locations within the magnetosphere where the decametric emissions originate helps to pin down the needed magnetic field strength and electron density to fit the Juno data. “The radio emission is likely constant,” says Martos, “but Juno has to be in the right spot to listen.”
The paper is Martos et al, “Juno Reveals New Insights Into Io?Related Decameter Radio Emissions,” Journal of Geophysical Research: Planets (18 June 2020). Abstract.
Yesterday we looked at the behavior of ice on Enceladus, a key to making long range plans for a lander there. But as we saw with Kira Olsen and team’s work, learning about the nature of ice on worlds with interior oceans has implications for other ice giant moons. This morning we look at the hellish surface environment of Europa, as high-energy radiation sleets down inside Jupiter’s magnetic field.
Europa’s surface radiation will complicate operations there and demand extensive shielding for any lander. But below the ice, that interior ocean should be shielded and warm enough to offer the possibility of life. With Europa Clipper on pace for a 2024 launch, we need to ask how the surface ice has been shaped and where we might find biosignatures that could have been churned up from below.
Tidal stresses on the ice leading to fracture are one way to force material up, but small impacts from above — debris in the Jovian system — also roil the surface. If we’re looking for potential biosignatures, we have to consider this surface churn and the effects of radiation upon what it produces. This is the subject of new work from a team led by Emily Costello (University of Hawai?i at Manoa), which has been studying the effects of electron radiation accelerated by Jupiter on complex molecules.
Image: The University of Hawai?i at Manoa’s Costello. Credit: UH Manoa.
The operative term in this paper, just published in Nature Astronomy, is impact gardening. The authors estimate through their modeling that the surface of Europa has been affected up to an average depth of 30 centimeters (about 12 inches). Over millions of years, the impacts add up even as surface material mixes with the subsurface, all bathed by radiation.
“If we hope to find pristine, chemical biosignatures, we will have to look below the zone where impacts have been gardening,” says Costello. “Chemical biosignatures in areas shallower than that zone may have been exposed to destructive radiation.”
Image: In this zoomed-in area (Figure 2) of Europa’s surface, an inset to Figure 1, a cliff runs across the middle of the image, revealing the interiors of the ridges leading up to it. The thin, bright layer at the top of the cliff is at least 20 to 40 feet (6 to 12 meters) thick. This thin surface layer, and possibly layers like it elsewhere over Europa’s surface, is where a process called “impact gardening” is thought to occur. Impact gardening is the small-scale mixing of the surface by space debris, such as asteroids and comets. Scientists are studying the cumulative effects of small impacts on Europa’s surface as NASA prepares to explore the moon with the upcoming Europa Clipper mission. New research and modeling estimate that the surface of Europa has been churned by small impacts to an average depth of about 12 inches (30 centimeters), within the layer of the surface that is visible here. Credit: NASA/JPL-Caltech.
The study looks not only at surface impacts but goes on to consider secondary impacts when debris returns to the surface after the initial strike. We learn there is a case for a particular zone on Europa — the moon’s mid- to high-latitudes — that would be less affected by radiation. In any case, a robotic lander may need to probe at least 30 centimeters down to find material unaffected by the ongoing impact gardening.
Rebecca Ghent (Planetary Science Institute, Tucson) is a co-author on the study:
“The work in this paper could provide guidance for design of instruments or missions seeking biomolecules; it also provides a framework for future investigation using higher-resolution images from upcoming missions, which would help to generate more precise estimates on the depth of gardening in various specific regions. The key parameters in this study are the impact flux and cratering rates. With better estimates of these parameters, and higher-resolution imaging resulting from upcoming missions, it will be possible to better predict the depths to which gardening has affected the shallow ice in specific regions.”
As a side note, I found when looking through Costello’s other papers that she and Ghent have done work on impact gardening at Ceres as well as Mercury and our own Moon. The paper on Ceres argues that the phenomenon is orders of magnitude less intense on Ceres than on the Moon, involving a much thinner regolith and leaving surface ice to be affected primarily by sublimation rather than impacts. It seems clear that our work on icy gas giant moons will need to take impact gardening into consideration, just as we monitor the movement of crustal ice.
The paper is Costello et al., “Impact gardening on Europa and repercussions for possible biosignatures,” Nature Astronomy 12 July 2021 (abstract). The paper on Ceres is Costello et al., “Impact Gardening on Ceres,” Geophysical Research Letters 11 April 2021 (abstract).
In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).
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