Centauri Dreams
Imagining and Planning Interstellar Exploration
Large Exomoons Shown to Be Detectable
The search for sub-planetary scale features in other solar systems continues, with encouraging news from the Hunt for Exomoons with Kepler project. A moon around a distant exoplanet is a prize catch, but as we’ve also seen recently, scientists are weighing the possibilities in detecting exoplanetary ring systems (see Searching for Exoplanet Rings). Confirming either would be a major observational step, but exomoons carry the cachet of astrobiology. After all, a large moon around a gas giant in the habitable zone might well be a living world.
David Kipping (Harvard University) and colleagues at HEK have released a new study that tackles the question of how detectable exomoons really are. Published online today by the Astrophysical Journal, the paper examines 41 Kepler Objects of Interest, bringing the total number of KOIs surveyed by HEK thus far up to 57. The paper demonstrates that the process is beginning to move out of the realm of computer simulations and assumption-laden theory to actual data from Kepler. The paper’s goal is to determine how small a moon could be detected in each case given the kind of signatures that flag an exomoon’s presence.
Image: After surveying nearly 60 exoplanets for moons, the HEK team have derived empirical limits for each world, demonstrating an ability to detect even the smallest moons capable of sustaining an Earth-like atmosphere (“Mini-Earths”) for 1 in 4 cases studied. Whilst a confirmed discovery remains elusive, the painstaking survey of 60 planets spanning several years reveals what is possible with current technology. Credit: The Hunt for Exomoons with Kepler (HEK) Project.
An examination of an exoplanet that does not turn up an exomoon thus leads to a statement of how massive a moon has been excluded by the current data, which means the team is learning much about the sensitivity of its methods. From the paper:
… based on empirical sensitivity limits, we show for the first time that the HEK project is sensitive to even the smallest moons capable of being Earthlike for 1 in 4 cases (after accounting for false-positives). In terms of planet-mass ratios, we find even that the Earth-Moon mass-ratio is detectable for 1 in 8 of cases, posing a challenge but not an insurmountable barrier. Mass ratios of ? 10?4, such as that of the Galilean satellites, have never been achieved. However, if Galilean-like satellites reside around lower-mass planets than Jupiter, of order ? 20 M?, then we do find sensitivity, as demonstrated by the limit of 1.7 Ganymede masses achieved for Kepler-10c.
This is encouraging news, for the team can now make statements about the actual mass of a detectable exomoon. In 1 of 3 planets surveyed, an exomoon with Earth’s mass is detectable. Kipping believes that we can move down to the smallest moon thought capable of supporting an Earth-like atmosphere and still detect it in 1 of 4 of the cases studied. No exomoons have yet been detected but we are learning just what our capabilities are. Says Kipping:
“Here we report on our null results and the first estimate of empirical sensitivities. Ultimately, we would like to actually discover a clear signal and are pursuing some interesting candidates we hope will pan out. Either way though, I like to recall what the Nobel Prize winning American physicist Richard Feynman said about science: ‘Nature is there and she’s going to come out the way she is, and therefore when we go to investigate it we shouldn’t pre-decide what it is we’re trying to do except to find out more about it’.”
Image: The Moon has about 1% the mass of the Earth posing a challenge for the HEK team, since such configurations are detectable for 1 in 8 planets surveyed. The much larger Pluto-Charon mass-ratio of 11.6% is much more detectable. Credit: Hunt for Exomoons with Kepler Project.
No exomoons turn up in the 41 KOIs surveyed in the study, with four, KOI-0092.01, KOI-0458.01, KOI-0722.01 and KOI-1808.01, showing up as false positives for an exomoon. Stellar activity is a likely cause, as the paper comments:
When dealing with a handful of transits, quasi-periodic distortions to the transit profile, such as those due to spots… can be well fitted by the flexible exomoon model. However, since an exomoon is not the underlying cause, this model lacks any predictive power and thus should fail F2a [a follow-up test described in the paper]. We therefore suggest that stellar activity is likely responsible for these four instances.
KOI-1808.01, in fact, passes the basic criteria for an exomoon detection, but the paper explains that the observed transit signal is distorted by the effects of star spots. Transit timing variations observed at KOI-0072.01 (Kepler-10c) seem to point to an additional planet in the system rather than an exomoon.
Thirteen of the KOIs produce some kind of spurious detection, assigned by the paper to effects like perturbations from unseen bodies, stellar activity or instrumental artifacts. Through the range of KOIs the project has studied thus far (57), 46 null detections are found from which upper limits on an exomoon’s mass can be derived. The paper reminds us that “…exomoons live in the regime where correlated noise is present and one must employ methods to guard against it when seeking such signals.”
The declared purpose of the Hunt for Exomoons with Kepler project is to ‘determine the occurrence rate of large moons around viable planet hosts,’ a task with implications for the abundance of life in the universe, for if habitable moons are common, there could be more of them than habitable planets, and conceivably more than one orbiting a single planet. An additional benefit of studying exomoons is that they can teach us about solar systems formation by showing us planet/moon systems in a variety of configurations.
The paper is Kipping et al., “The Hunt for Exomoons with Kepler (HEK): V. A Survey of 41 Planetary Candidates for Exomoons,” submitted to the Astrophysical Journal (preprint).
Chariklo & Chiron: Centaurs with Possible Rings
You may be forgiven if you aren’t familiar with the name Chariklo. Discovered in 1997, 10199 Chariklo is a ‘centaur,’ an outer system body with an orbit that moves between the orbits of Saturn and Uranus, just nudging the orbit of the latter. Its odd name (we’re big on names and their derivations here) comes from a nymph who in Greek mythology was the wife of Chiron and daughter of Apollo. No centaur is larger than Chariklo (estimated diameter 250 kilometers), and until just the other day, no other centaur was known to have what Chariklo did: A system of rings.
We’ve just learned, though, that the second largest centaur, 2060 Chiron, may have a set of rings of its own, although there are alternative ways of interpreting the data. Whether Chiron’s rings are confirmed or not, what was once thought to be an unusual phenomenon, a feature of Saturn alone, is now turning out to be far more common, with rings known to orbit Jupiter, Uranus and Neptune as well as Chariklo. So we have full-scale planets with rings of various size and density, and centaurs, which have been found to share the characteristics of both comets and asteroids. There may well be more ring systems out there, as there are estimated to be 44,000 centaurs in the Solar System with a diameter larger than 1 kilometer.
Image: An artist’s impression of the dense and narrow rings around Chariklo. The origin of these rings remains a mystery, but they may be the result of a collision that created a disc of debris. Data from a recent occultation implies that another centaur, Chiron, may also have a ring system, although other explanations are still being examined. Credit: European Southern Observatory.
The discovery of rings around Chariklo and the work on Chiron came about because of stellar occultations as the objects in question passed in front of a bright star, Chariklo in 2013 and Chiron in 2011. In both cases, the occultation produced a useful light signature for researchers studying the brief shadow. A disk of debris circling Chiron is one reading of the data from that centaur’s occultation, but we can’t rule out jets of material from the surface or even a shell of gas and dust enveloping the object. Amanda Bosh (MIT), a co-author of the paper on the discovery in the journal Icarus, calls that an intriguing result because of Chiron’s location, “…part of that middle section of the solar system, between Jupiter and Pluto, where we originally weren’t thinking things would be active, but it’s turning out things are quite active,”
Traces of activity on Chiron actually trace back to the early 1990s, when MIT’s James Elliot studied a similar stellar occultation by the centaur and was able to make estimates of its size. At that time, features in the data also implied the existence of jets of water and dust coming from the centaur’s surface. The new data, drawn from NASA’s Infrared Telescope Facility on Mauna Kea and the Las Cumbres Observatory Global Telescope Network at Haleakala, give us a more precise readings on an event that in its entirety lasted no more than a few minutes.
MIT’s Jessica Ruprecht, lead author of the paper, notes the range of possibilities involved in this work:
“If we want to make a strong case for rings around Chiron, we’ll need observations by multiple observers, distributed over a few hundred kilometers, so that we can map the ring geometry. But that alone doesn’t tell us if the rings are a temporary feature of Chiron, or a more permanent one. There’s a lot of work that needs to be done.”
According to this MIT news release, the two features observed in the data are each about 300 kilometers from the center of the object and are not dissimilar to what Elliot observed in the 1990s. These may be symmetrical jets of gas and dust rather than rings, perhaps the result of the centaur’s having moved inward from the Kuiper Belt, warming enough to turn frozen gases into jets that throw dust and other material off the surface. Ruprecht also notes that debris from a nearby object could conceivably be captured by a centaur like Chiron to produce rings. At the moment, then, we don’t know if we’re looking at a long-lasting feature or a transient event.
Back to Chariklo, whose stellar occultation in 2013 revealed the existence of two rings, one about 3 kilometers and the other about 7 kilometers wide, separated by about 9 kilometers. The find was startling because no previous ring systems around minor bodies had been discovered, and there were questions about how stable a ring system could be around such a small object. The two rings have received nicknames (Olapoque for the larger, Chui for the smaller) derived from the names of rivers in Brazil, but the IAU will at some point confer official names on both.
The Chiron paper is Ruprecht et al., “29 November 2011 stellar occultation by 2060 Chiron: Symmetric jet-like features,” Icarus Vol. 252 (15 May 2015), pp. 271-276 (abstract). For more on Chiron, see Ortiz et al., “Possible ring material around centaur (2060) Chiron,” Astronomy & Astrophysics 576 (2015) A18 (preprint). For the Chariklo work, see Braga-Ribas et al., “A ring system detected around the Centaur (10199) Chariklo,” Nature Vol. 508, Issue 7494 (2014), pp. 72-75 (preprint).
Can We Find Exoplanets Using the Titius-Bode Relation?
The Titius-Bode law has always been a curiosity, one often attributed to little more than happenstance. But recently this numerological curiosity, which predicts that planets in a solar system appear with a certain ratio between their orbital periods, has been the subject of renewed investigation. Francois Graner (Ecole Normale Superieure, Paris) and Berengere Dubrulle (Observatoire Midi Pyrenees, Toulouse) revisited Titius-Bode in the 1990s, asking whether it actually flagged symmetry properties that most solar systems should exhibit.
And now continuing work out of Australian National University and the University of Copenhagen has made predictions using a modified version of the law that can be tested against observation of known exoplanetary systems. So we need to refresh our memory on the formulation, which shows us a relationship that predicts planetary orbits. Take a sequence where each number is double the number that preceded it. Thus 0, 3, 6, 12 and so on. Add 4 to each of these numbers, then divide the result by 10. If you examine our Solar System in terms of astronomical units (AU), the planets follow the sequence in many respects.
Enough so, at least, that the lack of a planet in what we now call the main asteroid belt led Johann Bode to suggest that a planet should appear at 2.8 AU between Mars and Jupiter, where the dwarf planet Ceres was subsequently found. The Titius-Bode formulation, developed in the 18th Century by Johann Titius and later discussed by Johann Elert Bode, had also received a boost in 1781 when the outer system planet it predicted at 19.6 AU was discovered at 19.2 AU (Uranus). Neptune, however, turned out to be 30.1 AU out instead of the Titius-Bode prediction of 38.8, while Pluto was found at 40 AU instead of 77.2.
Can some form of the Titius-Bode formulation still be useful to us in exoplanet work? Steffen Kjær Jacobsen (Niels Bohr Institute, Copenhagen) and fellow researchers Charles Lineweaver and Timothy Bovaird (the latter both at ANU) wondered whether a modified form of Titius-Bode might be of use in predicting exoplanet orbits. Developing work first presented in a 2013 paper on the subject, the authors believe the results can be applied to existing data. Says Jacobsen:
“We decided to use this method to calculate the potential planetary positions in 151 planetary systems, where the Kepler satellite had found between 3 and 6 planets. In 124 of the planetary systems, the Titius-Bode law fit with the position of the planets as good as or better than our own solar system. Using T-B’s law we tried to predict where there could be more planets further out in the planetary systems. But we only made calculations for planets where there is a good chance that you can see them with the Kepler satellite.”
Image: Exoplanetary systems where the previously known planets are marked with blue dots, while the red dots show the planets predicted by the Titius-Bode law on the composition of planetary systems. 124 planetary systems in the survey – based on data from the Kepler satellite, fit with this formula. Credit: Timothy Bovaird et al., 2015.
In their paper in Monthly Notices of the Royal Astronomical Society, the team explains that they took the 27 planetary systems that did not fit the Titius-Bode requirements and added planets where Titius-Bode predicted they would be located. By adding planets between the already known planets, their work predicted a total of 228 planets in the 151 planetary systems. From this the team produced a priority list of 77 planets in 40 systems.
These are planets the researchers suggest should be searched for in the Kepler data, a chance to falsify the Titius-Bode predictions drawing on existing datasets. This work follows up co-authors Lineweaver and Bovaird’s previous investigation of Titius-Bode possibilities in exoplanets, which predicted in 2013 the existence of 141 new exoplanets in 68 systems. The following year Changcheng Huang and Gaspar Bakos performed a search of Kepler data for 97 of the planets thus predicted, resulting in the confirmation of five of them. The current paper tunes up the 2013 paper’s methodology, as discussed within:
In this paper, we perform an improved TB [Titius-Bode] analysis on a larger sample of Kepler multiple-planet systems to make new exoplanet orbital period predictions. We use the expected coplanarity of multiple-planet systems to estimate the most likely inclination of the invariable plane of each system. We then prioritize our original and new TB-based predictions according to their geometric probability of transiting. Comparison of our original predictions with the HB14 [the Huang/Bakos paper] confirmations shows that restricting our predictions to those with a high geometric probability to transit should increase the detection rate by a factor of ?3.
If the Titius-Bode predictions were to hold up, from 1 to 3 habitable-zone planets should exist in each of the systems. The team ran an additional study of the 31 systems out of the 151 studied where planets have been found close to the habitable zone, finding there should be an average of two planets in the habitable zone. If the implications of Titius-Bode are broadly true, then the potential exists for billions of stars with planets in the habitable zone throughout the galaxy, a finding that subsequent analysis of Kepler data and future work may support.
The paper is Bovaird, Lineweaver and Jacobsen, “Using the inclinations of Kepler systems to prioritize new Titius–Bode-based exoplanet predictions,” Monthly Notices of the Royal Astronomical Society Vol. 448, Issue 4, pp. 3608-3627 (abstract). The 2013 paper is Bovaird and Lineweaver, “Exoplanet predictions based on the generalized Titius–Bode relation,” MNRAS Vol. 435, Issue 2, pp. 1126-1138 (abstract). The Huang/Bakos paper is “Testing the Titius–Bode law predictions for Kepler multiplanet systems,” MNRAS Vol. 442, Issue 1, pp. 674-681 (abstract).
The Colors of Extraterrestrial Life
One of these days we’ll have the instruments in place to examine light from a terrestrial-class world around another star. This opens up the possibility of identifying atmospheric gases like oxygen, ozone, carbon dioxide and methane. All of these can occur in an atmosphere in the absence of life, but if we find them existing simultaneously in great enough quantities, we will have detected a possible biosignature, for without life’s activity to replenish them, these gases would recombine and leave us with a much less tantalizing atmospheric mix.
But tackling planetary atmospheres for biosignatures is only one way to proceed. An interdisciplinary team led by Cornell University’s Lisa Kaltenegger and Siddharth Hegde (Max Planck Institute for Astronomy), is examining life detection based on the characteristic tint of lifeforms. An alien organism covering large parts of the planet — think forests, for example, on Earth — would reflect light at particular wavelengths, light that could be measured spectrally.
Image: In this composite satellite image from NASA, you can see a greenish tint in the reflected sunlight, a direct signature of plant life present on Earth’s surface. Similarly, if microbial life with a particular pigmentation covered larges swathes of an exoplanet’s surface, its presence could in principle be measured directly through its tint in reflected starlight viewed through our telescopes. Credit: NASA Earth Observatory.
The challenge, and thus the burden of preliminary work on this concept, is to figure out what spectral signatures different kinds of organism might throw. Working with colleagues at NASA Ames, the researchers have put together a catalog drawn from cultures of 137 different species of microorganisms, seeking a wide range of pigmentation in species occurring in environments as diverse as Chile’s Atacama desert, Hawaiian seawater, old woodwork found in a Missouri state park and hot springs in the Yellowstone National Park. Focusing on extremophiles — life pushed to its limits — allowed the team to investigate the widest possible range for physical and geo-chemical conditions on the surface of exoplanets.
The method, examined in a new paper in Proceedings of the National Academy of Sciences, is to measure the chemical ‘fingerprints’ of each microorganism culture and make the findings available in an online catalog. Reflectance spectra are produced at optical and near-infrared wavelengths and assembled in the first such collection dedicated to surface biosignatures. The catalog was designed to reflect as wide a range of life as possible, understanding that on our own planet, dominant species have undergone huge changes.
From the paper:
Although there is a considerable knowledge base of the spectral properties of land plants, very little information is present in the literature on the reflectance properties of microorganisms. Land plants are widespread on present-day Earth and are easily detected from high-resolution spacecraft observations. However, they occupy only a small niche in the environmental parameter space that brackets known terrestrial life. Additionally, land plants have been widespread on Earth for only about 460 My, whereas much of the history of life has been dominated by single-celled microbial life. Within the prokaryotic and eukaryotic microbes there is a far greater diversity of pigmentation than in land plants. For this reason, any hypotheses about extraterrestrial life based solely on land plants ignore much of the diversity of known life.
Image: Eight of the 137 microorganism samples used to measure biosignatures for the catalog. In each panel, the top is a regular photograph of the sample and the bottom is a micrograph, a 400x zoomed-in version of the top image. The scientists were aiming to achieve diversity in color and pigmentation. Top left to bottom right: Unknown species of genus bacillus (Sonoran desert, AZ, USA); unknown species of genus Arthrobacter (Atacama desert, Chile); Chlorella protothecoides (sap of a wounded white poplar); unknown species of genus Ectothiorhodospira (Big Soda Lake, NV, USA); unknown species of genus Anabaena (with green fluorescent protein; stagnant freshwater); unknown species of genus Phormidium (Kamori Channel, Palau); Porphyridium purpureum (old woodwork at salt spring, Boone’s Lick State Park, MO, USA); Dermocarpa violacea (aquarium outflow, La Jolla, CA, USA). Credit: Hegde et al. / MPIA.
The single-celled microorganisms that have dominated Earth’s history have flourished for 3.5 billion years and probably longer, and have demonstrated again and again that they can be found in the most extreme conditions, from inside nuclear reactors (Chernobyl) to deserts and polar regions. Their particular pigmentation will depend upon local environmental conditions, and thus their future detection by space-based telescopes will tell us something about the environment on the planet they inhabit. The reflectance from surface lifeforms also plays into models for exoplanets that can be used to study chemical processes in their atmospheres.
This news release from the MPIA distills the team’s methods for measuring biosignatures, a task performed by Hegde working with Lynn Rothschild and other researchers from NASA Ames:
Hegde, [Ivan] Paulino-Lima and [Ryan] Kent measured the sample biosignatures at the Center for Spatial Technologies and Remote Sensing (CSTARS) at the University of California, Davis. They used a setup called an integrating sphere, which is hollow and lined on the inside with a reflective coating. The integrating sphere contained a hole for the light source, the microorganism sample, and a detector to measure the fingerprint in the reflected light from the sample. The effect of the sphere shape is as follows: when light shines through the hole and reflects off the sample, it is distributed evenly in all directions. Therefore, the detector can be placed anywhere in the sphere, against any part of the wall, and still measure the same averaged (“integrated”) fingerprint. This is important because for the foreseeable future, telescopes will only be able to measure reflected light from an exoplanet that has been averaged over the whole of the visible part of the planet’s surface.
Lisa Kaltenegger, who heads up Cornell’s Institute for Pale Blue Dots, points to the wide range of possible life including extremophiles that occurs in the database, saying that it “…gives us the first glimpse at what diverse worlds out there could look like… On Earth these are just niche environments, but on other worlds, these life forms might just have the right make to dominate, and now we have a database to know how we could spot that.” The database, which is open for the free use of researchers worldwide, is located at the Institute.
Further additions to the database are expected in the future as more samples become available to catalog microbial reflectance spectra. The paper is Hegde et al., “Surface biosignatures of exo-Earths: Remote detection of extraterrestrial life,” in Proceedings of the National Academy of Sciences, published online before print March 16, 2015 (abstract). The catalog is Surface biosignatures of exo-Earths, now available online.
The Search for ‘Chaotic Earths’
As we get the next generation of space-based telescopes into operation, one of our more significant problems is going to be knowing where to look. After all, once we’ve identified potentially interesting planets for follow-up with spectroscopic analysis of their atmospheres, we’re still faced with the need to focus on the most likely targets. Telescope time is precious, and the ability to rule out planets so as to whittle down our list is a necessary skill to refine.
On that score, Rory Barnes (University of Washington) and colleagues have weighed in with a particular type of planetary configuration we may want to avoid. Barnes is interested in solar systems where gravity plays a significant role in disrupting what might otherwise be a circular orbit in the habitable zone. Some of these effects may be relatively small, but if. over time, we elongate the orbit of an otherwise habitable planet by these small interactions, we can all but eliminate its chances for life.
The particular focus here is mean motion resonances, which occur when two planets periodically reach the same positions in their orbit relative to one another. An example right here in our Solar System is Neptune and Pluto/Charon, where the former goes around the Sun three times for every two orbits of the latter. In cases like these, the orbital periods of the two planets are an integer ratio of each other, and the gravitational tug the resonance causes occurs each time the planets reach the same relative orbital position, an effect that grows with time.
But mean motion resonances (MMRs) are only half the story, for Barnes and team also examine mutual inclination. While the planets in our Solar System have a relatively low inclination (i.e., they are ‘coplanar,’ found on roughly the same plane around our Sun), we’ve found that exoplanets aren’t necessarily configured this way. We have many exoplanet systems with mean mutual resonances, and have found planets with a large inclination to each other. MMRs with inclined orbits have been studied in terms of how they form from the protoplanetary disk and through later gravitational scattering, but we’ll need further work to find out how common planets in mean mutual resonance with mutually inclined orbits really are, and what happens to them.
Image: A “chaotic Earth” could exist in a planetary system in which a neighboring planet has a “year” that is an integer multiple of another planet’s “year,” and if the orbital planes are not aligned. The affected planet’s orbit can become very elongated and can even flip all the way over, so that the two planets are revolving in opposite ways. These planets would have unpredictable climates, perhaps becoming inhospitable for millions of years at a time. Here, the potentially habitable planet is perturbed by a Neptune-mass planet on a three-year orbit and has an elongated orbit, which would make it relatively hot. As such it is mostly dry, but some seas remain, including one which contains the stellar glint, a feature astronomers will look for as it reveals the presence of surface liquids. Credit: Rory Barnes.
In the new paper, Barnes discusses computer simulations his team performed to simulate planetary systems with resonance and mutually inclined orbits. The results are not kind to the prospects for astrobiology, for dramatic orbital fluctuations can occur, including planets being pushed into near-collisions with the host star. The researchers demonstrate the characteristics of planets in inclined MMRs and also discuss four worlds known to be in MMRs but whose inclination is not known: HD 128311 and HD 73426 (2:1 resonance), HD 60532 (3:1) and HD 45364 (3:2). Inclined MMR systems are found to evolve chaotically for at least 10 billion years.
The authors liken what happens to planets in this configuration to a pendulum:
We have simulated the orbital evolution of exoplanets in mean motion resonances and inclinations and found the orbits can evolve chaotically for at least 10 Gyr. We hypothesize that these systems behave like compound pendula, which are naturally chaotic systems that can switch between modes of oscillation, as seen in our simulations… We find this chaotic motion over a range of mass ratios and for the 2:1, 3:2 and 3:1 resonance. We also tested different N-body codes using different integration schemes, and conclude the results are robust. Inclined MMRs can be produced by planet-planet scattering and the resultant systems are qualitatively similar to our simulations of known systems in an MMR.
We learn from these simulations that inclined MMR systems can produce planets in short-period orbits misaligned with the spin axis of the host star. We also learn that planets in mean mutual resonance may episodically go through periods of migration inward toward the star. The argument is that if we are looking for potentially habitable planets, we should consider that systems with MMRs and mutual inclination may be in a state of chaotic evolution where habitable conditions on a particular world will not be maintained long enough for life to emerge.
The results were something of a surprise for the researchers, as Barnes notes in this UWA news release:
“What happens when you have planets that are in this resonance and with mutual inclinations? … [W]hat we found was that things go all haywire. Those little perturbations that keep happening at the same point cause one of the orbits to do some crazy things — even flip over entirely — and then kind of come back to where it was before. It was pretty unexpected for us.”
These potential effects are all shown through simulation, as we have no examples of planets that are known to be in both mean mutual resonance as well as mutually inclined orbits. The authors believe the best way to detect such systems is through astrometric measurements, since identification through transits or radial velocity methods is difficult. The paper suggests that the European Space Agency’s Gaia instrument is the most likely to discover inclined MMRs through astrometry, offering us an observational test of these theories on chaotic orbital evolution.
The paper is Barnes et al., “Long-lived Chaotic Orbital Evolution of Exoplanets in Mean Motion Resonances with Mutual Inclinations,” accepted for publication in The Astrophysical Journal (preprint).
Evidence Mounts for Ganymede’s Ocean
Yesterday’s discussion of hydrothermal activity inside Saturn’s moon Enceladus reminds us how much we can learn about what is inside an object by studying what is outside it. In Enceladus’ case, Cassini’s detection of tiny rock particles rich in silicon as the spacecraft arrived in the Saturnian system led to an investigation of how these grains were being produced inside Enceladus through interactions between water and minerals. If correctly interpreted, these data point to the first active hydrothermal system ever found beyond Earth.
Now Ganymede swings into the spotlight, with work that is just as interesting. Joachim Saur and colleagues at the University of Cologne drew their data not from a spacecraft on the scene but from the Hubble Space Telescope, using Ganymede’s own auroral activity as the investigative tool. Their work gives much greater credence to something that has been suspected since the 1970s: An ocean deep within the frozen crust of the moon.
Image: NASA’s Hubble Space Telescope observed a pair of auroral belts encircling the Jovian moon Ganymede. The belts were observed in ultraviolet light by the Space Telescope Imaging Spectrograph and are colored blue in this illustration. They are overlaid on a visible-light image of Ganymede taken by NASA’s Galileo orbiter. The locations of the glowing aurorae are determined by the moon’s magnetic field, and therefore provide a probe of the moon’s interior, where the magnetic field is generated. The amount of rocking of the magnetic field, caused by its interaction with Jupiter’s own immense magnetosphere, provides evidence that the moon has a subsurface ocean of saline water. Credit: NASA, ESA, and J. Saur (University of Cologne, Germany). Ganymede Globe Credit: NASA, JPL, and the Galileo Project
The early work on a Ganymede ocean grew out of computer models of the interior, but the Galileo spacecraft was able to measure the moon’s magnetic field in 2002, offering enough evidence for an ocean to keep the idea in play. The problem was that the Galileo measurements were too brief to produce an overview of the field’s long-term cyclical activity.
It was Saur’s idea to look at the idea afresh. Given that Ganymede is deeply embedded in Jupiter’s magnetic field, the aurorae that are produced in its polar regions are going to be influenced by any changes to that field, changes that produce a ‘rocking’ movement in the aurorae. These movements, Saur reasoned, would be a useful marker, one that, like the silica grains near Enceladus, could tell a story about activity deep below the surface. Says Saur:
“I was always brainstorming how we could use a telescope in other ways. Is there a way you could use a telescope to look inside a planetary body? Then I thought, the aurorae! Because aurorae are controlled by the magnetic field, if you observe the aurorae in an appropriate way, you learn something about the magnetic field. If you know the magnetic field, then you know something about the moon’s interior.”
The ‘rocking’ of the aurorae on Ganymede depends upon what’s inside the moon, and by the researchers’ calculations, a saltwater ocean would create a secondary magnetic field that would act against Jupiter’s field, tamping down the motion of the aurorae. The Hubble data show us that this is happening, for Saur’s models indicate the auroral activity is reduced to 2 degrees as opposed to the 6 we would expect if an ocean were not present. Ganymede thus joins Europa and Enceladus as an outer planet moon with increasing evidence for an ocean.
Does the likelihood of an ocean now mean we’ll shift more resources toward Ganymede as a possible venue for life? Remember that the European Space Agency’s JUICE mission (Jupiter Icy Moons Explorer) is still on track for a possible 2022 launch. Current planning calls for flybys of Callisto and Europa followed by an extended period of orbital operations around Ganymede. The mission would reach Jupiter in 2030, if these plans come to fruition.
For all its interest, though, Ganymede’s ocean seems less accessible than Europa’s, as it’s evidently sheathed in a crust of rock and ice that is 150 kilometers thick. Beneath that crust is an ocean scientists believe to be as much as 100 kilometers deep. Ganymede is a world that may well hold more water than all the water on the surface of our planet. But even if we could get to it, it’s also an ocean still thought to be trapped between two layers of ice, meaning the interesting interactions with the rocky core (as at Enceladus) would not be occurring.
Image: This is an illustration of the interior of Jupiter’s largest moon, Ganymede. It is based on theoretical models, in-situ observations by NASA’s Galileo orbiter, and Hubble Space Telescope observations of the moon’s aurorae, which allows for a probe of the moon’s interior. The cake-layering of the moon shows that ices and a saline ocean dominate the outer layers. A denser rock mantle lies deeper in the moon, and finally an iron core beneath that. Credit: NASA, ESA, and A. Feild (STScI).
What excites us about Enceladus is the prospect that hydrothermal vents at the ocean floor could be producing an environment with enough sources of energy and nutrients to make life possible. We know that the dark seafloor vents on our own planet are now considered a serious candidate for the place where life originated. If our current models are correct, Ganymede would lack the ability to develop this kind of ecosystem, but we still have much to learn about all these icy moon environments and the oceans they apparently conceal.
And as a draft of the paper on this work notes, whether Ganymede is life-bearing or not is of less consequence than the method employed here, which has implications not only within our own Solar System but elsewhere:
The method introduced here can also be applied to other planetary moons and planets to study their electrical conductivity structure. If these bodies are exposed to time-variable magnetic fields and exhibit auroral emissions, then their auroral patterns will be modified by any electrically conductive layers. Observations of the auroral emission responses combined with appropriate models for the responses will provide valuable information about these conductive layers, such as subsurface oceans. The method might one day even be applicable to exoplanets (and exomoons), once appropriate objects and associated magnetic fields are observationally confirmed.
The paper is Saur et al., “The Search for a Subsurface Ocean in Ganymede with Hubble Space Telescope Observations of its Auroral Ovals,” accepted at the Journal of Geophysical Research. Full publication information and links as soon as I have them. Meanwhile, this news release is helpful.
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