Centauri Dreams
Imagining and Planning Interstellar Exploration
Liquid Methane in Titan’s Canyons
It was in 2012 that Cassini data showed us the presence of the river system now called Vid Flumina, which empties into Titan’s Ligeia Mare after a journey of more than 400 kilometers. Given surface temperatures on this largest of Saturn’s moons, researchers assumed liquid methane would be the key player here. The question was whether the river — and the eight canyons that branched off from it along its course — were still filled with liquid or long dry.
Now we have the answer, thanks to new work from Valerio Poggiali (La Sapienza University, Rome) and colleagues. Using radar signals bounced off Titan’s surface in May of 2013, the researchers probed the deep gorges near Titan’s north pole and were able to distinguish rocky material from smooth liquid. We’re clearly looking at a surface that is actively eroding, one with striking comparisons to the landscapes of Utah and Arizona as well as the Nile River gorge.
Key to the work here is the use of Cassini’s radar as an altimeter, measuring the height of features on the surface. Poggiali and team were able to use the altimetry data in combination with previous radar imagery of the area to analyze the Vid Flamina channels. Radar returns from the channels are highly reflective, producing a telltale glint, and the radar backscatter in relation to nearby terrain implicates smooth surfaces. From the paper:
…we interpret these smoothness constraints as requiring liquid surfaces. This represents the first direct detection of liquid-filled channels on Titan. Furthermore, channels exhibit canyon-like morphology, with the liquid surface elevations of the higher-order tributaries of the Vid Flumina network…occurring at the same elevation as Ligeia Mare. We also find lower order tributaries with liquid surface elevations above the level of Ligeia Mare, consistent with elevated tributary networks feeding into the main channel system.
The canyons branching off from Vid Flumina are less than a kilometer wide, with walls as high as 570 meters. They were likely carved by the liquid methane as it drained into Vid Flumina, a process we see on Earth in the shaping of river gorges. These are steep walls, rising as sharply as 40 degrees. What remains unknown is the age of the processes at work here, and the depth of the liquid methane. What we can assume is that the presence of liquid in these canyons reflects a process of canyon formation that is ongoing on this active geological surface.
Image: Liquid methane and ethane flowing through Vid Flumina, a 400-kilometer long river often compared to Earth’s Nile River, is fed by canyon channels running hundreds of meters deep. Credit: NASA, JPL-Caltech, Agenzia Spaziale Italiana.
Such steep clefts in Titan’s landscape could be the result of several processes including terrain uplift and changes in sea level. This JPL news release draws comparisons with the Grand Canyon, where rising terrain caused the Colorado River to cut deeply into the landscape below over a timespan of several million years. But canyons formed from changes in water level are also found on Earth, as is evident at Lake Powell, a reservoir that straddles the border between Arizona and Utah. Here, the Colorado’s rate of erosion increases when the water level in the reservoir drops. On Titan, both process may be in play.
“It’s likely that a combination of these forces contributed to the formation of the deep canyons,” says Poggiali, “but at present it’s not clear to what degree each was involved. What is clear is that any description of Titan’s geological evolution needs to be able to explain how the canyons got there.”
Image: NASA’s Cassini spacecraft pinged the surface of Titan with microwaves, finding that some channels are deep, steep-sided canyons filled with liquid hydrocarbons. One such feature is Vid Flumina, the branching network of narrow lines in the upper-left quadrant of the image. Credit: NASA/JPL-Caltech/ASI.
We have other channels to study on Titan, and many may be hidden below the resolution of the Cassini instruments. But bear in mind as well that when the Cassini mission ends on September 15 of next year, we will have used its radar in imaging mode to cover a total area of 67 percent of the surface. That’s a triumph for the mission, but also a reminder of how much we leave unseen. No matter how successful the mission, it always points to what needs study next.
The paper is Poggiali et al., “Liquid-filled canyons on Titan,” published online by Geophysical Research Letters 9 August 2016 (abstract).
Stromatolites: Astrobiological Implications
Oil companies involved in astrobiology? It doesn’t seem likely, but in a roundabout way, it’s true. A consortium including Chevron, Repsol, BP and Shell have a natural interest in developing better models for subsurface reservoirs and source rocks in microbe-rich carbonate environments. At the same time, NASA’s Astrobiology Program is intrigued with how we could find bacterial structures on other worlds, and their role in planetary habitability.
The result: Both Big Oil and NASA are supporting research into stromatolites, the calcium-carbonate rock structures built up by lime-secreting bacteria (technically, cyanobacteria, that draw their energy from photosynthesis). We can probe ancient life on Earth by studying these accreted structures, some of which go back more than 3.5 billion years.
Erica Suosaari works for Bush Heritage Australia, an organization involved in conservation and land management. The Hamelin Station Reserve in Western Australia borders a nature reserve with vast quantities of marine stromatolites. Suosaari’s work in the area, funded by the above sources, is encapsulated in a paper in Scientific Reports (citation below), as noted in an article in Astrobiology Magazine, from which this:
“Looking for evidence of life in the rocks is like finding a needle in the haystack,” wrote Suosaari in an e-mail. “If stromatolites have definitive bio-signatures — such as self organized morphologies that are indicative of life processes — then it may be possible to look for that ‘signature’ in rocks on the surface of other planets and significantly reduce the size of that haystack.”
The Hamelin Pool Marine Nature Reserve offers stromatolites in extraordinary abundance, providing the opportunity to study analogs to the earliest such formations on Earth. Suosaari and team have discovered that modern stromatolites in their sample have created structures similar to stromatolites that emerged billions of years ago. The cyanobacteria that formed ancient stromatolites are believed to be the first organisms to use photosynthesis, with the significant side-effect of producing the oxygen so useful in the development of complex life.
Image: Modern stromatolites in Shark Bay, Western Australia. Credit: Paul Harrison (Reading, UK) using a Sony CyberShot DSC-H1 digital camera., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=714512
Is Hamelin Pool, then, a window into the early Earth? The processes at work here involve microbes that draw from the same lineage, with the same oxygen-producing result. What’s intriguing for research in the near-term is that it may be possible to transplant microbial communities like these to other places. Let me quote from the conclusion of the Astrobiology Magazine article:
Suosaari said she thought of stromatolites when reading about SpaceX founder Elon Musk’s plans to bring life to the planet Mars. She suggested that because these stromatolite-building microbial communities produce oxygen, they could potentially make the Red Planet more life-friendly.
“Obviously with Elon Musk’s plans, we don’t have billions of years to shape the atmosphere if he is planning to move life there in the coming years, and Mars has less than 1 percent of the atmosphere of Earth,” she acknowledged. “But I begin to think about photosynthesizing microbial mats and how they have prevailed for billions of years; it’s a kind of resilience and longevity that our species hasn’t yet achieved. Perhaps we should look to these microbial communities to generate oxygen on the Red Planet at a small scale.”
An intriguing thought, as is the idea that fossil evidence of life on far more distant exoplanets may eventually tell our probes something about how life took hold there. In the case of Earth-based stromatolites, we see microbial mats that colonize the surface of the structure, mats that vary according to where the stromatolite is situated in the tidal zone. Consider the stromatolite as a record of previous surface mats, while the shapes of the various kinds of stromatolite relate to differing conditions in the broader pool.
This is an evidently global record, if we can learn to read it. Note this from the paper:
New insights regarding stromatolite growth in Hamelin Pool present opportunities for comparative sedimentological research advancing understanding of early Earth. The diversity of morphologies in the eight Stromatolite Provinces… provides a unique opportunity for investigating environmental and/or biological processes determining stromatolite morphology…. Of particular note, previously unreported, elongate nested subtidal structures of Spaven Province are remarkably similar to 1.9 billion year old longitudinal stromatolites at Great Slave Lake, Northwest Territories.
Could we find structures like these on distant planets? I’m reminded of the enigmatic ‘stolid, dark tower images’ returned to Earth by AXIS, a probe sent by a future Earth to Alpha Centauri B in Greg Bear’s Queen of Angels (1990). The AXIS data convince some that an intelligent life-form must have created the towers, but later the view shifts:
“The only news we have from AXIS may or may not be significant. A recently received analysis shows that at least three of the circular tower formations discovered by AXIS on Alpha Centauri B-2 are made up of mixes of minerals and organic materials, the minerals being calcium carbonate and aluminum and barium silicates, and the organic materials being amorphous carbohydrate polymers similar to cellulose found in terrestrial plant tissue. AXIS has told its Earth-based maters that, in its opinion, the towers may not be artificial structures…”
With knowledge of life’s development limited to our own planet, we can’t yet know what kind of structures are implicated in the development of life elsewhere, allowing habitable conditions to gradually develop. But we may eventually find structures just as rich in their own way as the stromatolites at the Hamelin Pool Marine Nature Reserve, and as enigmatic as the formations the orbiting AXIS probe investigates from its perch high above a habitable Centauri planet.
The paper is Suosaari et al., “New multi-scale perspectives on the stromatolites of Shark Bay, Western Australia,” published online by Scientific Reports 3 February 2016 (full text).
Atmospheric Collapse on Io
I suspect most scientists would like to have a moment like the one Stanton Peale, Patrick Cassen and Ray Reynolds experienced when Voyager flew past Io in 1979. How many of us get to see a major idea vindicated in such short order? It was on March 5 that Voyager 1 passed within 22,000 kilometers of Io. A scant three days before, Peale, Cassen and Reynolds had published their prediction that tidal forces should keep the moon’s interior roiling, resulting in volcanic activity. Linda Morabito, on the Voyager navigation team, analyzed Voyager imagery shortly thereafter to discover that volcanoes were indeed active on the surface.
What a triumph for the power of thorough analysis and prediction. Voyager, in fact, found nine plumes on Io, while demonstrating that its surface was dominated by sulfur and sulfur dioxide frost, with extensive lava fields extending for hundred of kilometers. The moon was also observed to have a thin atmosphere consisting primarily of sulfur dioxide.
That thin envelope is now the subject of a new paper from Constantine Tsang (SwRI) and colleagues. We learn that gas emitted from Io’s volcanoes freezes onto the surface when Jupiter moves between Io and the Sun. Sublimation re-creates the atmosphere when sunlight returns.
Image: An artist’s rendering depicts the atmosphere on Io, Jupiter’s volcanic moon, as it collapses during daily eclipses. Credit: Southwest Research Institute.
This was the first time that atmospheric collapse and re-building on Io had been observed, a tricky procedure indeed. The scientists used the Gemini North telescope in Hawaii with the Texas Echelon Cross Echelle Spectrograph (TEXES). The latter is a high resolution mid-infrared spectrometer that was able to detect the heat signature of the atmospheric collapse.
Temperatures on Io’s surface drop from -145 degrees Celsius to -170 degrees C when a full eclipse occurs, followed by atmospheric collapse as the sulfur dioxide settles onto the surface. From the paper:
These observations provide the first direct evidence that Io’s primary molecular SO2 atmosphere collapses in eclipse, with SO2 condensing on the surface as SO2 frost. Our observations also allow us to constrain the spatial distribution of Io’s atmosphere. The fact that these observations show SO2 band depths decreasing until they almost disappear implies the atmosphere, at least on the hemisphere centered around 340° that is observed during eclipses, is global and not centered over volcanic hotspots; i.e., localized SO2 over active volcanoes is a minor component of the atmosphere on this hemisphere.
What we see is a moon whose tenuous atmosphere is in a continual cycle of collapse and restoration. It’s known that the volcanoes of Io are the source of the sulfur dioxide in the atmosphere, but we now have a demonstration of the power of sunlight to control atmospheric pressure as it changes the temperature of surface ice, resulting in the sublimation that rebuilds the atmosphere after an eclipse. This is a daily phenomenon on Io, where the day is 1.7 Earth days long, and an eclipse by Jupiter occurs for two hours out of every day.
Image (click to enlarge): Atmospheric collapse on Io in a series of images. Credit: Southwest Research Institute.
These findings are still a work in progress, for recent observations by the Hubble instrument have contradicted them by suggesting that Io’s atmosphere does not respond quickly to sunlight as the moon comes out of eclipse. There may, in other words, be longitudinal variations here. Where the atmosphere is thickest, emission from the volcanoes may mask the sublimation seen in the Gemini North data. It will take further observations to resolve the matter.
The paper is Tsang et al., “The Collapse of Io’s Primary Atmosphere in Jupiter Eclipse,” published online by the Journal of Geophysical Research 2 August 2016 (full text).
SETI, Astrobiology and Red Dwarfs
If you’ve been following the KIC 8462852 story, you’ll want to be aware of Paul Carr’s Dream of the Open Channel blog, as well as his Wow! Signal Podcast, both of which make for absorbing conversation. In his latest blog post, Carr offers sensible advice about how to look at anomalies in our astronomical data. Dysonian SETI tries to spot such anomalies in hopes of uncovering the activities of an extraterrestrial civilization, but as Carr makes clear, this is an enterprise that needs to be slowly and patiently done, without jumping to any unwarranted assumptions.
Let me quote Carr on this important point:
…we will have to be patient, since we will be almost certainly be wrong at first, or perhaps just unlucky in our search. We don’t need to nail it exactly, but we will need to develop rough models of ET activity that distinguishes it from nature. These models would more or less fit the data that we think anomalous, would make testable predictions, and would show how to rule out at least known natural phenomena. Such a family of models may be available next year, or it may be in 100 years, but the more anomalous data we have, the more the models can be constrained.
This paragraph gets it right, taking it as a given that we have no idea whether there are extraterrestrial civilizations or, for that matter, life of any kind around other stars. We certainly have no idea how widespread either form of life might be, and in the case of Dysonian SETI, we would be looking at technologies so far in advance of ours that recognizing them for what they are (or might be) creates myriad challenges. So while we try to distinguish natural phenomena from the possibility of intelligent activity, we need to keep these profound limitations in mind.
Tabby’s Star, then, is a wonderful case in point, certainly a motivator for this kind of research (and, as we’ve seen, one capable of being sustained at least modestly by public funding), but we should also consider it in a broader perspective. The goal will be to build a catalog of unusual phenomena that can be consulted as we begin to differentiate among such targets. We may discover that all of these can be accounted for by natural processes, and if so, then we have learned something valuable about the universe. No small accomplishment, that.
Red Dwarfs and Astrobiology
Looking beyond SETI to more fundamental questions of astrobiology, we find ourselves in that unsettling period when we have instruments in the pipeline that can tell us much about the exoplanets we observe, but we’re not yet receiving the data that can make a definitive call on the existence of life elsewhere. Astrobiology will accumulate data at increasingly fine levels of detail as we move from missions like Kepler to searches around closer stars. Meanwhile, we have to tune up our models for detecting biosignatures as we wait for the technology to test them.
Here the Transiting Exoplanet Survey Satellite (TESS) comes to mind, as does PLATO (PLAnetary Transits and Oscillations of stars), and of course the James Webb Space Telescope. TESS is due for a 2017 launch, JWST for 2018 and PLATO for 2024. WFIRST (Wide Field Infrared Survey Telescope), scheduled for the mid-2020s, is likewise going to provide key exoplanet observations, and let’s not neglect the small photometric platform CHEOPS (CHaracterising ExOPlanet Satellite), which will sharpen the target lists of future ground-based observatories. We need to continue refining our answers to this question: What does life do to a planet that offers a key observable, and what are the best instruments to detect it.
Red dwarfs make excellent targets if we’re studying a planetary atmosphere to learn whether or not there are biomarkers there, and now we have a new paper from Avi Loeb (Harvard-Smithsonian Center for Astrophysics) that asks whether such stars may ultimately become home to the vast majority of cosmic civilizations. Working with Rafael Batista and David Sloan (both at Oxford University), Loeb acknowledges the obvious: We don’t know if stars like these can support life, and the authors call for building the datasets to find out. But if they can, then the implications are that most life in deep space will eventually be around such stars.
I say ‘eventually’ because M-dwarfs have lifetimes measured in the trillions of years, much greater than the 10 billion years or so that G-class dwarfs like our Sun can expect. And of course, around our own star life gets problematic within about a billion years. We have a planet that cannot be expected to remain habitable all the way to the last days of the Sun.
If life can form on planets around red dwarfs, then the probability of life grows much higher as we go further and further into the future, for these small stars are the most common kind of star in the galaxy, comprising as much as 80 percent of the stellar population. That would mean we are early to the dance, and a densely populated galaxy has simply not had time to develop. Loeb’s paper calculates the relative formation probability per unit time of habitable Earth-like planets within a fixed comoving volume of the Universe and finds red dwarfs favored:
“If you ask, ‘When is life most likely to emerge?’ you might naively say, ‘Now,'” says Loeb. “But we find that the chance of life grows much higher in the distant future.”
Image: This artist’s conception shows a red dwarf star orbited by a pair of habitable planets. Because red dwarf stars live so long, the probability of cosmic life grows over time. As a result, Earthly life might be considered “premature.” Credit: Christine Pulliam (CfA).
Hence the importance of a biosignature detection. If we find such markers in the atmosphere of a red dwarf, we have learned something not only about that particular star, but about the prospect of life in later cosmic eras up to the ten trillion year lifetime of the average red dwarf. The universe we see has had 13.7 billion years to produce life, but we can only imagine what kinds of life might emerge in the future. As for the probability of our own emergence, let me quote from the paper:
One can certainly contend that our result presumes our existence, and we therefore have to exist at some time. Although our result puts the probability of finding ourselves at the current cosmic time within the 0.1% level, rare events do happen. In this context, we reiterate that our results are an order of magnitude estimate based on the most conservative set of assumptions within the standard ?CDM model.
Conservative indeed, and if we tweak the assumptions, it gets more extreme:
If one were to take into account more refined models of the beginning of life and observers, this would likely push the peak even farther into the future, and make our current time less probable. As an example, one could consider that the beginning of life on a planet would not happen immediately after the planet becomes ‘habitable’. Since we do not know the circumstances that led to life on Earth, it would be more realistic to assume that some random event must have occurred to initiate life, corresponding to a Poisson process [in probability theory, used to model random points in time and space]. This would suppress early emergence and thus shift the peak probability to the future.
Are we truly premature, or are we simply going to learn that life is not possible around stars in an M-dwarf habitable zone? We’ve considered all the possibilities many times in these pages. Tidally locked to its star, a planet like this would experience constant day on one side, constant night on the other, with ramifications for climate and habitability that remain controversial. Extreme radiation from solar flares in young M-dwarfs may scour the surface of life (or, on the other hand, act as an evolutionary spur). And such planets may be home to volcanic activity that can lead to runaway greenhouse effects (see A Mini-Neptune Transformation?).
In other words, life’s chances around G-class stars may be profoundly greater than around M-dwarfs, in which case the chance of life emerging does not increase as we move into the distant future. For these reasons, using our upcoming space missions to search for life around small red stars can help us place ourselves in the cosmic hierarchy. We need to learn what conditions a planet in the habitable zone of an M-dwarf can support, and the discovery of biosignatures there would cause us to re-evaluate our thoughts on ‘average’ life and its existence around Sun-like stars.
The paper is Loeb, Batista and Sloan, “Relative Likelihood for Life as a Function of Cosmic Time,” accepted for publication in Journal of Cosmology and Astroparticle Physics (preprint). A CfA news release is also available. Ben Guarino writes up Loeb’s findings in a helpful essay for the Washington Post.
KIC 8462852: Fading in the Kepler Data
Those of you who have been following the controversy over the dimming of KIC 8462852 (Tabby’s Star) may remember an interesting note at the end of Bradley Schaefer’s last post on Centauri Dreams. Schaefer (Louisiana State University) had gone through his reasoning for finding a long-term dimming of the star in the DASCH (Digital Access to a Sky Century@Harvard) database. His third point about the star had to do with the work of Ben Montet (Caltech) and Joshua Simon (Carnegie Observatories).
Montet and Simon’s work relied on an interesting premise. Tabby’s Star had been discovered because it was in the Kepler field, and thus we had high-quality data on its behavior, the unusual light curves that the Planet Hunters team brought to the attention of Tabetha Boyajian. As the researchers note in a new paper, Kepler found ten significant dips in the light curve over the timespan of the Kepler mission, dips that were not only aperiodic but irregular in shape, and that varied enormously, from fractions of one percent up to 20% of the total flux of KIC 8462852.
Image: Montage of flux time series for KIC 8462852 showing different portions of the 4-year Kepler observations with different vertical scalings. Panel ‘(c)’ is a blowup of the dip near day 793, (D800). The remaining three panels, ‘(d)’, ‘(e)’, and ‘(f)’, explore the dips which occur during the 90-day interval from day 1490 to day 1580 (D1500). Credit: Boyajian et al., 2015.
Schaefer noted in his Centauri Dreams post (see Further Thoughts on the Dimming of KIC 8462852) that if Tabby’s Star were actually fading at a rate of 0.164 mag/cen, then it should have undergone fading during the period it was under observation by Kepler (in fact, it should have faded by 0.0073 mag over the Kepler lifetime on the main Cygnus field). Montet and Simon have now presented us with their analysis in a paper just up on the arXiv server.
A fading of the kind Schaefer described would be well above the photometric precision of the Kepler instrument. Montet and Simon realized they could search for long-term trends by using the full-frame images (FFI) collected during the Kepler mission. Eight of these were recorded at the beginning of the mission, with another FFI recorded each month throughout the mission. Given that the mission lasted four years, a star dimming at the rate Schaefer suggests should decrease in brightness by 0.6% over the Kepler baseline. And as the authors point out, using FFI data avoids the removal of the dimming trend by the data processing pipeline.
The results: The study, which worked with KIC 8462852 and seven nearby comparison stars, found that in the first three years of the Kepler mission, Tabby’s Star dimmed at a rate of 0.341%±0.041% per year. Over the next six months, it decreased in brightness by 2.5%, and then stayed at that level during the duration of the primary Kepler mission. The paper continues:
We then compare this result to a similar analysis of other stars of similar brightness on the same detector, as well as stars with similar stellar properties, as listed in the KIC, in the Kepler field. We find that 0.5% of stars on the same detector and 0.7% of stars with similar stellar properties exhibit a long-term trend consistent with that observed for KIC 8462852 during the first three years of the Kepler mission. However, in no cases do we observe a flux decrement as extreme as the 2.5% dip observed in Quarters 12-14 of the mission. The total brightness change of KIC 8462852 is also larger than that of any other star we have identified in the Kepler images.
Image: Photometry of KIC 8462852 as measured from the FFI data. The four colors and shapes (green squares, black circles, red diamonds, and blue triangles) represent measurements from the four separate channels the starlight reaches as the telescope rolls. The four subpanels show flux from each particular detector individually. The main figure combines all observations together; we apply three linear offsets to the data from different channels to minimize the scatter to a linear fit to the first 1100 days of data. In all four channels, the photometry is consistent with a linear decrease in flux for the first three years of the mission, followed by a rapid decrease in flux of ≈ 2.5% over the next six months. The light gray curve represents one possible Kepler long cadence light curve consistent with the FFI photometry created by fitting a spline to the FFI photometry as described in Section 4. The large dips observed by Boyajian et al. (2016) are visible but narrow relative to the cadence of FFI observations. The long cadence data behind this figure are available online. Credit: Montet & Simon.
M. A. Thompson (University of Hertfordshire) and colleagues published a recent study in Monthly Notices of the Royal Astronomical Society reporting their findings using millimetre and sub-millimetre photometry. The paper finds that a dust cloud orbiting Tabby’s Star would have to be no larger than 7.7 Earth masses of material within a radius of 200 AU, adding “Such low limits for the inner system make the catastrophic planetary disruption hypothesis unlikely.”
Montet and Simon don’t necessarily agree, but in any case there are other problems. The authors think the light curve is “…consistent with the transit of a cloud of optically thick material orbiting the star,” and that such a cloud could be small enough to meet Thompson and team’s requirements. The breakup of a small body or a recent collision producing a large dust cloud could also produce a cometary family that transited the host star as a single group. But we’re still not out of the woods:
To explain the transit ingress timescale, the cloud would need to be at impossibly large distances from the star or be slowly increasing in surface density. The flat bottom of the transit would then suggest a rapid transition into a region of uniform density in the cloud, which then continues to transit the star for at least the next year of the Kepler mission. Moreover, such a model does not naturally account for the long-term dimming in the light curve observed in both DASCH and the Kepler FFI data, suggesting that this idea is, at best, incomplete.
A deeply mysterious star, our KIC 846285. Montet and Simon call for alternative hypotheses and new data to help us explain existing observations, and we can be glad to have Tabetha Boyajian’s team on the case thanks to the success of the recent Kickstarter campaign. Observations are already in progress at the Las Cumbres Observatory Global Telescope Network, and the Kickstarter funds will take us deep into 2017. For more on the Las Cumbres work, see Corey Powell’s recent interview with Boyajian for Discover Magazine, from which this:
From our new observations, we’ll be able to tell a lot about the material that’s passing in front of the star: if it’s some kind of dusty thing, some kind of solid thing. [Boyajian’s working hypothesis is that the dimming is caused by a huge swarm of comets, set loose perhaps by some cataclysmic event around the star.] What’s also important is that we will also get a baseline of spectral observations so we can look at if there’s any radial velocity shift or if there’s any variable emission of the lines, things we’d expect comets to have.
The paper is Montet and Simon, “KIC 8462852 Faded Throughout the Kepler Mission,” submitted to the AAS Journals and available as a preprint. The Thompson paper on circumstellar dust in this system is “Constraints on the circumstellar dust around KIC 8462852,” published online by Monthly Notices of the Royal Astronomical Society 25 February 2016.
Antimatter Acquisition: Harvesting in Space
Talking about antimatter, as we’ve done in the past two posts, leads to the question of why the stuff is so hard to find. When we make it on Earth, we do so by smashing protons into targets inside particle accelerators of the kind found at the Fermi National Accelerator Laboratory in Batavia, IL and CERN (the European Organization for Nuclear Research). It’s not exactly an efficient process from the antimatter production standpoint, as it produces a zoo of particles, anti-particles, x-rays and gamma rays, but it does give us enough antimatter to study.
But there is another way to find antimatter, for it occurs naturally in the outer Solar System and even closer to home. James Bickford (Draper Laboratory, Cambridge MA) has looked at how we might trap antimatter that occurs in the Earth’s radiation belts. In a report for NIAC back in 2006 (available here), Bickford laid out a strategy for using high temperature superconductors to form two pairs of RF coils with a radius of 100 meters, to be powered by nuclear or solar power. The idea is that the magnetic field created through the RF coils will concentrate and trap the incoming antiproton stream.
Now the model changes from production on Earth to harvesting natural antimatter in space. We get antimatter in the Solar System because high-energy galactic cosmic rays (GCR) bombard the upper atmosphere of the planets, causing ‘pair production,’ which is the creation of an elementary particle and its antiparticle. The kinetic energy of the cosmic ray particle is converted into mass when it collides with another particle. According to Bickford’s calculations, about a kilogram of antiprotons enter our Solar System every second, and any planet with a strong magnetic field is fair game for collection.
As the planet’s magnetic field holds the antimatter particles, they spiral along the magnetic field lines. This is a process that continually replenishes itself both for matter and antimatter. Jupiter is a source, but Saturn is even better, for a larger flux enters its atmosphere. Saturn is, in fact, the place where the largest total supply of antiprotons appears, with reactions in its rings injecting 250 micrograms per year into the planet’s magnetosphere. But we can start with the Earth, for the antimatter production process was confirmed here in 2011.
These results came from the PAMELA (Payload for Antimatter/Matter Exploration and Light-nuclei Astrophysics) satellite, a joint mission among scientists from Italy, Germany, Russia, and Sweden (see Antimatter Source Near the Earth). The most abundant source of antiprotons near us is found to be in a thin belt that extends from a few hundred to about 2000 kilometers above Earth, moving along Earth’s magnetic field lines and bouncing between the north and south magnetic poles.
Image: An antimatter reservoir near our planet in the form of a belt of antiprotons that lies within the innermost portion (pink) of Earth’s magnetosphere, the large bubble-like region interior to the blue arc that is controlled by the planet’s magnetic field. Credit: Aaron Kaase/NASA GSFC.
Compared to harvesting antimatter on Earth, space harvesting is five orders of magnitude more cost effective, and Bickford’s report suggests we could be collecting 25 nanograms of antimatter per day near our planet. And here’s a spectacular mission concept that can grow out of this, also drawn from the Bickford report:
The baseline concept of operations calls for a magnetic scoop to be placed in a low-inclination orbit, which cuts through the heart of the inner radiation belt where most antiprotons are trapped. Placing the vehicle in an orbit with an apogee of 3500 km and a perigee of 1500 km will enable it to intersect nearly the entire flux of the Earth’s antiproton belt. The baseline mission calls for a fraction of the total supply to be trapped over a period of days to weeks and then used to propel the vehicle to Saturn or other solar system body where there is a more plentiful supply. The vehicle then fully fills its antiproton trap and propels itself on a mission outside of our solar system.
We can imagine fuel depots in the Solar System that could support our growing infrastructure with missions to Mars and the asteroids. There is even the possibility, tantalizingly referenced in the report, of using the galactic cosmic ray flux enroute to a destination to further bulk up the fuel supply. It’s bracing stuff, and a reminder that when we talk about gathering antimatter for a mission, we aren’t necessarily limited to the sparse production from today’s colliders.
Symmetry Violations
But back to the original question. Why is antimatter so hard to find? If it is truly ‘mirror matter,’ as the title of Robert Forward’s book suggests, shouldn’t there be equal amounts of it, and shouldn’t that equality have prevailed from the beginning of the universe? It seems logical to think so, but of course if that had occurred, we would not be here to contemplate the problem.
Now we’re entering the realm of charge-parity (CP) symmetry, which asserts that physics should be unchanged if we plug in antiparticles where particles currently are. Most particle interactions show this charge-parity symmetry to hold, and it carries the implication that the universe should have begun with equal amounts of matter and antimatter. Why and where CP symmetry does fail is a serious question, one that has us looking for any observable violation of the principle.
We have no definitive answer, but we do have interesting results from the T2K experiment in Japan, as reported in New Scientist following their discussion at Neutrino 2016 (the XXVII International Conference on Neutrino Physics and Astrophysics), held in London in early July. The researchers at T2K have been monitoring the oscillations that occur when neutrinos spontaneously change ‘flavors’, from electron to muon to tau. Neutrinos as well as antineutrinos each come in these three types, and all three types can undergo such oscillations.
Image: The inside of the Super-Kamiokande detector in Japan. Credit: T2K.
Observing that 32 muon neutrinos that traveled between the J-PARC accelerator in Tokai to the Super-Kamiokande neutrino detector in Kamioka had turned into electron neutrinos, the team ran the same experiment with muon antineutrinos. Charge-parity symmetry says that the rates of change should be the same, but the researchers report just four muon antineutrinos have changed into the anti-electron neutrino. The numbers are small but the possible violation of CP symmetry is provocative. Results from NoVA, a similar experiment sending neutrinos between Illinois and Minnesota, are showing roughly similar values for apparent CP violation.
More data are needed to reach any firm conclusions, but these results point to the direction of future work at both installations. Some process that violates CP symmetry has to be in place to explain the overwhelming difference between the amount of matter and antimatter in the universe. Thus we can expect any results showing deviations from this symmetry will make news. Meanwhile, from a propulsion standpoint, we have to reckon with the paucity of antimatter by imagining creative ways of creating or finding enough to use in our future experiments. Space-based antimatter harvesting may prove to be the most cost effective way to proceed.
I’ll close by quoting James Bickford in a 2014 interview, where I think he strikes just the right note about the need for small scale experiments as well as avoiding antimatter hype:
For the most part, propelling spacecraft to near the speed of light with antimatter lives in the realm of Star Trek. The technical obstacles are non-trivial and probably won’t be solved in the near future, if ever. From this perspective, the potential for antimatter probably has been overhyped. However, the small scale experiments are just the first baby steps that could help us down the long path. More importantly, research and development in this area is part of a broader framework that could help fundamental science and our understanding of the universe. Antimatter plays a central role in some of the Holy Grail problems of physics, such as the nature of dark matter and why matter dominates over antimatter.
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