No Heat Signature of Europan ‘Plumes’

One of the youngest surface features on Europa draws attention because of its possible connection with what lies beneath the Jovian moon’s ice. The dark center of Pwyll crater, visible in the image below, is some 40 kilometers across, with a central peak reaching about 600 meters. At issue is the terrain resulting from the impact causing the crater. An impact perhaps 20 million years ago seems to have blown water and ice across the Europan surface. Evidence of a possible plume from Europa’s ocean in this area is the subject of continuing work.

The bright terrain around the crater suggests water ice, and note, too that the Pwyll impact left ejecta rays as far as the Conamara Chaos region 1000 kilometers to its north. Conamara Chaos features themselves have been studied extensively for terrain suggestive of melting and refreezing ice. We saw recently how Xianzhe Jia (University of Michigan), working with the SETI Institute’s Melissa McGrath, used data from the Galileo mission to support later Hubble evidence of possible plumes on Europa (see Galileo Evidence for Plumes on Europa).

Image: This enhanced color image of the region surrounding the young impact crater Pwyll on Jupiter’s moon Europa was produced by combining low resolution color data with a higher resolution mosaic of images obtained on December 19, 1996 by the Solid State Imaging (CCD) system aboard NASA’s Galileo spacecraft. This region is on the trailing hemisphere of the satellite, centered at 11 degrees South and 276 degrees West, and is about 1240 kilometers across. North is toward the top of the image, and the sun illuminates the surface from the east. The 26 kilometer diameter impact crater Pwyll, just below the center of the image, is thought to be one of the youngest features on the surface of Europa. The diameter of the central dark spot, ejecta blasted from beneath Europa’s surface, is approximately 40 kilometers, and bright white rays extend for over a thousand kilometers in all directions from the impact site. Credit: NASA/JPL-Caltech.

Plumes on Europa would be a momentous discovery, one reason the elusive evidence for their existence is proving so controversial. For if we had active plumes venting seawater from the deep Europan ocean, we could study their composition without the need for landing and drilling through the ice. Europa, like Enceladus, could prove a target for astrobiologically-focused missions in the near future, and indeed, the two researchers I cited above are both associated with the Europa Clipper science team, which will perform multiple flybys of the moon.

This morning we learn that Julie Rathbun (Planetary Science Institute) has dug into the evidence for Europan plumes in a talk titled “A closer look at Galileo Thermal data from possible plume sources near Pwyll, Europa,” with news that makes these plumes more unlikely. For what we would expect at Europa is similar to what we see at Enceladus, the signature of hotspots that flag the energy source driving the plume activity. Similar hotspots can be found on Earth at geysers like Yellowstone and its associated hot springs. But no Europan hotspots can be found.

Rathbun states the issue concisely, so let me just quote her on this:

“We searched through the available Galileo thermal data at the locations proposed as the sites of potential plumes. Reanalysis of temperature data from the Galileo mission does not show anything special in the locations where plumes have possibly been observed. There are no hotspot signatures at either of the sites.”

Such plumes may exist, but appear only rarely. Rathbun continues:

“This is surprising because the Enceladus plumes have a clear thermal signature at their site of origin, so this suggests that either the Europa plumes are very different, or the plumes are only occasional, or that they don’t exist, or that their thermal signature is too small to have been detected by current data.”

Rathbun presented these findings at a Division for Planetary Sciences press conference held at the American Astronomical Society 50th annual meeting in Knoxville, TN. Plume research is ongoing, and bear in mind that the possible plume in the area near Pwyll is not the only one being considered. There is evidence in the Xianzhe Jia and Melissa McGrath work for plume activity about 1,000 kilometers northeast of the first site. Indeed, Jia and McGrath found that magnetic perturbations found by Galileo during its E12 flyby were consistent with a rising plume.

So the jury is still out. If we can get Europa Clipper off, perhaps as early as 2022, its flybys could prove conclusive one way or another. While we wait, analysis and simulations of the interactions between possible plumes and the plasma environment around Europa continue. We have only two Galileo flybys of the Moon containing magnetometer data that came closer than 400 kilometers from the surface, where we might expect a plasma and magnetic field signature if plumes exist, but Europa Clipper will ramp up the magnetic and thermal dataflow considerably.

The Xianzhe Jia paper is “Evidence of a plume on Europa from Galileo magnetic and plasma wave signatures,” published online at Nature Astronomy 14 May 2018 (abstract).


Birth of a Supercluster

Long-time Centauri Dreams readers know that I love things that challenge our sense of scale, the kind of comparison that, for example, tells us that if we traveled the distance from the Earth to the Sun, we would have to repeat that distance 268,770 times just to reach the nearest star. It’s much simpler, of course, to say that Proxima Centauri is 4.25 light years from us, but it’s the relating of distances to things that are closer to us that gets across scale, especially for those who are just beginning their astronomical explorations. And I have to admit that the scales involved in going interstellar still pull me up short at times when I ponder them.

So how about this for scale: We have somewhere between 200 billion and 300 billion stars in our galaxy (the number is flexible enough that you’ll see a wide range in the literature). Relate that to the Local Group, the gathering of galaxies that includes both the Milky Way and M31, the Andromeda Galaxy. These are the two most massive members of the Local Group, but depending on how we count dwarf galaxies, it contains more than 30 members spread out over a diameter of 10 million light years. Both the Milky Way and M31 have their own dwarf galaxies.

Then consider the concept of a ‘supercluster,’ which contains galaxy groups within it. Thus the Milky Way is considered part of both the Local Group as well as the Laniakea Supercluster, which is itself home to approximately 100,000 galaxies and subsumes the Virgo Supercluster. The Laniakea Supercluster emerged in the literature in 2014 in a paper examining the relative velocity of galaxies. Laniakea is a Hawaiian word meaning ‘immense heaven.’ R. Brent Tully (University of Hawaii at Manoa) and team identified this structure some 520 million light years in diameter, containing 100,000 galaxies, with a mass of one hundred million billion Suns.

Now a team of astronomers working with data from the European Southern Observatory’s Very Large Telescope (VLT) using its VIMOS (VIsible Multi-Object Spectrograph) instrument has identified a proto-supercluster that formed in the early universe 2.3 billion years after the Big Bang (i.e., its redshift of 2.45 means that astronomers observe it as it was 2.3 billion years after the Big Bang). ESO is describing the discovery, which they have nicknamed Hyperion, as the most massive structure yet found so early in the formation of the Universe.

“This is the first time that such a large structure has been identified at such a high redshift, just over 2 billion years after the Big Bang,” explained the first author of the discovery paper, Olga Cucciati. “Normally these kinds of structures are known at lower redshifts, which means when the Universe has had much more time to evolve and construct such huge things. It was a surprise to see something this evolved when the Universe was relatively young!”

Image: An international team of astronomers using the VIMOS instrument of ESO’s Very Large Telescope have uncovered a titanic structure in the early Universe. This galaxy proto-supercluster — which they nickname Hyperion — was unveiled by new measurements and a complex examination of archive data. This is the largest and most massive structure yet found at such a remote time and distance — merely 2 billion years after the Big Bang. Credit: ESO/Luis Calçada and Olga Cucciati.

Hyperion emerged in the analysis of a field in the constellation Sextans carried out by researchers in the VIMOS Ultra-deep Survey, which has been creating a 3D map of the distribution of over 10,000 galaxies. Hyperion contains seven high-density regions connected by thin ‘filaments’ of galaxies. The average supercluster, says Brian Lemaux (University of California, Davis), an astronomer and co-leader of the team behind this result, shows more concentrated distribution of mass and clear structure. “But in Hyperion,” Lemaux adds, “the mass is distributed much more uniformly in a series of connected blobs, populated by loose associations of galaxies.”

The mass distribution makes sense when you consider that nearby superclusters have had billions of years to create the observed clumping into denser regions with more defined structure. We might expect Hyperion to evolve into something more like the Virgo Supercluster, and studying it should provide insights into how galactic superclusters evolve. It offers a rare glimpse into the early era of supercluster formation, and another signpost of immensity.

What’s ahead for the study of Hyperion? Calling it a “unique possibility to study a rich supercluster in formation 11 billion years ago,” the paper adds this:

This impressive structure deserves a more detailed analysis. On the one hand, it would be interesting to compare its mass and volume with similar findings in simulations, because the relative abundance of superclusters could be used to probe deviations from the predictions of the standard ?CDM model [Lambda cold dark matter, a model that includes a cosmological constant, dark energy and cold dark matter]. On the other hand, it is crucial to obtain a more complete census of the galaxies residing in the proto-supercluster and its surroundings. With this new data, it would be possible to study the co-evolution of galaxies and the environment in which they reside, at an epoch (z ? 2 ? 2.5) when galaxies are peaking in their star-formation activity.

The Hyperion findings are being compared to the results of the Observations of Redshift Evolution in Large Scale Environments (ORELSE) survey, led by Lori Lubin (UC-Davis), who was on the team that discovered Hyperion. ORELSE studies superclusters closer to Earth using data from the W.M. Keck Observatory in Hawaii. The next step will be to map out Hyperion in greater detail.

The paper is Cucciati, et al., “The progeny of a Cosmic Titan: a massive multi-component proto-supercluster in formation at z=2.45 in VUDS,” accepted at Astronomy & Astrophysics. Preprint.


A Signature of Planetary Migration

Earlier in the week I talked about Astronomy Rewind, an ambitious citizen science project dedicated to recovering old astronomical imagery and digitizing it for comparison with new data. Now I’ve learned that another citizen science effort, Planet Finders, is working with simulated data from the Transiting Exoplanet Survey Satellite (TESS), planning to transition into real TESS data as soon as they become available. Have a look at this effort here if you are interested in becoming a beta tester. TESS will be a hugely significant exoplanet mission particularly in terms of nearby stars, so becoming a part of this project should be an exciting venture indeed.

On with today’s post, which I would have actually run yesterday if I had read the paper soon enough, as it offers insights into Wednesday’s entry on protoplanetary disks. As we’ve seen, these can become the discovery grounds for young planets. In the case of the 2-million year old CI Tau, that meant an already confirmed gas giant in a ‘hot Jupiter’ configuration, along with three other gas giants, two of them far from the central star. Hence the question: Where did the hot Jupiter CI Tau b form? Because if it migrated, it did so early in the history of this system.

Now we have a separate research effort attempting to show that planet migration within a protoplanetary disk can be identified through markers within the disk itself. The idea is to produce an observational marker that can guide future research and tell us whether, within a given system, a planet is moving inward through the disk or staying within its existing orbit.

Image: Astrophysicist Farzana Meru: Credit: University of Warwick.

Lead author Farzana Meru (University of Warwick, UK), working with colleagues at Cambridge University, describes an observational signature in young stellar system dust rings. As with the CI Tau work, the new study involves the Atacama Large Millimeter/submillimeter Array (ALMA), which is able to probe such disks in intricate detail, revealing structure in the form of clumps, gaps, spiral arms, crescents and rings. Even more to the point, by working at different millimeter frequencies, ALMA can identify clustering material in different particle sizes.

Image: Example of a protoplanetary disk. This is an ALMA image of the young star HL Tau and its disk. The image reveals multiple rings and gaps that herald the presence of emerging planets as they sweep their orbits clear of dust and gas. The image was released Nov. 6, 2014. ALMA (NRAO/ESO/NAOJ); C. Brogan, B. Saxton (NRAO/AUI/NSF).

The question, then, is what can variations in dust particle size tell us? The researchers perform simulations of movement within a ring of gas and dust in the presence of migrating planets of relatively low mass, from 12 to 60 Earth masses. The results show that the gas pressure profile of the disk varies significantly depending on whether the planet is migrating or stationary.

From the paper:

…the pressure perturbation exterior to the planet is weaker while that interior to the planet becomes more important for migrating planets. Dust can therefore be enhanced both interior or exterior to the planet and the result is governed by the relative values of the planet and dust velocities. For small sizes, the dust velocity in the outer disc is too small to keep up with the moving pressure maximum while in the inner disc it moves faster and can collect, forming a dust density enhancement interior to the planet.

For larger dust particles, the dust velocity in the disk exterior to the planet’s orbit is high enough to keep up with the pressure perturbation, producing what the authors describe as a ‘dust density enhancement’ in this region. We would thus expect smaller particles in the interior ring, larger particles in the exterior. By studying the inner and outer rings around the planetary orbit at different wavelengths, we should be able to show whether or not the planet is migrating.

We’re talking about small differences indeed — a migrating planet at 30 AU that is 30 times the mass of the Earth should, according to this modeling, be associated with an inner ring consisting of particles less than a millimeter in size, while those in the outer ring would measure slightly over a millimeter. But this is a workable observable, for as ALMA increases the wavelengths at which it observes, the inner dust ring fades while the exterior ring becomes brighter. This is because the emissivity of dust grains depends on the maximum grain size in the mixture.

Image: This is the paper’s Figure 7. Caption: “Dust density rendered simulation image of the disc with a 30M? migrating planet at Rp = 0.75 for dust with Stokes numbers of 0.02 (left) and 0.2 (right). The small dust forms a ring interior to the planet while the large dust forms a ring exterior to it.” The ‘Stokes number’ refers to the movement of particles in flow. What is significant here is the separation of small- and large-particle dust accumulations. Credit: Meru et al.

The movement of the dust is the key. In the inner ring, we are seeing smaller particles because these move inwards more slowly than the planet itself, accumulating in the inner ring, while large dust particles are found in the exterior ring because they move at higher velocity than the smaller particles and keep pace with the planet in its movement inward. Over time, the composition of the two rings is distinctive enough to become an ALMA observable.

We’re entering an era when protoplanetary disks are becoming the subject of intense scrutiny thanks to facilities like ALMA, along with the Spectro- Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument on the Very Large Telescope (VLT) and the Gemini Planet Instrument (GPI) on the Gemini Telescope. And we’re learning that most such disks show sub-structure susceptible to such analysis. We need observational benchmarks like these because a hot Jupiter like CI Tau’s can be the result of migration, but it could also achieve its tight orbit because of gravitational interactions early in the development of the system.

Applying observational constraints to planetary migration could become a potent tool in untangling a young system’s evolution. The work is promising but still in its early stages. In principle, write the researchers, “…it may be possible to use the location of dust rings in order to detect planetary migration, although the feasibility of this measurement is yet to be established.”

The paper is Meru et al., “Is the ring inside or outside the planet?: The effect of planet migration on dust rings,” accepted at Monthly Notices of the Royal Astronomical Society (preprint).


Red Dwarfs, ‘Superflares’ and Habitability

Given their ubiquity in the Milky Way, red dwarfs would seem to offer abundant opportunities for life to emerge. But we’re a long way from knowing how habitable the planets that orbit them might be. While mechanisms for moderating the climate on tidally locked worlds in tight habitable zones continue to be discussed, the issue of flares looms large. That makes a new survey of 12 young red dwarfs, and the project behind it, of unusual interest in terms of astrobiology.

What jumps out at the reader of Parke Loyd and team’s paper is the superflare their work caught that dwarfed anything ever seen from our own Sun, a much larger star. It was enough to set Loyd, a postdoctoral researcher at Arizona State University, back on his heels.

“When I realized the sheer amount of light the superflare emitted, I sat looking at my computer screen for quite some time just thinking, ‘Whoa.'” He adds: “With the Sun, we have a hundred years of good observations. And in that time, we’ve seen one, maybe two, flares that have an energy approaching that of the superflare. In a little less than a day’s worth of Hubble observations of these young stars, we caught the superflare. This means that we’re looking at superflares happening every day or even a few times a day.”

Image: Violent outbursts of seething gas from young red dwarfs may make conditions uninhabitable on fledgling planets. In this artist’s rendering, an active, young red dwarf (right) is stripping the atmosphere from an orbiting planet (left). ASU astronomers have found that flares from the youngest red dwarfs they surveyed — approximately 40 million years old — are 100 to 1000 times more energetic than when the stars are older. They also detected one of the most intense stellar flares ever observed in ultraviolet light — more energetic than the most powerful flare ever recorded from our Sun. Credit: NASA, ESA, and D. Player (STScI)

Loyd’s work is under the aegis of a program called HAZMAT, which stands for HAbitable Zones and M dwarf Activity across Time (ASU’s Evgenya Shkolnik is principal investigator for this project). The issue of time is significant, for HAZMAT will survey young, intermediate and old M-dwarfs using data from the Hubble Space Telescope, and this initial paper focuses on stars that are roughly 40 million years old, mere infants given that this category of star can burn for as long as a trillion years.

As to that superflare, it’s easy to see why it gave Loyd pause. His team detected 18 flares from its 12 target stars, but the superflare swamped them all, emitting 1032.1 erg in the far ultraviolet. That exceeds the most energetic flare from an M-dwarf previously observed by Hubble by a factor of 30.

Have a look at the paper’s description of what the authors call the ‘Hazflare’ in comparison to other flare observations from the past:

This observation is of particular value because superflares are common on stars (e.g., Davenport 2016), yet spectrophotometry of such flares in the UV, the band most relevant to planetary atmospheric photochemistry, is rare. Superflares are estimated from Kepler data to occur on M0-M4 dwarfs at a frequency of a few per day (Yang et al. 2017). Photochemical models exploring the effects of flares on planetary atmospheres have thus far relied primarily on observations of the 1985 Great Flare on AD Leo (Hawley & Pettersen 1991; Segura et al. 2010; Tilley et al. 2017), a flare estimated to emit a bolometric energy of 1034 erg. The Great Flare also showed a clear continuum in FUV emission, and overall the continuum was responsible for at least an order of magnitude more overall energy emitted by the flare than lines, consistent with the Hazflare. However, the Great Flare observations, made with the International Ultraviolet Explorer, saturated in the strongest emission lines, degrading their accuracy.

Image: Observations with the Hubble Space Telescope discovered a superflare (red line) that caused a red dwarf star’s brightness in the far ultraviolet to abruptly increase by a factor of nearly 200. Credit: P. Loyd/ASU.

So the HAZMAT observations prove valuable indeed. Superflares like the one described in this paper are far more common in young dwarfs, which can erupt up to 1,000 times more powerfully in their youth than after they have aged, and the Kepler data on their frequency (above) are noteworthy. The mechanism: Strong magnetic fields twisted by the churning atmosphere of the young star, causing them to break and reconnect, a process producing huge amounts of energy. Such violent activity is associated with M-dwarfs in the first 100 million years of their lifetime.

You would think we could just wait out flare activity and assume that older M-dwarfs were our best bet for habitable conditions, but a key question is what kind of damage will already have been done. Pummeling from flares like these could cause atmospheric damage, perhaps even stripping the atmosphere from what might have been promising worlds in the zone where liquid water could exist on the surface. Ultraviolet and X-ray radiation, even if it leaves the atmosphere more or less intact, would also be a huge factor in determining the emergence of surface life, possibly acting as an evolutionary spur or conceivably preventing it from appearing at all.

We also have to learn what processes can replenish the atmosphere of a planet if it undergoes a period of intense UV bombardment followed by a gradually calmer stellar environment. The sheer longevity of red dwarfs gives reason to hope that life could eventually emerge, but we won’t know until we can make the kind of atmospheric observations that will be coming our way through missions like the Transiting Exoplanet Survey Satellite (TESS), the James Webb Space Telescope and the European Space Agency’s ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) observatory. The latter, to be launched in 2028, will make a large-scale survey of the chemistry of exoplanet atmospheres using transit methods.

But back to HAZMAT, which will move on next to intermediate-age red dwarfs some 650 million years old, followed by analysis of the radiation environment around much older M-dwarfs. The evolution of that environment will help us refine the target list for the above missions as we focus on star systems more likely to have life. Given that most of the habitable zone planets in the galaxy will have had to withstand high flare activity, we need to make modeling the effects of flare erosion on atmospheres a high priority task.

The paper is Loyd et al., “HAZMAT. IV. Flares and Superflares on Young M Stars in the Far Ultraviolet,” accepted for publication at the Astrophysical Journal (preprint).


An Infant System Laden with Gas Giants

We’ve never found a ‘hot Jupiter’ around a star as young as CI Tau. This well studied system, some 2 million years old, has drawn attention for its massive disk of dust and gas, one that extends hundreds of AU from the star. But radial velocity examination recently revealed CI Tau b, a hot Jupiter that in and of itself raises questions. Couple that to the likelihood of three other gas giant planets emerging in the disk with extreme differences in orbital radii and it’s clear that CI Tau challenges our ideas of how gas giants, especially hot Jupiters, emerge and evolve.

Can a hot Jupiter form in place, or is migration from a much more distant orbit the likely explanation? The latter seems likely, and in that case, what was the mechanism here around such a young star? Most hot Jupiter host stars have lost their protoplanetary disks, which means that astronomers have been working with theoretical formation models to produce the observed tight orbits. And because about 1 percent of main sequence solar type stars (CI Tau is a K4IV object) have hot Jupiters around them, the riddle of their formation demands resolution.

The new work on CI Tau comes via high resolution imaging at submillimeter wavelengths at the Atacama Large Millimeter/submillimeter Array (ALMA), where Cathie Clarke (Cambridge Institute of Astronomy) and colleagues have found three distinct gaps in the star’s protoplanetary disk at ? 13, 39 and 100 au. The team’s paper in Astrophysical Journal Letters reports on computer modeling showing that these gaps are likely caused by additional gas giant planets.

Image: From Figure 1 of the paper, showing the protoplanetary disk around CI Tau. Credit: Clarke et al.

Making the find even more intriguing is that while CI Tau b is in an orbit not dissimilar from Mercury’s, the farthest putative planet orbits at a distance three times that of Neptune. All are large objects — the two outer worlds are roughly the mass of Saturn, while the two inner planets weigh in at 1 Jupiter mass and 10 Jupiter masses for the hot Jupiter. We’re left with the question of how these other planets affected the hot Jupiter’s orbital position, and whether there is a mechanism at work here that could apply to hot Jupiters in other systems.

For that matter, how did the two Saturn-class planets emerge where they are?

“Planet formation models tend to focus on being able to make the types of planets that have been observed already, so new discoveries don’t necessarily fit the models,” said Clarke. “Saturn mass planets are supposed to form by first accumulating a solid core and then pulling in a layer of gas on top, but these processes are supposed to be very slow at large distances from the star. Most models will struggle to make planets of this mass at this distance.”

No doubt. Complicating the picture further is that we have only learned about these planet candidates because of their effects on the protoplanetary disk, so whether or not extreme orbital parameters like these are common in hot Jupiter systems remains an open question. After all, older systems like those we’ve found other hot Jupiters in have already lost their disks.

The Jovian-class worlds may turn out to be easier to explain. Despite CI Tau’s relative youth, the authors argue that its hot Jupiter would still have had time to make the migration into hot Jupiter range. From the paper:

The hot Jupiter… could have been formed by a variety of mechanisms; from the modeled masses in disc and planets and from the accretion on to the star the inferred timescale for its inward migration is ? 0.4 Myr (Dürmann & Kley 2015) so that there would have been plenty of time for it to have migrated from a range of outward lying locations. The roughly Jovian mass planet inferred at 14 au is also easy to account for in terms of existing planet formation models (i.e. core accretion models involving either planetesimal or pebble accretion.

But those two outer ‘Saturns’ will need further work. We have a number of planetary disks with well-defined substructure to look at (HL Tau, HD 163296 and HD 169142, among others), but none with a hot Jupiter. Is this orbital configuration one that can survive on billion-year timescales? The authors believe these planets may still end up at small radii, suggesting they could be eventually ejected from the system by gravitational interactions. It will take future imaging surveys to tell us whether systems like the emerging one at CI Tau can be long-lived.

The paper is Clarke et al., “High-resolution Millimeter Imaging of the CI Tau Protoplanetary Disk: A Massive Ensemble of Protoplanets from 0.1 to 100 au,” Astrophysical Journal Letters Vol. 866, No. 1 (4 October 2018). Abstract / preprint.