July 1, 2016 | Science Of Cycles News and Research

Earth’s Mantle Appears To Have A Driving Role In Plate Tectonics

Deep down below us is a tug of war moving at less than the speed of growing fingernails. Keeping your balance is not a concern, but how the movement happens has been debated among geologists.

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New findings from under the Pacific Northwest Coast by University of Oregon and University of Washington scientists now suggest a solution to a mystery that surfaced when the theory of plate tectonics arose: Do the plates move the mantle, or does the mantle move the plates.

The separation of tectonic plates, the researchers proposed in a paper online ahead of print in the journal Nature Geoscience, is not simply dictating the flow of the gooey, lubricating molten material of the mantle. The mantle, they argue, is actually fighting back, flowing in a manner that drives a reorientation of the direction of the plates.

The new idea is based on seismic imaging of the Endeavor segment of the Juan de Fuca Plate in the Pacific Ocean off Washington and on data from previous research on similar ridges in the mid-Pacific and mid-Atlantic oceans.

“Comparing seismic measurements of the present mantle flow direction to the recent movements of tectonic plates, we find that the mantle is flowing in a direction that is ahead of recent changes in plate motion,” said UO doctoral student Brandon P. VanderBeek, the paper’s lead author. “This contradicts the traditional view that plates move the mantle.”

While the new conclusion is based on a fraction of such sites under the world’s oceans, a consistent pattern was present, VanderBeek said. At the three sites, the mantle’s flow is rotated clockwise or counterclockwise rather than in the directions of the separating plates. The mantle’s flow, the researchers concluded, may be responsible for past and possibly current changes in plate motion.

The research—funded through National Science Foundation grants to the two institutions – also explored how the supply of magma varies under mid-ocean ridge volcanoes. The researchers conducted a seismic experiment to see how seismic waves moved through the shallow mantle below the Endeavor segment.

They found that the middle of the volcanic segment, where the seafloor is shallowest and the inferred volcanic activity greatest, the underlying mantle magma reservoir is relatively small. The ends, however, are much deeper with larger volumes of mantle magma pooling below them because there are no easy routes for it to travel through the material above it.

Traditional thinking had said there would be less magma under the deep ends of such segments, known as discontinuities.

“We found the opposite,” VanderBeek said. “The biggest volumes of magma that we believe we have found are located beneath the deepest portions of the ridges, at the segment ends. Under the shallow centers, there is much less melt, about half as much, at this particular ridge that we investigated.

“Our idea is that the ultimate control on where you have magma beneath these mountain ranges is where you can and cannot take it out,” he said. “At the ends, we think, the plate rips apart much more diffusely, so you are not creating pathways for magma to move, build mountains and allow for an eruption.”

How Comets Are Born

Detailed analysis of data collected by Rosetta show that comets are the ancient leftovers of early Solar System formation, and not younger fragments resulting from subsequent collisions between other, larger bodies.

Understanding how and when objects like Comet 67P/Churyumov-Gerasimenko took shape is of utmost importance in determining how exactly they can be used to interpret the formation and early evolution of our Solar System.

A new study addressing this question led by Björn Davidsson of the Jet Propulsion Laboratory, California Institute of Technology in Pasadena (USA), has been published in Astronomy & Astrophysics.

If comets are primordial, then they could help reveal the properties of the solar nebula from which the Sun, planets and small bodies condensed 4.6 billion years ago, and the processes that transformed our planetary system into the architecture we see today.

The alternative hypothesis is that they are younger fragments resulting from collisions between older ‘parent’ bodies such as icy trans-Neptunian objects (TNOs). They would then provide insight into the interior of such larger bodies, the collisions that disrupted them, and the process of building new bodies from the remains of older ones.

“Either way, comets have been witness to important Solar System evolution events, and this is why we have made these detailed measurements with Rosetta – along with observations of other comets – to find out which scenario is more likely,” says Matt Taylor, ESA’s Rosetta project scientist.

During its two-year sojourn at Comet 67P/Churyumov-Gerasimenko, Rosetta has revealed a picture of the comet as a low-density, high-porosity, double-lobed body with extensive layering, suggesting that the lobes accumulated material over time before they merged.

The unusually high porosity of the interior of the nucleus provides the first indication that this growth cannot have been via violent collisions, as these would have compacted the fragile material. Structures and features on different size scales observed by Rosetta’s cameras provide further information on how this growth may have taken place.

Earlier work showed that the head and body were originally separate objects, but the collision that merged them must have been at low speed in order not to destroy both of them. The fact that both parts have similar layering also tells us that they must have undergone similar evolutionary histories and that survival rates against catastrophic collision must have been high for a significant period of time.

Merging events may also have happened on smaller scales. For example, three spherical ‘caps’ have been identified in the Bastet region on the small comet lobe, and suggestions are that they are remnants of smaller cometesimals that are still partially preserved today.

At even smaller scales of just a few metres across, there are the so-called ‘goosebumps’ and ‘clod’ features, rough textures observed in numerous pits and exposed cliff walls in various locations on the comet.

While it is possible that this morphology might arise from fracturing alone, it is actually thought to represent an intrinsic ‘lumpiness’ of the comet’s constituents. That is, these ‘goosebumps’ could be showing the typical size of the smallest cometesimals that accumulated and merged to build up the comet, made visible again today through erosion due to sunlight.

According to theory, the speeds at which cometesimals collide and merge change during the growth process, with a peak when the lumps have sizes of a few metres. For this reason, metre-sized structures are expected to be the most compact and resilient, and it is particularly interesting that the comet material appears lumpy on that particular size scale.

Further lines of evidence include spectral analysis of the comet’s composition showing that the surface has experienced little or no in situ alteration by liquid water, and analysis of the gases ejected from sublimating ices buried deeper within the surface, which finds the comet to be rich in supervolatiles such as carbon monoxide, oxygen, nitrogen and argon.

These observations imply that comets formed in extremely cold conditions and did not experience significant thermal processing during most of their lifetimes. Instead, to explain the low temperatures, survival of certain ices and retention of supervolatiles, they must have accumulated slowly over a significant time period.

“While larger TNOs in the outer reaches of the Solar System appear to have been heated by short-lived radioactive substances, comets don’t seem to show similar signs of thermal processing. We had to resolve this paradox by taking a detailed look at the time line of our current Solar System models, and consider new ideas,” says Björn.

Björn and colleagues propose that the larger members of the TNO population formed rapidly within the first one million years of the solar nebula, aided by turbulent gas streams that rapidly accelerated their growth to sizes of up to 400 km.

Around three million years into the Solar System’s history, gas had disappeared from the solar nebula, only leaving solid material behind. Then, over a much longer period of around 400 million years, the already massive TNOs slowly accreted further material and underwent compaction into layers, their ices melting and refreezing, for example. Some TNOs even grew into Pluto or Triton-sized objects.

Comets took a different path. After the rapid initial growth phase of the TNOs, leftover grains and ‘pebbles’ of icy material in the cold, outer parts of the solar nebula started to come together at low velocity, yielding comets roughly 5 km in size by the time gas has disappeared from the solar nebula. The low speeds at which the material accumulated led to objects with fragile nuclei with high porosity and low density.

This slow growth also allowed comets to preserve some of the oldest, volatile-rich material from the solar nebula, since they were able to release the energy generated by radioactive decay inside them without heating up too much.

The larger TNOs played a further role in the evolution of comets. By ‘stirring’ the cometary orbits, additional material was accreted at somewhat higher speed over the next 25 million years, forming the outer layers of comets. The stirring also made it possible for the few kilometre-sized objects in size to bump gently into each other, leading to the bi-lobed nature of some observed comets.

“Comets do not appear to display the characteristics expected for collisional rubble piles, which result from the smash-up of large objects like TNOs. Rather, we think they grew gently in the shadow of the TNOs, surviving essentially undamaged for 4.6 billion years,” concludes Björn.

“Our new model explains what we see in Rosetta’s detailed observations of its comet, and what had been hinted at by previous comet flyby missions.”
“Comets really are the treasure-troves of the Solar System,” adds Matt.

“They give us unparalleled insight into the processes that were important in the planetary construction yard at these early times and how they relate to the Solar System architecture that we see today.”

Loneliest Young Star Seen By Spitzer And WISE

Alone on the cosmic road, far from any known celestial object, a young, independent star is going through a tremendous growth spurt.

The unusual object, called CX330, was first detected as a source of X-ray light in 2009 by NASA’s Chandra X-Ray Observatory while it was surveying the bulge in the central region of the Milky Way. Further observations indicated that this object was emitting optical light as well. With only these clues, scientists had no idea what this object was.

But when Chris Britt, postdoctoral researcher at Texas Tech University in Lubbock, and colleagues were examining infrared images of the same area taken with NASA’s Wide-field Infrared Survey Explorer (WISE), they realized this object has a lot of warm dust around it, which must have been heated by an outburst.

Comparing WISE data from 2010 with Spitzer Space Telescope data from 2007, researchers determined that CX330 is likely a young star that had been outbursting for several years. In fact, in that three-year period its brightness had increased by a few hundred times.

Astronomers looked at data about the object from a variety of other observatories, including the ground-based SOAR, Magellan, and Gemini telescopes. They also used the large telescope surveys VVV and the OGLE-IV to measure the intensity of light emitted from CX330. By combining all of these different perspectives on the object, a clearer picture emerged.

“We tried various interpretations for it, and the only one that makes sense is that this rapidly growing young star is forming in the middle of nowhere,” said Britt, lead author of a study on CX330 recently published in the Monthly Notices of the Royal Astronomical Society.

The lone star’s behavior has similarities to FU Orionis, a young outbursting star that had an initial three-month outburst in 1936-7. But CX330 is more compact, hotter and likely more massive than the FU Orionis-like objects known. The more isolated star launches faster “jets,” or outflows of material that slam into the gas and dust around it.

“The disk has probably heated to the point where the gas in the disk has become ionized, leading to a rapid increase in how fast the material falls onto the star,” said Thomas Maccarone, study co-author and associate professor at Texas Tech.

Most puzzling to astronomers, FU Orionis and the rare objects like it—there are only about 10 of them—are located in star-forming regions. Young stars usually form and feed from their surrounding gas and dust-rich regions in star-forming clouds. By contrast, the region of star formation closest to CX330 is over a thousand light-years away.

“CX330 is both more intense and more isolated than any of these young outbursting objects that we’ve ever seen,” said Joel Green, study co-author and researcher at the Space Telescope Science Institute in Baltimore. “This could be the tip of the iceberg—these objects may be everywhere.”

In fact, it is possible that all stars go through this dramatic stage of development in their youth, but that the outbursts are too short in cosmological time for humans to observe many of them.

How did CX330 become so isolated? One idea is that it may have been born in a star-forming region, but was ejected into its present lonely pocket of the galaxy. But this is unlikely, astronomers say. Because CX330 is in a youthful phase of its development—likely less than 1 million years old—and is still eating its surrounding disk, it must have formed near its present location in the sky.

“If it had migrated from a star-forming region, it couldn’t get there in its lifetime without stripping its disk away entirely,” Britt said.

CX330 may also help scientists study the way stars form under different circumstances. One scenario is that stars form through turbulence. In this “hierarchical” model, a critical density of gas in a cloud causes the cloud to gravitationally collapse into a star. A different model, called “competitive accretion,” suggests that stars begin as low-mass cores that fight over the mass of material left in the cloud. CX330 more naturally fits into the first scenario, as the turbulent circumstances would theoretically allow for a lone star to form.

It is still possible that other intermediate- to low-mass stars are in the immediate vicinity of CX330, but have not been detected yet.

When CX330 was last viewed in August 2015, it was still outbursting. Astronomers plan to continue studying the object, including with future telescopes that could view it in other wavelengths of light.

Outbursts from a young star change the chemistry of the star’s disk, from which planets may eventually form. If the phenomenon is common, that means that planets, including our own, may carry the chemical signatures of an ancient disk of gas and dust scarred by stellar outbursts.

But as CX330 is continuing to devour its disk with increasing voracity, astronomers do not expect that planets are forming in its system.

“If it’s truly a massive star, its lifetime is short and violent, and I wouldn’t recommend being a planet around it,” Green said. “You could experience some pretty intense heat for a few centuries.”

The Role Of Magnetic Fields In Star Formation

The star forming molecular clump W43-MM1 is very massive and dense, containing about 2100 solar masses of material in a region only one-third of a light year across (for comparison, the nearest star to the Sun is a bit over four light years away).

Previous observations of this clump found evidence for infalling motions (signaling that material is still accumulating onto a new star) and weak magnetic fields. These fields are detected by looking for polarized light, which is produced when radiation scatters off of elongated dust grains aligned by magnetic fields. The Submillimeter Array recently probed this source with high spatial resolutions and found evidence for even stronger magnetic fields in places. One of the outstanding issues in star formation is the extent to which magnetic fields inhibit the collapse of material onto stars, and this source seems to offer a particularly useful example.

CfA astronomers Josep Girart and TK Sridharan and their colleagues have used the ALMA submillimeter facility to obtain images with spatial scales as small as 0.03 light years. Their detailed polarization maps show that the magnetic field is well ordered all across the clump, which itself is actually fifteen smaller fragments, one of which (at 312 solar masses) appears to be the most massive fragment known.

The scientists analyze the magnetic field strengths and show that, even in the least massive fragment the field is not strong enough to inhibit gravitational collapse. In fact, they find indications that gravity, as it pulls material inward, drags the magnetic field lines along. They are, however, unable to rule out possible further fragmentation. The research is the most precise study of magnetic fields in star forming massive clumps yet undertaken, and provides a new reference point for theoretical models.

Astronomers Gain New Insight Into Magnetic Field Of Sun And Its Kin

Astronomers have used data from NASA’s Chandra X-ray Observatory to make a discovery that may have profound implications for understanding how the magnetic field in the Sun and stars like it are generated.

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Researchers have discovered that four old red dwarf stars with masses less than half that of the Sun are emitting X-rays at a much lower rate than expected.

X-ray emission is an excellent indicator of a star’s magnetic field strength so this discovery suggests that these stars have much weaker magnetic fields than previously thought.

Since young stars of all masses have very high levels of X-ray emission and magnetic field strength, this suggests that the magnetic fields of these stars weakened over time. While this is a commonly observed property of stars like our Sun, it was not expected to occur for low-mass stars, as their internal structure is very different.

The Sun and stars like it are giant spheres of superheated gas. The Sun’s magnetic field is responsible for producing sunspots, its 11-year cycle, and powerful eruptions of particles from the solar surface. These solar storms can produce spectacular auroras on Earth, damage electrical power systems, knock out communications satellites, and affect astronauts in space.

“We have known for decades that the magnetic field on the Sun and other stars plays a huge role in how they behave, but many details remain mysterious,” said lead author Nicholas Wright of Keele University in the United Kingdom. “Our result is one step in the quest to fully understand the Sun and other stars.”

The rotation of a star and the flow of gas in its interior both play a role in producing its magnetic field. The rotation of the Sun and similar stars varies with latitude (the poles versus the equator) as well as in depth below the surface. Another factor in the generation of magnetic field is convection. Similar to the circulation of warm air inside an oven, the process of convection in a star distributes heat from the interior of the star to its surface in a circulating pattern of rising cells of hot gas and descending cooler gas.

Convection occurs in the outer third (by radius) of the Sun, while the hot gas closer to the core remains relatively still. There is a difference in the speed of rotation between these two regions. Many astronomers think this difference is responsible for generating most of the magnetic field in the Sun by causing magnetic fields along the border between the convection zone and the core to wind up and strengthen. Since stars rotate more slowly as they age, this also plays a role in how the magnetic field of such stars weakens with time.

“In some ways you can think of the inside of a star as an incredibly complicated dance with many, many dancers,” said co-author Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “Some dancers move with each other while others move independently. This motion generates magnetic field, but how it works in detail is extremely challenging to determine.”

For stars much less massive than the Sun, convection occurs all the way into the core of the star. This means the boundary between regions with and without convection, thought to be crucial for generating magnetic field in the Sun, does not exist. One school of thought has been that magnetic field is generated mostly by convection in such stars. Since convection does not change as a star ages, their magnetic fields would not weaken much over time.

By studying four of these low-mass red dwarf stars in X-rays, Wright and Drake were able to test this hypothesis. They used NASA’s Chandra X-ray Observatory to study two of the stars and data from the ROSAT satellite to look at two others.

“We found that these smaller stars have magnetic fields that decrease as they age, exactly as it does in stars like our Sun,” said Wright. “This really goes against what we would have expected.”

These results imply that the interaction along the convection zone-core boundary does not dominate the generation of magnetic field in stars like our Sun, since the low mass stars studied by Wright and Drake lack such a region and yet their magnetic properties are very similar.

A paper describing these results by Wright and Drake appears in the July 28th issue of the journal Nature. NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra’s science and flight operations.

Scientists Observe A Superluminous Supernova That Appears To Have Exploded Twice

Supernovae are among the most violent phenomena in the universe. They are huge explosions that end the lives of certain types of stars. These explosions release immense amounts of energy, so much that some can be observed from Earth with the naked eye, appearing as points of light that are briefly brighter than all the millions of stars in the galaxies where they are found. Following an intense burst of light lasting a few weeks, supernovae start to fade gradually until they have effectively burned out.

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There are several types of supernovae. The astronomers classify them by their observable characteristics, which give clues about their origin. Among the most well known are those of type Ia .When a white dwarf, the final state of a star slightly more massive than the sun, absorbs mass from another nearby star or merges with another white dwarf, its mass grows until it becomes unstable and a thermonuclear explosion occurs. As these events produce a characteristic luminosity, they can be used by astronomers as “standard candles” to measure large distances in the universe, like sailors to inferring the distance of a known lighthouse at night by estimating its brightness.

The other types of supernovae are produced when very massive stars exhaust their fuel, so that nuclear fusion in their interiors comes to an end. This fusion not only causes stars to emit light and heat, but keeps them in equilibrium so that they don’t collapse under their own gravity. When the fusion stops, the centre of the star collapses and the outer layers are flung outwards with violence, causing a supernova, while the centre implodes, leaving a neutron star—or for very massive stars, a black hole.

In recent years, a new type of supernova has been discovered, about which very little is yet known, and which are brighter and longer-lasting. Astronomers call them superluminous supernovae (SLSN). Although only about a dozen of them are known, an international group of researchers has used the Gran Telescopio CANARIAS (GTC) to observe a superluminous supernova almost from the moment it occurred. The research has revealed surprising behaviour, because this supernova showed an initial increase in brightness that later declined for a few days, and then increased again much more strongly. The scientists have combined from the GTC with other observations in order to try to explain the origin of the phenomenon.

“Superluminous supernovas are up to a hundred times more energetic than type 1a supernovae because they can remain bright for up to six months before fading, rather than just a few weeks,” explains Mathew Smith, a postdoctoral researcher at the University of Southampton (UK) and the person directing this study, whose results have been published in the specialized journal The Astrophysical Journal Letters. “What we have managed to observe, which is completely new, is that before the major explosion, there is a shorter, less luminous outburst, which we can pick out because it is followed by a dip in the light curve, and which lasts just a few days.”

It is the first time that something like this has been observed in a supernova. “From our data, we have tried to determine if this is a characteristic unique to this object, or whether it is a common feature of all superluminous supernovae, but has not been observed before, which is perfectly possible given their unpredictable nature,” says the scientist.

This new, intriguing object, given the cryptic name of DES14X3taz by the astronomers, was discovered on December 21, 2014 by the Dark Energy Survey, an international project that surveys the night sky, making precision measurements of over 300 million galaxies that are situated thousands of millions of light years from Earth, and incidentally detecting thousands of supernovae and other transient phenomena. The objective of this survey is to explain the expansion of the universe, and to find clues to the nature of dark energy. To do this, astronomers are using an extremely sensitive 570 megapixel digital camera on the four metre Victor M. Blanco telescope at the Inter-American Observatory at Cerro Tololo (Chile).

Once DES14X3taz had been identified as a possible superluminous supernova, an immediate observation was requested on the GTC, which turned its powerful eye toward it over two nights of observation, January 26 and February 6, 2015. GTC devotes some of its observing time to “targets of opportunity,” so that other programmed observations can postponed to prioritize such transient phenomena, which may offer unrepeatable opportunities.

“The GTC, with its huge 10.4m mirror, and its OSIRIS instrument, is the ideal tool to observer this SNSL, which is at a vast distance and because we are looking for information in the visible and the near infrared,” says Smith, who is a participant in the Dark Energy Survey. Thanks to the observations made with the GTC and other telescopes, Smith and his collaborators could reconstruct the evolution of the brightness of DES14X3taz from almost the moment of its detection. They have also determined its absolute brightness with great precision, as well as its distance, some 6,400 million light years.

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After comparing their observations with several physical models, the astronomers concluded in their article that the most plausible explanation is that the mechanism causing this supernova is the birth of a “magnetar,” a neutron star that rotates very rapidly on its axis. In the data, the initial peak of the brightness graph is followed by rapid cooling of the object, after which there is a quicker rise in brightness. This is consistent with the emission of a huge bubble of material into the surrounding space, which cools rapidly as it grows in size.

“We think that a very massive star, some 200 times the mass of the Sun, collapses to form a magnetar. In the process, the first explosion occurs, which expels into space a quantity of matter equivalent to the mass of our sun, and this gives rise to the first peak of the graph. The second peak occurs when the star collapses to form the magnetar, which is a very dense object rotating rapidly on its axis, and which heats up the matter expelled from the first explosion. This heating is what generates the second peak in the luminosity,” explains Smith.

This understanding may allow us to “standardize” superluminous supernovae as has occurred for the type Ia supernovae for use as a reference source for distance measurement on large scales in the universe. Its high luminosity may make these objects useful for calculating distances on larger scales, and with greater accuracy than current techniques. However, before we get to that point, we need a much deeper understanding of their origin and their nature.

Another mystery about this new type of supernova is that, until recently, all the examples detected have been in small galaxies with low metallicity (low content in heavy elements), which is not well understood. “It is a part of the mystery of these objects,” says Smith, and adds that among future priorities, we need to detect more superluminous supernovae and observe them from the moment they explode in real time with a telescope the size of the GTC.

Experimentation Suggests Vikings Could Have Used Sunstone To Navigate

A team of researchers from several institutions in Hungary has conducted experiments meant to test the possibility that the Vikings actually did use sunstones to navigate. In their paper published in Proceedings of the Royal Society A, the team describes the experiments they carried out, their results and why they now believe it is possible to use a sunstone as a navigational aid during times when the skies are covered with clouds.

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The exploits of the Vikings have been well documented—they conducted raids across Europe from the late 790s till 1066, when the Normans famously conquered England. But as more recent research has established, they were also long-distance seafaring travelers, venturing as far as the Middle East and North America. But how they found their way across vast stretches of ocean has been a bit of a mystery, particularly during times when there were no stars or sun in the sky to guide them. Some historical evidence such as Icelandic legends have mentioned travel under snowy skies using sunstones and a study of a Viking wreck conducted in 2002 revealed that a crystal (Icelandic spar) had been onboard that was found near other implements used for navigation.

Modern sunstone is a type of crystal that, when viewed from different angles, offers a spangled optical effect. In this new effort, the researchers have conducted a study designed to test the possibility that such crystals could really have helped Vikings find their way across the ocean.

They believe it was a three step process: (1) determine the direction of light from the sky using the sunstone held up to the sky, (2) use that information to determine the direction of sunlight and then (3) use a shadow stick to determine which direction was north. The team previously conducted tests to measure the accuracy of the first two steps and, apparently satisfied with the results, have now conducted experiments with the third.

To test the third step, the researchers asked 10 volunteers to try to work out the position of the sun in a digital planetarium using dots to stand in for results of using a sunstone. After conducting a total of 2,400 trials, the researchers report that 48 percent resulted in producing an accurate reading to within just one degree. They noted also that the volunteers did best when the virtual sun was near the horizon, showing that the method worked best at dawn and dusk. The team suggests their results indicate that it was possible that the Vikings used sunstones to navigate under cloudy skies.