A Nearby River Of Stars

Astronomy & Astrophysics publishes the work of researchers from the University of Vienna, who have found a river of stars, a stellar stream in astronomical parlance, covering most of the southern sky. The stream is relatively nearby and contains at least 4000 stars that have been moving together in space since they formed, about 1 billion years ago. Due to its proximity to Earth, this stream is a perfect workbench on which to test the disruption of clusters, measure the gravitational field of the Milky Way, and learn about coeval extrasolar planet populations with upcoming planet-finding missions. For their search, the authors used data from the ESA Gaia satellite.

Our own host galaxy, the Milky Way, is home to star clusters of variable sizes and ages. We find many baby clusters within molecular clouds, fewer middle-age and old age clusters in the Galactic disk, and even fewer massive, old globular clusters in the halo. These clusters, regardless of their origin and age, are all subject to tidal forces along their orbits in the Galaxy. Given enough time, the Milky Way gravitational forces relentlessly pull them apart, dispersing their stars into the collection of stars we know as the Milky Way.

“Most star clusters in the Galactic disk disperse rapidly after their birth as they do not contain enough stars to create a deep gravitational potential well, or in other words, they do not have enough glue to keep them together. Even in the immediate solar neighborhood, there are, however, a few clusters with sufficient stellar mass to remain bound for several hundred million years. So, in principle, similar, large, stream-like remnants of clusters or associations should also be part of the Milky Way disk.” says Stefan Meingast, lead author of the paper published in Astronomy & Astrophysics.

Thanks to the precision of the Gaia measurements, the authors could measure the 3D motion of stars in space. When carefully looking at the distribution of nearby stars moving together, one particular group of stars, as yet unknown and unstudied, immediately caught the eye of the researchers. It was a group of stars that showed precisely the expected characteristics of a cluster of stars born together but being pulled apart by the gravitational field of the Milky Way.

“Identifying nearby disk streams is like looking for the proverbial needle in a haystack. Astronomers have been looking at, and through, this new stream for a long time, as it covers most of the night sky, but only now realize it is there, and it is huge, and shockingly close to the Sun” says João Alves, second author of the paper. “Finding things close to home is very useful, it means they are not too faint nor too blurred for further detailed exploration, as astronomers dream.”

Due to sensitivity limitations of the Gaia observations, their selection only contained about 200 sources. An extrapolation beyond these limits suggests the stream should have at least 4000 stars, thereby making the structure more massive than most know clusters in the immediate solar neighborhood. The authors also determined the stream’s age to be around one billion years. As such, it already has completed four full orbits around the Galaxy, enough time to develop the stream-like structure as a consequence of gravitational interaction with the Milky Way disk.

“As soon as we investigated this particular group of stars in more detail, we knew that we had found what we were looking for: A coeval, stream-like structure, stretching for hundreds of parsecs across a third of the entire sky.” Says Verena Fürnkranz, co-author and Masters student at the University of Vienna. “It was so thrilling to be part of a new discovery” she adds.

This newly discovered nearby system can be used as a valuable gravity probe to measure the mass of the Galaxy. With follow-up work, this stream can tell us how galaxies get their stars, test the gravitational field of the Milky Way, and, because of its proximity, become a wonderful target for planet-finding missions. The authors hope to unravel even more such structures in the future with the help of the rich Gaia database.

Earth’s Magnetic Pole Shifting at Unexpected Speed

Rapid shifts in the Earth’s north magnetic pole are forcing researchers to make an unprecedented early update to a model that helps navigation by ships, planes and submarines in the Arctic, scientists said.

Compass needles point towards the north magnetic pole, a point which has crept unpredictably from the coast of northern Canada a century ago to the middle of the Arctic Ocean, moving towards Russia.

Arnaud Chulliat is a scientist at the University of Colorado in Boulder, Colorado. He is also the lead researcher for the newly updated World Magnetic Model. Chulliat told the Associated Press the continuous movement of magnetic north is a problem for compasses in smartphones and other electronic devices.

“It is moving at about 50 km (30 miles) a year. It did not move much between 1900 and 1980 but it is really accelerated in the past 40 years,” reports Ciaran Beggan, of the British Geological Survey in Edinburgh.

A five-year update of a World Magnetic Model was due in 2020 but the U.S. military requested an unprecedented early review, he said. The BGS runs the model with the U.S. National Oceanic and Atmospheric Administration.

Beggan said the moving pole affected navigation, mainly in the Arctic Ocean north of Canada. NATO and the U.S. and British militaries are among those using the magnetic model, as well as civilian navigation.

The wandering pole is driven by unpredictable changes in liquid iron deep inside the Earth. An update will be released on January 30, the journal Nature said, delayed from January 15 because of the U.S. government shutdown.

“The fact that the pole is going fast makes this region more prone to large errors,” Arnaud Chulliat, a geomagnetist at the University of Colorado Boulder and NOAA’s National Centers for Environmental Information, told Nature.

Beggan said the recent shifts in the north magnetic pole would be unnoticed by most people outside the Arctic, for instance using smartphones in New York, Beijing or London.

Many smartphones have inbuilt compasses to help to orientate maps or games such as Pokemon Go. In most places, however, the compass would be pointing only fractionally wrong, within errors allowed in the five-year models, Beggan said.

Enormous Earthquake Reveals Hidden ‘Mountains’ 410 Miles Underground That Could Be Bigger Than Any On Earth’s Surface

In 1994, a huge 8.2-magnitude earthquake struck a sparsely populated region in Bolivia at a depth of around 400 miles below sea level. Now, an international team of scientists has analyzed data from this event to uncover previously unidentified “mountains” deep within Earth’s interior.

Most of us were taught in school that Earth is divided into different layers: an inner and outer core, the mantle and the crust. But this simplifies the picture slightly because, according to scientists, there is another layer called the “transition zone,” which splits the mantle in two.

For a study published in the journal Science, the team from Princeton University wanted to determine the roughness of the transition zones at the top and bottom—which lie at depths of 410 kilometers (255 miles) and 660 kilometers (410 miles) respectively. (The bottom of the transition zone is often referred to as the “660-km boundary.”)

To do this, the team had to look deep into Earth’s interior. But since we aren’t able to physically see below the surface, the scientists analyzed the behavior of shockwaves created by earthquakes as they scatter inside our planet to create a picture of what’s going on beneath the surface.

When it comes to this technique, the more powerful the earthquake the better, because stronger shockwaves can travel farther, hence why the team chose to examine the 1994 Bolivia event—the second largest deep quake ever recorded. In fact, shockwaves from quakes with a magnitude of 7.0 or higher are so powerful, that they can travel from one side of the planet to the other and back again.

“You want a big, deep earthquake to get the whole planet to shake,” Jessica Irving, an author of the study from Princeton, said in a statement. “Earthquakes this big don’t come along very often.”

Using Princeton’s Tiger supercomputer, the team examined shockwave data to determine what the top and bottom of the transition zone may look like. This technique works in a similar way to how our eyes enable us to see objects in the environment by detecting scattering light waves.

“We know that almost all objects have surface roughness and therefore scatter light,” said lead author of the study Wenbo Wu, from Princeton. “That’s why we can see these objects—the scattering waves carry the information about the surface’s roughness. In this study, we investigated scattered seismic waves traveling inside Earth to constrain the roughness of Earth’s 660-km boundary.”

Their results show that while the top of the transition zone is mostly smooth, the bottom is very rough in some places, such as the mountainous terrain on Earth’s surface.

“In other words, stronger topography than the Rocky Mountains or the Appalachians is present at the 660-km boundary,” Wu said.

While the scientists could not conduct precise measurements of the height of this terrain, they suggest that these mountains could potentially be bigger than anything similar on Earth’s surface.

Satellite Images Reveal Interconnected Plumbing System That Caused Bali Volcano To Erupt

A team of scientists, led by the University of Bristol, has used satellite technology provided by the European Space Agency (ESA) to uncover why the Agung volcano in Bali erupted in November 2017 after 50 years of dormancy.

Their findings, published today in the journal Nature Communications, could have important implications for forecasting future eruptions in the area.

Two months prior to the eruption, there was a sudden increase in the number of small earthquakes occurring around the volcano, triggering the evacuation of 100,000 people.

The previous eruption of Agung in 1963 killed nearly 2,000 people and was followed by a small eruption at its neighboring volcano, Batur.

Because this past event was among the deadliest volcanic eruptions of the 20th Century, a great effort was deployed by the scientific community to monitor and understand the re-awakening of Agung.

During this time, a team of scientists from the University of Bristol’s School of Earth Sciences, led by Dr Juliet Biggs used Sentinel-1 satellite imagery provided by the ESA to monitor the ground deformation at Agung.

Dr Biggs said: “From remote sensing, we are able to map out any ground motion, which may be an indicator that fresh magma is moving beneath the volcano.”

In the new study, carried out in collaboration with the Center for Volcanology and Geological Hazard Mitigation in Indonesia (CVGHM), the team detected uplift of about 8-10 cm on the northern flank of the volcano during the period of intense earthquake activity.

Dr Fabien Albino, also from Bristol’s School of Earth Sciences, added: “Surprisingly, we noticed that both the earthquake activity and the ground deformation signal were located five kilometres away from the summit, which means that magma must be moving sideways as well as vertically upwards.

“Our study provides the first geophysical evidence that Agung and Batur volcanoes may have a connected plumbing system.

“This has important implications for eruption forecasting and could explain the occurrence of simultaneous eruptions such as in 1963.”

Earth’s Magnetic Shield Booms Like A Drum When Hit By Impulses

The Earth’s magnetic shield booms like a drum when it is hit by strong impulses, according to new research from Queen Mary University of London.

As an impulse strikes the outer boundary of the shield, known as the magnetopause, ripples travel along its surface which then get reflected back when they approach the magnetic poles.

The interference of the original and reflected waves leads to a standing wave pattern, in which specific points appear to be standing still while others vibrate back and forth. A drum resonates like this when struck in exactly the same way.

This study, published in Nature Communications, describes the first time this effect has been observed after it was theoretically proposed 45 years ago.

Movements of the magnetopause are important in controlling the flow of energy within our space environment with wide-ranging effects on space weather, which is how phenomena from space can potentially damage technology like power grids, GPS and even passenger airlines.

The discovery that the boundary moves in this way sheds light on potential global consequences that previously had not been considered.

Dr Martin Archer, space physicist at Queen Mary University of London, and lead author of the paper, said: “There had been speculation that these drum-like vibrations might not occur at all, given the lack of evidence over the 45 years since they were proposed. Another possibility was that they are just very hard to definitively detect.

“Earth’s magnetic shield is continuously buffeted with turbulence so we thought that clear evidence for the proposed booming vibrations might require a single sharp hit from an impulse. You would also need lots of satellites in just the right places during this event so that other known sounds or resonances could be ruled out. The event in the paper ticked all those quite strict boxes and at last we’ve shown the boundary’s natural response.”

The researchers used observations from five NASA THEMIS satellites when they were ideally located as a strong isolated plasma jet slammed into the magnetopause. The probes were able to detect the boundary’s oscillations and the resulting sounds within the Earth’s magnetic shield, which agreed with the theory and gave the researchers the ability to rule out all other possible explanations.

Many impulses which can impact our magnetic shield originate from the solar wind, charged particles in the form of plasma that continually blow off the Sun, or are a result of the complicated interaction of the solar wind with Earth’s magnetic field, as was technically the case for this event.

The interplay of Earth’s magnetic field with the solar wind forms a magnetic shield around the planet, bounded by the magnetopause, which protects us from much of the radiation present in space.

Other planets like Mercury, Jupiter and Saturn also have similar magnetic shields and so the same drum-like vibrations may be possible elsewhere.

Further research is needed to understand how often the vibrations occur at Earth and whether they exist at other planets as well. Their consequences also need further study using satellite and ground-based observations.

NASA Finds Possible Second Impact Crater Under Greenland Ice

A NASA glaciologist has discovered a possible second impact crater buried under more than a mile of ice in northwest Greenland.

his follows the finding, announced in November 2018, of a 19-mile-wide crater beneath Hiawatha Glacier — the first meteorite impact crater ever discovered under Earth’s ice sheets. Though the newly found impact sites in northwest Greenland are only 114 miles apart, at present they do not appear to have formed at the same time.

If the second crater, which has a width of over 22 miles, is ultimately confirmed as the result of a meteorite impact, it will be the 22nd largest impact crater found on Earth.

“We’ve surveyed the Earth in many different ways, from land, air and space — it’s exciting that discoveries like these are still possible,” said Joe MacGregor, a glaciologist with NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who participated in both findings.

Before the discovery of the Hiawatha impact crater, scientists generally assumed that most evidence of past impacts in Greenland and Antarctica would have been wiped away by unrelenting erosion by the overlying ice. Following the finding of that first crater, MacGregor checked topographic maps of the rock beneath Greenland’s ice for signs of other craters. Using imagery of the ice surface from the Moderate Resolution Imaging Spectroradiometer instruments aboard NASA’s Terra and Aqua satellites, he soon noticed a circular pattern some 114 miles to the southeast of Hiawatha Glacier. The same circular pattern also showed up in ArcticDEM, a high-resolution digital elevation model of the entire Arctic derived from commercial satellite imagery.

“I began asking myself ‘Is this another impact crater? Do the underlying data support that idea?’,” MacGregor said. “Helping identify one large impact crater beneath the ice was already very exciting, but now it looked like there could be two of them.”

MacGregor reported the discovery of this second possible crater in Geophysical Research Letters on Feb.11.

To confirm his suspicion about the possible presence of a second impact crater, MacGregor studied the raw radar images that are used to map the topography of the bedrock beneath the ice, including those collected by NASA’s Operation IceBridge. What he saw under the ice were several distinctive features of a complex impact crater: a flat, bowl-shaped depression in the bedrock that was surrounded by an elevated rim and centrally located peaks, which form when the crater floor equilibrates post-impact. Though the structure isn’t as clearly circular as the Hiawatha crater, MacGregor estimated the second crater’s diameter at 22.7 miles. Measurements from Operation IceBridge also revealed a negative gravity anomaly over the area, which is characteristic of impact craters.

“The only other circular structure that might approach this size would be a collapsed volcanic caldera,” MacGregor said. “But the areas of known volcanic activity in Greenland are several hundred miles away. Also, a volcano should have a clear positive magnetic anomaly, and we don’t see that at all.”

Although the newly found impact craters in northwest Greenland are only 114 miles apart, they do not appear to have been formed at the same time. From the same radar data and ice cores that had been collected nearby, MacGregor and his colleagues determined that the ice in the area was at least 79,000 years old. The layers of ice were smooth, suggesting the ice hadn’t been strongly disturbed during that time. This meant that either the impact happened more than 79,000 years ago or — if it took place more recently — any impact-disturbed ice had long ago flowed out of the area and been replaced by ice from farther inland.

The researchers then looked at rates of erosion: they calculated that a crater of that size would have initially been more half a mile deep between its rim and floor, which is an order of magnitude greater than its present depth. Taking into account a range of plausible erosion rates, they calculated that it would have taken anywhere between roughly a hundred thousand years and a hundred million years for the ice to erode the crater to its current shape — the faster the erosion rate, the younger the crater would be within the plausible range, and vice versa.

“The ice layers above this second crater are unambiguously older than those above Hiawatha, and the second crater is about twice as eroded,” MacGregor said. “If the two did form at the same time, then likely thicker ice above the second crater would have equilibrated with the crater much faster than for Hiawatha.”

To calculate the statistical likelihood that the two craters were created by unrelated impact events, MacGregor’s team used recently published estimates that leverage lunar impact rates to better understand Earth’s harder-to-detect impact record. By employing computer models that can track the production of large craters on Earth, they found that the abundance of said craters that should naturally form close to one another, without the need for a twin impact, was consistent with Earth’s cratering record.

“This does not rule out the possibility that the two new Greenland craters were made in a single event, such as the impact of a well separated binary asteroid, but we cannot make a case for it either,” said William Bottke, a planetary scientist with the Southwest Research Institute in Boulder, Colorado, and co-author of both MacGregor’s paper and the new lunar impact record study.

Indeed, two pairs of unrelated but geographically close craters have already been found in Ukraine and Canada, but the ages of the craters in the pairs are different from one another.

“The existence of a third pair of unrelated craters is modestly surprising but we don’t consider it unlikely,” MacGregor said. “On the whole, the evidence we’ve assembled indicates that this new structure is very likely an impact crater, but presently it looks unlikely to be a twin with Hiawatha.”

Innovative Method Enables New View Into Earth’s Interior

An innovative X-ray method enables new high-pressure investigations of samples under deep mantle conditions. The technique, which was developed by a team led by Georg Spiekermann from DESY, the German Research Centre for Geosciences GFZ and the University of Potsdam, extends the range of instruments available to high-pressure researchers. Successful tests of the new method at DESY’s X-ray light source PETRA III support the idea that heavy elements have to accumulate in magmas so that they could be stable at depths of Earth’s lower mantle. The scientists present their work in the journal Physical Review X.

The so-called standard conditions of chemistry, i.e. a temperature of 25 degrees Celsius and a pressure of 1013 millibar, are actually rare in nature. Most of the matter in the universe exists under completely different conditions. In Earth’s interior, for example, pressure and temperature rise rapidly to many times the standard conditions. “However, even with the most elaborate deep drilling, only the uppermost part of the Earth’s crust is accessible,” Spiekermann emphasises. Researchers therefore simulate the conditions of Earth’s interior in the laboratory in order to investigate the behaviour of matter under these conditions.

Such experiments often involve determining the inner structure of the samples, which in many materials changes with increasing pressure. This inner structure can be explored with X-rays that are energetic enough to penetrate the sample and short enough in wavelength to resolve the tiny details of atomic distances. For this purpose, usually two X-ray based methods exist in high-pressure research: absorption and diffraction of X-rays through the sample.

Based on X-ray emission, Spiekermann and his team have now developed a third method that can be used to determine both the bonding distances in compressed amorphous (disordered) matter and the so-called coordination number, which indicates how many direct neighbours an atom has. These parameters can be read from the energy and intensity of the radiation of a certain emission line of the sample, called Kβ” (“K-beta-doubleprime”). The Kβ” radiation is generated when the sample is excited with X-rays. The energy of the emission line depends on the coordination number, the intensity on the bonding distance.

Experiments at the experimental station P01 at DESY’s X-ray source PETRA III have confirmed the new method. “We have shown this, using the spectrum of germanium in compressed amorphous germanium dioxide, but this procedure can also be applied to other chemical systems,” says Spiekermann.

The method will provide scientists with an additional technique for investigating the structure of high-pressure samples. “The insight provided by a new measuring method is particularly welcome when different methods have so far produced significantly different results so far, as in the case of compressed amorphous germanium dioxide,” explains DESY researcher Hans-Christian Wille, head of the measuring station P01 at which the experiments took place.

For their experiments, the researchers exposed samples of germanium dioxide (GeO2) to a pressure of up to 100 gigapascals, about one million times as much as the atmospheric pressure at sea level. This pressure corresponds to a depth of 2200 kilometres in the lower mantle of Earth. The measurements show that the coordination number of germanium dioxide does not rise higher than six even under this extreme pressure. This means that even in the high-pressure phase, the germanium atoms each still have six neighbouring atoms as already at 15 gigapascals.

This result is of great interest for the exploration of Earth’s interior, because germanium dioxide has the same structure and behaves like silicon dioxide (SiO2), which is the main component of natural magmas in general. Since melts such as magma generally have a lower density than the solid form of the same material, it has long been a mystery why magmas at great depth do not rise towards the surface over geological periods.

“There are two possible explanations for this, one chemical, the other structural,” Spiekermann explains. “Either heavy elements such as iron accumulate in the melt, or there is a special compacting mechanism in melts that makes melts denser than crystalline forms of the same composition.” The latter would be noticeable, among other things, by an increase in the coordination number under high pressure.

“Our investigations show that up to 100 gigapascals the coordination number in non-crystalline germanium dioxide is not higher than in the corresponding crystalline form,” reports the researcher. Applied to silicon dioxide, this means that magma with a higher density can only be produced by enriching relatively heavy elements such as iron. The composition and structure of the lower mantle have far-reaching consequences for the global transport of heat and the propagation of Earth’s magnetic field.