Month: October 2017

Seeing Double: Scientists Find Elusive Giant Black Hole Pairs

Astronomers have identified a bumper crop of dual supermassive black holes in the centers of galaxies. This discovery could help astronomers better understand how giant black holes grow and how they may produce the strongest gravitational wave signals in the Universe.

The new evidence reveals five pairs of supermassive black holes, each containing millions of times the mass of the Sun. These black hole couples formed when two galaxies collided and merged with each other, forcing their supermassive black holes close together.

The black hole pairs were uncovered by combining data from a suite of different observatories including NASA’s Chandra X-ray Observatory, the Wide-Field Infrared Sky Explorer Survey (WISE), and the ground-based Large Binocular Telescope in Arizona.

“Astronomers find single supermassive black holes all over the universe,” said Shobita Satyapal, from George Mason University in Fairfax, Virginia, who led one of two papers describing these results. “But even though we’ve predicted they grow rapidly when they are interacting, growing dual supermassive black holes have been difficult to find.”

Before this study fewer than ten confirmed pairs of growing black holes were known from X-ray studies, based mostly on chance detections. To carry out a systematic search, the team had to carefully sift through data from telescopes that detect different wavelengths of light.

Starting with the Galaxy Zoo project, researchers used optical data from the Sloan Digital Sky Survey (SDSS) to identify galaxies where it appeared that a merger between two smaller galaxies was underway. From this set, they selected objects where the separation between the centers of the two galaxies in the SDSS data is less than 30,000 light years, and the infrared colors from WISE data match those predicted for a rapidly growing supermassive black hole.

Seven merging systems containing at least one supermassive black hole were found with this technique. Because strong X-ray emission is a hallmark of growing supermassive black holes, Satyapal and her colleagues then observed these systems with Chandra. Closely-separated pairs of X-ray sources were found in five systems, providing compelling evidence that they contain two growing (or feeding) supermassive black holes.

Both the X-ray data from Chandra and the infrared observations suggest that the supermassive black holes are buried in large amounts of dust and gas.

“Our work shows that combining the infrared selection with X-ray follow-up is a very effective way to find these black hole pairs,” said Sara Ellison of the University of Victoria in Canada, who led the other paper describing these results. “X-rays and infrared radiation are able to penetrate the obscuring clouds of gas and dust surrounding these black hole pairs, and Chandra’s sharp vision is needed to separate them”.

The paper led by Ellison used additional optical data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey to pinpoint one of the new black hole pairs. One member of this black hole pair is particularly powerful, having the highest X-ray luminosity in a black hole pair observed by Chandra to date.

This work has implications for the burgeoning field of gravitational wave astrophysics. While scientists using the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the VIRGO interferometer have detected the signals of merging black holes, these black holes have been of the smaller variety weighing between about eight and 36 times the mass of the Sun.

The merging black holes in the centers of galaxies are much larger. When these supermassive black holes draw even closer together, they should start producing gravitational waves. The eventual merger of the dual supermassive black holes in hundreds of millions of years would forge an even bigger black hole. This process would produce an astonishing amount of energy when some of the mass is converted into gravitational waves.
“It is important to understand how common supermassive black hole pairs are, to help in predicting the signals for gravitational wave observatories,” said Satyapal. “With experiments already in place and future ones coming online, this is an exciting time to be researching merging black holes. We are in the early stages of a new era in exploring the universe.”

LIGO/VIRGO is not able to detect gravitational waves from supermassive black hole pairs. Instead, pulsar timing arrays such as the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) are currently performing this search. In the future, the Laser Interferometer Space Antenna (LISA) project could also search for these gravitational waves.

Four of the dual black hole candidates were reported in a paper by Satyapal et al. that was recently accepted for publication in The Astrophysical Journal, and appears online. The other dual black hole candidate was reported in a paper by Ellison et al., which was published in the September 2017 issue of the Monthly Notices of the Royal Astronomical Society and appears online.

Study Suggests Hydrogen, Oxygen, Water And Carbon Dioxide Generated In Earth’s Mantle

Research from the University of Texas at Arlington and the Wadia Institute of Himalayan Geology suggests that hydrogen, oxygen, water and carbon dioxide are being generated in the earth’s mantle hundreds of kilometers below the earth’s surface.

“This discovery is important as it shows how earth’s planetary evolution may have happened,” said Asish Basu, UTA professor of earth and environmental sciences and co-author of the cover paper published in Geology in August.

The researchers focused their attention on a seven-kilometer thick portion of the earth’s upper mantle now found in the High Himalayas, at altitudes between 12,000 and 16,000 feet. This section of the mantle was pushed upwards to the top of the mountains as a result of the Indian Plate pushing north into Asia, displacing the ancient Tethys ocean floor and underlying mantle to create the Himalayan Mountain Belt around 55 million years ago.

“This is important as it means that we can analyze the nature of the mantle under the earth’s crust, at depths where drilling cannot reach,” Basu explained. “One key initial discovery was finding microdiamonds whose host rocks originated in the mantle transition zone, at depths between 410 and 660 kilometers below the earth’s surface.”

By studying the host rocks and associated minerals, the scientists had a unique opportunity to probe the nature of the deep mantle. They found primary hydrocarbon and hydrogen fluid inclusions along with microdiamonds by using Laser Raman Spectroscopic study. The discovery also showed that the environment in the deep mantle transition zone depths where the diamond is formed is devoid of oxygen.

The researchers suggest that during the advective transport or mantle up-welling into shallower mantle zones, the hydrocarbon fluids become oxidized and precipitate diamond, a mechanism that may also be responsible for forming larger diamonds like the world’s most valuable, Koh-i-Noor or Mountain of Light diamond, now in the Queen of England’s crown.

“We also found that the deep mantle upwelling can oxidize oxygen-impoverished fluids to produce water and carbon dioxide that are well-known to produce deep mantle melting,” said Souvik Das, UTA post-doctoral research scholar.

“This means that many of the key compounds affecting evolution like carbon dioxide and water are generated within the mantle,” he added.

Explosive Bursts Of Methane Helped Ancient Mars Keep Liquid Water Flowing, Study Finds

In a drying time, Mars may have been kept warm enough for liquid water to remain stable on the surface thanks to explosive bursts of methane gas, a new study finds.

The simulations, described in the journal Nature Geoscience, could explain how Mars managed to sustain a series of lakes in a climate that at first glance seems too cold and arid to have done so.

Since landing on the Red Planet in August 2012, NASA’s rover Curiosity has discovered that 96-mile-wide Gale Crater held a series of lakes around 3.5 billion years ago. The rocks it has drilled and X-rayed and lasered have also revealed environments that would have been potentially habitable for Earth-like life.

Keep in mind, however, that Mars’ wettest period was likely the first billion years of its 4.6 billion-year life, the Noachian period, when it had a thicker atmosphere that would have been better able to keep liquid water stable on the planet’s surface.

“Although the climate was relatively cold compared to Earth, there is evidence that liquid water flowed in streams and rivers, formed alluvial fans and deltas, and ponded in big lakes and possibly seas,” Alberto Fairen of the Centro de Astrobiologia in Spain and Cornell University, who was not involved in the paper, wrote in a commentary.

Then came the 600 million-year Hesperian period, when the Red Planet began to transform from a cold, wet world to a cold, icy one, as the protective atmosphere thinned and the planet’s interior cooled. The next 3 billion years until now are known as the Amazonian period, during which Mars solidified its reputation as the cold, dry planet we see today.

So here’s the thing that’s puzzled planetary scientists: Gale Crater’s rocks bear evidence of liquid water on Mars during the Hesperian period, including lakes (perhaps protected by a layer of surface ice) and deltas. But that means these lakes and deltas persisted during a period that was markedly drier, with a thinner atmosphere less capable of sustaining liquid water. How do these two facts square up?

“Previous hypotheses have struggled to explain lake-forming climates that are both rare and long-lasting,” Fairen wrote. “For example, volcanism and impacts can produce episodes of climate warming, but not of sufficiently long duration.”

Now, lead author Edwin Kite, a planetary scientist the University of Chicago, and his colleagues say that after running climate models they’ve come up with an explanation: explosive bursts of methane.

Here’s how it works. The Red Planet’s obliquity, or tilt on its axis, can vary far more dramatically than Earth’s does. The researchers think that occasional dramatic shifts in that tilt (perhaps around 10 to 20 degrees) would have exposed ice-covered parts of the Martian surface to the sun, causing that cover to shrink fairly quickly. The ice’s retreat would have exposed clathrates filled with pockets of methane, allowing the methane to burst out of the ground and into the atmosphere.

Methane is a powerful greenhouse gas – about 25 times as powerful as carbon dioxide. So if enough of it were to emerge from the ground at the same time, it could actually result in a significant amount of warming, the thinking goes.

Now, eventually, methane gets broken down by sunlight. But in the meantime, Kite and his colleagues found that it could lead to warming lasting hundreds of thousands of years – long enough to explain the extended presence of liquid water during this otherwise dry time in Martian history, scientists say.

What does this mean for the possibility that life could have emerged on Mars? Kite was quick to point out that if any microbes ever lived on the Red Planet, they would likely have done so during its earliest days, when water was far more abundant.

“If life ever established itself on Mars, then it would have probably done so before the relatively young (less than 3.6 billion years ago) features modeled in our paper,” Kite wrote in an email.

Regardless, the study reveals an increasingly complex portrait of a planet in transition, scientists said.

“The methane burst scenario proposed by Kite et al. contributes to an emerging view that the existence of liquid water on early Mars arose from a combination of diverse astronomical, geochemical and geological factors,” Fairen wrote. “Although it seems unlikely that a single mechanism can explain not only the presence of liquid water, but its recurrence and persistence, the methane burst hypothesis provides a means to episodically tip the Hesperian climate over the edge.”

Siberian Volcanic Eruptions Caused Extinction 250 Million Years Ago, New Evidence Shows

A team of scientists has found new evidence that the Great Permian Extinction, which occurred approximately 250 million years ago, was caused by massive volcanic eruptions that led to significant environmental changes.

The study, which appears in the journal Scientific Reports, reports a global spike in the chemical element nickel at the time of extinction. The anomalous nickel most likely came from emanations related to the concurrent huge volcanic eruptions in what is now Siberia. These eruptions, the researchers say, are associated with nickel-rich magmatic intrusions — rocks formed from the cooling of magma — that contain some of the greatest deposits of nickel ore on the planet.

Using an Inductively Coupled Plasma Mass Spectrometer, which measures the abundance of rare elements at their atomic level, the scientists documented anomalous peaks of nickel in regions ranging from the Arctic to India at the time of the Great Permian Extinction — distributions that suggest these nickel anomalies were a worldwide phenomenon.

This new evidence of a nickel fingerprint at the time of the extinctions convinced the scientists that it was the volcanic upheaval in Siberia that produced intense global warming and other environmental changes that led to the disappearance of more than 90 percent of all species.

“The Siberian volcanic eruptions and related massive intrusions of nickel-rich magmas into Earth’s crust apparently emitted nickel-rich volatiles into the atmosphere, where they were distributed globally,” explains New York University geologist Michael Rampino, the paper’s senior author. “At the same time, explosive interactions of the magma with older coal deposits could have released large amounts of carbon dioxide and methane, two greenhouse gases, which would explain the intense global warming recorded in the oceans and on land at the time of the mass extinctions. The warm oceans also became sluggish and depleted in dissolved oxygen, contributing to the extinction of many forms of life in the sea.”

“This new finding, which contributes further evidence that the Siberian Trap eruptions were the catalyst for the most extensive extinction event Earth has ever endured, has exciting implications,” says Sedelia Rodriguez, a co-author of the paper and lecturer in the department of Environmental Science at Barnard College. “We look forward to expanding our research on nickel and other elements to delineate the specific areas affected by this eruption. In doing so, we hope to learn more about how these events trigger massive extinctions that affect both land and marine animals. Additionally, we hope this research will contribute to determining whether an event of this magnitude is possible in the future.”

Large Volcanic Eruptions In Tropics Can Trigger El Niño Events

Explosive volcanic eruptions in the tropics can lead to El Niño events, those notorious warming periods in the Pacific Ocean with dramatic global impacts on the climate, according to a new study.

Enormous eruptions trigger El Niño events by pumping millions of tons of sulfur dioxide into the stratosphere, which form a sulfuric acid cloud, reflecting solar radiation and reducing the average global surface temperature, according to the study co-authored by Alan Robock, a distinguished professor in the Department of Environmental Sciences at Rutgers University-New Brunswick.

The study, published online today in Nature Communications, used sophisticated climate model simulations to show that El Niño tends to peak during the year after large volcanic eruptions like the one at Mount Pinatubo in the Philippines in 1991.

“We can’t predict volcanic eruptions, but when the next one happens, we’ll be able to do a much better job predicting the next several seasons, and before Pinatubo we really had no idea,” said Robock, who has a doctorate in meteorology. “All we need is one number — how much sulfur dioxide goes into the stratosphere — and you can measure it with satellites the day after an eruption.”

The El Niño Southern Oscillation (ENSO) is nature’s leading mode of periodic climate variability. It features sea surface temperature anomalies in the central and eastern Pacific. ENSO events (consisting of El Niño or La Niña, a cooling period) unfold every three to seven years and usually peak at the end of the calendar year, causing worldwide impacts on the climate by altering atmospheric circulation, the study notes.

Strong El Niño events and wind shear typically suppress the development of hurricanes in the Atlantic Ocean, the National Oceanic and Atmospheric Administration says. But they can also lead to elevated sea levels and potentially damaging cold season nor’easters along the East Coast, among many other impacts.

Sea surface temperature data since 1882 document large El Niño-like patterns following four out of five big eruptions: Santa María (Guatemala) in October 1902, Mount Agung (Indonesia) in March 1963, El Chichón (Mexico) in April 1982 and Pinatubo in June 1991.

The study focused on the Mount Pinatubo eruption because it’s the largest and best-documented tropical one in the modern technology period. It ejected about 20 million tons of sulfur dioxide, Robock said.

Cooling in tropical Africa after volcanic eruptions weakens the West African monsoon, and drives westerly wind anomalies near the equator over the western Pacific, the study says. The anomalies are amplified by air-sea interactions in the Pacific, favoring an El Niño-like response.

Climate model simulations show that Pinatubo-like eruptions tend to shorten La Niñas, lengthen El Niños and lead to unusual warming during neutral periods, the study says.

If there’s a big volcanic eruption tomorrow, Robock said he could make predictions for seasonal temperatures, precipitation and the appearance of El Niño next winter.

“If you’re a farmer and you’re in a part of the world where El Niño or the lack of one determines how much rainfall you will get, you could make plans ahead of time for what crops to grow, based on the prediction for precipitation,” he said.

Earth’s Tectonic Plates Are Weaker Than OnceThought

No one can travel inside Earth to study what happens there. So scientists must do their best to replicate real-world conditions inside the lab.

“We are interested in large-scale geophysical processes, like how plate tectonics initiates and how plates move underneath one another in subduction zones,” said David Goldsby, an associate professor at the University of Pennsylvania. “To do that, we need to understand the mechanical behavior of olivine, which is the most common mineral in the upper mantle of Earth.”

Goldsby, teaming with Christopher A. Thom, a doctoral student at Penn, as well as researchers from Stanford University, the University of Oxford and the University of Delaware, has now resolved a long-standing question in this area of research. While previous laboratory experiments resulted in widely disparate estimates of the strength of olivine in Earth’s lithospheric mantle, the relatively cold and therefore strong part of Earth’s uppermost mantle, the new work, published in the journal Science Advances, resolves the previous disparities by finding that, the smaller the grain size of the olivine being tested, the stronger it is.

Because olivine in Earth’s mantle has a larger grain size than most olivine samples tested in labs, the results suggest that the mantle, which comprises up to 95 percent of the planet’s tectonic plates, is in fact weaker than once believed. This more realistic picture of the interior may help researchers understand how tectonic plates form, how they deform when loaded with the weight of, for example, a volcanic island such as Hawaii, or even how earthquakes begin and propagate.

For more than 40 years, researchers have attempted to predict the strength of olivine in Earth’s lithospheric mantle from the results of laboratory experiments. But tests in a lab are many layers removed from the conditions inside Earth, where pressures are higher and deformation rates are much slower than in the lab. A further complication is that, at the relatively low temperatures of earth’s lithosphere, the strength of olivine is so high that it is difficult to measure its plastic strength without fracturing the sample. The results of existing experiments have varied widely, and they don’t align with predictions of olivine strength from geophysical models and observations.

In an attempt to resolve these discrepancies, the researchers employed a technique known as nanoindentation, which is used to measure the hardness of materials. Put simply, the researchers measure the hardness of a material, which is related to its strength, by applying a known load to a diamond indenter tip in contact with a mineral and then measuring how much the mineral deforms. While previous studies have employed various high-pressure deformation apparatuses to hold samples together and prevent them from fracturing, a complicated set-up that makes measurements of strength challenging, nanoindentation does not require such a complex apparatus.

“With nanoindentation,” Goldsby said, “the sample in effect becomes its own pressure vessel. The hydrostatic pressure beneath the indenter tip keeps the sample confined when you press the tip into the sample’s surface, allowing the sample to deform plastically without fracture, even at room temperature.”

Performing 800 nanoindentation experiments in which they varied the size of the indentation by varying the load applied to the diamond tip pressed into the sample, the research team found that the smaller the size of the indent, the harder, and thus stronger, olivine became.

“This indentation size effect had been seen in many other materials, but we think this is the first time it’s been shown in a geological material,” Goldsby said.

Looking back at previously collected strength data for olivine, the researchers determined that the discrepancies in those data could be explained by invoking a related size effect, whereby the strength of olivine increases with decreasing grain size of the tested samples. When these previous strength data were plotted against the grain size in each study, all the data fit on a smooth trend which predicts lower-than-thought strengths in Earth’s lithospheric mantle.

In a related paper by Thom, Goldsby and colleagues, published recently in the journal Geophysical Research Letters, the researchers examined patterns of roughness in faults that have become exposed at Earth’s surface due to uplifted plates and erosion.

“Different faults have a similar roughness, and there’s an idea published recently that says you might get those patterns because the strength of the materials on the fault surface increases with the decreasing scale of roughness,” Thom said. “Those patterns and the frictional behavior they cause might be able to tell us something about how earthquakes nucleate and how they propagate.”

In future work, the Penn researchers and their team would like to study size-strength effects in other minerals and also to focus on the effect of increasing temperature on size effects in olivine.

Meteorite Tells Us That Mars Had A Dense Atmosphere 4 Billion Years Ago

Exploration missions have suggested that Mars once had a warm climate, which sustained oceans on its surface. To keep Mars warm requires a dense atmosphere with a sufficient greenhouse effect, while the present-day Mars has a thin atmosphere whose surface pressure is only 0.006 bar, resulting in the cold climate it has today. It has been a big mystery as to when and how Mars lost its dense atmosphere.

An old meteorite has been known to contain the ancient Martian atmosphere. The researchers simulated how the composition of the Martian atmosphere changed throughout history under various conditions. By comparing the results to the isotopic composition of the trapped gas, the researchers revealed how dense the Martian atmosphere was at the time when the gas became trapped in the meteorite.

The research team concluded that Mars had a dense atmosphere 4 billion years ago. The surface air pressure at the time was at least 0.5 bar and could have been much higher. Because Mars had its magnetic field about 4 billion years ago and lost it, the result suggests that stripping by the solar wind is responsible for transforming Mars from a warm wet world into a cold desert world.

NASA’s MAVEN spacecraft is orbiting Mars to explore the processes that removed the Martian atmosphere. The Japan Aerospace Exploration Agency (JAXA) is planning to further observe the removal processes by the Martian Moons eXploration (MMX) spacecraft. These missions will reveal how the dense atmosphere on ancient Mars predicted in this study was removed over time.