Mapping Dark Energy – Accelerating the Expansion of the Universe

On 21 June 2019 the Spektrum-Röntgen-Gamma (Spektr-RG / SRG) spacecraft will be launched from the Kazakh steppe, marking the start of an exciting journey. SRG will be carrying the German Extended ROentgen Survey with an Imaging Telescope Array (eROSITA) X-ray telescope and its Russian ART-XC partner instrument. A Proton rocket will carry the spacecraft from the Baikonur Cosmodrome towards its destination – the second Lagrange point of the Sun-Earth system, L2, which is 1.5 million kilometers from Earth.

In orbit around this equilibrium point, eROSITA will embark upon the largest-ever survey of the hot universe. The space telescope will use its seven X-ray detectors to observe the entire sky and search for and map hot sources such as galaxy clusters, active black holes, supernova remnants, X-ray binaries and neutron stars.

Walther Pelzer, executive board member for the Space Administration at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), says, “eROSITA’s X-ray ‘eyes’ are the best that have ever been launched as part of a space telescope. Their unique combination of light-collecting area, field-of-view and resolution makes them approximately 20 times more sensitive than the ROSAT telescope that flew to space in the 1990s. ROSAT also incorporated advanced technology that was ‘made in Germany’. With its enhanced capabilities, eROSITA will help researchers gain a better understanding of the structure and development of the universe, and also contribute towards investigations into the mystery of dark energy.”

The universe has been expanding continuously since the Big Bang. Until the 1990s, it was thought that this cosmic expansion would slow down and eventually come to a halt. Then, the astrophysicists Saul Perlmutter, Adam Riess and Brian Schmidt observed stellar explosions that were visible from a great distance and always emitted the same amount of light. They measured their distances and could hardly believe their findings.

“The Type 1a supernovae observed exhibited lower brightness levels than expected. It was clear that the universe was not slowing down as it expanded – quite the opposite, in fact. It is gathering speed and its components are being driven further and further apart at an ever-increasing rate,” explains Thomas Mernik, eROSITA Project Manager at the DLR Space Administration. With this discovery, the three researchers turned science upside and were awarded the Nobel Prize in Physics in 2011. Yet Perlmutter, Riess and Schmidt have left us with one crucial question: “What is the ‘cosmic fuel’ that powers the expansion of the universe? Since no one has yet been able to answer this question, and the ingredients of this catalyst are unknown, it is simply referred to as dark energy. eROSITA will now attempt to track down the cause of this acceleration,” explains Mernik.

Galaxy clusters – a key to dark energy

Very little is known about the universe. The ingredients that make up 4 percent of its energy density – ‘normal’ material such as protons and neutrons – is only a very small part of the ‘universe recipe’. What the other 96 percent is composed of remains a mystery. Today it is believed that 26 percent is dark matter. However, the largest share, estimated at 70 percent, is comprised of dark energy.

To track this down, scientists must observe something unimaginably large and extremely hot: “Galaxy clusters are composed of up to several thousand galaxies that move at different velocities within a common gravitational field. Inside, these strange structures are permeated by a thin, extremely hot gas that can be observed through its X-ray emissions. This is where eROSITA’s X-ray ‘eyes’ come into play. They allow us to observe galaxy clusters and see how they move in the universe, and above all, how fast they are travelling. We hope that this motion will tell us more about dark energy,” explains Thomas Mernik.

Scientists are not just interested in the movement patterns of galaxy clusters. They also want to count and map these structures. Up to 10,000 such clusters should be ‘captured’ by eROSITA’s X-ray ‘eyes’ – more than have ever been observed before. In addition, other hot phenomena such as active galactic nuclei, supernova remnants, X-ray binaries and neutron stars will be observed and identified.

eROSITA will scan the entire sky every six months for this purpose, and create a deep and detailed X-ray map of the universe over four years. It will thus produce the largest-ever cosmic catalog of hot objects and thus improve the scientific understanding of the structure and development of the universe.

The German telescope consists of two core components – its optics and the associated detectors. The former consists of seven mirror modules aligned in parallel. Each module has a diameter of 36 centimeters and consists of 54 nested mirror shells, whose surface is composed of a para-boloid and a hyper-boloid (Wolter-I optics).

“The mirror modules collect high-energy photons and focus them onto the CCD X-ray cameras, which were specially developed for eROSITA at our semiconductor laboratory in Garching. These form the second core component of eROSITA and are located at the focus of each of the mirror systems. The highly sensitive cameras are the best of their kind and, together with the mirror modules, form an X-ray telescope featuring an unrivaled combination of light-collecting area and field-of-view,” explains Peter Predehl, eROSITA principal investigator at MPE.

Moving Closer to Understanding of Universe’s Most Powerful Explosions

Good fortune and cutting-edge scientific equipment have allowed scientists to observe a Gamma Ray Burst jet with a radio telescope and detect the polarization of radio waves within it for the first time – moving us closer to an understanding of what causes the universe’s most powerful explosions.

Gamma Ray Bursts (GRBs) are the most energetic explosions in the universe, beaming out mighty jets which travel through space at over 99.9% the speed of light, as a star much more massive than our Sun collapses at the end of its life to produce a black hole. The study was published in Astrophysical Journal Letters.

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Studying the light from Gamma Ray Burst jets as we detect it travelling across space is our best hope of understanding how these powerful jets are formed, but scientists need to be quick to get their telescopes into position and get the best data. The detection of polarized radio waves from a burst’s jet, made possible by a new generation of advanced radio telescopes, offers new clues to this mystery.

The light from this particular event, known as GRB 190114C, which exploded with the force of millions of Suns’ worth of TNT about 4.5 billion years ago, reached NASA’s Neil Gehrels Swift Observatory on Jan 14, 2019.

A rapid alert from Swift allowed the research team to direct the Atacama Large Millimeter/Sub-millimeter Array (ALMA) telescope in Chile to observe the burst just two hours after Swift discovered it. Two hours later the team was able to observe the GRB from the Karl G. Jansky Very Large Array (VLA) telescope when it became visible in New Mexico, USA.

Combining the measurements from these observatories allowed the research team to determine the structure of magnetic fields within the jet itself, which affects how the radio light is polarized. Theories predict different arrangements of magnetic fields within the jet depending on the fields’ origin, so capturing radio data enabled the researchers to test these theories with observations from telescopes for the first time.

The research team, from the University of Bath, Northwestern University, the Open University of Israel, Harvard University, California State University in Sacramento, the Max Planck Institute in Garching, and Liverpool John Moores University discovered that only 0.8% of the jet light was polarized, meaning that jet’s magnetic field was only ordered over relatively small patches – each less than about 1% of the diameter of the jet. Larger patches would have produced more polarized light.

These measurements suggest that magnetic fields may play a less significant structural role in GRB jets than previously thought. This helps us narrow down the possible explanations for what causes and powers these extraordinary explosions.

First author Dr. Tanmoy Laskar, from the University of Bath’s Astrophysics group, said: “We want to understand why some stars produce these extraordinary jets when they die, and the mechanism by which these jets are fuelled – the fastest known outflows in the universe, moving at speeds close to that of light and shining with the incredible luminosity of over a billion Suns combined.

“I was in a cab on my way to O’Hare airport in Chicago, following a visit with collaborators when the burst went off. The extreme brightness of this event and the fact that it was visible in Chile right away made it a prime target for our study, and so I immediately contacted ALMA to say we were going to observe this one, in the hope of detecting the first radio polarization signal.

“It was fortuitous that the target was well placed in the sky for observations with both ALMA in Chile and the VLA in New Mexico. Both facilities responded quickly and the weather was excellent. We then spent two months in a painstaking process to make sure our measurement was genuine and free from instrumental effects. Everything checked out, and that was exciting.

Dr. Kate Alexander, who led the VLA observations, said: “The lower frequency data from the VLA helped confirm that we were seeing the light from the jet itself, rather than from the interaction of the jet with its environment.”

Dr. Laskar added: “This measurement opens a new window into GRB science and the studies of energetic astrophysical jets. We would like to understand whether the low level of polarization measured in this event is characteristic of all GRBs, and if so, what this could tell us about the magnetic structures in GRB jets and the role of magnetic fields in powering jets throughout the universe.”

Professor Carole Mundell, Head of Astrophysics at the University of Bath, added: “The exquisite sensitivity of ALMA and rapid response of the telescopes has, for the first time, allowed us to swiftly and accurately measure the degree of polarization of microwaves from a GRB afterglow just two hours after the blast and probe the magnetic fields that are thought to drive these powerful, ultra-fast outflows.”

The research team plans to hunt for more GRBs to continue to unravel the mysteries of the biggest explosions in the universe.

Scientists Use X-Rays from Faraway Galaxy Cluster to Reveal Secrets of Plasma

Most visible matter in the universe doesn’t look like our textbook picture of a nucleus surrounded by tethered electrons. Out beyond our borders, inside massive clusters, galaxies swim in a sea of plasma – a form of matter in which electrons and nuclei wander unmoored.

Though it makes up the majority of the visible matter in the universe, this plasma remains poorly understood; scientists do not have a theory that fully describes its behavior, especially at small scales.

However, a University of Chicago astrophysicist led a study that provides a brand-new glimpse of the small-scale physics of such plasma. Using NASA’s Chandra X-ray Observatory, scientists took a detailed look at the plasma in a distant galaxy cluster and discovered the flow of plasma is much less viscous than expected and, therefore, turbulence occurs on relatively small scales – an important finding for our numerical models of the largest objects in the universe.

“High-resolution X-ray observations allowed us to learn some surprising truths about the viscosity of these plasmas,” said Irina Zhuravleva, an assistant professor of astrophysics and first author of the study, published June 17 in Nature Astronomy. “One might expect that variations in density that arise in the plasma are quickly erased by viscosity; however, we saw the opposite – the plasma finds ways to maintain them.”

Scattered around the universe are massive clusters of galaxies, some of them millions of light-years across containing thousands of galaxies. They sit in a type of plasma that we cannot recreate on Earth. It is extremely sparse – on the order of a sextillion times less dense than air on Earth – and has very weak magnetic fields, tens of thousands of times weaker than we experience on the Earth’s surface. To study this plasma, therefore, scientists must rely on cosmic laboratories such as clusters of galaxies.

Zhuravleva and the team chose a relatively nearby galaxy cluster called the Coma Cluster, a gigantic, bright cluster made up of more than 1,000 galaxies. They chose a less dense region away from the cluster center, where they hoped to be able to capture the average distance that particles travel between interactions with NASA’s Chandra X-ray Observatory. In order to build a high-quality map of the plasma, they observed the Coma cluster for almost 12 days – much longer than a typical observing run.

One thing that jumped out was how viscous the plasma was – how easily it’s stirred. “One could expect to see the viscosity resisting chaotic motions of plasma as we zoom in to smaller and smaller scales,” Zhuravleva said. But that didn’t happen; the plasma was clearly turbulent even on such small scales.

“It turned out that plasma behavior is more similar to the swirling motions of milk stirred in a coffee mug than the smoother ones that honey makes,” she said.

Such low viscosity means that microscopic processes in plasma cause small irregularities in the magnetic field, causing particles to collide more frequently and making the plasma less viscous. Alternately, Zhuravleva said, viscosity could be different along and perpendicular to magnetic field lines.

Understanding the physics of such plasmas is essential for improving our models of how galaxies and galaxy clusters form and evolve with time.

“It is exciting that we were able to use observations of clusters of galaxies to understand fundamental properties of intergalactic plasmas,” said Zhuravleva. “Our observations confirm that clusters are great laboratories that can sharpen theoretical views on plasmas.”

Other scientists on the study were affiliated with Stanford University, the Max Planck Institute for Astrophysics in Germany, the Space Research Institute in Russia, the University of Oxford, Niels Bohr International Academy, SLAC National Accelerator Laboratory, Harvard-Smithsonian Center for Astrophysics, Masaryk University, Eötvös Loránd University and Hiroshima University.

What Have We Learned from 20 Years of X-rays?

This year marks the 20th anniversary of two landmark missions: the Chandra X-ray Observatory, one of NASA’s Great Observatories, which launched July 23, 1999, and the X-ray Multi-Mirror mission (XMM-Newton), which launched a few months later on December 10th. Together, these satellites revolutionized X-ray astronomy, bringing it on par with astronomy at other wavelengths. We celebrate the history of X-ray astronomy in the August 2019 issue; here, we mark seven discoveries heralded by these two missions.

The Anatomy of a Supernova

To astronomers’ surprise, Chandra’s image of Cassiopeia A, the bloom of gas left over after a massive star went supernova some 340 years ago, revealed a star turned inside out. While massive stars fuse the heaviest elements in their cores and lighter elements in surrounding, onion-like layers, the Cas A explosion had flung clumps of iron to the outermost regions. The find suggests the star’s contents mixed together right before or after its core collapsed (or both).

Stellar Nurseries

XMM-Newton surveyed low-mass stars forming in the Taurus Molecular Cloud while Chandra examined massive stars coming together in the Orion Nebula. Most of the X-rays in these images, including the Chandra Ultra-deep Orion Project pictured above, come from young stars. In some cases interacting stellar winds from massive young stars produce the X-rays. The surveys have given astronomers a wealth of data on the newborn stars’ magnetic fields.

Black Hole Physics

With Chandra and XMM-Newton, astronomers could for the first time estimate black hole spin. By measuring how a black hole’s strong gravity smears the emissions from iron ions, astronomers can see how close the gas comes to the event horizon — the closer it comes, the faster the black hole is spinning. Astronomers have used this and other X-ray-based methods to gauge the spins of dozens of black holes.

Monitoring by Chandra and XMM-Newton has also shed light on the slumbering beast at the center of the Milky Way known as Sgr A*. While Sgr A* doesn’t seem to be devouring gas in the manner of the supermassive black holes that power distant quasars, it’s doing something that sets off roughly daily X-ray flares. Sometimes they’re accompanied by infrared sizzles, but other times the X-rays pop on their own. The flares may originate in snapping magnetic fields, the occasional ingestion of an asteroid, or something else entirely — the jury’s still out.

Jets of Change

The combination of X-ray and radio observations of galaxy clusters solved a long-term mystery: The hot gas between galaxies in clusters ought to cool over time, raining down on the clusters’ central galaxies and forming stars by the handful. But in many clusters astronomers haven’t found the expected stellar newborns. Turns out radio-emitting jets from the central galaxies’ supermassive black holes blow bubbles into the surrounding X-ray-emitting gas, sending out pressure waves that pump heat back into the surrounding medium, which prevents it from cooling. Astronomers soon realized that this concept of “black hole feedback” might affect everything from galaxy evolution to cosmology.

Extragalactic X-ray Background

From the launch of the Aerobee rocket in 1962, astronomers had known that the X-ray sky wasn’t dark, instead teeming with high-energy photons. The Einstein Observatory showed that supermassive black holes, too far away or faint to be seen individually, could explain this background. But it was Chandra that sharpened the view, resolving almost all of the background into its individual sources. Data from Chandra and XMM-Newton suggest that most of the sources that remain undetected are shrouded in gas and dust.

Hot Jupiters and Habitability

X-ray observations have provided direct evidence of star-planet interactions, such as when XMM-Newton caught flares from the HD 17156 system that appeared whenever the hot Jupiter came closest to its star. X-ray data also temper ideas of habitability: XMM-Newton observations revealed that high-energy radiation irradiates the three Earth-size planets in Trappist-1’s so-called habitable zone and has probably long ago stripped them of their atmospheres. Likewise, observations showed that Proxima Centauri b receives 250 times more X-rays from its star than Earth does from the Sun; its habitability, too, is uncertain.

Dark Matter, Dark Energy

Galaxy clusters have proven key to testing dark matter and understanding dark energy. X-ray observations first revealed the wildly hot gas within clusters — gas that would have drifted away if it weren’t for the cluster’s dark matter, which gravitationally holds it in place. Then, astronomers observed clusters dating back to when the universe was less than half its current age, estimating the growth of these huge structures over cosmic time. The result: solid evidence for the existence of dark energy and a unique way to gauge its density and equation of state.

 

Tsunamis Warning Lifted In Japanese Coastal Regions After Earthquake

Japan’s meteorological agency has lifted an earlier tsunami warning, after a 6.8 magnitude earthquake was recorded at 10:22 p.m. Tuesday (9:22 a.m. ET) off the coast of Yamagata Prefecture in the north of the country.

The agency had originally warned that tsunamis were “expected to arrive imminently” in the coastal areas of Yamagata, Niigata and Ishikawa, with an advisory issued for four coastal regions.

The meteorological agency has advised residents to evacuate those coastal regions immediately and to not enter the sea or approach the coastal regions until the advisory has been lifted.

Slight sea-level changes may be observed in coastal regions, but no tsunami damage is expected, it added.

In March 2011, Japan was hit by a 9.0 magnitude earthquake, its worst ever. The massive quake rocked the country and triggered an enormous tsunami that resulted in the country’s worst nuclear disaster.

The earthquake was so strong that it permanently moved Japan’s main island, Honshu, more than two meters to the east. Its impact also raised huge waves up to 40 meters (approximately 131 feet) high that, as people were still reeling from the aftershocks, began crashing into the coast.

In Fukushima, approximately 256 kilometers (159 miles) north of Toyko, three reactors at the Fukushima Daiichi nuclear plant melted down, releasing radioactive materials into the air.

Nearly 20,000 people died in the earthquake and subsequent tsunami and nuclear meltdown, with more than 100,000 people evacuated from the area.

Sun’s History Found Buried In Moon’s Crust

When the Sun was just a baby four billion years ago, it went through violent outbursts of intense radiation, spewing scorching, high-energy clouds and particles across the solar system. These growing pains helped seed life on early Earth by igniting chemical reactions that kept Earth warm and wet. Yet, these solar tantrums also may have prevented life from emerging on other worlds by stripping them of atmospheres and zapping nourishing chemicals.

Just how destructive these primordial outbursts were to other worlds would have depended on how quickly the baby Sun rotated on its axis. The faster the Sun turned, the quicker it would have destroyed conditions for habitability.

This critical piece of the Sun’s history, though, has bedeviled scientists, said Prabal Saxena, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Saxena studies how space weather, the variations in solar activity and other radiation conditions in space, interacts with the surfaces of planets and moons.

Now, he and other scientists are realizing that the Moon, where NASA will be sending astronauts by 2024, contains clues to the ancient mysteries of the Sun, which are crucial to understanding the development of life.

“We didn’t know what the Sun looked like in its first billion years, and it’s super important because it likely changed how Venus’ atmosphere evolved and how quickly it lost water. It also probably changed how quickly Mars lost its atmosphere, and it changed the atmospheric chemistry of Earth,” Saxena said.

The Sun-Moon Connection

Saxena stumbled into investigating the early Sun’s rotation mystery while contemplating a seemingly unrelated one: Why, when the Moon and Earth are made of largely the same stuff, is there significantly less sodium and potassium in lunar regolith, or Moon soil, than in Earth soil?

This question, too, revealed through analyses of Apollo-era Moon samples and lunar meteorites found on Earth, has puzzled scientists for decades — and it has challenged the leading theory of how the Moon formed.

Our natural satellite took shape, the theory goes, when a Mars-sized object smashed into Earth about 4.5 billion years ago. The force of this crash sent materials spewing into orbit, where they coalesced into the Moon.

“The Earth and Moon would have formed with similar materials, so the question is, why was the Moon depleted in these elements?” said Rosemary Killen, an planetary scientist at NASA Goddard who researches the effect of space weather on planetary atmospheres and exospheres.

The two scientists suspected that one big question informed the other — that the history of the Sun is buried in the Moon’s crust.

Killen’s earlier work laid the foundation for the team’s investigation. In 2012, she helped simulate the effect solar activity has on the amount of sodium and potassium that is either delivered to the Moon’s surface or knocked off by a stream of charged particles from the Sun, known as the solar wind, or by powerful eruptions known as coronal mass ejections.

Saxena incorporated the mathematical relationship between a star’s rotation rate and its flare activity. This insight was derived by scientists who studied the activity of thousands of stars discovered by NASA’s Kepler space telescope: The faster a star spins, they found, the more violent its ejections. “As you learn about other stars and planets, especially stars like our Sun, you start to get a bigger picture of how the Sun evolved over time,” Saxena said.

Using sophisticated computer models, Saxena, Killen and colleagues think they may have finally solved both mysteries. Their computer simulations, which they described on May 3 in the The Astrophysical Journal Letters, show that the early Sun rotated slower than 50% of baby stars. According to their estimates, within its first billion years, the Sun took at least 9 to 10 days to complete one rotation.

They determined this by simulating the evolution of our solar system under a slow, medium, and then a fast-rotating star. And they found that just one version — the slow-rotating star — was able to blast the right amount of charged particles into the Moon’s surface to knock enough sodium and potassium into space over time to leave the amounts we see in Moon rocks today.

“Space weather was probably one of the major influences for how all the planets of the solar system evolved,” Saxena said, “so any study of habitability of planets needs to consider it.”

Life Under the Early Sun

The rotation rate of the early Sun is partly responsible for life on Earth. But for Venus and Mars — both rocky planets similar to Earth — it may have precluded it. (Mercury, the closest rocky planet to the Sun, never had a chance.)

Earth’s atmosphere was once very different from the oxygen-dominated one we find today. When Earth formed 4.6 billion years ago, a thin envelope of hydrogen and helium clung to our molten planet. But outbursts from the young Sun stripped away that primordial haze within 200 million years.

As Earth’s crust solidified, volcanoes gradually coughed up a new atmosphere, filling the air with carbon dioxide, water, and nitrogen. Over the next billion years, the earliest bacterial life consumed that carbon dioxide and, in exchange, released methane and oxygen into the atmosphere. Earth also developed a magnetic field, which helped protect it from the Sun, allowing our atmosphere to transform into the oxygen- and nitrogen-rich air we breathe today.

“We were lucky that Earth’s atmosphere survived the terrible times,” said Vladimir Airapetian, a senior Goddard heliophysicist and astrobiologist who studies how space weather affects the habitability of terrestrial planets. Airapetian worked with Saxena and Killen on the early Sun study.

Had our Sun been a fast rotator, it would have erupted with super flares 10 times stronger than any in recorded history, at least 10 times a day. Even Earth’s magnetic field wouldn’t have been enough to protect it. The Sun’s blasts would have decimated the atmosphere, reducing air pressure so much that Earth wouldn’t retain liquid water. “It could have been a much harsher environment,” Saxena noted.

But the Sun rotated at an ideal pace for Earth, which thrived under the early star. Venus and Mars weren’t so lucky. Venus was once covered in water oceans and may have been habitable. But due to many factors, including solar activity and the lack of an internally generated magnetic field, Venus lost its hydrogen — a critical component of water. As a result, its oceans evaporated within its first 600 million years, according to estimates. The planet’s atmosphere became thick with carbon dioxide, a heavy molecule that’s harder to blow away. These forces led to a runaway greenhouse effect that keeps Venus a sizzling 864 degrees Fahrenheit (462 degrees Celsius), far too hot for life.

Mars, farther from the Sun than Earth is, would seem to be safer from stellar outbursts. Yet, it had less protection than did Earth. Due partly to the Red Planet’s weak magnetic field and low gravity, the early Sun gradually was able to blow away its air and water. By about 3.7 billion years ago, the Martian atmosphere had become so thin that liquid water immediately evaporated into space. (Water still exists on the planet, frozen in the polar caps and in the soil.)

After influencing the course for life (or lack thereof) on the inner planets, the aging Sun gradually slowed its pace and continues to do so. Today, it revolves once every 27 days, three times slower than it did in its infancy. The slower spin renders it much less active, though the Sun still has violent outbursts occasionally.

Exploring the Moon, Witness of Solar System Evolution

To learn about the early Sun, Saxena said, you need to look no further than the Moon, one of the most well-preserved artifacts from the young solar system.

“The reason the Moon ends up being a really useful calibrator and window into the past is that it has no annoying atmosphere and no plate tectonics resurfacing the crust,” he said. “So as a result, you can say, ‘Hey, if solar particles or anything else hit it, the Moon’s soil should show evidence of that.'”

Apollo samples and lunar meteorites are a great starting point for probing the early solar system, but they are only small pieces in a large and mysterious puzzle. The samples are from a small region near the lunar equator, and scientists can’t tell with complete certainty where on the Moon the meteorites came from, which makes it hard to place them into geological context.

Since the South Pole is home to the permanently shadowed craters where we expect to find the best-preserved material on the Moon, including frozen water, NASA is aiming to send a human expedition to the region by 2024.

If astronauts can get samples of lunar soil from the Moon’s southernmost region, it could offer more physical evidence of the baby Sun’s rotation rate, said Airapetian, who suspects that solar particles would have been deflected by the Moon’s erstwhile magnetic field 4 billion years ago and deposited at the poles: “So you would expect — though we’ve never looked at it — that the chemistry of that part of the Moon, the one exposed to the young Sun, would be much more altered than the equatorial regions. So there’s a lot of science to be done there.”

Big Earthquakes Might Make Sea Level Rise Worse

A GEOLOGIC ONE-TWO punch rocked the South Pacific in September 2009, as a magnitude 8.1 earthquake struck off the coast of the island nation of Samoa, followed mere moments later by a similarly intense temblor. A towering tsunami soon crashed onto the shores of islands nearby, leaving more than 180 dead and communities in ruins in Samoa, the neighboring U.S. territory of American Samoa, and surrounding islands.

But a new study, published in the Journal of Geophysical Research: Solid Earth, reveals that the quakes also sparked a slow-burning danger for the more than 55,000 residents of American Samoa: sea level rise that is five times as fast as the global average.

Like other island and coastal regions around the world, Samoa and American Samoa are facing encroaching waters as our warming world sends sea levels soaring at accelerating rates. In the wake of the mega-quakes, though, the researchers discovered that these Pacific islands are also sinking. The situation is particularly concerning for American Samoa, where the team estimates that, over the next 50 to a hundred years, local sea levels could rise by roughly a foot in addition to the anticipated effects of climate change.

While the contributions of big earthquakes won’t be the same everywhere, the discovery emphasizes the sometimes overlooked effects that geology can have on the increasing number of people around the world who call coastlines home. (Also find out how powerful quakes are priming the region around Mount Everest for a huge disaster.)

“Everybody is talking about climate change issues … but they overlooked the impact of the earthquake and associated land subsidence,” says study leader Shin-Chan Han of the University of Newcastle, Australia, referring to documents from regional governments on sea level rise.

“This is a really important thing to point out,” says geophysicist Laura Wallace of the geoscience consultancy firm GNS Science, Te Pū Ao, in New Zealand, who was not involved in the study. “It obviously has a big impact on the relative sea level changes people are going to see in places like [the Samoan islands].”

Geologic geometry

Plate tectonics is constantly reshaping the surface of our planet—a role particularly evident during an earthquake. Generally speaking, these events occur where tectonic plates are colliding or sliding against each other, building up geologic stress. When that pent-up energy is released suddenly, it can send blocks of the planet’s crust careening out of place. (Find out how smaller “hidden” earthquakes are affecting California.)

But not all the change from a big earthquake is immediate. Unlike the rigid crust, the rocks of the mantle below flow like cold molasses and gradually adjust to the sudden surface jolt, Wallace says. This can cause either sinking or uplift of the land that can continue for decades after a temblor strikes.

This prolonged landscape deformation is what intrigues Han. For years, he’s scoured data from the Gravity Recovery and Climate Experiment, or GRACE, satellites to hunt for the rise and fall of land after a quake. This satellite duo orbited Earth in a line from 2002 to 2017 and precisely tracked the gap between the spacecraft. As they passed over zones with slightly more mass, and thus stronger gravity, the leading craft would feel the tug just before the trailing one. This tweaked the space in between and registered as a wobble in the planet’s gravitational field that can reveal changes in the landmass below.

In the case of the 2009 earthquake, such changes were minute on a day-to-day basis. But eventually, the effects were large enough that Han saw something strange happening in the Samoan islands while poring over the GRACE data.

A rare coincidence

The 2009 event was a particularly unusual earthquake that initially baffled scientists, since the pair of powerful temblors ripped through the Earth nearly at the same time. One broke along a so-called normal fault, created due to the flex of the oceanic crust as it plunges under another tectonic plate in what’s known as a subduction zone. Another quake broke within the subduction zone due to the compressive forces of the colliding plates.

The researchers investigated the lingering impacts of these quakes using a combination of GRACE data and local GPS and tide gauge records. They then built a computer model to tease apart the complex interplay between the temblors and what is happening at the surface.

This data showed slow sinking of the landscape, driven primarily by the normal-fault quake. This particular earthquake causes one side of the landscape to fall in relation to the other, which sent the nearby islands sinking downward.

The team found that nearly a decade after the event, the island of Samoa has sunk by roughly 0.4 inches a year. The situation is particularly acute for American Samoa, which has seen more than 0.6 inches of subsidence each year, and it doesn’t look like it’s stopping anytime soon.

The pace outstrips the estimated rate of global sea level rise, which is creeping upward at some 0.13 inches a year. Flooding and seawater intrusion in freshwater aquifers are already grave concerns for residents of American Samoa, Han says, and the latest find only adds to the worry.

Bathtub oceans

This latest study emphasizes the need for greater awareness and continued monitoring to mitigate the potential effects of mega-quakes, Wallace says. However, predicting such sea level effects before an earthquake strikes is not feasible, since earthquake prediction itself remains elusive.

“This might be a problem that suddenly causes you heartburn next week,” Freymueller says, “or it might not cause any problem for the next century.”

This latest study emphasizes the need for greater awareness and continued monitoring to mitigate the potential effects of mega-quakes, Wallace says. However, predicting such sea level effects before an earthquake strikes is not feasible, since earthquake prediction itself remains elusive.

“This might be a problem that suddenly causes you heartburn next week,” Freymueller says, “or it might not cause any problem for the next century.”