Month: July 2017

Harvesting Earthquake Fault Slip f­rom Laser Images of Napa’s Vineyards

A new U.S. Geological Survey-led study suggests that earthquake-related deformation just below the Earth’s surface can be quite different from how it is expressed at the surface. Scientists using laser images of grapevine rows deformed by the 2014 South Napa earthquake have found that the amount of surface displacement caused by the earthquake could be significantly less than estimates of the actual slip across the fault plane. The laser images show the amount that the portion of a vine row on one side of the fault was shifted horizontally with respect to the portion on the other side.

The findings are important because they provide unprecedented details of the process of earthquake-related fault slip reaching near the Earth’s surface, the place where structures are built and where people reside.

The team used computer models to relate the measured surface distortion to fault slip at depth and verified their results with trenches cut across sections of the West Napa Fault that produced surface disruption associated with the 2014 earthquake.

“If fault slip at a couple of meters’ depth could be different than slip right at the surface, and we can infer that using these types of high-resolution laser images and slip models, then we can use this information to make better estimates of the rates at which faults slip over multiple earthquake cycles,” said lead author USGS geophysicist Ben Brooks.

This, in turn, would lead to more accurate seismic hazard assessments and improved methodologies for mitigating and monitoring the possible interruption of underground infrastructures such as pipelines by near-surface faulting.

The multi-institutional team, including researchers from the California Geological Survey and geological consultants, used the same type of LIDAR (Light Detection and Ranging) laser technology that is mounted on the roofs of self-driving vehicles for navigation.

“We’re using the same imaging and navigation technology mounted on the roofs of the test robotic vehicles that people see driving around these days,” said co-author professor Craig Glennie of the University of Houston’s National Center for Airborne Laser Mapping. “Only we are processing the data to higher precision and accuracy so that we can detect ground motions on the order of centimeters.”

Before the ground-based laser scanning technology employed by the authors became available, geologists had to rely on satellite-based measurements of ground deformation that could only be made with much less spatial resolution and less frequently — measurements which wouldn’t have revealed the detail necessary to make the study’s breakthrough.

“What’s so exciting about these new imaging technologies is that we can now learn how much earthquake slip happens very close to the surface, which is where all the people and infrastructure are located,” said USGS geophysicist and co-author Sarah Minson.

The Outer Galaxy

The Sun is located inside one of the spiral arms of the Milky Way galaxy, roughly two-thirds of the way from the galactic center to the outer regions. Because we are inside the galaxy, obscuration by dust and the confusion of sources along our lines-of-sight make mapping the galaxy a difficult task. Astronomers think that the galaxy is a symmetric spiral, and about 10 years ago, CfA astronomers Tom Dame and Pat Thaddeus, using millimeter observations of the gas carbon monoxide, discovered symmetric components to the spiral arms deep in the inner galaxy that lent support to this model.

The galaxy is not perfectly flat. It has a slight warp that allows some distant structures, at least in the direction of the constellations of Scutum and Centaurus, to be seen more distinctly above much of the foreground confusion. In 2011 the same CfA astronomers were the first to discover a large-scale spiral feature within this distant warp which they called the “Outer Scutum–Centaurus Arm (OSC).” Subsequent studies placed the OSC at a distance from the galactic center of over forty thousand light-years; it appears to be a symmetric counterpart to a spiral arm on the opposite side, in the direction of Perseus.

CfA astronomer Tom Dame has joined with a set of collaborators to probe the extent of massive star formation in the OSC; sadly, his colleague Pat Thaddeus passed away earlier this year. Using radio measurements of ionized gas, which traces the hot ultraviolet from massive young stars, as well as bright emission from masers associated with massive star formation, the scientists observed 140 candidate locations and discovered evidence for massive young stars in about sixty percent of them. The study shows that the OSC is forming new stars, some with as much as forty solar masses each. These stars and their associated ionized environments, at least as far as we know now, mark the outer boundary for massive star formation in the Milky Way.

Ocean Circulation, Coupled With Trade Wind Changes, Efficiently Limits Shifting Of Tropical Rainfall Patterns

The Intertropical Convergence Zone (ITCZ), also known as the doldrums, is one of the dramatic features of Earth’s climate system. Prominent enough to be seen from space, the ITCZ appears in satellite images as a band of bright clouds around the tropics. Here, moist warm air accumulates in this atmospheric region near the equator, where the ocean and atmosphere heavily interact. Intense solar radiation and calm, warm ocean waters produce an area of high humidity, ascending air, and rainfall, which is fed by converging trade winds from the Northern and Southern Hemispheres. The convected air forms clusters of thunderstorms characteristic of the ITCZ, releasing heat before moving away from the ITCZ—toward the poles—cooling and descending in the subtropics. This circulation completes the Hadley cells of the ITCZ, which play an important role in balancing Earth’s energy budget—transporting energy between the hemispheres and away from the equator.

However, the position of the ITCZ isn’t static. In order to transport this energy, the ITCZ and Hadley cells shift seasonally between the Northern and Southern Hemispheres, residing in the one that’s most strongly heated from the sun and radiant heat from the Earth’s surface, which on average yearly is the Northern Hemisphere. Accompanying these shifts can be prolonged periods of violent storms or severe drought, which significantly impacts human populations living in its path.

Scientists are therefore keen to understand the climate controls that drive the north-south movement of the ITCZ over the seasonal cycle, as well as on inter-annual to decadal timescales in Earth’s paleoclimatology up through today. Researchers have traditionally approached this issue from the perspective of the atmosphere’s behavior and understanding rainfall, but anecdotal evidence from models with a dynamic ocean has suggested that the ocean’s sensitivity to climate changes could affect the ITCZ’s response. Now, a study from MIT graduate student Brian Green and the Cecil and Ida Green Professor of Oceanography John Marshall from the Program in Atmospheres, Oceans and Climate in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS) published in the American Meteorological Society’s Journal of Climate, investigates the role that the ocean plays in modulating the ITCZ’s position and appreciates its sensitivity when the Northern Hemisphere is heated. In so doing, the work gives climate scientists a better understanding of what causes changes to tropical rainfall.

“In the past decade or so there’s been a lot of research studying controls on the north-south position of the ITCZ, particularly from this energy balance perspective. … And this has normally been done in the context of ignoring the adjustment of the ocean circulation—the ocean circulation is either forcing these [ITCZ] shifts or passively responding to changes in the atmosphere above,” Green says. “But we know, particularly in the tropics, that the ocean circulation is very tightly coupled through the trade winds to atmospheric circulation and the ITCZ position, so what we wanted to do was investigate how that ocean circulation might feedback on the energy balance that controls that ITCZ position, and how strong that feedback might be.”

To examine this, Green and Marshall performed experiments in a global climate model with a coupled atmosphere and ocean, and observed how the ocean circulation’s cross-equatorial energy transport and its associated surface energy fluxes affected the ITCZ’s response when they imposed an inter-hemispheric heating contrast. Using a simplified model that omitted landmasses, clouds, and monsoon dynamics, while keeping a fully circulating atmosphere that interacts with radiation highlighted the ocean’s effect while minimized other confounding variables that could mask the results. The addition of north-south ocean ridges, creating a large and small basin, mimicked the behavior of the Earth’s Atlantic’s meridional overturning circulation and the Pacific Ocean.

Green and Marshall then ran the asymmetrically heated planet simulations in two ocean configurations and compared the ITCZ responses. The first used a stationary “slab ocean,” where the thermal properties were specified so that it mimicked the fully coupled model before perturbation, but was unable to respond to the heating. The second incorporated a dynamic ocean circulation. By forcing the models identically, they could quantify the ocean circulation’s impact on the ITCZ.

“We found in the case where there’s a fully coupled, dynamic ocean, the northward shift of the ITCZ was damped by a factor of four compared to the passive ocean. So that’s hinting that the ocean is one of the leading controls on the position of the ITCZ,” Green says. “It’s a significant damping of the response of the atmosphere, and the reason behind this can just be diagnosed from that energy balance.”

In the dynamic ocean model, they found that when they heat the simulated ocean-covered planet, eddies export some heat into the tropical atmosphere from the extra-tropics, which causes the Hadley cells to respond—the Northern Hemisphere cell to weaken and the Southern Hemisphere cell to strengthen. This transports heat southward through the atmosphere. Concurrently, the ITCZ shifts northward; associated with this are changes in the trade winds—the surface branch of the Hadley cells—and the surface wind stress near the equator. The surface ocean feels this change in winds, which energizes an anomalous ocean circulation and moves water mass southwards across the equator in both hemispheres, carrying heat with it. Once this surface water reaches the extra-tropics, the ocean pumps it downwards where it returns northward across the equator, cooler and at depth. This temperature contrast between the warm surface cross-equatorial flow and the cooler deeper return flow sets the heat transported by the ocean circulation.

“In the slab ocean case, only the atmosphere can move heat across the equator; whereas in our fully coupled case, we see that the ocean is the most strongly compensating component of the system, transporting the majority of the forcing across the equator.” Green says. “So from the atmosphere’s perspective, it doesn’t even feel the full effect of that heating that we’re imposing. And as a result, it has to transport less heat across the equator and shift the ITCZ less.” Green adds that the response of the large basin ocean circulation broadly mimics the Indian Ocean’s yearly average circulation.

Marshall notes that the ability of the wind-driven ocean circulation to damp ITCZ shifts represents a previously unappreciated constraint on the atmosphere’s energy budget: “We showed that the ITCZ cannot move very far away from the equator, even in very ‘extreme’ climates,” indicating that the position of the ITCZ may be much less sensitive to inter-hemispheric heating contrasts than previously thought.”

Green and Marshall are currently expanding upon this work. With the help of David McGee, the Kerr-Mcgee Career Development Assistant Professor in EAPS, and postdoc Eduardo Moreno-Chamarro, the pair are applying this to the paleoclimate record during Heimrich events, when the Earth experiences strong cooling, looking for ITCZ shifts.

They’re also investigating the decomposition of heat and mass transport between the atmosphere and the ocean, as well as between the Earth’s oceans. “The physics that control each of those oceans’ responses are dramatically different, certainly between the Pacific and the Atlantic oceans,” Green says. “Right now, we’re working to understand how the mass transports of the atmosphere and ocean are coupled. While we know that they’re constrained to overturn in the same sense, they’re not actually constrained to transport an identical amount of mass, so you could further enhance or weaken the damping by the ocean circulation by affecting how strongly the mass transports are coupled.”

Gallium In Lunar Samples Explains Loss Of Moon’s Easily Vaporized Elements

A pair of researchers with Institut Universitaire de France has found more evidence of a large evaporative event in the moon’s past. In their paper published on the open access site Science Advances, Chizu Kato and Frédéric Moynier describe their study of gallium isotopes from lunar samples, what they found, and why they believe it sheds some light on how the moon was formed.

The prevailing theory by astronomers who study the moon is that it was created by a celestial body slamming into the Earth, flinging debris into space. That debris, they believe, eventually coalesced into what is now the moon. That theory depends on samples of lunar rocks and soil matching material found on Earth. However, some isotope amounts, especially the lighter ones, do not match well with those found on Earth, which means they either originated elsewhere, or something caused a change in their original amounts. Many scientists agree with the latter theory, as do Kato and Moynier. They, like many of their colleagues, believe that for a period of time after it was formed, the moon was so hot it was completely covered in magma. They suggest this caused many of the lighter isotopes to evaporate into space. To back up their theory, researchers have studied isotopes of elements such as potassium and zinc found in lunar rocks. They compare the ratio of those that are heavier with those here on Earth to learn more about how much of the lighter ones may have been lost to evaporation and the events that could have led to the differences.

In this new effort, the researchers looked at the gallium isotope in moon samples and found that its low boiling point, its resilience to evaporation during magmatic events and other characteristics suggest that the differences in amounts of gallium on the moon versus the Earth could, indeed, be explained by a huge evaporative event—such as hot magma covering the surface of the moon. This finding bolsters the theory that the moon was created by something colliding with the Earth.

Quasars May Answer How Starburst Galaxies Were Extinguished

Some of the biggest galaxies in the universe are full of extinguished stars. But nearly 12 billion years ago, soon after the universe first was created, these massive galaxies were hotspots that brewed up stars by the billions.
How these types of cosmic realms, called dusty starburst galaxies, became galactic dead zones is an enduring mystery.

Astronomers at the University of Iowa, in a new study published in the Astrophysical Journal, offer a clue. They say quasars, powerful energy sources believed to dwell at the heart of galaxies, may be responsible for why some dusty starburst galaxies ceased making stars.

The study could help explain how galaxies evolve from star makers to cosmic cemeteries and how various phenomena scientists know little about—quasars and supermassive black holes that are believed to exist deep within all galaxies, for example—may propel those changes.

The scientists arrived at their theory after locating quasars inside four dusty starburst galaxies that still are creating stars.

“These quasars may play an important role in making the dusty starbursts extinct in the cosmic history,” says Hai Fu, assistant professor in the UI’s Department of Physics and Astronomy and the paper’s first author. “This is because quasars are energetic enough to eject gas out of the galaxy, and gas is the fuel for star formation, so quasars provide a viable mechanism to explain the transition between a starburst and an extinct elliptical (galaxy).”

Quasars shouldn’t be detectable in dusty starburst galaxies because their light would be absorbed, or blocked, by the grit churned up by the intense star-forming activity taking place there, Fu says.

“So, the fact that we saw any such quasars implies that there must be more quasars hidden in dusty starbursts,” Fu says. “To push this to the extreme, maybe every dusty starburst galaxy hosts a quasar and we just cannot see the quasars.”

Fu and his team located the quasars in March 2016 with the Atacama Large Millimeter/submillimeter Array (ALMA), a bank of radio telescopes located more than 16,000 feet above sea level in northern Chile. It was the first time Fu’s team reserved time on ALMA, brought into full operation in 2013 and funded by international partners, including the U.S. National Science Foundation.

The scientists then mapped the quasars with other telescopes and at wavelengths ranging from ultraviolet to far infrared. Based on these observations, they confirmed the quasars are the same as those located with ALMA. The question then became: Why are these quasars visible when they should be enshrouded?

The researchers have a theory. They think the quasars are peeking out from deep holes in each galaxy, a debris-less vacuum that allows light to escape amid the cloudy surroundings. The specific shape of these galaxies is unclear because even ALMA isn’t powerful enough to provide a clear look at regions of the cosmos where light being detected was emitted 12 billion years ago, when the universe was roughly one-seventh its current age. But the team imagines the galaxies may be doughnut shaped and oriented in such a way that their holes (and, thus, the quasar) can be seen.

“It’s a rare case of geometry lining up,” says Jacob Isbell, a UI senior from Garrison, Iowa, majoring in physics and astronomy and the paper’s second author. “And that hole happens to be aligned with our line of sight.”
The scientists now think most quasars inside dusty starburst galaxies can’t be seen because they’re oriented in a way that keeps them hidden. But finding four examples of dusty starburst galaxies with viewable quasars does not seem random; in fact, it suggests more exist.

The paper is titled, “The circumgalactic medium of submillimeter galaxies. II. Unobscured QSOS within dusty starbursts and QSO sightlines with impact parameters below 100 kiloparsec.”

Earth-Like Atmosphere May Not Survive Proxima B’s Orbit

Proxima b, an Earth-size planet right outside our solar system in the habitable zone of its star, may not be able to keep a grip on its atmosphere, leaving the surface exposed to harmful stellar radiation and reducing its potential for habitability.

At only four light-years away, Proxima b is our closest known extra-solar neighbor. However, due to the fact that it hasn’t been seen crossing in front of its host star, the exoplanet eludes the usual method for learning about its atmosphere. Instead, scientists must rely on models to understand whether the exoplanet is habitable.

One such computer model considered what would happen if Earth orbited Proxima Centauri, our nearest stellar neighbor and Proxima b’s host star, at the same orbit as Proxima b. The NASA study, published on July 24, 2017, in The Astrophysical Journal Letters, suggests Earth’s atmosphere wouldn’t survive in close proximity to the violent red dwarf.

“We decided to take the only habitable planet we know of so far — Earth — and put it where Proxima b is,” said Katherine Garcia-Sage, a space scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the study. The research was supported by NASA’s NExSS coalition — leading the search for life on planets beyond our solar system — and the NASA Astrobiology Institute.

Just because Proxima b’s orbit is in the habitable zone, which is the distance from its host star where water could pool on a planet’s surface, doesn’t mean it’s habitable. It doesn’t take into account, for example, whether water actually exists on the planet, or whether an atmosphere could survive at that orbit. Atmospheres are also essential for life as we know it: Having the right atmosphere allows for climate regulation, the maintenance of a water-friendly surface pressure, shielding from hazardous space weather, and the housing of life’s chemical building blocks.

Garcia-Sage and her colleagues’ computer model used Earth’s atmosphere, magnetic field and gravity as proxies for Proxima b’s. They also calculated how much radiation Proxima Centauri produces on average, based on observations from NASA’s Chandra X-ray Observatory.

With these data, their model simulates how the host star’s intense radiation and frequent flaring affect the exoplanet’s atmosphere.

“The question is, how much of the atmosphere is lost, and how quickly does that process occur?” said Ofer Cohen, a space scientist at the University of Massachusetts, Lowell and co-author of the study. “If we estimate that time, we can calculate how long it takes the atmosphere to completely escape — and compare that to the planet’s lifetime.”

An active red dwarf star like Proxima Centauri strips away atmosphere when high-energy extreme ultraviolet radiation ionizes atmospheric gases, knocking off electrons and producing a swath of electrically charged particles. In this process, the newly formed electrons gain enough energy that they can readily escape the planet’s gravity and race out of the atmosphere.

Opposite charges attract, so as more negatively charged electrons leave the atmosphere, they create a powerful charge separation that pulls positively charged ions along with them, out into space.

In Proxima Centauri’s habitable zone, Proxima b encounters bouts of extreme ultraviolet radiation hundreds of times greater than Earth does from the sun. That radiation generates enough energy to strip away not just the lightest molecules — hydrogen — but also, over time, heavier elements such as oxygen and nitrogen.

The model shows Proxima Centauri’s powerful radiation drains the Earth-like atmosphere as much as 10,000 times faster than what happens at Earth.

“This was a simple calculation based on average activity from the host star,” Garcia-Sage said. “It doesn’t consider variations like extreme heating in the star’s atmosphere or violent stellar disturbances to the exoplanet’s magnetic field — things we’d expect provide even more ionizing radiation and atmospheric escape.”

To understand how the process can vary, the scientists looked at two other factors that exacerbate atmospheric loss. First, they considered the temperature of the neutral atmosphere, called the thermosphere. They found as the thermosphere heats with more stellar radiation, atmospheric escape increases.

The scientists also considered the size of the region over which atmospheric escape happens, called the polar cap. Planets are most sensitive to magnetic effects at their magnetic poles. When magnetic field lines at the poles are closed, the polar cap is limited and charged particles remain trapped near the planet. On the other hand, greater escape occurs when magnetic field lines are open, providing a one-way route to space.

“This study looks at an under-appreciated aspect of habitability, which is atmospheric loss in the context of stellar physics,” said Shawn Domagal-Goldman, a Goddard space scientist not involved in the study. “Planets have lots of different interacting systems, and it’s important to make sure we include these interactions in our models.”

The scientists show that with the highest thermosphere temperatures and a completely open magnetic field, Proxima b could lose an amount equal to the entirety of Earth’s atmosphere in 100 million years — that’s just a fraction of Proxima b’s 4 billion years thus far. When the scientists assumed the lowest temperatures and a closed magnetic field, that much mass escapes over 2 billion years.

“Things can get interesting if an exoplanet holds on to its atmosphere, but Proxima b’s atmospheric loss rates here are so high that habitability is implausible,” said Jeremy Drake, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics and co-author of the study. “This questions the habitability of planets around such red dwarfs in general.”

Red dwarfs like Proxima Centauri or the TRAPPIST-1 star are often the target of exoplanet hunts, because they are the coolest, smallest and most common stars in the galaxy. Because they are cooler and dimmer, planets have to maintain tight orbits for liquid water to be present.

But unless the atmospheric loss is counteracted by some other process — such as a massive amount of volcanic activity or comet bombardment — this close proximity, scientists are finding more often, is not promising for an atmosphere’s survival or sustainability.

Planetary Defense Campaign Will Use Real Asteroid For The First Time

For the first time, NASA will use an actual space rock for a tabletop exercise simulating an asteroid impact in a densely populated area. The asteroid, named 2012 TC4, does not pose a threat to Earth, but NASA is using it as a test object for an observational campaign because of its close flyby on Oct. 12, 2017.

NASA has conducted such preparedness drills rehearsing various aspects of an asteroid impact, such as deflection, evacuation, and disaster relief, with other federal entities in the past. Traditionally, however, these exercises involved hypothetical impactors, prompting Vishnu Reddy of the University of Arizona’s Lunar and Planetary Laboratory to propose a slightly more realistic scenario, one that revolves around an actual near-Earth asteroid, or NEA.

“The question is, how prepared are we for the next cosmic threat?” says Reddy, an assistant professor of planetary science at the Lunar and Planetary Laboratory. “So we proposed an observational campaign to exercise the network and test how ready we are for a potential impact by a rogue asteroid.”

NASA’s Planetary Defense Coordination Office (PDCO), the federal entity in charge of coordinating efforts to protect Earth from hazardous asteroids, accepted Reddy’s proposal to conduct an observational campaign as part of assessing its Earth-based defense network. Reddy will assist Michael Kelley, who serves as a program scientist with NASA PDCO and the civil servant lead on the exercise.

The goal of the TC4 exercise is to recover, track, and characterize 2012 TC4 as a potential impactor in order to exercise the entire system from observations, modeling, prediction, and communication.

Measuring between 30 and 100 feet, roughly the same size as the asteroid that exploded over Chelyabinsk, Russia, on Feb. 15, 2013, TC4 was discovered by the Pan-STARRS 1 telescope on Oct. 5, 2012, at Haleakala Observatory on Maui, Hawaii. Given its orbital uncertainty, the asteroid will pass as close as 6,800 kilometers (4,200 miles) above Earth’s surface.

“This is a team effort that involves more than a dozen observatories, universities, and labs across the globe so we can collectively learn the strengths and limitations of our planetary defense capabilities,” said Reddy, who is coordinating the campaign for NASA PDCO.

Since its discovery in 2012, the uncertainty in the asteroid’s orbit has slowly increased, as it would for any asteroid as time passes. Therefore, the first order of business will be to “recover” the object—in other words, nail down its exact path. Reddy and his collaborators hope that depending on its predicted brightness, the asteroid would be visible again to large ground-based telescopes in late August.

“One of the strengths of UA research is partnering with federal agencies or industry to work together in solving some of the grand challenges we face,” said Kimberly Andrews Espy, senior vice president for research. “This project is a perfect example of matching UA capabilities—from our world-class imaging to our expertise in space sciences—with an external need.”

The UA is home to the Catalina Sky Survey, one of the most prolific asteroid discoverers, and the Spacewatch project that recovers and tracks faint NEAs. Both teams will take part in the planetary defense exercise.