Gerald Wasserburg, 1927–2016

Gerald J. Wasserburg, John D. MacArthur Professor of Geology and Geophysics, Emeritus, passed away on June 13, 2016. He was 89 years old.

Gerry Wasserburg's research focused on the origins and history of the solar system and its component bodies. His work established a time scale for the development of the early solar system including the end of the process of nucleosynthesis and the formation about 4.5 billion years ago of solid objects such as the earth and the moon, other planets, and certain meteorites. He also is acknowledged widely for his isotope studies of lunar materials collected by the Apollo missions and his involvement in U.S. space research programs. He and his colleagues also did important work on the dating of rocks, on the evolution of the earth through time, and on the modern oceans.

Wasserburg earned his SB, SM, and PhD degrees from the University of Chicago (in 1951, 1952, and 1954), and joined the Caltech faculty in 1955 as an assistant professor of geology. He became an associate professor in 1959, professor of geology and geophysics in 1963, MacArthur Professor in 1982, and he retired in 2001.

He served as chair of the Division of Geological and Planetary Sciences from 1987 to 1989 and as executive officer for geochemistry over the same time period.

He was the recipient of numerous honors, including the Arthur L. Day Medal from the Geological Society of America (1970), the NASA Distinguished Public Service Medal (1972 and 1978), the Wollaston Medial of the Geological Society of London (1985), the Crafoord Prize in Geosciences (1986), the Gold Medal of the Royal Astronomical Society (1991), and the William Bowie Medal of the American Geophysical Union (2008). He was a member of the National Academy of Sciences, the American Philosophical Society, the American Academy of Arts and Science, and the Norwegian Academy of Science and Letters.

A full obituary will be published at a later date.

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Natural Quasicrystals May Be the Result of Collisions Between Objects in the Asteroid Belt

Naturally formed quasicrystals—crystal-like solids with supposedly impossible symmetries—are among the rarest structures on Earth. Only two have ever been found.

A team led by Paul Asimow (MS '93, PhD '97), professor of geology and geochemistry at Caltech, may have uncovered one of the reasons for that scarcity, demonstrating in laboratory experiments that quasicrystals could arise from collisions between rocky bodies in the asteroid belt with unusual chemical compositions.

A paper on their findings was published on June 13 in the advance online edition of the Proceedings of the National Academy of Sciences.

At an atomic level, crystals are both ordered and periodic, meaning that they have a defined geometric structure, with that structure repeating itself over and over. To grow such a repeating structure without the original organization breaking down, the crystal can only exhibit one of four types of rotational symmetry: two-fold, three-fold, four-fold, or six-fold.

The number refers to how many times an object will look exactly the same within a full 360-degree rotation about an axis. For example, an object with two-fold symmetry appears the same twice, or every 180 degrees; an object with three-fold symmetry appears the same three times, or every 120 degrees; and an object with four-fold symmetry appears the same four times, or every 90 degrees.

Prior to 1984, it was believed that it would be impossible for a crystal to grow with any other type of symmetry; no examples of crystals with other symmetries had been discovered in nature or grown in a lab. In that year, however, Princeton physicist Paul Steinhardt (BS '74) theorized a set of conditions under which other types of symmetry could potentially exist and Dan Shechtman of the Israel Institute of Technology published a paper announcing the creation of a crystal-like structure with a five-fold rotational symmetry.

These structures were ordered enough to produce recognizable diffraction patterns when shot with high-energy beams of electrons and X-rays—unlike disordered structures, which produce no patterns. However, the crystal-like structures were not periodic—that is, their organization shifted and changed as they grew. The materials were dubbed "quasiperiodic crystals," or "quasicrystals" for short.

Over the next few decades, researchers figured out how to manufacture more than 100 different varieties of quasicrystals by melting and homogenizing certain elements and then cooling them at very specific rates in the lab. Still, though, no naturally existing quasicrystals were known. Indeed, researchers suspected their formation would be impossible. That is because most lab-grown quasicrystals were metastable, meaning that the same combination of elements could arrange themselves into a crystalline structure using less energy.

Everything changed in the late 2000s, when Steinhardt and colleague Luca Bindi from the Museum of Natural History at the University of Florence (currently in the Faculty of the Department of Earth Sciences of the same University) found a tiny grain of an aluminum, copper, and iron mineral that exhibited five-fold symmetry. The grain came from a small sample of the Khatyrka meteorite, an extraterrestrial object known only from a few pieces found in Russia's Koryak Mountains. Steinhardt and his collaborators found a second natural quasicrystal from the same meteorite in 2015, confirming that the natural existence of quasicrystals was possible, just very rare.

A microscopic analysis of the meteorite indicated that it had undergone a major shock at some point in its lifetime before crashing to Earth – likely from a collision with another rocky body in space. Such collisions are common in the asteroid belt and release high amounts of energy.

Asimow and colleagues hypothesized that the energy released by the shock could have caused the quasicrystal's formation by triggering a rapid cycle of compression, heating, decompression, and cooling.

To test the hypothesis, Asimow simulated the collision between two asteroids in his lab. He took thin slices of minerals found in the Khatyrka meteorite and sandwiched them together in a sample case that resembles a steel hockey puck. He then screwed the "puck" to the muzzle of a four-meter-long, 20-mm-bore single-stage propellant gun, and blasted it with a projectile at nearly one kilometer per second, about equal to the speed of the fastest rifle-fired bullets.

It is important to note that those minerals included a sample of a metallic copper-aluminum alloy, which has only been found in nature in the Khatyrka meteorite.

After the sample was shocked with the propellant gun, it was sawed open, polished, and examined. The impact smashed the sandwiched elements together and, in several spots, created microscopic quasicrystals.

Armed with this experimental evidence, Asimow says he is confident that shocks are the source of naturally formed quasicrystals. "We know that the Khatyrka meteorite was shocked. And now we know that when you shock the starting materials that were available in that meteorite, you get a quasicrystal."

Sarah Stewart (PhD '02)—a planetary collision expert from the University of California, Davis, and reviewer of the PNAS paper—admits she was surprised by the findings. "If you had called me before the study and asked if this would work I would have said 'no way.' The astounding thing is that they did it so easily," she says. "Nature is crazy."

Asimow acknowledges that the experiments leave many questions unanswered. For example, it is unclear at what point the quasicrystal formed during the shock's pressure and temperature cycle. A bigger mystery, Asimow says, is the origin of the copper-aluminum alloy in the meteorite, which has never been seen elsewhere in nature.

Next, Asimow plans to shock various combinations of minerals to see what key ingredients are necessary for natural quasicrystal formation.

These results are published in a paper titled "Shock synthesis of quasicrystals with implications for their origin in asteroid collisions." In addition to Asimow, Steinhardt, and Bindi, other coauthors on the paper are Chi Ma, director of analytical facilities in the Geological and Planetary Sciences division at Caltech; Lincoln Hollister (PhD '66) and Chaney Lin from Princeton University; and Oliver Tschauner from the University of Nevada, Las Vegas. Their work was supported by the National Science Foundation (NSF), the University of Florence, and the NSF-Materials Research Science & Engineering Centers Program through New York University and the Princeton Center for Complex Materials.

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Aliso Canyon, Methane, and Global Climate: A Conversation with Paul Wennberg

On October 23, 2015, the Aliso Canyon underground storage facility for natural gas in the San Fernando Valley—the fourth largest of its kind in the United States—had one of its wells blow out, leading to a large release of methane. The leak was not fully under control until February 11, 2016. In the interim, residents of nearby neighborhoods were sickened by the odorants added to the gas, thousands of households were displaced, and California's governor declared a state of emergency for the area. The story made international headlines; the BBC's headline, for example, read, "California methane leak 'largest in US history.'"

The leak was indeed large and undoubtedly difficult for the residents of the area. However, Caltech's Paul Wennberg says there is also a bigger picture to keep in mind: enormous methane and carbon dioxide (CO2) emissions occur all the time, with troubling implications for global climate. Wennberg is Caltech's R. Stanton Avery Professor of Atmospheric Chemistry and Environmental Science and Engineering, executive officer for Environmental Science and Engineering, and director of the Ronald and Maxine Linde Center for Global Environmental Science.

We recently sat down with him to talk about methane emissions and how to put the Aliso Canyon event into perspective.

What was your involvement with the Aliso Canyon event?

We have a greenhouse gas remote sensing system here at Caltech that is part of TCCON—the Total Carbon Column Observing Network. The day after the Aliso Canyon leak started, we observed something really weird in the air above Pasadena. There was a large, big plume of methane and ethane gas that came over. We now know that it was from the Aliso Canyon facility. We are providing data for the final analyses of the leak.

In the past you have suggested that the methane emissions from Los Angeles are much larger than was previously included in models.

Right. Thankfully, models are now catching up as we learn more from the data.

What does the Aliso Canyon event suggest about Los Angeles's methane emissions in general?

Aliso Canyon was a very dramatic event. Everyone heard about it worldwide. The leak continued for about 100 days, and yet it only doubled the amount of methane being emitted by LA during that period. This was a tragedy for the people living next to it, who had to deal with horrible nausea and other side effects of the chemicals associated with the natural gas. But from a climate point of view, the methane leak was actually quite trivial.

There are enormous amounts of methane being released into the atmosphere globally as a result of human activity. That is certainly true of LA, but as far as climate goes, it doesn't matter whether it's released in LA or New Zealand. On the timescale that methane sticks around in the atmosphere, it gets well mixed and affects the entire planet.

How much methane is emitted per year?

About three hundred teragrams [Tg; one teragram is equivalent to one billion kilograms] of methane are emitted every year by people and the activities of people, like agriculture and energy. Los Angeles emits about 0.4 Tg. That means that of the human methane emissions, LA as a total is one part in a thousand—not nothing, but a pretty small amount.

For perspective, Aliso Canyon emitted around 0.1 Tg. It was a big event, but what it really illustrates is how big a challenge we truly face. There are many sources emitting methane into the atmosphere and they are very diffuse. Reducing them will require hard work on many, many fronts. So it's not just, "If we solve this one problem, everything will be beautiful in the world."

You could imagine the response to the Aliso Canyon leak might be that we would all of a sudden focus all of our efforts trying to prevent leaks in natural gas storage facilities. That would not be the right answer from a climate perspective.

How should people go about eliminating methane emissions?

There is not "one" fix. Each source requires a different strategy for mitigation.

First, there is fixing leaks in the pipelines and storage facilities.

Then, it turns out that ruminants like cows and sheep produce a lot of methane—probably a third, if not more, of the human emissions. A paper about this, recently in Science, suggests that an important part of the recent increases in methane is coming from agriculture. Depending on what you feed these ruminants, they produce less methane. They eat grass, but they can't metabolize it: they have a fermenter going in their bellies—a whole microbiome that breaks the grass down into smaller things like acetate that they can metabolize. Depending on the microbiome of their guts, the cows and sheep make more or less methane. And it turns out that you can manage this.

Then there are the wetlands used for rice agriculture. Methane is produced anaerobically—in places with no oxygen—by Archaea. If you have a flooded rice paddy, the methane is produced at the roots and is transpired through the rice plants into the atmosphere. Quite a few studies now show that if you can change your rice agricultural practices to allow the fields to dry periodically, the methane emissions drop hugely.

If you were able to fix all of these things what would the impact be in terms of climate change?

If we could really knock the methane emissions back to what they were before people started emitting methane, it would be a large change. It would be a half a watt per meter squared. The total global warming would drop by around 25 percent.

How does the importance of reducing methane emissions compare to the importance of reducing carbon dioxide emissions?

Globally, methane is important. It's maybe a third of the climate forcing of CO2—that is, the increase in methane has contributed about one third of the total change in Earth's climate over the last 100 years. In terms of climate impact, however, the methane emissions from people in Los Angeles are absolutely dwarfed by their CO2 emissions—all of our driving, going on airplanes, and everything else that we do. Still, if we are to reduce our global warming potential and the amount of greenhouse gasses we emit to the atmosphere, methane has to be part of the equation.

We like to think that we can solve these problems by fixing singular events, but climate doesn't work that way. We're talking about the emissions of 7 billion people. If it were that this was produced by 100 events like Aliso Canyon, this would be a simple problem: we solve the 100 problems, and we're done. But it's all of us, and it's all of what we eat, it's all of the energy that we use, it's all of the miles that we drive. It's a much more complex problem.

What work is your group currently doing in terms of methane?

One of the things we've been doing is long-term monitoring. Natural gas is mostly methane (CH4) but there's also ethane (C2H6) in it and this provides a way of separating the signature of methane emitted from agriculture, which has no ethane, and emissions from natural gas, which does.

Over the last five years or so, the production of oil in the United States has increased hugely, and associated with that oil production is natural gas, and therefore methane and ethane. Traditionally, most of the ethane produced at a wellhead was pulled off and sent to the plastic industry. With the changing oil production, the market has become flooded in ethane: there's simply not enough plastic to be made. When the industry can't sell the ethane to the plastic industry, they simply leave it in the natural gas. We see this in the natural gas delivered to Los Angeles. Five years ago natural gas had about 2 percent ethane. Now it's 5 percent—it's more than doubled. What we've seen—and this has nothing to do with Aliso Canyon—is that over the last five years, the amount of ethane in the air over Pasadena has increased.

That's important because it tells us that a significant fraction of the methane that's being released in LA is coming from natural gas brought into Los Angeles. This has been a topic of a lot of debate. Is the big methane emitter the oil production down in the Long Beach area? Is it waste treatment plants? Is it garbage dumps? What we find is that about half of all the methane emitted in this part of LA is gas that originally came in on a pipeline.

How do you know that?

We actually know from the gas company how much ethane is in the natural gas. They report this publically from one of their storage fields and this matches the ethane in samples of the natural gas coming into our buildings.

Are there other projects under way at Caltech to study methane emissions?

Christian Frankenberg [associate professor of environmental science and engineering at Caltech and a JPL research scientist] has been leading an effort to build remote sensing instruments that allow imaging of methane plumes. Using small spectrometers on airplanes, he has flown over areas where you might have a lot of methane emissions and identified individual sources. Last year they were able to find individual pipelines that were leaking in Colorado and in New Mexico. They found several big leaks from pipelines and were able to tell the pipeline operators, who shut them down and fixed them.

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We recently sat down with Paul Wennberg to talk about methane emissions and how to put the Aliso Canyon event into perspective.
Monday, May 23, 2016
Brown Gymnasium – Scott Brown Gymnasium

Animal magnetism

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Brown Gymnasium – Scott Brown Gymnasium

Animal magnetism

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TA Workshop: Getting the Biggest ‘Bang for Your Buck’ - Teaching strategies for busy TAs

White House Puts Spotlight on Earthquake Early-Warning System

Since the late 1970s, Caltech seismologist Tom Heaton, professor of engineering seismology, has been working to develop earthquake early-warning (EEW) systems—networks of ground-based sensors that can send data to users when the earth begins to tremble nearby, giving them seconds to potentially minutes to prepare before the shaking reaches them. In fact, Heaton wrote the first paper published on the concept in 1985. EEW systems have been implemented in countries like Japan, Mexico, and Turkey. However, the Unites States has been slow to regard EEW systems as a priority for the West Coast.

But on February 2, 2016, the White House held the Earthquake Resilience Summit, signaling a new focus on earthquake safety and EEW systems. There, stakeholders—including Caltech's Heaton and Egill Hauksson, research professor in geophysics; and U.S. Geological Survey (USGS) seismologist Lucy Jones, a visiting associate in geophysics at Caltech and seismic risk advisor to the mayor of Los Angeles—discussed the need for earthquake early warning and explored steps that can be taken to make such systems a reality. 

At the summit, the Gordon and Betty Moore Foundation announced $3.6 million in grants to advance a West Coast EEW system called ShakeAlert, which received an initial $6 million in funding from foundation in 2011. The new grants will go to researchers working on the system at Caltech, the USGS, UC Berkeley, and the University of Washington.

"We have been successfully operating a demonstration system for several years, and we know that it works for the events that have happened in the test period," says Heaton. "However, there is still significant development that is required to ensure that the system will work reliably in very large earthquakes similar to the great 1906 San Francisco earthquake. This new funding allows us to accelerate the rate at which we develop this critical system."

In addition, the Obama Administration outlined new federal commitments to support greater earthquake safety including an executive order to ensure that new construction of federal buildings is up to code and that federal assets are available for recovery efforts after a large earthquake.

The commitments follow a December announcement from Congressman Adam Schiff (D-Burbank) that Congress has included $8.2 million in the fiscal year 2016 funding bill specifically designated for a West Coast earthquake early warning system.

"By increasing the funding for the West Coast earthquake early-warning system, Congress is sending a message to the Western states that it supports this life-saving system. But the federal government cannot do it alone and will need local stakeholders, both public and private, to get behind the effort with their own resources," said Schiff, in a press release. "The early warning system will give us critical time for trains to be slowed and surgeries to be stopped before shaking hits—saving lives and protecting infrastructure. This early warning system is an investment we need to make now, not after the 'big one' hits."

ShakeAlert utilizes a network of seismometers—instruments that measure ground motion—widely scattered across the Western states. In California, that network of sensors is called the California Integrated Seismic Network (CISN) and is made up of computerized seismometers that send ground-motion data back to research centers like the Seismological Laboratory at Caltech.

Here's how the current ShakeAlert works: a user display opens in a pop-up window on a recipient's computer as soon as a significant earthquake occurs in California. The screen lists the quake's estimated location and magnitude based on the sensor data received to that point, along with an estimate of how much time will pass before the shaking reaches the user's location. The program also gives an approximation of how intense that shaking will be. Since ShakeAlert uses information from a seismic event in progress, people living near the epicenter do not get much—if any—warning, but those farther away could have seconds or even tens of seconds' notice.

The goal is an improved version of ShakeAlert that will eventually give schools, utilities, industries, and the general public a heads-up in the event of a major temblor.

Read more about how ShakeAlert works and about Caltech's development of EEW systems in a feature that ran in the Summer 2013 issue of E&S magazine called Can We Predict Earthquakes?

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On February 2, 2016, the White House held the Earthquake Resilience Summit, signaling a new focus on earthquake safety and EEW systems.

Developing a Picture of the Earth's Mantle

Deep inside the earth, seismic observations reveal that three distinct structures make up the boundary between the earth's metallic core and overlying silicate mantle at a depth of about 2,900 kilometers—an area whose composition is key to understanding the evolution and dynamics of our planet. These structures include remnants of subducted plates that originated near the earth's surface, ultralow-velocity zones believed to be enriched in iron, and large dense provinces of unknown composition and mineralogy. A team led by Caltech's Jennifer Jackson, professor of mineral physics has new evidence for the origin of these features that occur at the core-mantle boundary.

"We have discovered that bridgmanite, the most abundant mineral on our planet, is a reasonable candidate for the material that makes up these dense provinces that occupy about 20 percent of the core-mantle boundary surface, and rise up to a depth of about 1,500 kilometers. Integrated by volume that's about the size of our moon!" says Jackson, coauthor of a study that outlines these findings and appears online in the Journal of Geophysical Research: Solid Earth. "This finding represents a breakthrough because although bridgmanite is the earth's most abundant mineral, we only recently have had the ability to precisely measure samples of it in an environment similar to what we think the materials are experiencing inside the earth."

Previously, says Jackson, it was not clear whether bridgmanite, a perovskite structured form of (Mg,Fe)SiO3, could explain seismic observations and geodynamic modeling efforts of these large dense provinces. She and her team show that indeed they do, but these structures need to be propped up by external forces, such as the pinching action provided by cold and dense subducted slabs at the base of the mantle.

Jackson, along with then Caltech graduate student Aaron Wolf (PhD '13), now a research scientist at the University of Michigan at Ann Arbor, and researchers from Argonne National Laboratory, came to these conclusions by taking precise X-ray measurements of synthetic bridgmanite samples compressed by diamond anvil cells to over 1 million times the earth's atmospheric pressure and heated to thousands of degrees Celsius.

The measurements were done utilizing two different beamlines at the Advanced Photon Source of Argonne National Laboratory in Illinois, where the team used powerful X-rays to measure the state of bridgmanite under the physical conditions of the earth's lower mantle to learn more about its stiffness and density under such conditions. The density controls the buoyancy—whether or not these bridgmanite provinces will lie flat on the core-mantle boundary or rise up. This information allowed the researchers to compare the results to seismic observations of the core-mantle boundary region.

"With these new measurements of bridgmanite at deep-mantle conditions, we show that these provinces are very likely to be dense and iron-rich, helping them to remain stable over geologic time," says Wolf.

Using a technique known as synchrotron Mössbauer spectroscopy, the team also measured the behavior of iron in the crystal structure of bridgmanite, and found that iron-bearing bridgmanite remained stable at extreme temperatures (more than 2,000 degrees Celsius) and pressure (up to 130 gigapascals). There had been some reports that iron-bearing bridgmanite breaks down under extreme conditions, but the team found no evidence for any breakdown or reactions.

"This is the first study to combine high-accuracy density and stiffness measurements with Mössbauer spectroscopy, allowing us to pinpoint iron's behavior within bridgmanite," says Wolf. "Our results also show that these provinces cannot possibly contain a large complement of radiogenic elements, placing strong constraints on their origin. If present, these radiogenic elements would have rapidly heated and destabilized the piles, contradicting many previous simulations that indicate that they are likely hundreds of millions of years old."

In addition, the experiments suggest that the rest of the lower mantle is not 100 percent bridgmanite as had been previously suggested. "We've shown that other phases, or minerals, must be present in the mantle to satisfy average geophysical observations," says Jackson. "Until we made these measurements, the thermal properties were not known with enough precision and accuracy to uniquely constrain the mineralogy."

"There is still a lot of work to be done, such as identifying the dynamics of subducting slabs, which we believe plays a role in providing an external force to shape these large bridgmanite provinces," she says. "We know that the earth did not start out this way. The provinces had to evolve within the global system, and we think these findings may help large-scale geodynamic modeling that involves tectonic plate reconstructions."

The results of the study were published in a paper titled "The thermal equation of state of (Mg,Fe)SiO3bridgmanite (perovskite) and implications for lower mantle structures." In addition to Jackson and Wolf, other authors on the study are Przemeslaw Dera and Vitali B. Prakapenka from the Center for Advanced Radiation Sources at Argonne National Laboratory. Support for this research was provided by the National Science Foundation, the Turner Postdoctoral Fellowship at the University of Michigan, and the California Institute of Technology.

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A team led by Caltech's Jennifer Jackson, professor of mineral physics has new evidence for the origin of features that occur at the core-mantle boundary.
Monday, November 30, 2015

Microbial diners, drive-ins, and dives: deep-sea edition

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Probing the Mysteries of Europa, Jupiter's Cracked and Crinkled Moon

New research identifies possible sites of frozen, watery deposits.

Jupiter's moon Europa is believed to possess a large salty ocean beneath its icy exterior, and that ocean, scientists say, has the potential to harbor life. Indeed, a mission recently suggested by NASA would visit the icy moon's surface to search for compounds that might be indicative of life. But where is the best place to look? New research by Caltech graduate student Patrick Fischer; Mike Brown, the Richard and Barbara Rosenberg Professor and Professor of Planetary Astronomy; and Kevin Hand, an astrobiologist and planetary scientist at JPL, suggests that it might be within the scarred, jumbled areas that make up Europa's so-called "chaos terrain."

A paper about the work has been accepted to The Astronomical Journal.

"We have known for a long time that Europa's fresh icy surface, which is covered with cracks and ridges and transform faults, is the external signature of a vast internal salty ocean," Brown says. The areas of chaos terrain show signatures of vast ice plates that have broken apart, shifted position, and been refrozen. These regions are of particular interest, because water from the oceans below may have risen to the surface through the cracks and left deposits there.

"Directly sampling Europa's ocean represents a major technological challenge and is likely far in the future," Fischer says. "But if we can sample deposits left behind in the chaos areas, it could reveal much about the composition and dynamics of the ocean below." That ocean is thought to be as deep as 100 kilometers.

"This could tell us much about activity at the boundary of the rocky core and the ocean," Brown adds.

In a search for such deposits, the researchers took a new look at data from observations made in 2011 at the W. M. Keck Observatory in Hawaii using the OSIRIS spectrograph. Spectrographs break down light into its component parts and then measure their frequencies. Each chemical element has unique light-absorbing characteristics, called spectral or absorption bands. The spectral patterns resulting from light absorption at particular wavelengths can be used to identify the chemical composition of Europa's surface minerals by observing reflected sunlight.

The OSIRIS instrument measures spectra in infrared wavelengths. "The minerals we expected to find on Europa have very distinct spectral fingerprints in infrared light," Fischer says. "Combine this with the extraordinary abilities of the adaptive optics in the Keck telescope, and you have a very powerful tool." Adaptive optics mechanisms reduce blurring caused by turbulence in the earth's atmosphere by measuring the image distortion of a bright star or laser and mechanically correcting it.

The OSIRIS observations produced spectra from 1600 individual spots on Europa's surface. To make sense of this collection of data, Fischer developed a new technique to sort and identify major groupings of spectral signatures.

"Patrick developed a very clever new mathematical tool that allows you to take a collection of spectra and automatically, and with no preconceived human biases, classify them into a number of distinct spectra," Brown says. The software was then able to correlate these groups of readings with a surface map of Europa from NASA's Galileo mission, which mapped the Jovian moon beginning in the late 1990s. The resulting composite provided a visual guide to the composition of the regions the team was interested in.

Three compositionally distinct categories of spectra emerged from the analysis. The first was water ice, which dominates Europa's surface. The second category includes chemicals formed when ionized sulfur and oxygen­­—thought to originate from volcanic activity on the neighboring moon Io­­—bombard the surface of Europa and react with the native ices. These findings were consistent with results of previous work done by Brown, Hand and others in identifying Europa's surface chemistry.

But the third grouping of chemical indicators was more puzzling. It did not match either set of ice or sulfur groupings, nor was it an easily identified set of salt minerals such as they might have expected from previous knowledge of Europa. Magnesium is thought to reside on the surface but has a weak spectral signature, and this third set of readings did not match that either. "In fact, it was not consistent with any of the salt materials previously associated with Europa," Brown says.

When this third group was mapped to the surface, it overlaid the chaos terrain. "I was looking at the maps of the third grouping of spectra, and I noticed that it generally matched the chaos regions mapped with images from Galileo. It was a stunning moment," Fischer says. "The most important result of this research was understanding that these materials are native to Europa, because they are clearly related to areas with recent geological activity."

The composition of the deposits is still unclear. "Unique identification has been difficult," Brown says. "We think we might be looking at salts left over after a large amount of ocean water flowed out onto the surface and then evaporated away. He compares these regions to their earthly cousins. "They may be like the large salt flats in the desert regions of the world, in which the chemical composition of the salt reflects whatever materials were dissolved in the water before it evaporated."

Similar deposits on Europa could provide a view into the oceans below, according to Brown. "If you had to suggest an area on Europa where ocean water had recently melted through and dumped its chemicals on the surface, this would be it. If we can someday sample and catalog the chemistry found there, we may learn something of what's happening on the ocean floor of Europa and maybe even find organic compounds, and that would be very exciting." 

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Researchers have mapped what may be salt deposits from the ocean below the ice onto the Jovian moon's surface.

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