In a paper publishing today in the SIAM Journal on Imaging Sciences, Matan Leibovich (New York University), George Papanicolaou (Stanford University), and Chrysoula Tsogka (University of California, Merced) introduce a new method for taking high-resolution images of fast-moving and rotating objects in space, such as satellites or debris in LEO. They created an imaging process that first utilizes a novel algorithm to estimate the speed and angle at which an object in space is rotating, then applies those estimates to develop a high-resolution picture of the target.

Leibovich, Papanicolaou, and Tsogka used a theoretical model of a space imaging system to construct and test their imaging process. The model depicts a piece of fast-moving debris as a cluster of very small, highly reflective objects that represent the strongly reflective edges of an item in orbit, such as the solar panels on a satellite. The cluster of reflectors all move together with the same speed and direction and rotate about a common center. In the model, multiple sources of radiation on the Earth’s surface — such as the ground control stations of global navigation satellite systems — emit pulses that are reflected by target pieces of space debris. A distributed set of receivers then detects and records the signals that bounce off the targets.

The model focuses on sources that produce radiation in the X-band, or from frequencies of 8 to 12 gigahertz. “It is well known that resolution can be improved by using higher frequencies, such as the X-band,” Tsogka said. “Higher frequencies, however, also result in distortions to the image due to ambient fluctuations from atmospheric effects.” Signals are distorted by turbulent air as they travel from the target to receivers, which can make the imaging of objects in LEO quite challenging. The first step of the authors’ imaging process was thus to correlate the data taken at different receivers, which can help reduce the effects of these distortions.

The diameter of the area encompassed by the receivers is called the physical aperture of the imaging system — in the model, this is about 200 kilometers. Under normal imaging conditions, the physical aperture’s size determines the resolution of the resulting image; a larger aperture begets a sharper picture. However, the quick movement of the imaging target relative to the receivers can create an inverse synthetic aperture, in which the signals that were detected at multiple receivers as the target moved throughout their field of view are synthesized coherently. This configuration can effectively improve the resolution, as if the imaging system had a wider aperture than the physical one.

Objects in LEO can spin on timescales that range from a full rotation every few seconds to every few hundred seconds, which complicates the imaging process. It is thus important to know — or at least be able to estimate — some details about the rotation before developing the image. The authors therefore needed to estimate the parameters related to the object’s rotation before synthesizing the data from different receivers. Though simply checking all of the possible parameters to see which ones yield the sharpest image is technically feasible, doing so would require a lot of computational power. Instead of employing this brute force approach, the authors developed a new algorithm that can analyze the imaging data to estimate the object’s rotation speed and the direction of its axis.

After accounting for the rotation, the next step in the authors’ imaging process was to analyze the data to develop a picture of the space debris that would hopefully be as accurate and well-resolved as possible. One method that researchers often employ for this type of imaging of fast-moving objects is the single-point migration of cross correlations. Though atmospheric fluctuations do not usually significantly impair this technique, it does not have a very high resolution. A different, commonly-used imaging approach called Kirchhoff migration can achieve a high resolution, as it benefits from the inverse synthetic aperture configuration; however, the trade-off is that it is degraded by atmospheric fluctuations. With the goal of creating an imaging scheme that is not too heavily affected by atmospheric fluctuations but still maintains a high resolution, the authors proposed a third approach: an algorithm whose result they call a rank-1 image. “The introduction of the rank-1 image and its resolution analysis for fast-moving and rotating objects is the most novel part of this study,” Leibovich said.

To compare the performance of the three imaging schemes, the authors gave simulated data of a rotating object in LEO to each one and compared the images that they produced. Excitingly, the rank-1 image was much more accurate and well-resolved than the result of single-point migration. It also had similar qualities to the output of the Kirchhoff migration technique. But this result was not entirely surprising, given the problem’s configuration. “It is important to note that the rank-1 image benefits from the rotation of the object,” Papanicolaou said. Though a rotating object generates more complex data, one can actually incorporate this additional information into the image processing technique to improve its resolution. Rotation at certain angles can also increase the size of the synthetic aperture, which significantly improves the resolution for the Kirchhoff migration and rank-1 images.

Further simulations revealed that the rank-1 image is not easily muddled by errors in the new algorithm for the estimation of rotation parameters. It is also more robust to atmospheric effects than the Kirchhoff migration image. If receivers capture data for a full rotation of the object, the rank-1 image can even achieve optimal imaging resolution. Due to its good performance, this new imaging method could improve the accuracy of imaging LEO satellites and space debris. “Overall, this study shed light on a new method for imaging fast-moving and rotating objects in space,” Tsogka said. “This is of great importance for ensuring the safety of the LEO band, which is the backbone of global remote sensing.”

These reconstructions are important for understanding how and when oxygen became available in Earth’s atmosphere and ultimately increased to the levels that support life as we know it today.

Yet reconstructions that rely on signals from sedimentary rocks but ignore the impact of local sedimentary processes do so at their own peril, according to geoscientists including David Fike in Arts & Sciences at Washington University in St. Louis.

Their new study published Feb. 26 in Science Advances is based on analyses of a mineral called pyrite (FeS₂) that is formed in the presence of bacteria. With its chemical-reduced iron (Fe) and sulphur (S), the burial of pyrite in marine sediments is one of the key controls on oxygen levels in Earth’s atmosphere and oceans.

The researchers compared pyrite in sediments collected in a borehole drilled in the shelf just off the eastern coast of New Zealand with sediments drilled from the same ocean basin but hundreds of kilometers out into the Pacific.

“We were able to get a gradient of shallow to deep sediments and compare the differences between those isotopic compositions in pyrite between those sections,” said Fike, professor of Earth and planetary sciences and director of environmental studies at Washington University.

“We demonstrate that, for this one basin in the open ocean, you get very different signals between shallow and deep water, which is prima facie evidence to argue that these signals aren’t the global fingerprint of oxygen in the atmosphere,” said Fike, who also serves as director of Washington University’s International Center for Energy, Environment and Sustainability (InCEES).

Instead of pointing directly to oxygen, the same signals from pyrite could be reinterpreted as they relate to other important factors, Fike said, such as sea level change and plate tectonics.

Fike and first author Virgil Pasquier, a postdoctoral fellow at the Weizmann Institute of Sciences in Israel, first questioned the way that pyrite has been used as a proxy in a study published in PNAS in 2017 using Mediterranean Sea sediments. For his postdoctoral research, Pasquier has been working with professor Itay Halevy at the Weizmann Institute to understand the various controls on the isotopic composition of pyrite. Their results raise concerns about the common use of pyrite sulfur isotopes to reconstruct Earth’s evolving oxidation state.

“Strictly speaking, we are investigating the coupled cycles of carbon, oxygen and sulfur, and the controls on the oxidation state of the atmosphere,” Pasquier said.

“It’s much more sexy for a paper to reconstruct past changes in ocean chemistry than to focus on the burial of rocks or what happened during the burial,” he said. “But I find this part even more interesting. Because most microbial life — especially back when oxygen was initially accumulating in the atmosphere — occurred in sediments. And if our ultimate goal is to understand oxygenation of the oceans, then we have to understand this.”

For this study, the team conducted 185 sulfur isotope analyses of pyrite along the two boreholes. They determined that changes in the signals in pyrite from the nearshore borehole were more controlled by sea level-driven changes in local sedimentation, rather than any other factor.

In contrast, sediments in the deeper borehole were immune to the sea-level changes. Instead, they recorded a signal associated with the long-term reorganization of ocean currents.

“There is a water depth threshold,” said Roger Bryant, a co-author and PhD graduate of Fike’s laboratory at Washington University, now a postdoctoral fellow at the University of Chicago. “Once you go below that water depth, sulfur isotopes apparently are not sensitive to things like climate and environmental conditions in the surface environment.”

Fike added: “The Earth is a complicated place, and we need to remember that when we try to reconstruct how it has changed in the past. There are a number of different processes that impact the kinds of signals that get preserved. As we try to better understand Earth’s long-term evolution, we need to have a more nuanced view about how to extract information from those signals.”

Led by scientists who collaborate as part of the International Research Network for Nuclear Astrophysics (IReNA)  and the Joint Institute for Nuclear Astrophysics — Center for the Evolution of the Elements (JINA-CEE), the study is published in the latest issue of the journal Science.

Heavy elements we encounter in our everyday life, like iron and silver, did not exist at the beginning of the universe, 13.7 billion years ago. They were created in time through nuclear reactions called nucleosynthesis that combined atoms together. In particular, iodine, gold, platinum, uranium, plutonium, and curium, some of the heaviest elements, were created by a specific type of nucleosynthesis called the rapid neutron capture process, or r process.

The question of which astronomical events can produce the heaviest elements has been a mystery for decades. Today, it is thought that the r process can occur during violent collisions between two neutron stars, between a neutron star and a black hole, or during rare explosions following the death of massive stars. Such highly energetic events occur very rarely in the universe. When they do, neutrons are incorporated in the nucleus of atoms, then converted into protons. Since elements in the periodic table are defined by the number of protons in their nucleus, the r process builds up heavier nuclei as more neutrons are captured.

Some of the nuclei produced by the r process are radioactive and take millions of years to decay into stable nuclei. Iodine-129 and curium-247 are two of such nuclei that were pro-duced before the formation of the sun. They were incorporated into solids that eventually fell on the earth’s surface as meteorites. Inside these meteorites, the radioactive decay generat-ed an excess of stable nuclei. Today, this excess can be measured in laboratories in order to figure out the amount of iodine-129 and curium-247 that were present in the solar system just before its formation.

Why are these two r-process nuclei are so special? They have a peculiar property in common: they decay at almost exactly the same rate. In other words, the ratio between iodine-129 and curium-247 has not changed since their creation, billions of years ago.

“This is an amazing coincidence, particularly given that these nuclei are two of only five radioactive r-process nuclei that can be measured in meteorites,” says Benoit Côté from the Konkoly Observatory, the leader of the study. “With the iodine-129 to curium-247 ratio being frozen in time, like a prehistoric fossil, we can have a direct look into the last wave of heavy element production that built up the composition of the solar system, and everything within it.”

Iodine, with its 53 protons, is more easily created than curium with its 96 protons. This is because it takes more neutron capture reactions to reach curium’s higher number of protons. As a consequence, the iodine-129 to curium-247 ratio highly depends on the amount of neutrons that were available during their creation.

The team calculated the iodine-129 to curium-247 ratios synthesized by collisions between neutron stars and black holes to find the right set of conditions that reproduce the composition of meteorites. They concluded that the amount of neutrons available during the last r-process event before the birth of the solar system could not be too high. Otherwise, too much curium would have been created relative to iodine. This implies that very neutron-rich sources, such as the matter ripped off the surface of a neutron star during a collision, likely did not play an important role.

So what created these r-process nuclei? While the researchers could provide new and insightful information regarding how they were made, they could not pin down the nature of the astronomical object that created them. This is because nucleosynthesis models are based on uncertain nuclear properties, and it is still unclear how to link neutron availability to specific astronomical objects such as massive star explosions and colliding neutron stars.

“But the ability of the iodine-129 to curium-247 ratio to peer more directly into the fundamental nature of heavy element nucleosynthesis is an exciting prospect for the future,” said Nicole Vassh from the University of Notre Dame, coauthor of the study.

With this new diagnostic tool, advances in the fidelity of astrophysical simulations and in the understanding of nuclear properties could reveal which astronomical objects created the heaviest elements of the solar system.

“Studies like this are only possible when you bring together a multidisciplinary team, where each collaborator contributes to a distinct piece of the puzzle. The JINA-CEE 2019 Frontiers meeting provided the ideal environment to formalize the collaboration that led to the current result,” Côté said.

This work was supported in part by JINA-CEE a U.S. National Science Foundation (NSF) Physics Frontiers Center operating under grant No. PHY- 1430152, and by IReNA, an NSF AccelNet Network of Networks operating under grant OISE-1927130.

A cell contains several hundred thousand tRNA molecules, each of which consists of only 70 to 90 nucleotides folded into a cloverleaf-like pattern. At one end, tRNAs carry one of the twenty amino acids that serve as protein building blocks, while the opposite end pairs with the codon specifying this amino acid in messenger RNA during translation. Although there are only 61 codons for the twenty amino acids, cells from different organisms can contain hundreds of unique tRNA molecules, some of which differ from each other by only a single nucleotide. Many nucleotides in tRNAs are also decorated with chemical modifications, which help tRNAs fold or bind the correct codon.

The levels of individual tRNAs are dynamically regulated in different tissues and during development, and tRNA defects are linked to neurogical diseases and cancer. The molecular origins of these links remain unclear, because quantifying the abundance and modifications of tRNAs in cells has long remained a challenge. The team of Danny Nedialkova at the MPI of Biochemistry has now developed mim-tRNAseq, a method that accurately measures the abundance and modification status of different tRNAs in cells.

Modification roadblocks and resolutions

To measure the levels of multiple RNAs simultaneously, scientists use an enzyme called reverse transcriptase to first rewrite RNA into DNA. Millions of these DNA copies can then be quantified in parallel by high-throughput sequencing. Rewriting tRNAs into DNA has been tremendously hard since many tRNA modifications block the reverse transcriptase, causing it to stop synthesizing DNA.

“Many researches have proposed elegant solutions to this problem, but all of them relieve only a fraction of the modification roadblocks in tRNAs,” explains Danny Nedialkova, Max Planck Research Group Leader at the Max Planck Institute of Biochemistry. “We noticed that one specific reverse transcriptase seemed to be much better at reading through modified tRNA sites. By optimizing the reaction conditions, we could significantly improve the enzyme’s efficiency, enabling it to read through nearly all tRNA modification roadblocks,” adds Nedialkova. This made it possible to construct DNA libraries from full-length tRNA copies and use them for high-throughput sequencing.

The mim-tRNAseq computational toolkit

The analysis of the resulting sequencing data also presented significant challenges. “We identified two major issues: the first one is the extensive sequence similarity between different tRNA transcripts,” explains Andrew Behrens, PhD student in Nedialkova’s group and first author of the paper. “The second one comes from the fact that an incorrect nucleotide (a misincorporation) is introduced at many modified sites during reverse transcription. Both make it extremely challenging to assign each DNA read to the tRNA molecule it originated from,” adds Behrens.

The team tackled these issues with novel computational approaches, including the use of modification annotation to guide accurate read alignment. The resulting comprehensive toolkit is packaged into a freely available pipeline for alignment, analysis and visualization of tRNA-derived sequencing data . Researchers can use mim-tRNAseq to not only measure tRNA abundance, but also to map and quantify tRNA modifications that induce nucleotide misincorporations by the reverse transcriptase. “mim-tRNAseq opens up myriad possibilities moving forward,” says Nedialkova. “We expect it will help us and others to tackle many outstanding questions about tRNA biology in health and disease.”

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The discovery by researchers from Rice University, Austria’s Vienna University of Technology (TU Wien), Switzerland’s Paul Scherrer Institute and Canada’s McMaster University is detailed in a paper in the Proceedings of the National Academy of Sciences. Of interest are both the origins of the effect, which is typically associated with magnetism, and its gigantic magnitude — more than 1,000 times larger than one might observe in simple semiconductors.

Rice study co-author Qimiao Si, a theoretical physicist who has investigated quantum materials for nearly three decades, said, “It’s really topology at work,” referring to the patterns of quantum entanglement that give rise the unorthodox state.

The material, an exotic semimetal of cerium, bismuth and palladium, was created and measured at TU Wien by Silke Bühler-Paschen, a longtime collaborator of Si’s. In late 2017, Si, Bühler-Paschen and colleagues discovered a new type of quantum material they dubbed a “Weyl-Kondo semimetal.” The research laid the groundwork for empirical investigations, but Si said the experiments were challenging, in part because it wasn’t clear “which physical quantity would pick up the effect.”

In April 2018, Bühler-Paschen and TU Wien graduate student Sami Dzsaber, the study’s first author, dropped by Si’s office while attending a workshop at the Rice Center for Quantum Materials (RCQM). When Si saw Dzsaber’s data, he was dubious.

“Upon seeing this, everybody’s first reaction is that it is not possible,” he said.

To appreciate why, it helps to understand both the nature and the 1879 discovery of Edwin Hall, a doctoral student who found that applying a magnetic field at a 90-degree angle to conducting wire produced a voltage difference across the wire, in the direction perpendicular to both the current and the magnetic field. Physicists eventually discovered the source of the Hall effect: The magnetic field deflects the motion of passing electrons, pulling them toward one side of the wire. The Hall effect is a standard tool in physics labs, and devices that make use of it are found in products as diverse as rocket engines and paintball guns. Studies related to the quantum nature of the Hall effect captured Nobel Prizes in 1985 and 1998.

Dzsaber’s experimental data clearly showed a characteristic Hall signal, even though no magnetic field was applied.

“If you don’t apply a magnetic field, the electron is not supposed to bend,” Si said. “So, how could you ever get a voltage drop along the perpendicular direction? That’s why everyone didn’t believe this at first.”

Experiments at the Paul Scherrer Institute ruled out the presence of a tiny magnetic field that could only be detected on a microscopic scale. So the question remained: What caused the effect?

“In the end, all of us had to accept that this was connected to topology,” Si said.

In topological materials, patterns of quantum entanglement produce “protected” states, universal features that cannot be erased. The immutable nature of topological states is of increasing interest for quantum computing. Weyl semimetals, which manifest a quasiparticle known as the Weyl fermion, are topological materials.

So are the Weyl-Kondo semimetals Si, Bühler-Paschen and colleagues discovered in 2018. Those feature both Weyl fermions and the Kondo effect, an interaction between the magnetic moments of electrons attached to atoms inside the metal and the spins of passing conduction electrons.

“The Kondo effect is the quintessential form of strong correlations in quantum materials,” Si said in reference to the correlated, collective behavior of billions upon billions of quantum entangled particles. “It qualifies the Weyl-Kondo semimetal as one of the rare examples of a topological state that’s driven by strong correlations.

“Topology is a defining characteristic of the Weyl-Kondo semimetal, and the discovery of this spontaneous giant Hall effect is really the first detection of topology that’s associated with this kind of Weyl fermion,” Si said.

Experiments showed that the effect arose at the characteristic temperature associated with the Kondo effect, indicating the two are likely connected, Si said.

“This kind of spontaneous Hall effect was also observed in contemporaneous experiments in some layered semiconductors, but our effect is more than 1,000 times larger,” he said. “We were able to show that the observed giant effect is, in fact, natural when the topological state develops out of strong correlations.”

Si said the new observation is likely “a tip of the iceberg” of extreme responses that result from the interplay between strong correlations and topology.

He said the size of the topologically generated Hall effect is also likely to spur investigations into potential uses of the technology for quantum computation.

“This large magnitude, and its robust, bulk nature presents intriguing possibilities for exploitation in topological quantum devices,” Si said.

Si is the Harry C. and Olga K. Wiess Professor in Rice’s Department of Physics and Astronomy and director of RCQM. Bühler-Paschen is a professor at TU Wien’s Institute for Solid State Physics.

Today, Rottinghaus, an associate professor in the University of Missouri College of Education, is helping young people eager to find their place in the world by identifying their strengths and connecting them with careers that match their skillset, interests and personality. As the father of three children, including a daughter soon to enter high school, he wants to ensure they are equipped with the knowledge and skills to succeed as they prepare to enter the modern workforce.

In an effort to fill the United States labor market’s current “skills gap,” Rottinghaus and graduate student Chan Jeong Park collaborated to study how certain tools could better help students to know their strengths. In his study, Rottinghaus partnered with YouScience, a web-based aptitude assessment system, to distribute an online career aptitude test to more than 7,000 high school students across 14 states. The skills gap is defined as the disconnect between the skills employers look for when recruiting potential employees and the number of job-seekers with those skills.

While most career exploration surveys focus mainly on students’ interests, the aptitude test Rottinghaus distributed also inquired about the strengths and skills the students possess, which enabled them to explore more potential careers than they had originally considered. The aptitude assessments helped identify female students with the talent for careers in construction, technical health care, manufacturing and computer technology, areas they may not have previously considered based solely on their interests. The assessments also helped identify males with the talent to pursue jobs in patient-centered health care.

“When you look at rapidly growing employment sectors like manufacturing, computer technology, health care and construction, there is a pipeline concern, as we need more young people equipped with the skills to enter these fields,” Rottinghaus said. “Not only does the aptitude test help high school students identify potential career paths, but it also helps them identify classes they can take now or in college that will strengthen their skillset and potentially open up doors for their future.”

According to Rottinghaus, one approach to mitigating the “skills gap” is to encourage more women and underrepresented groups to pursue high-demand careers in science, technology, engineering and mathematics, or STEM.

“Due to traditional societal norms and gender stereotypes, young women have historically not been as likely to pursue STEM careers,” Rottinghaus said. “But when we looked at their aptitude scores, the system would often indicate many of the young women surveyed have the aptitude to be successful in these areas. We can also help men consider more nontraditional fields, too, such as nursing or health care, which tend to be predominantly female.”

Rottinghaus believes providing aptitude tests to young students and having a trained counselor review the results with them can help in overcoming gender stereotypes. He encourages his own children to expand their career possibilities by exploring fields not traditionally considered in the past.

“I don’t want my daughter to feel constricted in her career exploration by only considering fields traditionally held by women. I want her to consider a full array of opportunities,” Rottinghaus said. “My overall goal is to help people intentionally identify aptitudes and interests to find their fit with educational pathways and labor market needs so they can be happy and productive members of society. There are also implications for institutions of higher education, as students who don’t know what they want to major in are more likely to drop out before graduation.”

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