Jupiter’s turbulence is not just skin deep. The giant planet’s visible storms and blemishes have roots far below the clouds, researchers report in the June 3 Science. The new observations offer a preview of what NASA’s Juno spacecraft will see when it sidles up to Jupiter later this year.
A chain of rising plumes, each reaching nearly 100 kilometers into Jupiter, dredges up ammonia to form ice clouds. Between the plumes, dry air sinks back into the Jovian depths. And the famous Great Red Spot, a storm more than twice as wide as Earth that has churned for several hundred years, extends at least dozens of kilometers below the clouds as well.
Jupiter’s dynamic atmosphere provides a possible window into how the planet works inside. “One of the big questions is what is driving that change,” says Leigh Fletcher, a planetary scientist at the University of Leicester in England. “Why does it change so rapidly, and what are the environmental and climate-related factors that result from those changes?”
To address some of those questions, Imke de Pater, a planetary scientist at the University of California, Berkeley, and colleagues observed Jupiter with the Very Large Array radio observatory in New Mexico. Jupiter emits radio waves generated by heat left over from its formation about 4.6 billion years ago. Ammonia gas within Jupiter’s atmosphere intercepts certain radio frequencies. By mapping how and where those frequencies are absorbed, the researchers created a three-dimensional map of the ammonia that lurks beneath Jupiter’s clouds. Those plumes and downdrafts appear to be powered by a narrow wave of gas that wraps around much of the planet.
The depths of Jupiter’s atmospheric choppiness isn’t too surprising, says Scott Bolton, a planetary scientist at the Southwest Research Institute in San Antonio. “Almost everyone I know would have guessed that,” he says. But the observations do provide a teaser for what to expect from the Juno mission, led by Bolton. The spacecraft arrives at Jupiter on July 4 to begin a 20-month investigation of what’s going on beneath Jupiter’s clouds using tools similar to those used in this study.
The new observations confirm that Juno should work as planned, Bolton says.
By getting close to the planet — just 5,000 kilometers from the cloud tops — Juno will break through the fog of radio waves from Jupiter’s radiation belts that obscures observations made from Earth and limits what telescopes like the Very Large Array can see. But the spacecraft will see only a narrow swath of Jupiter’s bulk at a time. “That’s where ground-based work like the research de Pater has been doing is really essential,” Fletcher says. Observations such as these will let Juno scientists know what’s going on throughout the atmosphere so they can better understand what Jupiter is telling them.
Two days after my first daughter was born, her pediatrician paid a house call to examine her newest patient. After packing up her gear, she told me something alarming: “For the next few months, a fever is an emergency.” If we measured a rectal temperature at or above 100.4° Fahrenheit, go to the hospital, she said. Call her on the way, but don’t wait.
I, of course, had no idea that a fever constituted an emergency. But our pediatrician explained that a fever in a very young infant can signal a fast-moving and dangerous bacterial infection. These infections are rare (and fortunately becoming even rarer thanks to newly created vaccines). But they’re serious, and newborns are particularly susceptible.
I’ve since heard from friends who have been through this emergency. Their newborns were poked, prodded and monitored by anxious doctors, in the hopes of quickly ruling out a serious bacterial infection. For infants younger than two months, it’s “enormously difficult to tell if an infant is seriously ill and requires antibiotics and/or hospitalization,” says Howard Bauchner, a pediatrician formerly at Boston University School of Medicine and now editor in chief of the Journal of the American Medical Association.
A new research approach, described in two JAMA papers published in August, may ultimately lead to better ways to find the cause of a fever.
These days, for most (but not all) very young infants, their arrival at a hospital will trigger a workup that includes a urine culture and a blood draw. Often doctors will perform a lumbar puncture, more commonly known as a spinal tap, to draw a sample of cerebrospinal fluid from the area around the spinal cord.
Doctors collect these fluids to look for bacteria. Blood, urine and cerebrospinal fluid are smeared onto culture dishes, and doctors wait and see if any bacteria grow. In the meantime, the feverish infant may be started on antibiotics, just in case. But this approach has its limitations. Bacterial cultivation can take several days. The antibiotics may not be necessary. And needless to say, it’s not easy to get those fluids, particularly from a newborn.
Some scientists believe that instead of looking for bacteria or viruses directly, we ought to be looking at how our body responds to them. Unfortunately, the symptoms of a bacterial and viral infection are frustratingly similar. “You get a fever. You feel sick,” says computational immunologist Purvesh Khatri of Stanford University. Sadly, there are no obvious telltale symptoms of one or the other, not even green snot. In very young infants, a fever might be the only sign that something is amiss. But more subtle clues could betray the cause of the fever. When confronted with an infection, our immune systems ramp up in specific ways. Depending on whether we are fighting a viral or bacterial foe, different genes turn up their activity. “The immune system knows what’s going on,” Khatri says. That means that if we could identify the genes that reliably get ramped up by viruses and those that get ramped up by bacteria, then we could categorize the infection based on our genetic response.
That’s the approach used by two groups of researchers, whose study results both appear in the August 23/30 JAMA. One group found that in children younger than 2, two specific genes could help make the call on infection type. Using blood samples, the scientists found that one of the genes ramped up its activity in response to a viral infection, and the other responded to a bacterial infection.
The other study looked at immune responses in even younger children. In infants younger than 60 days, the activity of 66 genes measured in blood samples did a pretty good job of distinguishing between bacterial and viral infections. “These are really exciting preliminary results,” says Khatri, who has used a similar method for adults. “We need to do more work.”
Bauchner points out that in order to be useful, “the test would have to be very, very accurate in very young infants.” There’s very little room for error. “Only time will tell how good these tests will be,” he says. In an editorial that accompanied the two studies, he evoked the promise of these methods. If other experiments replicate and refine the results of these studies, he could envision a day in which the parents of a feverish newborn could do a test at home, call their doctor and together decide if the child needs more care.
That kind of test isn’t here yet, but scientists are working on it. The technology couldn’t come soon enough for doctors and parents desperate to figure out a fever.
Muscles don’t have long-term memory for exercises like running, biking and swimming, a new study suggests. The old adage that once you’ve been in shape, it’s easier to get fit again could be a myth, at least for endurance athletes, researchers in Sweden report September 22 in PLOS Genetics.
“We really challenged the statement that your muscles can remember previous training,” says Maléne Lindholm of the Karolinska Institute in Stockholm. But even if muscles forget endurance exercise, the researchers say, other parts of the body may remember, and that could make retraining easier for people who’ve been in shape before. Endurance training is amazingly good for the body. Weak muscle contractions, sustained over a long period of time — as in during a bike ride — change proteins, mainly ones involved in metabolism. This translates into more energy-efficient muscle that can help stave off illnesses like diabetes, cardiovascular disease and some cancers. The question is, how long do those improvements last?
Previous work in mice has shown that muscles “remember” strength training (SN: 9/11/10, p. 15). But rather than making muscles more efficient, strength-training moves like squats and push-ups make muscles bigger and stronger. The muscles bulk up as they develop more nuclei. More nuclei lead to more production of proteins that build muscle fibers. Cells keep their extra nuclei even after regular exercise stops, to make protein easily once strength training restarts, says physiologist Kristian Gundersen at the University of Oslo in Norway. Since endurance training has a different effect on muscles, scientists weren’t sure if the cells would remember it or not. To answer that question, Lindholm’s team ran volunteers through a 15-month endurance training experiment. In the first three months, 23 volunteers trained four times a week, kicking one leg 60 times per minute for 45 minutes. Volunteers rested their other leg. Lindholm’s team took muscle biopsies before and after the three-month period to see how gene activity changed with training. Specifically, the scientists looked for changes in the number of mRNAs (the blueprints for proteins) that each gene was making. Genes associated with energy production showed the greatest degree of change in activity with training. At a follow-up, after participants had stopped training for nine months, scientists again biopsied muscle from the thighs of 12 volunteers, but didn’t find any major differences in patterns of gene activity between the previously trained legs and the untrained legs. “The training effects were presumed to have been lost,” says Lindholm. After another three-month bout of training, this time in both legs, the researchers saw no differences between the previously trained and untrained legs. While this study didn’t find muscle memory for endurance — most existing evidence is anecdotal — it still might be easier for former athletes to get triathalon-ready, researchers say. The new result has “no bearing on the possible memory in other organ systems,” Gundersen says. The heart and cardiovascular system could remember and more easily regain previous fitness levels, for example, he says.
Even within muscle tissue, immune cells or stem cells could also have some memory not found in this study, says molecular exercise physiologist Monica Hubal of George Washington University in Washington, D.C. Lindholm adds that well-trained connections between nerves and muscles could also help lapsed athletes get in shape faster than people who have never exercised before. “They know how to exercise, how it’s supposed to feel,” Lindholm says. “Your brain knows exactly how to activate your muscles, you don’t forget how to do that.”
To human observers, bumblebees sipping nectar from flowers appear cheerful. It turns out that the insects may actually enjoy their work. A new study suggests that bees experience a “happy” buzz after receiving a sugary snack, although it’s probably not the same joy that humans experience chomping on a candy bar.
Scientists can’t ask bees or other animals how they feel. Instead, researchers must look for signs of positive or negative emotions in an animal’s decision making or behavior, says Clint Perry, a neuroethologist at Queen Mary University of London. In one such study, for example, scientists shook bees vigorously in a machine for 60 seconds — hard enough to annoy, but not hard enough to cause injury — and found that stressed bees made more pessimistic decisions while foraging for food. The new study, published in the Sept. 30 Science, is the first to look for signs of positive bias in bee decision making, Perry says. His team trained 24 bees to navigate a small arena connected to a plastic tunnel. When the tunnel was marked with a blue “flower” (a placard), the bees learned that a tasty vial of sugar water awaited them at its end. When a green “flower” was present, there was no reward. Once the bees learned the difference, the scientists threw the bees a curveball: Rather than being blue or green, the “flower” had a confusing blue-green hue.
Faced with the ambiguous color, the bees appeared to dither, meandering around for roughly 100 seconds before deciding whether to enter the tunnel. Some didn’t enter at all. But when the scientists gave half the bees a treat — a drop of concentrated sugar water — that group spent just 50 seconds circling the entrance before deciding to check it out. Overall, the two groups flew roughly the same distances at the same speeds, suggesting that the group that had gotten a treat first had not simply experienced a boost in energy from the sugar, but were in a more positive, optimistic state, Perry says.
In a separate experiment, Perry and colleagues simulated a spider attack on the bees by engineering a tiny arm that darted out and immobilized them with a sponge. Sugar-free bees took about 50 seconds longer than treated bees to resume foraging after the harrowing encounter.
The researchers then applied a solution to the bees’ thoraxes that blocked the action of dopamine, one of several chemicals that transmit rewarding signals in the insect brain. With dopamine blocked, the effects of the sugar treat disappeared, further suggesting that a change in mood, and not just increased energy, was responsible for the bees’ behavior.
The results provide the first evidence for positive, emotion-like states in bees, says Ralph Adolphs, a neuroscientist at Caltech. Yet he suspects that the metabolic effects of sugar did influence the bees’ behavior. Geraldine Wright, a neuroethologist at Newcastle University in England, shares that concern. “The data reported in the paper doesn’t quite convince me that eating sucrose didn’t change how they behaved, even though they say it didn’t affect flight time or speed of flight,” she says. “I would be very cautious in interpreting the responses of bees in this assay as a positive emotional state.”
It may sound like science fiction, but it’s not: Scientists have created the first time crystal, using a chain of ions. Just as a standard crystal repeats in a regular spatial pattern, a time crystal repeats in time, returning to a similar configuration at regular intervals.
“This is a remarkable experiment,” says physicist Chetan Nayak of Microsoft Station Q at the University of California, Santa Barbara. “There is a ‘wow factor.’”
Scientists at the University of Maryland and the University of California, Berkeley created a chain of 10 ytterbium ions. These ions behave like particles with spin, a sort of quantum mechanical version of angular momentum, which can point either up or down. Using a laser, the physicists flipped the spins in a chain of ions halfway around, from up to down, and allowed the ions to interact so that the spin of each ion would influence the others. The researchers repeated this sequence at regular intervals, flipping the ions halfway each time and letting them interact. When scientists measured the ions’ spins, on average the ions went full circle, returning to their original states, in twice the time interval at which they were flipped halfway. This behavior is sensible — if each flip turns something halfway around, it takes two flips to return to its original position. But scientists found that the ions’ spins would return to their original orientation at that same rate even if they were not flipped perfectly halfway. This result indicates that the system of ions prefers to respond at a certain regular period — the hallmark of a time crystal — just as atoms in a crystal prefer a perfectly spaced lattice. Such time crystals are “one of the first examples of a new phase of matter,” says physicist Norman Yao of UC Berkeley, a coauthor of the new result, posted online September 27 at arXiv.org.
Time crystals take an important unifying concept in physics — the idea of symmetry breaking — and extend it to time. Physical laws typically treat all points in space equally — no one location is different from any other. In a liquid, for example, atoms are equally likely to be found at any point in space. This is a continuous symmetry, as the conditions are the same at any point along the spatial continuum. If the liquid solidifies into a crystal, that symmetry is broken: Atoms are found only at certain regularly spaced positions, with voids in between. Likewise, if you rotate a crystal, on a microscopic level it would look different from different angles, but liquid will look the same however it’s rotated. In physics, such broken symmetries underlie topics ranging from magnets to superconductors to the Higgs mechanism, which imbues elementary particles with mass and gives rise to the Higgs boson.
In 2012, theoretical physicist Frank Wilczek of MIT proposed that symmetry breaking in time might produce time crystals (SN: 3/24/12, p. 8). But follow-up work indicated that time crystals couldn’t emerge in a system in a state of equilibrium, which is settled into a stable configuration. Instead, physicists realized, driven systems, which are periodically perturbed by an external force — like the laser flipping the ions — could create such crystals. “The original examples were either flawed or too simple,” says Wilczek. “This is much more interesting.”
Unlike the continuous symmetry that is broken in the transition from a liquid to a solid crystal, in the driven systems that the scientists used to create time crystals, the symmetry is discrete, appearing at time intervals corresponding to the time between perturbations. If the system repeats itself at a longer time interval than the one it’s driven at — as the scientists’ time crystal does — that symmetry is broken.
Time crystals are too new for scientists to have a handle on their potential practical applications. “It’s like a baby, you don’t know what it’s going to grow up to be,” Wilczek says. But, he says, “I don’t think we’ve heard the last of this by a long shot.” There probably are related systems yet to be uncovered, says Nayak. “We’re just kind of scratching the surface of the kinds of amazing phenomena — such as time crystals — that we can have in nonequilibrium quantum systems. So I think it’s the first window into a whole new arena for us to explore.”
There’s much more to the universe than meets the eye, and a new web-based app lets you explore just how much our eyes are missing. Gleamoscope presents the night sky across a range of electromagnetic frequencies. Spots of gamma rays pinpoint distant feeding black holes. Tendrils of dust glow with infrared light throughout the Milky Way. A supernova remnant — the site of a star that exploded roughly 11,000 years ago — blasts out X-rays and radio waves.
Many of these phenomena are nearly imperceptible in visible light. So astronomers use equipment, such as specialized cameras and antennas, that can detect other frequencies of electromagnetic radiation. Computers turn the data into images, often assigning colors to certain frequencies to highlight specific details or physical processes.
In Gleamoscope, a slider smoothly transitions the scene from one frequency of light to another, turning the familiar star-filled night sky into a variety of psychedelic landscapes. Pan and magnification controls allow you to scan all around the night sky and zoom in for a closer look. The interactive map combines images from many observatories and includes new data from the Murchison Widefield Array, a network of radio antennas in Australia. Over 300,000 galaxies appear as dots in images of the new radio data, described in an upcoming issue of Monthly Notices of the Royal Astronomical Society. The radio map by itself can also be explored on mobile devices in a separate app called GLEAM, available on Google Play.
There’s no bow or festive wrap, but I hope that you will consider this issue a gift of sorts. That is how the staff of Science News thinks of it, our year-end recap of the top science stories. In these pages, you’ll find the stories that continued to resonate well after we first covered them and many that we expect will resonate for years to come — all collected in one easy-to-read, extremely portable, no-batteries-required package (unless you are reading this on a smartphone or tablet, that is). Gravitational waves, of course, occupy the top spot on our list this year. The “of course” reflects the fundamental importance of the detection of this elusive form of energy, announced in February. The finding confirmed key theories in physics, sure, but even more exciting is what it promises for the future. Gravitational waves are powerful tools for probing the universe. Just as the Hubble Space Telescope revealed cosmic beauty in electromagnetic radiation, gravitational wave detectors may show scientists an unprecedented view of far-off Closer to home, the Zika virus became one of our most closely watched stories this year, as the extent of human suffering caused by the mosquito-borne virus became clear. But it’s also a tale of progress: Scientists have responded swiftly, creating a robust literature on the virus in a short time. We still don’t have all the answers, but we’ve come a long way in terms of creating the knowledge urgently needed to inform health recommendations.
Other stories made this year’s list with a more mixed pedigree. The discovery of a (relatively) nearby exoplanet energized many of our science fiction–fueled fantasies of other worlds, for instance. Research moved ahead on what some call “three-parent babies” — using mitochondrial donors to replace a woman’s own disease-prone mitochondria in egg cells — despite a lack of clarity on the procedure’s efficacy. Melting Arctic sea ice has led to a historically significant opening of passageways between the Pacific and the Atlantic oceans. New hope for the battle against Alzheimer’s disease seemed worthy of mention. All these developments and more were regarded by Science News reporters and editors as milestones of discovery or news of importance to society.
We also decided to add some other elements to our year-in-review coverage for 2016. Guided by the deft hand of Beth Quill, our enterprise editor, we augmented our Top 10 list with an essay by managing editor Tom Siegfried about two of physics’s noteworthy recent failures and how the two are related. Science journalist and author Sonia Shah offers a roundup of 2016 in public health, reminding us of the thorny problems associated with infectious diseases, from antibiotic resistance to the resurgence of yellow fever. Other pieces illustrate some of the challenges facing the driverless car revolution, as well as what Science News reporters see on the horizon for the coming year.
We have tried to pack as much science as possible into this issue, from the biggest stories to the more obscure nuggets of discovery and surprise. I can’t think of a gift I’d more like to receive.
Fossils of a giant otter have emerged from the depths of an open-pit mine in China.
The crushed cranium, jaw bone and partial skeletons of at least three animals belong to a now-extinct species of otter that lived some 6.2 million years ago, scientists report January 23 in the Journal of Systematic Palaeontology.
At roughly 50 kilograms in weight, the otter would have outclassed today’s giant otter, a river-dwelling South American mammal weighing in at around 34 kilograms. Scientists named the new species Siamogale melilutra, a nod to its unusual mix of badger and otter features. Melilutra is a mash-up of the Latin words for both creatures.
Badgers and otters both belong to a group of carnivorous animals called Mustelidae, but scientists have had trouble figuring out where to place extinct members in the mammalian family tree. (European badgers and modern otters share similar-looking teeth and skulls.) Still, Siamogale melilutra, however badgerlike, is indeed an otter, researchers concluded after CT scanning, reconstructing and analyzing the fossil skull.
Based on plant and animal fossils found near the collection site, scientists believe that the ancient otter probably lived in the shallow lake of a warm and humid swamp, lush with broad-leaved evergreens and grasses.
WASHINGTON — Lone neutrons quickly decay, but scientists don’t agree on how long the particles stick around before their demise. New experiments could resolve the dispute — or deepen the mystery.
Outside of a nucleus, neutrons survive only about 15 minutes on average. They quickly decay into a proton, an electron and an antineutrino. Two methods used for measuring the neutron lifetime disagree, leaving scientists uncertain about the subatomic particle’s true longevity (SN: 5/19/12, p. 20). One technique involves containing chilled neutrons in a trap, or “bottle,” waiting awhile, and counting the remaining neutrons to determine how many decayed. Other experiments monitor beams of neutrons and count the number of decays by detecting the protons emitted. Bottle measurements come up with lifetimes about 9 seconds shorter than beam measurements.
“This is actually fairly important for a number of things,” physicist Robert Pattie said January 29 at a meeting of the American Physical Society. In particular, pinning down the neutron’s lifetime is necessary for understanding how atomic nuclei began forming after the Big Bang. Scientists’ befuddlement makes it harder to calculate the properties of the early universe.
One drawback of typical bottle experiments is that neutrons can be absorbed or otherwise lost when they hit the wall of the bottle. So Pattie, of the Los Alamos National Laboratory in New Mexico, and colleagues are working on an updated bottle-style measurement using a magnetic field to keep neutrons from hitting the bottom of the trap, while gravity keeps them from flying out of the top.
Physicist Craig Huffer of North Carolina State University in Raleigh and colleagues are working on a bottle experiment that uses a magnetic field to trap neutrons. Rather than counting neutrons at the end, the researchers detect flashes of light produced as neutrons inside the bottle decay away.
In beam experiments, accuracy depends on making a careful count of the neutrons beamed in and the protons produced in the decays, physicist Shannon Fogwell Hoogerheide of the National Institute of Standards and Technology in Gaithersburg, Md., explained at the meeting on January 30. She and colleagues are refining their beam measurement to better enumerate protons and neutrons. Some scientists have suggested the discrepancy could have deeper meaning. The short lifetime in bottle measurements could indicate that neutrons are somehow disappearing unexpectedly, making the lifetime appear shorter than it really is. “Kind of an out-there mechanism is that they’ve gone into some kind of alternative reality, which we call the mirror world,” says physicist Ben Rybolt of the University of Tennessee. In such a world, all the particles we know of would be duplicated — mirror protons, neutrons and electrons could exist, which would interact only very slightly with the particles we know.
Jumping to such an explanation for the neutron lifetime discrepancy is “a little bit of a leap,” Rybolt acknowledges, but such mirror particles could also explain the conundrum of dark matter, an unseen substance indicated by the motions of stars inside galaxies. To test the idea, Rybolt and colleagues are proposing to shoot beams of neutrons at a barrier and check if any make it through, which could indicate the particles had briefly become mirror neutrons.
The problem of measuring the neutron’s lifetime is complex enough that a variety of new techniques are under preparation to unravel the issue. “I don’t think one additional experiment can resolve the discrepancy,” says Huffer. Instead, multiple new measurements with different techniques should eventually converge on the correct value.
Almost every night that the constellation Orion is visible, physicist Mark Vagins steps outside to peer at a reddish star at the right shoulder of the mythical figure. “You can see the color of Betelgeuse with the naked eye. It’s very striking, this red, red star,” he says. “It may not be in my lifetime, but one of these days, that star is going to explode.”
With a radius about 900 times that of the sun, Betelgeuse is a monstrous star that is nearing its end. Eventually, it will no longer be able to support its own weight, and its core will collapse. A shock wave from that collapse will speed outward, violently expelling the star’s outer layers in a massive explosion known as a supernova. When Betelgeuse detonates, its cosmic kaboom will create a light show brighter than the full moon, visible even during the daytime. It could happen tomorrow, or a million years from now. Countless stars like Betelgeuse — any of which could soon explode — litter the Milky Way. Scientists estimate that a supernova occurs in our galaxy a few times a century. While brilliantly gleaming supernovas in far-flung galaxies are regularly spotted with telescopes trained on the heavens, scientists eagerly hope to capture two elusive signatures that are detectable only from a supernova closer to home. These signatures are neutrinos (subatomic particles that stream out of a collapsing star’s center) and gravitational waves (subtle vibrations of spacetime that will also ripple out from the convulsing star). “These two signals, directly from the interior of the supernova, are the ones we are really longing for,” says Hans-Thomas Janka, an astrophysicist at the Max Planck Institute for Astrophysics in Garching, Germany. Unlike light, which is released from the star’s surface, stealthy neutrinos and gravitational waves would give scientists a glimpse of the processes that occur deep inside a collapsing star.
Supernovas offer more than awe-inspiring explosions. When they erupt, the stars spew out their guts, seeding the cosmos with chemical elements necessary for life to exist. “We clearly wouldn’t be here without supernovas,” says Vagins, of the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo. But the processes that occur within the churning mess are still not fully understood. Computer simulations have revealed much of the physics of how stars explode, but models are no substitute for a real-life nearby blast.
One inspiration for scientists is supernova 1987A, which appeared in the Large Magellanic Cloud, a satellite galaxy of the Milky Way, 30 years ago (SN: 2/18/17, p. 20). That flare-up hinted at the unparalleled information nearby supernovas could provide. With the detectors available at the time, scientists managed to nab just two dozen of 1987A’s neutrinos (SN: 3/21/87, p. 180). Hundreds of papers have been written analyzing that precious handful of particles. Calculations based on those detections confirmed scientists’ hunch that unfathomably large numbers of neutrinos are released after a star’s core collapses in a supernova. In total, 1987A emitted about 1058 neutrinos. To put that in perspective, there are about 1024 stars in the observable universe — a vastly smaller number.
Since 1987, neutrino detectors have proliferated, installed in exotic locales that are ideal for neutrino snagging, from the Antarctic ice sheet to deep mines across the globe. If a supernova went off in the Milky Way today, scientists could potentially nab thousands or even a million neutrinos. Gravitational wave detection has likewise come on the scene, ready to pick up a slight shift in spacetime stirred up by the blast. Detecting such gravitational waves or a surfeit of supernova neutrinos would lead to a distinct leap in scientists’ knowledge, and provide new windows into supernovas. All that’s needed now is the explosion.
Early warning Despite estimates that a few stars explode in the Milky Way every century, no one has glimpsed one since the early 1600s. It’s possible the explosions have simply gone unnoticed, says physicist John Beacom of Ohio State University in Columbus; light can be lost in the mess of gas and dust in the galaxy. A burst of neutrinos from a supernova would provide a surefire signal.
These hermitlike elementary particles shun most interactions with matter. Produced in stars, radioactive decay and other reactions, neutrinos are so intangible that trillions of neutrinos from 1987A’s explosion passed through the body of every human on Earth, yet nobody felt a thing. For supernovas like 1987A, known as type 2 or core-collapse supernovas, about 99 percent of the explosion’s energy goes into the tiny particles. Another, less common kind of supernova, type 1a, occurs when a remnant of a star called a white dwarf steals matter from a companion star until the white dwarf explodes (SN: 4/30/16, p. 20). In type 1a supernovas, there’s no core collapse, so neutrinos from these explosions are much less numerous and are less likely to be detectable on Earth.
For scientists studying supernovas, neutrinos’ reluctance to interact is an advantage. The particles don’t get bogged down in their exit from the star, so they arrive at Earth several hours or even more than a day before light from the explosion, which is released only after the shock wave travels from the star’s core up to its surface. That means the particles can tip off astronomers that a light burst is imminent, and potentially where it is going to occur, so they can have their telescopes ready.
Most neutrino experiments (there are more than a dozen) weren’t built for the purpose of taking snapshots of unpredictable supernovas; they were built to study neutrinos from reliable sources, like the sun, nuclear reactors or particle accelerators. Nevertheless, seven neutrino experiments have joined forces to create the SuperNova Early Warning System, SNEWS. If neutrino detectors in at least two locations report an unexpectedly large burst of neutrinos, SNEWS will send an e-mail alert to the world’s astronomers. The experiments involved are a weirdly diverse bunch, including IceCube, a detector composed of light sensors frozen deep in the ice of Antarctica (SN: 12/27/14, p. 27); Super-Kamiokande, which boasts a tank filled with 50,000 tons of water stationed in a mine in Hida, Japan; and the Helium and Lead Observatory, or HALO — with the motto “astronomically patient” — made of salvaged lead blocks in a mine in Sudbury, Canada. Their common thread: The experiments are big to provide a lot of material for neutrinos to interact with — such as lead, water or ice.
With light sensors sunk kilometers deep into the ice sheet, IceCube’s detector is so huge that it could pick up traces of a million neutrinos from a Milky Way supernova. Because it was designed to capture only the highest energy neutrinos that are rocketing through space, it’s not sensitive enough to detect individual neutrinos emitted during a supernova. Instead, IceCube’s focus is on the big picture: It catalogs an increase of light in its detectors produced by neutrinos interacting in the ice in time slices of two billionths of a second, says IceCube leader Francis Halzen of the University of Wisconsin–Madison. “We make a movie of the supernova.”
Super-Kamiokande is the neutrino detector that can pinpoint the location of the impending stellar paroxysm. It is a successor to Kamiokande-II, one of a few detectors to spot a handful of neutrinos from 1987A. Shortly after a burst of neutrinos from a nearby supernova, the detector could direct astronomers to zero in on a few degrees of sky. If that happens, says neutrino physicist Kate Scholberg of Duke University, “I expect anybody with the capability will be zooming in.” On the lookout Various neutrino detectors await signals emitted from a supernova, including Super-Kamiokande in Japan, IceCube in Antarctica and HALO in Canada. They are joined by a gravitational wave observatory, LIGO, with detectors in Louisiana (shown) and Washington state.
Languages of a supernova Light and neutrinos are two of several languages that a supernova speaks. In that sense, supernova 1987A was a “Rosetta stone,” Beacom says. By scrutinizing 1987A’s light and its handful of neutrinos, scientists began piecing together the theoretical physics that explains what happened inside the star. In a future supernova, another language, gravitational waves, could add nuance to the tale. But the explosion has got to be close.
If neutrinos are elusive, gravitational waves border on undetectable. Minute tremors in space itself, predicted by Einstein’s general theory of relativity, are generated when massive objects accelerate. In 2016, scientists with the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, announced the first direct detection of gravitational waves, produced by two merging black holes (SN: 3/5/16, p. 6). That milestone required a pair of detectors so precise that they can sense quivers that squish the detectors’ 4-kilometer-long arms by just a tiny fraction of the diameter of a proton.
Gravitational waves from a supernova should be even harder to tease out than those from merging black holes. The pattern of ripples is less predictable. Surveys of the properties of the many supernovas detected in other galaxies indicate that the explosions vary significantly from one to the next, says astroparticle physicist Shunsaku Horiuchi of Virginia Tech in Blacksburg. “We ask, ‘Is there a standard supernova?’ The answer is ‘No.’ ”
Despite the challenges, finding gravitational waves from supernovas is a possibility because the explosions are chaotic and asymmetrical, producing lumpy, lopsided bursts. An explosion that expands perfectly symmetrically, like an inflating balloon, would produce no gravitational waves. The gravitational wave signature thus can tell scientists how cockeyed the detonation was and how fast the star was spinning.
Gravitational waves might also reveal some of the physics of the strange stew of neutrons that makes up a protoneutron star — the beginnings of an incredibly dense star formed in a supernova. Scientists would like to catalog the compressibility of the neutron-rich material — how it gets squeezed and rebounds in the collapse. “The gravitational wave signature would have an imprint in it of this stiffness or softness,” says computational astrophysicist Tony Mezzacappa of the University of Tennessee.
There’s a chance the supernova would collapse into an even more enigmatic state — a black hole, which has a gravitational field so strong that not even light can escape. When a black hole forms, the flow of neutrinos would abruptly drop, as their exit route is cut off. Detectors would notice. “Seeing the moment that a black hole is born,” says Vagins, “would be a tremendously exciting thing.”
While neutrinos can be oracles of supernovas, a stellar explosion could reveal a lot about neutrinos themselves. There are three types of neutrinos: electron, muon and tau. All are extremely light, with masses less than a millionth that of an electron (SN: 1/26/13, p. 18). But scientists don’t know which of the three neutrinos is the lightest; a nearby supernova could answer that question.
Supernovas, with all the obscure physics at their hearts, have a direct connection to Earth. They are a source of many of the elements from which planets eventually form. As stars age, they fuse together heavier and heavier elements, forging helium from hydrogen, carbon from helium and so on up the periodic table to iron. Those elements, including some considered essential to life, such as carbon and oxygen, spew out from a star’s innards in the explosion.
“All the elements that exist — that are here on Earth — that are heavier than iron were either made in supernovas or other cataclysmic events in astronomy,” says physicist Clarence Virtue of Laurentian University in Sudbury, Canada. Gold, platinum and many other elements heavier than iron are produced in a chain of reactions in which neutrons are rapidly absorbed, known as the r-process (SN: 5/14/16, p. 9). But scientists still argue about whether the r-process occurs in supernovas or when neutron stars merge with one another. Pulling back the curtain on supernovas could help scientists resolve the dispute.
Even the reason supernovas explode and sow their chemical seeds has been vigorously debated. Until recently, computer simulations of supernovas have often fizzled, indicating that something happens in a real explosion that scientists are missing. The shock wave seems to need an extra kick to make it out of the star and produce the luminous explosion. The most recent simulations indicate that the additional oomph is most likely imparted by neutrinos streaming outward. But, says Mezzacappa, “At the end of the day, we’re going to need some observations against which we can check our models.”
Hurry up and wait Supernovas’ potential to answer such big questions means that scientists are under pressure not to miss a big break. “You better be ready. If it happens and you’re not ready, then you will be sad,” Scholberg says. “We have to be as prepared as possible to gather as much information as we possibly can.” If a detector isn’t operating at the crucial moment, there’s no second chance. So neutrino experiments are designed to run with little downtime and to skirt potential failure modes — a sudden flood of data from a supernova could crash electronics systems in an ultrasensitive detector, for example. Gravitational wave detectors are so finicky that interference as subtle as waves crashing on a nearby beach can throw them out of whack. And in upcoming years, LIGO is scheduled to have detectors off for months at a time for upgrades. Scientists can only hope that, when a supernova comes, everything is up and running.
Some even hope that neighboring stars hold off a little longer. “It seems like I’m always telling people that I’d like Betelgeuse to go off one year from now,” jokes Bronson Messer, a physicist who works on supernova simulations at Oak Ridge National Laboratory in Tennessee. With each improvement of the simulations, he’s eager for a bit more time to study them.
Messer keeps getting his wish, but he doesn’t want to wait too long. He’d like to see a supernova in the Milky Way during his lifetime. But it could be many decades. Just in case, Vagins, who’s been taking those nightly peeks at Betelgeuse, is doing his part to prepare the next generation. He no longer scans the skies alone. “I’ve already taught my 6-year-old son how to find that star in the sky,” he says. “Maybe I won’t get to see it go, but maybe he’ll get to see it go.”
