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      A meteor may have exploded over Antarctica 430,000 years ago

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      Seventeen tiny particles recovered from a flat-topped mountain in eastern Antarctica suggest that a space rock shattered low in the atmosphere over the ice-smothered continent about 430,000 years ago.

      The nickel- and magnesium-rich bits were sifted from more than 6 kilograms of loose sediments collected atop the 2,500-meter-tall summit of Walnumfjellet, says Matthias van Ginneken, a cosmochemist at the University of Kent in England. Their exotic chemistry doesn’t match Earth rocks, but it does match the proportions of elements seen in a type of meteorite called a carbonaceous chondrite, van Ginneken and his colleagues report March 31 in Science Advances.

      Most of the particles range in size from 0.1 to 0.3 millimeters across, and more than half consist of spherules that are fused together into odd-shaped globs. The elemental mix in the spherules closely matches that of particles found at two other far-flung sites in Antarctica— one more than 2,750 kilometers away — which suggests that all of the materials originated in the same event. Because the other particles were found in ice cores and dated to about 430,000 years ago, the team presumes that the newly found particles from Walnumfjellet fell then too.  

      The chemistry of nickel- and magnesium-rich spherules (pictured) found on a mountaintop in Antarctica match that of a certain type of stony meteorites.Scott Peterson/micro-meteorites.com

      The chemistry of nickel- and magnesium-rich spherules (pictured) found on a mountaintop in Antarctica match that of a certain type of stony meteorites.Scott Peterson/micro-meteorites.com

      The meteor that broke up over Antarctica was between 100 to 150 meters across, the team’s simulations suggest, and probably burst at low altitude. Blast waves may have pummeled a 100,000-square-kilometer area of the ice sheet, the team estimates. The explosion left no crater, but peak temperatures where the plume of hot gases reached Earth’s surface would have hit 5,000° Celsius and may have melted up to a few centimeters of ice. A similar airburst over a densely populated area today would result in millions of casualties and severely damage an area hundreds of kilometers across (SN: 5/2/17). More

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      Physicists’ devotion to symmetry has led them astray before

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      Second of two parts

      Physicists have a lot in common with Ponce de León and U2’s Bono. After decades of searching, they aren’t getting any younger. And they still haven’t found what they’re looking for.

      In this case, the object of the physicists’ quest is SUSY. SUSY is not a real person or even a fountain relevant to aging in any way. It’s a mathematical framework based on principles of symmetry that could help physicists better explain the mysteries of the universe. Many experts believe that particles predicted by SUSY are the weakly interacting massive particles, or WIMPs, that supposedly make up the invisible “dark matter” lurking throughout the cosmos.

      So far, though, SUSY has been something of a disappointment. Despite multiple heroic searches, SUSY has remained concealed from view. Maybe it is a mathematical mirage.

      If SUSY does turn out to be a myth, it won’t be the first time that symmetry has led science on a wild WIMP chase. Reasoning from the symmetry of circular motion originally suggested the existence of a new form of matter out in space more than two millennia ago. Devotion to that symmetry blinded science to the true nature of the solar system and planetary motion for the next 19 centuries.

      You can blame Plato and Aristotle. In their day, ordinary matter supposedly consisted of four elements: earth, air, fire and water. Aristotle built an elaborate theory of motion based on those elements. He insisted that they naturally moved in straight lines; earth and water moving straight down (toward the center of the world), air and fire moving straight up. In the heavens, though, Aristotle noticed that motion appeared to be circular, as the stars rotated around the nighttime sky. “Our eyes tell us that the heavens revolve in a circle,” he wrote in On the Heavens. Since the known four elements all moved in a straight line, Aristotle deduced that the heavens must consist of a fifth element, called aether — absent on Earth but predominant in space.

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      Plato, on theoretical rather than observational grounds, had already insisted that circularity’s symmetry signified perfection, and therefore circular motion should be required in the heavens. And so for centuries, the assumption that celestial motion must be circular held a stranglehold on natural philosophers attempting to understand of the universe. As late as the 16th century, Copernicus was willing to depose Aristotle’s Earth from the middle of everything but still believed that the Earth and other planets revolved around the sun with a combination of circular motions. Another half century passed before Kepler established that planetary orbits are elliptical, not circular.

      Aristotle’s belief in an exotic form of matter in space is not so different from the picture scientists paint of the heavens today, albeit in a rather more rigorous and sophisticated theoretical way. Dark matter predominates in space, astronomers believe; it is inferred to exist from gravitational effects altering the motions of stars and galaxies. And physicists have determined that the dark matter cannot (for various noncircular reasons) be made of the same ordinary matter found on Earth.

      SUSY particles have long been one of the most popular proposals for the identity of this cosmic dark matter, based on more complicated notions of symmetry than those available to Plato and Aristotle. And since the onset of the 20th century, symmetry math has generated an astounding string of scientific successes. From Einstein’s relativity to the theory of elementary particles and forces, symmetry considerations now form the core of science’s understanding of nature.  

      These mathematical forms of symmetry are more elaborate examples of symmetry as commonly understood: a change that leaves things looking like they did before. A perfectly symmetric face looks the same when a mirror swaps left with right. A perfect sphere’s appearance is not altered when you rotate it to see the other side. Rotate a snowflake by any multiple of 60 degrees and you see the same snowflake.

      In a similar way, more sophisticated mathematical frameworks, known as symmetry groups, describe aspects of the physical world, such as time and space or the families of subatomic particles that make up matter or transmit forces. Symmetries in the equations of such math can even predict previously unknown phenomena. Symmetry in the equations describing subatomic particles, for instance, revealed that for each particle nature allowed an antimatter particle, with opposite electric charge.

      In fact, all the known ordinary matter and force particles fit neatly into the mathematical patterns described by symmetry groups. But none of those particles can explain the dark matter.

      SUSY particles as a dark matter possibility emerged in the 1970s and 1980s, when theorists proposed an even more advanced symmetry system. That math, called supersymmetry (hence SUSY), suggested the existence of a “super” partner particle for each known particle: a force-particle partner for every matter particle, and a matter-particle partner for every force particle. It was an elegant concept mathematically, and it solved (or at least ameliorated) some other vexing theoretical problems. Plus, of the super partner particles it predicted, the lightest one (whichever one that was) seemed likely to be a perfect dark matter WIMP.

      Alas, efforts to detect WIMPs (which should be hitting the Earth all the time) have almost all failed to find any. One experiment that did claim a WIMP detection seems to be on shaky ground — a new experiment, using the same method and materials, reports no such WIMP evidence. And attempts to produce SUSY particles in the world’s most powerful particle accelerator, the Large Hadron Collider, have also come up empty.

      Some physicists have therefore given up on SUSY. And perhaps supersymmetry has been as misleading as the Greek infatuation with circular motion. But the truth is that SUSY is not a theory that can be slain by a single experiment. It is a more nebulous mathematical notion, a framework within which many specific theories can be constructed.   

      “You can’t really kill SUSY because it’s not a thing,” physicist Patrick Stengel of the International Higher School of Advanced Studies in Trieste, Italy, said at a conference in Washington, D.C., in 2019. “It’s not an idea that you can kill. It’s basically just a framework for a bunch of ideas.”

      At the same conference, University of Texas at Austin physicist Katherine Freese pointed out that there was never any guarantee that the Large Hadron Collider would discover SUSY. “Even before the LHC got built, there were a lot of people who said, well, it might not go to a high enough energy,” she said.

      So SUSY may yet turn out to be an example of symmetry that leads physics to success. But just in case, physicists have pursued other dark matter possibilities. One old suggestion that has recently received renewed interest is a lightweight hypothetical particle called an axion (SN: 3/24/20).

      Of course, if axions do exist, symmetry fans could still rejoice — the motivation for proposing the axion to begin with was resolving an issue with yet another form of symmetry. More

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      Uranium ‘snowflakes’ could set off thermonuclear explosions of dead stars

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      Tiny crystals of uranium could set off massive explosions within a dead star, physicists propose, making for a cosmic version of a thermonuclear bomb.

      Expired stars called white dwarfs slowly cool as they age. In the process, heavy elements such as uranium begin to crystalize, forming “snowflakes” in the stars’ cores. If enough uranium clumps together — about the mass of a grain of sand — it could initiate a chain of nuclear fission reactions, or the splitting of atomic nuclei.

      Those reactions could raise temperatures within the star, setting off nuclear fusion — the merging of atomic nuclei — and generating an enormous explosion that destroys the star, two physicists calculate in a paper published March 29 in Physical Review Letters. The effect is akin to a hydrogen bomb, a powerful thermonuclear weapon in which fission reactions trigger fusion, says Matt Caplan of Illinois State University in Normal. The scenario is still hypothetical, Caplan admits — more research is needed to determine if uranium snowflakes could really spur a stellar detonation.

      White dwarfs are already known to be explosion-prone: They’re the source of blasts called type 1a supernovas. Typically, these explosions happen when a white dwarf pulls matter off a companion star (SN: 3/23/16). The researchers’ uranium snowflake proposal is an entirely new mechanism that might explain a small fraction of type 1a supernovas, without the need for another star. More

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      Here’s why humans chose particular groups of stars as constellations

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      The Big Dipper’s stars make up a conspicuous landmark in the sky of the Northern Hemisphere. Even novice stargazers can easily pick out the shape, part of the Ursa Major constellation. Now, scientists have shown that three factors can explain why certain groups of stars form such recognizable patterns.

      To replicate how humans perceive the celestial sphere, a team of researchers considered how the eye might travel randomly across this night sky. Human eyes tend to move in discrete jumps, called saccades (SN: 10/31/11), from one point of interest to another. The team created a simulation that incorporated the distribution of lengths of those saccades, combined that with basic details of the night sky as seen from Earth — namely the apparent distances between neighboring stars and their brightnesses.

      The technique could reproduce individual constellations, such as Dorado, the dolphinfish. And when used to map the whole sky, the simulation generated groupings of stars that tended to align with the 88 modern constellations recognized by the International Astronomical Union, Sophia David and colleagues reported March 18 at an online meeting of the American Physical Society.

      “Ancient people from various cultures connected similar groupings of stars independently of each other,” said David, a high school student at Friends’ Central School in Wynnewood, Penn., who worked with network scientists at the University of Pennsylvania. “And this indicates that there are some fundamental aspects of human learning … that influence the ways in which we organize information.” More

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      The ‘USS Jellyfish’ emits strange radio waves from a distant galaxy cluster

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      Something’s fishy in the southern constellation Phoenix.

      Strange radio emissions from a distant galaxy cluster take the shape of a gigantic jellyfish, complete with head and tentacles. Moreover, the cosmic jellyfish emits only the lowest radio frequencies and can’t be detected at higher frequencies. The unusual shape and radio spectrum tell a tale of intergalactic gas washing over galaxies and gently revving up electrons spewed out by gargantuan black holes long ago, researchers report in the March 10 Astrophysical Journal.

      Spanning 1.2 million light-years, the strange entity lies in Abell 2877, a cluster of galaxies 340 million light-years from Earth. Researchers have dubbed the object the USS Jellyfish, because of its ultra-steep spectrum, or USS, from low to high radio frequencies.

      “This is a source which is invisible to most of the radio telescopes that we have been using for the last 40 years,” says Melanie Johnston-Hollitt, an astrophysicist at Curtin University in Perth, Australia. “It holds the record for dropping off the fastest” with increasing radio frequency.

      Johnston-Hollitt’s colleague Torrance Hodgson, a graduate student at Curtin, discovered the USS Jellyfish while analyzing data from the Murchison Widefield Array, a complex of radio telescopes in Australia that detect low-frequency radio waves. These radio waves are more than a meter long and correspond to photons, particles of light, with the lowest energies. Remarkably, the USS Jellyfish is about 30 times brighter at 87.5 megahertz — a frequency similar to that of an FM radio station — than at 185.5 MHz.

      The Murchison Widefield Array consists of 4,096 radio antennas grouped into 256 “tiles” (one pictured) spanning several kilometers in a remote region of Western Australia.Pete Wheeler, ICRAR

      “That is quite spectacular,” says Reinout van Weeren, an astronomer at Leiden University in the Netherlands who was not involved with the work. “It is quite a neat result, because this is really extreme.”

      The USS Jellyfish bears no relation to previously discovered jellyfish galaxies. “This is absolutely enormous compared to those other things,” Johnston-Hollitt says. Indeed, jellyfish galaxies are a very different kettle of celestial fish. Although they also inhabit galaxy clusters, they are individual galaxies passing through hot gas in a cluster. The hot gas tears the galaxy’s own gas out of it, creating a wake of tentacles. The much larger USS Jellyfish, on the other hand, appears to have formed when intergalactic gas and electrons interacted.

      Hodgson and his colleagues note that two galaxies in the Abell 2877 cluster coincide with the brightest patches of radio waves in the USS Jellyfish’s head. These galaxies, the researchers say, probably have supermassive black holes at their centers. The team ran computer simulations and found that the black holes were probably accreting material some 2 billion years ago. As they did so, disks of hot gas formed around each of them, spewing huge jets of material into the surrounding galaxy cluster.

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      This ejected material had electrons that whirled around magnetic fields at nearly the speed of light, so the electrons emitted radio waves. Over time, though, the electrons lost energy, and the most energetic electrons, which had been emitting the highest radio frequencies, faded the most. Then a wave of gas sloshed through the entire cluster, reaccelerating the electrons around the two galaxies.

      “It’s a very gentle process,” Johnston-Hollitt says. “The electrons don’t get that much energy, which means they don’t light up at high frequencies.” Instead, the gentle gas wave caused electrons to emit radio waves with the lowest energies and frequencies, giving the USS Jellyfish the extreme spectrum it has today. More

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      50 years ago, experiments hinted at the possibility of life on Mars

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      Organics on Mars — Science News, March 27, 1971

      [Researchers] have exposed a mixture of gases simulating conditions believed to exist on the surface of Mars to ultraviolet radiation. The reaction produced organic compounds. They conclude that the ultraviolet radiation bombarding the surface of Mars could be producing organic matter on that planet.… The fact that such organic compounds may be produced on the Martian surface increases the possibility of life on Mars.

      Update

      In 1976, a few years after those experiments, NASA took its search for organic molecules to the Red Planet’s surface. That year, the Viking landers became the first U.S. mission to land on Mars. Though the landers failed to turn up evidence in the soil, NASA has continued the hunt. In 2018, the Curiosity rover found hints of life: organic molecules in rocks and seasonal shifts in atmospheric methane. A new phase of the hunt began in February when the Perseverance rover landed on Mars (SN Online: 2/17/21). It will find and store rocks that might preserve signs of past life for eventual return to Earth. More

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      A new black hole image reveals the behemoth’s magnetic fields

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      Astronomers have gotten their first glimpse of the magnetic fields tangled around a black hole.

      The Event Horizon Telescope has unveiled the magnetism of the hot, glowing gas around the supermassive black hole at the heart of galaxy M87, researchers report in two studies published online March 24 in the Astrophysical Journal Letters. These magnetic fields are thought to play a crucial role in how the black hole scarfs down matter and launches powerful plasma jets thousands of light-years into space (SN: 3/29/19).

      “We’ve known for decades that jets are in some sense powered by accretion onto supermassive black holes, and that the in-spiraling gas and the outflowing plasma are highly magnetized — but there was a lot of uncertainty in the exact details,” says Eileen Meyer, an astrophysicist at the University of Maryland, Baltimore County not involved in the work. “The magnetic field structure of the plasma near the event horizon [of a black hole] is a completely new piece of information.”

      The supermassive black hole inside M87 was the first black hole to get its picture taken (SN: 4/10/19). That image showed the black hole’s shadow against its accretion disk — the bright eddy of superhot gas spiraling around the black hole’s dark center. It was created using observations taken in April 2017 by a global network of observatories, which collectively form one virtual, Earth-sized radio dish called the Event Horizon Telescope (SN: 4/10/19).

      [embedded content]
      Using data from 2017, scientists created the first real picture of the supermassive black hole at the center of galaxy M87. How? We explain.

      The new analysis uses the same observations. But unlike the black hole’s initial portrait, the new image accounts for the polarization of the light waves emitted by gas around the black hole. Polarization measures a light wave’s orientation — whether it wiggles up and down, left and right or at an angle — and can be affected by the magnetic field where the light originated. So, by mapping the polarization of light around the edge of M87’s black hole, researchers were able to trace the structure of the underlying magnetic fields.

      The team found evidence that some magnetic fields loop around the black hole along with the disk of material swirling into it. That’s to be expected because “when gas is rotating, it’s basically able to carry along the magnetic field with it,” says Jason Dexter, an astrophysicist at the University of Colorado Boulder.

      But, he says, “there’s some interesting component of this magnetic field which is not just following the motion of the gas.” At least some of the magnetic field lines are sticking up or down perpendicularly from the accretion disk, or pointing directly toward or away from the black hole, Dexter and colleagues found. These magnetic fields must be very strong to resist being dragged around by the whirl of infalling gas, he says.

      Such strong magnetic fields may actually push back against some of the material spiraling in toward the black hole, helping it resist gravity’s pull, says study coauthor Monika Mościbrodzka, an astrophysicist at Radboud University in Nijmegen, the Netherlands. Magnetic fields pointed up and down from the accretion disk could also help launch the black hole’s plasma jets, by channeling material toward the black hole’s poles and giving it a boost in speed, she says.

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      The dark matter mystery deepens with the demise of a reported detection

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      First of two parts

      In mystery stories, the chief suspect almost always gets exonerated before the end of the book. Typically because a key piece of evidence turned out to be wrong.

      In science, key evidence is supposed to be right. But sometimes it’s not. In the mystery of the invisible “dark matter” in space, evidence implicating one chief suspect has now been directly debunked. WIMPs, tiny particles widely regarded as prime dark matter candidates, have failed to appear in an experiment designed specifically to test the lone previous study claiming to detect them.

      For decades, physicists have realized that most of the universe’s matter is nothing like earthly matter, which is made mostly from protons and neutrons. Gravitational influences on visible matter (stars and galaxies) indicate that some dark stuff of unknown identity pervades the cosmos. Ordinary matter accounts for less than 20 percent of the cosmic matter abundance.

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      For unrelated reasons, theorists have also long suggested that nature possesses mysterious types of tiny particles predicted by a theoretical mathematical framework known as supersymmetry, or SUSY for short. Those particles would be massive by subatomic standards but would interact only weakly with other matter, and so are known as Weakly Interacting Massive particles, hence WIMPs.

      Of the many possible species of WIMPs, one (presumably the lightest one) should have the properties necessary to explain the dark matter messing with the motion of stars and galaxies (SN: 12/27/12). Way back in the last century, searches began for WIMPs in an effort to demonstrate their existence and identify which species made up the dark matter.

      In1998, one research team announced apparent success. An experiment called DAMA (for DArk MAtter, get it?), consisting of a particle detector buried under the Italian Alps, seemingly did detect particles with properties matching some physicists’ expectations for a dark matter signal.

      It was a tricky experiment to perform, relying on the premise that space is full of swarms of WIMPs. A detector containing chunks of sodium iodide should give off a flash of light when hit by a WIMP. But other particles from natural radioactive substances would also produce flashes of light even if WIMPs are a myth.

      So the experimenters adopted a clever suggestion proposed earlier by physicists Katherine Freese, David Spergel and Andrzej Drukier, known formally as an annual modulation test. But let’s just call it the June-December approach.

      As the Earth orbits the sun, the sun also moves, traveling around the Milky Way galaxy, carried by a spiral arm in the direction of the constellation Cygnus. If the galaxy really is full of WIMPs, the sun should be constantly plowing through them, generating a “WIMP wind.” (It’s like the wind you feel if you stick your head out of the window of a moving car.) In June, the Earth’s orbit moves it in the same direction as the sun’s motion around the galaxy — into the wind. But in December, the Earth moves the opposite direction, away from the wind. So more WIMPs should be striking the Earth in June than in December. It’s just like the way your car windshield smashes into more raindrops when driving forward than when going in reverse.

      As the sun moves through space, it should collide with dark matter particles called WIMPs, if they exist. When the Earth’s revolution carries it in the same direction as the sun, in summer, the resulting “WIMP wind” should appear stronger, with more WIMP collisions detected in June than in December.GEOATLAS/GRAPHI-OGRE, ADAPTED BY T. DUBÉ

      At an astrophysics conference in Paris in December 1998, Pierluigi Belli of the DAMA team reported a clear signal (or at least a strong hint) that more particles arrived in June than December. (More precisely, the results showed an annual modulation in frequency of light flashes, peaking around June with a minimum in December.) The DAMA data indicated a WIMP weighing in at 59 billion electron volts, roughly 60 times the mass of a proton.

      But some experts had concerns about the DAMA team’s data analysis. And other searches for WIMPs, with different detectors and strategies, should have found WIMPs if DAMA was right — but didn’t. Still, DAMA persisted. An advanced version of the experiment, DAMA/LIBRA, continued to find the June-December disparity.

      Perhaps DAMA was more sensitive to WIMPs than other experiments. After all, the other searches did not duplicate DAMA’s methods. Some used substances other than sodium iodide as a detecting material, or watched for slight temperature increases as a sign of a WIMP collision rather than flashes of light.

      For that matter, WIMPs might not be what theorists originally thought. DAMA initially reported 60 proton-mass WIMPs based on the belief that the WIMPs collided with iodine atoms. But later data suggested that perhaps the WIMPs were hitting sodium atoms, implying a much lighter WIMP mass — lighter than other experiments had been optimally designed to detect. Yet another possibility: Maybe trace amounts of the metallic element thallium (much heavier atoms than either iodine or sodium) had been the WIMP targets. But a recent review of that proposal found once again that the DAMA results could not be reconciled with the absence of a signal in other experiments.

      And now DAMA’s hope for vindication has been further dashed by a new underground experiment, this one in Spain. Scientists with the ANAIS collaboration have repeated the June-December method with sodium iodide, in an effort to reproduce DAMA’s results with the same method and materials. After three years of operation, the ANAIS team reports no sign of WIMPs.

      To be fair, the no-WIMP conclusion relies on a lot of seriously sophisticated technical analysis. It’s not just a matter of counting light flashes. You have to collect rigorous data on the behavior of nine different sodium iodide modules. You have to correct for the presence of rare radioactive isotopes generated by cosmic ray collisions while the modules were still under construction. And then the statistical analysis needed to discern a winter-summer signal difference is not something you should try at home (unless you’re fully versed in things like the least-square periodogram or the Lomb-Scargle technique). Plus, ANAIS it still going, with plans to collect two more years of data before issuing a final analysis. So the judgment on DAMA’s WIMPs is not necessarily final.

      Nevertheless, it doesn’t look good for WIMPs, at least for the WIMPs motivated by belief in supersymmetry.   

      Sadly for SUSY fans, searches for WIMPs from space are not the only bad news. Attempts to produce WIMPs in particle accelerators have also so far failed. Dark matter might just turn out to consist of some other kind of subatomic particle.

      If so, it would be a plot twist worthy of Agatha Christie, kind of like Poirot turning out to be the killer. For symmetry has long been physicists’ most reliable friend, guiding many great successes, from Einstein’s relativity theory to the standard model of particles and forces.

      Still, failure to find SUSY particles so far does not necessarily mean they don’t exist. Supersymmetry just might be not as simple as it first seemed. And SUSY particles might just be harder to detect than scientists originally surmised. But if supersymmetry does turn out not to be so super, scientists might need to reflect on the ways that faith in symmetry can lead them astray. More