The biggest ice shelf collapse on record was set in motion years earlier than previously thought, new research reveals.

Analyzing declassified images from spy satellites, researchers discovered that the downhill flow of ice on Antarctica’s Larsen B ice shelf was already accelerating as early as the 1960s and ’70s. By the late 1980s, the average ice velocity at the front of the shelf was around 20 percent faster than in the preceding decades, the researchers report in a paper to be published in Geophysical Research Letters.
Rising temperatures since the 1950s probably quickened the ice flow, which in turn put more strain on the ice and further weakened the shelf, says study coauthor Hongxing Liu, a geographer at the University of Cincinnati. Previous work had suggested that the ice shelf’s downward slide began only a few years before a Rhode Island-sized region of ice disintegrated into thousands of icebergs in 2002.

The new data will help scientists more confidently predict how Antarctic ice will fare in the coming decades, says Penn State glaciologist Richard Alley, who was not involved in the work. The early response of Larsen B to warming “is consistent with this ice shelf system being sensitive, and gives a target for future modeling studies to learn how sensitive, and for what reasons,” he says.

Ice shelves such as Larsen B line Antarctica’s coast and slow the flow of the continent’s glaciers and ice sheets into the sea. Rising temperatures are shrinking Antarctica’s ice, with several ice shelves on track to disappear completely within 100 years (SN Online: 3/26/15). Tracking the long-term decline of ice shelves is tricky, though. Scientific satellite images are sparse prior to the 1990s and next to nonexistent before the 1980s.

Liu and colleagues turned to another group that peered at Antarctica, a U.S. intelligence agency called the National Reconnaissance Office. In 1963, the agency photographed the continent as part of an intelligence-gathering mission. While these images were declassified in 1995, the photos were too distorted by effects such as the camera used and Earth’s curvature to use for ice flow measurements.

Making the photographs usable required identifying stationary landmarks for reference, a difficult task on a continent covered with shifting white ice. Comparing the spy photos with later scientific images, Liu and colleagues identified 44 potential landmarks. Then, using the locations as anchor points, the researchers unwarped the images. Along with additional satellite images snapped in 1979 and the 1980s, the modified images allowed the researchers to track Larsen B’s ice flow over time.
The ice on Larsen B’s front flowed at around 400 meters per year on average between 1963 and 1986, calculations using images from those years indicate. From 1986 to 1988, the average was 490 meters per year. That speed boost suggests that the ice flow accelerated between the 1963 to 1986 satellite images. Several glaciers that feed into Larsen B underwent similar accelerations, the researchers found.

Larsen B’s early acceleration hints that the ice shelf was already weakening well before the 1990s, says Ted Scambos, a polar scientist at the National Snow and Ice Data Center in Boulder, Colo., who was not involved in the study. Previous studies suggested that balmy surface temperatures caused Larsen B’s demise by forming meltwater pools on top of the ice shelf that forced open cracks in the ice (SN: 10/18/14, p. 9). The new satellite data suggest that this fracturing was a finishing blow following long-term weakening by forces such as relatively warm seawater eroding the ice shelf’s underside, Scambos says.

Any parent trying to hustle a school-bound kid out the door in the morning knows that her child’s skull possesses a strange and powerful form of black magic: It can repel parents’ voices. Important messages like “find your shoes” bounce off the impenetrable fortress and drift unheeded to the floor.

But when this perplexing force field is off, it turns out that mothers’ voices actually have profound effects on kids. Children’s brains practically buzz when they hear their moms’ voices, scientists report in the May 31 Proceedings of the National Academy of Sciences. (Fun and not surprising side note: Babies’ voices get into moms’ brains, too.)

The parts of kids’ brains that handle emotions, face recognition and reward were prodded into action by mothers’ voices, brain scans of 24 children ages 7 to 12 revealed. And words were not required to get this big reaction. In the study, children listened to nonsense words said by either their mother or one of two unfamiliar women. Even when the words were fake, mothers’ voices still prompted lots of neural action.

The study was done in older kids, but children are known to tune into their mothers’ voices early. Really early, in fact. One study found that fetuses’ heart rates change when they hear their moms read a story. For a fetus crammed into a dark, muffled cabin, voices may take on outsized importance.

And voices carry particularly powerful messages throughout childhood. “A tremendous amount of emotional information is conveyed to children through auditory channels,” says University of Wisconsin-Madison child psychologist Seth Pollak. And, he points out, kids are small. “Kids’ faces are down around our knees. And children who are crawling are looking at the ground,” he says. This obvious point means that facial expressions and other visual signals might not pack as much punch as a voice.

Of course, voices other than those belonging to moms are also important. Pollak says that voices of fathers — or any other caregiver who spends lots of time around a child — probably affect children’s brains in a similar way. It’s just that those studies haven’t been done yet.

The results of the latest brain scan study make a lot of sense, says Pollak. Some of the brain regions activated are those involved in feeling good. “A caregiver’s voice is actually rewarding. It activates the systems that make us feel calm,” he says.
And the new study might help explain some of Pollak’s earlier results. He and his colleagues stressed out 68 girls, who happened to be the same ages as those in the brain scanning study, by making them do math and word problems in front of three unsmiling adult strangers — a terrifying prospect for most kids. (And adults.) After their ordeal, the girls either talked to their moms in person, on the phone or by instant messenger.

Compared with the instant messenger typers, the girls who saw their moms in person or talked to them on the phone were more soothed, showing lower levels of stress hormones. That finding, published in 2012 in Evolution and Human Behavior, suggests that to a kid, there’s something especially calming about hearing her own mother’s voice.

And now, by showing the widespread reaction to a mother’s voice, the brain data back that up. “It all kind of hangs together in a way that I think is very intuitive,” Pollak says. In other words, a mother’s voice is powerful, perhaps even strong enough to overcome a force field.

Thanks to modern laser technology, Southeast Asia’s Khmer Empire is rising from forest floors for the first time in centuries.

New findings show the remarkable extent to which Khmer people built cities and transformed landscapes from at least the fifth to the 15th century, and perhaps for several hundred years after that, says archaeologist Damian Evans of Cambodia’s Siem Reap Center. Laser mapping in 2015 of about 1,910 square kilometers of largely forested land in northern Cambodia indicates that gridded city streets and extensive canals emerged surprisingly early, by around A.D. 500, Evans reports June 13 in the Journal of Archaeological Science. Researchers have generally assumed that large-scale urban development began later at Greater Angkor, capital of the Khmer Empire from the ninth to 15th centuries (SN: 5/14/16, p. 22).
A helicopter carrying light detection and ranging equipment, lidar for short, flew sorties over seven Khmer sites in the vicinity of Greater Angkor. Lidar’s laser pulses gathered data on the contours of jungle- and vegetation-covered land. Lidar maps revealed city blocks, canals and other remnants of past settlements.
Mysterious ground features previously identified by lidar surveys at Angkor Wat temple in Greater Angkor also turned up at several sites, some located as many as 100 kilometers from Greater Angkor. Those sites include the eighth to ninth century city of Mahendraparvata and a 12th century city, Preah Khan of Kompong Svay. Fields of precisely arranged earthen mounds at these settlements may have been used to collect rainwater, Evans speculates. Earthen embankments forming coiled or spiral patterns might have been gardens or ceremonial spaces.

“It’s humbling to see the lidar data and realize how much was previously missed in ground surveys at Preah Khan,” says archaeologist Mitch Hendrickson of the University of Illinois at Chicago. Hendrickson conducts research at Preah Khan, one of several ancient cities that provided food and other services to Greater Angkor via an extensive road system.

Before the 2015 lidar survey, Mahendraparvata was known “only from inscription texts and a few bits of broken-down masonry,” adds archaeologist Charles Higham of the University of Otago in Dunedin, New Zealand. Mahendraparvata’s laser-traced layout indicates it was an early, small-scale version of Greater Angkor, Higham says.
A military invasion and sacking of Greater Angkor in the 15th century apparently did not result in most of its roughly 750,000 residents abandoning the site, as many investigators have thought. Lidar data from 2015 indicate that Khmer capitals established after Greater Angkor’s defeat, such as Longvek and Oudong, show no signs of dense populations created by mass relocations from the former capital, Evans says.

That suggests that the political state collapsed at Greater Angkor, but hundreds of thousands of rice farmers carried on, Hendrickson says. “Lots of fish and rice were still available,” he says. “Local farmers were more resilient than the state was.”

Coral reefs won’t be out of hot water anytime soon. A global bleaching event that began in June 2014 is the longest on record and now covers a larger area than ever before. What’s worse, it shows no signs of ending.

Global warming exacerbated by the latest El Niño is to blame, National Oceanic and Atmospheric Administration scientists reported Monday at the 13th International Coral Reef Symposium in Honolulu. Since 1979, periodic mass bleachings covering hundreds of kilometers have only lasted for “a year or so,” said NOAA Coral Reef Watch Coordinator Mark Eakin. But this one has dragged on for two years, threatening more than 40 percent of reefs globally, and more than 70 percent in the United States.

When corals are stressed by heat, they reject the colorful algae living inside them and turn a ghostly white. Those algae are a major source of food, so reefs can die if conditions don’t improve.

NOAA scientists aren’t sure what will end this episode. It could extend into 2017, and more frequent events are possible in the future, the scientists said. “Climate models suggest that most coral reefs may be seeing bleaching every other year by mid-century,” Eakin added. “How much worse that gets will depend on how we deal with global warming.”

In Colorado’s Rocky Mountains, male and female valerian plants have responded differently to hotter, drier conditions, a new study shows. Rapidly changing ratios of the sexes could be a quick sign of climate change, the researchers say.

Valerian (Valeriana edulis) plants range from hot, scrubby lowlands to cold alpine slopes. In each patch of plants, some are male and some are female. The exact proportion of each sex varies with elevation. High on the mountain, females are much more common than males; they can make up 80 percent of some populations.
Four decades ago, in patches of valerian growing in the middle of the plant’s elevation range, 33.4 percent of the plants were males. Those patches grew in the Rockies at elevations around 3,000 meters. Today, you would have to hike considerably higher to find the same proportion of male plants. Males, now 5.5 percent more common on average, are reaching higher elevations than in the past, researchers report in the July 1 Science.

“We think climate is acting almost like a filter on males and females,” says Will Petry of ETH Zurich, who led the study while at the University of California, Irvine. “The settings on this filter are controlling the sex ratio.” Those settings are sweeping up the mountainside like a rising tide at a rate of 175 meters per decade, Petry and colleagues found.
Ecologists already knew that the ratio of male to female plants can vary with altitude or water availability, says ecologist Spencer Barrett of the University of Toronto, who was not involved in this study. But “the idea that a sex ratio is moving upslope — nobody’s ever done that before.”

Those moving sex ratios have kept pace with climate change since the late 1970s. Today, winter snows are melting earlier and summers are hotter, with less rain. As a result, the same amount of precipitation that would have fallen at one elevation in 1978 now falls at higher elevations instead; it has moved upslope by 133 meters per decade. Soil moisture has moved up the mountain, too, by 195 meters per decade.

The parallel shifts mean that changing sex ratios could be a marker of climate change, says population biologist Tom Miller of Rice University in Houston, a coauthor of the study. Today, movements of whole species — often up in latitude or altitude — are a hallmark of climate change. But proportions of males and females are changing “substantially faster than species are moving,” Miller says. They “might be a much more rapid fingerprint of climate change than where species are migrating to.”
Petry’s team found that fingerprint while hiking around the Rocky Mountain Biological Laboratory in Crested Butte, Colo. As the scientists walked through the mountains in Chaffee and Gunnison counties, they counted flowering males and females at 31 sites in 2011, then compared their modern data with historical counts from nine of the same populations, made by coauthor Judy Soule from 1978 to 1980. When Petry saw that the percentage of males and females had changed, “we also started thinking about the consequences,” he says.

If one sex vastly outnumbers the other, populations could die out. “Imagine if it became an Amazonia situation,” says Kailen Mooney, whose lab at UC Irvine led the new study. A 100 percent female population wouldn’t be pollinated, and would disappear once the mature females died, he says.

If those female-only populations grew above a certain altitude and died out because males couldn’t reach them, then male plants would set the upper boundary for the whole species. Sex ratios “add nuance” to the way scientists think about climate-driven migration, Mooney says, because one sex could determine geographic limits for whole species.

NASA’s Juno spacecraft has sent back its first picture of Jupiter since arriving at the planet July 4 (SN: 7/23/16, p. 14). The image, taken July 10 when the spacecraft was 4.3 million kilometers from Jupiter, shows off the planet’s clouds, its Great Red Spot (a storm a bit wider than Earth) and three of its moons (Io, Europa and Ganymede).

Juno is on the outbound leg of its first of two 53.5-day orbits of the gas giant (Juno will then settle into 14-day orbits). During orbit insertion, all of Juno’s scientific instruments were turned off while the spacecraft made its first dive through the harsh radiation belts that encircle the planet. This first image indicates that Juno is in good health and ready to study the largest planet in the solar system.

The probe is the ninth to visit Jupiter and the second to stay in orbit (SN: 6/25/2016, p. 32). For the next 20 months, Juno will investigate what lurks beneath the opaque clouds that enshroud the planet (SN: 6/25/2016, p. 16). The spacecraft won’t take its first intimate pictures of Jupiter until August 27, when it flies within 5,000 kilometers of the cloud tops.

U.S. drivers love to hit the road. The problem is doing so safely.

In 2013, 32,894 people in the United States died in motor vehicle crashes. Although down since 2000, the overall death rate — 10.3 per 100,000 people — tops 19 other high-income countries, the U.S. Centers for Disease Control and Prevention reported July 8. Belgium is a distant second with 6.5 deaths per 100,000. Researchers reviewed World Health Organization and other data on vehicle crash deaths, seat belt use and alcohol-impaired driving in 2000 and 2013.
Canada had the highest percentage of fatal crashes caused by drunk drivers: 33.6 percent. New Zealand and the United States tied for second at 31 percent. But Canada and 16 other countries outperformed the United States on seat belt use — even though, in 2013, 87 percent of people in the United States reported wearing safety belts while riding in the front seat.

Spain saw the biggest drop — 75 percent — in its crash death rate. That country improved nearly all aspects of road safety, including decreasing alcohol-impaired driving and increasing seat belt use, the researchers say.

Thirst drove one of the last populations of woolly mammoths to extinction.

A small group of holdouts on an isolated Alaskan island managed to last about 8,000 years longer than most of their mainland-dwelling brethren. But by about 5,600 years ago, the island’s lakes — the only source of freshwater — became too small to support the mammoths (Mammuthus primigenius), scientists report online the week of August 1 in the Proceedings of the National Academy of Sciences.
“I don’t think I’ve ever seen something so conclusive about an extinction before,” says Love Dalén, an evolutionary geneticist at the Swedish Museum of Natural History in Stockholm who was not involved in the research. The study highlights “how sensitive small populations are and how easily they can become extinct.”

Surprisingly recent woolly mammoth bones had previously been discovered in a cave on St. Paul Island, which became isolated from the mainland roughly 14,000 years ago. Since there’s no evidence that prehistoric humans lived on St. Paul, the find provided a chance to study extinction in the absence of human influence, says Russell Graham, a paleontologist at Penn State who led the study.

The scientists extracted a core of sediment from a lake bed near the cave to see how environmental conditions had changed over the last 11,000 years. The team found remnants of ancient plants, animals and fungi in the sediment — including traces of mammoth DNA in some layers. By analyzing and dating the different sediment layers, the team could infer when and how the mammoths went extinct.

“We initially thought that vegetation change and habitat would be the major driving factor,” Graham says. Instead, his team found a wealth of evidence — including an increase in salt-tolerant algae and crustaceans 6,000 years ago — suggesting freshwater shortages as the culprit.
A warmer climate after the last Ice Age ended contributed to the St. Paul mammoths’ downfall. Sea level rise shrank the mammoths’ island habitat and cut into their freshwater supplies by raising the water table and making the lake saltier over time, the team concluded. Warmer, drier conditions also caused water to evaporate more quickly from the lake surface.

The study highlights an often-overlooked vulnerability of island and coastal communities. Some islands in the South Pacific are currently experiencing similar freshwater shortages thanks to rising seas, Graham says – and Florida could be next in line. That’s particularly bad news for large island-dwelling and coastal mammals, which tend to need more water to survive than smaller species.

In a few highly specialized laboratories, scientists bombard matter with the world’s most powerful electrical pulses or zap it with sophisticated lasers. Other labs squeeze heavy-duty diamonds together hard enough to crack them.

All this is in pursuit of a priceless metal. It’s not gold, silver or platinum. The scientists’ quarry is hydrogen in its most elusive of forms.

Several rival teams are striving to transform hydrogen, ordinarily a gas, into a metal. It’s a high-stakes, high-passion pursuit that sparks dreams of a coveted new material that could unlock enormous technological advances in electronics.
“Everybody knows very well about the rewards you could get by doing this, so jealousy and envy [are] kind of high,” says Eugene Gregoryanz, a physicist at the University of Edinburgh who’s been hunting metallic hydrogen for more than a decade.

Metallic hydrogen in its solid form, scientists propose, could be a superconductor: a material that allows electrons to flow through it effortlessly, with no loss of energy. All known superconductors function only at extremely low temperatures, a major drawback. Theorists suspect that superconducting metallic hydrogen might work at room temperature. A room-temperature superconductor is one of the most eagerly sought goals in physics; it would offer enormous energy savings and vast improvements in the transmission and storage of energy.

Metallic hydrogen’s significance extends beyond earthly pursuits. The material could also help scientists understand our own solar system. At high temperatures, compressed hydrogen becomes a metallic liquid — a form that is thought to lurk beneath the clouds of monstrous gas planets, like Jupiter and Saturn. Sorting out the properties of hydrogen at extreme heat and high pressure could resolve certain persistent puzzles about the gas giants. Researchers have reported brief glimpses of the liquid metal form of hydrogen in the lab — although questions linger about the true nature of the material.

While no lab has yet produced solid metallic hydrogen, the combined efforts of many scientists are rapidly closing in on a more complete understanding of the element itself — as well as better insight into the complex inner workings of solids.
Hydrogen, the first element in the periodic table and the most common element in the universe, ought to be easy to understand: a single proton paired with a single electron. “What could be more simple than an assembly of electrons and protons?” asks theoretical physicist Neil Ashcroft of Cornell University. But at high pressures, the physics of hydrogen rapidly becomes complex.

At room temperature and atmospheric pressure, hydrogen is a gas. But like other materials, altered conditions can transform hydrogen into a solid or a liquid. With low enough temperatures or a sufficiently forceful squeeze, hydrogen shape-shifts into a solid. Add heat while squeezing, and it becomes a liquid.

If subjected to still more extreme conditions, hydrogen can — at least theoretically — undergo another transformation, into a metal. All metals have one thing in common: They conduct electricity, due to free-flowing electrons that can go where they please within the material.
Squeeze anything hard enough and it will become a metal. “Pressure does a great job of dislodging the outer electrons,” Ashcroft says. This is what scientists are aiming to do with hydrogen: create a sloshing soup of roving electrons in either a liquid or a solid.

When hydrogen is compressed, many atoms begin to interact with one another, while paired in molecules of two hydrogen atoms each. The underlying physics becomes a thorny jumble. “It is amazing; the stuff takes up incredibly complex arrangements in the solid state,” says Ashcroft, the first scientist to propose, in 1968, that metallic hydrogen could be a high-temperature superconductor.

Hydrogen’s complexity fascinates scientists. “It’s not just the metallization question that’s of interest to me,” says Russell Hemley, a chemist at the Carnegie Institution for Science in Washington, D.C., and Lawrence Livermore National Laboratory in California. Studying the intricacies of hydrogen’s behavior can help scientists refine their understanding of the physics of materials.

In 1935, when physicists Eugene Wigner and Hillard Bell Huntington of Princeton University first predicted that compressed solid hydrogen would be metallic, they thought the transition to a metal might occur at a pressure 250,000 times that of Earth’s atmosphere. That may sound like a lot, but scientists have since squeezed hydrogen to pressures more than 10 times as high — and still no solid metal.

Scientists originally expected that the transition would be a simple flip to metallic behavior. Not so, says theoretical physicist David Ceperley of the University of Illinois at Urbana-Champaign. “Nature has a lot more possibilities.” Solid hydrogen exists in multiple forms, each with a different crystal structure. As the pressure climbs, the wily hydrogen molecules shift into ever-more-complex arrangements, or phases. (For physicists, the “phase” of matter goes deeper than the simple states of solid, liquid or gas.) The number of known solid phases of hydrogen has grown steadily as higher pressures are reached, with four phases now well established. The next phase scientists find could be a metal — they hope.

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Timeline: The race to make metallic hydrogen
Rival research teams are rushing to transform solid or liquid hydrogen into a metal. With each experiment, the pressure rises. They have two very different aims: a room-temperature superconductor and a window into the gas giants.
If solid metallic hydrogen turns out to be a room-temperature superconductor, it would have to be crushed to work, making it impractical for many applications. But if hydrogen could hold its metallic form after the pressure is released, as some researchers have suggested, “it would be revolutionary,” says physicist Isaac Silvera, who leads the metallic hydrogen hunt at Harvard University. Such a material could be used in electrical wires to reduce loss of energy and decrease the world’s power consumption. And it might lead to efficient, magnetically levitated trains and technological advances in nuclear fusion, supercomputing and more.

While one group of would-be metallurgists is searching for solid metal, other investigators seek the scientifically intriguing liquid hydrogen metal. Their techniques differ in timescale and size. To produce liquid metal, scientists violently slam hydrogen for fractions of a second at a time, using enormous machines at national laboratories. Scientists searching for a solid metal, on the other hand, use fist-sized devices to capture hydrogen between the tips of two tiny diamonds and slowly squeeze.

Diamonds have it rough
Crushing an ethereal, normally gaseous substance between two diamonds sounds nearly impossible. Such tricky experiments make for a field where researchers are in regular disagreement over their latest results. “We’re still missing high-quality, reliable data,” says physicist Alexander Goncharov. “The issue is the experiments are too challenging.”

In his office at the Geophysical Laboratory of the Carnegie Institution, Goncharov opens a desk drawer and pulls out a device called a diamond anvil cell. The cylinder of metal is small enough for Goncharov to cradle in his palm. Bits of precisely machined steel and tough tungsten carbide are held together with four screws. Through portals in the top and bottom, bright sparkles shimmer: diamonds.

Inside the capsule, two diamonds taper to tiny points a few hundredths of a millimeter wide. They pinch the material within, squashing it at over a million times atmospheric pressure. The gap between the minuscule anvils can be as small as a few thousandths of a millimeter, about the size of a human red blood cell.

Once they’ve been pressurized, diamond anvil cells will hold the pressure almost indefinitely. The prepared cells can be carried around — inspected in the laboratory, transported to specialized facilities around the world — or simply stored in a desk drawer. Goncharov regularly travels with them. (Tip from the itinerant scientist: If questions arise at airport security, “never use the word ‘diamonds.’ ”) The diamond anvil can squeeze more than hydrogen — materials from iron to sodium to argon can be crushed in the diamond vise.
To identify new, potentially metallic phases of solid hydrogen within the pressurized capsules, scientists shine laser light onto the material, measuring how molecules vibrate and rotate — a technique called Raman spectroscopy (SN: 8/2/08, p. 22). If a new phase is reached, molecules shift configurations, altering how they jiggle. Certain types of changes in how the atoms wobble are a sign that the new phase is metallic. If the material conducts electricity, that’s another dead giveaway. A final telltale sign: The normally translucent hydrogen should acquire a shiny, reflective surface.

Significant hurdles exist for diamond anvil cell experiments. Diamonds, which cost upwards of $600 a pop, can crack under such intense pressures. Hydrogen can escape from the capsule, or diffuse into the diamonds, weakening them. So scientists coat their diamonds with thin layers of protective material. The teams each have their own unique recipe, Goncharov says. “Of course, everyone believes that their recipe is the best.”

Three phases of solid hydrogen have been known since the late 1980s. With the discovery of a fourth phase in 2011, “the excitement was enormous,” Gregoryanz says. In Nature Materials, Mikhail Eremets and Ivan Troyan at the Max Planck Institute for Chemistry in Mainz, Germany, reported that a new phase appeared when they squashed room-temperature hydrogen to over 2 million times atmospheric pressure. Goncharov, Gregoryanz and colleagues created the new phase and deduced its structure in Physical Review Letters in 2012. In phase IV, as it’s known, hydrogen arranges itself into thin sheets — somewhat like the single-atom-thick sheets of carbon known as graphene, the scientists wrote.

Progress doesn’t come easy. With each new paper, scientists disagree about what the results mean. When Eremets and colleagues discovered the fourth phase, they thought they also had found metallic hydrogen (SN: 12/17/11, p. 9). But that assertion was swiftly criticized, and it didn’t stand up to scrutiny.

The field has been plagued by hasty claims. “If you look at the literature for the last 30 years,” Gregoryanz says, “I think every five years there is a claim that we finally metallized hydrogen.” But the claims haven’t been borne out, leaving scientists perpetually skeptical of new results.

In a recent flurry of papers, scientists have proposed new phases — some that might be metallic or metal-like precursors to a true metal — and they are waiting to see which claims stick. Competing factions have volleyed papers back and forth, alternately disagreeing and agreeing.

A paper from Gregoryanz’s group, published in Nature in January, provided evidence for a phase that was enticingly close to a metal (SN Online: 1/6/16), at more than 3 million times atmospheric pressure. But other scientists disputed the evidence. In their own experiments, Eremets and colleagues failed to confirm the new phase. In a paper posted online at arXiv.org just days after Gregoryanz’s paper was published, Eremets’ team unveiled hints of a “likely metallic” phase, which occurred at a different temperature and pressure than Gregoryanz’s new phase.

A few months later, Silvera’s group squeezed hydrogen hard enough to make it nearly opaque, though not reflective — not quite a metal. “We think we’re just below the pressure that you need to make metallic hydrogen,” Silvera says. His findings are consistent with Eremets’ new phase, but Silvera disputes Eremets’ speculations of metallicity. “Every time they see something change they call it metallic,” Silvera says. “But they don’t really have evidence of metallic hydrogen.”

All this back and forth may seem chaotic, but it’s also a sign of a swiftly progressing field, the researchers say. “I think it’s very healthy competition,” Gregoryanz says. “When you realize that somebody is getting ahead of you, you work hard.”

The current results are “not very well consistent, so everybody can criticize each other,” Eremets says. “For me it’s very clear. We should do more experiments. That’s it.”

There are signs of progress. In 2015, Eremets and colleagues discovered a record-breaking superconductor: hydrogen sulfide, a compound of hydrogen and sulfur. When tightly compressed into solid form, hydrogen sulfide superconducts at temperatures higher than ever seen before: 203 kelvins (−70° Celsius) (SN: 8/8/15, p. 12).

Adding sulfur to the mix stabilizes and strengthens the hydrogen structure but doesn’t contribute much to its superconducting properties. Hydrogen sulfide is so similar to pure hydrogen, Eremets says, that, “in some respects we already found superconductivity in metallic hydrogen.”

Brief glimmers
Giant, gassy planets are chock-full of hydrogen, and the element’s behavior under pressure could explain some of these planets’ characteristics. A sea of flowing liquid hydrogen metal may be the source of Jupiter’s magnetic field (SN: 6/25/16, p. 16). Learning more about metallic hydrogen’s behavior deep inside such planets could also help resolve a long-standing puzzle regarding Saturn: The ringed behemoth is unexpectedly bright. The physics of hydrogen’s interactions with helium inside the planet could provide the explanation.

Using a radically different set of technologies, a second band of scientists is on the hunt for such liquid metallic hydrogen. These researchers have gone big, harnessing the capabilities of new, powerful machines designed for nuclear fusion experiments at government-funded national labs. These experiments show the most convincing evidence of metallic behavior so far — but in hydrogen’s liquid, not solid, form. These enormous machines blast hydrogen for brief instants, temporarily sending pressures and temperatures skyrocketing. Such experiments reach searing temperatures, thousands of kelvins. With that kind of heat, metallic hydrogen appears at lower, more accessible pressures.
Creating such conditions requires sophisticated equipment. The Z machine, located at Sandia National Laboratories in Albuquerque, generates extremely intense bursts of electrical power and strong magnetic fields; for a tiny instant, the machine can deliver about 80 terawatts (one terawatt is about the total electrical power–generating capacity in the entire United States).

A group of scientists recently used the Z machine to launch a metal plate into a sample of deuterium — an isotope of hydrogen with one proton and one neutron in its nucleus — generating high pressures that compressed the material. The pummeled deuterium showed reflective glimmers of shiny metal, the scientists reported last year in Science. “It starts out transparent, it goes opaque, and then later we see this reflectivity increase,” says Marcus Knudson of Sandia and Washington State University in Pullman.

Another group is pursuing a different tactic, using some of the most advanced lasers in the world, at the National Ignition Facility at Lawrence Livermore. Scientists there zap hydrogen to produce high pressures and temperatures. Though the conclusions of this experiment are not yet published, says physicist Gilbert Collins of Lawrence Livermore, one of the leaders of the experiment, “we have some really beautiful results.”

The first experiment to show evidence of liquid metallic hydrogen was performed at Lawrence Livermore in the 1990s. A team of physicists including William Nellis, now at Harvard, used a sophisticated gunlike apparatus to shoot projectiles into hydrogen at blisteringly fast speeds. The resulting hydrogen briefly conducted electricity (SN: 4/20/96, p. 250).

These experiments face hurdles of their own — it’s a struggle to measure temperature in such systems, so scientists calculate it rather than measuring it directly. But many researchers are still convinced by these results. Metallic hydrogen “certainly has been produced by shock techniques,” Cornell’s Ashcroft says.

Some scientists still have questions. “It’s certainly difficult to tell if something is a metal or not at such high temperature,” Collins says. Although they need high temperatures to reach the metal liquid phase, some physicists define a metal based on its behavior at a temperature of absolute zero. Current experiments hit high pressures at temperatures as close to zero as possible to produce relatively cool liquid metallic hydrogen.

Scientists who conduct palm-sized diamond anvil cell experiments refuse to be left out of the liquid-metal action. They’ve begun to use laser pulses to heat and melt hydrogen crammed into the cells. The results have stirred up new disagreements among competing groups. In April’s Physical Review B, Silvera and coauthors reported forming liquid metallic hydrogen. But under similar conditions, Goncharov and others found only semiconducting hydrogen, not a metal. They reported their results in Physical Review Letters in June.

“There’s kind of a crisis now with these different experiments,” says Illinois theorist Ceperley. “And there’s a lot of activity trying to see who’s right.” For now, scientists will continue refining their techniques until they can reach agreement.

The major players have managed to reach consensus before. Four phases of solid hydrogen are now well established, and researchers agree on certain conditions under which solid hydrogen melts.

Solid metallic hydrogen, however, has perpetually seemed just out of reach, as theoretical predictions of the pressure required to produce it have gradually shifted upward. As the goalposts have moved, physicists have reached further, achieving ever-higher pressures. Current theoretical predictions put the metal tantalizingly close — perhaps only an additional half a million times atmospheric pressure away.

The quest continues, propelled by a handful of hydrogen-obsessed scientists.

“We all love hydrogen,” Collins says. “It has the essence of being simple, so that we think we can calculate something and understand it, while at the same time it has such a devious nature that it’s perhaps the least understandable material there is.”

Fractions of a second after food hits the mouth, a specialized group of energizing nerve cells in mice shuts down. After the eating stops, the nerve cells spring back into action, scientists report August 18 in Current Biology. This quick response to eating offers researchers new clues about how the brain drives appetite and may also provide insight into narcolepsy.

These nerve cells have intrigued scientists for years. They produce a molecule called orexin (also known as hypocretin), thought to have a role in appetite. But their bigger claim to fame came when scientists found that these cells were largely missing from the brains of people with narcolepsy.
People with narcolepsy are more likely to be overweight than other people, and this new study may help explain why, says neuroscientist Jerome Siegel of UCLA. These cells may have more subtle roles in regulating food intake in people without narcolepsy, he adds.

Results from earlier studies hinted that orexin-producing nerve cells are appetite stimulators. But the new results suggest the opposite. These cells actually work to keep extra weight off. “Orexin cells are a natural obesity defense mechanism,” says study coauthor Denis Burdakov of the Francis Crick Institute in London. “If they are lost, animals and humans gain weight.”

Mice were allowed to eat normally while researchers eavesdropped on the behavior of their orexin nerve cells. Within milliseconds of eating, orexin nerve cells shut down and stopped sending signals. This cellular quieting was consistent across foods. Peanut butter, mouse chow, a strawberry milkshake and a calorie-free drink all prompted the same response. “Foods with different flavors and textures had a similar effect, implying that it is to do with the act of eating or drinking, rather than with what is being eaten,” Burdakov says. When the eating ended, the cells once again resumed their activity.
When Burdakov and colleagues used a genetic technique to kill orexin nerve cells, mice ate more food than normal, behavior that led to weight gain, the team found. But a reduced-calorie diet slimmed these mice down.
The results suggest that giving orexin to people who lack it may reduce obesity. But that might not be a good idea. An overactive orexin system has been tied to stress and anxiety, Burdakov says. Orexin’s link to stress raises a different possibility —that anxiety can be reduced by curbing orexin nerve cell activity. “And our study suggests that the act of eating can do just that,” Burdakov says. “This provides a candidate explanation for why people turn to eating at times of anxiety.”