爱上海龙凤

Water ice lies just beneath the cratered surface of dwarf planet Ceres and in shadowy pockets within those craters, new studies report. Observations from NASA’s Dawn spacecraft add to the growing body of evidence that Ceres, the largest object in the asteroid belt between the orbits of Mars and Jupiter, has held on to a considerable amount of water for billions of years.

“We’ve seen ice in different contexts throughout the solar system,” says Thomas Prettyman, a planetary scientist at the Planetary Science Institute in Tucson and coauthor of one of the studies, published online December 15 in Science. “Now we see the same thing on Ceres.” Ice accumulates in craters on Mercury and the moon, an icy layer sits below the surface of Mars, and water ice slathers the landscape of several moons of the outer planets. Each new sighting of H2O contributes to the story of how the solar system formed and how water was delivered to a young Earth.
A layer of ice mixed with rock sits within about one meter of the surface concentrated near the poles, Prettyman and colleagues report. And images of inside some craters around the polar regions, from spots that never see sunlight, show bright patches, at least one of which is made of water ice, a separate team reports online December 15 in Nature Astronomy.

“Ceres was always believed to contain lots of water ice,” says Michael Küppers, a planetary scientist at the European Space Astronomy Center in Madrid, who was not involved with either study. Its overall density is lower than pure rock, implying that some low-density material such as ice is mixed in. The Herschel Space Observatory has seen water vapor escaping from the dwarf planet (SN Online: 1/22/14), and the Dawn probe, in orbit around Ceres since 2015, spied a patch of water ice in Oxo crater, though the amount of direct sunlight there implies the ice has survived for only dozens of years (SN Online: 9/1/16). The spacecraft has also found minerals on the surface that formed in the presence of water.

But researchers would like to know where Ceres’ water is. Knowing whether it is blended throughout the interior or segregated from the rock could help piece together the story of where Ceres formed and how the tiny world was put together. That, in turn, could provide insight into how diverse the worlds around other stars might be.

To map the subsurface ice, Prettyman and colleagues used a neutron and gamma-ray detector onboard Dawn. As Ceres is bombarded with cosmic rays — highly energetic particles that originate outside the solar system — atoms in the dwarf planet spray out neutrons. The amount and energy of the neutrons can provide a clue to the abundance of hydrogen, presumably locked up in water molecules and hydrated minerals.

Finding patches of ice was a bit more straightforward. Planetary scientist Thomas Platz and colleagues pinpointed permanently shadowed spots on Ceres, typically in crater floors near the north and south poles. The team then scoured images of those locations for bright patches. Out of the more than 600 darkened craters they identified, the researchers found 10 with bright deposits that could be surface ice. One had a chunk sticking out into just enough sunlight for Dawn to measure the spectrum of the reflected light and detect signs of water.
Water vapor escaping from inside the dwarf planet likely falls back to Ceres, where some of it gets trapped in these cold spots, says Platz, of the Max Planck Institute for Solar System Research in Göttingen, Germany.

Just because there is water doesn’t mean Ceres is a good place for life to take hold. Temperatures in the shadows don’t get above –216° Celsius. “It’s pretty cold, there’s no sunlight. We don’t think that’s a habitable environment,” Platz says. Although, he adds, “one could mine for future missions to get fuel.”

Ceres is now the third major heavily cratered body, along with Mercury and the moon, with permanently shadowed regions where ice builds up. “All the ones we’ve got info on to test this show you’ve accumulated something,” says Peter Thomas, a planetary scientist at Cornell University, who is not a part of either research team. Those details improve researchers’ understanding of how water interacts with a variety of planetary environments.

Certain marine worms spend their larval phase as little more than a tiny, transparent “swimming head.” A new study explores the genes involved in that headfirst approach to life.

A mud flat in Morro Bay, Calif., is the only known place where this one species of acorn worm, Schizocardium californicum, is found. After digging up the creatures, Paul Gonzalez, an evolutionary developmental biologist at Stanford University, raised hordes of the larvae at Stanford’s Hopkins Marine Station in Pacific Grove, Calif.
Because a larva and an adult worm look so different, scientists wondered if the same genes and molecular machinery were involved in both phases of development. To find out, Gonzalez and colleagues analyzed the worm’s genetic blueprint during each phase, they report online December 8 in Current Biology.

Genes linked to trunk development were switched off during the larval phase until just before metamorphosis. Instead, most of the genes switched on were associated with head development, Gonzalez says.

The larvae hatch from eggs laid on the mud. When tides flood the area, the squishy, gel-filled animals use hairlike cilia to swim upwards to devour bits of algae. “They’re feeding machines,” Gonzalez says. He speculates that being balloon-shaped noggins, rather than wriggling noodles, may help the organisms float and feed more efficiently.

After about two months of gorging at the algae buffet, the larvae, which grow to roughly 2 millimeters across, transform and sink back into the muck. There, they eventually grow a body that can stretch up to about 40 centimeters.

An asteroid bombardment that some say triggered an explosion of marine animal diversity around 471 million years ago actually had nothing to do with it.

Precisely dating meteorites from the salvo, researchers found that the space rock barrage began at least 2 million years after the start of the Great Ordovician Biodiversification Event. So the two phenomena are unrelated, the researchers conclude January 24 in Nature Communications.

Some scientists had previously proposed a causal link between the two events: Raining debris from an asteroid breakup (SN: 7/23/16, p. 4) drove evolution by upsetting ecosystems and opening new ecological niches. The relative timing of the impacts and biodiversification was uncertain, though.
Geologist Anders Lindskog of Lund University in Sweden and colleagues examined 17 crystals buried alongside meteorite fragments. Gradual radioactive decay of uranium atoms inside the crystals allowed the researchers to accurately date the sediment layer to around 467.5 million years ago. Based in part on this age, the researchers estimate that the asteroid breakup took place around 468 million years ago. That’s well after fossil evidence suggests that the diversification event kicked off.

Other forces such as climate change and shifting continents instead promoted biodiversity, the researchers propose.

Locked inside a human brain protein, the hallucinogenic drug LSD takes an extra-long trip.

New X-ray crystallography images reveal how an LSD molecule gets trapped within a protein that senses serotonin, a key chemical messenger in the brain. The protein, called a serotonin receptor, belongs to a family of proteins involved in everything from perception to mood.

The work is the first to decipher the structure of such a receptor bound to LSD, which gets snared in the protein for hours. That could explain why “acid trips” last so long, study coauthor Bryan Roth and colleagues report January 26 in Cell. It’s “the first snapshot of LSD in action,” he says. “Until now, we had no idea how it worked at the molecular level.”
But the results might not be that relevant to people, warns Cornell University biophysicist Harel Weinstein.

Roth’s group didn’t capture the main target of LSD, a serotonin receptor called 5-HT2A, instead imaging the related receptor 5-HT2B. That receptor is “important in rodents, but not that important in humans,” Weinstein says.

Roth’s team has devoted decades to working on 5-HT2A, but the receptor has “thus far been impossible to crystallize,” he says. Predictions of 5-HT2A’s structure, though, are very similar to that of 5-HT2B, he says.

LSD, or lysergic acid diethylamide, was first cooked up in a chemist’s lab in 1938. It was popular (and legal) for recreational use in the early 1960s, but the United States later banned the drug (also known as blotter, boomer, Purple Haze and electric Kool-Aid).

It’s known for altering perception and mood — and for its unusually long-lasting effects. An acid trip can run some 15 hours, and at high doses, effects can linger for days. “It’s an extraordinarily potent drug,” says Roth, a psychiatrist and pharmacologist at the University of North Carolina School of Medicine in Chapel Hill.
Scientists have known for decades that LSD targeted serotonin receptors in the brain. These proteins, which are also found in the intestine and elsewhere in the body, lodge within the outer membranes of nerve cells and relay chemical signals to the cells’ interiors. But no one knew exactly how LSD fit into the receptor, or why the drug was so powerful.

Roth and colleagues’ work shows the drug hunkered deep inside a pocket of the receptor, grabbing onto an amino acid that acts like a handle to pull down a lid. It’s like a person holding the door of a storm cellar closed during a tornado, Roth says.

When the team did additional molecular experiments, tweaking the lid’s handle so that LSD could no longer hang on, the drug slipped out of the pocket faster than when the handle was intact. That was true whether the team used receptor 5-HT2B or 5-HT2A, Roth says. (Though the researchers couldn’t crystallize 5-HT2A, they were able to grow the protein inside cells in the lab for use in their other experiments.) The results suggest that LSD’s grip on the receptor is what keeps it trapped inside. “That explains to a great extent why LSD is so potent and why it’s so long-lasting,” Roth says.

David Nutt, a neuropsychopharmacologist at Imperial College London, agrees. He calls the work an “elegant use of molecular science.”

Weinstein remains skeptical. The 5-HT2A receptor is the interesting one, he maintains. A structure of that protein “has been needed for a very long time.” That’s what would really help explain the hallucinogenic effects of LSD, he says.

The topic of time is both excruciatingly complicated and slippery. The combination makes it easy to get bogged down. But instead of an exhaustive review, journalist Alan Burdick lets curiosity be his guide in Why Time Flies, an approach that leads to a light yet supremely satisfying story about time as it runs through — and is perceived by — the human body.

Burdick doesn’t restrict himself to any one aspect of his question. He spends time excavating what he calls the “existential caverns,” where philosophical questions, such as the shifting concept of now, dwell. He describes the circadian clocks that keep bodies running efficiently, making sure our bodies are primed to digest food at mealtimes, for instance. He even covers the intriguing and slightly insane self-experimentation by the French scientist Michel Siffre, who crawled into caves in 1962 and 1972 to see how his body responded in places without any time cues.
In the service of his exploration, Burdick lived in constant daylight in the Alaskan Arctic for two summery weeks, visited the master timekeepers at the International Bureau of Weights and Measures in Paris to see how they precisely mete out the seconds and plunged off a giant platform to see if time felt slower during moments of stress. The book not only deals with fascinating temporal science but also how time is largely a social construct. “Time is what everybody agrees the time is,” one researcher told Burdick.
That subjective truth also applies to the brain. Time, in a sense, is created by the mind. “Our experience of time is not a cave shadow to some true and absolute thing; time is our perception,” Burdick writes. That subjective experience becomes obvious when Burdick recounts how easily our brains’ clocks can be swayed. Emotions, attention (SN: 12/10/16, p. 10) and even fever can distort our time perception, scientists have found.

Burdick delves deep into several neuroscientific theories of how time runs through the brain (SN: 7/25/15, p. 20). Here, the story narrows somewhat in an effort to thoroughly explain a few key ideas. But even amid these details, Burdick doesn’t lose the overarching truth  — that for the most part, scientists simply don’t know the answers. That may be because there is no one answer; instead, the brain may create time by stitching together a multitude of neural clocks.
After reading Why Time Flies, readers will be convinced that no matter how much time passes, the mystery of time will endure.

First germanium integrated circuits

Integrated circuits made of germanium instead of silicon have been reported … by researchers at International Business Machines Corp. Even though the experimental devices are about three times as large as the smallest silicon circuits, they reportedly offer faster overall switching speed. Germanium … has inherently greater mobility than silicon, which means that electrons move through it faster when a current is applied. — Science News, February 25, 1967

UPDATE:
Silicon circuits still dominate computing. But demand for smaller, high-speed electronics is pushing silicon to its physical limits, sending engineers back for a fresh look at germanium. Researchers built the first compact, high-performance germanium circuit in 2014, and scientists continue to fiddle with its physical properties to make smaller, faster circuits. Although not yet widely used, germanium circuits and those made from other materials, such as carbon nanotubes, could help engineers make more energy-efficient electronics.

Helium — the recluse of the periodic table — is reluctant to react with other elements. But squeeze the element hard enough, and it will form a chemical compound with sodium, scientists report.

Helium, a noble gas, is one of the periodic table’s least reactive elements. Originally, the noble gases were believed incapable of forming any chemical compounds at all. But after scientists created xenon compounds in the early 1960s, a slew of other noble gas compounds followed. Helium, however, has largely been a holdout.
Although helium was known to hook up with certain elements, the bonds in those compounds were weak, or the compounds were short-lived or electrically charged. But the new compound, called sodium helide or Na2He, is stable at high pressure, and its bonds are strong, an international team of scientists reports February 6 in Nature Chemistry.

As a robust helium compound, “this is really the first that people ever observed,” says chemist Maosheng Miao of California State University, Northridge, who was not involved with the research.

The material’s properties are still poorly understood, but it is unlikely to have immediate practical applications — scientists can create it only in tiny amounts at very high pressures, says study coauthor Alexander Goncharov, a physicist at the Carnegie Institution for Science in Washington, D.C. Instead, the oddball compound serves as inspiration for scientists who hope to produce weird new materials at lower pressures. “I would say that it’s not totally impossible,” says Goncharov. Scientists may be able to tweak the compound, for example, by adding or switching out elements, to decrease the pressure needed.

To coerce helium to link up with another element, the scientists, led by Artem Oganov of Stony Brook University in New York, first performed computer calculations to see which compounds might be possible. Sodium, calculations predicted, would form a compound with helium if crushed under enormously high pressure. Under such conditions, the typical rules of chemistry change — elements that refuse to react at atmospheric pressure can sometimes become bosom buddies when given a squeeze.

So Goncharov and colleagues pinched small amounts of helium and sodium between a pair of diamonds, reaching pressures more than a million times that of Earth’s atmosphere, and heated the material with lasers to temperatures above 1,500 kelvins (about 1200° Celsius). By scattering X-rays off the compound, the scientists could deduce its structure, which matched the one predicted by calculations.
“I think this is really the triumph of computation,” says Miao. In the search for new compounds, computers now allow scientists to skip expensive trial-and-error experiments and zero in on the best candidates to create in a laboratory.

Na2He is an unusual type of compound known as an electride, in which pairs of electrons are cloistered off, away from any atoms. But despite the compound’s bizarre nature, it behaves somewhat like a commonplace compound such as table salt, in which negatively charged chloride ions alternate with positively charged sodium. In Na2He, the isolated electron pairs act like negative ions in such a compound, and the eight sodium atoms surrounding each helium atom are the positive ions.

“The idea that you can make compounds with things like helium which don’t react at all, I think it’s pretty interesting,” says physicist Eugene Gregoryanz of the University of Edinburgh. But, he adds, “I would like to see more experiments” to confirm the result.

The scientists’ calculations also predicted that a compound of helium, sodium and oxygen, called Na2HeO, should form at even lower pressures, though that one has yet to be created in the lab. So the oddball new helium compound may soon have a confirmed cousin.

Temperatures across Earth’s mantle are about 60 degrees Celsius higher than previously thought, a new experiment suggests. Such toasty temperatures would make the mantle runnier than earlier research suggested, a development that could help explain the details of how tectonic plates glide on top of the mantle, geophysicists report in the March 3 Science.

“Scientists have been arguing over the mantle temperature for decades,” says study coauthor Emily Sarafian, a geophysicist at the Woods Hole Oceanographic Institution in Massachusetts and at MIT. “Scientists will argue over 10 degree changes, so changing it by 60 degrees is quite a large jump.”
The mostly solid mantle sits between Earth’s crust and core and makes up around 84 percent of Earth’s volume. Heat from the mantle fuels volcanic eruptions and drives plate tectonics, but taking the mantle’s temperature is trickier than dropping a thermometer down a hole.

Scientists know from the paths of earthquake waves and from measures of how electrical charge moves through Earth that a boundary in the mantle exists a few dozen kilometers below Earth’s surface. Above that boundary, mantle rock can begin melting on its way up to the surface. By mimicking the extreme conditions in the deep Earth — squeezing and heating bits of mantle that erupt from undersea volcanoes or similar rocks synthesized in the lab — scientist can also determine the melting temperature of mantle rock. Using these two facts, scientists have estimated that temperatures at the boundary depth below Earth’s oceans are around 1314° C to 1464° C when adjusted to surface pressure.

But the presence of water in the collected mantle bits, primarily peridotite rock, which makes up much of the upper mantle, has caused problems for researchers’ calculations. Water can drastically lower the melting point of peridotite, but researchers can’t prevent the water content from changing over time. In previous experiments, scientists tried to completely dry peridotite samples and then manually correct for measured mantle water levels in their calculations. The scientists, however, couldn’t tell for sure if the samples were water-free.

The measurement difficulties stem from the fact that peridotite is a mix of the minerals olivine and pyroxene, and the mineral grains are too small to experiment with individually. Sarafian and colleagues overcame this challenge by inserting spheres of pure olivine large enough to study into synthetic peridotite samples. These spheres exchanged water with the surrounding peridotite until they had the same dampness, and so could be used for water content measurements.

Using this technique, the researchers found that the “dry” peridotite used in previous experiments wasn’t dry at all. In fact, the water content was spot on for the actual wetness of the mantle. “By assuming the samples are dry, then correcting for mantle water content, you’re actually overcorrecting,” Sarafian says.
The new experiment suggests that, if adjusted to surface pressure, the mantle under the eastern Pacific Ocean where two tectonic plates diverge, for example, would be around 1410°, up from 1350°. A hotter mantle is less viscous and more malleable, Sarafian says. Scientists have long been puzzled about some of the specifics of plate tectonics, such as to what extent the mantle resists the movement of the overlying plate. That resistance depends in part on the mix of rock, temperature and how melted the rock is at the boundary between the two layers (SN: 3/7/15, p. 6). This new knowledge could give researchers more accurate information on those details.

The revised temperature is only for the melting boundary in the mantle, so “it’s not the full story,” notes Caltech geologist Paul Asimow, who wrote a perspective on the research in the same issue of Science. He agrees that the team’s work provides a higher and more accurate estimate of that adjusted temperature, but he doesn’t think the researchers should assume temperatures elsewhere in the mantle would be boosted by a similar amount. “I’m not so sure about that,” he says. “We need further testing of mantle temperatures.”

Dental plaque preserved in fossilized teeth confirms that Neandertals were flexible eaters and may have self-medicated with an ancient equivalent of aspirin.

DNA recovered from calcified plaque on teeth from four Neandertal individuals suggest that those from the grasslands around Beligum’s Spy cave ate woolly rhinoceros and wild sheep, while their counterparts from the forested El Sidrón cave in Spain consumed a menu of moss, mushrooms and pine nuts.

The evidence bolsters an argument that Neandertals’ diets spanned the spectrum of carnivory and herbivory based on the resources available to them, Laura Weyrich, a microbiologist at the University of Adelaide in Australia, and her colleagues report March 8 in Nature.

The best-preserved Neandertal remains were from a young male from El Sidrón whose teeth showed signs of an abscess. DNA from a diarrhea-inducing stomach bug and several gum disease pathogens turned up in his plaque. Genetic material from poplar trees, which contain the pain-killing aspirin ingredient salicylic acid, and a plant mold that makes the antibiotic penicillin hint that he may have used natural medication to ease his ailments.

The researchers were even able to extract an almost-complete genetic blueprint, or genome, for one ancient microbe, Methanobrevibacter oralis. At roughly 48,000 years old, it’s the oldest microbial genome sequenced, the researchers report.

The Martian atmosphere definitely had more gas in the past.

Data from NASA’s MAVEN spacecraft indicate that the Red Planet has lost most of the gas that ever existed in its atmosphere. The results, published in the March 31 Science, are the first to quantify how much gas has been lost with time and offer clues to how Mars went from a warm, wet place to a cold, dry one.

Mars is constantly bombarded by charged particles streaming from the sun. Without a protective magnetic field to deflect this solar wind, the planet loses about 100 grams of its now thin atmosphere every second (SN: 12/12/15, p. 31). To determine how much atmosphere has been lost during the planet’s lifetime, MAVEN principal investigator Bruce Jakosky of the University of Colorado Boulder and colleagues measured and compared the abundances of two isotopes of argon at different altitudes in the Martian atmosphere. Using those measurements and an assumption about the amounts of the isotopes in the planet’s early atmosphere, the team estimates that about two-thirds of all of Mars’ argon gas has been ejected into space. Extrapolating from the argon data, the researchers also determined that the majority of carbon dioxide that the Martian atmosphere ever had also was kicked into space by the solar wind.

A thicker atmosphere filled with carbon dioxide and other greenhouse gases could have insulated early Mars and kept it warm enough for liquid water and possibly life. Losing an extreme amount of gas, as the results suggest, may explain how the planet morphed from lush and wet to barren and icy, the researchers write.