Glass frogs often start life with some tender care from a source scientists didn’t expect: frog moms.

Maternal care wouldn’t be news among mammals or birds, but amphibian parenting intrigues biologists because dads are about as likely as moms to evolve as the caregiver sex. And among New World glass frogs (Centrolenidae), what little parental care there is almost always is dad’s job — or so scientists thought, says Jesse Delia of Boston University.
Months of strenuous nights searching streamside leaves in five countries, however, have revealed a widespread world of brief, but important, female care in glass frogs. In examining 40 species, Delia and Laura Bravo-Valencia, now at Corantioquia, a government environmental agency in Santa Fe de Antioquia, Colombia, found that often mothers lingered over newly laid eggs for several hours. By pressing maternal bellies against the brood, moms hydrated the jelly-glop of eggs and improved offspring chances of survival, Delia, Bravo-Valencia and Karen Warkentin, also of BU, report online March 31 in the Journal of Evolutionary Biology.

Glass frogs take their name from see-through skin on their bellies and, in certain cases, transparent organ tissues. (Some have clear hearts that reveal blood swishing through.) These frogs aren’t exactly obscure species, but until this field project, which stretched over six rainy seasons, female care in the family was unknown.

Female glass frogs may not cuddle their eggs for long, but it’s enough to matter, the researchers found. As is common in frogs, the mothers don’t drink with their mouths but absorb water directly through belly skin into a bladder. Moms pressing against a mass of newly laid eggs caused the protective goo to swell — perhaps by osmosis or peeing — and the mass to quadruple in size. For some of the glass frogs in the study, the youngsters were on their own once mom left. But at least hydration created an unpleasant amount of slime for a predator to bite through before getting to frog embryos.
Night-hunting katydids in captivity, when offered a choice, barely nibbled at a hydrated mass of frog offspring, concentrating instead on eating an unhydrated clutch. In the field, when researchers removed about two dozen moms from their clutches in two species, mortality at least doubled to around 80 percent. Predators and dehydration caused the most deaths.

There are still more than 100 glass frog species that Delia and Bravo-Valencia haven’t yet watched in the wild. But the researchers did track down maternal care in 10 of 12 genera. Such a widespread form of maternal care probably evolved in the ancestor of all glass frogs, the researchers propose after analyzing glass frog family trees several ways.

In contrast, prolonged care from glass frog dads — rehydrating the egg mass as needed and fighting off predators such as hungry spiders — seems to have arisen independently later, at least twice. Across evolution in the animal kingdom, “usually we don’t see transitions from female to male care,” Delia says. “The pattern we found is completely bizarre.”

Why females started hanging around their eggs at all fascinates Hope Klug, an evolutionary biologist at the University of Tennessee at Chattanooga who studies parental care. In frogs, with eggs mostly fertilized externally, females could easily leave any care to dad.

“Parental care is perhaps more common and diverse in animals than we realize,” she says. “We just might have to look a little bit harder for it.”

Got a mouse in the house? Blame yourself. Not your housekeeping, but your species. Humans never intended to live a mouse-friendly life. But as we moved into a settled life, some animals — including a few unassuming mice — settled in, too. In the process, their species prospered — and took over the world.

The rise and fall of the house mouse’s fortunes followed the stability and instability of the earliest human settlements, a new study shows. By analyzing teeth from ancient mice and comparing the results to modern rodents hanging out near partially settled groups, scientists show that when humans began to settle down, one mouse species seemed to follow. When those people moved on, another species moved in. The findings reveal that human settlement took place long before agriculture began, and that vermin didn’t require a big storehouse of grain to thrive off of us.

Between 15,000 and 11,000 years ago (a time called the Natufian period), people began to form small stone settlements in what is now Israel and Jordan. They were not yet farming or storing grain, but they were living in a single place for a season or two, and coming back to that place relatively often. Those early settlers changed the ecosystem of the world around them — presenting new opportunities for local flora and fauna.

Lior Weissbrod, an archaeologist at the University of Haifa in Israel, started his career wanting to search for clues to the history of animal-human relationships. He was especially interested in animal remains. But, he admits, mouse teeth weren’t exactly his first choice. “[At] the site I was going to work on, the remains of larger animals were already studied,” he says. “I was left with the small mammals.”

Small mammals have even smaller teeth. The largest mouse molars are only about 1 millimeter long. This meant a lot of time sifting dirt through very fine mesh for Weissbrod. He collected 372 mouse teeth from the dirt of five different archaeological sites in modern-day Israel and Jordan, with remains dating from 11,000 to 200,000 years ago. He gave the teeth to his colleague Thomas Cucchi of the National Museum of Natural History in Paris, who developed a technique to classify the mouse teeth by species based on tiny differences in their shape.
The first human settlements were only a few houses, but that’s a seismic shift if you’re the size of a mouse. Before the settlements were built, the mouse species Mus macedonicus
scurried through the undergrowth. But after stone buildings arrived, another species dominated — Mus domesticu s. When humans left the settlements for about 1,000 years around 12,000 years ago, the reverse occurred: M. domesticus left and M. macedonicus moved back in.
The two species probably competed against each other when humans were absent, Weissbrod hypothesizes. But when people were around, M. domesticus was better at taking advantage of human presence. They may have had more flexible diets — making it easier to live off our leavings — and less fear of people. “Once you get humans into the picture and human settlements … there’s a creation of a new habitat,” he explains. “This species [M. domesticus] is finally getting a break from the competition.”

Ancient samples alone would not be enough to establish whether one mouse species could out-compete another based on human settlement alone. For a modern approximation of the ancient, partially settled lifestyle, Weissbrod turned to the Maasai, a nomadic cattle-herding group in southern Kenya. “Maasai are nothing like ancient hunter-gatherers,” he notes. “But we look at specific things that make them comparable.” Maasai herders tend to live in small groups, and use those settlements over and over. “The Maasai aren’t sedentary, they are on the verge perhaps,” he explains. “So they move [often] but not to the extent of highly mobile nomadic groups, they move on a seasonal basis.”

Weissbrod headed to Kenya to collect 192 live, spiny mice, living around Maasai settlements. These mice could be divided into two species, Acomys wilsoni — the short-tailed version — and Acomys ignitus, with longer tails. An archaeologist used to working with dry and dusty fossils, Weissbrod had to adjust to working with live, wiggling rodents. “It took a conscious process of pushing myself to get over the ingrained aversion,” he says.
Like their ancient kin, one modern spiny mouse had a competitive advantage when people were present. A. ignites made up 87 percent of the mice caught hanging out with the Maasai, but only 45 percent of the mice in non-settled area.
The associations between mice and men allowed Weissbrod, Cucchi and their colleagues to show how house mice came to live alongside us — no grain or farming required. “We can show with a high degree of certainty now that mice became attached to us 3,000 years before farming,” Weissbrod says. “From that we learned hunter-gatherers were making the transition to sedentary lives before farming. They didn’t need the crops to make that transition.” Weissbrod and his colleagues published their findings March 27 in the Proceedings of the National Academy of Sciences.

“I thought it was wonderful,” Melinda Zeder, an archaeologist at the Smithsonian Institution in Washington, D.C., says of the study. “I really appreciated it on a number of levels.” Zeder said she was impressed that such a big finding could come from samples so incredibly small. “It’s not a keystone or early writing, it’s something most people overlook,” she says. “Out of the mouths of mice, we’re getting a wonderful view of a pivotal time in history.”

Other species certainly took advantage of human settlement, Zeder notes. Wolves and wild boar arrived and “auditioned themselves for a starring role as a domesticate.” A few of the friendlier ones began to interact more with people. Those people began to see the advantages of the animals. From wolves were born dogs, and from wild boar our pigs.

But when humans weren’t paying attention, other species moved in and thrived on their own. Mice aren’t the only vermin that have arrived to, as Weissbrod says, “domesticate themselves.” Sparrows, pigeons, rats and other species have come to take advantage of our presence. We may have never seen a use for them, but that doesn’t stop them from using us.

The juice saga continues. The American Academy of Pediatrics updated their official ruling on fruit juice, recommending none of the sweet stuff before age 1. Published in the June Pediatrics, the recommendation is more restrictive than the previous one, which advised no juice before age 6 months.

The move comes from the recognition that whole fruits — not just the sweet, fiberless liquid contained within — are the most nutritious form of the food. Babies under 1 year old should be getting breast milk or formula until they’re ready to try solid foods. After their first birthdays, any extra liquids they drink should be water or milk. (These updated guidelines may not apply to babies who might need fruit juice to help with constipation.)

Whole fruits — or, mashed up clumps of them — have more fiber and protein than juice. The only benefit that juice has over its former whole form is that it’s way easier for a kid to slurp down.

The potential risks of cavities and obesity in part prompted the updated guidelines. It’s worth saying that neither of these outcomes are guaranteed with juice drinking. In fact, a recent study failed to find a link between juice drinking and excessive weight gain in children. Still, juice offers no nutritional advantage over whole fruits, so the reasoning of the AAP seems to be, “Why risk it?”

The AAP gave additional, more nuanced advice for parents who do decide to give juice to children age 1 and older:

Don’t give kids juice in bottles or sippy cups, especially at bedtime. That ease of drinkability would encourage kids to drink juice for long periods of time, prolonging sugar baths for teeth.

Look out for unpasteurized juice. Harmful forms of E. coli bacteria can appear in unpasteurized apple cider, for instance, posing a particular risk for young people.

Give 1- to 3-year-old kids no more than 4 ounces of juice a day. That recommendation drops 2 ounces from earlier guidance that limited juice to between 4 and 6 ounces daily. Children ages 4 to 6 years old get those extra 2 ounces back, with a daily limit of between 4 and 6 ounces.

Make sure kids are drinking 100 percent juice, not those sneaky “cocktails” or “drinks.” Those are often nutritional wastelands, packed with even more sugar and devoid of other nutrients.
Though the guidelines don’t mention habit formation, I suspect this also came into play in the AAP’s encouragement of whole fruits over juice. Children’s taste preferences get shaped early. Really early, actually. Fetuses learn to love flavors their mothers ate while pregnant. So babies who grow accustomed to sweet juice might be less impressed with water or milk. And while that might not be a problem in the early years of life, years or decades of drinking sweet liquids will catch up with them eventually.

Anyone who’s dragged roller luggage knows it’s liable to fishtail. To most people, this is a nuisance. To a few scientists, it’s a physics problem. Researchers detail the precise interplay of forces that set suitcases shimmying in a study published online June 21 in Proceedings of the Royal Society A.

The researchers simulated and observed the motion of a toy model suitcase on a treadmill. They found that the suitcase’s side-to-side motion at any given moment is related to its tilt and distance off-center from the line of travel.
For instance, imagine a suitcase rolling straight ahead, but then hitting a bump or cutting a corner that causes the right wheel to lift. The suitcase’s tilt makes the left wheel steer the suitcase rightward. When the right wheel falls back to the ground and the left wheel lifts off, the suitcase — now positioned and tilted to the right — banks left. Switch wheels, swing, repeat.

“It’s a pretty good analysis of the system,” says Andy Ruina, a physicist at Cornell University who was not involved in the research.

This swaying motion is “a bit funny and counterintuitive,” says study coauthor Sylvain Courrech du Pont. It actually gets smaller when the suitcase rolls faster. Lowering the angle of the suitcase can get the rocking to stop altogether, he says.

Understanding the physics of this system could be useful for more than designing stable suitcases, because it also applies to other two-wheel carriers — like car-pulled trailers. “In the near future, maybe we will have a car without a driver,” says Courrech du Pont, a physicist at Paris Diderot University. “It would be a good thing if the car knows how to stop this kind of motion.”

When it comes to smelling pretty, petunias are pretty pushy.

Instead of just letting scent compounds waft into the air, the plants use a particular molecule called a transporter protein to help move the compounds along, a new study found. The results, published June 30 in Science, could help researchers genetically engineer many kinds of plants both to attract pollinators and to repel pests and plant eaters.

“These researchers have been pursuing this transporter protein for a while,” says David Clark, an expert in horticultural biotechnology and genetics at the University of Florida in Gainesville. “Now they’ve got it. And the implications could be big.”
Plants use scents to communicate (SN: 7/27/02, p. 56). The scent compounds can attract insects and other organisms that spread pollen and help plants reproduce, or can repel pests and plant-eating animals. The proteins found in the new study could be used to dial the amount of scent up or down so that plants can attract more pollinators or better protect themselves. Currently unscented plants could be engineered to smell, too, giving them a better shot at reproduction and survival, Clark says.
Plants get their scents from volatile organic compounds, which easily turn into gases at ambient temperatures. Petunias get their sweet smell from a mix of benzaldehyde, the same compound that gives cherries and almonds their fruity, nutty scent, and phenylpropanoids, often used in perfumes.

But nice smells have a trade-off: If these volatile compounds build up inside a plant, they can damage the plant’s cells.
About two years ago, study coauthor Joshua Widhalm, a horticulturist at Purdue University in West Lafayette, Ind., and colleagues used computer simulations to look at the way petunias’ scent compounds moved. The results showed that the compounds can’t move out of cells fast enough on their own to avoid damaging the plant. So the researchers hypothesized that something must be shuttling the compounds out.

In the new study, led by Purdue biochemist Natalia Dudareva, the team looked for genetic changes as the plant developed from its budding stage, which had the lowest levels of volatile organic compounds, to its flower-opening stage, with the highest levels. As flowers opened and scent levels peaked, the gene PhABCG1 went into overdrive; levels of the protein that it makes jumped to more than 100 times higher than during the budding stage, the researchers report.

The team then genetically engineered petunias to produce 70 to 80 percent less of the PhABCG1 protein. Compared with regular petunias, the engineered ones released around half as much of the scent compounds, with levels inside the plant’s cells building to double or more the normal levels. Images of the cells show that the accumulation led to deterioration of cell membranes.

A lot of work has been done to identify the genes and proteins that generate scent compounds, says Clark. But this appears to be the first study to have identified a transporter protein to shuttle those compounds out of the cell. “That’s a big deal,” he says.

Humpback whales, those innovative foodies, have discovered their own pop-up restaurants.

Migrant humpbacks returning to southeastern Alaska in spring are the first of their kind known to make routine visits to fish hatcheries releasing young salmon into the sea, says marine ecologist Ellen Chenoweth.

The whales are “40 feet long and they’re feeding on fish that are the size of my finger,” says Chenoweth, of the Juneau fisheries center of University of Alaska Fairbanks. For tiny prey to be worthwhile to humpbacks, it’s good to find crowds — such as young salmon streaming out of hatchery nets.

Six years of systematic observations of whales at five hatcheries at Baranof Island reveal a pattern of humpbacks visiting during springtime releases, Chenoweth and her colleagues report June 12 in Royal Society Open Science.

Whale visits to the salmon buffet enhance humpbacks’ reputation for innovation in feeding, Chenoweth says. And since the water is relatively shallow, visits also make it easier to film humpback lunge feeding. More often, studying whale food dives means watching animals surface from the depths to catch their breath before the next exciting plunge. “It’s like going to a basketball game but you can only really watch players on the bench,” Chenoweth says.

Underwater, a humpback opens its jaws and rushes into the mass of prey as its throat balloons out “like a parachute opening,” she says. The toothless whale then filters the mouthful, swooshing it out through dangling fringes of baleen, which snag what’s worth swallowing. In one lunge, a humpback can take in about 27,600 liters of seawater — about 28 metric tons — roughly doubling the whale’s weight.

An unlikely hero has emerged in the quest to fight HIV: the cow. In a first for any animal, including humans, four cows injected with a type of HIV protein rapidly produced powerful antibodies against the virus, researchers report. Learning how to induce similar antibodies in humans may be key to a successful HIV vaccine.

The antibodies, called broadly neutralizing antibodies, can stop infection from a variety of HIV types. The cows generated these antibodies as soon as 42 days after immunization, the researchers report online July 20 in Nature. For the small percentage of people estimated to develop these antibodies after a natural infection, it can take several years.
The work identifies “a new and much more efficient method to generate broadly active antibodies against HIV,” says immunologist Justin Bailey of Johns Hopkins University School of Medicine, who was not involved in the study.

Making an HIV vaccine has proved difficult because the virus changes all the time. Different strains exist throughout the world, and the virus even mutates within an infected person’s body. Most often, people develop antibodies that are specific to one strain but ineffective against others. HIV vaccines tested so far have not led to the production of broadly neutralizing antibodies.

About 1 percent of HIV-infected people eventually generate broadly neutralizing antibodies that are especially potent and effective against many types of HIV. The development of these antibodies doesn’t seem to help infected people. But when given to monkeys before exposure to a virus similar to HIV, the antibodies prevent infection.

Broadly neutralizing antibodies specific to HIV have a few quirky features, one of which is the presence of a long stretch of amino acids that sticks out from the antibody surface. This protruding part of the antibody binds to a viral site that remains the same between strains, because the virus needs it to gain entry to a cell. HIV’s thick coat of surface sugars makes the viral binding site difficult to access. A longer stretch of amino acids seems to be able to pierce through “and reach in, almost like the long arm of the law,” says Vaughn Smider, a molecular immunologist at the Scripps Research Institute in La Jolla, Calif.

In people infected with HIV who develop broadly neutralizing antibodies, this antibody region — called HCDR3 — has about 30 amino acids, about twice as long as what is usual for human antibodies. Although on the long side for a human, “that’s actually kind of short for a cow,” Smider says.
And so the idea to immunize cows was born. Since cows naturally make longer HCDR3s, Smider explains, perhaps they’d have this sought-after response to HIV.

Smider and colleagues took serum — blood with the cells removed, leaving antibodies behind — from four immunized cows and tested it against different types of HIV virus in a test tube. All of the cows developed broadly neutralizing antibodies. The researchers then tested one cow’s antibodies on an even larger number of virus types. After 381 days, this cow’s antibodies prevented 96 percent of the 117 HIV types from infecting cells in a lab dish. The researchers also isolated an antibody from this cow that had a long HCDR3 of 60 amino acids and stopped infection by 72 percent of the HIV types.

If researchers could induce antibodies with long HCDR3s in humans, “then that could be the basis of getting a vaccine to work,” Smider says. “You need a step before the immunization that helps expand the rare antibodies.” Since cows are so good at making broadly neutralizing antibodies, it also might be possible to turn the cow’s handiwork into drugs for HIV treatment, if bovine antibodies are effective at stopping the virus in other animals, he says.

Galaxies may grow by swiping gas from their neighbors.

New simulations suggest that nearly half the matter in the Milky Way may have been siphoned from the gas of other galaxies. That gas provides the raw material that galaxies use to build their bulk. The finding, scheduled to appear in the Monthly Notices of the Royal Astronomical Society, reveals a new, unexpected way for galaxies to acquire matter and could give clues to how they evolve.
“These simulations show a huge amount of interaction among galaxies, a huge dance that’s going on,” says astronomer Romeel Davé of the University of Edinburgh. That dance, and the subsequent exchange of atoms, could be what establishes a galaxy’s character — whether it’s small or big, elliptical or spiral, quiet or bursting with star formation. If the simulation results are confirmed with observations, it could be a major advancement in understanding galaxy formation, Davé says.

It makes sense that much of the material in one galaxy actually came from other galaxies, says study coauthor Claude-André Faucher-Giguère, a theoretical astrophysicist at Northwestern University in Evanston, Ill. “Still, the result was really unexpected,” he says.

Astronomers thought galaxies got their matter in two main ways. First, atoms clumped together to form stars and then galaxies, not long after the Big Bang about 13.8 billion years ago. Then some of those atoms were eventually ejected by supernovas but rained back onto the same galaxy, recycling the gas again and again.

The new simulations showed a third way galaxies could score gas. Powerful supernova explosions would eject atoms, in the form of gas, far from their home galaxies into intergalactic space. Those atoms would then travel through space, pushed toward other galaxies by galactic winds that move at several hundred kilometers per second. When the particles neared a galaxy’s gravitational pull, they would get sucked in, where they would serve as the basis for stars, planets, dust and other material in their new galactic home. Still, this exchange of atoms is extremely difficult to spot in space because the gas atoms, don’t give off light like stars do.
Faucher-Giguère and colleagues spotted the exchange in computer simulations that show how galaxies formed just after the Big Bang and how they have evolved over time. The team tracked gas atoms as they moved through the model universe, formed stars and then were ejected from galaxies as those stars exploded.

In the simulations, up to half of the atoms in large galaxies were pulled in from other galaxies. Because more massive galaxies have more gravity, they tended to pull atoms from the ejected material of small galaxies. The exchange appears to take billions of years as atoms travel the vast space between galaxies, the team notes.

“It’s that not surprising to see a galaxy kick out matter, which is then pulled in by other galaxies,” Davé says. What is surprising, he says, is the amount of material that’s transferred. Before seeing the simulations, he would have guessed that about 5 percent of gas was transferred among galaxies this way. “To see that it is up to 50 percent is pretty remarkable,” he says.

Already, astronomers are searching for evidence of this material-swapping behavior among galaxies. Faucher-Giguère and colleagues, working with researchers using the Hubble Space Telescope, hope to observe intergalactic transfer of gas among galaxies soon.

Single-celled microbes may have taught plants and animals how to pack their genetic baggage.

Archaea, a type of single-celled life-form similar to bacteria, keep their DNA wrapped around proteins much in the same way as more complex organisms, researchers report in the Aug. 11 Science. This finding provides new insight into the evolutionary origins of the DNA-packing process and the secret to archaea’s hardiness, which enables some to live in acid, boiling water or other extreme environments.
All eukaryotes, including plants and animals, store their genetic material in cell compartments called nuclei. Such organisms cram meters of genetic material into the tiny nuclei by wrapping strands of DNA around clusters of proteins called histones (SN: 1/10/15, p. 32). “It doesn’t really matter which eukaryote you look at, whether it’s amoebas or plants or humans or fish or insects or anything,” says coauthor John Reeve, a microbiologist at Ohio State University. “They all have exactly the same structure.”

Unlike bacteria, some archaea also contain histones, but researchers weren’t sure whether these microbes spool DNA around the protein bobbins the way eukaryotes do. So Reeve and colleagues used a method called X-ray crystallography to discern, for the first time, the precise shape of archaea DNA bound to histones.

The researchers saw that archaea DNA coils around the histones, similar to the way it does in eukaryotes. “It’s a big deal actually seeing this,” says Steven Henikoff, a molecular biologist at the Fred Hutchinson Cancer Research Center in Seattle who was not involved in the work. The resemblance between archaea and eukaryote DNA wrapping means that the first organism that used this storage scheme was an ancestor of both modern eukaryotes and archaea, the researchers conclude.

But the way archaea DNA twists around histones isn’t identical to the coils of DNA seen in eukaryotes. In eukaryotes, a strand of DNA loops twice around a cluster of eight histones to create what’s called a nucleosome, and connects many of these nucleosomes like beads on a string. Archaea DNA string together bundles of proteins, too. But while eukaryotes always tether eight-protein clumps, archaea DNA can spiral around stacks of many more histones to create rod-shaped structures of various lengths. “So it’s not as uniform as in eukaryotes,” says coauthor Karolin Luger, a biophysicist and Howard Hughes Medical Institute investigator at the University of Colorado Boulder.

Researchers tested the importance of that rodlike architecture by tampering with the histone-DNA structures of some archaea and then observing how these mutant archaea fared in different conditions. “We tried to mimic some real-life situations that some of these organisms could get into,” Luger says.
For instance, some archaea that live in volcanic vents that emit sulfurous gases sometimes get spewed out and have to survive sans sulfur. Archaea with normal histone-DNA shapes can handle that kind of midlife crisis. But when researchers cut their mutant microbes off sulfur, the microorganisms’ growth was stunted. These microbes may not have been able to adapt to sulfur deprivation as well as their wild counterparts “because they can’t unpackage their DNA as readily if the structure has been changed,” Reeve says.

Henikoff calls it “a pretty cool experiment.” It showed that the archaea’s particular DNA-histone architecture was “biologically relevant, not just a novelty,” he says.

A total solar eclipse shines a light on the sun’s elusive atmosphere. When the moon blocks the sun, it’s finally possible to see how this diffuse cloud of plasma, called the corona, is magnetically sculpted into beautiful loops. The material there is about a trillionth the density of the solar surface. From its delicate and diaphanous appearance, you might expect the corona to be where the sun goes to cool off.

That couldn’t be more wrong. The corona is a mysteriously sizzling inferno where the temperature jumps from a mere few thousand degrees to several million degrees. Why?
“It’s one of the longest unanswered questions in all of solar physics,” says Paul Bryans of the High Altitude Observatory at the National Center for Atmospheric Research in Boulder, Colo. “There are a bunch of different ideas about what’s going on there, but it’s still highly debated.” Data collected during the Aug. 21 solar eclipse may bring scientists closer to settling that debate.

The sun simmers at about 5,500° Celsius at its visible surface, the photosphere. But the gas just above the photosphere is heated to about 10,000° C. Then in the corona, the temperature makes an abrupt jump to several million degrees.

“It’s counterintuitive that as you move away from a heat source, it gets warmer,” Bryans says. The corona’s diffuseness makes its heat even stranger — the most basic ways to heat a material rely on particles crashing into each other, but the corona is too tenuous for that to work.

An eclipse first brought this abnormal arrangement to light. German astronomer Walter Grotrian observed spectral lines — the fingerprints of elements that show up when light is split into its component wavelengths — emitted by the corona during a total solar eclipse in 1869.

Astronomers at first assumed those lines were due to a new element they dubbed coronium. But Grotrian realized that iron atoms stripped of several of their electrons by the heat were responsible. These iron lines in the corona are still used to measure its temperature: The more electrons lost, the hotter the material in the corona (SN Online: 6/16/17).
Such extreme temperatures have something to do with the corona’s magnetic field, which is probably where all that energy is stored. Once the energy is there, the corona has a hard time radiating it away, so it builds up. Most of the ways that materials release energy — stripping electrons from atoms, accelerating those electrons so they release X-rays and ultraviolet particles of light — are already maxed out in the corona.
“We know there’s energy coming in, and it’s hard to get it out unless you get very hot,” says Amir Caspi of the Southwest Research Institute in Boulder, Colo. “What we don’t understand is how that energy gets into the corona in the first place.”

Physicists have several ideas. Maybe loops of magnetic field lines in the corona vibrate like guitar strings, heating things up, sort of like how a microwave oven heats food. Maybe the magnetic anchors of those loops on the sun’s surface braid and twist the magnetic field above them, dumping in energy that is then continually radiated away like the heating element in a toaster.

Or maybe tiny explosions called nanoflares or jets called spicules carry energy away from the photosphere and into the corona. The formation of new coronal loops that connect to existing ones could dump in enough extra energy to heat the plasma up.

During the solar eclipse, dozens of groups of scientists across the country will deploy telescopes equipped with filters to pick out polarized light, infrared light or those electron-deprived iron atoms in search of answers. Bryans and his colleagues will be on a mountaintop near Casper, Wyo., in the path of totality. There, the team will take images at a fast clip in both visual and infrared wavelengths to map how the corona changes as the moon moves across the sun. (I will be in Wyoming with this team on the day of the eclipse and will be sharing more about how the experiments went.)

“We can look at how things change as we move from the surface up into the atmosphere,” Bryans says. “How that changes is tied to understanding how the corona is heated.”

Probably all of those mechanisms scientists have thought up contribute to the corona’s extreme heat. It’s difficult to declare just one the most important. But ultimately, the solar eclipse is the best chance scientists have to test them. It’s the only time the corona is the star of the solar show.