The quest to identify the nature of the neutrino’s alter ego is heating up

Galaxies, stars, planets and life, all are formed from one essential substance: matter.

But the abundance of matter is one of the biggest unsolved mysteries of physics. The Big Bang, 13.8 billion years ago, spawned equal amounts of matter and its bizarro twin, antimatter. Matter and antimatter partners annihilate when they meet, so an even stephen universe would have ended up full of energy — and nothing else. Somehow, the balance tipped toward matter in the early universe.
A beguiling subatomic particle called a neutrino may reveal how that happened. If neutrinos are their own antiparticles — meaning that the neutrino’s matter and antimatter versions are the same thing — the lightweight particle might point to an explanation for the universe’s glut of matter.

So scientists are hustling to find evidence of a hypothetical kind of nuclear decay that can occur only if neutrinos and antineutrinos are one and the same. Four experiments have recently published results showing no hint of the process, known as neutrinoless double beta decay (SN: 7/6/02, p. 10). But another attempt, set to begin soon, may have a fighting chance of detecting this decay, if it occurs. Meanwhile, planning is under way for a new generation of experiments that will make even more sensitive measurements.

“Right now, we’re standing on the brink of what potentially could be a really big discovery,” says Janet Conrad, a neutrino physicist at MIT not involved with the experiments.
Each matter particle has an antiparticle, a partner with the opposite electric charge. Electrons have positrons as partners; protons have antiprotons. But it’s unclear how this pattern applies to neutrinos, which have no electric charge.

Rather than having distinct matter and antimatter varieties, neutrinos might be the lone example of a theorized class of particle dubbed a Majorana fermion (SN: 8/19/17, p. 8), which are their own antiparticles. “No other particle that we know of could have this property; the neutrino is the only one,” says neutrino physicist Jason Detwiler of the University of Washington in Seattle, who is a member of the KamLAND-Zen and Majorana Demonstrator neutrinoless double beta decay experiments.

Neutrinoless double beta decay is a variation on standard beta decay, a relatively common radioactive process that occurs naturally on Earth. In beta decay, a neutron within an atom’s nucleus converts into a proton, releasing an electron and an antineutrino. The element thereby transforms into another one further along the periodic table.
In certain isotopes of particular elements — species of atoms characterized by a given number of protons and neutrons — two beta decays can occur simultaneously, emitting two electrons and two antineutrinos. Although double beta decay is exceedingly rare, it has been detected. If the neutrino is its own antiparticle, a neutrino-free version of this decay might also occur: In a rarity atop a rarity, the antineutrino emitted in one of the two simultaneous beta decays might be reabsorbed by the other, resulting in no escaping antineutrinos.

Such a process “creates asymmetry between matter and antimatter,” says physicist Giorgio Gratta of Stanford University, who works on the EXO-200 neutrinoless double beta decay experiment. In typical beta decay, one matter particle emitted — the electron — balances out the antimatter particle — the antineutrino. But in neutrinoless double beta decay, two electrons are emitted with no corresponding antimatter particles. Early in the universe, other processes might also have behaved in a similarly asymmetric way.

On the hunt
To spot the unusual decay, scientists are building experiments filled with carefully selected isotopes of certain elements and monitoring the material for electrons of a particular energy, which would be released in the neutrinoless decay.

If any experiment observes this process, “it would be a huge deal,” says particle physicist Yury Kolomensky of the University of California, Berkeley, a member of the CUORE neutrinoless double beta decay experiment. “It is a Nobel Prize‒level discovery.”

Unfortunately, the latest results won’t be garnering any Nobels. In a paper accepted in Physical Review Letters, the GERDA experiment spotted no signs of the decay. Located in the Gran Sasso underground lab in Italy, GERDA looks for the decay of the isotope germanium-76. (The number indicates the quantity of protons and neutrons in the atom’s nucleus.) Since there were no signs of the decay, if the process occurs it must be extremely rare, the scientists concluded, and its half-life must be long — more than 80 trillion trillion years.

Three other experiments have also recently come up empty. The Majorana Demonstrator experiment, located at the Sanford Underground Research Facility in Lead, S.D., which also looks for the decay in germanium, reported no evidence of neutrinoless double beta decay in a paper accepted in Physical Review Letters. Meanwhile, EXO-200, located in the Waste Isolation Pilot Plant, underground in a salt deposit near Carlsbad, N.M., reported no signs of the decay in xenon-136 in a paper published in the Feb. 16 Physical Review Letters.

Likewise, no evidence for the decay materialized in the CUORE experiment, in results reported in a paper accepted in Physical Review Letters. Composed of crystals containing tellurium-130, CUORE is also located in the Gran Sasso underground lab.

The most sensitive search thus far comes from the KamLAND-Zen neutrinoless double beta decay experiment located in a mine in Hida, Japan, which found a half-life longer than 100 trillion trillion years for the neutrinoless double beta decay of xenon-136.

That result means that, if neutrinos are their own antiparticles, their mass has to be less than about 0.061 to 0.165 electron volts depending on theoretical assumptions, the KamLAND-Zen collaboration reported in a 2016 paper in Physical Review Letters. (An electron volt is particle physicists’ unit of energy and mass. For comparison, an electron has a much larger mass of half a million electron volts.)

Neutrinos, which come in three different varieties and have three different masses, are extremely light, but exactly how tiny those masses are is not known. Mass measured by neutrinoless double beta decay experiments is an effective mass, a kind of weighted average of the three neutrino masses. The smaller that mass, the lower the rate of the neutrinoless decays (and therefore the longer the half-life), and the harder the decays are to find.

KamLAND-Zen looks for decays of xenon-136 dissolved in a tank of liquid. Now, KamLAND-Zen is embarking on a new incarnation of the experiment, using about twice as much xenon, which will reach down to even smaller masses, and even rarer decays. Finding neutrinoless double beta decay may be more likely below about 0.05 electron volts, where neutrino mass has been predicted to lie if the particles are their own antiparticles.

Supersizing the search
KamLAND-Zen’s new experiment is only a start. Decades of additional work may be necessary before scientists clinch the case for or against neutrinos being their own antiparticles. But, says KamLAND-Zen member Lindley Winslow, a physicist at MIT, “sometimes nature is very kind to you.” The experiment could begin taking data as early as this spring, says Winslow, who is also a member of CUORE.

To keep searching, experiments must get bigger, while remaining extremely clean, free from any dust or contamination that could harbor radioactive isotopes. “What we are searching for is a decay that is very, very, very rare,” says GERDA collaborator Riccardo Brugnera, a physicist at the University of Padua in Italy. Anything that could mimic the decay could easily swamp the real thing, making the experiment less sensitive. Too many of those mimics, known as background, could limit the ability to see the decays, or to prove that they don’t occur.

In a 2017 paper in Nature, the GERDA experiment deemed itself essentially free from background — a first among such experiments. Reaching that milestone is good news for the future of these experiments. Scientists from GERDA and the Majorana Demonstrator are preparing to team up on a bigger and better experiment, called LEGEND, and many other teams are also planning scaled-up versions of their current detectors.

Antimatter whodunit
If scientists conclude that neutrinos are their own antiparticles, that fact could reveal why antimatter is so scarce. It could also explain why neutrinos are vastly lighter than other particles. “You can kill multiple problems with one stone,” Conrad says.

Theoretical physicists suggest that if neutrinos are their own antiparticles, undetected heavier neutrinos might be paired up with the lighter neutrinos that we observe. In what’s known as the seesaw mechanism, the bulky neutrino would act like a big kid on a seesaw, weighing down one end and lifting the lighter neutrinos to give them a smaller mass. At the same time, the heavy neutrinos — theorized to have existed at the high energies present in the young universe — could have given the infant cosmos its early preference for matter.

Discovering that neutrinos are their own antiparticles wouldn’t clinch the seesaw scenario. But it would provide a strong hint that neutrinos are essential to explaining where the antimatter went. And that’s a question physicists would love to answer.

“The biggest mystery in the universe is who stole all the antimatter. There’s no bigger theft that has occurred than that,” Conrad says.

n the future, an AI may diagnose eye problems

The computer will see you now.

Artificial intelligence algorithms may soon bring the diagnostic know-how of an eye doctor to primary care offices and walk-in clinics, speeding up the detection of health problems and the start of treatment, especially in areas where specialized doctors are scarce. The first such program — trained to spot symptoms of diabetes-related vision loss in eye images — is pending approval by the U.S. Food and Drug Administration.

While other already approved AI programs help doctors examine medical images, there’s “not a specialist looking over the shoulder of [this] algorithm,” says Michael Abràmoff, who founded and heads a company that developed the system under FDA review, dubbed IDx-DR. “It makes the clinical decision on its own.”
IDx-DR and similar AI programs, which are learning to predict everything from age-related sight loss to heart problems just by looking at eye images, don’t follow preprogrammed guidelines for how to diagnose a disease. They’re machine-learning algorithms that researchers teach to recognize symptoms of a particular condition, using example images labeled with whether or not that patient had that condition.
IDx-DR studied over 1 million eye images to learn how to recognize symptoms of diabetic retinopathy, a condition that develops when high blood sugar damages retinal blood vessels (SN Online: 6/29/10). Between 12,000 and 24,000 people in the United States lose their vision to diabetic retinopathy each year, but the condition can be treated if caught early.
Researchers compared how well IDx-DR detected diabetic retinopathy in more than 800 U.S. patients with diagnoses made by three human specialists. Of the patients identified by IDx-DR as having at least moderate diabetic retinopathy, more than 85 percent actually did. And of the patients IDx-DR ruled as having mild or no diabetic retinopathy, more than 82.5 percent actually did, researchers reported February 22 at the annual meeting of the Macula Society in Beverly Hills, Calif.

IDx-DR is on the fast-track to FDA clearance, and a decision is expected within a few months, says Abràmoff, a retinal specialist at the University of Iowa in Iowa City. If approved, it would become the first autonomous AI to be used in primary care offices and clinics.

AI algorithms to diagnose other eye diseases are in the works, too. An AI described February 22 in Cell studied over 100,000 eye images to learn the signs of several eye conditions. These included age-related macular degeneration, or AMD — a leading cause of vision loss in adults over 50 — and diabetic macular edema, a condition that develops from diabetic retinopathy.

This AI was designed to flag advanced AMD or diabetic macular edema for urgent treatment, and to refer less severe cases for routine checkups. In tests, the algorithm was 96.6 percent accurate in diagnosing eye conditions from 1,000 pictures. Six ophthalmologists made similar referrals based on the same eye images.

Researchers still need to test how this algorithm fares in the real world where the quality of images may vary from clinic to clinic, says Aaron Lee, an ophthalmologist at the University of Washington in Seattle. But this kind of AI could be especially useful in rural and developing regions where medical resources and specialists are scarce and people otherwise wouldn’t have easy access to in-person eye exams.

AI might also be able to use eye pictures to identify other kinds of health problems. One algorithm that studied retinal images from over 284,000 patients could predict cardiovascular health risk factors such as high blood pressure.

The algorithm was 71 percent accurate in distinguishing eye images between smoking and nonsmoking patients, according to a report February 19 in Nature Biomedical Engineering. And it predicted which patients would have a major cardiovascular event, such as a heart attack, within the next five years 70 percent of the time.

With AI getting more adept at screening for a growing list of conditions, “some people might be concerned that this is machines taking over” health care, says Caroline Baumal, an ophthalmologist at Tufts University in Boston. But diagnostic AI can’t replace the human touch. “Doctors will still need to be there to see patients and treat patients and talk to patients,” Baumal says. AI will just help people who need treatment get it faster.

Cosmic dust may create Mars’ wispy clouds

The seeds for Martian clouds may come from the dusty tails of comets.

Charged particles, or ions, of magnesium from the cosmic dust can trigger the formation of tiny ice crystals that help form clouds, a new analysis of Mars’ atmosphere suggests.

For more than a decade, rovers and orbiters have captured images of Martian skies with wispy clouds made of carbon dioxide ice. But “it hasn’t been easy to explain where they come from,” says chemist John Plane of the University of Leeds in England. The cloud-bearing layer of the atmosphere is between –120° and –140° Celsius — too warm for carbon dioxide clouds to form on their own, which can happen at about –220° C.
Then in 2017, NASA’s MAVEN orbiter detected a layer of magnesium ions hovering about 90 kilometers above the Martian surface (SN: 4/29/17, p. 20). Scientists think the magnesium, and possibly other metals not yet detected, comes from cosmic dust left by passing comets. The dust vaporizes as it hits the atmosphere, leaving a sprinkling of metals suspended in the air. Earth has a similar layer of atmospheric metals, but none had been observed elsewhere in the solar system before.

According to the new calculations, the bits of magnesium clump with carbon dioxide gas — which makes up about 95 percent of Mars’ atmosphere — to produce magnesium carbonate molecules. These larger, charged molecules could attract the atmosphere’s sparse water, creating what Plane calls “dirty” ice crystals.

At the temperatures seen in Mars’ cloud layer, pure carbon dioxide ice crystals are too small to gather clouds around them. But clouds could form around dirty ice at temperatures as high as –123° C, Plane and colleagues report online March 6 in the Journal of Geophysical Research: Planets.

False alarms may be a necessary part of earthquake early warnings

Earthquake warning systems face a tough trade-off: To give enough time to take cover or shut down emergency systems, alerts may need to go out before it’s clear how strong the quake will be. And that raises the risk of false alarms, undermining confidence in any warning system.

A new study aims to quantify the best-case scenario for warning time from a hypothetical earthquake early warning system. The result? There is no magic formula for deciding when to issue an alert, the researchers report online March 21 in Science Advances.
“We have a choice when issuing earthquake warnings,” says study leader Sarah Minson, a seismologist at the U.S. Geological Survey, or USGS, in Menlo Park, Calif. “You have to think about your relative risk appetite: What is the cost of taking action versus the cost of the damage you’re trying to prevent?”

For locations far from a large quake’s origin, waiting for clear signs of risk before sending an alert may mean waiting too long for people to be able to take protective action. But for those tasked with managing critical infrastructure, such as airports, trains or nuclear power plants, an early warning even if false may be preferable to an alert coming too late (SN: 4/19/14, p. 16).

Alerts issued by earthquake early warning systems, called EEWs, are based on several parameters: the depth and location of the quake’s origin, its estimated magnitude and the ground properties, such as the types of soil and rock that seismic waves would travel through.

“The trick to earthquake early warning systems is that it’s a misnomer,” Minson says. Such systems don’t warn that a quake is imminent. Instead, they alert people that a quake has already happened, giving them precious seconds — perhaps a minute or two — to prepare for imminent ground shaking.
Estimating magnitude turns out to be a sticking point. It is impossible to distinguish a powerful earthquake in its earliest stages from a small, weak quake, according to a 2016 study by a team of researchers that included Men-Andrin Meier, a seismologist at Caltech who also coauthors the new study. Estimating magnitude for larger quakes also takes more time, because the rupture of the fault lasts perhaps several seconds longer – a significant chunk of time when it comes to EEW. And there is a trade-off in terms of distance: For locations farther away, there is less certainty the shaking will reach that far.
In the new study, Minson, Meier and colleagues used standard ground-motion prediction equations to calculate the minimum quake magnitude that would produce shaking at any distance. Then, they calculated how quickly an EEW could estimate whether the quake would exceed that minimum magnitude to qualify for an alert. Finally, the team estimated how long it would take for the shaking to strike a location. Ultimately, they determined, EEW holds the greatest benefit for users who are willing to take action early, even with the risk of false alarms. The team hopes its paper provides a framework to help emergency response managers make those decisions.

EEWs are already in operation around the world, from Mexico to Japan. USGS, in collaboration with researchers and universities, has been developing the ShakeAlert system for the earthquake-prone U.S. West Coast. It is expected be rolled out this year, although plans for future expansion may be in jeopardy: President Trump’s proposed 2019 budget cuts the USGS program’s $8.2 million in funding. It’s unclear whether Congress will spare those funds.

The value of any alert system will ultimately depend on whether it fulfills its objective — getting people to take cover swiftly in order to save lives. “More than half of injuries from past earthquakes are associated with things falling on people,” says Richard Allen, a seismologist at the University of California, Berkeley who was not involved in the new study. “A few seconds of warning can more than halve the number of injuries.”

But the researchers acknowledge there is a danger in issuing too many false alarms. People may become complacent and ignore future warnings. “We are playing a precautionary game,” Minson says. “It’s a warning system, not a guarantee.”

Powerful New England quake recorded in pond mud

The history of New England’s most damaging earthquake is written in the mud beneath a Massachusetts pond. Researchers identified the first sedimentary evidence of the Cape Ann earthquake, which in 1755 shook the East Coast from Nova Scotia to South Carolina. The quake, estimated to have been at least magnitude 5.9, took no lives but damaged hundreds of buildings.

Within a mud core retrieved from the bottom of Sluice Pond in Lynn, Mass., a light brown layer of sediment stands out amid darker layers of organic-rich sediment, the researchers report March 27 in Seismological Research Letters. The 2-centimeter-thick layer contains tiny fossils usually found near the shore, as well as types of pollen different from those found in the rest of the core. Using previous studies of the pond’s deposition rates, geologist Katrin Monecke of Wellesley College in Massachusetts and her colleagues determined the layer dates to between 1740 and 1810.
That light-brown layer is likely a turbidite, sediment jumbled up by a sudden lake slope failure, the study says. There are no other turbidites in the core, which spans about 400 years, suggesting the slopes held fast through floods and hurricanes. But the Cape Ann quake was likely a strong enough trigger to cause the slope failure.

Though the eastern United States is not at the seismically active edge of a tectonic plate, it has occasionally had its ground-shakers (SN Online: 8/23/11). The study suggests other East Coast lakes and ponds may contain evidence of prehistoric quakes, giving researchers a new way to estimate their frequency.

The Cape Ann quake also left its mark on the colonists, inspiring poems that suggested the temblor was a warning from a wrathful God. Harvard University scientist John Winthrop chronicled witness accounts of the quake in a 1757 paper to the Royal Society of London. “The earthquake began with a roaring noise,” Winthrop quoted one man as saying, “like thunder at a distance.”

Ardi walked the walk 4.4 million years ago

A famous 4.4-million-year-old member of the human evolutionary family was hip enough to evolve an upright gait without losing any tree-climbing prowess.

The pelvis from a partial Ardipithecus ramidus skeleton nicknamed Ardi (SN: 1/16/10, p. 22) bears evidence of an efficient, humanlike walk combined with plenty of hip power for apelike climbing, says a team led by biological anthropologists Elaine Kozma and Herman Pontzer of City University of New York. Although researchers have often assumed that the evolution of walking in hominids required at least a partial sacrifice of climbing abilities, Ardi avoided that trade-off, the scientists report the week of April 2 in the Proceedings of the National Academy of Sciences.
“Ardi evolved a solution to an upright stance, with powerful hips for climbing that could fully extend while walking, that we don’t see in apes or humans today,” says Pontzer, who is also affiliated with CUNY’s Hunter College. Ardi’s hip arrangement doesn’t appear in two later fossil hominids, including the famous partial skeleton known as Lucy, a 3.2-million-year-old Australopithecus afarensis.

Ardi’s lower pelvis is longer than that of humans, which led some researchers to argue that Ardipithecus mainly climbed in trees and walked slowly with bent knees and hips, or perhaps not at all. But the new study shows it “would not have impeded its ability to walk upright in a humanlike fashion,” says paleoanthropologist Carol Ward of the University of Missouri in Columbia.
Unlike other hominids and living apes, Ardi’s upper pelvis is positioned behind the lower pelvis, enabling a straight-legged gait, Pontzer and his colleagues find. An evolutionary reorienting of the pelvis in that way enabled back muscles to support an upright spine, W­­ard suggests.
A relatively large gluteus maximus works with hamstring muscles to push humans into a straight-legged stance. Ardi may have had a small rear-end muscle for her size, making a forward-positioned lower pelvis especially critical for walking, Pontzer says.

Using previous data from present-day humans, chimps and monkeys, Pontzer’s group documented a relationship between the shape and orientation of the lower pelvis and the energy available for a range of motions involved in walking and climbing. They used those findings to examine fossil pelvises of Ardi, Lucy and a 2.5-million-year-old Australopithecus africanus. No other fossil hominids from that long ago included a pelvis complete enough for analysis.

The researchers also evaluated a nearly 18-million-year-old fossil pelvis from an African ape, Ekembo nyanzae.

A. afarensis and A. africanus displayed pelvic arrangements for upright walking, but not for Ardi’s apelike climbing power. In particular, the lower pelvis of the two Australopithecus species was nearly as short as the walking-specialized lower pelvis of people today. E. nyanzae’s pelvis was specialized for climbing, as in modern apes and monkeys. Its long, straight pelvis enabled walking with bent hips and knees.

The new study coincides with previous evidence that Ardi’s lower back was flexible enough to support straight-legged walking, says paleoanthropologist Owen Lovejoy of Kent State University in Ohio. Lovejoy, who led an initial investigation of Ardi’s lower-body bones, has long contended that ancient hominids had a humanlike gait (SN: 7/17/10, p. 5).

“A. afarensis and A. africanus walked much like we do, and for the most part that goes for Ardi as well,” Lovejoy says.

Ardi’s unusual mix of walking and climbing abilities spurred the evolution of hominid bodies geared toward minimizing lower-limb injuries, Lovejoy proposes. Ardi’s long lower pelvis and apelike, opposable big toe were replaced in Lucy’s kind by a short lower pelvis connected to smaller hamstring muscles, a humanlike big toe and a fully developed arch (SN: 3/12/11, p. 8). Those changes made climbing harder for A. afarensis, but stabilized its upright stance, helping to prevent foot injuries and hamstring tears when stopping suddenly or accelerating quickly, Lovejoy says.

These seals haven’t lost their land ancestors’ hunting ways

Some seals still eat like landlubbers.

Just like lions, tigers and bears, certain kinds of seals have claws that help the animals grasp prey and tear it apart. X-rays show that the bones in these seals’ forelimbs look like those found in the earliest seals, a new study finds.

Ancestors of these ancient seals transitioned from land to sea at some point, preserving clawed limbs useful for hunting on land. But clawed paws in these northern “true seals,” which include harbor and harp seals, seem to be more than just a holdover from ancient times, says David Hocking, a marine zoologist at Monash University in Melbourne, Australia. Instead, retaining the claws probably helps northern true seals catch a larger meal than they could with the stiff, slippery fins of other pinnipeds such as sea lions and fur seals, Hocking and his colleagues report April 18 in Royal Society Open Science.
Hocking and his colleagues spent 670 hours observing wild harbor and gray seals hunting salmon in Scotland. Tests with three captive seals, two harbor seals born in captivity and one spotted seal born in the wild allowed the team to observe eating behaviors at closer range.
While some of the captive seals seemed to prefer swallowing their prey whole, both the wild and captive animals relied heavily on their claws overall, the scientists found. The critters were frequently spotted using their slashers to hold onto prey and rip off smaller bites, much as a land animal like a wolverine or a bear might. Up-close observations revealed seals caught prey underwater, but ripped it apart at the surface. That probably lets them breathe while eating without inhaling gulps of seawater — a challenge when devouring a large meal underwater.
Northern true seals have flexible joints that allow the animals to curl their claws to grasp prey. These flexible joints are also seen on early pinnipeds such as Enaliarctos mealsi, a seal that lived 23 million years ago, Hocking and his colleagues found. Fur seals and sea lions, however, “have inflexible fingers that help them to maintain a stiff flipper,” Hocking says.

The evolution of flipperlike forelimbs helped some pinnipeds propel themselves through the water more efficiently. But slippery flippers aren’t as useful for grasping prey. That could explain why fur seals and sea lions tend to target smaller fish that they can swallow whole underwater without needing to grasp, Hocking says.

But this fully aquatic feeding style might have been a challenge for the earliest pinnipeds, who probably used their clawed paws to hunt more like today’s true seals, the researchers say. Catching prey underwater and then shredding it at the surface was probably a smaller behavioral leap from full-on land feeding than other aquatic hunting strategies.

Documenting seals using their paws to grasp food is a “nice observation,” says Frank Fish, a biologist at West Chester University in Pennsylvania. Without knowing what early seals ate, though, it’s hard to say for sure whether they actively used their claws to hold onto large prey, he says.

Other scientists have documented true seals using their pawlike forelimbs in stereotypically terrestrial ways, too, such as using the claws to dig out lairs in ice or uncovering buried fish from the seafloor.

Masses of shrimp and krill may play a huge role in mixing oceans

When it comes to tiny ocean swimmers, the whole is much greater than the sum of its parts. Ocean turbulence stirred up by multitudes of creatures such as krill can be powerful enough to extend hundreds of meters down into the deep, a new study suggests.

Brine shrimp moving vertically in two different laboratory tanks created small eddies that aggregated into a jet roughly the size of the whole migrating group, researchers report online April 18 in Nature. With a fluid velocity of about 1 to 2 centimeters per second, the jet was also powerful enough to mix shallow waters with deeper, saltier waters. Without mixing, these waters of different densities would remain isolated in layers.
The shrimp represent centimeter-sized swimmers, including krill and shrimplike copepods, found throughout the world’s oceans that may together be capable of mixing ocean layers — and delivering nutrient-rich deep waters to phytoplankton, or microscopic marine plants, near the surface, the researchers suggest.
“The original thinking is that these animals would flap their appendages and create little eddies about the same size as their bodies,” says John Dabiri, an expert in fluid dynamics at Stanford University. Previous work, including acoustic measurements of krill migrations
in the ocean ( SN: 10/7/06, p. 238 ) and theoretical simulations of fluid flow around swimmers such as jellyfish and shrimplike copepods ( SN: 8/29/09, p. 14 ), had suggested that they may be stirring up more turbulence than thought.
In 2014, Dabiri coauthored a study that debuted the laboratory tank setup also used in the new research. That paper noted that migrating brine shrimp created jets and eddies much larger than themselves. “But there was skepticism about whether those lab results were relevant to the ocean,” Dabiri says. The 2014 study didn’t account for how ocean water stratifies into layers that don’t easily mix, due to differences in salinity or temperature. It wasn’t clear if shrimp-generated turbulence could be strong enough and extend deep enough to overcome the physical barriers and mix the layers.

The new research used a 1.2-meter-deep tank and a 2-meter-deep tank. Each held tens of thousands of wiggly brine shrimp in two layers of water of different densities. The researchers used LED lights to prompt the shrimp to migrate upward or downward, mimicking the massive daily, vertical migrations of krill, copepods and other ocean denizens. The shrimp migrated in close proximity to one another – and that helped to magnify their individual efforts, the scientists found.

“As one animal swims upward, it’s kicking backward,” Dabiri says. That parcel of water then gets kicked downward by another nearby animal, and then another. The result is a downward rush that gets stronger as the migration continues, and eventually extends about as deep as the entire migrating group. In the ocean, that could be as much as hundreds of meters.“At the heart of the investigation is the question about whether life in the ocean, as it moves about the environment, does any important ‘mixing,’ ” says William Dewar, an oceanographer at Florida State University in Tallahassee. “These results argue quite compellingly that they do, and strongly counter the concern that most marine life is simply too small in size to matter.”

The team’s finding opens the door to a host of interesting questions, Dewar adds. Ocean mixing is an important part of the global climate cycle: It churns up nutrients that feed phytoplankton blooms and aids the exchange of gases with the atmosphere. Adding biologically driven mixing to physical processes in the ocean makes the equation even more complex, he says.

The next step will be to try to observe the effect at sea, using shipboard measurements, Dabiri says. “Previous studies looked for turbulence or eddies on the scale of the animals’ size,” he says, instead of large downward jets. “This paper tells us for the first time what to look for.”

Asteroids could have delivered water to the early Earth

Shooting small rocks from a high-speed cannon showed that some asteroids could have brought water to the early Earth — without all the water boiling away on impact, a new study finds.

“We can’t bring an asteroid to Earth and crash it into the Earth, bad things would happen,” says planetary geologist R. Terik Daly, who did the research while a graduate student at Brown University in Providence, R.I. “So we went into the lab and tried to re-create the event as best we can.”
After the solar system formed about 4.6 billion years ago, Earth grew up relatively close to the sun, where it was too hot for water to condense out of the gas phase. And Earth was too small to hold on to much nearby gas anyway. So scientists think the pale blue dot may have received its water from somewhere else — although exactly how that happened is still up for debate (SN: 5/16/15, p. 18).

Daly, now at Johns Hopkins University, and Brown planetary scientist Peter Schultz made marble-sized pellets of antigorite, a mineral found in Japan that is similar to the kinds of rocks that may have brought water to Earth billions of years ago. To simulate a dry planetary surface, the team baked pumice at 850° Celsius for 90 minutes. Then the team shot the pellets at the pumice at about 5 kilometers per second using the NASA Ames Vertical Gun Range in California.
That speed is similar to those at which asteroids probably crashed into each other when the planets were forming, Daly says. Previous simulations suggested that all of an asteroid’s water would vaporize upon impact if the asteroid had been traveling faster than 3.1 kilometers per second. On a planet like the early Earth, which lacked an atmosphere, that water vapor would then have been lost to space.
But Daly and Schultz found that some of the water vapor released by the pellets’ impacts was captured within glass created from shocked rock, or conglomerates of “busted-up” rocks called breccias. Asteroids could have delivered up to 30 percent of their stored water to growing planets, the scientists conclude April 25 in Science Advances.
The next step is working out how the water could escape from rocks to create oceans and other water bodies, Daly says.

“I really like this work,” says planetary scientist Yang Liu of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who was not involved in the study. “The experimental setup is very clever.”

Liu studies water in lunar material, and one frequent question about her work is how the moon can have water at all (SN: 10/24/09, p. 10). Earth’s nearest celestial neighbor lacks a thick atmosphere where vapor can accumulate, which means the moon should have had an even harder time keeping impact-delivered water than the Earth did.

“This work demonstrates that this is feasible even for airless bodies,” she says. The finding even suggests a way for future crewed missions to find water on the moon: “Perhaps we should just look for impact melts to get the water we need.”

50 years ago, starving tumors of oxygen proposed as weapon in cancer fight

Animal experiments demonstrate for the first time that transplanted tumors release a chemical into the host’s bloodstream that causes the host to produce blood vessels to supply the tumor.… If such a factor can be identified in human cancers … it might be possible to prevent the vascularization of tumors. Since tumors above a certain small size require a blood supply to live, they might by this method be starved to death. — Science News, May 4, 1968

By the 1990s, starving tumors had become a focus of cancer research. Several drugs available today limit a tumor’s blood supply. But the approach can actually drive some cancer cells to proliferate, researchers have found. For those cancers, scientists have proposed treatments that open up tumors’ gnarled blood vessels, letting more oxygen through. Boosting oxygen may thwart some cancer cell defenses and promote blood flow — allowing chemotherapy drugs and immune cells deeper access to tumors (SN: 3/4/17, p. 24).